Current trends in optical and X-ray metrology of advanced materials for nanoscale devices VI

Resume : Organic-inorganic halide perovskites (HP) are remarkable semiconductors that combine features of hard and soft matter and have potential applications in many areas. Two-dimensional (2D) HPs – where organic spacer cations can be engineered to microscopically isolate nanometer-thick HP layers, offers tunable physical properties and improved device stability [Nature 536, 312 (2016)]. Recent studies have hinted at the importance of dynamic interactions between electronic states and the lattice in these materials. It is thus critical to understand the interplay between structure and properties in 2D-HPs during light excitation and in device operation conditions. Here, we investigate these effects by exploring the 2D-HP phase space both in terms of HP layer thickness and organic spacer rigidity. We reveal the structure-properties interplay by correlating state-of-the-art characterization tools, i.e. grazing-angle wide angle x-ray diffraction, ultrafast electron-diffraction, and optoelectronic spectroscopy at the sub-micron scale. Our work demonstrates that the characteristics of excitons can be modulated not only by the thickness of the HP layers as in classic quantum-wells but also through structural distortions. Similar effects are linked to local compositional changes or artificial interfaces in heterostructures of 2D-HP and WS2 [Nano Lett. 19, 4852 (2019)]. We also report on the transient structural changes under light excitation and discuss their implications for applications.

Resume : Analyzing the photoluminescence (PL) of semiconductors complementarily in time and wavelength allows to derive their key optoelectronic and transport properties. Up to now, separate acquisitions have to be performed. We developed a 4D imaging set-up that allows the acquisition of spectral and temporal luminescence intensity with micrometric spatial resolution. Very notably, this dataset is acquired simultaneously and hence with exact same experimental conditions. This novel set-up relies on single pixel imaging technology, an approach that enables the reconstruction of the spatial information recorded from a higher resolution non-imaging detector. The sample PL signal is spatially modulated with different patterns by a digital micro-mirror device1. We make use of this technique for the first time with a streak camera as a detector, allowing to record the PL intensity decays and spectrum for each pixel with very high temporal (<100ps) and spectral resolutions (< 1nm). A patent application has been filled. We already performed successful measurements and 4D numerical reconstructions and we demonstrate the use of this setup by characterizing hybrid mixed halide perovskite samples. We extract and analyze the material properties (gap) at different time scales as well as the carrier recombination (lifetime, radiative coefficient) and transport (diffusion coefficient) properties with a single experiment. 1 Duarte et al, IEEE Signal Process. Mag. 25, 83 (2008).

Resume : Wide-bandgap materials, which are interesting for opto-electronic devices, like Ga2O3, exhibit often a low crystal symmetry. Here, we present the determination of the optical properties of such materials by spectroscopic ellipsometry. For the investigation of the nature of the underlying excitations, we present a general approach for the decomposition of the dielectric function by taking into account the dipole orientation distribution of each excitation into account. The application of this approach is demonstrated exemplary on an orthorhombic KTiOPO4, monoclininc Ga2O3 and triclinic K2Cr2O7. Without making any assumptions, we show that the tensor elements of the dielectric function in general are not independent of each other. The dielectric function can be described by a superposition of electronic transitions. For each transition an eigensystem exists in which the contribution to the dielectric function is given by a diagonal susceptibility tensor. The relative magnitude of these tensor elements is given by the dipole orientation distribution of the corresponding transition. In the limiting case, the oriented dipole approach is obtained as well as the tensor for uniaxial and isotropic materials. It is also shown that the imaginary part of the non-diagonal tensor elements can become negative, which is forbidden in scalar systems.

Resume : Exceptional points (EPs) correspond to non-Hermitian degeneracies and represent topological charges, being of great interest in the context of research on topologically non-trivial photonic systems due to their possible applications for e.g. optical isolators. EPs can occur in the momentum space of a photonic system, representing directions along which the eigenmodes coalesce, yielding a degeneracy in energy, broadening and polarization. Furthermore, the eigenenergies exhibit a complex-square-root topology around such EPs. A well-known related phenomenon is the splitting of each classic optic axes of optically biaxial material into two singular optic axes, in the absorptive regime [1,2]. However, the presence of such EPs is not limited to bulk single crystals but can occur in a variety of systems described by non-Hermitian Hamiltonians [3]. Here, we report on EPs in planar dielectric heterostructures with broken cylindrical symmetry, realized by using anisotropic cavity layer materials, and present different approaches for experimental proving of the EPs in such photonic systems. Furthermore, we show how the occurrence and direction of EPs can be controlled by the structure design. Finally, we describe approaches for breaking the in-plane reciprocity of the system, thereby paving the way for more complex topologically non-trivial photonic systems. [1] W. Voigt, ‎Ann.Phys 314 (1902) [2] C.Sturm, M. Grundmann, Phys.Rev.A 93 (2016) [3] S. Richter et. al., Phys.Rev.Lett. 123 (2019)

Resume : Chiral organic semiconductors offer unique potential for chiroptical applications when the thin film semiconducting properties combine with a sizable excitonic circular dichroism. This is the case for a series of naturally chiral prolinol-derived squaraines with variable side chain length for fine-tunability of the chiroptical responsen [1]. For an accurate, quantitative determination of the chiroptical response from their homogeneous thin films, a partial Mueller matrix polarimetry measurement is sufficient. Many modern ellipsometers are capable of measuring 12 out of the 16 elements of a Mueller matrix, which can be completed by symmetry considerations for non-depolarizing samples [2]. This allows extraction of the true circular dichroism not overlaid by possible anisotropy effects from the sample. For an ultimate quantitative evaluation of the circular dichroism a series of measurements for samples with varying layer thickness is necessary. Furthermore, for determination of the true dissymmetry factor, which is the absorbance normalized circular dichroism, a correction for reflection losses due to the thin film nature of the samples is required [3]. [1] Schulz, Balzer, Scheunemann, Arteaga, Lützen, Meskers, Schiek. Adv. Funct. Mater. 29 (2019) 1900684. [2] Arteaga, Ossikovski. JOSA A 36 (2019) 416. [3] Schulz, Zablocki, Abdullaeva, Brück, Balzer, Lützen, Arteaga, Schiek. Nat. Commun. 9 (2018) 2413.

