New materials for photonics

Resume : The Self-Assembled Monolayers (SAMs) are well recognised for their application as the system for nanofabrication of modern photonic devices, not excluding production of graphene. In particular, the alkanethiol SAMs spontaneously deposited from the alcoholic solution on GaAs and InP surface can be used for bio- and chemical sensing, molecular electronics and passivation of the surface. Generally, passivation is the most effective method to improve the performance of devices, especially the third generation of infrared detectors based on type-II InAs/GaSb superlattice (T2SL). Various concepts of passivation have been reported in the literature, including the deposition of the dielectric films, polyimide capping, the overgrowth of a wide-bandgap semiconductor layer and the modification of the atomic structure. The spontaneously deposited thiol SAMs may nondestructively reduce the superficial native oxide and efficiently protect the surface as both chemical and electrical sophisticated passivation system. In this work, the novel two-step passivation with the octadecanethiol-based wet treatment and followed by SiO2 deposition was proposed for T2SL detector. In order to understand the mechanism of passivation, the GaSb and InAs surfaces covered with alkane- and aromatic thiols were characterized by the AFM, Raman spectroscopy and contact angle measurement. This novel conception of passivation allowed for the reduction of the dark current by one order of magnitude for T2SL detector.

Resume : Transition metal nitrides (TMNs) have recently attracted a great deal of interest due to their promising plasmonic behavior, robust physicochemical performance and easy integration into already established technologies. This work provides a discussion on an alternative plasmonic TMNs material, in particular, titanium nitride (TiN) and its extensive sensing capabilities for disease diagnosis and airborne pollutants assessment. It is known that TiN exhibits similar but broader plasmonic absorption compared to gold. However, the broadband absorption feature of TiN impedes the plasmonic sensing application owning to its difficulty in monitoring the plasmonic angular or wavelength response. By using the interferometric phase modulation method, the broadband plasmonic TiN nanofilm (30 nm in thickness) synthesized by reactive magnetron sputtering can be directly applied as a high-performance plasmonic sensing matrix. The computational and experimental results demonstrated that TiN exhibited large phase increase within the visible region, and the phase modulated TiN sensors showed good sensitivity and tunability from 520nm to 750nm, which proved TiN to be a promising refractory plasmonic material that could replace and even outperform conventional plasmonic materials. Furthermore, TiN nanofilms with broadband tunability can be functionalized to achieve highly sensitive biosensing in both aqueous and gaseous media. Thanks to its wide tunability, TiN nanofilm, as a new plasmonic material can be applied directly in the fields of tumor exosomes detection, disease biomarker identification and airborne pollutants quantification.

Resume : The state-of-the-art optical detectors are based on either bolometers or quantum photodetectors (i.e. photovoltaic and photoconductive detectors). Bolometers are uncooled thermal detectors with lower cost. They are commonly resistive bolometers based on temperature-dependent resistors or thermoelectric bolometers based on thermocouples or thermopiles. Quantum infrared (IR) photodetectors have high performance, but they are expensive and require cooling. Exotic and toxic materials (e.g. HgCdTe) are needed at longer wavelengths. Compared to the cooled detectors, bolometers are much slower and less sensitive. Recently we have demonstrated that both ultra-thin single-crystalline and poly-crystalline silicon membranes can be employed in fast and sensitive electro-thermal detectors [A. Varpula et al., Appl. Phys. Lett. 110, 262101 (2017) & Proceedings 2 (13), 894 (2018)]. The speed of these devices stems from the small thermal mass of the nanomembranes, while their sensitivity is based the remarkable reduction of the thermal conductivity of the Si nanomembrane with the decreasing membrane thickness [S. Neogi et al., ACS Nano 9, 3820 (2015)]. While those devices were not capable of optical detection, here we present a full high-speed infrared optical detector. Experimental optical characterization results, factors determining the detector performance, improvement prospects, and detector applications are discussed.

Resume : Light trapping and the broadband absorption of the solar radiation is of interest to various applications related to absorption-based photonic devices. During the last couple of years we developed a novel paradigm for light trapping and broadband absorption of the solar radiation using arrays of subwavelength non-imaging light concentrators (NLC)1–34. We numerically show that silicon NLC arrays provide >75% broadband absorption enhancement of the solar radiation compared with that of optimized nanopillar arrays. Our calculations show that NLC arrays function as anti-transmission layers in contrast to nanopillar arrays that function as anti-reflection layers. We show that the absorption enhancement in NLC arrays is due to efficient occupation of Mie modes which is motivated by the unique NLC geometry, and we demonstrate light trapping at the Yablonovitch limit. We examine the performance of a photovoltaic cell based on NLC, and we show that the short-circuit current density of the NLC-based cell is >75% higher than the short-circuit current density of a photovoltaic cell based on optimized nanopillar arrays. Finally, we present a recent near-field optical microscopy examination of NLC arrays. References 1. Prajapati, A., et al. Nano Energy 54, 447–452 (2018). 2. Marko, G., et al. Nano Energy 61, 275–283 (2019). 3. Konedana, S. S. P., et al. Nano Energy 59, 321–326 (2019). 4. Prajapati, A., et a. Nanoscale 11, 3681–3688 (2019).

Resume : Achieving high-power and high-beam-quality (namely, high-brightness) semiconductor lasers is important for applications including smart mobility (LiDAR) and smart production. Here, I will show that photonic-crystal lasers can exhibit brightnesses of over 300MWcm-2sr-1. I will also show that mechanical-free, two-dimensional beam scanning can be achieved by introducing modulated photonic crystal structures.

Resume : Last years’ research on silicon photonics has been focused on the engineering of new optical materials for hybrid photonic platforms, aiming at more power efficient systems with additional functionalities. In this regard, functional oxides are perfect candidates to implement active elements in complex systems [1]. Transparent oxides within a wavelength span from the ultra-violet (UV) to the mid-infrared (mid-IR) wavelengths are targeted for hybrid integration. Furthermore, these functional oxides often have refractive index values suitable for photonic applications. In the nanophotonics field, yttria-stabilized zirconia (YSZ) remains an understudied material for integrated optics despite its refractive index around 2.15, and promising optical properties as it belongs to the popular and interesting mid-index contrast material family (MiDex) [2]. Another MiDex material, SiNx is largely employed due to a number of interesting characteristics such as: Low propagation loss, absence of two photon absorption (TPA), large transparency window from the ultraviolet to the mid-infrared, and good Kerr effect [3-5]. We recently demonstrated around 2 dB/cm propagation losses in waveguides etched on YSZ at a wavelength of 1380 nm [6]. Based on these promising preliminary results, we hereby explore the doping of Er3+ ions in the YSZ matrix as active elements with an emission wavelength in correspondence with C-band of telecommunication window (=1530 nm) and in the visible range by means of up-conversion. In our study, we have designed low-loss SiNx waveguides on a silicon-based platform and experimentally grown an active cladding by pulsed layer deposition (PLD). We prove guided emission of erbium ions within the visible and near-IR wavelengths. These results pave the way towards the implementation of light amplification functionalities by means of earth-doped oxides into hybrid photonic platforms. [1]. Lorenz, M., et al. Journal of Physics D: Applied Physics, 49(43), 433001 (2016). [2]. Kimberling, L.C. et al. “Monolithic Silicon Microphotonics”, in Silicon Photonics edited by L. Pavesi, D.J. Lockwood (Springer Berlin Heidelberg New York, 2004), pp. 89-119. [3]. Castillo-Vega, G. et al. Opt. Mater. Express, 2, 1416-1424, (2012) [4]. Yamane, H. et al. Journal of Crystal Growth, 94(4),880-884, (1989). [5]. Nakamura, R. et al. Review of Scientific Instruments, 75(3), 636-644, (2004). [6]. Marcaud, G., et al. Phys. Rev. Materials 2, 035202 (2018).

