Semiconductor nanostructures towards opto-electronic and photonic device applications – VIII

Resume : Colloidal lead halide perovskite nanocrystals (LHP NCs, formula APbX3, A=Cs+, formamidinium; X=Cl, Br, I) exhibit spectrally narrow (?100 meV) fluorescence, tunable over the entire visible spectral region of 400-800 nm. Owing to their high oscillator strength, slow dephasing (long coherence times of up to 80 ps), minimal inhomogeneous broadening of emission lines, and a bright triplet exciton character with orthogonal dipole orientation, these NCs make for a highly versatile platform for creating controlled, aggregated states exhibiting collective phenomena. Long-range ordered superlattices (SLs) with the simple cubic packing of cubic perovskite NCs exhibit sharp red-shifted lines in their emission spectra and superfluorescence (a fast collective emission resulting from coherent multi-NCs excited states) [1]. We now present perovskite-type ABO3 binary and ternary NC SLs by a shape-directed co-assembly of CsPbBr3 nanocubes (occupying B- and/or O-sites) with spherical dielectric Fe3O4 or NaGdF4 NCs (A-sites) and truncated-cuboid PbS NCs (B-site) [2]. Such ABO3 SLs, as well as other newly obtained SL structures (binary NaCl- and AlB2-types, columnar assemblies with disks etc.), exhibit a high degree of orientational ordering of CsPbBr3 nanocubes. These mesostructures exhibit superfluorescence, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behaviour with a strongly accelerated build-up time. Far greater structural space, beyond the realm of known lattices, is anticipated from combining NCs of various shapes. Here, we present also on the co-assembly of steric-stabilized CsPbBr3 nanocubes (5.3 nm) with disk-shaped LaF3 NCs (9.2?28.4 nm in diameter, 1.6 nm in thickness) into binary SLs, yielding six columnar structures with AB, AB2, AB4, and AB6 stoichiometry, not observed before and in our reference experiments with NC systems comprising spheres and disks [3]. 1. G. Rainò et al. Nature 2018, 563, 671?675 2. I. Cherniukh et al. Nature 2021, 593, 535?542 3. I. Cherniukh et al. ACS Nano, 2021, 15, 16488?16500

Resume : Research interest in all inorganic lead halide perovskite nanocrystals (LHP-NCs), featuring general chemical formula of CsPbX3, has recently grown fast thanks to their outstanding chemical and physical properties that make them optimal candidates for a wide range of technological applications such as photovoltaics1, light emitting devices2 and photodetectors3. In this fast-growing and heterogeneous playground, we report a robust, reproducible, and easy scalable synthetic method allowing the production of more than 2.5 g - until now - of high quality CsPbBr3 NCs for either fundamental studies or in-solution and device applications. To this aim, we modified the synthetic procedure reported by Akkerman et al. 4 by introducing, for the first time, the use of a turbo emulsifier (Ultra Turrax Homogeneizer) usually employed for the preparation of large batches of formulates, to improve the reaction mixture homogenization and overcome concentration gradients and reproducibility issues that usually affect LHP-NCs liter-scale reaction volumes. We also introduced tetrabutylammonium bromide (TBAB) as extra bromide precursor: working in halogen rich environment is known to help reducing defectivity and this specific quaternary ammonium salt is too bulky to be competitive with other cations in the perovskite crystal lattice. We demonstrated that the amount of recovered solid material is proportional to the volume of solution used at every scale, suggesting that the process is well controlled up to the biggest scale. There are also evidences that increasing the scale, magnetic stirring becomes insufficient where turbo-emulsifier remains reliable: we are willing to further extend the procedure to 7 g of output materials to deepen this aspect and reach amounts considerably higher than the general ones5. We further pushed the limit by demonstrating that, with our approach, the low-boiling solvents and the excess reactants, in particular lead bromide, can be recovered and reused, thus reducing the environmental impact connected to waste production and moving a step towards final industrial application. We finally preliminary tested radioluminescence properties both in solution and in polymer matrix to apply them to scintillation field: the big amount that can be easily produced, in one pot, with low waste favors many applications, especially the production of wide scintillating windows even with at high concentration. REFERENCES [1]. Banerjee S. K. Et al., Stability Improvement of Perovskite Solar Cells by Compositional and Interfacial Engineering, Chem. Mater., 33, 1540-70 (2021). [2]. Demir H. V. et al., Light Generation in Lead Halide Perovskite Nanocrystals: LEDs, Color Converters, Lasers, and Other Applications, Small, 15 (47), 1902079 (2019). [3]. Manna L., Brovelli S., Beverina L. et al., Efficient, fast and reabsorption-free perovskite nanocrystal-based sensitized plastic scintillators, Nat. Nanotechnol., 15(6), 462-468 (2020). [4]. Akkerman, Q. A. et al. Strongly emissive perovskite nanocrystal inks for high-voltage solar cells. Nat. Energy 2, (2017). [5]. Deng, Z. et al., Ligand-Mediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature, ACS Nano, 10 (3), 3648–3657 (2016).