Resume : There is significant research interest in the field of soft matter that focuses on studying slow relaxation processes in systems that are close to a phase transition or are dynamically arrested. Yoghurt, gelatin and various food additives are example of such systems. In some cases, accessing dynamic properties in time scales of seconds or even minutes is crucial. One method that allows for studying slowly relaxing systems is to perform many DLS measurements, to obtain time averaged correlation functions. Between each measurement the sample is rotated such that a different speckle is observed. The results are then averaged to provide the ensemble average [1]. Additionally, when working with concentrated samples, the suppression of multiple scattering is required to obtain meaningful results. This is achieved through the 3D modulated cross-correlation technique [2,3], where two temporally separated light scattering experiments are performed at the same scattering vector on the same sample volume in order to extract only the single scattering information common to both. In this work, we present modulated 3D static and dynamic light scattering (SLS and DLS) measurements on Polystyrene particles suspended in a gel matrix. Using a sample goniometer, we rotate the sample to record a subsection of the configuration phase space. We then average over many sub-ensembles to capture the full phase space. We demonstrate that to follow such systems undergoing dynamical relaxation, as well as to study dynamically arrested samples, modulated 3D cross-correlation combined with a sample goniometer is a perfectly suited experimental tool that can record information otherwise not accessible. [1] K. N. Pham, S. U. Egelhaaf, A. Moussaı̈d, and P. N. Pusey, “Ensemble-averaging in dynamic light scattering by an echo technique”, Rev. Sci. Instrum. 75, 2419 (2004). [2] Patent EP2365313 A1 [3] Ian D. Block and Frank Scheffold, “Modulated 3D cross-correlation light scattering: Improving turbid sample characterization”, Rev. Sci. Instrum. 81, 123107 (2010).

Resume : Optical spectroscopy represents one of the primary methods to characterize electron states in mono- and few-layer (ML & FL) two-dimensional (2D) materials. It enables quality control of the 2D material during growth and processing, e.g., upon ambient exposure or layer transfer. However, when covering the few-ML film by another material to form heterojunctions or metallic contacts, observation of the 2D film optical spectra becomes problematic. We will show that the optical spectra of a 1 ML MOCVD-grown WS2 can still be observed by measuring transient photocurrent spectra in a Si/SiO2/WS2 capacitor even upon evaporation of a relatively thick (10-20 nm) metal layer overlayer. The photoconductivity (PC) signal arises in this case due to separation of electron-hole pairs optically generated in WS2 with subsequent trapping and detrapping at the interface with the underlying oxide. Characteristic excitonic peaks (A, B, C) observed in the optical spectra are reproduced in the PC spectra taken from unmetallized (air-exposed) 1ML WS2 and after evaporation of a 15-nm thick Al overlayer indicating that the 2D film is not modified. By contrast, evaporation of 15-nm Au or Cu results in attenuation of the PC signal and almost complete disappearance of the A exciton peak. Similar observations have also been made on MoS2 films (1.5-3.5 ML thick) indicating the PC method to be a viable technique in assessing the processing-induced changes in the ML-thin semiconductor.

Resume : Hyperspectral luminescence imaging technique is an optical measurement, which can spatially and spectrally resolved photoluminescence (PL) spectrum emitted from semiconductors. This is a powerful technique to map the surface of semiconductors and to investigate electrical and optical properties of photo-generated carriers in the system. The information about emitting particles, such as temperature and quasi-Fermi level splitting, can be extracted via fitting their PL spectrum with the generalized Planck’s law. However, in nanostructure materials, such as quantum wells (QWs), the fitting analysis can be challenging, especially at high excitation powers when the influence of band filling and discrete energy levels are not negligible. Therefore, it is required a careful analysis of emitted PL spectra considering complexities imposed by quantum confinements in such structures. Here, we present our results in hyperspectral imaging of InGaAs quantum well (QW) structures and fitting their full PL spectra with the Generalized Planck’s law. We have extracted temperature and quasi-Fermi level splitting of photo-generated carriers at various lattice temperatures and excitation powers. Via this information we have determined the strength of hot carrier thermalization and Seebeck effects in the QW structures. Moreover, we will present our advancements in extraction of absorption coefficients and linewidth broadenings from multiple transitions in the samples.

Resume : Solar water splitting converting the solar energy into green hydrogen fuel is one significant milestone on the road to a sustainable energy future and has driven many researches in the past years [1]. III-V/Si tandem material combining the good optical properties of III-V semiconductor with the low cost of Si substrate is considered as one kind of promising material for efficient water splitting reactions [2-3]. In this work, we demonstrate a new photoelectrode material GaPSb/Si for solar water splitting. The band structure of GaPSb alloy are determined by combining experimental absorption measurements with tight binding (TB) theoretical calculations, which shows good bandgap and appropriate band alignment for water splitting. Besides, the PEC characterizations were made on GaPSb/Si sample, which show very good PEC performances. The GaPSb/Si tandem material is thus a potential candidate for low-cost high-efficiency solar water splitting. References: [1] A. J. Bard and M. A. Fox, “Artificial photosynthesis: solar splitting of water to hydrogen and oxygen”, Accounts of Chemical Research, 28(3), 1995. [2] I. Lucci et al., “A Stress-Free and Textured GaP Template on Silicon for Solar Water Splitting”, Advanced Functional Materials, 28(30):1801585, 2018. [3] M. Alqahtani et al., “Photoelectrochemical water oxidation of GaP(1− x)Sbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting”, Sustainable Energy & Fuels 3, 2019.

Resume : Tin antimony sulfide Sn-Sb-S (TAS) thin films were deposited on glass and Si substrates using vacuum evaporation technique. Spectroscopic ellipsometry (SE) is a non-destructive method that we used to study the optical properties of the films. The SE measured data were analyzed by considering double layer optical model for all the samples, with the two Tauc-Lorentz oscillators and Gaussian dispersion relations. From the ellipsometric study, we extracted the thickness, absorption coefficient, band gap energy, refractive index and extinction coefficient of all samples. All the films exhibited high absorption coefficient a in the visible range (>105 cm-1). The values of the band gap energy Eg of Sn-Sb-S thin films deposited on glass varied from 1.13 to 1.48 eV. For the samples deposited on silicon, the band gap energy Eg varied from 1.44 to 1.72 eV. The spectral dependencies of the refractive index and the extinction coefficient of Sn-Sb-S thin films were determined and analyzed.