Resume : Quantum dot light-emitting diodes (QD-LEDs) are an attractive alternative to organic light-emitting diodes (OLEDs) that have attracted attention for next generation optoelectronic devices due to their high colour-saturated photoluminescence, processability, and stability. Existing QD-LEDs often employ toxic or rare metals (e.g., Cd, Pb, In, etc.). Although colloidal silicon nanocrystals (SiNCs) are an attractive alternative because of their abundance, biocompatibility and tailorable surface chemistry, they exhibit comparatively wide bandwidth photoluminescence that is not readily narrowed using size selection methods. One alternative approach is to prepare optical cavities that preferentially select specific emission wavelengths. This poster will outline our investigation into SiNC-polymer hybrid-based structures that offer tunable narrow photoluminescence.

Resume : In this contribution, we discuss the luminescence enhancement phenomena that were observed in photonic crystal (PhC) slabs and cavities fabricated on Si structures with the self-assembled Ge(Si) nanoislands. The luminescence response of slabs and cavities with the photonic crystal parameters varied over a wide range of values was analyzed. The strong (by more than two orders of magnitude) enhancement of the photoluminescence (PL) response related with Ge(Si) nanoislands was observed both in PhC slabs and PhC cavities though for different parameters of photonic crystals. We show that manifestation of phenomena related with Ge(Si) nanoislands interactions with the photonic crystal modes and the modes of PhC cavities depends directly on PhC parameters. So, the strong PL enhancement at room temperature was observed for the cavities realized in PhCs, periods of which were varied between 350 and 500 nm. In the PhCs with the larger periods, Ge(Si) nanoislands interactions with photonic crystal modes dominate. In this latter case, we emphasize two different types of interactions, namely: i) the interactions with the radiative modes of PhC slabs, and ii) the interactions with the so-called bound states in the continuum (BIC). The modes related with the bound states in the continuum are characterized by their extremely high Q values reaching 5000. The experimental data are discussed together with the results of theoretical simulatons carried out for these structures. The work was supported by the Russian Foundation for Basic Research (grant #18-42-520047_r-a) and the Russian Academy of Sciences.

Resume : Silver (Ag) nanoplates show strong nonlinear optical properties with ultrafast response, needed in a variety of devices [1]. They are chemically produced in liquid phase and do not require any purification, significantly simplifying device processing [2]. Here, we report the linear and non-linear optical characterization of Ag nanoplates synthesized by reduction of AgNO3 by NaBH4 and further modified by hydrazine, a process that allows tuning plasmon resonances across the visible and near IR [2]. Their nonlinear optical absorption decreases for increasing incident laser power, with the characteristic behavior of saturable absorbers. The nanoplates are then integrated by laser-induced deposition in a fiber laser cavity, to generate ultrafast pulses. We obtain Q-switched pulses at 1033 nm, with durations of ~2.2 μs and output power of 3.49 mW. These room temperature, solution-processed nanoplates are a viable alternative to saturable absorbers from the visible to the IR. [1] R. W. Boyd, Nonlinear Optics (Academic Press, 2008) [2] G. Compagnini, et al., J. Nanomater. 7084731 (2019)

Resume : Dewetting is a ubiquitous phenomenon in nature: many different thin films of organic and inorganic substances share this shape instability driven by surface tension and mass transport. This spontaneous phenomenon leads a thin film to break and drip in isolated islands. Here, I will address two distinct cases of solid-state dewetting: 1) templated dewetting of silicon and 2) spontaneous dewetting of silicon-germanium. Templated solid-state dewetting can be used to frame complex nanoarchitectures, nanowires (up to 0.75 mm long) and connected circuits of monocrystalline silicon on insulator with unprecedented precision and reproducibility over large scales [1]. Phase-field simulations quantitatively benchmark the experimental results revealing the dominant role of surface diffusion as a driving force for dewetting and the role of faceting in stabilizing the nanostructures. I will discuss the use of these ordered structures as dielectric Mie resonators for visible and NIR light manipulation [2,3]. Spontaneous dewetting of thick SiGe layers leads to the onset of spinodallike structures as accounted for by the features of Minkowski-functionals and evolution of Betti numbers [4]. The formation of these disordered structures is interpreted in the framework of the Cahn-Hilliard-Cook theory of phase separation in analogy with spinodal dewetting of polymers and liquid-metals. I will discuss the possibility to exploit this bottom-up, self-assembly method to form hyper-uniform, dielectric metasurfaces at visible and near-infrared frequencies [5] over ultra-large scales. References [1] M. Naffouti et al., Sci. Adv. 3 : eaao1472 (2017) [2] M. Abbarchi et al. ACS nano 8, 11181 (2014) [3] T. Wood et al. ACS photonics 4, 873 (2017). [4] S. Herminghaus et al. Science 282, 916 (1998) [5] Z. Ma et al., Journal of Applied Physics 121, 244904 (2017)

Resume : GeSn / GeSi multi quantum wells with high Sn content, direct bandgap GeSn wells and GeSi barriers might be of use in optoelectronics. As GeSn has to be grown in the 300°C–375°C range (to avoid Sn segregation/precipitation), GeSi barriers should also be grown in that T range. GeSi might also be used to passivate, at low Temperatures, the surface of Ge-rich imagers and photo-detectors. Thanks to X-Ray Reflectivity (thickness) and X-Ray Diffraction (Si content), I have therefore explored, in a 200mm CVD tool, the 100 Torr growth of tens of nm thick GeSi layers on Ge(001) buffers. Precursor gases were Ge2H6 (Ge), SiH2Cl2 and/or Si2H6 (Si). Whatever the chemistry, a temperature increase in the 350°C-500°C range resulted in an increase, (a stabilization) then a decrease of the GeSi Growth Rate (GR), together with an almost linear increase of the Si content. For a given Si flow, GR (Si contents) were definitely higher (lower) with Ge2H6 and SiH2Cl2 than with Ge2H6 and Si2H6, however. Adding SiH2Cl2 to Si2H6 had, quite surprisingly, almost no impact on the GeSi growth kinetics, which was mainly governed by the Si2H6 flow. With the proper precursor flows, I succeeded in having (i) with Ge2H6 and SiH2Cl2, GeSi GR increasing from 2 up to 32 nm min.-1 as T increased from 325°C up to 375°C, with an almost steady Si content (close to 7.5%) (ii) GR increasing from 0.6 up to 19 nm min.-1 and Si contents decreasing from 24 down to 22% over the same T range with Ge2H6, SiH2Cl2 and Si2H6.