Resume : Lead-based semiconductors are among the most explored compounds for the synthesis of colloidal nanomaterials, mainly due to the appealing optoelectronic properties demonstrated by lead halide perovskites in the UV-VIS and by lead chalcogenides in the IR spectral ranges. The mature research on both classes of materials has recently led to the exploration of systems where such properties can coexist and interact. One promising direction is that of heterostructures, which are composite materials formed by intimately connected domains of two different compounds. They are generally challenging to obtain, mostly because of the poor compatibility in terms of required synthetic conditions and crystal structures of the two materials. Another direction to investigate is represented by compounds that are chemically related to both lead halides and lead chalcogenides, namely the lead chalcohalides. These materials, with general formula PbaEbXc ,(E=S, Se, Te, X=F, Cl, Br) are expected to show intermediate properties in between those of lead halides and chalcogenides, and more importantly might feature the chemical and structural compatibility needed to interface with both. We pioneered the investigation of lead chalcohalides at the nanoscale, discovering additional colloidal nanocrystals (NCs): the new compounds are semiconductors with a band gap in between those of lead halide perovskites and of lead sulfide, were obtained in relatively mild reaction conditions and feature a remarkable chemical stability. Moreover, we achieved the synthesis of colloidal heterostructures formed by epitaxially connected domains of Pb4S3Br2 sulfobromide and CsPbX3 perovskite, thus demonstrating a synthetic and structural compatibility between lead halide perovskites and lead chalcohalides. Stability of the system is highly increased respect to CsPbX3 NCs We take advantage of the domains? compatibility and exploit CsPbCl3 perovskite NCs as disposable and phase-selective epitaxial templates to drive the synthesis of lead sulfochloride NCs. As a result, we expand the family of lead chalcohalides with two new phases: Pb3S2Cl2 and Pb4S3Cl2.The perovskite domain is called a disposable template because, at a later stage, it can be etched from the heterostructures by exploiting the solubility of CsPbCl3 in polar solvents, while leaving the Pb4S3Cl2 domains intact. Hence, the full procedure delivered colloidally stable Pb4S3Cl2 NCs that could not be obtained by direct synthesis due to the competitive nucleation. Our use of perovskite NCs as disposable and phase-selective epitaxial templates parallels that of reaction-directing groups in traditional organic chemistry and catalysis. Such an approach to a deterministic synthesis of NCs might be extended to other pairs of materials with known or predictable epitaxial relations, taking advantage of the vast library of already reported nanomaterials as starting templates. This approach could open new routes for the colloidal syntheses of materials which are now hindered by an excessive activation energy for the homogeneous nucleation, or by the competitive formation of undesired phases.

Resume : Metal halide perovskite materials are currently of high interest due to their excellent electro-optical properties. Their high damage threshold makes them of special interest for high-fluence applications like lasing devices, and the possibility of vacuum deposition is promising for a future large-scale industrial production. When utilizing vacuum deposition for perovskite production, precise thickness control allows production of multiple quantum well (MQW) structures. Such structures allow for high charge carrier concentration and enhanced electron-hole recombination, and thus open up a great path towards high performance and tunable materials. Herer, we present a comprehensive study of the optical properties of vacuum-deposited CsPbBr3 perovskite MQWs with organic (TPBi) barrier layers. Blue shifts in absorption and emission spectra are observed with decreasing well width, as expected for quantum confinement. The shifts agree well with simulations of the confinement energies. The photoluminescence quantum yield increases by up to 32 times from bulk material to the thinnest well layers. Amplified spontaneous emission (ASE) measurements show very low thresholds down to 7.3 ?J cm-2 for a perovskite thickness of 8.7 nm, significantly lower than previously observed for CsPbBr3 thin-films. With their increased photoluminescence efficiency and low ASE thresholds, MQW structures with CsPbBr3 are excellent candidates for moderate efficient perovskite-based LEDs and lasers.

