Semiconductor nanostructures towards electronic and opto-electronic device applications – VI

Resume : Phosphorescent iridium complexes have long been thought to have random orientation when doped in an emitting layer due to their octahedron structures. Recently, however, some heteroleptic iridium complexes have been reported to have preferred emitting dipoles orientation (EDO) along horizontal direction (parallel to substrates). The outcoupling efficiency of the emitted light from the horizontally oriented emitting dipoles in an OLED can reach 45% which is much higher than isotropically oriented transition dipoles. In this talk, we will present that the preferred EDO of Ir complexes in OLEDs originates from the preferred direction of the triplet transition dipole moments and the intermolecular interaction with host molecules at the vacuum-organic interface. The EDO is influenced by many factors which can be summarized as follows: (1) Heteroleptic iridium complexes are more likely to have preferred orientation in host materials than homoleptic iridium complexes. (2) (2) The EDO of heteroleptic Ir-complexes varies from horizontal to isotropic, or even to vertical direction depending on host molecules. (3) The preferred molecular orientation of the host molecules does not induce the preferred molecular orientation of the dopant molecules. (4) Molecules with a bulky ancillary ligand and substituents at the para-position of the pyridine in the main ligands of Ir complexes plays pivotal role inducing the orientation of heteroleptic Ir complexes. Finally high efficiency OLEDs with external quantum efficiencies over 35% will be presented using Ir and Pt based phosphorescent dyes possessing highly oriented emitting dipoles and high photoluminescent quantum yields.

Resume : Enhancing electroluminescence (EL) efficiency might be a never ending pursuit from displays to lightings where OLEDs are being used as sources. For past years, phosphorescent OLEDs (PhOLEDs) based on heavy metal complexes have been considered as an only solution to realize high efficiencies by harvesting both singlet and triplet excitons as light. However, recent progress utilizing delayed fluorescence challenges the conventional idea of achieving high EL efficiencies and further has even proved its competency by achieving 100% internal quantum efficiency (IQE). There are two different mechanisms reported that drive delayed fluorescence which are triplet-triplet annihilation (TTA) and thermally activated delayed fluorescence (TADF). Fluorescent material showing TADF phenomenon enables additional harvest of triplet excitons as well as singlet excitons via reverse intersystem crossing (RISC) from triplet (T1) to singlet (S1) state due to thermal activation and small energy gap between the two excited states. Replacing phosphorescent material to fluorescent material will eventually lower cost, clear stability issues and help manufacture environment friendly products. Therefore, there has been tremendous need to synthesize fluorescent dye showing TADF phenomenon. Especially, among the three primary colors, blue dye has been considered as the most crucial due to its importance role on creating white light with purity and longer lifetime than phosphorescent material, however still technically challanging to synthesize. Therefore, commercially, blue fluorescent materials are being used for OLEDs in spite of their deficient IQE of ~25%. In order to enhance EL efficiency, TADF phenomenon must be involved in to take advantage of triplet manifold where excitons are rich. In this study, our approach to achieve high EL efficiency in blue fluorescence OLEDs will be demonstrated. With different design concepts on blue TADF materials, we were able to achieve high EL efficiency and color purity for blue fluorescence OLEDs.

Resume : The performance of organic semiconductor lasers in the quasi continuous-wave (qCW) and CW regimes has, up to now, remained far below that of inorganic semiconductor lasers. Consequently, the realization of CW lasing from organic semiconductor films is highly anticipated for practical applications in the areas of spectroscopy, data communication and sensing but still remains a challenging objective. In this study, low-threshold surface-emitting organic distributed feedback lasers operating in the qCW regime at 80 MHz as well as under CW photoexcitation of 30 ms have been demonstrated by combining a mixed-order grating with an organic thin film of a host material 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) blended with an organic laser dye 4,4’-bis[(N-carbazole)styryl]biphenyl (BSBCz). These superior CW lasing properties are accomplished using BSBCz with high optical gain, high photoluminescence quantum yield and no triplet absorption losses at the lasing wavelength. Simple encapsulation of laser devices with CYTOP greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium taking place otherwise under intense CW photo-excitation. This study provides evidence that the development of a CW organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture and for the realization of an electrically-pumped organic laser diode.

Resume : In organic light-emitting transistors the structural properties such as the in-plane geometry and the lateral charge injection are the key elements which enable the monolithic integration of multiple electronic, optoelectronic and photonic functions within the same device [1]. In this contribution, we report on the realization of hybrid highly-integrated multifunctional optoelectronic organic device by introducing an inorganic high-capacitance photonic crystal [2] as a gate dielectric into a transparent single-layer ambipolar OLET. By engineering the photonic crystal multistack and band gap, we show that the integration of the inorganic photonic structure has a twofold effect on the optoelectronic performance of the device, i.e. (i) to modulate the spectral profile and outcoupling of the emitted light and (ii) to enhance the transistor source-drain current by a 25-fold factor. Consequently, the photonic-crystal integrated OLET showed an order-of-magnitude higher brightness with respect to the corresponding polymer-dielectric device, while presenting as-designed electroluminescence spectral and spatial distribution. Our results validate the efficacy of the proposed approach that is expected to unravel the technological potential for the realization of hybrid highly-integrated optoelectronic smart systems based on organic light-emitting transistors. [1] M. Muccini el al, Laser & Photon. Rev., 6, 2, 258–275 (2012); [2] L. Passoni et al. ACS Nano, 8, 12, 12167-12174 (2014).

Resume : We report on a series of highly fluorescent fluorene derivatives that are functionalized with siloxane chains to become non-volatile liquid at room-temperature (RT). As RT neat liquids, these materials exhibit excellent performances in terms of PLQY, Amplified Stimulated Emission (ASE) threshold and charge-carrier mobilities. In particular, unprecedented ambipolar charge transport with hole and electron mobilities exceeding 10-4 cm2/V.s could be observed in the RT liquid state. Finally, these materials led to the first demonstration of an organic distributed feedback (DFB) laser based on a monolithic liquid molecular semiconductor. [ref] J.-C. Ribierre, L. Zhao, M. Inoue, P.-O. Schwartz, J.-H. Kim, K. Yoshida, A.S.D. Sandanayaka, H. Nakanotani, L. Mager, S. Méry, C. Adachi, Chem. Commun. 2016, 52, 3103

Resume : Since two-photon sterolithography (TPS) first came out as a novel technique over a decade ago, a great number of diverse micro-objects have been fabricated using TPS with a variety of effective two-photon chromophores. In TPS, when a near-infrared ultrashort-pulsed laser is closely focused into a volume of photocurable resins, real 3D microstructures can be fabricated using a layer-by-layer accumulating technique; therefore, TPS is considered to be a promising technique for 3D nano/microfabrication. In particular, 3D microstructures containing noble metals or semiconducting quantum dots are of interest for applications in optoelectronics, photonics and biophotonics. Quantum size effects become relevant in noble metal nanoparticles. Because of this there are a many issues regarding the use of photo-lithography in fabricating noble metal containing microstructures including undesirable optical and thermal effects. Realizing well defined ordered structures containing noble metals and quantum dots are hence a challenge in the realm of microfabrication. In this lecture, various approaches for achieving 2D and 3D organic-inorganic hybrid structures is presented. In addition, recent developments of novel 3D cancer cell chips using a three-floor hierarchical 3D pyramid structure for the in vitro 3D cell growth simulation of tumor cells and detection of anticancer drugs is also reported.

Resume : Semiconductor nanowires (NWs) have been intensively studied in recent years due to their potential applications in the new generation of extraordinary tiny devices [1]. Among various III-V semiconductors, particularly, InAs has attracted much research attention aiming at high-frequency nano-electronics and long-wavelength nano-photonics due to its exceptionally high electron mobility and narrow energy band-gap [2]. However, tuning the performances of such nano-devices is another challenge aside from the fabrication of these objects. In NWs as the surface to volume ratio is large, the surface charge concentration and surface defect states play a significant role in their electronic properties. Therefore, modification of NW surface can be used to tune the electronic property of nanostructure materials. Focused laser beams have been used for a long time for annealing, doping and oxidation of materials [3,4]. Here, we have investigated the feasibility of using a focused laser beam with variable power to tune the hole density in InAs nanowires by simultaneous photogeneration and local heating [5]. We used micro-Raman spectroscopic technique for such studies because the laser irradiation during Raman measurement can cause local sample heating and can create electron-hole pairs. The oxide layer formed on the surface of InAs NWs, due to laser-induced heating; decouples electron-hole pairs by trapping electrons. The interplay between optical phonons and the photogenerated charge carriers is realized by monitoring coupled plasmon-LO phonon (CPLP) modes at different laser powers. The study shows a significant correlation between the oxide layer thickness and hole density in InAs NW [5]. References: [1] W. Lu and C. M. Lieber, Nat. Mater., 2007, 6, 841-850. [2] S.Adachi, Properties of Group-IV, III–V and II–VI Semiconductors, Wiley, 2005. [3] D Bäuerle, Appl. Surf. Sci. 2002 186, 1–6 [4] A. Chimmalgi, D. J. Hwang and P. Grigoropoulos Nano Lett. 2005, 5, 1924–30 [5] D. Majumdar, D. Ercolani, L. Sorba and A. Singha, Journal of Materials Chemistry C 2016, 4 , 2339-2344

Resume : Heterostructured semiconductor nanoparticles, including core-shell and dot-in-rod structures, have been garnering interest for their potential in applications including lighting/displays, solar concentration, photocatalysis, and photodetectors. One important aspect is that their structure and electronic properties can be synthetically tuned to control quantum yields, optical polarization, self-assembly, photocatalytic activity, etc. This work focuses on nanorod heterostructures containing a CdSe quantum dot core with surrounding CdS shell. It aims to elucidate various structural and electronic properties and their effects on assembly through a combination of nanoparticles and characterization techniques providing a more complete understanding of the materials. To be able to take advantage of the optical properties of these anisotropic materials and use them in the fabrication of nanocrystal-based devices, we need to be able to direct their orientation into large-scale organized and close-packed layers. One of the mechanisms of doing this is with the nanocrystals self-assembling into ordered structures through the minimization of energy. Another alignment method of interest is with the use of an external stimulus, such as an electric field (electrophoretic deposition), to direct the assembly of the nanocrystals. The work presented here will show the control over alignment that can be achieved with either method, as well as delve into the various parameters that can influence the alignment, such as nanorod length, surface chemistry, and particle charge. Additionally, the optical and electronic properties of the assembled structures will be discussed.

Resume : We will discuss a lithographic method that employs the technique of cation exchange in colloidal nanocrystals for structuring surfaces at a nanoscale. Specifically, we present the limits in spatial resolution of this technique and a design for exploiting the method for the production of opto-electronic devices. Cation exchange has been established as a post-synthesis method for altering the chemical composition of colloidal nanocrystals without changing the morphology.1 2 Apart from facilitating synthesis schemes for various colloidal materials this technique can also be employed to films of nanoparticles.3 In this case the cation exchange can be combined with lithography methods. The effect of high-energy radiation on the surfactants prevents the underlying nanoparticles from being cation-exchanged and thus allows for a structuring of the films at high resolution. In particular we will discuss examples of cation exchange from Cd to Cu or Pb in films and wires of CdSe or CdS and discuss the opto-electronic applications of such structures. 1. Rivest, J. B. & Jain, P. K. Cation exchange on the nanoscale: an emerging technique for new material synthesis, device fabrication, and chemical sensing. Chem Soc Rev 42, 89–96 (2013). 2. Beberwyck, B. J., Surendranath, Y. & Alivisatos, A. P. Cation Exchange: A Versatile Tool for Nanomaterials Synthesis. J. Phys. Chem. C 117, 19759–19770 (2013). 3. Miszta, K. et al. Nanocrystal Film Patterning by Inhibiting Cation Exchange via Electron-Beam or X-ray Lithography. Nano Lett 14, 2116–2122 (2014).

Resume : Thermal scanning probe lithography (t-SPL) is a technique that has achieved resolutions of <10 nm, linear patterning speeds of >10 mm/s1 and less than 5 nm error for overlay accuracy on nanoscale devices buried under the resist stack2. A dedicated pattern transfer process has been developed to successfully achieve pattern transfer by reactive ion etching of 14 nm half pitch lines and metal lift¬-off of sub 20 nm features3. Recently, mix and match with laser writing has been added to the t-SPL tool to enable fast processing of bigger fields such as contact pads defined by laser beam. We demonstrate the capabilities of the tool by creating devices with high resolution features in various substrates. In silicon we fabricated single electron memory devices with sub 25 nm dimensions in a 12 nm thick silicon layer. Leads to the outside were fabricated in the same patterning step by laser writing. On MoS2 substrates we patterned large arrays of nanoribbons with sub 20 nm dimensions and a length of 100-300 nm to create nanoribbon FET devices. Using InAs nanowires, we fabricated source and drain contacts with down to 50 nm distance and observed quantized conductance in the channels at 80 K with a 90% injection efficiency and a steep switch on-off behavior using top gates. Acknowledgement: Funding was provided by the EU FP7 program FP7/2007-2013 No. 318806 (SNM). 1P. Paul et al., Nanotechnology 23, 385307 (2012) 2C. Rawlings et al., ACS Nano 9, 6188-6195 (2015) 3H. Wolf et al., J. Vac. Sci. Technol. B 33, 02B102 (2015)

Resume : Developing scalable methods for the deposition of two-dimensional (2D) semiconductors onto substrates continues to be an important area of research for potential future electronic and optical devices. Here we develop an entirely new technique for the controllable deposition of wafer-scale 2D post-transition metal chalcogenides, by depositing native interfacial metal oxide layer of low melting temperature metal precursors (e.g metallic Ga, In and Sn).[1] These metals readily form an atomically-thin protective oxide layer in a self-limiting reaction which can be transferred onto oxygen terminated substrates using our devised method. This atomically thin oxide layer is then transformed into chalcogenide compounds using a low temperature reaction (~300 C). The result is semiconducting bi-layer GaS on wafer scale, featuring strong photoluminescence and a bandgap of 3.0 eV. We furthermore demonstrate a surface patterning technique utilizing photolithography and organic functionalization agents that allow for the controlled and targeted deposition of 2D GaS. Finally we investigate the synthesized bilayer GaS for transistor and photodetector devices. The developed method was found to be also suitable for the deposition of atomically thin indium and tin based compounds.

Resume : In this paper we will present some of the results obtained at i3N/CENIMAT in the area of functional metal oxide nanoparticles to be used in a wide range of sensor applications. ZnO nanostructures have been produced either under microwave irradiation using low cost domestic microwave equipment or by conventional heating, both under hydrothermal conditions. X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, room/low temperature photoluminescence and Raman spectroscopy have been used to investigated the structure, morphology and optical properties of the produced ZnO nanorods. The hexagonal wurtzite structure has been identified, and a red-orange emission has been detected in the presence of UV irradiation for all the conditions studied. The as-prepared ZnO nanorods were test as a UV sensor on paper substrate, which lead to a fast response and a rapid recovery times, as well a sensitivity up to 10 indicating that these materials may have a high potential to be employed in low cost, disposable UV applications. The capability to control in an smart way the infrared reflectance to environmental temperature variations, can be achieved with thermochromic materials like VO2. In this paper we report by the first time a new application of VO2 on ceramic tiles, aiming to control the reflected IR radiation on smart roofs and thus improving the energy efficiency as possible and reducing the carbon dioxide emissions. The VO2 NPs have been produced by hydrothermal synthesis, providing a new, quicker and cleanner production route. The superior thermochromic characteristics of VO2 nanoparticles in conjunction with this new application to a smart roof offers a great potential to regulate the energy in an intelligent way. Electrochemically active bacteria have the capability to transfer electrons to cell exterior, a feature that is currently explored for important applications in bioremediation and biotechnology fields. However, the number of isolated and characterized EAB species is still very limited regarding their abundance in nature. Colorimetric detection has emerged recently as an attractive mean for fast identification and characterization of analytes based on the use of electrochromic materials. In this work, WO3 nanoparticles were synthesized by microwave assisted hydrothermal synthesis and used to functionalize non-treated regular copy-paper substrates. This allowed the production of a paper-based colorimetric sensor able to detect EABs in a simple, effective, low-cost and eco-friendly manner.

Resume : Oxide semiconductors as the active layer have attracted much attention for various applications including display due to their high electrical and optical characteristics available at low process temperatures. Indeed, InGaZnO, as a representative, has been adopted as an active material in the backplane of high-resolution display in mass production since 2012. Here we report the device instability under the AC bias stress on the drain of thin-film transistors (TFT) which may be one of the main origins of the failure of integrated circuits (providing the control signal for the gate in the active matrix). The effects of the driving frequency, pulse shape and the strength of lateral electric field as well as the channel current are investigated. We employ bottom gate, top contact structure with etch stop layer for InGaZnO TFT. A 150 nm thick Mo prepared by RF sputtering was used as a gate electrode and a 200 nm thick silicon dioxide deposited by plasma enhanced chemical vapor deposition as a gate dielectric. A 40 nm thick InGaZnO film was deposited by RF-magnetron sputtering at room temperature on the gate insulator and 100 nm think silicon dioxide film as an etch stop and passivation layer film was deposited. A 150 nm thick Mo was used as a source and drain electrode and patterned by conventional lithography process. The detection of a current degradation is carried out through current-voltage measurements of transfer curve at the intervals during AC drain stress. The results are also compared with the reliability test under the negative bias, temperature, and illumination stress which is known to be most severe stress condition for oxide semiconductor TFTs. This investigation reports that the device degradation is accelerated drastically by transient properties such as the pulse shape and frequency as well as the the strength of the lateral electric field. We believe that these findings should be considered for the reliable design of integrated circuits which consist of oxide semiconductor TFTs, such as gate driver circuits in the active-matrix display device.

Resume : Back-Side Illuminated CMOS Image Sensors (BSI CIS) requires an anti reflective coating layer (ARC) at the backside interface. Moreover this ARC have to play the role of passivation layer in order to reduce the dark current level due to thermal generation of electrons at the backside Si/SiO2 interface. Indeed, defects at the Si/SiO2 interface imply energy levels into the silicon band gap, commonly called interface traps (Dit), and then assist the generation of electrons. In this study we focus on the passivation layer properties, i.e the reduction in electron’s generation rate at the interface, Us. To do this, there are two possibilities. First, reduce the density of interface traps which is the root cause of electrons’ generation. Secondly, by introducing negative charge in the ARC. Indeed, this permits to accumulate holes at the Si/SiO2 interface which allows to pull the Fermi level toward the valence band and thus to empty the interface states. The aim of our work is to understand origin of defects and charges in ARC to control their quantity and so reduce dark current. For this we review here the results concerning charges (Qtot) and density of interface traps (Dit) obtained using two passivation materials, HfO2 and Al2O3, deposited by ALD. These results are obtained from COCOS (Corona Oxide Characterization Of Semiconductor) measurements on full sheet wafers. In particular we focus on the impact of ALD process, anneal and nature of the underlying SiO2.

Resume : The unique optical and electrical properties of ZnO nanotubes (NTs), due to their high aspect ratio and confinement effect, can lead to high optical UV efficiency light emitting diodes (LED). Furthermore, self-assembled ZnO NTs are easier to form, with larger exciton binding energy (60 meV) and inexpensive compared to GaN materials. In this study, to fabricate high-brightness ZnO/GaN NTs-based UV-Blue LED, high optical and structural quality vertical n-ZnO NTs without catalyst doped with Gadolinium (Gd) (2 wt%) were grown on MOCVD p-GaN films with c-sapphire substrate using pulsed laser deposition. The deposition parameters are optimized to grow high quality hexagonal doped ZnO NTs to be 660 C and 150 mTorr. X-ray diffraction, scanning electron microscopy and transmission electron microscopy confirmed highly textured single crystal hexagonal-shaped ZnO NT with thin wall (>20 nm) and wide hollow (>200 nm diameter) and 800-1000 nm height grown along c-axis. We show an intense UV–blue electroluminescence emission at room temperature arising from our ZnO NT-based LEDs. Micro-photoluminescence (μPL) reveals an intense bandedge emission with a very weak defect band, indicating high optical quality. The strong blue emission can be due to donor presence as a result of Gd doping. Electrical measurements performed using four-probe station show distinct diode characteristics. We demonstrate that Gd dopants significantly improve the device performance due to donor injection. We demonstrate a fabrication method for extracting the emission from GdZnO/GaN NTs. We investigate carrier dynamics by time resolved PL spectroscopy using femtosecond Ti:sapphire laser with a frequency tripled wavelength (266 nm), that is attached to a streak camera.

Resume : The CuCrO2 delafossite phase has been attracting high interest in the last two decades [1]. Its direct wide band gap energy of 3.1 eV together with its high conductivity, up to 217 S.cm-1 with the incorporation of Mg [2], are relevant for its integration as a p-type transparent semiconductor. The use of CuCrO2 in p-n photodiodes has previously been assessed in thin film heterostructures, with ZnO as the n-type semiconductor by pulsed laser deposition [3,4] and magnetron sputtering [5]. However, the photodiodes produced thereof presented low rectifying behavior, with a ratio smaller than 100 between the forward current to the reverse current at 1.5 V [3]. The use of ZnO nanowires to create photodiodes has already been developed in combination with a variety of p-type semiconductors, such as GaN [6] or CuSCN [7], through the fabrication of core shell nanowire heterostructures. These radial heterostructures offer a number of advantages related to their high surface-to-volume ratio such as efficient light trapping and charge carrier management [6-8]. In this work, we present a new UV photodetector obtained by depositing a thin shell of CuCrO2 over an array of vertically aligned ZnO nanowires. The ZnO nanowire arrays are grown by chemical bath deposition (CBD) on a ZnO seed layer deposited by sol-gel on commercial ITO-covered glass substrate. The CuCrO2 delafossite phase is deposited on top of the ZnO nanowire arrays by aerosol-assisted metal organic chemical vapor deposition (AA-MOCVD) at 400°C using copper and chromium acetylacetonate precursors. It is found that the 40 nm-thick CuCrO2 shell conformally and uniformly covers the 1 micron-height ZnO nanowires. The structural morphology of the grains is analyzed by automated crystal phase and orientation mapping with precession (ASTAR) in a TEM, showing a columnar grain growth oriented perpendicularly to the surface of the ZnO nanowires. These core shell nanowire heterostructures present a high absorbance in the UV region (i.e., below 400nm), due to the wide band-gap energy of both oxides. The J-V characteristic curve shows a significant rectifying behavior, with a reverse current density below 2x10-4 A/cm2, a leakage current of 5x10-6 A/cm2, and a forward current at +1V of 0.9 A/cm2. The ratio between the forward current to the reverse current at 1V is as high as 5500, which is much larger than for the thin film photodiode [3], and reveals the high interface quality of the present heterostructures. Moreover, J−V measurements under dark and AM1.5G solar illumination conditions show a photovoltaic behavior with open-circuit voltage (Voc) and short-circuit current density (Jsc) of 39 mV and 8 x10-7 A/cm2, respectively. The combination of the high absorbance in the UV region, of the high rectifying ratio and of the photocurrent generated highlights promising possibilities for its use as a self-powered UV photodetector. The fabrication of this device is further enabled by the use of AA-MOCVD and CBD at temperatures below 400 °C, a set of processing conditions which could favor the implementation of all oxide-based UV photodetector with reduced costs and large scale production. [1] Nagarajan R. et al (2001) J. Appl. Phys., 89, 8022-8025 [2] Tripathi T. S. et al (2017) Adv. Electron. Mater., 1600341 [3] Tonooka K. et al (2006) Thin Solid Films, 515, 2415–2418 [4] Chiu T. W. et al. (2008) Thin Solid Films 516 5941–5947 [5] Chen L. F. et al. (2013) Jpn. J. Appl. Phys. 52 05EC02 [6] Chen C.-H. et al. (2009) Chemical Physics Letters, 476, 69–72 [7] Garnier J. et al. (2015) ACS Appl. Mater. Interfaces, 7, 5820−5829 [8] Michallon J. et al. (2014) Optics Express, 22, A1174

Resume : ZnSe layers were grown on ZnO substrates by the metal organic chemical vapor deposition technique. A new structure appeared at lower thicknesses films. The structural properties of the thin films were studied by the X-ray diffraction (XRD) and Raman spectroscopy methods. First, Raman selection rules are explicitly put forward from a theoretical viewpoint. Second, experimentally- retrieved- intensities of the Raman signal as a function of polarization angle of incident light are fitted to the obtained theoretical dependencies in order to confirm the crystallographic planes of zinc blend ZnSe thin film, and correlate with DRX measurements. Raman spectroscopy has been used to characterize the interfacial disorder that affects energy transport phenomena at ZnSe/ZnO interfaces and the Photoluminescence (PL) near the band edge of ZnSe thin films.