Resume : The search for efficient group-IV-based light emitters that preserve their optical properties well above room-temperature (RT) and that can be fabricated on Si is ongoing. For integration on Si-platforms, electrical charge carrier injection and convenient coupling to other Si photonic components are clearly essential. Epitaxial quantum dots (QDs) offer benefits of quantum confinement and all-crystalline matrix material, important for doping of e.g. LEDs. We introduce a new class of group-IV light-sources consisting of epitaxial Ge/Si QDs into which intentionally extended point defects are introduced upon low-energy ion-implantation. For such defect-enhanced quantum dots (DEQDs) the charge carriers are confined inside the QD by two interlaced zero-dimensional structures. Holes experience quantum confinement due to the large bandoffsets between Si and Ge, while electrons are trapped by introduced defects for which the split-[110] self-interstitial was found to be a configuration with minimum formation energy. Prominent features of DEQDs are clear signs for optically-pumped lasing, absence of thermal quenching of the light-emission up to RT, and the demonstration of LEDs, grown directly on Si without that need of thick buffer layers that exhibit at 100°C still ~30% of their maximum emission. Nevertheless, in view of the intriguing but poorly explored DEQD-properties and the perspectives of using them for Si-photonics, we discuss strategies towards (1) improvement of their light-emission efficiency, (2) understanding the microstructural nature of DEQDs upon their formation, robustness, optical properties, strain, etc. and (3) fostering the potential of DEQDs concerning devices for Si-based optoelectronics and beyond.

Resume : Zinc oxide (ZnO) has become interesting due to its wide band-gap of 3.3 eV and high exciton binding energy ~60 meV which makes it applicable in optoelectronic devices. Since it is a n-type semiconductor, p-type doping seemed to be quite a challenge but due to the constant development of technique and increasing control of the parameters of growth high-quality p-ZnO appears to be more achievable than ever. One of the groups of potential candidates for p-type dopants in ZnO are group V elements. The presentation includes the results obtained for ZnO samples with varying concentration of Sb dopant and grown on Al2O3 with the use of Molecular Beam Epitaxy technique. Investigations concern the measurement techniques such as Secondary Ion Mass Spectrometry, X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction, and Raman spectroscopy that provided the information on the properties of the doped layers. The XPS spectra revealed the presence of three chemical states of Sb: one dominant correlated to Sb3+ and a small amount of Sb5+ as well as Sb3-. Raman scattering measurements, beside numerous sapphire, and ZnO modes, revealed three additional modes ~ 509, 532 and 575 cm-1 associated with the presence of Sb and with intensities increasing with Sb doping. Moreover the strain in the material was also studied. The research was supported by the NCN project DEC-2017/27/N/ST7/02022, and by the statutory grant of Wroclaw University of Science and Technology, 0401/0009/17 and 0402/0023/18.

Resume : Artificial neural networks lie at the heart of many solutions to tackle the huge amounts of data in deep learning applications, autonomous driving or speech recognition and are one of the key enabling technologies of this century. To date most of the implementations of brain-inspired computing are software-based and therefore suffer from the limitations of conventional computer architectures, especially the separation of memory and processor. A promising alternative to achieving fast and efficient computation of neuromorphic tasks is building direct hardware mimics of neurons and synapses that can be connected to larger neural networks and operate – in contrast to conventional computers – highly parallel and more analogous to brains. Here we present the implementation of an artificial neural network on the integrated photonics platform with phase-change materials capable of basic pattern recognition tasks that can be trained in a supervised as well as in an unsupervised way. By working entirely in the optical domain and exploiting wavelength division multiplexing techniques, such systems are promising for direct processing of visual data without electro-optic conversions accessing the high bandwidth and speed inherent to optical systems.

Resume : The metal-insulator transition (MIT) in transition metal oxides, and in particular in vanadium dioxide (VO2), has been the subject of numerous research works due to the potential for novel applications such as memory devices and field effect transistors. In VO2, the five orders of magnitude resistance drop at the MIT temperature (TMIT ~ 70 C) is accompanied by a large change in refractive index. This, together with the fact that the transition can be triggered by different external knobs (temperature, electric field, light, …) make this material very interesting for creating a new generation of photonic devices. In this talk, I will present the capabilities of using the MIT in VO2 in different types of photonic devices demonstrated so far.

Resume : A considerable amount of research has been dedicated to the development of three-dimensional (3D) display technologies and holographic display has been thought an ideal technology to reconstruct the wavefront of 3D objects without eye fatigue. [1] In digital holography, spatial light modulator (SLM) is necessary and pixel pitch of the SLM have to be reduced to enhance viewing angle of holographic image. [2] Although the LCoS has been leading the SLM technology, reducing the pixel size of the LCoS SLMs are limited due to crosstalk of liquid crystals. Therefore, we propose phase-change device to modulate light, which can be reduced sub micrometer without crosstalk between nearby pixels. Ge2Sb2Te5 (GST) is well-known phase-change material and widely used in optical recording and phase change memory. [3] It provide remarkable changes in both optical and electrical properties when switched between amorphous and crystalline phase. In this study, we fabricated 1.5 ?m pixel pitch 8K×2K array of GST device with a structure of ITO/GST/ITO/Oxide/Reflector. Computer-generated hologram (CGH) patterns are shaped by changing the GST phase selectively, which was achieved by irradiating a XeCl excimer laser pulse. Several irradiation conditions of laser were tested and optimum condition that the void or phase separation is not generated GST was applied. We define the location of the CGH pattern as z = 0 cm. The CGH image is designed to reconstruct a checkboard image at z = 25 cm, and letters ????? at z=30 cm. Hologram image from the pattern was successfully reconstructed as shown in Fig. 2 Future, we will fabricate 1.5 ?m pixel pitch 15K×2K array of GST device with transistor and reconstruct moving 3D image through electric operations. Acknowledgement This work was supported by 'The Cross-Ministry Giga KOREA Project' grant from the Ministry of Science, ICT and Future Planning, Korea. References 1. J. Hong, Y. Kim, H.-J. Choi, J. Hahn, J.-H Park, H. Kim, S.-W Min, N. Chen, B. Lee, Appl. Opt. 50, H87 (2011). 2. P. S. Chiu, C. -H. Chen, and W. W.-C. Chiang, Eurodisplay 2015 S7.3 (2015) 3. M. H. R. Lankhorst, B. W. S. M. M. Ketelaars and R. A. M. Wolters, Nat. Mater. 4, 347 (2005)

Resume : The unique properties of AlN and GaN including their solid solutions AlxGa1-xN, if combined in the convenient form of sintered pellets, are expected to yield promising materials for many optoelectronic and high power/frequency applications. In this work, presented is a study on the mechanical and structural properties of nanoceramics made by no-additive high temperature and high pressure sintering in such a bimetallic nitride system. The composite nitride nanopowders were synthesized in-situ by utilizing transamination/deamination ammonolysis chemistry of the mixed Al/Ga tris(dimethyl)amide system. The individual metal amides were mixed, Al/Ga=1/1, and stirred for 3 h under reflux in a hexane solution. Subsequently, the volatiles were evacuated and the mixture was reacted with excess liquid ammonia. Upon volatiles removal, the resulting solid metal amide/imide precursor was nitrided at 800 and 950 °C towards the Al/Ga nitride nanopowders. In the final step, the nanopowders were sintered at 650-1000 °C, 7.7 GPa, 3 minutes. The pellets were investigated using the Vickers hardness test Hv, XRD, 27Al/71Ga MAS NMR, and SEM/EDX. The final materials were the mixtures of the nanocrystalline hexagonal nitrides AlN/GaN/AlxGa1-xN. The amounts of the component AlxGa1-xN were significant and depended on processing conditions. The nanoceramics had the favorable Vickers hardness values in the range 7-12 GPa. Acknowledgement. The study was supported by Polish NCN Grant No. 2017/25/B/ST5/01032.