Resume : In this work, we report the extraction of the real part of the third order nonlinear optical susceptibility of a c-cut paratellurite (TeO2-α) single crystal using the detected intensity of the nonresonant background (NRB) in multiplex Coherent Anti-Stokes Raman Scattering (M-CARS) measurements. Using fused silica and SF57 as nonlinear reference materials, we particularly show that the pure electronic contribution of the third order nonlinear susceptibility can be obtained alone without significant impact of the vibrational response, provided some precautions in the selected frequency range. The results are then confronted with the data obtained by Duclère et al. from Z-scan experiments. The (a,b) in-plane modulation of the nonlinear refractive index of TeO2-α is finally evaluated.

Resume : Electrochromism is defined as the ability of a material to change its optical state in response to an applied DC voltage. Tungsten and Molybdenum oxides are widely studied for this application purpose, however little attention is paid for their mixed oxides. Thin films of Tungsten and Molybdenum mixed oxides were prepared by reactive DC magnetron sputtering using combinatorial technique (i.e. simultaneous sputtering from different targets). Samples were made with different composition as well as with different morphologies (i.e. superlattice type and amorphous mixed oxide type samples.) The deposited films were characterized by Spectroscopic Ellipsometry, SEM and TEM microscopy for morphology and amorphous content. Furthermore electrochromic coloration efficiency and response times were studied in electrolyte cells using propylene carbonate – lithium perchlorate electrolyte. Correlation between the sputtering parameters (oxygen pressure, power), layer morphology and electrochromic properties is studied for establishing enhanced composition-dependent electrochromic performance. Authors acknowledge the funding of Hungarian National Science Fund OTKA NN131269 and K129009

Resume : We investigated the enhanced indium tin oxide(ITO)/Ag/Polytetrafluoroethylene(PTFE) tri-layer through the improved wettability between the oxide layer and metal layer by the oxygen plasma treatment. Using a successive multi-cathode magnetron sputtering system, the ITO/Ag/PTFE tri-layer electrode with the fixed thickness (50nm/9nm/40nm) was prepared on a colorless polyimide substrate. The oxygen plasma treatment was conducted with increasing treatment time at fixed power of 20 W. The structural and morphological properties of the oxide layer were analyzed by a contact angle, scanning electron microscopy(SEM), X-ray photoelectron spectroscopy, and atomic force microscopy. The improved wettability of oxide layer showed a lower pinhole size of the thin Ag layer on the oxide layer through the SEM analysis. As these result, the oxygen plasma treated ITO/Ag/PTFE tri-layer electrode had a higher optical transmittance of 87.4% at visible region wavelength(550nm) and lower sheet resistance of 13.5 Ohm/square than untreated ITO/Ag/PTFE due to oxygen plasma treatment effect.

Resume : As an essential part of a transparent electrode for highly flexible electronic devices, Oxide/polymer composite and the sandwich structure transparent electrode of oxide-polymer hybrid/metal/hybrid(OPHMH) structure have been proposed. Amorphous oxide embedded in polytetrafluoroethylene(PTFE) was fabricated on flexible substrates using a continuous magnetron sputtering system with RF and DC simultaneously. Oxide/polymer composite layer showed that the higher the amount of PTFE in oxide/PTFE composite, the higher the water contact angle and flexibility. As part of our commitment to uniform deposition of the sandwich structure, plasma and ion beam pre-treatment effects were also investigated in order to control surface energy and wettability. This pre-treatment resulted in surface activation and hydrophilicity improvement of both oxide and polymer, and uniform thin film deposition was possible. Due to the stable structure of tri-layer interfaces, we could demonstrate that the OPHMH structure with a hydrophobic surface ensures high flexibility and optical properties in the visible wavelength region (400 ~ 800 nm), and low sheet resistance applicable to highly flexible electronic devices.

Resume : FOTON Institute - INSA Rennes is part of the DROP-IT1 consortium. DROP-IT aims at combining optoelectronics and photonics in a single flexible drop-on demand inkjet technology platform by means of exploiting the potential of lead-free perovskite materials. A survey of lead-free perovskite materials performed by the perovskite team in Rennes will be presented based on the literature2,3. The simulation team has developed in addition density functional theory (DFT) simulations of perovskite lead free materials and a DFT methodology based on to extract band alignments and dielectric mismatch between perovskite materials and hole and electron transporting layers in thin film device architectures4,5. This project has received funding from the European Union’s Horizon 2020 research and innovation Programme under the grant agreement No 862656. The information and views set out in the abstracts and presentations are those of the authors and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein. References: (1) DROPIT https://www.uv.es/dropit/ (accessed Jan 13, 2020). (2) Volonakis, G.; Filip, M. R.; Haghighirad, A. A.; Sakai, N.; Wenger, B.; Snaith, H. J.; Giustino, F. Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals. J. Phys. Chem. Lett. 2016, 7 (7), 1254–1259. https://doi.org/10.1021/acs.jpclett.6b00376. (3) Volonakis, G.; Haghighirad, A. A.; Milot, R. L.; Sio, W. H.; Filip, M. R.; Wenger, B.; Johnston, M. B.; Herz, L. M.; Snaith, H. J.; Giustino, F. Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap. J. Phys. Chem. Lett. 2017, 8 (4), 772–778. https://doi.org/10.1021/acs.jpclett.6b02682. (4) Traore, B.; Pedesseau, L.; Blancon, J.-C.; Tretiak, S.; Mohite, A. D.; Even, J.; Katan, C.; Kepenekian, M. Importance of Vacancies and Doping in Hole Transporting Nickel Oxide Interface with Halide Perovskites. ACS Appl. Mater. Interfaces 2020. https://doi.org/10.1021/acsami.9b19457. (5) Canicoba, N. D.; Zagni, N.; Liu, F.; McCuistian, G.; Fernando, K.; Bellezza, H.; Traoré, B.; Rogel, R.; Tsai, H.; Le Brizoual, L.; et al. Halide Perovskite High-k Field Effect Transistors with Dynamically Reconfigurable Ambipolarity. ACS Materials Lett. 2019, 1 (6), 633–640. https://doi.org/10.1021/acsmaterialslett.9b00357.