Resume : Self-powered, High-performance, ultraviolet photodetector based on hydrogenated doped zinc oxide nanoflakes Buddha Deka Boruah and Abha Misra Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, Karnataka, India 560012 Recently, self-powered opto-electronic devices have drawn great research interest in the scientific community due to the wide ranges of applications.1,2 Among those ultraviolet (UV) photodetector displays potential impact due to the various application in industrial, water sterilization, environmental, military, flame sensing, early missile plume detection, radiation detection etc.3,4 In our work, hierarchal structure is fabricated for self-powered UV photodetector by combining hydrogenated doped zinc oxide nanoflakes (H:ZnO NFs) and poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS) where H:ZnO NFs acts as a UV radiation sensitive material as well as electron transportation layer and PEDOT:PSS is hole transportation layer. ZnO NFs was directly grown on transparent indium tin oxide coated glass substrate. As-synthesized NFs display superior UV light absorption efficiency as compared to other nanostructures where the light harvesting efficiency for ZnO NFs was measured 98%. Due to the larger size of NFs than radiation UV light, multiple reflections take place in the cavity of NFs that resulted in increased light absorption efficiency. Moreover, large number of free charge electrons in hydrogenated NFs significantly reduces the recombination probability of photo generated charge carriers under UV illumination. Therefore, outstanding increase of response current (photo current – dark current) up to 25 times was noticed in hydrogenated NFs as compared to NRs. Fabricated self-powered UV photodetector showed superior response current of 1000 nA under low UV illumination signal of 3 mW/cm2 in absence of external bias voltage along with the outstanding photon detection speed in the order of ms which is much faster than any conventional ZnO based photodetectors. Cyclic and spectral photoresponse studies of the photodetector reveal stable and visible-blind UV sensitive photo response nature. Furthermore, the higher values of photo response parameters such as responsivity (2.6 mA/W), external quantum efficiency (1%) and specific detectivity (1010 Jones) of the self-powered photodetector under low UV illumination intensity of 3 mW/cm2 demonstrate the high-performance self-powered photodetector. This work opens novel avenues in the field of self-powered energy conversion device such as photodetector by providing structural advantages. References 1. F. R. Fan, W. Tang and Z. L. Wang, Adv. Mater. 2016, 28, 4283. 2. F. R.Fan, L. Lin, G. Zhu, W. Wu, R. Zhang and Z. L. Wang, Nano Lett. 2012, 12, 3109. 3. H. Chen, K. Liu, L. Hu, A. A. Al-Ghamdi and X. Fang, Mater. Today 2015, 18, 493. 4. B. D. Boruah, A. Mukherjee, S. Sridhar and A. Misra, ACS Appl. Mater. Interfaces 2015, 7, 10606.

Resume : ZnTe homojunction diode doped with oxygen is a very promising material for photovoltaic applications. It is shown, by numerical optimization, that the properties of an intermediate band solar cell present in the ZnTe:O (Energetic position of the IB, oxygen doping concentration...) play a crucial role in its performance. In the present study the intermediate band solar cell (IBSC) parameters are optimized using a solar cell device simulator (SCAPS). We have considered the parasitic effects such as the shunt resistance and series resistance which dramatically influence the performance of the cell. We observed that when the open circuit voltage and fill factor decrease, the short circuit current is increased by more than 100 % and we have an improvement of approximately 30 % in the conversion efficiency compared to the ZnTe homojunction without oxygen.

Resume : In present work, Molybdenum disulfide (MoS2) nanostructured thin films (NTFs) were synthesized by sulfurizing MoO3 NTFs in three different non-conventional methods (named methods 1-3). Method 1 uses sulfur vapors, second utilizes H2S/Ar gas and third adopts plasma of H2S/Ar gas. The effect of sulfurizing ambient on its efficiency to convert MoO3 into MoS2 has been studied. And parameters such as crystallinity, purity, uniformity and stoichiometry control have been basis of this study. The samples showed uniform nanoflakes (NFs) structures throughout sample, revealed by SEM, same as their precursor MoO3. XRD and Raman disclosed crystalline MoS2 in all three methods, however the degree of crystallinity was greater in case of sulfurization in H2S/Ar plasma ambient. HR-TEM revealed formation of core-shell nanostructures comprising of MoO2 in core and MoS2 making shell. Maximum shell thickness (~ 20 nm) was obtained in method 3. Quantitative analysis of sulfurized films carried out by XPS, shows the presence of MoS2 in methods 1,2 and 3 with percentage found to be 18%, 87% and ~100% respectively. Although all three methods have resulted in synthesis of MoS2, However plasma ambient has resulted in high quality of MoS2 NTFs based on parameters such as crystallinity and stoichiometry control. Hydrogen sulfide plasma provides reducing environment as well as source of reactive sulfur species for sulfurization. The advantage of using plasma is three-fold. First the synthesis temperature is reduced due to the presence of sulfur’s ionic species in plasma. Secondly there is significant decline in the duration required to convert MoO3 into MoS2. And thirdly, as reduced temperatures are involved for synthesis, we have greater choices for substrates to be used.

Resume : Vanadium oxides (V-O) have enticed scientists and researchers due to their physical, chemical, optical and electrical properties offering wide optical band gap, good thermal and chemical stabilities. This makes oxides of vanadium, highly suitable, to be used in catalysis, electrochromic, power storage and gas sensing devices applications. It is essential for any device to be employed in sensing applications to have high aspect ratio and high surface area. We report the synthesis of vanadium oxide nanostructured thin films (NTFs) over wide area (400 mm2) using a novel technique plasma assisted sublimation process (PASP). Vanadium oxide NTFs were synthesized on glass substrates in N2 plasma and N2 gas at 550° C for a duration of 45 min. V2O5 pellets used as precursor, were kept in vacuum chamber at optimized chamber pressure. Nitrogen plasma accelerates the reduction of vanadium oxide which is then sublimated (heating by controlled supply) and deposited on substrates. SEM micrographs show rod like morphology of microstructures with flake like nanostructures on the surface revealing very high surface area of NTFs. HRTEM images of film revealed that flake like morphology as was observed in SEM, therefore confirming results obtained on morphological studies. XRD and Raman spectroscopic studies confirm long range order in reduced V2O5 (O/V ratio ~ 2.2) indicating structures are highly crystalline in nature. In absence of plasma, the crystallinity as well as surface morphology deteriorated, thereby inferring that plasma plays an important role in microstructure, nanostructure as well as crystal structure of vanadium oxides NTFs with different stoichiometry (controlled by plasma). Hence, high surface area nanostructured thin films are obtained for sensing applications.

Resume : Recently, research for surface nano-patterns for structural color realization has been highlighted. The color from surface nanostructures exhibits unique features such as broad blue iridescence, high resistance to discoloration and angle independent spectra, which are dissimilar to the color from pigment dye. Therefore structural color stands a chance of using prevention of forgery and security. However present nanostructure manufacturing processes following high cost, especially expensive nano-patterning process, are not suitable for mass production. Therefore, the nanostructure fabrication process of low cost and high speed needs to be conducted in order to make the desired shapes easily on the surface for structural color realization. In this presentation, the high-aspect-ratio nanostructure was fabricated by deposition of Ni and Al2O3 thin films on Si substrate and controlled wet-etching process. In addition, effects of surface structure, such as various thicknesses of each film, and interface property on structural color were also investigated.

Resume : The control of liquid crystal (LC) molecules on the alignment films is important for fundamental research and industrial applications. The electro-optical properties of LC devices (LCDs) depend on the alignment films for orientating LC molecules. It has been a mature technology in LCDs industry by using buffed organic polyimide (PI) films for aligning LCs nearly parallel or perpendicular to the substrates. Applications of inorganic alignment films for aligning LCs should be very promising for LCDs operated in severe conditions, where a highly durable material is needed. In this work, we will demonstrate that the LCs can be aligned on the buffed ZnO nanoparticle arrays (ZnO NPAs) and ZnO films fabricated by the hydrothermal method. The pretilt angle between LCs and alignment films is found to be controllable by annealed and UV treated ZnO films. Our results show that the pretilt angle of LCs on ZnO films depends on their surface wettability, and it can be successfully adjusted over a wide range from 97° to 60° water contact angle by UV and annealing treatment.

Resume : Colloidal quantum dots (QDs), semiconductor nanocrystals which exhibit size-dependent bandgap tunability arising from quantum confinement effect, has been drawing attention of researchers from various disciplines for the past decade since its discovery in 1980s. Especially in the field of display technology, unique narrow band emission characteristic of QDs has been considered as an advantage in realizing large color gamut and sharp color reproduction of displays. However, to use such materials in QD emissive displays, method of patterning differently colored QDs on to substrates is one of the bottlenecks in QD application to industrial level. In our previous study[1], we have demonstrated a way of patterning stacked QDs using layer-by-layer assembly and photolithography technique, which is a widely-used, conventional method of forming high-resolution patterns in present day semiconductor industry. In this presentation, we perform optical characterization of QDs using photoluminescence and time-resolved decay measurements using single-photon counting technique, to further expand our understanding of QDs undergoing through such layer-by-layer assembly and photolithography process. Through observing consequent optical properties of QDs, we comprehend how such processes affect the surface states of QDs. [1] J-S. Park et al., Nano Lett., 2016, 16 (11), pp 6946-6953

Resume : Hybrid materials (HMs) consisting nanoparticles of an organic functional agent dispersed in an insulating matrix, are widely used in photonics as active and passive elements as well as integral optics' components High efficient phosphors used in OLED technology are promising organic agents for HMs. One of the problems which can be solved by the HM creation is the protection of organic phosphors from environmental conditions (oxygen, moisture, UV light) which lead to organic phosphors degradation. In the synthesis of HM we used various organic materials: β-diketone lanthanide complex phosphors, complexes of 8-hydroxyquinoline with metals of the I, II and III group of the Periodic Table, individual phenanthroline and β-diketone ligands. As an insulating matrix we used the glass (62 mol.%)PbO-26(mol.%)B2O3-12(mol.%)SiO2, with the lowest Tm=484 C in the quasi-ternary system. HM's synthesis has been carried out in two stages: 1) pre-synthesized glass was melted; 2) powdered organometallic component (0.10 - 0.15 wt%) was added to the melt. The mixture was stirred during 10-20 seconds and cast into a mold. HMs had received a wide band photoluminescence in the 450 - 600 nm. The band shifted to longer wavelengths and significantly broadened compared with original phosphors' PL. HM showed a high stability when exposed to air and by heating to the crystallization temperature of the glass matrix. The research was financially supported by Russian Science Foundation (grant N 14-13-01074)

Resume : AC-driven electroluminescent device with dual functions of lighting panel in nighttime or mirror in daytime was achieved on the structure of transparent electrode/Zn2SiO4:Mn2 phosphor film/dielectric film/reflective metal electrode. To maximize the green electroluminescence from Zn2SiO4:Mn2 phosphor on the assumption that excitation sources are charges injected from the interfacial traps to the phosphor film, various transparent electrodes and dielectric materials were demonstrated. Tungsten or titanium metal was deposited on quartz substrate as a reflective rear electrode due to higher reflectance and higher melting point. The optimized device showed the high green electroluminescence over the threshold voltage of about 50 V, which acts as a lighting lamp. In addition, it has the high reflectance about 70 % when it is off, which works as mirror. Especially, the device showed an excellent temperature-independent behavior compared with ZnS-based device, and the uniform surface light emission of the large size of 10 by 10 mm2.

Resume : Interactive displays involve the interfacing of a stimuli-responsive sensor with a visual human-readable response. Here, we describe a polymeric electroluminescence-based stimuli-responsive display method that simultaneously detects external stimuli and visualizes the stimulant object. This “organic light-emitting board” is capable of both sensing and direct visualization of a variety of conductive information. Simultaneous sensing and visualization of the conductive substance is achieved when the conductive object is coupled with the light emissive material layer upon application of alternating current. A variety of conductive materials can be detected regardless of their work functions, and thus information written by a conductive pen is clearly visualized, as is a human fingerprint with natural conductivity. Furthermore, we demonstrate that integration of the organic light-emitting board with a fluidic channel readily allows for dynamic monitoring of metallic liquid flow through the channel, which may be suitable for biological detection and imaging applications.

Resume : ZnO is one of the most promising optical materials for its unique properties of large excitonic binding energy (60 meV), relatively low recombination loss and high electron mobility as well as robust growth. ZnO nanowires were shown to exhibit lasing previously. In this work, we combine a ZnO nanowire and single-crystalline metal films to fabricate Fabry-Perot type surface plasmon polariton (SPP) lasers to overcome the diffraction limit of conventional optics (λ/2n)3. ZnO nanowires with different lengths were first synthesized by a hydrothermal method. The ZnO nanowires were placed on a single-crystal Al film, which was grown with molecular beam expitaxy. Group velocity refractive index (group index) is a key parameter for evaluating the laser performance since larger group index means stronger interaction between plasmons and gain material. Group indices were obtained with various lasing mode spacings according to the formula ng=(λ2/2 ΔλL). The group indices measured for photonic lasers and plsmonic lasers are compared. The plasmonic lasers exhibit much larger group indices than photonic lasers ensuring ultra-strong confinement of optical field in the subwavelength regime. Plasmonic lasers are potentially useful in applications in biosensing, photonic circuits, and high-capacity signal processing.

Resume : We present an indoor positioning system were trichromatic white LEDs are used as transmitters, and an optical processor based on a-SiC:H technology as a mobile receiver. The optical processor is implemented using a double p-i-n photodetector with two UV light biased gates. The optoelectronic characterization of both transmitter and receiver is performed. The relationship between the optical inputs (transmitted data) and the associated digital output levels (received data) is established and decoded. An algorithm to decode the information is developed. The received signal is used in coded multiplexing techniques for supporting communications and navigation concomitantly on the same channel. The position of the device is estimated using the visible multilateration method by measuring the signal strength from several non-collinear transmitters. The location and motion information is calculated by position mapping and estimating the location areas. Since the transmitted data of the different LED light sources and its location is known, the corresponding transmitted data information, indoor position and motion direction of the mobile device can be determined. Data analysis showed that by using a double pin photodiode based on a a-SiC:H heterostucture as receiver and RBG-LEDs as transmitters it is possible not only to determine the position of a mobile target but also to infer the motion direction over time and also the transmitted information received in each position.

Resume : Colloidal semiconductor nanocrystals or quantum dots (QDs) are promising materials for the application in light-emitting devices, photodetection and solar energy conversion. Most of the previous fundamental studies and potential applications have been focused on Group II-VI QDs such as cadmium (Cd) based chalcogenides. However, the toxicity associated with cadmium could impose critical constraints for practicable applications. Indium phosphide (InP) QDs have been suggested as the most promising candidate to replace Cd-based QDs due to their band gap tunability covering the entire visible range and narrow spectral bandwidth. In recent times, considerable advancement has been made in the synthesis of InP QDs and InP/ZnS QDs featuring emission lines with full width at half maximum (FWHM) of <50 nm have already been synthesized. Still the optical properties of InP QDs are inferior to the well-developed cadmium-based QDs. In this work, we present an improved synthetic method to synthesize high quality InP/ZnSe quantum dots. The emission spectra of the QDs can be continuously tuned from 488 to 640 nm. The obtained QDs have an emission peak FWHM of as low as 35 nm in the shorter wavelengths and 44 nm in the longer wavelengths. The quantum yields of the InP/ZnSe QDs are in the range of 40 – 60%.

Resume : Organic-inorganic hybrid photoelectronic devices have attracted extensive attention in recent years owing to their unique features by combining the high carrier mobility of conductive organic molecules and broadband light absorption of inorganic semiconductors. As compared to Si and GaAs, zinc selenide (ZnSe), a II–VI compound n-type semiconductor with a wide direct bandgap of ~2.70 eV (~460 nm), is more sensitive to blue/UV light. Photodetectors operating in the blue/UV light region are important devices which can be used in many commercial and military applications such as flame detection, ozone layer monitoring and missile warning systems. Meanwhile, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is widely used in hybrid photoelectronic devices because of its high transparency, cost-effective processing method, low sheet resistance and high hole-transport rate. In this work, single-crystalline ZnSe nanowires with 50-100 nm in diameter and up to 30 µm in length were grown on silicon substrate by chemical vapor deposition (CVD). High-aspect-ratio ZnSe nanowires were successfully transferred on flexible polyimide substrate via the contact printing method. The PEDOT:PSS layer was then deposited on ZnSe nanowires by spin-coating. Under 405 nm blue laser irradiance with a power density of 20 mW·cm−2 and 3 V bias, the fabricated photodetector exhibits high on/off ratios between photoresponse current and dark current and fast response speed. The improved results are attributed to function of PEDOT:PSS as p-type conjugated polymer which transports holes and forms a p-n junction with ZnSe nanowires. The highly transparent polyimide (PI) substrate provides excellent mechanical flexibility and thermal stability so that our photodetector can be effectively operated under bending up to almost 180o. The results indicate that PEDOT:PSS/ZnSe nanowires hybrid photodetector on flexible polyimide substrate are very promising for applications in high-quality flexible electronic devices.

Resume : Amorphous oxide semiconductor thin film transistors (TFTs) have attracted much attention with distinguished features of high optical transparency, high mobility, good compatibility, and solution process ability compared to traditional a-Si TFT process. Solution-process fabricated TFTs are attracting increased interest due to the advantages of low cost and high throughput compared to conventional vacuum techniques. The solution-processed zinc tin oxide (ZTO) TFTs based on SiO2 gate dielectric that are easily inclined to make oxygen vacancies do not show good device performances. Also, the leakage current of SiO2 of nanometer scale increased dramatically owing to the tunneling effect. Thus, if a mixed system of ZrO2 and Al2O3 (ZAO) is used as the gate dielectric, electrical properties of TFTs will be improved because of the good interface state and oxygen vacancy suppression in the nearby ZTO system through aluminum and zirconium. There have been reports on double or stacked semiconductors, however, almost no report was found on mixed or stacked soluble gate dielectric system so far. Solution-processed gate dielectrics were fabricated with the combined ZrO2 and Al2O3 (ZAO) in the form of mixed and stacked types for oxide thin film transistors (TFTs). ZAO thin films prepared with double coatings for solid gate dielectrics were characterized by analytical tools. For the first time, the capacitance of the oxide semiconductor was extracted from the capacitance-voltage properties of the zinc-tin oxide (ZTO) TFTs with the combined ZAO dielectrics by using the proposed metal-insulatorsemiconductor (MIS) structure model. The capacitance evolution of the semiconductor from the TFT model structure described well the threshold voltage shift observed in the ZTO TFT with the ZAO (1:2) gate dielectric. The electrical properties of the ZTO TFT with a ZAO (1:2) gate dielectric showed low voltage driving with a field effect mobility of 37.01 cm2/Vs, a threshold voltage of 2.00 V, an on-to-off current ratio of 1.46 × 105, and a subthreshold slope of 0.10 V/dec.

Resume : Two dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous attention for optoelectronic applications due to their suitable bandgap that covers wide energy range. It is often required to form low resistance contact in order to have efficient optoelectronic devices and graphene is always used to form Ohmic and transparent contact with TMDCs. However, how the graphene electrode affects the optoelectronic properties of semiconducting TMDCs is still not clear and there are contradicting results that awaits further clarification. Here, we fabricated TMDC/graphene heterostructures with both p-type and n-type monolayer TMDCs. We achieved reversible and dynamic control of heterostructure properties through both electrostatic gating and photo irradiation. Using the heterostrucutres, we were able to clarify the controversy regarding to the interface interactions. Our results provide general guidelines to fabricate efficient and controllable optoelectronic devices with 2D TMDCs.

Resume : ZnO is a semiconductor material which is a hexagonal structure and a wide band gap of 3.37 eV at a room temperature, and has been studied for many attractive applications such as transistors, gas sensors, light-emitting diode and so on. Thin films of ZnO nanostructures have been synthesized by several techniques. In this study, non-doped ZnO thin films are deposited on glass by spray pyrolysis using Diethylzinc (DEZ) diluted with diisopropyl ether [1]. The major interests of this method are the operation at atmospheric pressure, the low-temperature growth and the deposition on a large area. The deposition of spray pyrolysis is occurred by a droplets stream. The properties of deposited films are controlled by the properties of the used solution and the thermodynamics at the interface between droplet and substrate. Depositions are conducted at the temperature between RT and 150°C using different DEZ concentration. The droplet impact marks are observed by a laser microscope. The optical and structural properties of ZnO films are characterized with spectrophotometer, XRD and TEM. The density of each layer is evaluated by X-ray reflectivity. The surface morphological analysis are carried out using SEM and AFM. The morphology of ZnO are changed sensitively with the spray pyrolysis condition, and affect the optical property and the film density. The change of morphology is explained in relating to the droplet impact remarks. [1] K. Yoshino et al, Jpn. J. Appl. Phys. 50 (2011) 040207

Resume : Recently, wearable photodetector and portable photoconductive devices have been widely studied. And as global warming accelerates, the rate of harmful ultraviolet rays entering the earth is increasing, so photodetector research is underway to prevent diseases caused by ultraviolet rays. In addition, research on materials for fabricating flexible photodetector on the plastic substrates by using low temperature process is necessary. ZnO, possible to process in low temperature condition and have fast electron mobility, is received attention for material used in the flexible electronic devices. ZnO, representative II-VI oxide semiconductor, is chemically stable material having high exciton energy and comparatively wide band gap energy. When Zn2 of ZnO nanorods are substituted with Mg2 ions, electrical and optical properties can be adjusted to a wide energy band gap of magnesium (7.8 eV), and It does not affect the structural properties because the ion radius of magnesium is similar to ZnO. So, we grew Mg-doped ZnO (MZO) nanorods on PES substrates for photodetector by hydrothermal methods. In order to investigate the growth of MZO nanorods according to the type of Mg precursor, we changed the Mg precursor and analyzed its properties. We used Mg(NO3)2 and (CH3COO)2Mg precursor to dope Magnesium to ZnO nanorods. And the precursor species and doping concentration were analyzed. As a results of SEM image, the diameter of the nanorods using Mg(NO3)2 precursor was about 40 times smaller and the density was higher. And as a results of XRD pattern, the FWHM characteristics of nanorods using Mg(NO3)¬2 precursors were better compare to (CH3COO)2Mg precursor.

Resume : The photocurrent of oxide semiconductor thin film transistors (TFTs) can be observed when the device is exposed to an ultra-violet light, because oxide semiconductor is a wide band gap semiconducting material. However, oxide TFTs can not generate photocurrent with the illumination of low-energy photon such as visible-light. Therefore, we decorated cadmium selenide (CdSe) quantum-dots (QDs) and metal nanoparticles in the oxide TFTs to increase the photocurrent with low-energy light, such as a visible light. A thin layer of QDs or metal nanoparticles were placed on or under the oxide semiconductors. We prepared CdSe QDs with sizes of ~6.3 nm, which can absorb red visible light. Metal nanoparticles were formed on a gate insulator using a thermal evaporator and a post-annealing processes. The prepared devices with QDs showed enhanced photocurrent upon exposure to visible-light. Measurements to construct an energy level diagram were made using ultraviolet photoelectron spectroscopy to determine the origin of the photocurrent, and we found that the small band gap of CdSe QDs enables the increase in photocurrent in the oxide semiconductor TFTs even with the visible-light. The device with metal nanoparticles showed enhanced photocurrent and modulation behavior under the visible-light, as well. The increased photocurrent showed a strong coupling between localized plasmons and electrical carriers in the active channel, where metal nanoparticles were existed. The device characteristics and origin of the photocurrent will be presented in detail. This result is relevant for developing highly transparent visible-light photosensors based on oxide semiconductors.

Resume : Presently the core-shell nanowire (NW) heterostructures shows exceptional performances towards photovoltaic application. The fabrication of the core-shell CdS/CdSe NW hetrostructure was done by simple, cheap and convenient bottom-up approaches. CdS NWs can be grown over the fluorinated tin oxide (FTO) glass substrate by placing it in the bath containing freshly prepared cadmium chloride (CdCl2) solution complexed with 25% liquid ammonia (NH4OH) solution. After 9 h the growth of white color cadmium hydroxide (Cd(OH)2) NWs found to be evident which was then dipped into the 0.02M solution of sodium sulfide flakes (Na2S) for 50 min, were the substitution of the hydroxyl to the sulfide takes place due to the negative free energy of formation. At last this resulting film was annealed for 250oC. For the growth of CdSe shell successive ionic layer adsorption and reaction (SILAR) was used which involves 1 complete cycle as immersion of CdS NWs film in 0.1 M of cadmium acetate (Cd2(CH3COO)) for 20 s and next 30 s in sodium hydrogen selenide (NaHSe) solution and finally rinsing in double distil water (DDW) for 10 s. Such cycles were repeated for 4, 8 and 12 times in order to acquire good coverage of CdSe over CdS NWs. For power conversion efficiency (PCE) photoelectrochemical cell was designed in which polyiodide electrolyte was injected through the capillary action in between the CdS/CdSe photoanode and platinum coated counter electrode which was separated by the tixo tape. As the SILAR cycles increases form 4, 8, 12 cycles the values of Jsc found to be 1.22, 1.30, 1.48 mA/cm2 whereas Voc changes from 562, 571, 581 mV as a result of which the efficiency changes from 0.299, 0.330 and 0.370 %. PCE for 12 cycles CdS/CdSe found to achieve 1.5 fold more than the value of bare CdS which was about 0.245%.

Resume : Although there are several ways of producing white light from blue light emitting diodes, LEDs, the preferred way for good colour rendering involves the use of green and red emitting conversion phosphors. Recently, many Eu3+-doped tungstate based lattices (as host for the Eu3+ activators that emit the red light) have been reported for possible application as red conversion phosphors. Some of these materials require high amounts of the expensive europium activator. However, it is notable that due to the price and critical resource of rare earth elements, it would be better to reduce the amount of these elements for an economic, energetic and environmental friendly synthesis procedure. The emission characteristic of a material strongly depends on the crystal structure of the host lattice as well as its uniformity, doping sites and the doping concentration of the activator that is emitting. Considering the doping sites, characteristics such as distance between dopants, coordinate numbers, relative spatial position and electrical environments are also important in order to fully understand the photoluminescence properties. Many tungstate lattices when acting as hosts for Eu3+ activators have been shown to manifest excellent luminescent efficiency and color purity as well as having high average refractive indexes. Also this class of tungsten oxides can be considered as ideal host lattices for dopants as they possess high thermal, chemical and physical stability and in addition they can often be synthesized at low temperatures. They also have broad and intense ligand-to-metal charge transfer (LMCT) band(s) in the UV or near-UV region that are able to capture the emission from an InGaNbased LED. α-Ag2-3xEuxWO4 (x = 0-4 mol%) powders were synthesized by the coprecipitation method at 90 °C for 20 minutes using Sodium tungstate dehydrate (Na2WO4.2H2O), Silver nitrate (AgNO3) and Europium Oxide (Eu2O3) as precursors. Trivalent rare earth ions presenting in different molar ratio concentrations were introduced into the α-Ag2WO4 lattice aiming to study their structural, optical and luminescence properties. This methodology was used because it is simple, cheap, does not need high production temperatures, is industrially favorable, as well as yields materials with homogeneous shape and size without deleterious phases. The nanorods were structurally characterised by X-ray powder diffraction, Rietveld refinement, low temperature (20°C to -190°C) micro-Raman spectroscopy. The morphology of the nanorods was confirmed by field emission scanning electron microscopy. α-Ag2-3xEuxWO4 phosphor crystalized as nanorods and it is suggested that it could be used for blue LED colour conversion as the red emitter for white light. Also, they have potential use in new applications based on its’ nano-rod morphology which allows for preferred alignment, dense packing and directional emission characteristics.