Resume : Ca3Mg3(PO4)4:Eu2+,Eu3+ phosphor was prepared by a sol-gel/Pechini method. The co-existence of Eu2+ and Eu3+ in the same matrix provided multichannel emissions resulting in a broad blue emission band and a few narrow red emission bands in the photoluminescence spectrum that ensued in the color tuning of its overall luminescence emission with changing temperature. With varying temperature, the as-prepared phosphor exhibited a variation in the overall color emission. The synergistic emissions of Eu2+ and Eu3+ produced color tuning from blue to red in the temperature range of 30 to 250 oC. This feature was found to be suitable for non-contact temperature-sensing. The relative and absolute temperature sensitivity for this phosphor is superior to most of the previously reported optical thermometric phosphors. Ca3Mg3(PO4)4:Eu2+,Eu3+ phosphor was found to be most effective in the temperature range of 30 oC (303 K) to 250 oC (523 K). The results suggest that Ca3Mg3(PO4)4:Eu2+,Eu3+ phosphor can be potentially used as a temperature sensor in fire alarms and fire-safety protection devices.

Resume : Tungsten oxide is one of the most intersting n-type semiconductor material with a wide variety of properties that have been rigorously studied due their essential properties for photocatalyst, electrochromic devices, gas sensors, photovoltaics. The WOx nanostructured thin films have attracted a lot of attention due to their small size and high surface-to-volume ratio. In this work we present the optical and structural properties of the WOx thin films obtained by pulsed laser deposition, using spectroscopic ellipsometry (SE), high resolution electron microscopy (HR-TEM) and X-ray diffraction (XRD) techniques. The optical properties and the structural quality of the films have been studied as a function of the deposition parameters such as oxygen partial pressure or laser wavelength. We will demonstrate in this study that any change in the crystalline structure induced by different deposition parameters lead at the changes in optical constants over a wide spectral range in the visible and infrared regions. The influence of the reactive gas partial pressure during thin film’s deposition on the NIR absortion range was revealed.

Resume : Dimethyldihydropyrene compounds could be applied in wide range of areas such as organic electronics, for example single molecule memory elements, and biology ? diagnostics, control of metabolic reactions. Photochromism of dimethyldihydropyrene derivatives is a reversible transformation under UV and visible light irradiation between two, opened-ring cyclophanediene and closed-ring dimethyldihydropyrene, isomers with different spectroscopic properties. To achieve even more suitable physical properties for different applications, DHP molecules could be modified by adding substitutes. However, the most common problem of these modified compounds is stability. To solve this problem, there is a need of deeper understanding of processes appearing during photochemical reaction. New dimethyldihydropyrene derivatives were synthesized, and their optical properties as well as excited state dynamics were investigated in the solutions. We focus on the emissive properties of dimethyldihydropyrene derivatives with the possibility to switch them between fluorescent and non-fluorescent states. During the first 100-300 ps after excitation under visible light the closed-ring cyclohexadiene isomer were opened. Reverse transformation took place through intermediate stage during several nanoseconds.

Resume : We performed comparative ab initio calculations for main ABO3 perovskite, namely SrTiO3, BaTiO3, PbTiO3, CaTiO3, SrZrO3, BaZrO3, PbZrO3 and CaZrO3 (001) surfaces as well as for polar YAlO3, ReO3, WO3 (001) and YAlO3 (111) surfaces [1,2]. The relaxation of (001) surface metal atoms for ABO3 perovskite upper two surface layers for both AO and BO2-terminations, in most cases, are considerably larger than that of oxygen atoms, what leads to a significant rumpling of the outermost plane. The ABO3 perovskite (001) surface energies are always smaller than the (011) and especially (111) surface energies. The ABO3 perovskite AO and BO2-terminated (001) surface band gaps are always reduced with respect to their bulk values. The B-O chemical bond population in ABO3 perovskite bulk always are smaller than near the (001) and especially (011) surfaces. The relaxation of atoms on the upper three layers of both YO and AlO2-terminated YAlO3 (001) surfaces were calculated using the slabs containing 22 and 23 atoms. We predict a significant increase of the Al-O chemical bond covalency on the AlO2-terminated YAlO3 (001) surface with respect to the YAlO3 bulk. Our calculated YO and AlO2-terminated YAlO3 polar (001) surface enegies are consideraly larger than the related structure ABO3 perovskite neutral (001) surface energies, but comparable with ABO3 perovskite polar (011) surface energies. References: 1. R.I. Eglitis and A.I. Popov, J. Saudi Chem. Soc. 22, 459-468 (2018) 2. R.I. Eglitis and A.I. Popov, Nucl. Instr. & Meth. Phys. Res. B 434, 1-5 (2018)

Resume : Over the last decade, Mn4+-activated materials have attracted much interest as potential non rare-earth red-emitting phosphors for application in white LEDs. One of the drawbacks of Mn-doped compounds is the formation of lower Mn valence states, such as Mn2+ and Mn3+, due to its interaction with intrinsic defects and impurities. We study the effect of an annealing in different atmospheres on Mn charge states and Mn4+ red photoluminescence (PL) in Mn-doped Mg2TiO4 phosphors. The phosphors with Mn concentrations about 1017 cm-3 and 1020 cm-3 were produced by solid state reaction at 1150 °C in the air and subsequently annealed at 600 °C in the O2 and N2 for several hours. The annealing in oxidizing atmosphere increased slightly the PL intensity, while the annealing in reducing atmosphere decreases by ten times PL intensity. The PL intensity decrease is concomitant with the decrease of UV peak intensity in the PL excitation spectra. In the phosphor with low Mn content, the Mn4+ red PL almost vanished and partial thermal transformation of Mg2TiO4 phase into MgTiO3 phase occurred. The electron paramagnetic resonance (EPR) study showed that annealing in a reducing atmosphere increased intensity of the EPR signal, caused by Mn2+ ions in Mg2TiO4 phase. No PL related to the optical transition of Mn2+ ions was found. It is proposed that formation of oxygen vacancies upon the annealing in reducing atmosphere promotes the decrease of concentration of Mn4+ emitting centers due to changes in Mn charge state.