Resume : Broadband light absorbers are highly desirable in numerous applications including solar-energy harvesting, thermo-photovoltaics, sensing and imaging, photodetectors, and thermal emitters. Metal-insulator-metal (MIM) layers has attracted enormous interest as a lithography-free and scalable structure for realizing planar super absorbers. However, typical MIM cavity exhibits a narrow absorption band, and efforts have thus been made to increase the absorption bandwidth. This study demonstrates that near-perfect absorption over a broad spectral range can be obtained from the MIM structure by using thermally evaporated Ag and Au thin films. A 55-nm-thick SiO2 spacer sandwiched between a 10-nm Ag top layer and a 100-nm Al back reflector exhibits absorption > 95% in the visible range of 400-700 nm. The broad absorption band shifts to a near-infrared range of 650-1000 nm by replacing the top layer with a 10-nm-thick Au film and increasing the SiO2 spacer thickness to 115 nm. The experimental results are supported by finite-difference time-domain (FDTD) simulation. The large absorption bandwidth is attributed to the lossy nature of the top metallic layer combined with the resonant absorption of the MIM cavity.

Resume : Organometallic halide perovskite solar cells have gained considerable interest in recent times due to their rapidly improving power conversion efficiencies. However, as with all technology the cost of ownership is key to its take up and viability, thus despite perovskite solar cells approaching the Shockley Queisser efficiency limit for a single junction device it cannot compete with established silicon technology. As a consequence, perovskite technology is shifting towards multi-junction solar cells where differing absorber layers respond to different spectral regions and hence negate this limitation. Such a development does come with complications, one issue is the standard transparent conducting oxide (TCO) currently employed is indium-tin oxide (ITO). Although ITO has ideal electrical and optical properties the material is deposited using sputtering methods and high temperatures (>400oC) heat treatments. Such a fabrication methodology is not compatible with thermal budget restrictions necessary to fabricate heterojunction devices, as a result an alternative material or methodology is required. Here we examine atomic layer deposition, an industry compatible large area low temperature growth technique, to grow ZnO and its doped variants as TCO layers. The materials properties were characterised and optimised. Finally, perovskite solar cells were fabricated in a P-I-N configuration and compared to the current state of the art devices.

Resume : Metal oxide nanowires have focused the interest of the scientific community due to their potential applications in fields such as photocatalysis, photodetectors, sensors, nanoscale electronics, etc. In the last years, great effort was made towards the preparation of metal oxide core-shell nanowire arrays with advanced functionalities provided by the radial heterojunction formed between the two metal oxides. Zinc oxide is a n-type semiconductor with a wide band gap, whereas copper oxide is a p-type metal oxide semiconductor with a narrow indirect band gap. By aligning these two metal oxide into a core-shell heterostructure, a p-n staggered gap heterojunction is achieved between the two semiconductors promoting a good control of the charge carrier generation at the interface. Thus, ZnO-CuO core-shell nanowire arrays were synthesized using two dry methods, thermal oxidation in air and magnetron sputtering. Morphological, structural, optical, compositional, surface chemistry, electrical and photocatalitycal properties of the ZnO-CuO nanowire arrays were investigated. An optimum copper oxide shell thickness was found for which the degradation of methylene blue under UV illumination was improved due to the heterojunction formed between the ZnO and CuO. Also, single ZnO-CuO core-shell nanowires were contacted using lithographic and thin film deposition techniques for applications in UV photodetectors.

Resume : In this paper, a simple and efficient approach is presented for the design in CST of a 10 GHz metamaterial based resonator used for high sensitive chemical sensor. The sensor was designed and fabricated as CPW line deposited on front side of high-resistivity Si/SiO2 substrate by incorporating four metamaterial Split Ring Resonator (SRR) on back side and covered in the center area with a thin film sensing MoS2 material. The simulation results demonstrate a very good quality factor of about 2000 for the MoS2 based metamaterial resonator being very useful for sensing applications. The proposed configuration is applied for chemical discrimination as NO2 gas sensing. A thin-film of MoS2 was deposited on the resonator using a pristine MoS2 nanoflake solution supplied by Graphene Supermaket. The response of the sensor have been study by passing different concentrations of NO2 gas through the gas chamber. The S-parameters have been measured as a function of frequency using a Vector Network Analyzer (VNA) for the MoS2 based resonator in air (without gas flow inside the chamber) and with gas flow into the chamber at different concentrations. In the literature was demonstrated that upon NO2 adsorption, the MoS2 based sensor resistance decreases. The change of the MoS2 based sensor resistance for different NO2 gas concentrations demonstrates a frequency of resonance shift of about 100 MHz resulting in a high sensitivity NO2 gas sensor. Acknoledgements; We are grateful for financial support of H2020 project NANOPOLY

Resume : The mechanochromic active device can measure stress or strain exerted by human body and exhibit visual warning signs when they are highly stressed or strained. These characteristic make the device be readily exploited for the applications where real-time interactivity is more required than unilateral information delivery, such as sports industry or medical application. Most of the mechanochromic researches are focusing on photonic crystal structure because the period of the photonic crystal can be easily changed by the tension and photonic crystal can show vivid color. However, photonic crystal is difficult to make uniform color distribution on large area and has highly angle-dependence property. Instead of photonic crystal structure, we fabricate the stretchable metal-insulator-metal planar cavity structure by using PDMS as substrate and insulator. Depending on the strain, the thickness of PDMS insulator layer is decreased and the MIM cavity resonance peak can be easily shifted. Also,compared with photonic crystal structure, this device has low angle-dependence property because the thickness of PDMS insulator layer is very thin. Consequently, we make mechanochromic active device using MIM planar cavity structure and tested on the human body for real-time interactivity application.