Resume : Over the last decade, zinc oxide (ZnO) has become a very attractive and promising II-VI semiconductor within the international scientific community due to its remarkable and unique properties: wide gap, high excitonic energy , High thermal and chemical stability, biocompatibility and abundance in nature, which places it among the most promising materials showing great potential for applications in various fields, such as piezoelectric or photovoltaic renewable energy, Electronic and optoelectronic systems, as well as biological and chemical sensors. This work focuses on the ZnO nanostructures for promising applications in the biological field, we are interested in the role of zinc oxide (ZnO) in the construction of biosensors and the principle of functioning based on their properties due to many Advantages of the (ZnO) surface which is very sensitive to the absorption of analytes. We use this material for the detection of substances present in biological solutions.

Resume : Schottky junctions with physisorbed and chemisorbed geometries in the same system preserve particular advantages in the application of attractive organic molecular devices. The interface geometries of Schottky diodes are either physisorbed with structural integrity or chemisorbed with large interface dipole in the previous cases. However, here, we design a single system with both. Density-functional theory calculations have been carried out to explore the interaction between Anthradithiophene (ADT) and the Cu(111) surface. Through geometry optimization of the adsorption system, we get bistable structure in single system. And in both systems, we find that the interface dipoles and molecule dipoles induced by adsorption of ADT are close related to the decrease of metal work function. Analysis of the projected density of states reveals electrons transfer from the adsorbates to the substrate indicating n-type diodes. Schottky barrier heights are obtained by calculating LUMO and HOMO level combination with interface diodes. Our results also show that the tunable junctions existing in ADT on Cu(111) take advantages of physisorbed geometry and chemisorbed geometry at the same time, which exhibits high potential in organic molecular devices.

Resume : We report a two-step solution-phase method to achieving controllable nanoscale heterogeneous structures consisting of a crystalline ZnO shell and an upconversional NaYF4: Yb, Tm core with little interface structural mismatches. By adjusting the reaction temperature, it is found that the interface, crystallinity, grain boundaries and thickness of ZnO shell layer in the core-shell structure can be precisely manipulated. With optimized core-shell heterogeneous structure, the well-prepared monodispersed NaYF4: Yb, Tm@ZnO nanoparticles possess much enhanced ultra-violet (UV) luminescence and large photocurrent response under low-energy near-infrared (NIR) irradiation, which could be ascribed to improved energy transfer efficiency caused by good crystallinity, less grain boundaries and less quenching centres in ZnO shell layer. These excellent properties ensure the establishment of NaYF4: Yb, Tm@ZnO as a highly promising material for biological labelling and efficient NIR photodetectors.

Resume : As the shrinkage of feature size in device continued, the contact resistance of metal/semiconductor has become a dominant parasitic factor in the transistor. To reduce the contact resistance of metal/semiconductor, the Schottky barrier heights have to be decreased. However, it is very difficult to decrease in the Schottky barrier heights of metal/semiconductor because of the strong surface Fermi-level pinning. The Fermi level pinning effect is caused by the metal induced gap state (MIGS). The MIGS was formed by the wave function of metal electron penetrated to silicon. Nickel silicide (NiSi) is the one of the solutions to reduce the contact resistance of metal/semiconductor. However, as the device structures have changed from 2D to 3D, it is difficult to form the NiSi within high aspect ratio structures. Also, the NiSi formation is difficult due to the lower contact area enhancement on Fin-type devices because NiSi is sensitive to temperature and NiSi2 is formed at low temperature in low contact area. This phase can create an increase of contact resistance. MIS contacts, which consist of the ultra-thin (typically under 1nm) dielectric layer inserted between the metal and semiconductor, have been proposed as a potential solution to solve these problems. The insulator inserted between metal and semiconductor prevent the wave function of metal electron from inducing the gap state in semiconductor and make Fermi level of the metal depinned. In this study, we investigated the contact resistance of metal/various interlayer thin films/semiconductor. Al2O3, ZrO2, ZnO2 and Si3N4 thin films were deposited by the ALD and deposited on various type substrates (n-type, p-type, n+-type, p+-type). The contact resistance was measured by TLM (transmission line model). The current-voltage (I-V) characteristics of the Schottky barrier diodes (SBD), and TLM were measured by the Agilent B1500. The presence of 0.5nm interlayer gives a significant increase in current density and converts the contacts from rectifying to the ohmic contact. As the thickness of interlayer increased, the current density also decreased significantly.

Resume : One of the possibilities to exploit the remarkable transport properties of graphene is to couple these layers with a ferroelectric gate dielectric with polarization oriented normal to the graphene. The interplay between the polarization and the carrier density in graphene needed to compensate the depolarization field has as a consequence the occurence of a hysteresis of the resistance of the graphene as function on the gate voltage. When the graphene is pre-doped, two stable resistance states are obtained, making it possible to design fast, easily accessible, non-volatile memory elements. However, the experimental results obtained with transferred graphene are contradictory, in the sense that the sense of the hysteresis is reversed, most probably owing to various adsorbates on graphene or at the interface with the ferroelectric. This work comments the first syntheses of graphene-like layers in ultrahigh vacuum on ferroelectric surfaces in absence of prior contamination [1]. We evidenced by X-ray photoelectron spectroscopy the formation of graphene-like layers with weak chemical interaction with the substrate, whose polarization state is preserved upon carbon deposition. Also, from linear dichroism in carbon K near-edge absorption fine structure these layers exhibit two-dimensional character. This is a first step towards the synthesis and the characterization of ideal graphene/ferroelectric heterostructures. [1] N.G. Apostol et al., RSC Adv. 6, 67883 (2016).

Resume : High-efficiency organic light emitting diode (OLED) with an external quantum efficiency (EQE) of greater than 50% and low roll-off was produced by inserting a vacuum nanohole array (VNHA) into phosphorescent OLED (PhOLED). The VNHA substrate was fabricated using a novel process called robust reverse-transfer (R2T). The R2T process was utilized to generate the vacuum nanohole array, which was then inserted to the PhOLED to maximize the refractive index (RI) contrast of the photonic crystal slab for a given background material of high RI. Since the VNHA substrate obtained by the proposed R2T process has a smooth surface comparable to the surface roughness of the polished silicon wafers, the electrical loss due to surface roughness is negligibly low, so the experimental results can be compared with the results of the optical modeling. The resultant extraction enhancement confirmed that VNHA extracts the entire waveguide loss into the air by comparing experimentally measured results with those produced from optical modeling analysis.

Resume : Atomically thin layers of elemental and compound materials have been studied extensively in the past several years due to their exciting properties. Like silicene, computational studies of some two-dimensional (2D) materials has been followed by synthesis of these materials via means of chemical vapor deposition (CVD). Here, we report CVD synthesis and characterization of a novel selenium based 2D material grown on oxidized silicon wafer. We investigated the properties of this material using transmission electron microscopy, X-ray photoelectron spectroscopy as well as optical and electrical characterization methods. Scanning micro-Raman spectroscopy shows that the characteristic Raman spectrum generates uniformly throughout the triangular crystals of the synthesized material. Photoluminescence spectroscopy along with the electrical measurements of back-gated two terminal devices of the monolayer crystals indicates a relatively large optical band gap of 1.75 eV. Magnetic force microscopy (MFM) reveals trace of a magnetic moment over the crystals . Such a fascinating property is quite unique among 2D materials and could open up a new door to investigate magnetism in 2D.

Resume : Typically lead sulfide quantum dots (PbS QDs) for solar cell incorporation are synthesized with the aid of the highly toxic and reactive sulfur precursor bis(trimethylsilyl) sulfide (TMS) (Hines et al. Adv. Mater. 2003). Recently, Hendricks et al. (Science 2015) proposed a new synthesis scheme that makes use of a wide variety of N,N’-substituted thioureas. By varying the thiourea substituents, the reaction rate can be tuned and this allows control over the QD size while maintaining extremely narrow size dispersions and full conversion of reactants. This, together with a high reproducibility makes it more appealing for larger volume synthesis than the TMS-based synthesis. However, various synthesis methods for the same material can give raise to different QD surface terminations which, in turn, will influence the final device performance. Perhaps, because of that, there have been no reports on PbS solar cells, which make use of this thiourea-based synthesis. In this contribution, we synthetized PbS QDs with a ~1.35 eV band gap by both TMS and thioureas and incorporated them into solar cells. Using nuclear magnetic resonance and Rutherford backscattering spectroscopy, we show that in both cases, synthesis and workup methods can be tuned to yield PbS with identical surface chemistry. Under such conditions, the solar cells show similar performance. That finding tells us that the resulting QD surface chemistry is key for high performance and not the synthesis route it-self.

Resume : In this paper carried out research the features of photoluminescence of the complex metal oxide nanopowders (ZnGa2O4, ZnGa2O4:Mn, Zn2SiO4:Mn) in vacuum and various gaseous ambient. The nanopowders were obtained using pulsed laser reactive technology. The influence of gas environment (О2, N2, H2, CO, CO2) on the photoluminescent intensity was investigated. Photoluminescent study was carried out at room temperature, excitation was performed using 365 nm UV LED. A change of gas pressure leads to a rather significant change in the intensity of the photoluminescence spectrum and deformation its. The most significant changes was observed for the intensity of the main peak (495 nm) of ZnGa2O4 nanopowders, with the increasing pressure of oxygen increases its intensity. This obviously is the result of a redistribution of existing centers of luminescence and the appearance of new adsorption centers of luminescence on the surface of nanopowders. The investigated material can be effectively used as materials for the construction of the multi-component photoluminescent sensing matrix.

Resume : Doped ZnO has attracted great attention as promising candidate for transparent conductive oxides (TCOs) due to its high transparency, non-toxicity, excellent thermal stability, abundance in nature and low cost [1]. In particular, Al-doped ZnO (ZAO) films have been extensively studied as a potential candidate and can replace the most widely used indium tin oxide (ITO). Aluminum (Al) is a n-type dopant that increases the concentration of free electrons and improves the conductivity of ZnO thin films [2]. Co-doping of Al with rare earth element like Gd can increase the transmittance and improve the optical properties of ZAO film [3]. In this study, Al and Gd co-doped ZnO thin films were deposited by reactive magnetron sputtering of Zn, Al and Gd targets at high pressure ( 10 Pa). The films were deposited at ambient temperature (without intentional heating of substrate) and also at high temperatures (substrate temperature varied from 573 K to 773 K) to investigate the influence of substrate temperature and film composition on the structure, morphology and optoelectronic properties of the films. The samples were characterized by X-ray diffraction (XRD), Field emission Scanning Electron Microscopy (FE-SEM), four point probe method and UV-VIS spectrophotometer. Keywords: ZnO, thin films, TCO, optoelectronics properties, reactive magnetron sputtering. [1] S. Fernández, A. M. Steele, J. J. Gandía, F. B. Naranjo, Thin Solid Films, 517 (2009), 3152–3156 [2] D. S. Kim, J. H. Park, S. J. Lee, K. J. Ahn, M. S. Lee, M. H. Ham, W. Lee, J. M. Myoung, Materials Science in Semiconductor Processing, 16 (2013), 997–1001. [3] W. Lin, R. Ma, W. Shao, B. Liu, Applied Surface Science, 253 (2007), 5179–5183.

Resume : From only few years ago, hybrid perovskite solar cell have been significantly developing with high power conversion efficiency, and its speed of progress is so rapid. because perovskite materials has unique properties itself such as absorption coefficient by direct band gap, long diffusion length of charge carriers by long life time and so forth. Along this rapid progress, perovskite nanocrystals have being recognize as very attractive materials in optoelectric device due to suitable properties such as high color purity , convenient bandgap tunability by simple combination/mixing of components and so on. In current, many different experimental procedures have been reported for synthesize perovskite nanocrystals. Among the suggested methods, we adapted a general synthesis method of Cesium lead bromide nanocrystal by using cesium oleate solution and lead(II) bromide solution with two type ligands Here, we find that a facile synthesis of 7-9nm CsPbBr3 nanocrystals with cubic shape by using additive at under 60 degrees. In this case, full width at half maximum(FWHM) is around 20nm and wavelength of maximum photoluminescence peak is 507nm. Moreover, by simple change of additive type and amount, samples have exhibited different the wavelength of PL peak and PL intensity. Accordingly, our method can synthesis cubic CsPbBr3 nanocrystal at lower temperature by using simple adduct and control the wavelength of photoluminescence peak by changing types and amount of adduct.

Resume : Noble metal (Au, Pd)/TiO2 core-shell or sandwich nanostructures are obtained by using two fabrication methods: pulsed laser deposition (PLD) and pulsed laser nanostructuring (PLN). The laser nanostructuring is performed in order to modify the morphology of the as-deposited nanostructures and hence to change their optical properties. Titania coating caused shift of the Localized Surface Plasmon Resonance (LSPR) band of the metal nanoparticles toward the longer wavelength side. The characterization of the metal/TiO2 nanocomposites consists of different parameters like: the weight ratio of metal to TiO2; the arrangements, the thickness and the sizes of the metal core and TiO2 shell in nanostructured systems; the light absorption, scattering and LSPR capacities of such nanoobjects as well. Change of the experimental parameters enables successful control of these features and tuning of the LSPR across the visible spectrum. The morphological, structural and optical properties are studied by using field emission scanning electron microscopy (FESEM), energy-dispersive x-ray (EDX) and UV–vis spectroscopy. Our work could render information for design of TiO2-coated core–shell nanomaterials with tunable plasmon absorption shifted to the longer (visible) wavelengths. These nanostructures represent improvement in the efficiency of the plasmonic solar cells.

Resume : Porous-Si (pSi) is a well-studied luminescence material that can present a strong yellow/red emission and combined with blue/green emitting material can be an alternative route toward the fabrication of white-light emitting nanostructures. The most common material used for the preparation of these structures is ZnO, a wide band gap semiconductor with its PL spectrum consisting of a broad visible (Vis) and a narrow Ultra-Violet (UV) emission component. Various vapor deposition techniques have been employed for the fabrication of pSi/ZnO nanostructures (NSs). In this work, we present a new all-electrochemical process, which might be superior in terms of process simplicity, scale-up potential, and versatility of design and performance of pSi/ZnO NSs. In particular, metal-assisted chemical etching, and electrochemical deposition have been employed. The microstructure, morphology and chemical features of the samples were correlated with their PL emission. Appropriate pSi fabrication conditions can lead to the preparation of NS that present a PL emission with a Gaussian-like profile, covering approximately all visible spectrum. A post deposition UV laser annealing (UV-LA) was employed to the pSi/ZnO. This process leads to the change of the PL emission spectrum with the appearance of a UV component. The influence of the laser pulse’s fluence, number of pulses, and the annealing environment was investigated.

Resume : CuInS2 nanocrystals (CIS NCs) are attracting attention as non-toxic alternatives to heavy metal–based chalcogenides for many technological applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs. The application of an electrochemical potential (VEC) enables us to externally tune the Fermi energy inside the NCs and thereby to reversibly control the occupancy of surface defects and intragap acceptor states involved in the nonradiative and radiative electron capture processes leading to exciton decay. The results indicate that, in analogy to Cu-doped II-VI NCs, emission occurs via radiative capture of a conduction-band electron in an intragap acceptor state most likely associated with Cu2+ centers that can be passivated by direct injection of electrons in the NC energy gap under reductive VEC. Temperature-controlled photoluminescence experiments highlight that the emission efficiency is limited by a non-radiative recombination process that affect both shelled and unshelled CIS NCs equally and that can be suppressed for T<100K. Finally, we formulate a model that describes the EC modulation of the PL efficiency in terms of the availability of radiative intragap acceptor sites and of surface electron traps that affect core-only NCs to a larger degree than CIS NCs with inorganic passivation.

Resume : Electronic band structure of strain-balanced GaAsN/GaAsBi type II double quantum wells (DQWs) was theoretically investigated by using the band anticrossing model combined with the envelope function approximation. The strain-balanced DQWs were coupled by the optimization of the well and barriers widths. We have discussed the coupling effect on the confined states energies and the oscillator strengths of inter-band transitions. The in-plane carrier effective masses and the optical absorption spectra of 1.3 and 1.55 µm GaAsN/GaAsBi type II DQWs are also examined.

Resume : The nanostructured materials have been extensively studied, not only because of the new properties and their possible technological applications, but also by the search for a better understanding of the physical and chemical processes that cause these changes. Among these materials, perovskite structure has received remarkable attention because of fundamental properties, versatility and potential for several technological applications. However, there is no consensus in the literature about the nature of the photoluminescence emission in various nanocrystalline semiconductor materials. Luminescent materials, or phosphors, are widely used in several technological applications, such as cathode-ray tubes, light bulbs, lasers, solar concentrators, vacuum fluorescent displays, medical radiology equipment (including scintillators), field emission displays and light-emitting diodes. In this context, this work aimed to perform the synthesis and the characterization of nanostructured CaTiO3:La, Pr and SrTiO3:La, Pr materials and to correlate with photoluminescent properties. It has been shown the incorporation of rare earth in perovskite materials improves the emission of photoluminescent properties. The polymeric precursor method was used to prepare CaTiO3:La, Pr and SrTiO3:La, Pr samples. The characterization using X-ray diffraction presents no spurious phases and the typical orthorhombic or cubic symmetries for these two perovskite structures. Moreover,this technique reveals broad diffraction peaks which are characteristic of nanostructured systems. Photoluminescence measurements at room temperature present a wide peakfor amorphous samples. As the temperature of calcination increases, the samples show crystallized structure and a large decrease of luminescent properties for samples without Pr doping. On the other hand, samples with Pr ions present a narrow and large emission centered in red wavelengths. Raman measurements and X-ray absorption spectroscopy at Ti K-edge were performed. The results obtained with these techniques show the evidence of some defects in TiO6 octahedra in both CaTiO3:La, Pr and SrTiO3:La, Pr systems as a function of La content. The enhancement of photoluminescent emission for CaTiO3:Pr,La samples is associated with disorder in the CaTiO3 or SrTiO3 lattice caused by La incorporation. In principle, more uneven crystal fields due to the lower symmetry at Pr3+ sites can mix opposite-parity into 4f configurational levels, subsequently increasing the 1D2-3H4 transition probabilities of Pr3+ ions. This disorder, as a function of the La content, was probed by both XANES and Raman measurements.

Resume : Memristors have recently generated significant interest due to their potential use in nanoscale logic and memory devices. Memristors are two-terminal memory resistors that retain internal resistance state according to the history of applied voltage and current. The memristive behavior was obtained in metal/oxide/metal ultra-thin-film structure. And devices based on transition metal oxide have been reported in the previous works. However, none of researches demonstrate the simple synthesis of memristors from metallic nanowires by taking advantage of Joule Heating . In our research, direct current-induced oxidation of Cu nanowires by high current density in atmospheric air at room temperature was investigated. Subsequently, reversible switching up to 100 cycles with large ON/OFF ratio (> 1000) and low threshold voltages (> 0.5V) was demonstrated. From TEM analysis of the manipulated nanowire devices, core-shell (Cu2O/Cu) nanowire structure was observed which provided an evidence of current-induced oxidation.

Resume : β-In2S3 thin films were prepared by chemical bath deposition (CBD) for application in solar cells reduced on cadmium as an alternative of window or buffer layer instead of CdS used in CdTe and CIGS devices. The forerunners to synthesize β-In2S3 were tioacetamide and Indium chloride of high purity in a solution aqueous solution. The samples were deposited on soda lime glass NSG TEC™ 15 (SnO2:F). The wide band gap value was around 2.5 eV. The morphological results from SEM showed a compact surface no voids. By X-ray diffraction was confirmed the stable phase β-In2S3. The resistivity of the samples was measured using four-point probe technique and the specific contact resistivity was determined by TLM method when the β-In2S3 as applied as buffer layer between the TCO and CdS thin film. The results obtained show our material as a potential candidate to be used as window or buffer layer in solar cells reduced or free in cadmium. This work was supported by CeMIE-SOL P25 and COFAA - Instituto Politecnico Nacional, Mexico. Key words: β-In2S3, CDB technique, cadmium reduced solar cells.

Resume : Recently, phosphor converted white light emitting diodes (pc-wLEDs) have been emerged as attractive technology, which combines InGaN-based blue chip with yellow-emitting YAG:Ce3+ phosphor to generate high-brightness white light, In this study, phosphor paste was prepared using YAG:Ce3+ nanoparticles with a uniformly spherical size distribution for white light modules. We synthesized monodispersed and spherical nano-sized YAG:Ce3+ particles by co-precipitation method. The phosphor paste with red component coated on a glass substrate was made using by a simple and convenient doctor blade method. We report the optimization luminous characteristics with CRI and CCT

Resume : Carrier transit in graphene-based Schottky diodes is fundamentally important for a deeper understanding of Dirac fermion behavior in graphene-based electronic devices. In this communication, we explore reverse tunneling current through field emission in graphene/oxide/n-semiconductor (G/Ox/n-S) Schottky junctions. Such junctions are not ideal (no oxide region), instead tunable oxide layer is considered as an excess layer between graphene and n-type semiconductor (Si). Under bias, mobile carriers are expected to traverse the oxide and junction barrier region to form current. A Landauer formalism method is proposed for calculating such currents through the combined barriers. We consider graphene replacing the metal region of a Schottky diode barrier and study carrier field emission (FE) through a double barrier formed from the oxide layer in contact with the semiconductor junction barrier of the Schottky device. Specifically, we model tunneling currents by means of two explicit transmission probability factors T(ox) and T(s) through the oxide and the semiconductor contact layer respectively and in a general Landauer formalism. Both of these transmission probabilities are explicitly derived for the two potential barriers at the junction between graphene and semi-metal. The calculated current J is a thermionic field-effect (TFE) current and essentially a combination of two fundamental mechanisms (a) thermionic emission from the graphene layer and (b) carrier tunneling through the oxide/semiconductor double barrier. The final current is a strong function of the two transmission probabilities, and of the temperature as T^5/2. The latter is seen as direct result of of the 2DOS of the graphene layer. Finalized currents are of the standard Schottky diode form, however with a different Richardson's constant, a different current pre-factor, and a T^5/2 temperature dependence as follows: J(TFE) = A** (T^5/2) T(ox) T(s) exp(-qVb/kT) (exp(qV/kT) - 1), where qVb is the junction barrier, and V is the applied voltage.

Resume : The aim of this work is to elaborate technology of nanoparticles formation in metal oxide semiconductors by laser radiation.Zn nanoparticles were grown at the surface of ZnO single crystal by using Nd:YAG laser radiation. The formation of Zn nanoparticles by the laser is explained by Thermogradient effect. According to the effect, the gradient of temperature induced by nanosecond laser radiation leads to generation of Zn interstitial atoms and to their concentration at the irradiated surface of the ZnO crystal. The experiments were performed on hydrothermally grown n-type ZnO single crystals with size 5.0x5.0x2.0 mm. The ZnO crystals were irradiated by the fourth harmonic of pulsed Nd:YAG laser with the following parameters: wavelength λ = 266 nm, pulse duration τ = 3 ns and laser intensity I max = 314 MW/cm^2 .Nanoparticles were studied by field emission scanning electron microscope (FESEM), Raman Spectroscopy and topography measurements and electrical conductivity mapping were performed by atomic force microscope (AFM). It was found that the formation of Zn nanoparticles characterizes by two thresholds intensities: the first threshold intensity I th1 = 3.5 MW/cm^2 at which conductivity starts to increase monotonously up to 10^3 times till the second threshold intensity I th2 = 290.0 MW/cm^2, due to the increase of the Zn interstitials concentration at the surface of the ZnO sample. At second treshold intensity „black ZnO” appears,which is caused by the emergency agglomeration of Zn nanoparticles with the size depending on number of laser pulses. An evidence of Zn phase formation in ZnO crystal is appearance of 70 cm -1 band in Raman spectra after irradiation by the laser. At the intensity of the laser equal or more than 290 MW/cm^2 agglomeration of Zn on nanoparticles take place. It is the second stage of the process. An evidence of the second stage of nanoparticles formation is the increase of the size of nanoparticles with numbers of the laser pulses.

Resume : From the initial research around the dye–sensitized solar cells, to the most recent conversion efficiency improvements, ab initio calculations have played an important role in the developed of the photovoltaic field, based on the use of hybrid organic–inorganic perovskite materials. Since the perovskite structures begun to be used in the development of photovoltaic devices, the movement of the organic cation inside the BI6 octahedra (B = Pb, Sn) kept attention, because of their possible impact on the opto–electronic properties of those materials. For this work, three different orientations of the methylammonium molecule were examined, looking for their influence on the electronic properties of the CH3NH3PbI3; particularly, the work was focused on the bands structures and the band gap for the tetragonal and cubic phases. The relativistic effects, mainly contributed by the lead ion, were taken into account along all the calculations made in the scheme of the density functional theory. The functional used was the proposed by Perdew, Burke and Ernzerhof, in the generalized gradient approximation, as implemented in the DMol3 code. The obtained results show clear changes on the densities of states, the bands structures and charge distribution, linked to the Pb–I bonds hybridization caused by each particular methylammonium molecule orientation. These results are limited to the periodic conditions but could contribute to have a time–depending picture of the intrinsic phenomena in the crystal. Acknowledgements: This work was partially supported by COFAA and the multidisciplinary projects IPN–SIP 20161770 and 20161771. I. Ornelas and J. Pilo want to acknowledge the graduate fellowship from CONACYT.