Resume : As the esthetic functions of metals have attracted increasing attention, their colorization is of scientific and technological significance. Here we show that vivid structural colors can be produced on stainless steel (STS) and Al, which are two of the most commonly used metals. It is well known that a transparent dielectric layer coated onto a substrate exhibits a rippled reflectance spectrum consisting of alternating reflectance minima and maxima due to multi-beam interference, making it appear colored. However, such a layer does not produce strong colors on a highly reflective metal like Al because the resulting interference ripple has small amplitudes. We were able to generate very vivid colors by coating a thin metal layer over the dielectic layer, where the metal layer plays a role to adjust the amount of light incident into the dielectric, ultimately strengthening the interference effect. The hue and saturation of the produced colors were controllable with the thicknesses of the dieletric and metallic layers, respectively. Simulation based on the finite-difference time-domain method supported the experimental results. Colors images could also be printed by locally controlling the thicknesses of the overlayers. This method has the potential for a variety of applications ranging from surface decoration and visual arts to optical filters and perfect absorbers.

Resume : Heterostructures based on TiO2 and dye-sensitizers that are sensitive to the light of the visible and near infrared bands of the spectrum indicate the importance of using for the transformation and accumulation of solar radiation energy, elimination of environmental pollution, the development of the non-traditional methods of low-tonnage synthesis of chemical compounds, etc. Therefore, the construction of heterostructures with high photocatalytic activity and a wide range of photosensitivity is a promising method of creating new multifunctional materials. Titanium (IV) oxide P25 (Degussa) was used to obtain light-sensitive heterostructures dye/TiO2. As a sensitizer was used a zwitterionic dye. The influence of the structure of the dye in its spectral and electrochemical properties was established. The oxidation and reduction potentials were determined by the method of cyclic voltammetry, there were calculated the values of the energy levels of HOMO and LUMO and it was been predicted the possibility of using a dye as a sensitizer of titanium dioxide. Photocatalytic activity of the heterostructures dye/TiO2 in the oxidation of iodide ions from the concentration of the dye when irradiated with visible light was investigated. The energy diagrams of electronic processes that occur when light is absorbed in different spectral ranges is analyzed. The mechanism of photocatalytic action of these complex substances is substantiated.

Resume : Cd1-xMnxTe (x = 0.1-0.45) and Cd1-xZnxTe (x = 0.05-0.15) crystals were grown by the Bridgman method in graphite containers for creation photosensitive elements. The processes of recrystallization of ordered epitaxial layers on the surface of these crystals under different modes of the millisecond laser were studied. Theoretical estimate of the surface temperature and the melting threshold of Cd1-xМnxTe crystals under the action of laser radiation was made The thickness of the melting layer was determined in SEM and AFM using the transverse chips of the obtained epitaxial structures. At critical values of the laser beam energy density, the inversion of conductivity type in the melted layers was obtained with the formation of a photosensitive barrier structure. For creation photosensitive elements on the base of laser-epitaxial layers, irradiation modes were developed, in which the planar boundary of the interface was formed. Such elements had the maximum values of the photodetection at wavelengths corresponding to the width of the band gap of Cd1-xMnxTe and Cd1-xZnxTe solid solutions depending on the composition x. The presence of additional photosensitivity peaks was associated with the photogeneration of nonequilibrium carriers in the laser-epitaxial layer, and the spectral location of these peaks depends on the modes of laser treatment. Spectral characteristics of the photosensitivity of the obtained epitaxial structures showed significant sensitivity in the spectral range of 700-960 nm.

Resume : For kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells to enter the market, in addition to efficiency improvements, the technological capability to produce flexible. In the Cu-Sn-Zn metallic precursor system, interdiffusion between precursors can cause the formation of alloys, such as Cu6Sn5 or Cu5Zn8, inside the precursor. The Cu6Sn5 and Cu5Zn8 alloys that were formed by reactions between Cu, Zn, and Sn, resulted in Cu2S (Cu2Se) at the initial stage of the sulfo-selenization process. The chalcogenization reactions with Cu6Sn5, Cu5Zn8, and Cu2S (Cu2Se) yield Cu vacancies (VCu). VCu diffuse to the lower absorber layer because of the Kirkendall effect, forming voids at the absorber layer–back contact interface. When doped into the bulk of CZTSSe, Na is most likely to form electrically inactive NaCu, and the formation of VCu can be suppressed at the initial stage of the sulfo-selenization process. In this case, the formation of voids at the absorber layer–back contact interface can be suppressed, and fill factor (FF) can be improved. Moreover, the suppression of forming VCu can restrain the formation of many types of defects and defect clusters. That way, the open circuit voltage (VOC) can be improved. After the sulfo-selenization process, the Na diffusion out of the NaCu sites leads to a high concentration of VCu formed in the CZTSSe grains. This would improve the current density (JSC). Here, we report a greater than 9% efficiency for a cell area of approximately 0.5 cm2 by changing NaF position in the precursors. Acknowledge This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) (No. 20173010012980) and by the DGIST R&D Programs of the Ministry of Science and ICT of Korea (19-BD-05).

Resume : Photovoltaic is regarded as one of the important sustainable energy technologies, due to the inexhaustible solar energy. Nonetheless, most solar cells suffer from high cost or the use of rare earth material or toxic material. In recent years, despite that CdTe solar cell and CuIn1-xGaxSe2 (CIGS) has respectively achieved 21 % and 22.6 % efficiency at research level, and are both available for mass production, CdTe and CIGS solar cell also has those issues mentioned above: rare, toxic materials, and high cost. These would potentially impact the value of CdTe and CIGS solar cell in industry, as well as the feasibility of their sustainable development. Low cost, non-toxic alternative material for CZTSe solar cells is very crucial for solar energy technologies. Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S, Se1−x)4 (CZTSSe) are very promising candidates as low cost, non-toxic alternative material for solar cell absorber layers. CZTSe materials are p-type semiconductor and possess high absorption coefficient (about 104 m-1), low band gap energy at around 1 eV , which is highly applicable to the absorber layer of a solar cell, as band gap energy at 1 eV covers photon absorption at wavelengths spanning almost the entire solar spectrum. Common methods to prepare CZTSe thin film includes co-evaporation, sputtering, electroplating, and spin coating using nano-particle ink solution, among which, CZTSe solar cell prepared by solution-based has the highest record efficiency of 9.7 %, surpassing efficiency of CZTSe solar cells prepared by all other methods. However, during solution-based fabrication process of CZTSe solar cells, hydrazine is needed for solution preparation. Due to that hydrazine is a highly flammable, highly explosive chemical, it needs to be processed in anoxic environment, and is still very risky. Also, hydrazine is corrosive to Mo electrode, which leads to deterioration on product homogeneousity and production yields, bringing great impact to mass production of CZTSe solar cell. CZTSe solar cells prepared by co-evaporation have the second highest record efficiency, 11.6 %. Yet, CZTSe solar cells prepared by co-evaporation suffer from slow production, homogeneousity for large area device, and high entry threshold for startups, which hinder co-evaporation CZTSe solar cells from commercialization. On the other hand, for CZTSe solar cells, sputtering can achieve production demand including large area, high throughput, and low cost. These are very preferable for commercialized mass production. To date, sputtering of CZTSe thin film has achieved the highest record efficiency of 9.7 % utilizing Cu10Sn90, Zn, and Cu targets, and post selenization at 460 oC under H2Se atmosphere. However, the cost for this method is higher, and H2Se is highly toxic, which raise the risk of fabrication, and the cost for waste treatment. In this study, CZTSe solar cells with 6.78 % efficiency has been achieved through sputtering, using target materials of CuxSe and ZnxSn1-x alloy for CZTSe precursor layers deposition, and Se tablet for selenization process. This approach using two sputtering target materials significantly lowers the cost and minimizes the pollution to the environment due to that the toxic H2Se is avoided, which makes it more sustainable in aspects of technology development and environment protection. The use of CuxSe and ZnxSn1-x alloy as target materials can improve compositional homogeneousity, and CZTSe grain size of the CZTSe thin film. This study presents a thorough investigation into the influence of precursor layers, and selenization temperature on CZTSe thin film and CZTSe solar cells characteristics, using measurements spanning x-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), reflectance measurement, Hall measurement, and energy dispersive x-ray spectrometer (EDS). The CZTSe solar cells are characterized by current-voltage (I-V) measurement, and external quantum efficiency (EQE) measurement.