Resume : Our research is based on two major scientific and technical problems: 1) the use of unique physicochemical properties of two-dimensional (2D) structures of graphene, BN and transition metals dichalcogenides of (TMD) MeX2, where Me - metals of IV, V and VI groups (Ti- Hf, V-Ta, Mo, W), and X = S, Se, Te, as well as 2) enhancing their capabilities during intercalation by impurity atoms and molecules. The results of a detailed study of the Raman spectra of natural MoS2 microcrystal (MC) powder and synthesized graphene-like 2H-MoS2 nanoparticles (NP) with additives 0.5 and 1 wt% of carbon excited by 488 nm and 632.8 nm laser radiation are highlighted here. A detailed numerical analysis of the shape of the observed D and G Raman bands, including their decomposition into spectral components, as well as comparison with the reference spectra of the detonation nanodiamonds of ~ 5 nm, is carried out. This revealed the formation of graphite and diamond-like nanostructures in MoS2 NC and their existence in molybdenite MC. Significant effect of 632.8 nm laser radiation that is in resonance with 2H-MoS2 excitons on synthesis and ordering of diamond and graphite-like structures was established for the first time

Resume : The properties of BCT-BZT system can be controlled by varying the Ba/Ca and Zr/Ti ratios, the amount of (Ca2+) in A-site and (Ti4+) in B-site being the key of tuning different functionalities of these materials. The phase diagram of (x(Ba0.7Ca0.3TiO3)-(1 − x)(BaZr0.2Ti0.8O3) system exhibit a morphotropic phase boundary (MPB) point in which the tetragonal and rhombohedral ferroelectric phases meet the paraelectric cubic phase. The structural phase diagram of BCTZ as a function of temperature is still under debate, the existence of an additional orthorhombic phase between rhombohedral and tetragonal ones being notices in some studies, but can be summarized as follows: for x ≤ 0.3 and x>0.8 a single phase transition is observed from R-phase to the high-temperature prototype cubic Pm3m structure (x ≤ 0.3) and from T to the C-phase for x>0.8. Between those limits, a multiple phase transitions through a R-T-C phase has been observed. In the present work we report the experimental values of transition temperature between R-T-C phases for the epitaxial thin films of BCTZ with x=0.45, 0.50, 0.55, obtained by spectrometric ellipsometry technique. Epitaxial thin films of (x(Ba0.7Ca0.3TiO3)-(1 − x)(BaZr0.2Ti0.8O3) were growth on SrTiO3, GdScO3 substrates by Pulsed Laser Deposition method. The structural properties of thin films were investigated by X -ray diffraction.The optical properties of BCTZ have been investigated in the 25-1000 C range of temperature.

Resume : VO2 polymorphs, such as VO2(B) and VO2(M), have a broad range of application in informational technology. The former is promising for lithium-ion batteries, while the later showed promise in ultrafast optical switchers and smart windows, mainly due to the reversible metal-insulator transition phenomena. In our work, we investigate the thermally-induced phase recrystallization from VO2(B) to VO2(M) by thermal vacuum annealing of initial VO2(B) thin films obtained by aerosol assisted CVD. According to the annealing conditions (temperature, time) highly textured and homogeneous VO2(M) layers with sharp and reversible metal-insulator phase transition (resistance ratio ΔR = (R30 °C – R90 °C)/R90 °C of around 10^4) were obtained. Phase recrystallization was confirmed by X-Ray diffraction (XRD), Raman spectroscopy (RS) and electrical measurements. In-situ XRD and in-situ RS measurements are deployed to study the phase transformation mechanism in vanadium dioxide system.

Resume : A semiconducting molybdenum ditelluride (MoTe2) monolayer, which is atomically-thin layered materials, has been attracting considerable attentions for development of various opto-electronic devices since it has a direct bandgap around 1.1 eV applicable to optical communications and exhibits an extraordinarily large exciton binding energy of several hundred meV owing to strong electron-hole confinement. However, the MoTe2 crystal has a critical disadvantage of formation of the surface decomposition-induced cluster defects and Te vacancies during thermal treatment even at low temperature of 200 ̊C. On the other hands, atomically-thin layered materials encapsulated by hexagonal boron nitride (hBN) are considered promising structures for enhancement of electrical and optical device performances. In this study, we investigated effects of thermal treatment on the crystal quality in the mechanically-exfoliated MoTe2 monolayer encapsulated by hBN using steady-state photoluminescence measurements, demonstrating suppression of desorption of Mo and Te atoms owing to existence of hBN capping layer. It was furthermore found that residues remained at the interfaces between hBN and MoTe2 for the as-fabricated samples, and the thermal treatment was effective for removal of the residues at the interfaces. These results indicate that combination of hBN encapsulation and thermal treatment is promising for realization of high-performance MoTe2-based opto-electronic devices.

Resume : Metamaterials is the artificially engineered materials which can tailor the properties depend on its structures. Recently, it has been greatly attractive for anticounterfeit applications due to its unique properties. In this study, we present metamaterial based broadband perfect absorbers in NIR wavelength for multiple, broadband NIR absorber fabrication. The NIR perfect absorbers were fabricated with high lossy metals. The it shows the complex, multiple absorption properties in NIR wavelength, and it can be tailored by the kind of materials, thickness, and nanostructures of perfect absorbers. The nanostructures of metamaterials were fabricated by nanoimprinting process on flexible substrate for industrial applications.

Resume : Liquid-metal jet X-ray sources promise to deliver high photon fluxes, which are unprecedented for laboratory based X-ray sources, because the regenerating liquid-metal anode is less sensitive to damage caused by an increased electron beam power density. For some quantitative X-ray analysis techniques, knowledge of the absolute photon flux is needed. However, a precise experimental determination of the photon flux of high-performance X-ray sources is challenging, because a direct measurement results in significant dead time losses in the detector or could even harm the detector. Indirect determinations rely on data base values of attenuation or scattering cross sections leading to large uncertainties. In this study we present an experimental determination of the photon flux of a liquid-metal jet X-ray source by means of elastic and inelastic photon scattering. Our approach allows for referencing the unpolarised output radiation of the liquid-metal jet X-ray source to polarized synchrotron radiation in a simple setup. Absolute photon fluxes per solid angle are presented with a detailed uncertainty budget for the characteristic emission lines of Ga Ka and In Ka for two different acceleration voltages of the X-ray source. (DOI: 10.1039/c9ja00127a)