Resume : Graphene has attracted great attention owing to its superb properties as an anode of organic or polymer light-emitting diodes (OLEDs or PLEDs). However, there are still barriers for graphene to replace existing indium tin oxide (ITO) due to relatively high sheet resistance and workfunction mismatch. In this study, PLEDs using molybdenum oxide (MoOx) nanoparticle-doped graphene are demonstrated on a plastic substrate to have a low sheet resistance and a high workfunction. Doping of graphene using MoOx is done with a simple and easy spin-coating process. After the doping, the sheet resistance of five-layer graphene with optical transmittance of ~88% was as low as ~180 Ω sq-1. Moreover, the surface roughness of MoOx-doped graphene becomes smoother than that of pristine graphene. Moreover, we found that thin MoOx film is acting a buffer layer for oxygen plasma treatment which can damage the graphene surface. A flexible PLED fabricated on MoOx-doped graphene anode shows high efficiency due to improved hole injection with decreased hole injection barrier. The maximum current efficiency and power efficiency of the PLED using the MoOx-doped graphene anode were 12.5 cd A−1 and 11.3 lm W−1, respectively. Compared with the pristine graphene anode, maximum current efficiency, and maximum power efficiency were enhanced by 255 %, and, 210 %, respectively, when the MoOx-doped graphene was applied as the anode.

Resume : We analyzed the transient photocurrent measurements after UV light excitation of ZnO nanowires in terms of (a) the sum of multiple exponential decays, (b) power law models, and (c) stretched exponential functions, as suggested by many researchers in a wide range of semiconductor materials. The best fits, however, with clear straight-line feature is obtained when the current decay is plotted against ln(ln(t)), a particular double-logarithmic representation. The ZnO nanowire samples were synthesized hydrothermally from a solution of zinc nitrate hexahydrate and sodium hydroxide at 90ºC, using quartz glass as a substrate. SEM micrographs show hexagonal nanorods with diameters between 30 and 125 nm and length of 1-2 microns. Coplanar Al contacts in a finger structure were added on the top surface by thermal evaporation. Both, the 266 nm UV line of a Nd:YAG laser for fast pulses, and Xe-light excitation during several tens of seconds were used for excitation. Current decays were monitored up to 6000 s. Taking the characteristic decay time of the initial, relatively fast exponential component, as a function of temperature, leads to an activation energy of some 150 meV. Next, based on the assumption of a broad decay time distribution, we find the barycenter of the product of current and time to be located near 600 to 1000 s, This would correspond to reemission times of photogenerated electrons from deep traps at 0.8 eV below the conduction band. However, in addition to this interpretation, the linear behaviour in the ln(ln(t)) description was already predicted in work from the Marburg group (R. Richert, H. Baessler, B. Ries, B. Movaghar, M. Gruenewald, Phil. Mag. Lett. 59, (1989) 95), who used Monte-Carlo simulation to describe kinetics of both photoluminescence and photoconductivity in amorphous and in organic semiconductors.

Resume : Molybdenum disulfide (MoS2), a photo-active layered material, have drawn great interest to the researcher for short-wave optoelectronic device applications due to layer dependent tunable bandgap and mobility, good absorption coefficient. MoS2 based optoelectronic devices usually fabricated using micromechanical exfoliated layers but the lack of area coverage directed to find us an inexpensive solvent-assisted chemical exfoliation technique, a promising route to large-scale production of variable size (?2-25 nm), highly luminescent MoS2 quantum dots (QDs) without damaging their structures. Below the excitonic Bohr radius, the photoluminescence and absorption characteristics of MoS2 QDs can systematically tunable and found an excellent blue shift with decreasing QD size. The coexistence of direct band gap emission from 2D MoS2 nanosheets and quantum confined nanocrystals has been achieved at room temperature. Yellow color QD dispersions have been showed a quantum confined absorption peak with two small shoulders at higher wavelength originating from interband transition. The PL spectra also exhibited size dependent characteristics (2.2-2.4 eV), consisting as 3 peaks similar as absorption spectra. Valence band splitted energy difference (150-180 meV) is very prominent and in excellent agreement with theoretically reported results. The temporal stability and decay dynamics of excited charge carriers in MoS2 quantum dots have been studied using time correlated single photon counting spectroscopy technique. The MSM-type photodetector (PD) device has been fabricated using MoS2 QDs at room temperature and the spectral responsivity and detectivity have been calculated to be ~133 mA/W and ~1011 cm.Hz1/2/W for -2V applied bias and 514 nm excitation source. The responsivity of the MSM PD is found to be highly sensitive to the QD size; maximum for ?18-26 nm. To enhance the device performance, the 0D/3D hybrid photodetector have been fabricated using colloidal MoS2 QDs and 3D Si. This type of PDs also exhibited size dependent responsivity for broadband spectral range. All the device exhibited a high photocurrent ratio of ~103 and the peak responsivity have been found to be ~850 mA/W, which is approx. 6 times larger than that of the only MoS2 QDs for -2 V applied bias. All the PD devices displayed excellent reproducibility with stability and fast response speed. The use of colloidal QD of 2D materials can be an excellent approach for large area device fabrication with enhanced output due to excellent light absorbing property of MoS2 QDs and short carrier diffusion path. We have also investigated the potentiality of different sized MoS2 QDs and their interaction with Si covering broad spectral range using COMSOL simulation based on FEM study which also exhibits the size dependent electric field enhancement and our theoretical predictions matched with the experimental results very well. Furthermore, the fabricated 0D/3D heterostructure also displayed white LEDs characteristic (~400-850 nm) in forward bias condition and the EL intensity increased with applied bias. The white EL emission is found to be steady in the 10-350 K temperature range make the LEDs potential for harsh condition. In summary, we have successfully established a lithography-free, efficient approach for the fabrication of large area PDs and LEDs on existing Si technology using colloidal MoS2 QDs as the key component for their size-tunable band gap, low-cost and ease of fabrication For high-speed optoelectronic circuits The above values are found to be superior to the reported results on large scale photodetector devices fabricated using chemically synthesized MoS2 QDs for next generation photonic devices.

Resume : In recent years great synthetic efforts have led to the development of new conjugated semiconducting molecules and polymers with improved optical and electrical properties. These advances in materials have in turn produced a series of solid-state organic optoelectronics devices with improved performance; yet, many devices with semiconductor layers processed from solution are comprised of conventional metal or metal oxide layers that act as charge collection interlayers and/or as electrodes that are processed from vacuum. Furthermore, these materials and their interfaces are often quite sensitive to exposure to the environment. Hence, there is a real need for new and versatile methods to control the electrical properties of materials near interfaces, in particular at semiconductor/electrode interfaces. In this talk we will discuss the importance of interfaces in several examples of organic optoelectronic devices including organic solar cells, organic field-effect transistors and photodiodes. We will show examples where that the electrical properties of the electrodes in such devices can be modified by either chemisorption or physisorption. We will also demonstrate that the active semiconductor layer can be modified near an interface with an electrode by using additives that vertically phase segregate during film formation. Finally, we will present a simple processing for the electrical doping of organic semiconductors over a limited depth near the surface of the film that is based on immersing the film into a polyoxometalate solution. Such approached can drastically reduce the fabrication cost of such devices and simplify device architecture.

Resume : The self-organization of pi-conjugated organic materials forming highly ordered supramolecular architectures has been extensively investigated in the last two decades in view of optoelectronic applications. Indeed, the control of both the mesoscopic and nanoscale organization within thin semiconducting films is the key issue for the improvement of charge transport properties and achievement of high charge carrier mobilities. These well-ordered materials are currently either self-organized semiconducting polymers or liquid crystals. In this context, we endeavored to investigate the self-organization of semiconducting liquid crystalline materials incorporating different kind of pi-conjugated systems in unique molecular or macromolecular architectures. Here we describe the design and synthesis of (i) dyads and triads combining discotic or calamitic pi-conjugated mesogens, and (ii) side-chain liquid crystal semiconducting polymers where the backbone is a pi-conjugated polymer and the side groups are pi-conjugated discotic mesogens. In this work, we will give the details on the synthesis, structural characterization and morphology studied by Polarized-light Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), Temperature-dependent small-angle X-ray diffraction, Grazing-incidence X-ray scattering (GIXS) and Atomic Force Microscopy (AFM). Moreover, their charge transport properties studied in OFET configuration will also be depicted.

Resume : Adhesion lithography (a-Lith) is a high throughput, scalable nanofabrication method that allows patterning of two electrodes in a co-planar geometry onto substrates of arbitrary type and area [1]. The attractiveness of this technique lies in the fact that the resulting asymmetric electrodes are separated only by a <15 nm nanogap, while the total width of the active nano-channel may reach hundreds of meters (m) confined in a small area. Deposition of a semiconducting material on top of these nanogap structures gives rise to electronic devices with very intriguing properties. Herein we present recent advances in a-Lith fabrication process, enabling versatile and tuneable manufacturing of metallic electrode nanogap structures. The type of substrate and metal electrodes as well as their respective size and shape can be tailored at will, following the requirements posed by the desired application. We show how the evolution of these unique architectures along with intelligent materials engineering paved the way to radiofrequency (RF) p- and n-type diodes [2] with large output currents, high AC-to-DC conversion efficiency and cut-off frequencies well within the GHz regime. Furthermore, we demonstrate nanoscale light-emitting diodes (LEDs) based on solution-processed light-emitting materials and introduce different approaches to engineer their metal-semiconductor interfaces to enhance light emission. The above examples showcase the high potential of a-Lith for industrial uptake, due to the scalability of the manufacturing process in a simple and low-cost manner, and for the development of high performance RF and opto-electronic devices. [1] Beesley et al, Nature Communications 5, 3933 (2014). [2] Semple et al, Small 12, 1993 (2016).

Resume : Novel multifunctional devices can be fabricated by combining organic semiconductors with molecules possessing different chemical, optical and electrical properties. Diarylethene molecules (DAEs) have attracted a great deal of attention thanks to their efficient interconversion between two thermodynamically stable isomers that exhibit remarkably different electronic properties, allowing to introduce controlled photo-modulable trap levels in the semiconductor bulk 1. We have achieved bi-functionality exploiting DAEs in combination with PDVT-8 or PCDTPT polymers as a bi-component active layer in organic thin-film transistors (OTFTs). These polymers have been chosen as p-type semiconductors because they feature high field effect mobility (up to 1 cm2V-1s-1) and HOMO energy levels that allow electronic interaction with the closed form of DAE-Me 2,3. These findings are of interest for the development of high-performing optically gated electronic devices, such as switches and multilevel non-volatile optical memories 4, therefore for a new generation of electronics. Finally, we explore the possibility of introducing a third functional component to control the modulation of the surface charge density at the semiconductor/dielectric interface. [1] Orgiu, E. et al., Nat. Chem. 4, 675–679 (2012). [2] Chen, H. et al, Adv. Mater. 24, 4618–22 (2012). [3] Ying, L. et al., J. Am. Chem. Soc 133, 18538–18541 (2011). [4] Leydecker, T. et al., Nat. Nanotechnol. (2016).

Resume : The standard platform for silicon photonics has been ridge or channel waveguides fabricated on silicon-on-insulator (SOI) wafers. SOI waveguides are so versatile and the technology built around it is so mature and popular that silicon photonics is almost regarded as synonymous with SOI photonics. However, due to several shortcomings of SOI photonics, novel platforms have been recently emerging. The shortcomings could be categorized into two sets: (a) those due to using silicon as the waveguide core material; and (b) those due to using silicon dioxide as the bottom cladding layer. Our team has been developing several heterogeneous platforms to address both set of shortcomings. This talk will focus on the first set of shortcomings, particularly the lack of true electrooptic effect and second-order optical nonlinearity in silicon due to the centrosymmetry of the material’s crystalline structure. To address this issue, the top silicon layer of SOI is replaced by a thin film of lithium niobate that possesses a noncentrosymmetric lattice structure. The thin films are transferred to silicon substrates by ion implantation and ion slicing techniques. Submicron waveguides are formed by rib-loading the thin films with index-matching materials such as silicon nitride. Our latest generation of devices have an on-chip insertion loss of ~ 2 dB. Low-frequency (kHz range) measurements yield extinction ratios of 18 dB and half-voltage length-product of 3.1 V.cm at 1550 nm. The modulators are characterized up to 50 GHz and attain a 3-dB modulation bandwidth of 33 GHz. The third-order intermodulation distortion spurious free dynamic range is 97.3 dBHz2/3 at 1 GHz and 92.6 dBHz2/3 at 10 GHz. The performance demonstrated by the thin film electrooptic modulators is on par with conventional lithium niobate devices, but with lower drive voltages, smaller device footprints, and potential compatibility for integration with large-scale silicon photonics. Also demonstrated on the heterogeneous thin-film lithium-niobate-on-silicon platform are two types of photonic devices for second-harmonic generators (SHG). Fabricated devices are characterized with pulsed-pumping in the near-infrared, showing second-harmonic generation at a signal wavelength of 780 to 800 nm and propagation loss of 1 dB/cm. The two types of second-order nonlinear devices include periodic poling and grating-assisted mode-shape modulation for quasi-phase matching of the fundamental (pump) and second-harmonic (signal) interacting waves. The pros and cons of the two approaches and the measured SHG data will be discussed.

Resume : Silicon nanowires (NWs) are attracting the interest of the scientific community as building blocks for a wide range of future nanoscaled devices. We demonstrate the realization of a 2D random fractal array of vertically aligned Si NWs by using metal assisted chemical etching without any lithographic process and by a cheap, fast and maskless approach compatible with Si technology. We were able to control and tune the optical properties of the system, by realizing different fractal geometry through the optimization of NW spatial arrangement [1-2]. In-plane multiple scattering and efficient light trapping related to the fractal structure were observed [3]. NW achieved by this technique exhibited a very bright room temperature PL and EL, tunable with NW size in agreement with the occurrence of quantum confinement effects. We fabricated a low-cost multiwavelength light source working at room temperature, achieved combining Si NWs and carbon nanotubes (CNT). The NW/CNT hybrid system exhibits a tunable emission both in the visible range, due to Si NWs, and in the IR range from CNTs. An innovative Si NW-based optical biosensor is realized, which exploits the PL properties for the detection of proteins in a wide range of concentrations, down to the femtomolar limit, demonstrating the great potentiality of this material for biosensing. 1. Light: Science & Applications 5 (4), e16062, 2016 2. Nano Letters, 16 (7), pp 4181–4188, 2016 3. Nature Photonics, in press

Resume : The development of novel materials and device designs has fueled the research on THz semiconductor THz detectors. Dyakonov et al [1] theoretically demonstrated THz detection based on the oscillations of plasma waves in the channel of sub-micron FETs. Room temperature detection of sub-THz radiation by FETs has been experimentally demonstrated using both arrays [2] and single devices [3]. This experimental work reports on the influence of the gate length on the sub-THz response of n-channel Strained Silicon (s-Si) MODFETs [4]. Transistors with gate lengths in the range 100-500nm were measured at room temperature using a dual-frequency electronic source based on frequency multipliers at 150 and 300 GHz with output power levels of 4 mW and 6 mW, respectively. The incoming radiation intensity was modulated by a mechanical chopper and the induced dc photoresponse was measured by a lock-in technique. Transistors showed very competitive values of noise-equivalent power (NEP) and responsivity (R); the 150nm gate ones exhibited a NEP of 150pW/Hz0.5 and R=15V/W at 150GHz. We found that gate pads play an antennae role coupling the THz radiation to the 2D electron channel; an optimal design of the pads must improve the performance of the detectors. [1] M. Dyakonov and M. S. Shur, Phys. Rev. Lett. 71 2465 (1993) [2] R. Tauk, et al, Appl. Phys. Lett. 89 253511 (2006) [3] A. Lisauskas, et al, J. Appl. Phys. 105 114511–8 (2009) [4] J. E. Velazquez, et al, Semicond. Sci. Technol. 19, S191 (2004)

Resume : Silicon-on-insulator (SOI) nanowire structures with nanowire diameters of less than 50 nm and nanowire lengths in the order of 300 nm were manufactured in a custom SOI process. An n pp junction was formed in each nanowire with dimensions and doping densities that allowed the pn junctions to operate in the reach-through bias condition. The nanowires were biased into avalanche multiplication and photon emission spectra were observed. By varying the nanowire lengths, the effect of the internal electric field on the avalanche electroluminescent spectra could be studied. Emission spectra covering the wavelength range from 350 nm to 1 700 nm were analysed. Decreasing the nanowire length, the VIS part of the spectrum (from 350 nm to 1 100 nm) showed an increase in optical output power, especially in the visible photopic region. At the same time the NIR part of the spectrum (wavelengths longer than 1 100 nm) decreased in optical output power. This illustrated that high energy photons are generated at higher electric fields, while low energy photons are generated in regions with low electric field. This result led to the conclusion that direct and indirect interband hot electron transitions in the conduction band are the dominant photon generating mechanisms in an avalanching junction. Since more than 150 nW of optical power could be observed at the front surface of the device, novel applications in the fields of near-to-eye displays and bio-sensors are envisaged.

Resume : Tin nanocrystals embedded in silicon are created by the growth of a tin-rich silicon layer via molecular beam epitaxy and subsequent annealing. Previous work has shown the emission of light in photoluminescence (PL) experiments, making these nanocrystals a potential candidate for on-chip integrated silicon optical devices[1]. Here, an electrical characterization of these structures with deep level transient spectroscopy (DLTS) measurements both before and after annealing is presented and compared to PL results. In the as-grown layers, DLTS measurements reveal the presence of intrinsic defects, specifically the silicon di-interstitial. After annealing, new DLTS peaks appear. Different annealing temperatures lead to significant changes in the DLTS spectra. The DLTS results will be compared to the PL data and possible origins of the peaks will be discussed. [1] S. Roesgaard et al., AIP Adv. 5, 077114 (2015)

Resume : We study thermal transport in isotopically disordered Si nanowires [1], discussing the feasibility of phonon engineering for thermoelectric applications within these systems. To this purpose, we carry out atomistic molecular dynamics and nonequilibrium Green’s function calculations to characterize the dependence of the thermal conductance as a function of the isotope concentration, isotope radial distribution and temperature [2]. We show that a reduction of the conductivity of up to 20% can be achieved with suitable isotope blends at room temperature and approximately 50% at low temperature. Interestingly, precise control of the isotope composition or radial distribution is not needed. An isotope disordered nanowire roughly behaves like a low-pass filter, as isotope impurities are transparent for long wave-length acoustic phonons, while only mid- and high-frequency optical phonons undergo significant scattering. [1] S. Mukherjee, U. Givan, S. Senz, A. Bergeron, S. Francoeur, M. de la Mata, J. Arbiol, T. Sekiguchi, K. M. Itoh, D. Isheim, D. N. Seidman and O. Moutanabbir, Nano Lett. 15, 3885 (2015) [2] M. Royo and R. Rurali, Phys.Chem.Chem.Phys. 18, 26262 (2016)

Resume : Semiconductor nanomaterials synthesized via wet chemical approach already demonstrated a great potential in a wide variety of applications including optoelectronics, energy conversion and storage, biomedicine, etc. Owing to a high versatility of the synthesis achieved after years of intensive development, nanomaterials can be prepared in wide size ranges, different morphologies and crystal structures. Among various shapes of nanoparticles designed to date, two dimensional (2D) structures have attracted particular interest during last decade, ignited by their unique properties resembling those of graphene (such as transition metal dichalcogenide nanosheets), as well as by new features which are not accessible in corresponding 0D or 1D nanoobjects (e.g., CdSe nanoplatelets vs. CdSe nanoparticles). Here we present two main types of semiconductor materials synthesized in the form of nanosheets via a facile colloidal method, CuSe and Bi2Te(Se)3. After the synthesis, CuSe nanosheets were processed into thin films via simple drop-casting yielding conducting coatings, whereas 2D nano-Bi2Te(Se)3 was characterized as a thermoelectric material being sintered into bulk pellets. Electrical properties of CuSe nanosheet films were compared with analogous coatings made of spherical nanoparticles demonstrating advantages of their 2D shape. Thermoelectrics prepared from bismuth chalcogenides also exhibited excellent performance. This work provides additional values of nanomaterials in 2D shape.

Resume : Group-IV-based semiconductor nanostructures are very promising for opto-electronic devices due to the possibility of monolithically integration. We have previously presented light emission from epitaxially grown Si(1-x-y)Sn(x)C(y), with x = 1.6 % and y = 0.04 %, where nanocrystals are formed by post-growth annealing. Here we discuss a promising method for analysis of scanning transmission electron microscopy (STEM) data for composition determination in nanocrystals. Using results from atom probe tomography (APT), we investigate how to analyze the STEM data for determination of the Sn composition in the nanocrystals and in the surrounding Sn-rich layer. Possible correlations between optical properties and nanocrystal composition will be discussed.

Resume : Three-dimensional (3D) islands or quantum dots (QDs) are typically grown by a self-assembly process known as StranskiKrastanov (SK) growth. The most extensively explored material system for SK growth is (In,Ga)As grown on GaAs(100). However, while SK growth occurs on GaAs(100) for a wide range of growth conditions and compositions, on the lower energy (110) surface two-dimensional (2D) growth is always favored. Here we explore using Bi as a surfactant in the growth of InAs on GaAs(110) by molecular beam epitaxy. The addition of surface segregating Bi during InAs deposition is found to alter the growth mode from 2D Frankvan der Merwe to 3D SK, thus inducing the self-assembly of InAs 3D islands directly on GaAs(110). Additionally, we demonstrate the ability to induce a morphological instability and thus 3D island nucleation on-demand, by exposing a previously grown 2D InAs layer to a Bi flux, causing a rapid rearrangement of the 2D layer into 3D islands. Density functional theory calculations reveal that surface Bi reduces the energetic cost of 3D island formation, by modifying the surface energies. Photoluminescence spectroscopy carried out on samples capped with GaAs demonstrates that these novel 3D nanostructures behave as optically active QDs. This work illustrates the game-changing role that surfactants can play in self-assembled nanostructure growth by allowing surface energies to be externally modified.

Resume : Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics, materials science and chemistry for many years to come. These materials offer unique properties combining ultra-thin thickness (one to few-atoms) with chemical and thermal stability. 2D materials cover a wide range of physical and chemical properties and they exist as metals, semi-metals, semiconductors and insulators, depending on their chemical composition, phase, stacking order and thickness (number of layers). Since all this parameters depend on the synthetic process, basic understanding of the growth is crucial to obtain the desired material. Here we aim to discuss the growth of MoS2 via chemical vapor deposition techniques and show the diversity of nanostructures that can be achieved as a function of the growth parameters. Closed inorganic MoS2 fullerene, MoS2-MoOx fullerene heterostructures, elongated hexagon crystals of MoS2 and MoS2-MoOx heterostructrues and finally single- and few-layer MoS2 films can be achieved by controlling the growth temperature, time and pressure in the process. The full characterization of the above nanomaterials by means of Raman and photoluminescence spectroscopy, adsorption spectroscopy, high-resolution transmission electron microscope and diffraction and electrical measurements will be described.

Resume : Efficient solar devices based on semiconductor nanoparticles require the use of conductive ligands; however, the exchange of the native, insulating ligands with conductive metal chalcogenide complexes introduces structural defects within the crystalline nanostructure that act as traps for charge carriers. We utilize atomically thin semiconductor nanoplatelets as a convenient platform for studying the development of defects during ligand exchange both microscopically, and spectroscopically. We demonstrate that these defects can be repaired by mild chemical or thermal routes, through the addition of L-type ligands or wet annealing, respectively. This results in a higher quality, conductive, colloidally stable nanomaterial that may be used for active films of optoelectronic devices.

Resume : Besides inorganic semiconducting and dielectric nanowires, pi-conjugated and hybrid 1D-nanostructures are promising systems for nano-optoelectronic and nanophotonic devices. Indeed, they allow to design complex architectures by combining different materials at the scale of the characteristic physical lengths in order to control optoelectronic properties, to get enhanced behaviors and ultimately to promote new paradigms for devices. Moreover, the 1D geometry promotes sub-wavelength optical propagation and cavity effects suitable for integrated nanophotonic devices (for a review, see [1]). Here, the design and the properties of different kinds of nanowires and nanotubes will be reported: - an original strategy to get highly luminescent nanosources and to control the color of emission of coaxial nanowires by preventing charge and energy transfer between the two types of luminophores - the experimental and modelled (FDTD) characteristics of light propagation along individual nanotubes in the visible range, as well as nanosources included within a single nanotube waveguide. Recent developments of hybrid nanowires and nanotubes containing plasmonic components or carbon nanotubes for tuning the functionalities will be introduced. [1] A. Garreau, J.L. Duvail Advanced Optical Materials 2, 1122-1140 (2014)

Resume : 2D nanoplatelets are a special class of colloidal nanocrystals thanks to their exceptionally well controlled optical features. For a while their synthesis was limited to wide band gap cadmium chalcogenides. Recently it was proposed by Izquierdo et al, to expand the growth of these 2D objects to mercury chalcogenides, while pushing the band gap in the near IR. We investigate the electronic structure and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be tuned from p to n type depending of the used capping ligands. In addition to control their majority carrier this also strongly affects the photoresponse of the material, switching from photoconductive to an almost blind material. These transport measurements are correlated with the measurement of the electronic structure by X-ray photoemission. We demonstrate that the photocurrent dynamic can be tuned between 100 µs and 2 ms thanks to the gate. We finally used time resolved photoemission as a probe of the transport relaxation to determine rather or not the observed dynamics are limited by fundamental process such as trapping. The pump probe surface photovoltage measurements present an even faster relaxation in the 100ns to 500ns range, which suggest that current performance are rather limited by geometrical factor.