Resume : We have prepared and evaluated a highly stretchable, transparent organic-inorganic hybrid elastomer for focus-tunable optical lenses. Crosslinked poly(dimethylsiloxane) (PDMS) has been well known for an elastomer material. The crosslinked PDMS films are obtained from a reaction between vinyl and hydridosilane groups in the PDMS via platinum (Pt)-catalyzed hydrosilylation. We synthesized high molecular weight poly(dimethylsiloxane-co-methylvinylsiloxane) (VPDMS) from diethoxydimethylsilane and diethoxymethylvinylsilane. As a crosslinker, we also prepared poly(dimethylsiloxane-co-methylsiloxane) (HPDMS) from diethoxydimethylsilane and diethoxymethylsilane. The number-average molecular weight and polydispersity of copolymers VPDMS and HPDMS were ca. 11.0×104 g/mol, 1.96 and 10.6×103 g/mol, 4.96, respectively. Crosslinked PDMS films were fabricated from the mixture of HPDMS and VPDMS containing Pt. The crosslinked PDMS films showed high transparency in a visible region (T > 90 % @ ca. 150 µm in film thickness), high strain (ɛ > 3.0) and low initial modulus values (E < 500 kPa), and low dielectric storage permittivity (ɛ' ~ 25 pF/m @ 1 kHz). In order to increase the dielectric storage permittivity, we introduce tri(ethylene glycol) divinyl ether (TEGDE) into the PDMS films. TEGDE not only shows high polarity but also has two vinyl moieties which can be chemically reacted to HPDMS. We investigate variations in electrical, optical, and mechanical properties of the organic-inorganic hybrid PDMS films. Finally, we will examine the hybrid elastomer as a candidate for focus-tunable optical lenses.

Resume : Application of secondary ion mass spectrometry (SIMS) in investigation and comparison of molybdenum disulphide (MoS2) films grown on SiO2, Al2O3 and BN substrates are presented. A SIMS measurement of the interface MoS2/substrate reveals oxygen out-diffusion from the substrates containing oxygen and formation in addition to MoS2 triangles also amorphous MoOS layer. Total area of MoS2 domains covering substrate is directly related to a type of substrate. For SiO2 small triangular domains of MoS2 separated by amorphous MoOS material are observed. For Al2O3 substrates the size of MoS2 domains are drastically improved due to higher stability of sapphire. For BN/Al2O3 substrates SIMS measurements reveal a uniform MoS2 coverage over whole 2-inch wafer. The BN layer grown on Al2O3 separates the sapphire, and apparently blocks oxygen diffusion to the grown MoS2 film. X-ray diffraction confirmed the presence of MoS2 film grown on BN/Al2O3 with the d002 distance of 0.613 nm between layers. Raman spectroscopy also confirmed the presence of MoS2 by observation of the A1g and E2g peaks. The final conclusion of the work is that the MoS2 coverage is dramatically influenced by the choice of the type of substrates. These results show the destructive role of oxygen released from the substrates such as SiO2 or Al2O3 during the growth process of MoS2. To obtain a large area of epitaxial MoS2 film it is necessary to use BN substrates.

Resume : The simultaneous influences of hydrostatic pressure and temperature combined to the size effects on the behavior of GaAs conical quantum dot with spherical edge on optical absorption coefficient are investigated. In the framework of the effective mass approximation, the Schrödinger equation of donor impurity have been solved analytically in an infinite potential barrier model, taking into account the dependence of the size, the dielectric constant and the effective masses on the hydrostatic pressure and the temperature. Our results show that the linear and nonlinear properties are strongly affected by the size of the structure, applied hydrostatic pressure and temperature.

Resume : An important building block in integrated photonic circuits is an efficient link between the optical and electrical domain. The two most widely used physical effects to enable such electro-optic links - the plasma-dispersion effect and Joule heating – suffer from limitations in low speed and high power dissipation. Over the past decade, we established an alternative electro-optic switching technology by embedding a Pockels material into silicon-based photonic devices using advanced epitaxial deposition techniques. By realizing integrated hybrid BTO/silicon devices, we demonstrated record-high, in-device Pockels coefficients of >900 pm/V. The Pockels effect in BTO-based photonic devices indeed enables extremely fast data modulation at rates beyond >40 Gbps and ultra-low-power electro-optic tuning of silicon and silicon-nitride waveguides. With the development of a wafer-level integration scheme of single-crystalline BTO layers to a 200 mm process, we could demonstrate a viable path to combine the BTO-technology with existing fabrication routes. With major breakthroughs in the past years, BTO has emerged as a strong candidate for a novel generation of electro-optic devices. Major achievements of the BTO technology will be covered in the presentation, ranging from important materials aspects, device development, integration concepts, and novel applications in the area of quantum computing, high-speed communication, and neuromorphic optical computing.

Resume : Perovskite oxide ferroelectrics are transparent in a VIS to UV range and possess a large refraction coefficient, which is extremely sensitive to electric field, mechanical stress, and temperature. Despite the high potential of ferroelectrics for photonics, mainstream well-established applications mainly employ a strong electrooptic effect in only some ferroelectric bulk crystals and ceramics from almost infinite family of these materials. The applications are hindered by difficulties in synthesis and integration of bulk optical ferroelectrics. This obstacle can be overcome by using single-crystal epitaxial films. However, although the present-day thin-film technology ensures epitaxial growth of numerous ferroelectric films on a variety of research and industrial substrates, effects of epitaxy on optical properties are largely unknown. Because of the high electrical, mechanical, and thermal sensitivity of ferroelectric refraction, the properties of epitaxial films can be dramatically altered by substrate-imposed misfit and/or thermal strain, interface-related in-built electric field, and possibly other yet unknown factors. Therefore, meticulous investigations of optical properties of epitaxial ferroelectric films are highly relevant. In this research we apply spectroscopic ellipsometry to experimentally determine optical constants of such films. In the presentation a critical overview of the methodological aspects will be given first. Next, the results obtained from epitaxial films of BaTiO3, PbTiO3, (Pb,Sr)TiO3, Pb(Zr,Ti)O3, KNbO3, and (K,Na)NbO3 will be summarized. Finally, unexpected findings that contradict existing models will be pointed out.