Resume : The aluminum nanoparticles (Al-NPs) are obtained by thermal evaporation on heated substrates at 500°C. The mean diameter of Al-NPs is controlled by the evaporated aluminum amount. The equivalent film thickness is varied from 4 to 20 nm. The used substrates are Quartz glass and crystalline silicon wafer oriented (100). For the characterization we used the Scanning Electron Microscopy to define the morphological properties and the optical transmission technique to define optical absorbance spectral. The Al-NPs diameter distribution reveals two kinds of population types : small and thick ones. For the aluminum evaporated amount with an equivalent film thickness less than 8 nm only the small population has been observed with a mean diameter around 20nm. The optical absorbance spectrum reveals a band absorption due to the Localized Surface Plasmon Resonance. The LSPR absorption band is closely linked to the nanoparticle's morphological properties. The position of the LSPR band varied from 274 to 355 cm-1.

Resume : Li ion battery is one of suitable energy storage in our life, such as mobile electronic devices and electric vehicles. For delightful and convenient existence, high speed charging and long battery life are very important. Charging time of smart phone is too long, which is about 5 hours for full-charge from empty state. Generally, 0.2C is used for charging current, otherwise battery capacity is reduced under higher C-rate (higher charging speed). This is because SEI layer, which is decomposed materials of electrolyte, LiF and organic solvent etc., is deposited on active cathode and anode materials. In this paper, we have achieved to obtain the ultrahigh speed charging and very tough cycling properties in LiCoO2 cathode thin film battery decorated with ferroelectric BaTiO3.[1] The important point is the blocking of creating SEI on cathode surface. In details, we have investigated interface reaction between cathode and electrolyte using epitaxial thin film battery. We have tried to insert artificial SEI of high dielectric constant materials, BaTiO3, on the cathode LiCoO2 epitaxial thin film. The high rate performance and cyclability are enhanced by existence of triple phase interface, cathode LiCoO2 – electrolyte LiPF6 (EC:DEC)– dot BaTiO3. The key point of this effect is that high dielectric constant material is better. [1] S. Yasuhara, S. Yasui et al., Nano Lett. 19 (2019)1688.

Resume : Polycrystalline thin film solar cells suffer from a laterally inhomogeneous grain structure, which may cause spatial variations of their electronic properties. For an improvement of the conversion efficiency, a better understanding of the limiting defects at sub-grain-size resolution is required. Therefore, we seek to correlate the electrical properties with the structure, composition, and deposition conditions. While measurement methods such as laser- or electron-beam induced current offer spatially resolved electrical information, they are not compatible with simultaneous non-destructive and high-sensitivity measurements of the composition and structure. Synchrotron-based multi-modal measurements using scanning X-ray microscopy enable the simultaneous measurement of several relevant aspects: with a spatial resolution on the nanoscale, hard X-ray ptychography maps the structure; X-ray fluorescence (XRF) gives insight into the elemental composition; and X-ray beam induced current (XBIC) and X-ray excited optical luminescence (XEOL) test the electrical and optical properties. For the first time, we have combined all these modalities in simultaneous measurements of a Cu(In,Ga)Se2 solar cell. Hence, we have been able to perform a point-by-point correlation of the electrical and optical solar cell performance with the structure and composition. In this contribution, we will report on these correlations with a focus on grain boundaries and voids in Cu(In,Ga)Se2.

Resume : We demonstrate, for the first time to our knowledge, a correlative 2D mapping of the second and third order nonlinear susceptibilities of a thermally poled borophosphate niobium glass. The experiment is conducted by means of ultrabroadband multiplex coherent anti-Stokes scattering (M-CARS) setup. By means of the latter, we already reported previously the measurement of the third order susceptibility of a paratellurite single crystal. This work represents a step further in regards to this previous work, by extending it to a fast and large scale imaging technique of the second and third order susceptibilities of a microstructured niobium borophosphate glass on which a thermal poling process was conducted. Thermal poling process consisted of the application of a static electric field in a micrometer scale under an eleveted temperature using microstructured anode electrodes. The nonlinear imaging results highlight a second harmonic generation and a decrease of the third order nonlinear susceptibility in the areas of the glass where a static electric field is written and where a density change occurs. These results are in total agreement with Marc Dussauze et al. previous works on the very exact same sample. This allows us to conclude that M-CARS is a suitable technique for a fast investigation of the third order nonlinearity of glasses and is also compatible with the second harmonic imaging process without any significant change in the setup. That bimodal investigation is useful in the field of material science for samples with complex structures.

Resume : Nowadays, mixed cation perovskites1,2 are one of the most efficient light harvesters in perovskite solar cells. The mixed cation perovskites are used mainly in their “bulk” form in the solar cell. However, confined perovskite nanostructures have been recently synthesized and seemed to be a promising route to efficient optoelectronic devices, taking advantage of the superior bulk properties of metal lead halide perovskite as well as the nanoscale properties. Recently, mixing two inorganic cations in the same Nanoparticles (NPs) was reported3,4. Indeed, Rb was incorporated into Cs-based NPs, forming all inorganic mixed cation perovskite NPs. A detailed structural study of mixed inorganic cation lead halide perovskite NPs at the atomic level is presented. We focused our investigations on mixed cation perovskite NPs having an RbxCs(1−x)PbBr3 composition5 (with x = 0, 0.2, 0.4, 0.6, and 0.8). A High-angle annular dark-field (HAADF) detector was used to obtain NPs imaging with atomic resolution. Density Functional Theory (DFT) calculations help understanding the role of Rb atoms into the stability of such NPs at various Rb concentrations. This work was supported within the framework of the Israeli−French scientific cooperation (Joint Research Projects 2019−2021 entitled ALLPOA (From Atomic Level to Layered Perovskite for Optoelectronic Applications)) by the Ministry of Science & Technology of the State of Israel (MOST) and France’s Centre National de la Recherche Scientifique (CNRS). References: (1) Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; et al. Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354 (6309), 206–209. https://doi.org/10.1126/science.aah5557. (2) Yi, C.; Luo, J.; Meloni, S.; Boziki, A.; Ashari-Astani, N.; Grätzel, C.; Zakeeruddin, S. M.; Röthlisberger, U.; Grätzel, M. Entropic Stabilization of Mixed A-Cation ABX3 Metal Halide Perovskites for High Performance Perovskite Solar Cells. Energy Environ. Sci. 2016, 9 (2), 656–662. https://doi.org/10.1039/C5EE03255E. (3) Amgar, D.; Binyamin, T.; Uvarov, V.; Etgar, L. Near Ultra-Violet to Mid-Visible Band Gap Tuning of Mixed Cation RbxCs1−xPbX3 (X = Cl or Br) Perovskite Nanoparticles. Nanoscale 2018, 10 (13), 6060–6068. https://doi.org/10.1039/C7NR09607K. (4) Baek, S.; Kim, S.; Noh, J. Y.; Heo, J. H.; Im, S. H.; Hong, K.-H.; Kim, S.-W. Development of Mixed-Cation CsxRb1–XPbX3 Perovskite Quantum Dots and Their Full-Color Film with High Stability and Wide Color Gamut. Advanced Optical Materials 2018, 6 (15), 1800295. https://doi.org/10.1002/adom.201800295. (5) Binyamin, T.; Pedesseau, L.; Remennik, S.; Sawahreh, A.; Even, J.; Etgar, L. Fully Inorganic Mixed Cation Lead Halide Perovskite Nanoparticles: A Study at the Atomic Level. Chem. Mater. 2019. https://doi.org/10.1021/acs.chemmater.9b04426.