Resume : CdSe based colloidal quantum dots are extensively studied for a variety of applications including the single photon emission at room temperature. While the blinking has been considerably reduced in CdSe quantum dots, the purity of single photon emission has been elusive at higher intensities due to the contribution of multiple excitons. We observe that the newly synthesized Cd-free InP based quantum dots have similar optical properties to CdSe. The broader ensemble spectra of InP was earlier believed due to the broader single dots but our characterization on the InP/ZnSe and InP/CdxZn1-xSe prove the inhomogeneity by the presence of narrow single dot spectra (16 nm). The single quantum dots also exhibit strong antibunching (g2(0)<0.02) at intensities below and above the saturation level proving the nature of single quantum emitter and a two-level system. We believe the stronger Auger rate of 70 ps is the preferred route for the multiple excitons, ultimately emission coming only from the single exciton. The simultaneous observation of the presence of more than 97% of on-states (suppression in blinking), mono-exponential photoluminescence decay (well-defined excited states), photostablity, tunable economical synthesis with higher quantum yield (65%) and the monolayer formation (suitability for integration) prove their potential to be used as the single photon sources and therefore applications in quantum information science.

Resume : All-inorganic cesium lead halide perovskite CsPbX3 (X=Cl, Br, I) have attracted as the promising candidates for many applications such as photovoltaics, photodetectors, and light emitting diodes (LEDs) because of their outstanding optoelectronic properties. Halide perovskite nanocrystals (NCs) are suitable for next generation lightning materials due to the properties of large spectral tunability (400-800 nm), high photoluminescence quantum yields (PLQYs), narrow full width at half maximum (FWHM) of emission band, and prospect for low economic cost. In this study, CsPbBr3 was synthesized through simple hot-injection method. In order to apply CsPbBr3 NCs for white LEDs, CsPbBr3 green film was fabricated with ethyl cellulose (EC) which have an important role to enhance the stability. Synthesized CsPbBr3 NCs film indicated high quality green CsPbBr3 perovskite NCs films with a FWHM of 21 nm and an absolutely high PLQY of 37.2%.

Resume : Colloidal inorganic semiconductor nanoparticles, or quantum dots (QDs), are strong candidates for low-cost, large-area solar energy absorbers and light-emitting materials due to their size-dependent optical absorption, strong photoluminescence, and solution-processability. QDs of size below ten nanometers only comprise hundreds or thousands of atoms. The large surface-to-volume ratio therefore makes the surface chemistry crucial to the behavior of these nanoparticles. An exciting frontier in QD science and engineering is the manipulation of physical and chemical properties via ligand surface chemistry. This presentation will review recent discoveries in the ligand chemistry of QD surfaces interacting with molecular ligand headgroups and metal-halide based perovskites. It will further detail their reaction mechanisms, and demonstrate solar-harvesting and light-emitting device applications.

Resume : Giant enhancement of short-range contact interactions between two carriers or a carrier with a magnetic impurity is an important consequence of the strong geometric confinement in low- dimensional semiconductor structures. The shrinkage of carrier wave functions increases the charge density, resulting in an increase of the interaction that is inversely proportional to the confinement volume. The electron-hole exchange interaction in quantum dots (QDs) is enhanced by up to three orders of magnitude thereby (up to tens of meV) so that the dark ground state exciton determines the optical properties of colloidal nanocrystals (NCs). Another example for interaction enhancement concerns the electron-nuclear Fermi contact interaction, evidenced impressively by the Overhauser effect in single quantum dots. A bright appearance of the contact interaction enhancement is the giant splitting of the exciton spin sublevels in NCs doped with magnetic Mn2+ ions at zero external magnetic field. This splitting is caused by magnetic polaron formation when the exchange field of about 100 Tesla induced by a hole confined in the NC polarizes the Mn2+ spins. The interaction of excitons with dangling bond spins (DBSs) at a NC surface could play an important role in the radiative recombination and spin dynamics of confined excitons. These spins always exist due to incomplete passivation of all dangling bonds of surface ions by the ligands. The direct proof of the DBS presence was demonstrated by SQUID measurements. The wave function mostly of the electrons spills out of the small size NCs, providing a high density of electron charge at the NC surface and enhancing the exciton interaction with the DBSs. This interaction is proportional to the square of the electron wave function at the NC surface and can be significant in small NCs, but decreases fast with increasing NC size, scaling almost as the inverse cube of the NC radius. The exciton interaction with the DBSs was suggested to be responsible for several experimental observations with still undisclosed origins: the lengthening of the dark exciton lifetime in magnetic field applied dominantly along the wurtzite axis, the anomalous Zeeman splitting of the exciton, the presence of the zero phonon line (ZPL) in fluorescence line narrowing (FLN) experiments, the temperature increase of the ZPL intensity, as well as the appearance of the dark exciton in single dot emission spectra. We report the DBMP observation in 2.8 nm diameter CdSe NCs at cryogenic temperatures. The polaron complex with a binding energy of 7 meV is revealed through a low- energy shift of the ZPL and its optical phonon replica (1PL) in FLN spectra using selective excitation. The shifts and the ratio of the emission intensities, IZPL/I1PL, of these spectral lines depend strongly on temperature, excitation density, and exposure time where changes are observed during minutes of illumination. The shifts and the intensity ratio collected for different experimental conditions show a universal parametric dependence in agreement with the DBMP theory. The strong temperature dependence of the emission decay below 10 K, where only dark excitons are thermally populated, is also explained by the DBMP formation. Modeling of the temperature dependence of the DBMP binding energy and its emission intensity shows that the DBMP is composed of a dark exciton and about 60 dangling bond spins. The exchange integral of one dangling bond spin with the electron confined in the NC is ∼ 0.12 meV. The magnetism originating from the spins of the surface dangling bonds should affect all types of colloidal nanostructures: nanowires, nanoplatelets, and nanorods synthesized not only from semiconductors, but also from metals and oxides. It should lead to unexpected magnetic behavior of nominally non-magnetic (i.e. diamagnetic) nanostructures.

Resume : Interest in 2D materials, originally sparked by the discovery of graphene, has been invigorated with the advent of single layers that exhibit semiconducting properties. Transition metal dichalcogenides (TMDC) are a particularly attractive class of semiconducting layered materials, owing to unconventional phenomena they exhibit such as topological superconductivity and charge density waves, as well as to their possible use in novel optoelectronic and energy conversion technologies. In these applications, variable composition and strain-induced effects could make a significant difference on the electronic and structural properties of the material. Materials modeling and first-principles simulation can be used to elucidate the physicochemical processes that govern these unique materials at the nanoscale. Here, we use electronic structure calculations to investigate strain effects on the electronic and optical properties of the common TMDC materials MoS2, MoSe2, WS2, and WSe2, and their mixed variants MoSSe, WSSe as limiting cases [1]. Our results give an account of how different components (dielectric constant, electron and hole effective masses, exciton binding energies) contribute to the overall changes in electronic structure induced by strain. In this sense, the present results offer a broad basis for designing atomically thick materials based on TMDCs with desirable optoelectronic properties. [1] Defo, R. K.; Fang, S.; Shirodkar, S. N.; Tritsaris, G. A.; Dimoulas, A.; Kaxiras, E. Physical Review B 2016, 94 (15).

Resume : By-layer excitons have a finite electric dipole and they can be driven by electrostatic fields, forming a new class of quantum ‘excitronic’ devices, with perspective opto-electronic applications and used to study dipolar bosonic quantum condensates. Here we report on the exact space-and-time-dependent dynamics of excitons in non-perturbative in-plane external fields in such devices, performing exact Schrödinger evolution of the two-particle system for tens of picoseconds, characterized by very different timescales of relative and centre-of-mass (CoM) degrees of freedom (DOFs), for gate-generated scattering potentials, including ramps, wells, barriers, quantum point contacts, anti-dots. Since gates produce an electrostatic field of opposite sign for the two particles, scattering substantially shakes internal DOFs. Therefore, intense energy transfer between CoM and internal DOFs induces complex transient phenomena, ultimately leading to strong renormalization of, e.g., transmission resonant energies and tunneling probabilities at asymptotic times and/or diffraction patterns in situations where none is expected in single-particle scattering. Common mean-field approaches are shown to be unable to reproduce this phenomenology. We show that internal virtual transition, the fundamental missing ingredient, are restored by a proper local self-energy. Such corrected mean-field calculations are in very good agreement with exact calculations at an extremely reduced computational cost.

Resume : Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials.[1-2] A prominent material for light emitting applications are CdSe/CdS core-shell dot-in-rods.[3] Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by non-resonant Raman spectroscopy with laser excitation energy lower than their bandgap.[4] With high signal-to-noise ratio in the frequency range from 5 - 50 cm-1, we reveal distinct Raman bands that can be related to confined extensional and radial-breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature of the acoustic vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an oscillation of a CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architecture leads to a reduction of the sound velocity in the core region of the rod, which results in a redshift of the rod RBM frequency and localization of the phonon induced strain in vicinity of the core where optical transitions occur. Such localized effects potentially can be exploited as a tool to tune exciton-phonon coupling in nanocrystal heterostructures. 1. D. Oron et al., Phys. Rev. Lett. 102, 177402 (2009) 2. G. del Águila, et. al., ACS Nano 8, 5921 (2014) 3. M. Zavelani Rossi et al., Laser & Photon. Rev. 6, 678-683 (2012) 4. M. Miscuglio et al., Nano Lett. 16, 7664 (2016)

Resume : CdSe quantum dots coated with an ultra-thick CdS shell, also known as Dot-in-Bulk nanocrystals (DiB NCs), are capable of emitting two-color light from core and shell excitons under weak optical excitation. This makes them unique model systems for fundamental studies of the exciton dynamics before carrier localization from the shell to the core states. Here, we use DiB NCs to investigate the photophysics of coexisting core and shell excitons by means of polarization- and time-resolved magneto-PL measurements conducted over a wide range of temperatures. We show that trapping of electrons by surface defects is thermally activated and its suppression for T<100K leads to over 500-fold brightening of the shell PL. Concomitantly, the core PL grows only by a factor of 1.5. This is explained by the formation of negative trions (T-), whose spin relaxation rate (ks) scales quadratically with temperature and magnetic field, B. Despite these similarities, the core and shell T- exhibit a dramatic difference in their ks values: at any B, ks of shell T- is x10 faster than the respective PL decay, while B=5T is needed for the ks of core T- to match the core PL rate. This indicates that the spin-flip mechanism for the shell holes is assisted by interactions with surface paramagnetic dangling bonds that are inaccessible to the core holes. These observations confirm recent theoretical predictions that surface spins play a critical role in spin dynamics of photo-excited carriers in NCs.

Resume : Charge carrier dynamics in atomically thin semiconductors is a critical concern in the formation of electronic and optoelectronic devices. In particular, understanding the dynamics involved in capturing and emission of charges from mid-gap states, and the process of recombination across heterostructures is of utmost importance for the informed design of efficient devices. In semiconducting TMDCs, large hysteresis observed during gate sweeps is analysed through current transient measurements of MoTe2 field-effect transistors. We demonstrate that the mechanism of threshold voltage transients governs the current response to charge trapping and can be attributed to changes in resistivity of the entire channel. We find that the dynamic behaviour of the trapped charges is significantly different than that experienced in conventional devices, a fact which carries dramatic implication on the performance of 2D devices. To further understand the effect of trapped charges on device performance, we study the formation of Schottky barriers between different metals and MoTe2. We find a strong dependence of the Schottky barrier height on the back gate, and an increase in barrier height with decrease in metal work function that gives strong indication to the pinning of the Fermi level at the contact, approaching the Bardeen limit. Using a graphene / MoS2 stacked structure, we gain insight into time dependent mechanisms of recombination and charge redistribution that pave the way to a new class of gated diodes, position sensors and logic components.

Resume : InP QD structures grown on GaAs substrates for operation in the 630–780 nm wavelength range have important advantages that make them ideally suited to integrated photonics for medical and biological applications. In addition to the low threshold current density Jth, 300K Jth for 2mm long lasers below 130Acm-2 and only 500Acm-2 for the same devices at 400K (127 ºC), InP QDs have broad gain spectra, which facilitate tuneable lasers. It is also possible to minimize or control the temperature tuning of the lasing wavelength between 0.03 and 0.17nm/ºC. While the quantum dot states do shift with temperature state filling, which is necessary due to a reduced gain at elevated temperature, moves the gain peak maximum in the opposite direction and this can result in a lasing wavelength that is almost temperature independent. The amount of state filling and hence the value of wavelength achieved depends on the optical loss. The broad gain spectrum and the use of optical loss to select the lasing wavelength combine to produce a novel way to realize dual-wavelength lasers [1], for use in sensing. Lasers are produced with wavelength separations between 8 and 63nm, where output intensity at each wavelength can be independently controlled. We demonstrate the use of such devices and the general functionality of the material system for the monolithic integration by combining on-chip capillary fill fluid delivery [2] and photonic elements in a system for particle detection and sizing. REFERENCES 1] S. Shutts, P.M. Smowton, A.B. Krysa, “Dual wavelength InP quantum dot lasers”, Applied Physics Letters, 104(24), article number: 241106. DOI: 10.1063/1.4883857. 2014 2] A. Sobiesierski, R. Thomas, P.D. Buckle, D.A. Barrow and P.M. Smowton. “A two-stage surface treatment for the long-term stability of hydrophilic SU-8”, Surface and Interface Analysis 47(13), pp. 1174-1179. DOI: 10.1002/sia.5870. 2015

Resume : The development of optical gain using colloidal semiconductor quantum dots (QD) has been intensively studied as it is a key point for the development of QD amplifiers and lasers. QDs offer several unique properties, such as broad and tunable gain bands and solution processability, making them ideal as gain material for integrated lasers. A difficulty in designing such devices is that it is not obvious which specific (material) architecture will give the best performance for a specific application as key parameters such as the threshold and peak material gain strongly depend on the QD’s architecture and composition. To compare different types of QDs on the same footing, we extend the well-known concept of the intrinsic absorption coefficient to an 'intrinsic gain coefficient' or QD material gain and we show that this material property can be conveniently measured using pump-probe spectroscopy. In particular, we have used state-of-the-art CdSe/CdS core/shell QDs to systematically study the effect of the QD core size and shell thickness on gain properties such as gain lifetime, threshold and peak material gain. At first glance, one expects a tradeoff between threshold and material gain as a function of QD size, where small dots show highest peak material gain while bigger dots show lower thresholds. However, because of strong exciton-exciton interactions, we found somewhat counterintuitive trends showing that thin-shelled QDs have both highest peak gain and lowest threshold.

Resume : Semiconductor nanostructures produced by top-down etching methods promise efficient in- and out-coupling of light that needs to be accompanied by material-specific surface treatments for applications into devices. Here, we report the fabrication of InGaN/GaN nanorod (NR LEDs) by reactive ion etching (RIE) followed by the wet chemical removal of the damaged material at the NRs’ sidewalls in a KOH solution and by the surface passivation with only 10 nm alumina using atomic layer deposition (ALD). Correlative characterization techniques are employed to understand the effects of these steps in terms of strain, optical, and electrical properties. Steady-state micro-photoluminescence (PL) reveals an increase by ~ 50% after KOH and up to ~ 80% after ALD with respect to RIE. Since the strain level probed by micro-Raman spectroscopy remains mostly constant after RIE, the PL enhancements can only be attributed to the reduction in the number of surface pathways for non-radiative recombination. These results are further confirmed by time-correlated single photon counting showing an increase in the carrier lifetime. Finally, complete devices with a suspended graphene contact are fabricated and an electroluminescence enhancement by ~ 30% after ALD with respect to KOH is measured, in very good agreement with the PL data. Our work demonstrate that the opto-electronic performance of InGaN/GaN nanostructures can be improved significantly by proper surface treatments up to device level.

Resume : The use of preformed inorganic nanoparticles has permitted the construction of optoelectronic devices by solution processing, and hence, the use of flexible substrates. Moreover, the application of nanocrystals with optical properties into the emissive layer has resulted in the observation of novel functionalities, especially in light emitting diodes where broadband wavelength emission or colour tuning have been reported. Despite the use of inorganic nanocomponents in the emissive layer, the devices are made of organic materials acting as charge transport layers. The resulting diodes are light weight and flexible but require protective films to avoid water and oxygen-induced degradation. This effect can be reduced using inverted charge injecting structures and substituting the organic components by transition metal oxides. On the other hand, the use of Carbon nanoparticles attracts interest of researchers and industry to the diodes due to their photoluminescent properties and the low environmental impact of their composition. In this work, we report the results obtained from devices made of emissive quantum dots sandwiched between transition metal oxide transport layers fabricated by solution-based process. Multicolour emission is observed with dots of similar nature and size when tuning the composition of the emissive layers. Moreover, the performance of the device is affected as well by the surface modification of the nanocrystals.

Resume : Quantum-dot-based semiconductor lasers have highly demanded operating parameters, such as temperature stability and low threshold current. However, in order to fully benefit from their advantages in telecom devices, it is necessary to increase their modulation speed, limited by hot carrier population. It can be achieved in a tunnel injection scheme, where carriers are supplied to the quantum dot (QD) ground state from an adjacent quantum well (QW), acting as a carrier reservoir and separated by a thin tunnelling barrier from QDs. We investigate molecular beam epitaxy structures grown on InP substrates and comprising of an InGaAs QW, separated by a thin InGaAlAs barrier from InAs QDs, with room temperature emission at 1.55 µm. To control the coupling between the QW and QD parts we tailor the thickness of the tunnelling barrier and QW width, which command the wave function overlaps and energy level separation. This affects the optical transitions’ rates, inducing changes from an uncoupled system, where the optical response is just a sum of responses from two isolated elements, to a strongly coupled system, exhibiting mixed 2D-0D transition characteristics, leading to the ground state transition indirect in the real space. The changes of the band structure are deduced from photoreflectance and photoluminescence spectra, supported by 8-band k·p modelling, and are directly reflected in the photoluminescence decay times for the ground state changing from 2 to 10 ns.

Resume : CdSe/CdS core-shell nanocrystals (NCs) show low-threshold optical gain. Yet, colloidal NCs still present a narrow gain band (full-width-half maximum around 10 nm) which limits their application to single-color lasers. Widening of the gain band by specifically engineered NCs can further improve the prospect of this class of materials toward the fabrication of solution-processed white-emitting or color-tunable lasers. Here, we will report on broadband amplified spontaneous emission (ASE) from wurtzite CdSe/CdS “giant-shell” nanocrystals (g-NCs) synthesized with an unprecedented large core up to 7.5 nm in diameter. The combination of large core and shell enable ASE from different CdSe optical transitions as well as from the CdS. Importantly, thin films of g-NCs with large CdSe core show ASE at different colours with a similar threshold, indicating that light emission amplification can be achieved from different optical transitions simultaneously. Tuning of the core diameter allows to obtain ASE in a wide spectral range, and blending of g-NCs with different core sizes gives rise to a continuous amplified spontaneous emission band from green to red (510 to 650 nm). Finally, films of CdSe/CdS g-NCs demonstrate simultaneous dual-color random lasing under ns-pulsed.[1] References: [1] Di Stasio, F.; Polovitsyn, A.; Angeloni, I.; Moreels, I.; Krahne, R. Broadband Amplified Spontaneous Emission and Random Lasing from Wurtzite CdSe/CdS “giant-Shell” Nanocrystals. ACS Photonics 2016, 3 (11), 2083–2088.

Resume : Semiconductor nanowires (NWs) have been considered promising building blocks for achieving the nano-lasers owing to their naturally formed cavity and gain medium, which are of great technological importance for integration with highly compact optoelectronic systems. Lasing from NWs have been successfully demonstrated on a wide variety of materials, covering a broad spectral range. Among the lasing spectrum, the near-infrared (NIR) region is currently drawing an intense research interest as NIR lasers are essential e.g. for telecommunications, sensing and medical diagnostic applications. So far, NIR lasing output are commonly realized based on GaAs/AlGaAs, GaAs/GaAsP and InGaAs/GaAs materials [1-4], where great care is taken to optimize the gain medium, as well as cavity design, because these factors determine the laser performance such as threshold pump density, differential gain, modulation bandwidth and thermal stability. For example, core-shell structure of NW with wider bandgap of outer shell is often used to both effectively confine the carriers and suppress the non-radiative recombination process at the surface of core, leading to pronounced optical amplification of cavity modes with high quantum yield. On the other hand, NW shell can also be utilized as gain by embedding radial multiple quantum wells, which interacts with higher order cavity modes with ring-like light distribution. The flexibility in engineering the gain material and laser cavity can be further extended by employing the dilute nitride material systems based on GaNAs alloy. A small amount of nitrogen (N) incorporated into the GaAs host lattice can induce a strong modification of the conduction band (CB) states, which gives rise to admixing of the CB minimum with the upper lying N-related localized states, and consequently leads to a significantly reduced bandgap. The benefit of this material is manifold. First the electron effective mass increases which enhances the electron confinement in the GaNAs QW structures. Second, the N in GaAs lattice can improve the surface quality of NW, likely due to partial nitridation of non-radiative surface states. All these will improve the radiative efficiency as desired for reliable device performance. In this study, we provide the first demonstration of lasing from a GaNAs shell co-axially grown on a GaAs core, i.e. from the GaAs/GaNAs core/shell nanowire. Single NWs with 400 nm diameter and 4.1 µm length were transferred to gold (Au) substrate. The low pump-power photoluminescence (PL) spectra at 5 K showed typical asymmetric emission from localized excitons trapped at N-related alloy fluctuations, which were coupled to fundamental HE11a/b Fabry-Perot cavity modes. With increasing excitation power, sharp lasing emission was observed from GaNAs band-to-band transition, which exhibits a characteristic non-linear ‘S’-shaped pump-power dependence with concomitant line narrowing. By using rate equation analysis, a threshold gain, g_th, of 3300 cm-1 and spontaneous emission coupling factor, β, of 0.045 were derived. Finite difference time domain (FDTD) simulation was performed, identifying HE21b cavity mode as the lowest threshold mode for lasing. This conclusion is supported by the polarization measurement of the lasing line, which shows predominant polarization along the NW axis, in agreement with the far-field polarization profile of HE21b mode. Further, the lasing emission from NWs can sustain up to ~ 150 K, even without intentional passivation of the GaNAs surface. Our work, therefore, points the potential of the GaNAs alloys as an active gain media and represents the first step towards the room temperature-operating GaNAs-based NW lasers. [1] D. Saxena, S. Mokkapati, P. Parkinson, N. Jiang, Q. Gao, H. H. Tan, C. Jagadish, Nat. Photonics 7, 963-968 (2013) [2] J. Tatebayashi, S. Kako, J. Ho, Y. Ota, S. Iwamoto, Y. Arakawa, Nat. Photonics 9, 501-505 (2015). [3] R. Chen, T. D. Tran, K. W. Ng, W. S. Ko, L. C. Chuang, F. G. Sedgwick, C. Chang-Hasnain, Nat. Photonics 5, 170-175 (2011). [4] B. Hua, J. Motohisa, Y. Kobayashi, S. Hara, T. Fukui, Nano Lett. 9, 112-116 (2008).

Resume : Semiconductor nanoparticles such as zinc selenide and manganese, copper or silver doped zinc selenide are used in many fields e.g in medicine, optoelectronics (production of LEDs), displays and solar panels. ZnSe:Mn nanoparticles were obtained in an aqueous medium using precursors such as zinc acetate, manganese acetate, selenide as product in reaction of selenium with sodium borohydride and 2-mercaptoethylamine hydrochloride as stabilizer. After completion of the reaction, the product was purified by membrane filtration and freeze dried. ZnSe:Mn nanoparticles had an powdery consistency without agglomerates and are characterized by hydrophilic properties. ZnSe:Mn nanoparticles were characterized by size of 20 nm, stable photoluminescence in the visible range corresponding to 580 nm and the light absorbance of 340 nm. ZnSe:Mn nanoparticles have the crystal structure of zinc blend and their surface are positively charged amino groups allowing a further modification of the particle. We have done the surface modification of ZnSe:Mn nanoparticles with organic semiconductors molecules. ZnSe:Mn nanocrystals are characterized by hydrophobic properties and can be used as element in organic matrix in production of many devices such as light emitting diodes, displays or solar panels. This work was financially supported by National Centre for Research and Development under Lider Program, contract no. LIDER/009/185/L-5/13/NCBR/2014.

Resume : A theoretical study of structural, electronic, magnetic and optical of Cr and Fe -doped zinc blende semiconductor CdTe have been investigated by spin-polarized calculations with full-potential augmented plane wave (FP-L/APW ) method within the generalized gradient approximation GGA for the exchange-correlation potential within the spin density functional theory (DFT). We find that the Cr- and Fe -doped zinc blende CdTe show half-metallic behavior with a total magnetic moment of 8.0 µB. It may be useful in semiconductor spintronics and other applications. Finally, we report also our results on optical properties like the complex dielectric functions and the refractive index n of the CdCrTe and CdFeTe compounds.

Resume : Hexagonal arsenene and antimonene have been fabricated very recently as new members of group V-enes. We provide the first investigation of the many-body effect, carrier mobility, and device performance of ML hexagonal arsenene and antimonene based on accurate ab initio method. The calculated quasi-particle band gaps of ML arsenene and antimonene by using ab initio many-body Green’s function approach are 2.47 and 2.38 eV, respectively. The calculated optical absorption band gaps of ML arsenene and antimonene from GW-Bethe-Salpeter equation are 1.6 and 1.5 eV, with exciton binding energies of 0.9 and 0.8 eV, respectively. We find that ML arsenene and antimonene have considerably low (21/66 cm2/V·s for electron/hole) and moderate carrier mobilities (150/510 cm2/V·s for electron/hole), respectively. The optimized sub-10 nm ML arsenene and antimonene field effect transistors can satisfy both low power and high performance requirements in the International Technology Roadmap for Semiconductors in the next decade. Together with their high stability under ambient condition, ML arsenene and antimonene are very attractive for nanoscale optoelectronic and electronic devices.