Resume : Radiation detection is of outmost importance in fundamental scientific research and applications including medical diagnostics, homeland security, environmental monitoring, and non-destructive inspection in industrial manufacturing. Lead halide perovskites (LHP) are rapidly emerging as high-Z materials for next generation of solution processable scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional scintillators, LHP urge to conjugate high scintillation yields with emission stability over prolonged exposure to high doses of ionizing radiation. To date, however, no definitive solution has been devised to suppress parasitic processes affecting the scintillation efficiency and kinetics and nothing is known of their radiation hardness for doses above a few kGy. Here, we demonstrate, for the first time, that CsPbBr3 nanocrystals (NCs) exhibit exceptional radiation hardness for 60Co ? radiation doses as high as 1 MGy. Side-by-side spectroscopic and radiometric experiments further highlight that, despite their defect tolerance, scintillators based on standard CsPbBr3 NCs suffer from electron trapping in highly dense surface traps. This limitation is effectively overcome through a post synthesis surface fluorination treatment resulting in over 500% enhancement of the scintillation efficiency which becomes comparable to commercial scintillator crystals, while still retaining exceptional levels of radiation harness. These results have profound implications for the widespread of LHPs in radiation detection schemes for high-energy physics, nuclear monitoring, nuclear batteries and space-grade solar cells where high radiation hardness is critical for successful and long-running operation, as well as for ultra-stable scintillators in medicine, environmental/industrial monitoring and border control.

Resume : Semiconductor titanium dioxide (TiO2) photocatalyst has ability of photocatalytic water splitting and can inactivate pathogenic microorganisms (viruses and bacteria) effectively. However, the large bandgap of TiO2 (ca. 3.0 eV) restricts its utilization of the whole solar spectrum. Recently, graphite-like carbon nitride g-C3N4, as a metal-free polymeric semiconductor with photoactivity in visible light, and a moderate band gap of 2.7 eV, has generated a lot of interest as photocatalytic material. In this work, we introduce a facile procedure to obtaining of g-C3N4/TiO2 binary composite films on titanium foil substrate by one-step CVD approach using melamine as precursor under air atmosphere. To produce partially oxidized product (TiO2) on the Ti surface we carried out the pyrolysis of precursor at the presence of a fixed volume of air. Deposited onto Ti surface the composite g-C3N4/TiO2 films were characterized by XRD, SEM, XPS and IR spectroscopy. It was found that the visible-light-induced photodegradation of methylene blue was remarkably increased upon coupling TiO2 with g-C3N4, possibly due to heterojunctions which enhanced electron-hole separation efficiency as a result of effective interfacial electron transfer between TiO2 and g-C3N4. The facile deposition method can be promising for the fabrication of efficient and low-cost photocatalyst based on g-C3N4/TiO2 composite films for microorganisms inactivation and H2-evolution by photocatalytic water splitting.

Resume : Silver nanowire electrodes (AgNWs) are increasingly studied in literature because of their excellent optoelectronic properties [1]. In addition, they are cheaper to manufacture compared to indium tin oxide (ITO) Electrodes [2]. They are therefore a promising alternative to ITO in many applications such as solar cells, OLEDs [3], touch screens, etc. In this work, the electrical and optical properties of AgNWs deposited on glass are investigated. We show that embedding AgNWs between two layers of ZnO nanoparticles (ZnONPs) leads to superior optical and electrical performances. As a result, we achieved a figure of merit (FoM) of 278 for ZnONPs/AgNWs/ZnOPs (ZAZ) electrode compared to 254 for ITO electrode. The integration of our ZAZ into an organic solar cell with PCE12:ITIC as active layer showed about 30% higher power conversion efficiency (PCE) than with an ITO electrode. References: [1] S. Yu et al., « Simultaneously improved conductivity and adhesion of flexible AgNW networks via a simple hot lamination process », Synth. Met., vol. 267, p. 116475, sept. 2020, doi: 10.1016/j.synthmet.2020.116475. [2] M. Lagrange, D. P. Langley, G. Giusti, C. Jiménez, Y. Bréchet, et D. Bellet, « Optimization of silver nanowire-based transparent electrodes: effects of density, size and thermal annealing », Nanoscale, vol. 7, no 41, p. 17410?17423, 2015, doi: 10.1039/C5NR04084A. [3] Y. Sun et al., « Flexible organic photovoltaics based on water-processed silver nanowire electrodes », Nat. Electron., vol. 2, no 11, p. 513?520, nov. 2019, doi: 10.1038/s41928-019-0315-1.