Resume : The microstructural parameters of electrode materials strongly influence the storage mechanisms and, besides, the overall performances of supercapacitors. Furthermore, the internal strain governs the stability during the long term cycling, and the strain engineering approach is lately a valuable method for development of energy storage electrodes. In this context, non-destructive characterization by X-ray based methods represents a necessity for both understanding the storage mechanisms and further progresses in this field. Starting from porous silicon substrate with high internal area obtained at different current densities and etching times, nanocomposite materials were fabricated through a polymer electrochemical deposition followed by thermal treatment partial carbonization, obtaining finally interconnected graphene-silicon nanofibrils networks. Analyzing the shape and of width of the diffuse scattering were obtained information regarding the pore morphology. Thus, it was found that the diffuse scattering is broadened after graphitization and consequently the decreasing of the crystallite sizes is connected with increasing of the internal strain, leading to an inherent emergence of grain boundaries in the PS lattice. Testing the new electrodes in a standard PVA-H 2 SO 4 electrolyte we observed a significant improvement of the specific capacitance after polymerization, accompanied by slight improvement of capacity retention, probably given by the increased lattice value.

Resume : X-ray excited optical luminescence (XEOL) and Time-resolved XEOL (TR-XEOL) have been developed successfully for the 23A X-ray nanoprobe beamline located at the Taiwan Photon Source (TPS). The advantages of the TR-XEOL include not only a nano-focused X-ray beam (<60 nm) with excellent spatial resolution, but also a streak camera that can simultaneously record the light-emitting spectrum and decay lifetime. Especially, the operating TR-XEOL in the hybrid bunch mode also can provide a high enough X-ray photon flux because the electron beam current reaches 500 mA and long time span (30 ps ~ 220 ns) can facilitate temporal domain measurements. Using XEOL and TR-XEOL to study optical properties of the light-emitting materials, such as MgZnO/ZnO MQWs, InGaN/GaN MQWs, and perovskite CsPbBr3, will be reported.

Resume : Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. In our work, the detailed crystallization process of mixed perovskites (FA0.83MA0.17Pb(I0.83Br0.17)3) during spin-coating is revealed by synchrotron-based in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, and three phase-formation stages are identified: I) precursor solution; II) hexagonal δ-phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI–PbI2–DMSO intermediate phases, and perovskite α-phase. The correlated device performance and ex situ characterizations suggest the existence of an “annealing window” covering the duration of stage II. The spin-coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs+ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs+ will bypass the formation of secondary phases in stage III by promoting the formation of α-phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed-perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.

Resume : 2D transition metal dichalcogenides (TMDCs) materials are considered of very high potential semiconductors for future nanosized electronic and optoelectronic devices. An information-rich nanoscale characterization technique is required to qualify these materials and assist in the deployment of 2D material-based applications. Scanning Probe Microscopy (SPM) is a powerful technique to image physical properties of 2D materials, such as topography, conductivity or other electrical properties. Combining SPM and Raman in a single instrumentation is extremely powerful as it makes imaging of both chemical and physical properties possible. As Raman is diffraction limited, only plasmon enhanced Raman and photoluminescence spectroscopies yield correlated electrical and chemical information down to the nanoscale. In this talk, we will report on Tip-Enhanced Photoluminescence (TEPL) and Tip-Enhanced Raman spectroscopy (TERS) data obtained on single crystal WS2 and WSe2 flakes directly grown on SiO2/Si. TEPL and TERS images will be correlated with contact potential difference and capacitance maps as results of Kelvin force probe microscopy acquisition. In addition, we will show the sensitivity of electronic properties (related to Fermi level and charge accumulation) upon light illumination. Beside these semiconductor/dielectric (SiO2) interfaces, probing TMCD/metal interfaces is also essential to integrate TMCDs in 2D or 3D complex structures of devices. We will show results from WS2 on silver and WSe2 and MoS2 on gold. Such transferred surfaces exhibit nanoscale inhomogeneities observed in correlated CPD and Raman maps. Finally, TEPL together with AFM topography data on a lateral single layer WS2/WSxSe1-x/WSe2 heterostructure grown on SiO2/Si will be presented: nanoscale PL response variations are observed beyond the smooth nano-resolution topography.