Resume : We report the growth of single crystal hexagonal ε-Fe3N(111) film on SrTiO3(100) substrate by ablating high-purity iron target in activated nitrogen. The measurements of reflection high-energy diffraction, x-ray diffraction, atomic force microscopy and vibrating sample magnetometer reveal that the ε-Fe3N(111) films have well crystallization, smooth surface, and strong ferromagnetism. The root mean square roughness of the ε-Fe3N film surface is 0.25 nm. The saturation magnetization values are ~900 and ~1050emu/cm3 at 300 and 10 K, respectively. The in-plane and out-of-plane coercivity forces are 550 and 675 Oe, respectively. Meanwhile, the film has obvious magnetic anisotropy.

Resume : Defects play an important role in the properties and performance of semiconductor devices. Depending on the application, they can be used to change the physical and chemical properties (e.g. the introduction of luminescent centres) or they need to be avoided (e.g. because they deteriorate conductivity). In the case of diamond, the intrinsic vacancy defect has, over the years, been studied extensively both in experiment and in theory. This vacancy gives rise to four dangling bonds which can couple; resulting in three possible spin states Sz= 0, 1, and 2. These states are expected to be strongly localized, with an energy located in the band gap of pristine diamond. Various theoretical studies have approached this system, and were able to validate the experimental stability order and electronic structure with varying success. In this work, we revisit the C-vacancy defect in diamond using current state of the art first-principles calculations. We show that the strong localization can be partially retrieved using standard DFT calculations, but computationally more expensive hybrid-functionals are needed to obtain the experimental electronic structure. Due to the cost of the latter and because of the role of electron localization, we set out to find the optimum parameters for a DFT U approach. We show that results of hybrid-functional quality can be obtained at a fraction of the computational cost, providing access to otherwise computationally too expensive properties.

Resume : Tunability of work function and optical nonlinearity is carried out by doping Au nanoparticles in TiO2. The work function and optical nonlinearity are found to be inversely related with respect to concentration of Au. The observed decrease in work function with increasing concentration of Au is due to charge transfer between TiO2 and Au nanoparticles in the presence of ultraviolet light irradiation during the formation of Au-TiO2 nanocomposites. The systematic change in the work function with Au concentration plays a major role in optical nonlinearity. The estimated optical nonlinearity was found to increase from 3.80×10-6 to 9.69×10-6 esu with increase in Au concentrations from 0 to 1.010-2 M. This observed increment in nonlinearity is due to the enhancement of local electric field created by excitation of surface plasmon resonance that affects the work function. Therefore, the surface plasmon resonance and work function help in tuning the optical nonlinearity. The tunable nonlinear optical response of the Au-TiO2 nanocomposites may find applications in nonlinear optics at wavelength 532 nm, a commonly available high power laser.

Resume : Large-area, free-standing and single-crystalline GaN nanomembranes are prepared by UV-assisted electroless chemical etching. As-prepared nanomembranes can act as seeding layers for subsequent overgrowth of high-quality GaN. High-resolution transmission electron microscopy coupled to electron energy loss spectroscopy as well as optical measurements were used to explore physical properties of these nanomembranes.

Resume : In this paper, we aim to present the impact of the atomic layer deposition of Titanium - Aluminum alloy annealed by different treatments on the effective work function of metal-oxide-semiconductor devices. By controlling the deposition cycle of Titanium - Aluminum, the composition of TiAl alloy could be altered. Hence, the work function of TiAl alloy as a metal gate stack could be tuned. To investigate the work function shift after annealing, we compared MOS capacitors with different gate stacks, proceeded post-metallization annealing (PMA) by MWA (1.2kWatt, 1.8kWatt and 2.4kWatt(~500 ℃)) and RTA (300, 400 and 500℃) respectively. The results revealed the capacitors with post-treatment by MWA can suppress work function shift and possess of lower interface trap density (Dit) and thinner EOT. The analysis by transmission electron microscopy (TEM) further explained the variation of EOT. MWA annealing efficiently suppresses work function shift of metal gate electrodes and decreases equivalent oxide thickness (EOT) of gate dielectrics after dopant activation process, it will become a ponderable and promising candidate to simplify and reduce cost of nowadays CMOS fabrication process.

Resume : Coupled InGaN QW-QDs nanostructures were investigated using variable-temperature TIPL and TRPL spectroscopic techniques. Compared with InGaN QW and InGaN QDs reference samples, significant enhancement of photoluminescence was observed at high temperature in InGaN QW-QDs nanostructure with a 4.5 nm barrier, while another sample with a 2 nm barrier showed no efficient enhancement. We attribute the enhancement to temperature dependent quantum tunneling of excitons from QW to QDs. Models were proposed to simulate strong phonon-exciton coupling in QW and temperature dependent quantum tunneling probability. Thus we suggest an optimum working temperature exists and the temperature can be tuned by barrier width.

Resume : In the last few years, an accelerated trend towards the miniaturization of nanoscale circuits has been recorded. In fact, this has been reflected by numerous enhancements at different levels of multi-gate structures such as the channel core or the gate material. Our aim in this work is to investigate the reliability performance of nanoscale junctionless DG MOSFET device including graded channel aspect. The behavior of the considered device is investigated numerically using ATLAS 2-D simulator, where the degradation mechanism and the reliability behavior of the proposed design are studied. In this context, the impact of the channel length and traps density on the variation of subthreshold criteria which are: threshold voltage, DIBL and swing factor is analyzed. The obtained responses indicate the superior immunity of the proposed device against traps induced degradation in comparison to the conventional structures. Thus, this work can offer more insights regarding the benefit of adopting the channel doping engineering for future reliable low-power nanoscale electronic applications.

Resume : We have fabricated AlGaN/GaN fin-shaped field-effect transistors (FinFETs) with different W/H (nanowire fin width/fin height) ratios. In the fin structure, a top 2-DEG channel is formed at AlGaN/GaN heterointerface and two MOS channels are formed on the etched side walls of GaN surface. Then, the effects of the sidewall MOS channels on the I-V and transconductance characteristics of FinFET devices have been investigated to resolve the short channel effect of nanowire fin structure while varying the temperature because electrons in the 2-DEG channel mainly experience polar-optical-phonon scattering from 100 to 350K. When the W/H ratio is large, the current through the 2-DEG channel is dominant. Decreasing the ratio, the contribution of the 2-DEG channel on device current decreases, but contribution of the sidewall MOS channels relatively increases. This characteristics can offer advantages for high temperature applications in electric vehicles. The narrow FinFET has the significant current contribution of the sidewall MOS channels. As a result, the total electron mobility has slightly positive temperature coefficient. Therefore, it is possible to achieve AlGaN/GaN Fin-FETs featuring temperature-insensitive current by controlling the W/H ratio and the gate biasing. .

Resume : In this work, we study the composition and elec¬tronic structure of Ga1-xAlxAs nanocrystals and films obtained on the GaAs surface by Al+ ion implantation with subsequent (laser + thermal) annealing. Test objects were n- and p-GaAs (111) films d = 500 nm thick. The films were subjected to Al+ ion bombardment with ion energy E0 varying between 0.5 and 5.0 keV and irradiation dose D in the range 1014–1017 cm–2. AES results showed that the implantation of ions Al+ in GaAs, combined with annealing in the subsurface layer is formed compound type of Ga0,5Al0,5As. The formation of ter¬nary compounds in photoelectron spectroscopies results in the following changes. (i) The spectrum becomes narrower by 0.3–0.4 eV; that is, peak EV becomes 0.3–0.4 eV farther from EB. (ii) Peak E1 shifts toward higher energies and broadens considerably. We think that 4p-electrons of Ga and 3p-electrons of Al contribute to the formation of this peak. (iii) Peak E2 due to the splitting of p-states in Ga, Al, and As shifts to the right by 0.1–0.2 eV, and its intensity grows. (iv) Arsenic peaks E3 and E4 remain almost the same: only their intensities change slightly. Varying the energy of Al ions in the interval 0.5– 5.0 keV, one can grow homogeneous Ga0.5Al0.5As films with thickness θ in the range from 2.0–2.5 to 6.0– 7.0 nm. Thus in the work obtained ternary Ga1-xAlxAs nanostructures 2–7 nm thick were grown on the surface of a GaAs crystal by means of 0.5 to 5.0-keV Al+ ion implantation followed by annealing. At low irradiation doses (D ≤ 1015 cm–2), nanocrystalline phases formed; at higher doses (D ≥ 1016 cm–2), Ga0.5Al0.5As nanofilms arose. Varying the post-implantation annealing temperature between 850 and 1000 K, one can vary x in the interval 0.5–0.2.

Resume : Bulk ingot material of the ternary mixture Ge10Se5Sb85 was prepared by direct fusion of high purity constituent elements in vacuum sealed silica tube. The glassy nature of the prepared sample was confirmed by the X-ray diffraction (XRD) technique. Current -Voltage characteristics of the investigated glass have been carried out at different thicknesses and temperatures. Switching phenomenon at the turn-over point (TOP) from a high-resistance state (OFF state) to a negative-differential resistance-state (NDRS) was detected where the threshold parameters such as threshold dissipated power (Pth) ,threshold voltage( Vth),threshold current (Ith),threshold electric field (Eth) and threshold resistance (Rth)were determined at different thicknesses and ambient temperatures of the investigated samples. At the turn-over point ,the activation energies (∆Ep, ∆Ev, ∆Ei ,∆Er and ∆Ef) caused by the threshold dissipated powers, threshold voltages, threshold currents, threshold resistances and threshold electric fields respectively, were deduced at different thicknesses of the samples. The increasing in the ambient temperature of the investigated material (∆TJ), the temperature of the conduction path (T\ ) and the Poole-Frenkel coefficient (βPF) were determined at different ambient temperatures and thicknesses of the samples on the basis of the Joule heating effects. The activation energy of hopping (W), the activation energy of conduction ΔEσ(eV), the hopping distance (d) of the charge carriers and the density of localized states N(E) were carried out due to Poole-Frenkel effect

Resume : Spectral sensitization of BDD was carried out by anchoring of 4-(bis-{4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl}-amino)-benzoic acid (P1) with polyethyleneimine as a linker or by covalent derivatization [1]. Alternative chemical modification was performed through a combination of diazonium electrografting and Suzuki cross-coupling reactions. The sensitized diamond exhibits stable cathodic photocurrents under visible light illumination in aqueous electrolyte solution with dimethylviologen serving as electron mediator. To enhance the light harvesting, nanotextured BDD was prepared via silica templating route by spherical or electrospun nanofiber templates. Cathodic photocurrents under solar light illumination are about 3-times larger on nanostructured electrodes compared to those on flat diamond. Illumination of the sensitized electrodes with chopped light at 1 sun intensity causes an increase of the cathodic photocurrent density to ca. 15-22 μA/cm2. Photocurrent densities scale linearly with light intensity (between 0.1 a 1 sun), and they represent the largest values reported so far for dye-sensitized diamond electrodes. The photoelectrochemical activation of the sensitized diamond electrodes is accompanied with characteristic changes of the dark voltammogram of the MV2+/MV+ redox couple and with gradual changes of the IPCE spectra. This work was supported by the Grant Agency of the Czech Republic (contract No. 13-31783S).

Resume : Conventional GaAs capped quantum dot (QD) based devices are limited to lower operating temperature due to higher thermionic emission. In this abstract, a combinational capping layer is proposed which improves the vertical confinement of carriers and reduces the thermionic emission probability. Compared to the GaAs capped structure, an improved intensity and redshift in the ground state (GS) photoluminescence (PL) peak are observed for the proposed structures. In the InAlGaAs/InGaAs/GaAs capped sample, the barrier potential has a better effect in terms of vertical confinement, which accommodate the QD ground and excited state with higher gap. This helps in a lower thermionic emission. However, the InGaAs/InAlGaAs/GaAs capped sample has a step type barrier potential and comparatively higher thermionic emission. A tenfold enhancement in the PL intensity is observed in the former case. This confirms better QD formation with higher density and reduced non radiative centers in the InAlGaAs/InGaAs/GaAs capped QDs. The proposed optimized structure has the highest activation energy (232.74 meV), and lower full width at half maximum (40.03 meV) at 300K, which is attributed to the formation of QDs with lower size dispersion. Hence, a lower thermionic emission probability along with a room temperature PL emission at 1.33 μm has been achieved. The proposed structure can be helpful for a better light emitting device with higher operating temperature and reduced dark current.

Resume : The intrinsic nanoscale point defects generating in the crystal lattice of ZnSe during the crystal growth and doping processes strongly determine the functional properties of the material as well as dopant (Fe) solubility. Nonstoichiometry and IR luminescence spectra of ZnSe:Fe powdered preparations and CVD-grown ZnSe:Fe crystals treated by high-temperature gas-static pressing (HIP) were changing depending on Fe doping level and preparation conditions. Whereas the nominally pure CVD-ZnSe crystals had excess of Zn over stoichiometric composition, all the Fe-doped ZnSe crystals had an excess of chalcogen. This correlates with the results of the Zn-Se-Fe phase diagram analysis. Isothermal sections of T-x-y diagrams of ternary Zn-Se-Fe were reconstructed and experimentally confirmed by X-ray diffraction and Energy Dispersive X-ray spectroscopy (EDS) in the temperature range of 873 K - 1088 K. In case of ZnSe:Fe phase with the Fe-concentration level about 1e19 cm-3 we observed the shift of the homogeneity range to Se-riched region with the nonstoichiometry level ranging from 5e17 to 1e19 cm-3. As a result of the control the intrinsic point defects and Fe-dopant level we achieved the large quantities of differential efficiency of the produced ZnSe:Fe2+ laser as ηslope = 55%. The research was financially supported by the Russian Science Foundation grant № 15-13-10028

Resume : Resistive Random Access Memory (RRAM) is one of the most promising nonvolatile memory because it has several advantages; for example, simple MIM (metal-insulator-metal) structure, fast operation speed, high endurance, high retention, and low energy consumption. However, the reliability is not persistent and completely switching mechanism is not fully understood. In this work, we deposited different metal oxide (HfO2, Al2O3) covering the Ni/NiO nanowire by atomic-layer-deposition (ALD). The different properties, including electrical characteristics, surface morphology, and elements distribution have been systematically investigated. The electrical characteristics were excellent, and the endurance could up to 205 cycles. It is pretty good for 1-D nanostructure. Also, we used focus-ion-beam (FIB) to prepare TEM sample subsequently following TEM observation. From the TEM analysis, we made sure where the conducting filament (CF) was and what the element components of CF were. The diffusion of oxygen vacancies formed the conducting filament, resulting in the change of morphology. The study enriched the understanding of the mechanism and could help achieve better switching properties of RRAM.

Resume : The crossbar structure of Resistive Random Access Memory (RRAM) is the most promising technology in the future nonvolatile memory devices, which is expected to develop nanoscale, high performance and low power consumption devices. However, little research has focused on the phenomenon of the switching process in much detail. The main purpose of this study is to develop an understanding of the reversible formation and disruption of the conductive filament (CF) occurring at crossbar center by real-time observation. The Ni/NiO nanowires was utilized to form a cross structure, which restricted the position of CF at the cross-center. From in situ TEM images, the significant change of morphology appearing at the crossbar center, which resulted from the absence and backfill of the oxygen-ions, was observed. In addition, EDS quantitative analysis showed that there was a lower O/ Ni ratio at the cross-center after the SET process; conversely, a higher O/Ni ratio would be present after the RESET process. The experimental results demonstrated that the CF is dominated by oxygen-ions movement followed with redox reactions. Moreover, the crossbar structure of RRAM device exhibited significant electrical characteristic during SET and RESET process no matter in high vacuum or at atmospheric pressure. This study provides the useful information for understanding the switching mechanism of crossbar nanowires structure, which is the most potential method to fabricate nanoscale RRAM devices applied to 3D stacked package technology and neuromorphic computing system in the future.

Resume : In recent years, ion implantation is often used to obtain nanocrystals and nanolayers on the surface and in the subsurface region of semiconductor and dielectric films. The staff of our group were obtained and studied composition, structure and electronic properties of single and multi-component nanostructures in different depths of silicon and gallium arsenide bombardment ions Ba+, Na+, Co+, Al+, O+, and Ar+. The optimum conditions of ion bombardment and annealing the formation of nanoscale structures type of MeSi2/Si, SiO2/Si, GaMeAs/GaAs, CaMeF2/CaF2, MeSi2/Si/MeSi2/Si, MeSi2/Si/CaF2 and others. Developed models and identified the main mechanisms for the formation of the nanocrystals and nanostructures in the process of ion implantation and subsequent annealing. A method for evaluating the composition, structure and properties of individual nanocrystals based on a comparative analysis of the Auger spectra of photo- and electron energy-loss electrons obtained for the substrate, nanofilms and nanocrystals. A method for estimating critical dimensions of nanocrystals which involve tunneling of electrons based on measuring changes in resistivity. The sizes of the nanocrystals in which silicide begin to manifest quantum size effects. In the case of Si nanocrystals with nanocrystals BaSi2 an average size nanocrystalline phases of BaSi2 (quantum wells) is 100 - 150 Å, and the distance between them 300 - 350 Å (quantum barrier). Estimated critical dimensions of nanofilms and nanocrystals, which begins to forming electron-band structure, typical for of massive materials.

Resume : Bipolar resistive switching in ZnO/SiOx bi-layer and ZnO/SiOx/ZnO tri-layer structures was investigated for nonvolatile memory applications. ZnO thin films were grown using the radio- frequency magnetron sputtering technique at room temperature. SiOx films were grown using plasma-enhanced chemical-vapor deposition at 200°C. Multiple high-resistance states were observed during the set process. The high/low resistance state ratio was ~10 during ~100 on/off cycles. The tri-layer memory device exhibited better endurance properties than the bi-layer device. Because an asymmetric conducting filament has a weak point for charge conduction at the oxide interfaces, we attributed the good endurance property to the reproducible formation/rupture of “micro”-conducting filaments. Moreover, the dynamics of the oxygen ions in the SiOx layer plays an important role in resistive switching.

Resume : Recently, researchers have shown an increased interest in fabrication of one-dimensional nanostructures (nanowires, nanorods (NRs) and nanotubes) based opto-electronic devices such as solar cells and photo-detectors, which offer significant advantages in terms of device performance and cost over their planer- counterparts. In particular, TiO2 NRs have attracted a great deal of attention for the fabrication of dye-sensitized solar cells, photo-catalysis and gas sensors due to their outstanding optical, chemical and physical properties. However, far too little attention has been paid to the use of TiO2 NRs for UV-photodetectors and semiconductor sensitized solar cells. Therefore, the aim of the present study is to fill this gap by employing TiO2 NRs for the fabrication of a CdTe/TiO2-NRs structured core-shell solar cell and Ag/TiO2-NRs based Schottky-type UV-photodetector. TiO2 NRs, diameter and length of which in the range of 90-120 nm and ~2 µm, respectively, were successfully synthesized on FTO (fluorine-doped tin-oxide) pre-coated glass substrates via hydrothermal technique. Prior to the fabrication of the aforementioned opto-electronic devices, the effect of growth parameters, including process-temperature, chemical precursor concentration and growth-time, on quality of synthesized NRs in terms of vertical-alignment and homogeneity over the substrate was investigated in detail. The results revealed that critical values for temperature (140 oC), growth-time (4h), precursor concentration (HCl:DI-water:1:1, V%) and angle between the substrate (conducting side facing down) and the wall of the reactor ( auto-clave) were required for the synthesis of well-aligned homogenously distributed TiO2 NRs. Following the optimization of growth parameters, the well-aligned TiO2 NRS were decorated with a thin sputtered layer of CdTe to form a core-shell type heterojunction completely inorganic sensitized solar cell. The device was then subjected to CdCl2-chemical treatment, an attempt to enlarge the grains size, saturate the dangling bonds at the grain boundaries and enhance the electrical properties of CdTe thin film. The performance of the fabricated solar cell was measured under standard test conditions (AM 1.5). The solar parameters such as open-circuit voltage, short-circuit current, fill factor and power conversion efficiency were extracted from the recorded current (I) vs voltage (V) characteristics under light illumination, which were found to be 0.37 V, 3.2x10-5 A, 25% and 0.42 %, respectively. The obtained 0.42 % efficiency is exactly 3.5 times larger than that we obtained for the same structure in our previous study, which was not subjected to CdCl2-chemical solution treatment. As a second part of this study, a Schottky type Ag/TiO2-NRs structured UV-photodetector was constructed. The I-V characteristics of the device was determined under different light wavelength to measure its UV-sensing performance. When compared with the dark current, the device exhibited a drastic increase in photocurrent under UV-light exposure. From the ON/OFF time dependent photoresponse measurements at 1 V with 380 nm (78 µW/cm2), rise time and decay time were calculated, which were found to be 34 ms and 0.5 s, respectively.

Resume : The structural, optical and gas sensing properties of pristine and swift heavy ion irradiated nanocrystalline SnO2 films have been studied. Structural characterization using X-ray diffraction (XRD) reveals systematic decrease in the crystallinity of the films with increasing ion fluence. The microstructural characterization performed using scanning electron microscopy reveals decrement in the grain size from 76.3 to 40.1nm with increasing ion fluence thus supporting the XRD results. The fluence dependent Rutherford backscattering spectra reveals sputtering of Sn as a result of ion bombardment. The optical studies performed using UV-Vis spectrometry show a blue shift in band gap from 4.65eV to 5.04eV with ion irradiation. Thermal spike simulations have been performed to understand the changes in the microstructural properties after ion bombardment. The simulated results are in good agreement with the experimental results. The gas sensing measurements performed using 1000, 3000 and 5000ppm of H2 gas at an operating temperature of 300degrees reveal an increase in sensing response with increase in ion fluence. The changes in the response characteristics have been discussed.

Resume : In this work, undoped and 1-5 at. % In-doped SnO2-δ films were deposited onto glass substrates at 350 °C by spray pyrolysis technique. The influence of dopant concentration was investigated using X-ray diffraction (XRD), Uv-Visible spectroscopy, and Hall Effect measurements using van der Pauw’s method. X-ray diffraction studies indicated that all films had preferred orientation along (200) plane and were polycrystalline with tetragonal rutile structure. The calculated average crystallite sizes increased after doping. Substitution of In into SnO2-δ thin films can be confirmed by the shifting of the peaks in the XRD patterns. Optical transmittance of the films showed high average transparency ~ 80-90% in the visible region. Hall measurements showed that the conduction type was dependent on In content. For low doped films (In ≤ 3 at.%), the films were n type, while at higher doping concentration the films were p type. The calculated values of the mean free path are very small compared to the average crystallite sizes calculated using XRD measurements. Therefore, we suggest that ionized and/or neutral impurity scattering are the main scattering mechanisms in these films. The above-mentioned characteristics render these In-doped SnO2 films potential candidates for their use in light-emitting diode and in optoelectronic devices, with the advantage that they are prepared by a simple and economical technique.

Resume : The solar cells with Cu2ZnSnS4 (CZTS) have been the objects of research over the last decade. The CZTS nanocrystals are a promising material for absorption layers, due to their narrow band gap (c. a. 1.4-1.5 eV), high absorption coefficient (10^4 cm^-1) and they the fact that don’t contain toxic elements. There are lots of methods to obtain CZTS nanocrystals, but there are only a few papers where the microwave radiation was used. In this work, the CZTS nanocrystals were obtained with microwave assisted synthesis and the resulting products were characterized. The reaction was carried out in different temperatures, times of reaction and the power of reactor, with ethylenediamine as a stabilizer of synthesis. The obtained materials were characterized with XRD, EDX, Raman, cyclic voltamperometry and NMR. The TEM images were also taken and the luminescent properties in Near Infrared (NIR) spectrum were investigated. The obtained nanocrystals have kesterite structures with diameter of about 5 nm. The surface of products were modified with derivatives of aminobenzene and aminonaphtalenes. The obtained materials were investigated with the previously mentioned methods. This work was financially supported by National Centre for Research and Development under Lider Program, contract no. LIDER/009/185/L-5/13/NCBR/2014.

Resume : Multishell coaxial semiconductor nanowires (CSNW) are attracting much interest due to their possible application as light harvesting devices, nanophotonic sources, and nanoscale FETs with novel geometries. Furthermore, recent technological advancements unlocked the possibility to modulate the CSNW composition in the radial directions. By using a self-consistent local density-functional approach, exploiting symmetry-compliant grids, we show how carrier density and localization can be tailored among the central core and the different tubular shells by a proper modulation of the overgrown layers of semiconductor. We also predict the impact of different localization patterns on the magneto-photoluminescence spectra of CSNWs.

Resume : ALD of SrTiO3 (STO) films has abnormally high growth rate at initial stage of film growth on Ru electrode was observed. In this study, very thin (1.5nm – 4.5nm) seed layers were deposited at a lower temperature to achieve well-controlled seed layers free from the adverse CVD effects. The ALD saturation curve was confirmed in SrO and TiO2 depositions, and the film growth showed a linear growth behavior with respect to the number of deposition cycles with no indication of abnormal growth at the initial stage. Furthermore, the thickness of the seed thin film was lowered to a certain level along with the formation of its micro-cracks and the dielectric constant was increased from 170 to 276 as compared with the case where the thick film was used as the seed layer. Also, the thin films grown on the thinner (1.5nm) seed STO films showed the higher density than those grown on the thicker (4.5nm) seed STO thin films after the main STO thin films were deposited even though the roughness showed the same tendency. This affected the crystal orientation and dielectric constant of the main layer deposited on the thin seed film. However, there was a disadvantage in that a Sr-rich thin film and an interfacial layer were formed at the bottom of the seed thin film. These electrical properties of the (top) RuO2 / STO / Ru (bottom) planar capacitor were estimated to confirm its feasibility for the next generation DRAM capacitor applications.