Resume : Lead Halide Perovskites (LHPs) have attracted attention over the last decades. However, Pb presents a great inconvenience in the development procedure of these materials, as it is highly toxic and contaminant. The Sn2 is the most obvious substitute for Pb because of the similar ionic radius and the similar electronic configuration to Pb2 , which makes it possible to form a perovskite with a formula ASnX3 similar to the lead-based counterparts. Precursor inks were prepared using commercially available precursors (DMSO and DMF) inside a nitrogen filled glovebox. In order to protect the precursor ink from oxidation (Sn2 ? Sn4 ) during handling and printing, an inert protecting agent was added on top of the precursor ink. The recipe for each perovskite composition was adopted from Jiang et al. [1] and modified to meet the inkjet printing requirements. The morphology of inkjet-printed films was characterized via SEM and optical microscopies to observe the uniformity of the films and the effect of printing different amounts of layers and different curing processes. This allowed obtaining the best conditions for producing pin-hole free layers. These results were combined with XPS measurements that allow characterizing the composition of the layers and XRD to determine crystalline structure and micro/nanocrystal size. Finally, optical analysis such as absorbance and PL (photoluminescence) spectra allows measuring band gap and confirm that the layers are optically active with their expected band gaps. [1] Jiang, X.; Wang, F.; Wei, Q.; Li, H.; Shang, Y.; Zhou, W.; Wang, C.; Cheng, P.; Chen, Q.; Chen, L.; Ning, Z., Ultra-high open-circuit voltage of tin perovskite solar cells via an electron transporting layer design. Nature Communications 2020, 11 (1).

Resume : Surface arrays of subwavelength photoactive structures have been demonstrated for photovoltaic (PV) applications with elevated omnidirectional and broadband absorption. However, this approach suffers from an increase in surface recombination and loss of photovoltage. Herein, surface decoration with subwavelength dielectric (SiO2) arrays that provide enhanced light trapping without compromising the photovoltage performance is proposed. An increase of
50% in broadband absorption is shown in an ultrathin film of 200 nm due to the presence of a top surface SiO2 nanopillar array in comparison with the same film decorated with an optimized antireflective coating of 50 nm of Si3N4. Interestingly, the broadband enhancement is not due to lower reflection, but rather the presence of the arrays forces a considerable lower transmission. The distribution of the optical power flux density suggests that the low transmission is due to appreciable refraction, which is induced by the presence of the dielectric arrays. Finally, the photovoltaic performance is examined for various array geometries and absorber acceptor concentrations and an overall increase of >60% in PV efficiency is calculated for a decorated photovoltaic cell in comparison with a PV cell with an antireflection coating of 50 nm Si3N4.

Resume : Mesocrystals are three-dimensional, macroscopic arrays of iso-oriented nanocrystals (NC). An open question is whether the structural order and orientation in such mesocrystals really matter in that they significantly change the optoelectronic properties of the array compared to a disordered ensemble of the same NCs? This presentation will detail how a combination of wide- and small angle X-ray scattering techniques in conjunction with X-ray cross-correlation analysis can answer this question. It is established that in ordered arrays of PbS quantum dots anisotropic charge transport prevails, which is attributed to the dominant effect of shortest interparticle distance. Similarly, it will be demonstrated that highly ordered micro-crystals of gold nanoclusters exhibit new optical transitions as well as a 100-fold increase in the charge carrier mobility in comparison to glassy, polycrystalline ensembles. A combination of laser scanning confocal microscopy and X-ray nanodiffraction reveals the impact of structural defects in caesium lead halide perovskite NC mesocrystals onto their collective fluorescence properties. These findings support the hypothesis that nanocrystals may be regarded as ?artificial atoms?, and that under certain conditions, analogies between atomic crystals and NC supercrystals may be drawn. The presentation will discuss how far these analogies can go and which potential applications arise from the exploitation of collective effects in mesocrystals.