Resume : Raman spectroscopy is an effective tool to study many properties without damaging the material. In general, the Raman signal is very small, and one of the methods to amplify the signal intensity is surface enhanced Raman scattering (SERS). SERS is a good amplification technique that enables single molecule detection, so much research has been done for decades. However, the chemical enhancement mechanism, which is one of the two enhancing mechanisms of SERS, is not completely understood. Since there is a limit to the signal enhancement by using electromagnetic enhancement only, the research of chemical enhancement mechanism can provide further clues to enhance the signal. In this study, we present results of chemical enhancement due to charge transfer transition to an excitonic state. First, we investigated the chemical contribution of the SERS effect of 4-mercaptopyridine (4-Mpy) molecules deposited on zinc oxide nanostructure substrates by resonance Raman scattering. The excitation energy is given from 1.7eV to 5.7eV, which covers all possible resonance contributions between the molecules and the substrate. We observed that the Raman intensities of 4-Mpy peaks at 5.14 eV in the UV region is about 20 times larger than that in the visible region. We contend that one of the reasons for the large amplification in this UV region is a charge transfer transition to the excitonic state and provide evidence of it. Furthermore, the comparison with GaN substrates without excitonic state at room temperature ensures the effect of the excitonic state on chemical SERS. Our results provide information regarding the role of charge transfer transition from the molecule HOMO to the excitonic state in SERS that can lead controllable enhancement of Raman signal.

Resume : There have been large number of theoretical and experimental studies of surface-enhanced Raman spectroscopy (SERS) over the last few decades. Unlike electromagnetic enhancement by surface plasmon resonance when using metallic substrate, chemical enhancement is responsible for selective enhancement of Raman response of different analyte molecules adsorbed on the same substrate materials. In this study, we use non-metallic substrates to clarify less studied effect of chemical enhancement in SERS. Rhodamine 6G (R6G) molecules are adsorbed on different structures of tungsten disulfide such as plane, flake and nanoflower. We estimate both electromagnetic enhancement by structure and chemical enhancement of R6G, by comparing finite-difference time-domain (FDTD) calculations to our Raman experimental results. We then discuss the contribution of both enhancement and crucial factors for attaining large enhancement factors. Based on our theoretical and experimental results, we suggest optimization conditions for SERS.

Resume : Spray flash evaporation (SFE) is a rapid and versatile technology to crystallize organic nanomaterials. This SFE processing was originally developed in the NS3E Laboratory about ten years ago and basically consists in injecting a pressurized hot liquid solution (10

Resume : Over the years, many studies on photovoltaic absorber materials have been conducted to improve the efficiency of photovoltaic devices. However, there are many challenging issues to overcome for the solar cell materials research ensuring longer lifetime of the cell and better chemical stability for practical application, for example. In particular, for organic-inorganic hybrid perovskite materials, the role and characteristics of organic cations that remain unclear. In this study, we focus on understanding the fundamental structural properties.One of the remarkable features of organic-inorganic hybrid perovskite is that it has different phases as the temperature varies. For example, CH3NH3PbBr3 shows a phase transition from cubic to tetragonal structure at ~235K and CH3NH3PbCl3 shows similar phase transition at ~179K. We measured temperature-dependent Raman spectra in single crystalline MAPbBr3 and MAPbCl3 samples and observed abrupt changes in spectra from both samples at the phase transition temperatures. Our results show that contributions to the phase transition in each atomic/molecular vibration are different but there are common features existing for different compounds. We show that Raman scattering spectroscopy is a very effective way for studying structural phase transitions in complex materials.

Resume : The X-ray excited optical luminescence (XEOL) and time-resolved X-ray excited optical luminescence (TR-XEOL) at 23A X-ray Nanorpobe (XNP) beamline of the Taiwan Photon Source (TPS) were applied to investigate the emission properties of the non-polar a-plane ZnO and MgZnO epi-films. Similar to our previous study[1], not only the intensity of near-band-edge (NBE) of a-MgZnO dramatically enhanced more than 10 times after using nano-focused beam irradiation, but also appeared an anomalous emission at 325nm, which was not observed in a-ZnO epi-films. We attribute that the high peak power density of the X-ray nanobeam may give rise to the anomalous excitation relative to the Mg energy states. The luminescence dynamic processes of the two samples were performed by TR-XEOL operating in the hybrid bunch mode, which provided the advantages that high enough X-ray photon flux as well as longer time interval (30 ps ~ 220 ns). The decay lifetime of a-MgZnO decreased gradually with the X-ray irradiation time. The detailed experimental processes will be reported. References: [1] Bi-Hsuan Lin, Yu-Hao Wu, Tai-Sing Wu, Yung-Chi Wu, Xiao-Yun Li, Wei-Rein Liu, Mau-Tsu Tang, and Wen-Feng Hsieh, Appl. Phys. Lett. 115, 171903 (2019).

Resume : Metamaterials is the artificially engineered materials which can tailor the properties depend on its structures. Recently, it has been greatly attractive for high performance ICT device such as super lens, holography, photonic integrated circuits, energy harvesting, sensor device, display device and so on. In this study, we present metamaterial based broadband perfect absorbers in visible wavelength for highly reflective, vivid color filter fabrication. The broadband perfect absorbers were fabricated with high lossy metals and coupled nanocrystals. The reflective color of metamaterial were tailored by the kind of materials, thickness, and nanostructures of perfect absorbers. The nanostructures of metamaterials were fabricated by nanoimprinting process for industrial applications. It also demonstrated on the flexible substrate for flexible display applications.

Resume : Fluorescent semiconductor nanocrystals are extremely promising and actively investigated materials that have various potential optoelectronic, photovoltaic, and biomedical applications. The optical and physico-chemical properties of the nanocrystals can be controlled not only by varying their shapes and sizes, but also by synthesizing complex core/multishell particles with controllable energy structure of the nanocrystals. The core/multishell nanocrystals with a fluorescence quantum yield reaching 100% have been designed, and the importance of the structure–optical properties correlation in these materials has been demonstrated. Raman spectroscopy has been proven to be one of the best approaches to the quality control of core/shell nanomaterials. Here, we have synthesized the extended series of CdSe(core)/multishell quantum dots (QDs) with different thicknesses and compositions of the shells and investigated them using the Raman spectroscopy technique. The CdSe Raman scattering bands characteristic of the longitudinal optical phonon of the cores, as well as the bands related to the vibrational states of different layers of the CdS, ZnS, and CdSe shells of the QDs, have been recorded and analyzed. The Raman spectroscopy approach to the quality control of the synthesized core/multishell nanomaterials is proposed.