Resume : In this work, we have proposed a simple Ni pattern guided growth method to fabricate GaN nanowire array for high responsive humidity sensing. As both device electrode and growth catalyst, Ni metal can directly guide the selective-area lateral growth of GaN nanowires, which successfully bridge across a wide gap between two adjacent electrodes up to ~20 μm. The obtained Ni/GaN nanowire array/Ni sensor device exhibits a typical Schottky-contacted MSM characteristic. Its electrical transport behavior has a great dependence on the physisorption of water molecules on GaN nanowires, which is highly sensitive to monitor the relative humidity (RH) in ambient environment. Based upon utilizing energy band diagrams, the response performances of GaN nanowire humidity sensor have been investigated under different gap distance, RH and UV power conditions at room temperature. As the RH increases from 15 % to 85 %, it is clearly seen that the output current of the MSM sensor decreases gradually. And the larger the Schottky barrier at negative bias of -5 V, the higher the RH sensitivity. With the increase of UV optical power, its RH sensitivity has a large improvement from 3208 % to 10066 % at 5V and from 7818 % to 24037 % at -5 V, respectively. Interestingly, it is noted that the UV photoexcitation modulation has provided an effective way for a great enhancement of the RH sensitivity of GaN nanowire humidity sensor.

Resume : We investigate the potential of bilayer graphene as a microwave photodetector by studying the microwave absorption in suspended clean bilayer graphene p−n junctions in the frequency range of 1−5 GHz at a temperature of 8 K. We observe a distinct photocurrent signal if the device is gated into the p−n regime, while there is almost no signal for unipolar doping in either the n−n or p−p regimes. Most surprisingly, the photocurrent strongly peaks when one side of the junction is gated to the Dirac point (charge-neutrality point CNP), while the other remains in a highly doped state. This is different to previous results where optical radiation was used. We propose a new mechanism based on the phototermal effect explaining the large signal. It requires contact doping and a distinctly different transport mechanism on both sides: one side of graphene is ballistic and the other diffusive. By engineering partially diffusive and partially ballistic devices, the photocurrent can drastically be enhanced.

Resume : Presented in this work are the results concerning formation of nano-heterostructures p-(Ag2O-Hg0.8Cd0.2Te) on the surface of solid solution Hg1–xCdxTe (х~0.22). Modification of this ternary chalcogenide semiconductor compound was performed using the method of doping the heterostructure samples with Ag+, which was followed by low-temperature treatment. The energy and dose of implanted ions were 100 keV and 4.8∙1013 сm-2, respectively [1]. Studied in this work are electro-physical and structural properties of the narrow-band semiconductor compound Hg(Cd)Te with the nano-heterostructure p-(Ag2O-Hg0.8Cd0.2Te) formed on its surface by using the method of ion implantation Ag+ in the top-down approach. Photo-emf properties were studied in the IR and MM range. Photo-emf sensitivity SV() in was obtained for the nano-heterostructure at temperature T= 77K (show Fig) in the wavelength range 1-2 μm and 5-7 μm. For SV() in MM (=2 mm) range were calculated at the room temperature NEP=2.7 10-8 (WHz -1/2) and specific directivity D=2*109 (cm Hz1/2W-1). The results indicate the use of this nanostructure for manufacturing of the Hg(Cd)Te multicolor photodetectors in sub-THz and IR regions. A. B. Smirnov, O. S. Litvin, V. O. Morozhenko, R. K. Savkina, M. I. Smoliy, R. S. Udovytska and F. F. Sizov,” Role of Mechanical Stresses at Ion Implantation of CdHgTe Solid Solutions,” Ukr.J.Phys., vol. 58, Num. 9, pp. 872-880, Oct. 2013.

Resume : CdSb and Cd1-xZnxSb thin films were obtained by high-frequency cathode sputtering in argon atmosphere. In4Se3 crystals, grown by the Czochralski method, and CdTe crystal, grown by the Bridgman method, were used as the substrates for heterojunction growing. Transmission electron microscope studies showed that films have quasi amorphous or polycrystalline structure, or sometimes a texture is formed, depending on the high-frequency power in the growth process. Properties of nanostructured CdSb and Cd1-xZnxSb (x= 0.1-0.2) films were studied by SEM and AFM methods. Mechanism of grain coagulation and film growth in Stranski-Krastanov mode was analyzed by calculating quantitative characteristics of obtained AFM images. Periodic system of CdSb islands was observed on In4Te3 crystal substrates. Chemical composition of the samples was studied by electron probe microanalysis EDX, that enables to set the conditions for obtaining stoichiometric films. Photosensitivity spectral characteristics of CdSb and Cd1-xZnxSb heterojunctions were studied. It was detected an increase of heterojunction photosensitivity in the case of doping the films by Te impurity. Laser treatment of CdSb and Cd1-xZnxSb films results in an increase of grain size and structure and also in improvement of current-voltage and capacitance-voltage characteristics of obtained heterojunctions. Designed photosensitive heterojunctions can be used as effective sensors of infrared radiation.

Resume : The tunable photoluminescence (PL) response was studied with rare earth (RE) doped barium zirconate titanate (BZT) ceramics. The PL response reduced with application of external electric field. The recovery of PL response after removal of E-field was found to be time-dependent. BaZrxTi1-xO3 (abbreviated as BZT-x) ceramics with x= 0.15, 0.2, 0.25 and 0.3 were fabricated for finding suitable host for Er3+ and Eu3+ doping. It was found that the Curie temperature (Tc) of BZT-0.15 is above room temperature which indicated it is rhombohedral phase. The Tc of BZT-0.2 is close to room temperature and BZT-0.25 and 0.3 are above room temperature. According to the temperature dependence dielectric properties measurement, the maximum of dielectric constant reduced with growing amount of Zr. Also, the piezoelectric coefficient was found decreasing with increasing Zr content. To study PL response under electric field at room temperature, BZT-0.15 is chosen to dope with 0.2 mol% of Er3+ (abbreviated as BZT-0.15-Er). BZT-0.15, 175 and 0.2 are chosen to dope with 0.2 mol% of Eu3+ (abbreviated as BZT-0.15-Eu, BZT-0.175-Eu and BZT-0.2-Eu). BZT-0.15-Er ceramics were fabricated. Excitation and emission spectrum of BZT-0.15-Er were measured with bulk and sample with ITO transparent electrode. The mechanism of the tunable photoluminescence is still under investigation. Strain and polarization are two of the possible reasons inducing symmetry increase. BZT-0.15, 0.175 and 0.2-Eu ceramics were fabricated. Using different zirconium content was aim to investigate the relation between the recovery time and Tc. Excitation and emission spectrum of BZT-0.15, 0.175 and 0.2-Eu were measured with bulk. Excitation and emission spectrum of BZT-0.15-Eu with ITO electrode were measured with E-field. The emission spectrum shown reduce with intensity with increasing E-field. However, the excitation spectrum did not have significant change.

Resume : As individual electron devices become smaller and smaller, with feature sizes reaching the length scale of a few atoms, standard approaches to designing and fabricating nanoelectronic devices by introduction of impurity atoms or “doping” and/or heterogeneous materials growth becomes impractical if not impossible. Here, it is demonstrated that these approaches are not required on the nanoscale. In this paper, we provide a presentation of a combined theoretical and experimental demonstration of the quantum confinement effect in single crystalline bismuth films of only a few nanometer thickness, showing for the first time that the quantum confinement effect can be used to provide the fundamental electronic function of rectification using a single elemental material and critically for realistic nanoelectronics applications, at room temperature. This is achieved by locally thinning a 12nm thick semimetal Bi film down to a few nanometers creating a band offset of ~130 meV at 12nm/4nm and ~340 meV at 12nm/2nm junctions. A mono-material hetero-dimensional (3D-2D) junction is demonstrated to provide rectification paving the way for general electronic functions achievable within single material layer.

Resume : In the forming of Ge fin structures on SOI, we found that the dry etching process must be carefully controlled. Otherwise, Ge over-etching or some special Ge fin profile will be produced. If the etching process is not well controlled, the top Ge/GeSi/Ge structure will be etched away, and only Si fin layer is left. In this case, the device will shows a abnormal characteristic. In this work, it shows the output and transfer characteristics of the enexpected Si FinFET after Ge overetching with Wfin of 40nm and Lg of 80nm. Ion/Ioff ratio of this device is as high as 108. However, its on current is only 17uA/um at -1V. This high Ion/Ioff ratio is actually from Si layer, not from the Ge layers. So etching process is emerging as a critical technology for scaling, packaging and continuing the drive to increase packing density and improve the performance of ICs. Therefore we can get the optimization factors by simulation model then to adjust not only parameters setting but also hardware modification. This will help us to improve the uniformity within whole run, to obtain the high quality / low manufacturing cost process and process optimization.

Resume : Synthesis of multicomponent nanocrystals (NCs) with accurate size and composition control represents a challenging task due to the different reactivity of the constituent elements. Often, NC composition changes simultaneously with NC size, whereas target NC compositions can only be achieved for large NC sizes prepared with extended growth times. Recently, we demonstrated that an amide-promoted approach is able to decouple the size and composition control for colloidal NCs. This work presents an extension of synthetic method to quaternary chalcogenide materials. As is conventional for amide-promoted synthesis, LiN(SiMe3)2 is co-injected along with selenium precursor in the hot reaction mixture containing metal halides. The presence of amide speeds up the nucleation rate, so that the target composition is achieved within seconds. We establish simple relations between initial concentrations of metal salts and the amounts of metals in the composition of Cu-Zn-In-Se NCs: corresponding to the chemistry law of mass action, the amounts of deposited Cu, Zn, and In are proportional to the power functions of starting molar concentrations of metal salts or a product of such power functions. This allows us to select a desired Cu-Zn-In-Se composition and obtain NCs of this composition with an error of 2-4 at.%. As complement to accurate composition tuning, independent size control, narrow size distributions, and long term colloidal stability are also achieved.

Resume : Transition metal dichalcogenides (TMDs) are predicted to be promising candidate for next generation electronics, optoelectronics and sensor devices. The intrinsic nature of carriers in two dimensional semiconductors offers numerous advantages such as high mobility, high sensitivity, fast response and chemical stability. Among several of the layered TMDs, MoS2 has attracted great interest because of its distinctive electronic, optical, and catalytic properties Two-dimensional molybdenum disulfide (MoS2) is a promising material for ultrasensitive photodetector owing to its favourable band gap and high absorption coefficient. However, their commercial applications are limited by the lack of high quality p-n junction and large wafer scale fabrication process. In this work, we report high speed Si/MoS2 (p-n) heterojunction based photodetector contrived from a very simple, reproducible and scalable fabrication process. To fabricate the heterojunction, we have adopted CVD process for the growth of MoS2 thin film. In which large area MoS2 thin film on silicon platform has been synthesized by sulfurization of RF-sputtered MoO3 films. The fabricated heterojunction exhibited excellent photoresponse properties in the visible to near-infrared wavelengths with a maximum responsivity of 8.75 A W –1 (at 580 nm and 3V bias) and fast rise time of 10 μs. Transient measurements of Si/MoS2 heterojunction under the modulated light reveal that the devices can function up to 50 kHz. The Si/MoS2 heterojunction is found to be sensitive to broadband wavelengths with maximum detectivity up to ≈1.4 x 1012 Jones (2V bias). Reproducible low dark current and high responsivity from over 20 devices in the same wafer has been measured. Additionally, the MoS2/Si photodetectors exhibit excellent stability in ambient atmosphere. The excellent performance of multilayered MoS2 heterostructure may lead the way for the fabrication of high speed photodetectors.

Resume : Recently, the research of atomically thin two-dimensional (2-D) heterostructures has attracted a lot of attention from material engineers. These heterostructures have opened the possibilities for optoelectronics and electrical properties beyond the unique properties of each layered materials. Especially, the MoS2/graphene heterostructure has been expected due to the synergetic effect of extremely high mobility of graphene and ultrasensitive and ultrafast detection of light of MoS2. The chemical or mechanical exfoliation and transfer of individual 2-D crystals are one of the most common methods to assemble a 2-D heterostructure. However, the drawback with this type of method is typically to produce small MoS2 flake. Also, it is easy to suffer from the defects or impurities on the interface of the heterostructures during the assembly process. These problems have a negative effect on the overall properties of the heterostructures by diminishing the interlayer coupling. Therefore, the well-established method for assembling high quality heterostructures is a key element to fabricate the uniform and high performance 2-D heterostructure-based optoelecronic devices. In this study, we successfully developed a direct growth method of layer-controlled MoS2 on graphene using the atomic layer deposition (ALD) process for the MoS2/graphene heterostructure. Also, we obtained the uniform and centimeter scale MoS2 films on graphene by alternating exposure of molybdenum fluoride (MoF6) and hydrogen disulfide (H2S) vapors. We utilized the atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photocurrent measurements to characterize the properties of the directly-grown MoS2/graphene heterostructure.

Resume : Germanium (Ge) is of interest in replacing silicon (Si) as a device substrate due to its higher electron and whole mobility that could lead to enhanced device performance without the need for further down-scaling. An important criteria for good device performance are highly activated carriers within operating regime of the pn junction. The fabrication of highly doped/activated n-type junction in Ge remains challenging, due to the interaction of ion implantation induced defects with dopant atoms during the thermal annealing process, which leads to low dopant activation and anomalous dopant diffusion. Therefore optimization of the process parameters, involving ion-implantation and thermal annealing, is required to achieve a higher activation level for highly-doped Ge n-type junction. In this work, phosphorus (P) with atomic size of 98 pm and tin (Sn) with atomic size of 145 pm were selected as a dopant and non-dopant atoms for a co-implantation process. Co-implantation has recently gained interest, due to its stress-induced activation attributed to the large atomic size of the co-implanted dopant. High concentration of dopant over the solubility limit can be achieved by this technique. Ultra-fast and low thermal budget annealing, such as laser annealing, enables high activation and a high degree of damage removal. Ion-implantation parameters were optimized using Transport of Ion in Matter (TRIM) software. Dose concentration of P were arranged to achieve maximum concentration of 1E20 cm-3. The energy and dose concentration for Sn atoms were then manipulated to achieve an overlapping depth profile of dopant. Laser annealing was carried out using a nanosecond KrF excimer laser at a wavelength of 248 nm. The laser energy densities were varied between 100 and 2000 mJ/cm-2 whilst the shot numbers varied between 1 and 1000. The depth of the implanted layer for 40 keV ion implantation energy was 50 nm, compared to 38 nm for 20 keV. Four-point-probe analysis shows similar activation level in both samples; with sheet resistance ranging between 240 Ω/□ ─ 300 Ω/□ for 40 keV and 20 keV implantation energies respectively annealed at laser energy densities between 500 mJ/cm2 ─ 1000 mJ/cm2 with 20-shot number. This finding is important to support Ge in replacing Si for future-generation microelectronics that requires continuous demand of enhanced performance without further down-scaling.

Resume : Tin nanocrystals embedded in silicon are created by the growth of a tin-rich silicon layer via molecular beam epitaxy and subsequent annealing. Previous work has shown the emission of light in photoluminescence (PL) experiments, making these nanocrystals a potential candidate for on-chip integrated silicon optical devices. Capacitance-voltage (CV) measurements are performed on the multilayer structure. The CV-curves are frequency and temperature dependent. From the derived doping profiles, it can be concluded that the silicon-tin layer and the interfaces contain electron trapping defects. The presence of tin nanocrystals increases the defect concentration significantly. The influence of different annealing treatments, which strongly influences the nanocrystal formation, will be discussed.

Resume : In the current trend of devices miniaturization, the control of heat dissipation at the nanoscale turned out to be a major issue. The manipulation of phonons as heat carriers in novel nanostructures is a potential candidate to address this problem. Phonons behaviour in nanostructures can open doors to unexplored physics of both fundamental and technological interest [1]. Semiconductor nanowires (NWs) are an ideal candidate to investigate phonon management since they: i) can be used as a growth template for complex architectures with high degree of freedom on composition/structure; ii) offer the possibility to engineer to a large extent the phonon properties by playing with different phonon scattering mechanisms at different length scales. In this work, micro-Raman was used to investigate the phonon structure in crystal phase and twin superlattice (SL) GaP NWs. Crystal phase SL is obtained by alternating crystal structure (between wurtzite, WZ, and zincblende, ZB) [2]. Twin SL is obtained by alternating ZB structures along [111] direction that are rotated by 60° with respect to each other. Both approaches ensure atomically sharp interfaces and no interface mixing, still providing effective interfaces for phonons. We measured single NWs transferred on different substrates by spatially-resolved Raman spectroscopy. As highlighted by polarization-dependent measurements, the structural quality of these NWs is unprecedented. Most importantly, we observe spectral features of the SL, ascribed to optical phonon confinement [3] as supported by numerical calculations. These results represent the first experimental observation of phonon confinement in superlattices nanowires and are an important step in the investigation of the quantum-mechanical nature of phonons. [1] M. Maldovan, Nature 503, 209 (2013) [2] S. Assali et al., Nano Letters 15, 8062 (2015) [3] M. Cardona et al., Superlattices and Microstructures 5, 27 (1989)

Resume : Indoor fire detectors are widely employed both in private and public buildings addressing an ever growing concern for safety and early assessment of danger and threats. However, commercially available devices are still very expensive and often subject to false alarms. Optical sensors, in particular, are much more reliable but still cannot reach a large market share due to their cost and complexity. Here we propose a novel optical flame sensor for indoor applications based on a visible-blind, near infrared PbS CQD photodetector. PbS CQD photodetectors are cost-effective, highly scalable alternative to devices based on III-V compound semiconductors; moreover, colloidal quantum dots can be easily deposited on a variety of substrates and even integrated with silicon electronics. Visible blindness allows the employment of a single sensor rejecting the ambient illumination as a source of false alarm (at least for indoor operation). Our devices can effectively detect a candle flame at a distance greater than 15m with a field of view of 120°. If equipped with a lens, the sensor can detect the same flame at more than 25m. Such characteristics allow the use of a single photodetector to monitor a 100m2 space.

Resume : The intersubband transition of single dopant confined in AlAs/GaAs core/shell quantum dots embedded in a glace matrix are studied theoretically. The binding energy and the linear and nonlinear optical properties associated to these transitions are determined. Combined effects such as geometrical confinement, the donor position in the structure and the pressure dependence of the physical parameters of the materials has been analyzed. Within the effective mass approximation, the Schrödinger equation has numerically solved by using the Ritz variational approach. The results show that the linear and nonlinear parts of the absorption coefficient and the refractive index associated to the 1s-1p intersubband transition undergo important changes. There are several interesting results to point out such as the shift of the absorption coefficients and refractive index to high values of photon energy.

Resume : Black phosphous (BP), consisting of a weak van der Waals interlayer interaction and strong in-plane bonds, has high carrier mobility and tunable band gap (0.3-2.0 eV), offering properties for electric and optoelectronic devices. Here we report a controllable thinning method by using hydrogen plasma etching to thin down mechanically exfoliated BP flakes. Atomic force microscope, optical microscopy and Raman techniques was used to identify process conditions. Not only the thickness of the BP flakes can be controlled, but also the defects of the exposed BP surface are removed after plasma treatment. It is expected to improve the electrical performance of BP based field-effect transistor (FET). This method provides a new way to fabricate BP-based electronic and optoelectronic devices in the future.

Resume : This work is concerned with the study of structural, optical, thermal and magnetic characteristics of the amorphous bulk As2Se3 glasses doped with Mn (concentration 2 and 5 wt. %). The studied glasses were synthesized by standard melt-quenched technique in evacuated quarts ampoules with subsequent air quenching. Structural studies were carried out using Raman and FTIR spectroscopy and X-ray diffraction. The radial distribution functions of doped and undoped bulk glasses were obtained and analyzed. In Raman spectra, main observed effect under the introduction of dopants was the change of relative concentration of main and non-stoichiometric structural units characteristic for As2Se3 glasses. Influence of transition metals on the optical properties of As2Se3 glass was studied in mid-IR region. Introduction of transitional elements changes magnetic properties of investigated chalcogenide glasses.

Resume : The evolution of the crystal, the microstructural and the optical properties of pulsed-laser deposited TiO2 films, investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy, optical transmittance and m-line spectroscopy measurements are reported. The samples were grown on (0 0 1) SiO2 substrates at temperatures from room temperature to 600 C in oxygen environment. The films grown up to 400 C are amorphous or consist of very fine crystallites. Crystalline films consisting of single anatase or anatase and rutile phases were obtained at temperatures higher than 400 C. Tendency towards columnar-like growth morphology was observed in all samples. All films revealed single- or multimode waveguiding and optically anisotropic properties. Considerable effective birefringence readings were estimated for the amorphous films. The observed optical anisotropy is much more related to the columnar microstructure rather than the crystalline features of the films. High birefringence materials are strongly desirable because of their potential applications in improved optical data storage and data transfer in communications technology.

Resume : Planar structures InSe-ZnSe composite / InSe were obtained by thermal annealing of n- and p-InSe plates in Zn vapors at 770-900 K. The presence of ZnSe crystallites in the composite was determined by X ray diffraction measurements, SEM microscopy and FTIR spectroscopy in the one-phonon region. The size of ZnSe and InSe crystallites from the composite vary from tens of nanometers up to micrometers as function of treatment temperature and duration. The ZnSe-InSe composite layer forming process contributes to creation of both new donor and acceptor levels in the band gap of InSe compound. At the same time the formation of electron trapping levels occurs. The influence of thermal regime and treatment duration on the structures’ photosensitivity is studied. By photovoltaic and photoluminescence spectra measurements was determined that non-equilibrium charge carriers occurs in the InSe layer from the junction’s interface. By photoluminescence and thermally stimulated luminescence studies the recombination and trapping levels’ diagram is determined for the InSe layer from the interface of the structure InSe - composite.

Resume : We have theoretically investigated the 1.55 µm p-type doped GaNAsBi-based Double Quantum Wells (DQWs) using the (16x16) BAC model combined with a self-consistent calculation. We have found that the coupling effect becomes more pronounced by reducing barriers width. The optical performance of the structure is enhanced when the DQWs are coupled and doped. Moreover, a p-i-n heterojunction based on GaNAsBi/GaAs (DQWs) designed for infrared photodetection was developed. The computed gain can reach the value 4.2 〖10〗^4 〖cm〗^(-1). The optimization of well parameters such as the Bismuth composition, the well width and the doping densities give rise to p-i-n heterojunction emitting at the wavelength 1.55 µm. The quantum confined stark effect on the optical properties of studied structures is also discussed.

Resume : Defect-engineered semiconductor nanostructures have been gaining interest in the past few years due to interesting mechanical, thermoelectrical and electric properties. The presence of defects could modify the phonon dispersion relation, thus change the phonon transport, which in turn affects the thermal conductivity of the nanowires (NWs).1,2 Therefore, the knowledge of thermal properties of twinned/polytype NWs is crucial for understanding their performance in future applications such as nanoscale electronics, sensors, photonic devices, solar cell and thermoelectric devices. Among different group 14 semiconductor NWs, germanium (Ge) NWs are of particular interest for high speed nanoelectronic applications due to its low band gap and high carrier mobility.3-5 We successfully introduced a noninvasive optical technique to study the temperature-dependent phonon behaviour of normal (non-defective) and twinned Ge NWs based on micro-Raman spectroscopy. Analysing the Raman spectra measured at different laser power, we derived the laser-induced local temperature rise during the Raman measurements for both the NWs and demonstrated the role of lamellar twinning and polytype phase on the heat transport properties of the NWs.6 We estimated the first-order temperature coefficient and the thermal conductivity of the polytype NWs was found to be lower than the normal NWs6, which could make them ideal candidate for thermoelectric applications. References 1. Murphy, K. F.et al; Nano Lett. 2014, 14, 3785. 2. Abramson, A. R.et al; J. Heat Transfer 2002, 124, 963. 3. Burchhart, T.et al; Nanotechnology 2011, 22, 035201. 4. Garnett, E.et al;. Nano Lett. 2010, 10, 1082−1087. 5. Cor Claeys, E. S. Germanium-Based Technologies from Materials to Devices, 1st ed.; Elsevier: New York, 2007. 6. Majumdar, D. et al; ACS Applied Materials and Interfaces 2015, 7, 24679

Resume : Recently, it has been established that the electronic properties of a graphane (hydrogenated graphene) sheet can be well tuned by foreign atom substitutions. This suggests that incorporating magnetic elements into its semiconducting environment could make graphane a good base for creating low dimensional dilute magnetic semiconductors for spintronic devices, desirable for information storage and processing. Using DFT including the GGA + Hubbard U correction, we investigate the effects on structural, electronic and magnetic properties of graphane upon incorporation of 3d transition metal (Cr, Mn and Fe) adatoms on different adsorption sites. It is found that the high-spin configurations are more favorable for all the considered systems regardless of the adsorption sites. The hydrogen-vacancy substitutional site is found to be the most favorable for the adsorption of Cr and Fe, resulting in half metallic magnetic ground states with supercell magnetic moments of 5 μB and 3 μB respectively. On the contrary, the Mn adatom shows appreciable preference to adsorb on the top of a carbon atom, with large magnetic moment of 5 μB. The observed half metallicity and the rich magnetic properties of these systems are particularly important for efficient spin injection and transport of high spin-polarized currents, desirable in spintronic device applications.

Resume : There has been analyzed theoretically the electronic structure of a coupled quantum dot (QD) – quantum well (QW) tunneling system, which is grown on InP substrate and consists of InAs self-assembled QDs and auxiliary In0.53Ga0.47As QW, separated by a thin lattice-matched AlGaInAs barrier. For the calculations, the specially developed program based on eight-band k·p theory has been used, where all partial differential equation are solved by finite difference method (FDM). The program is able to calculate the confined states of a QW in a 3D geometric numerical box and it can simultaneously handle QDs of even large sizes. Based on the calculated single-particle wave functions(WFs), many-body effects are studied by Hartree-Fock self-consistent field (HF SCF) theory. The HF equations are solved standardly by using variational WF. This allows solving the nonlinear Fock equation by SCF technique for density matrix. Because in the case of the FDM the values of the WFs are known only at the grid points, the Pulay method has been extended for the SCF calculations based only on WFs. Using this approach, the exciton states can be calculated for large symmetric and asymmetric QDs. In addition, the influence of the additional QW (the injector) of different widths on energy levels and WFs is investigated on the structures with different dimensionalities. The difference ininterband oscillator strengths calculated using single particle and excitonic WFs is examined.