Resume : Doping the well known BaTiO3 ferroelectric perovskite with Ca and Zr allows obtaining a rich variety of compositions with most different properties. Thin film of (x(Ba0.7Ca0.3TiO3)-(1???x)(BaZr0.2Ti0.8O3) where intensified study for application in microelectronic devices. The phase diagram of (x(Ba0.7Ca0.3TiO3)-(1???x)(BaZr0.2Ti0.8O3) (BCTZ) system exhibit a morphotropic phase boundary (MPB) point where the rhombohedral and tetragonal phases coexist. The properties of BCT-BZT system can be controlled by varying the Ba/Ca and Zr/Ti ratios. Depending on the amount of A-site (Ca2+) and B-site (Ti4+) isovalent substitutions, different properties can be obtained such as high dielectric constants, low losses, high piezoelectric coefficient, etc. Thin films of BCTZ growth on Pt/Si and on Nb doped SrTiO3 substrate by PLD, were investigated by X -ray diffraction, high resolution transmission electron microscopy (HRTEM) Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM). The optical properties of calcium titanate zirconate (x(Ba0.7Ca0.3TiO3)-(1???x)(BaZr0.2Ti0.8O3); BCTZ) thin films has been investigated in the 25-1000 C range of temperature. The phase transformation for BCTZ with x=45, 50, 55 was evidenced by spectroscopic ellipsometry technique and a comparison with the bulk properties has been done based on the strain effects and chemical substitution influences.

Resume : Transition-metal oxide perovskite single-crystalline thin films showing a rich variety of properties including resistive switching, photovoltaics, colossal magnetoresistance, multiferroicity, etc. are finding applications in a broad spectrum of information and energy technologies. However, the strict growth conditions of epitaxial oxide films such as high deposition temperature and use of lattice-match single crystal substrates result in great challenges of using such materials for flexible electronics or integration in conventional semiconductor technology. Transferable single-crystalline oxide perovskite films which can be integrated with arbitrary substrates could tackle the aforementioned obstacles and enable more exciting applications. Here, we will demonstrate the fabrication of freestanding epitaxial oxide perovskite thin films and their applications for electronic devices. Several typical oxide perovskites such as SrTiO3, ferroelectric Pb(Zr,Ti)O3 and multiferroic BiFeO3 were obtained in freestanding membrane form and ready to be transferred onto any surfaces by lift-off from the growth substrates after etching the water-soluble Sr3Al2O6 sacrificial buffer layer. Using these transferable function oxide membranes, novel electronic devices such as flexible single-crystalline ferroelectric tunnel memristors and high-performance top-gate two-dimensional transition-metal dichalcogenides transistors are fabricated.

Resume : Recently, high-speed low-loss electro-optic modulators on silicon nitride using a lead zirconate titanate (PZT) thin film have been demonstrated. These devices are based on co-planar electrodes structures on top of the ferroelectric thin film. Analysis of the in-plane dielectric and piezoelectric properties of the ferroelectric thin film should provide a better understanding of the operation of these devices. Here, we discuss the in-plane dielectric properties of the preferentially oriented PZT thin films using interdigitated electrodes (IDTs), where preferential orientation is obtained using a lanthanide based seed layer. The in-plane dielectric constant of the thin film is obtained from the combination of a finite element simulation and capacitance measurements. By simulating a period of the IDT structure, a relation between the capacitance and in-plane dielectric constant is obtained. A dielectric constant of about 400 is acquired after capacitance measurements. Moreover, a Sawyer-Tower circuit is used to investigate the polarization loops measured in-plane using IDT electrodes. A well-defined hysteresis loop is obtained indicating the switching of the polarization of the ferroelectric domains. From the hysteresis loop we are able to define poling conditions for an optimal operation of the electro-optic modulators. IV-measurements are performed to determine the leakage current. The rectifying property of the gold IDT-ferroelectric PZT contact is observed.

Resume : The integration of transparent conducting oxides (TCOs) in photonic circuits is a promising solution for enabling novel optoelectronics functionalities or improving their performance with respect to other approaches. TCOs can provide extraordinary tuning and modulation of their complex refractive indices by changing the carrier concentration with the application of an electric field. Ultra-compact and low power electro-optical modulators have been demonstrated based on the integration of TCOs, such as indium tin oxide (ITO) and indium oxide (In2O3), in silicon photonic structures. In this work, we investigate the use of ITO for new scopes such as thermo-optic tuning and non-volatile applications. Most relevant findings for thermo-optic tuning demonstrate state-of-the art results by achieving experimental power consumptions lower than 10 mW to achieve a π-phase shift. On the other hand, novel ultra-compact photonic memories are proposed by using ITO as a floating-gate in a flash-like memory structure and exploiting its epsilon-near-zero (ENZ) state.

Resume : Aluminum nitride is one of the III-V materials used in a new generation of electronic and optoelectronic devices especially in high power lasers. Indeed, thanks to its high thermal conductivity, up to 285 W.m-1.K-1, it is well qualified to evacuate heat from the devices; thus, improving their performances and reliability. One of the challenges is to succeed in integrating aluminum nitride films in real devices. In order to do so, the first challenge is to deposit well crystallized aluminum nitride films at low temperature (<200°C). The second consists of plasma etching of aluminum nitride films using ICP in a Cl2/BCl3/Ar plasma mixture. In the presentation, we will show that thanks to magnetron sputtering we have succeeded to deposit well crystallized aluminum nitride films at a temperature as low as 200°C. The film quality was checked by XRD and TEM analysis and also thanks to thermal characterizations. Thermal conductivity as high as 175 W.m-1.K-1 were found. Concerning the etching step, knowing that such films are very hard to etch; BCl3 gas was added to the plasma mixture to improve the etching process. Aluminum nitride films deposited on silicon and indium phosphide were successfully etched. A patterned SiO2 mask was used as a hard mask for selective etching. The etch rate reached 200 nm/min. In addition, a 0 D plasma kinetic model of Cl2/BCl3/Ar was used to study the effects of the reactor parameters on the etching process. Simulation results show that the positive ion to atomic chlorine flux ratio increases with the %BCl3. Such parameters could play an important role in the ion neutral synergy during the etching process. Finally, a III-V/SOI hybrid laser fabrication process is being developed in order to fully integrate the optimized aluminum nitride films, including a new etching process, which has been improved thanks to the simulation model.

Resume : We investigate the transport of excitons and trions in monolayer semiconductor WS2 subjected to controlled non-uniform mechanical strain. We actively control and tune the strain profiles with an AFM-based setup in which the monolayer is indented by an AFM tip. Optical spectroscopy is used to reveal the dynamics of the excited carriers. The non-uniform strain configuration locally changes the valence and conduction bands of WS2, giving rise to effective forces attracting excitons and trions towards the point of maximum strain underneath the AFM tip. We show that the completion between “funneling” of free excitons, charge excitons, and free carries leads to previously unobserved effects. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices.