Resume : The growth morphology, composition , structure of PbSe, PbTe, SnTe nanostructures grown on GaSe substrates and electrical properties of (III─VI)─(IV─VI) semiconductor heterostructures were studied. Thin (d< 100 nm) layers of the (IV─VI) semiconductor compounds were grown by van der Waals epitaxy from the vapor phase using hot-wall technique on the atomically smooth van der Waals (0001) surfaces of layered crystals GaSe [1]. The growth (IV─VI) quantum dots with a high surface density( of 1011 cm─2 ) was observed on these surfaces in the presence of Ga droplets and in nanocavities having the shape of trihedral pyramids (surface density of 108 cm─2 ) formed in the uppermost (6─7) layers of GaSe substrates at a high supersaturation in the condensation zone. The formation of nanocavities on the van der Waals (0001) GaSe surfaces and their morphology after the thermal annealing of layered crystals in molecular hydrogen atmosphere and vacuum annealing of GaSe substrates in the presence of a force perpendicular to the crystal layers has been studied. It is demonstrated that nucleation and growth of (IV─VI) quantum dots on the flat van der Waals (0001) surface and in nanocavities proceeds via the different (“droplet epitaxy” and” vapor-liquid-solid”) mechanisms. In contrast to conventional semiconductors (Si or GaAs) the extraordinary mechanical properties of 2D materials allows for applying strain of a few percent and strong nanoscale bending. This phenomenon was observed for nanostructures consisting of the layered nanoporous GaSe matrix, (IV─VI) 2D layers and quantum dots formed in nanocavities using the AFM cantilever loading perpendicularly to the vdW surface (0001) of quasi-2D layered crystal GaSe. Large built−in strain gradients in the normal direction causes the interfacial charge redistribution at heterojunction interfaces resulting in flexoelectric effect [2] based on the presence of curved regions in GaSe layers that are fixed between pyramidal ( IV─VI) PbSe nanoparticles . This effect can be applied for controlling the charge transport at the interface in nonplanar heterostructures. [1] Z.R. Kudrynskyi, A.P. Bakhtinov, V.N. Vodopyanov, Z.D. Kovalyuk, et al. Nanotechnology, 2015,26, 465601 [2] J. Zhang,C.Wang, C.Bowen, Nanoscale, 2014, 6, 13314

Resume : Solid state laser lighting, which is combines a laser diode (LD) with a YAG:Ce3+ phosphor have been attracted as next-generation light source due to their remarkable characteristics such as environmental-friendly device, long operating time, low energy-consumption and high brightness, compared to conventional light source. In this study, we prepared YAG:Ce3+ ceramic phosphor plate (CPP) with various amounts of Al2O3. The characteristics were investigated according to the addition amount of Al2O3 and optimized. The luminous characteristics of combination of YAG:Ce3+ and Al2O3 are improved in comparison with YAG:Ce3+ alone. We suggest that CPP is a next generation materials for application in LD lighting.

Resume : High efficiency photovoltaics with ultrathin films are a current matter of interest. Reduced active layer thickness can improve carrier collection efficiencies and diminish fabrication costs if the photon losses are avoided through a light trapping scheme. Our approach here is to shape the germanium active layer into a photonic crystal structure fabricated by a combination of nanoimprinting lithography and Molecular Beam Epitaxy. The fabricated architecture is designed employing FDTD computations aiming at maximum broadband absorption with potential applications in telecommunications, sensing and photovoltaics. We observe enhancements of absorption efficiency up to 70% in the nanostructured semiconductor sample respect to a flat film of Ge over a broad bandwidth [400-1800 nm] measured by VIS-NIR microspectroscopy. We demonstrate how to tune the optical response of the photonic architectures with the geometrical parameters of the architecture. The easy fabrication route by means of soft nanoimprinting lithography allows for seamless integration in many optoelectronic fabrication procedures.

Resume : III-V semiconductor nanostructures have attained large significance for high-performance power and optoelectronic applications, owing to the large electron mobilities, direct nature of the bandgap and large surface-to-volume ratio. In particular, axial III-V nanowire heterostructures enable elastic lattice relaxation in two dimensions without formation of defects and are therefore of interest for realizing devices such as single-electron transistors and single-photon emitters. However, in GaAs/InAs nanopillar heterostructures the lattice misfit of ~7% induces considerable strains in the lattice regions close to the heterointerface, which affect the electronic structure. In the present contribution we analyze the spatial distribution, partitioning and energy of misfit strains in axial-heteroepitaxial GaAs/InAs nanopillars by means of calculations using atomistic molecular statics and analytic approaches within the frame of continuum elasticity theory. It is found that the atomistic simulations predict a faster decay of the strain and strain energy with distance from the GaAs/InAs heterointerface as compared to the continuum approach of Zubia and Hersee [1]. Good agreement is found for the total strain energies obtained by molecular statics and the analytic continuum approach of Glas [2]. [1] D. Zubia, S.D. Hersee: J. Appl. Phys. 85 (1999) 6492 [2] F. Glas: Phys. Rev. B 74 (2006) 121302

Resume : Quality of low cost power generating material is depends on the Nano-mechanisms involved in the deposition Dhananjoy Roy Post Graduate Department of Physics, Barasat Govt. College (Affilated to West Bengal State University) 10 KNC Road, Barasat, 24 PG(N) - 700124, West Bengal, India Email: dhananjoy1964@rediffmail.com I like to present here the results of deposition, characterization and analysis of a series of samples of solar cell grade materials deposited under various conditions. We see that nano-mechanism involved in the process of formation of final product is very precise and selective by the deposition conditions. A small change in concentration of constituent gases, pressure of the deposition chamber, power, substrate temperature trigger different mechanism for the diffusion of atoms or molecules and transfer of energy among their neighbors before settlement. A clear evolution of samples from amorphous to crystalline samples via polycrystalline samples are observed either by changing substrate temperature or by changing dilution of constituent gases from which samples are formed. Substrate temperature and deposition rate play an important role for determining the grain size and structure of the grown samples. Junctions or interfacial can modulate the transport properties of individual materials. So it is necessary to understand the relationship between an interface and its concomitant properties. Characterization of samples and analysis of the process or mechanism involved has been tested through a series of measurements and observations. So nature and character of the samples are governed by the thermodynamical conditions of the depositing chamber. Moreover, different techniques for sandwich thin film production, characterization and interfacial reactions have been discussed in order to understand the kinetic behavior in the above systems. Thus identification of nanomechanism and optimization of deposition conditions are important for obtaining a low cost power generating materials. Keywords: nanomechanics, solar cell materials, transport properties, interfacial reactions

Resume : Lead-based materials are being dominantly used in defence and industrial applications owing to their excellent piezoelectric properties. However, such materials pose serious threat to environment and human life as lead is toxic. Therefore, there is an urgent need to develop eco-friendly lead-free systems with desirable properties. Out of all, Potassium Sodium Niobate (KNN), an alkaline Niobate ferroelectric material, is considered to be one of the most promising candidates as an alternative for lead based materials, due to its moderately high piezoelectric properties as well as a high Curie temperature (above 400˚Celsius) and large electromechanical coupling factors. Despite of having wide applications in sensors and actuators, KNN can also be exploited for opto-electronic devices like light modulators and Electro optic switches. In the present work, KNN thin film was successfully synthesized using Physical deposition technique on epitaxially matched Strontium Titanate (STO) substrate. The structural and optical properties were studied and analysed using X-Ray diffraction (XRD) technique and UV- Visible spectroscopy respectively. The dielectric constant of the film was found to be in accordance to the reported values in literature. KNN thin film was also investigated using Surface Plasmon Resonance technique investigated for the possible application in non-linear optics.

Resume : Chalcogenide glasses belong to chalcogen elements as S, Se and Te. Chalcogenide glasses have good infrared transparency, high refractive index and low optical loss, so the possible applications of these glasses are in optical wave guide, chemical sensors, bio-sensor, photoreceptors in copying machines and X-ray imaging plates, I.R. optical lenses, windows and high sensitivity ionic sensors, optoelectronics, integrated optics, electro-photography solar cells, non-volatile memory cell, electrical and optical memory devices. The phase transformation is necessarily accompanied by thermally activated processes of heating by optical radiation or electrical heating that be influenced by strongly on the temperature evolution through a switching event. It is essential to be aware of the thermal transport behavior of Se-Te-Sn-Ag system as the thermal response of materials controls the data storage capacity, the area of active region and device performance, still there has not been any endeavor. The Transient Plane Source (TPS) technique is one of the most specific devices for studying thermal transport phenomenon. TPS technique uses a resistive heater pattern that is cut from a thin sheet of metal and covered on both sides with thin layers of an insulating material. The TPS element is used as both heat source and temperature sensor. Present work is based on the introducing TPS technique and measured thermal transport properties named as effective thermal conductivity λe, effective thermal diffusivity χe, specific heat per unit volume ρCv, and related parameters as phonon free mean path τ and thermal inertia IT of melt quenched Se-Te-Sn-Ag alloy at room temperature. At the concentration of 2 wt. % of Ag the anomalous deviations of properties from main tendencies were observed. The Philips Thorpe constraint theory explains the percolation threshold due to formation of two dimensional layers structure convoyed by scattering of phonons. Keywords: Transient plane source technique, Chalcogenide materials, Transport properties.

Resume : This paper evaluates the output performance of Sub-Carrier Multiplexing (SCM) based Radio over Fiber (RoF) system based on theoretical analysis for different modulation techniques like Direct and External modulation based on Mach Zehender Modulator (MZM) and Optical Phase Modulator (OPM). Simulation has been carried out using different amplification techniques to understand and compare the performance of the system under the influence of inter-modulation and harmonic distortions. The signal subcarrier spacing affects the system output closely. It is also seen that 2nd & 3rd order inter-modulation distortion terms are almost suppressed by the use of OPM. Optical Phase Modulator along with EDFA greatly improves the system performance considerably by almost 25dB.

Resume : Silicon nanowires or nanowhiskers (NWs) are the unique one-dimensional nanostructures with the high aspect ratio, which are promising for the semiconductor logics, memory, solar energy, sensors and nanoelectromechanic devices applications. For the successful application of silicon NWs, it is necessary to know how to control such parameters as the diameter, height, crystal orientation and concentration of doping impurities in the nanostructures. We report on the experimental investigation of the Si:Au NWs growth by the pulsed laser deposition technique. NWs were produced in the vapor-liquid-solid process with diameter about 200 nm. Volt-ampere characteristics for an individual NWs with p-type conductivity on the n-type substrate were obtained. Also we demonstrate the method of silicon NWs doping by using the two-component catalyst (Cu:Au~1:60). During the NWs growth, copper is distributed inside the nanostructure, while the gold droplet remains on the top of the NWs.

Resume : Chalcogenide glasses belong to chalcogen elements as S, Se and Te. Chalcogenide glasses have good infrared transparency, high refractive index and low optical loss, so the possible applications of these glasses are in optical wave guide, chemical sensors, bio-sensor, photoreceptors in copying machines and X-ray imaging plates, I.R. optical lenses, windows and high sensitivity ionic sensors, optoelectronics, integrated optics, electro-photography solar cells, non-volatile memory cell, electrical and optical memory devices. The phase transformation is necessarily accompanied by thermally activated processes of heating by optical radiation or electrical heating that be influenced by strongly on the temperature evolution through a switching event. It is essential to be aware of the thermal transport behavior of Se-Te-Sn-Ag system as the thermal response of materials controls the data storage capacity, the area of active region and device performance, still there has not been any endeavor. The Transient Plane Source (TPS) technique is one of the most specific devices for studying thermal transport phenomenon. TPS technique uses a resistive heater pattern that is cut from a thin sheet of metal and covered on both sides with thin layers of an insulating material. The TPS element is used as both heat source and temperature sensor. Present work is based on the introducing TPS technique and measured thermal transport properties named as effective thermal conductivity λe, effective thermal diffusivity χe, specific heat per unit volume ρCv, and related parameters as phonon free mean path τ and thermal inertia IT of melt quenched Se-Te-Sn-Ag alloy at room temperature. At the concentration of 2 wt. % of Ag the anomalous deviations of properties from main tendencies were observed. The Philips Thorpe constraint theory explains the percolation threshold due to formation of two dimensional layers structure convoyed by scattering of phonons. Keywords: Transient plane source technique, Chalcogenide materials, Transport properties.

Resume : III-V semiconductor nanopillars have a great potential for optoelectronic applications such as light emitting devices because of the direct nature of the bandgap and a large surface-to-volume ratio. Recently, it has been reported that the photoluminescence (PL) energy of the GaAs core of GaAs-Al0.33Ga0.67As core-shell nanowire arrays red-shifts with increasing thickness of the heteroepitaxial shell, due to misfit strains [1]. Also, the PL intensity increases with rising nanowire areal density [1]. In the present contribution we analyze GaAs-In0.2Ga0.8As core-shell nanopillar arrays with focus on the correlation between the PL intensity on the one hand and the pillar size and areal density on the other hand. For the fabrication we used low-cost nanosphere lithography and reactive ion etching to obtain GaAs pillar arrays on GaAs substrates, followed by molecular beam epitaxial overgrowth of the shell layer. SEM image analysis reveals that the shell layers on nanopillars have triangular cross-sections and exhibit well-defined low-index facets on the radial surfaces, where the in-plane edge lengths of the facets vary systematically with the crystal orientation of the GaAs pillar sidewalls. Except for the pillar top region the shell layers show good structural quality as confirmed by TEM imaging. [1] P. Prete, I. Miccoli, N. Lovergine: Proc. of SPIE 9174 (2014) 91740P

Resume : III-V semiconductors are widely used to produce high performance optoelectronic devices operating in the technologically important mid-infrared spectral range. The GaSb and InAs substrates which these materials are typically grown on are expensive, small in size and have suboptimal physical properties such as low thermal conductivity. Integration of III-Vs onto silicon substrates offers the opportunity to overcome these shortcomings and opens the possibility of lab-on-chip MIR photonic integrated circuits. However, the unusual III-V/Si interface presents challenges to epitaxial growth. We report on the techniques employed to grow high quality GaSb optoelectronic devices on silicon using molecular beam epitaxy. Silicon substrates were prepared using an in-situ thermal oxide desorption technique. A ~5 nm AlSb nucleation layer followed by a low temperature GaSb buffer layer help to relieve the large lattice mismatch, improving the quality of the overlying epilayers. GaSb p-i-n devices were grown on top of this buffer layer and then processed into mesa etched LED and photodetectors. X-ray diffraction measurements revealed that the GaSb is fully relaxed and of good crystal quality with a narrow 250 arcsec peak. The devices demonstrated excellent performance, with room temperature electroluminescence and spectral response at 1700 nm. Growth of GaSb based quantum wells and InAsSb/InAs superlattices deposited on the GaSb/Si buffer layer will also be discussed.

Resume : Stochiometric ZnO thin films were deposited on n-Si(100) and Si(100)/SiO2 substrates at 200 oC by RF-magnetron sputtering of ZnO powder target in Ar/CH4 working gas. The effect of carbon incorporation on optical, electrical and photovoltaic properties of the ZnO thin films and n-Si/n+-ZnO heterojunction have been studied. Structural and optical properties of ZnO:C films were analyzed by SEM/EDS, FTIR, Raman scattering, photoluminescence and ellipsometry. It is demonstrated that carbon incorporation results in drastic change of charge transport mechanism in ZnO films: sheet resistance in ZnO:C films decreased four orders of magnitude up to 90 Ω/sq and appeared to be independent on the temperature in range of 150-300 K while undoped ZnO films exhibited classical semiconductor temperature behavior. It was shown that photoresponce of n-Si/n+-ZnO:C heterojunction as well as photo-induced open circuit voltage increases with increase of carbon incorporation. The value of Internal Quantum Yield in short-wavelength region was about 30 - 40% which indicates acceptable value of the velocity of surface recombination and hence of a high quality of the n-Si/ZnO interface. The I-V characteristics of the n-Si/ZnO heterostructures show a prominent photoresponse under illumination by light of incandescent bulb at reverse applied. Observed properties of n-Si/n+-ZnO:C heterostructures and effect of carbon incorporation in zinc oxide films will be discussed.

Resume : The growth of MoS2 layers of desired dimensions and thicknesses at predefined locations is essential for fabricating opto-electronic devices based solely on MoS2 or on hetero-structures based on MoS2. Here, a novel route for patterned growth of 2D MoS2 arrays by combining radio frequency (RF) magnetron sputtering, stencil mask lithography and vapour phase sulfurization is presented. The present method is further extended to grow MoS2 (2D)/WS2 heterojunctions arrays. Resulting heterojunctions have been characterised by Raman, Kelvin probe force microscopy (KPFM), conducting atomic force microscopy (CAFM) and spectroscopic ellipsometry (SE) techniques. Surface potential analysis using KPFM across individual MoS2 patterns indicates the Fermi level at 4.6 eV with n-type nature, whereas similar analysis on uncovered WS2 portion reveals that the Fermi level is located in the middle of band gap i. e. intrinsic nature of WS2. CAFM is employed to obtain localised I-V curves at random locations in the patterned arrays of MoS2/WS2 hetero-junctions. Rectifying I-V behaviour with a good point to point uniformity is observed. Optical constants of the heterojunction are obtained by SE technique. In WS2/MoS2 junction, interference fringes in the amplitude ratio (ψ) of p- and s- polarized component of reflected light almost dies at 1.8 eV indicating material absorbs all light above this energy. By carrying out KPFM under white light exposure a surface photo voltage of ~50 mV is observed in the junction which indicates efficient charge generation and separation under illumination. This junction is also characterized macroscopically by carrying out I-V characteristics under dark and light condition and is found to show a VOC of 50 mV consistent with the KPFM measurements.

Resume : Bipolar resistive switching behaviors of Pt/Nb-doped SrTiO3 (Nb:SrTiO3) junctions were studied. Pt/Nb:SrTiO3 junctions have shown clear multi-level resistance states with good endurance up to 103 cycles. Temperature dependence of resistance reveals insulating behavior at positive bias branch, on the contrary, metallic behavior at negative bias branch. Interestingly, after drastic current change during applying positive bias, clear negative differential resistance (NDR) phenomena are observed at negative bias. The NDR phenomena diminish as temperature increases and finally disappear at about 250 K. To find the origin of such NDR, electrical and optical measurements have been conducted.

Resume : We developed new architectures for photodetectors based on heterostructures n-Si/p-nanostructured semiconductors aiming at improving the resposivity in UV and extending the bandwidth in SWIR. We started from a Si Schottky detector with Ti/Au digitated electrodes. On top of this device we deposited the p-type semiconductor using the layer-by layer method. PbS quantum dots with short ligands and a reduced graphene oxide (RGO) nanocomposite with poly (3- hexylthiophene) (P3HT) were used as p-type semiconductor. Intermediate layers (self assembled monolayer and/or thin dielectric films) were introduced to tune the electrodes workfunction and to control the charge transfer and separation. Thus we were able to improve the resposivity, and reduce the dark current. The complex structure includes several types of components: Au/Si Schottky or MSM photodetectors, n-Si/p-nanocompozite heterostructures, and photoconductors. The lay-out allows different polarization schemes and combinations of these components, offering the possibility to improve the photocarriers collection, to tune the gain and the spectral responsivity of the whole system. The paper will present the fabrication process, the operation principles and the characterization results of these photodetectors with complex architecture. The RGO-P3HT/n-Si based photodetectors show higher responsivities in UV-Vis-IR range than Si-based photodiodes (~5-10 times), and an extended wavelength range in SWIR (responsivities of 30-50 mA/W). The devices based on n-Si and 5.5 nm diameter- PbS QDs) can achieve either very high responsivities in SWIR, up to 100 A/W, or a flat response over the entire bandwidth from UV to SVIR, function on the polarization scheme.

Resume : High sensitivity photodetectors based on PbS colloidal quantum dots (CQD) have been demonstrated by several research groups in the last years with performance similar to commercial III-V semiconductor devices. Nevertheless, investigation of the noise performance of such new photodetectors is still lacking. Complex transport phenomena in the CQD film, like variable range hopping and tunneling through dielectric barriers, could be responsible for significant 1/f noise thus hindering the employment of such devices in low noise applications. Many authors, conversely, assumed that shot noise is the dominating factor, thus neglecting 1/f noise and overestimating devices’ performance. Here we present an analysis of the noise power spectra of PbS CQD photoconductors fabricated with different deposition techniques. The devices have been characterized focusing on the low frequency regime (<100kHz) investigating the noise dependence on the light intensity and voltage bias and fabrication parameters such as deposition technique (drop-casting, spin coating) and oxidation process.

Resume : Nanostructured GeSi is a versatile material as its properties are easily manipulated by tuning composition and size [1,2]. In this work, we report on the photosensitivity of GeSi nanocrystals (NCs) /TiO2 multilayers. TiO2/(GeSi/TiO2)2 layers sequence was deposited by magnetron sputtering on oxidized Si wafers and then nanostructured by rapid thermal annealing - RTA (600–1000 oC). The thickness of GeSi and TiO2 layers was varied. XRD, SEM, TEM and Raman measurements for investigating structure and morphology and spectral distributions of photocurrent for studying films photoresponse were performed. We find that RTA at 750 - 850 oC leads to formation of both cubic GeSi NCs (60:40 composition) and rutile TiO2 NCs. HRTEM investigations evidence a variation of GeSi NCs size with their position in the 800 oC RTA multilayer structures, i.e. from 5–6 nm near the Si substrate to ~10 nm near the free surface. The photocurrent spectral distribution shows two maxima at ~ 985 nm and ~ 700 nm, and the edge from high wavelengths shifts toward higher energies when the measurement temperature decreases. References: [1] Nanotechnology 26, 375701 (2015); [2] Phys. Status Solidi B 251, 1340 (2014)

Resume : ZnO nanotube (NT) structures have attracted significant attention from both researchers and practitioners due to their higher aspect ratio and the fact that oxides have much better output efficiencies compared to other nanostructures. In this work, we successfully fabricated n-type ZnO NTs of high structural, optical, and electrical quality on p-type Si (001) substrate by pulsed laser deposition, which has, according to our knowledge, never been accomplished before. The scanning electron microscopy image shows hexagonal vertical ZnO NT array (grown along c-axis) of average ~ 650 nm length and wall thickness ~ 11-20 nm, obtained at 100 mTorr pressure and 650 oC growth temperature. Transmission electron microscopy confirms that the hollow of the NT starts from the bottom of the NT near the interface. We investigated the optical properties of these NTs by means of temperature dependence micro-photoluminescence (μPL) and time resolved spectroscopy using femtosecond Ti:sapphire laser attached to a streak camera with a repetition rate of 76 MHz and a frequency tripled wavelength (266 nm). Room temperature μPL measurements show a strong and sharp bandedge peak at 378.8 nm with a very weak defect band, indicating high optical quality. The radiative carrier lifetime increases linearly with temperature, allowing us to conclude that we accomplished one-dimensional exciton confinement. A photodetector based on these ZnO NTs/p-Si was fabricated. We showed current-voltage characteristics under dark and illuminated conditions. A 6-month illumination test confirms that our photodetector exhibits high stability and response. The detectivity and responsivity of our ZnO NT-based photodetector clearly demonstrate high performance UV photoelectric detection device.

Resume : Nowadays, one-dimensional (1D) semiconducting nanocrystals (NCs) have unique physical properties, such as small length scale, excellent flexibility, enhanced light absorption, and higher thermoelectric coefficient crystal size dependent electronic structure, which lead to tunable electrical or optical properties. In this group of materials, ZnSe:Ag NCs are one of the promising materials for manufacture of light-emitting devices or solar cells. ZnSe:Ag NCs were successfully synthesized by hot-injection method using octadecylamine (ODA) as coordinating solvent. Zinc stearate, selenourea and silver nitrate as precursors of zinc, selenium and silver were used for synthesis of nanocrystals. ZnSe:Ag NCs have strong luminescence in visible light range with maximum about 440 nm and 580 nm. Nanowires are characterized by hexagonal structure and have diameter of about 5-8 nm and 100-300 nm of length. The amino groups of ODA and zinc on the surface of nanocrystals form covalent bonds, which enables effective exchange of ligands. ODA was replaced by organic semiconductor molecules, such as derivatives of aminonaphthalenes. As a result, nanocrystals are characterized by an increased ability to absorb light in the visible spectrum by the contents of ZnSe:Ag nanocrystals additional absorbing compounds on the surface. This work was financially supported by National Centre for Research and Development under Lider Program, contract no. LIDER/009/185/L-5/13/NCBR/2014.

Resume : The nitrogen-vacancy (NV) center in diamond has garnered considerable attention as a platform for quantum information and quantum sensing applications. Nevertheless, several outstanding challenges still need to be addressed in order to implement NV center based functional devices. For instance, the entanglement of NV centers is based on their reciprocal dipolar interaction, requiring defects be located within few nanometers of each other. Not only is precisely positioning the qubits at the nanoscale level demanding, but their proximity to each other also complicates the ability to separately address them. Similarly, sensing applications rely on the detection of the dipolar magnetic fields generated by target spins external to the diamond lattice, calling for the NV center sensors to be within a few nanometers of the surface where their coherence properties considerably degrade. Here, we use a ferromagnetic film as a quantum bus to mediate the interactions between a microwave (MW) source and the NV centers, thereby relaxing the distance requirements. In particular, we show that surface spin-waves (SW) can amplify a MW signal detected by the NV centers by more than two orders of magnitude and establish that the SW-NV center interaction allows for the remote coherent control of the NV centers. These results demonstrate the potential of hybrid ferromagnet-ND systems for the practical implementation of interconnected quantum networks and nanoscale sensors.