Resume : Black phosphorus (BP), a 2D semiconducting material of interest in fields of electronics and photonics, exhibits physical properties characterized by a strong anisotropy and a band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging both because BP thin layers photooxidizes in ambient conditions and dielectric environments significantly shifts the energy of the electronic states. Using glovebox conditions, we prepared free-standing samples of thin BP and probed the dielectric response in vacuum using Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). From these experiments we extracted the optical bandgaps for the mono- bi- and trilayer BP, and compared the results with electronic structure calculations as a function of the dielectric environment. The dispersions of the plasmon versus momentum for 1-3 layers and bulk BP is also presented, with clear deviations from the parabolic dispersion and strong anisotropic fingerprints.

Resume : The two-dimensional materials with direct band gap are attractive for optoelectronics operated in the visible and near infrared spectral range. The number of new van der Waals crystals increases systematically but the doping and the modification of their optoelectronic properties remain challenging. Here we present the tuning of the fundamental properties of different 2D mono- and dichalcogenides using millisecond range flash lamp annealing (FLA) in the controlled atmosphere. Those investigated 2D flakes are made by mechanical exfoliation onto the SiN/Si substrates. The change of internal properties of 2D chalcogenides is monitored by micro-Raman, photoluminescence and photoreflectance spectroscopies as well as conductive atomic force microscope (c-AFM). Using ms-range FLA in N2 ambient the transition metal dichalcogenides are stable up to the annealing at 1200 oC, while upon high temperature annealing the group IV-dichalcogenides can be reduced to monochalcogenides which opens new route for the fabrication of heterostructures. The formation of SnSe/SnSe2 heterostructures is proven by micro-Raman spectroscopy and current-voltage characteristic obtained by c-AFM measurements.

Resume : Van der Waals (vdW) heterostructures, in which a wide range of unique atomic layers can easily be combined, offer novel prospects to engineer and manipulate quantum confined states. I will present evidence for quantum confined states engineered by the periodic potential landscape of interlayer valley excitons in a vdW heterostructure. Here, the moiré potential of a twisted MoSe2/WSe2 heterostructure traps interlayer excitons, and we show that the trapped interlayer excitons inherit the magneto-optical properties of the type-II band-structure. Finally, we will discuss the ability to charge the trapped states with electrons or holes in tunable electronic devices. These results highlight new opportunities to engineer quantum confined spins in vdW heterostructures.

Resume : The Li ions have been considered previously for modification of luminescence properties of the ZnO-based materials. The main reason is that Li-dopingof the ZnO can affecton its crystallinity. It can suppress or promote the growth of ZnO particles during a synthesis as well as the crystal grain sizes in the polycrystalline ZnO films upon thermal annealing. In the present work, we repot on the investigation of photoluminescent (PL) properties of the Li-doped ZnO films produced by screen-printing technique and sintered at 800-1000° C for 30-180 min in air. PL emission of the samples is observed in the wide spectral range from 350 to 800 nm. The spectra are complex and consist of two PL bands: narrow PL band in the 350 – 420 nm range and wide PL band in the 430 – 800 nm range. The relative intensities of these bands depend on Li doping, annealing temperature and annealing duration. The first band is related to the exciton emission. The wide PL band originates from the defects (native or Li-induced defects). This wide band consists of six PL components peaked at 430, 485, 540 (intensive), 575 (the most intensive), 650 and 760 (weak) nm. Analysis of temperature behavior of the spectra gives the evidences on the different origin of PL components contributing in the 430 – 800 nm band. The comparison of PL and structural properties of the samples showed that Li doping controls the ZnO grain sizes and, in consequence, the concentration of native and Li-induced defects. This work has received funding from the EU-H2020 grant No 654360 within the framework of the NFFA-Europe Transnational Access Activity.

Resume : The growth of well-ordered, controlled, periodic two-dimensional (2D) semiconductor nanostructure patterns with enhanced surface morphology and advanced Light-matter Interaction control i.e. absorption and emission enhancement is becoming increasingly important because of the need to understand the fundamental growth mechanism as well as its potential applications in nanoscale electronics, biological sensing and diagnosis, and nanophotonics. In this study, we demonstrate that by using a low-cost, chemical technique, one can synthesize nanostructures of ZnO on different types of seed layers, such as ZnO seed layer as well as a metal-catalyst (silver). Annealing of the seed layer is not required when a metal catalyst is used as a nucleation layer for the growth of the nanowires (NWs). Selective growth of ZnO using templates fabricated by bottom-up and top-down approach has been achieved. The bottom-up approach involves fabrication of self-organized templates of functionalized polystyrene beads for two purposes, namely, as masks for patterning silicon and for fabrication of urchin-like ZnO structures [1]. In the top-down approach, different kinds of silicon patterns in the form of cones or pillars can be obtained using Laser Interference Lithography technique. ZnO NWs are grown on these patterns (pine-tree like structures) [2,3]. The structural and optical properties of these patterned ZnO NW structures will be presented. Integration of the ZnO nanorods with 2-D materials will also be demonstrated [4]. [1]A. Gokarna, et al. RSC Adv, 4, 47234, 2014. [2]A. Gokarna, et al. Phys. Stat. Sol. C 13, 421, 2016. [3]A. Gwiazda, et al. Adv. Mater. Technol. 2,1700107, 2017. [4]H. Jeong, et al. Adv. Mater. 29,1700308, 2017.

Resume : Cubic ZnxMg1-xO have been proposed as wide band gap semiconductors for short wavelength optoelectronic applications operating in the deep UV region. A very small value of c/a in MgO (1.55) and the high iconicity of this material results in a tendency to crystalize in rocksalt structure. By combing MBE growth and HRTEM we were able to determine conditions in which the cubic phase of ZnO and cubic ZnxMg1-xO alloys can be grown on MgO substrates. In the case of ZnxMg1-xO compounds, it was found that the maximum of the layer thickness in cubic phase strongly depends on Zn concentration, decreasing with x, which reflects the alloy phase instability. The band structures of ZnxMg1-xO alloys in the rocksalt structure are obtained by ab-initio calculations based on the Local Density Approximation (LDA) to density functional theory. The values of total energies, atomic coordinates and lattice parameters are determined by minimization of the Hellman-Feynman forces using pseudopotentials as implemented in the Vienna ab-initio Simulation Package (VASP). In the second step of calculations, the band structures were obtained by the Linear-Muffin-Tin-Orbital method in a full-potential version with a semi-empirical correction (LDA+C) for the deficiency of LDA in predicting semiconductor gaps. The calculated band gaps for ZnxMg1-xO alloys as functions of composition x are compared with the experimental data with single quantum wells and super lattice grown by PA-MBE technique. Optical data was obtained by using deep UV cathodoluminescence. X-ray diffraction and high-resolution transmission electron microscopy were used for microstructure analysis of MgO/ZnMgO based rocksalt structure. The field of possible applications is broad, and should further increase due to the cheapness of MgO substrates.