Resume : Semiconductor Multi-Junction Solar Cells (MJSCs) hold the record power conversion efficiency among all photovoltaic technologies for 20 years, approaching 50% under concentration. The theory predicts that adding a 1 eV bandgap layer lattice-matched to GaAs/Ge to the standard triple-junction solar cell would allow the implementation of the optimum design and provide a significant efficiency improvement. We have recently proposed type-II GaAsSb/GaAsN Superlattices (SL) as an ideal pseudo-material for this purpose, since they allow overcoming the difficulties inherent to the growth of highly mismatched quaternary diluted nitride alloys and also provide long carrier lifetimes that result in enhanced collection efficiency. Indeed, we have demonstrated a 134% improvement in power conversion efficiency in a SL solar cell over the bulk counterparts. However, the strong Sb segregation produces a SL composition profile very different from the nominal design, leading to a far from an ideal potential profile that distorts the type-II nature of the structure. In this study, we introduce different growth strategies in order to avoid Sb segregation and have accurate control of the SL interfaces and potential profile. The samples have been grown by solid-source Molecular Beam Epitaxy. Detailed structural (X-Ray Diffraction and advanced STEM-related techniques) and optical (Photoluminescence (PL) as a function of temperature, time-resolved PL and Photoreflectance Spectroscopy) characterization has been performed. We have also fabricated SL solar cell devices and analysed them by means of I-V curves (under dark and AM1.5 conditions) and voltage-dependent Photocurrent Spectroscopy. The combination of all these techniques, together with finite differences calculations, allows us to correlate the microstructure of the superlattice in terms of interface abruptness and composition profiles with the electronic structure, radiative carrier lifetime and, ultimately, solar cell performance. EDX-TEM shows that the adequate growth strategies result in abrupt interfaces and a step-like potential profile, which also affects radiative carrier lifetime. All this has a strong impact on solar cell performance: monochromatic Voc and Jsc increase 47% and 155% respectively, compared to the reference sample without interface control strategies. Moreover, this effect is more relevant for narrow period samples, which are the critical ones for photovoltaic applications, since they allow the formation of minibands and improve carrier transport. We find a clear enhancement of solar cell performance as the period thickness is reduced, which allows reaching high external quantum efficiencies at 0 V in the 3 nm period sample with a narrow total SL thickness of 250 nm. This could be an important step towards the integration of a 1 eV GaAsSb/GaAsN superlattice sub-cell into MJSCs with the optimum structure.

Resume : The unique semiconducting properties of exfoliated transition metal dichalcogenides (TMDs) are of particular interest to researchers hoping to harness them for optoelectronic applications. 2D-TMDs are especially desirable for the tunability of their optoelectronic properties and their potential to revolutionize next-generation, ultrathin solar-energy conversion devices. However, a major challenge continues to be the large-scale production of these materials without compromising nanosheet quality[1]. To address this our group has developed solution-processable methods for the scalable fabrication of high-performance semiconducting TMD nanosheets dispersions and their processing into thin films with the aim of large-area solar energy conversion devices. Here we highlight our recent work developing a novel method for the powder-based exfoliation of TMDs using large molecule electrointercalation. The resulting nanosheet dispersion contains high yields of mono- to few-layer nanosheets with large lateral dimensions (>1 µm2). Using MoS2 as a model material, Impressive optoelectronic properties are demonstrated including tunable photoluminescence, high photocurrent densities (1.25 mA cm-2), and absorbed photon-to-current efficiencies as high as 90%, all without additional treatments or complex device structures. Not only do these nanosheet films outperform other conventional solution-processable methods, but they also strongly compete with chemical vapor (CVD) techniques. To examine the reason for their high-performance we use a recently developed, non-destructive fluorescence microscopy approach (2D-PAINT)[2] to estimate defect densities paired with atomic resolution scanning electron microscopy (STEM) using an integrated differential phase contrast (iDPC)[3] technique to precisely determine the defect types. These characterization techniques are thus demonstrated for the first time on solution-processed material and give fundamental insights into the origin of the promising optoelectronic properties showcased in this presentation. References: [1] Yu, X.; Sivula, K. ACS Energy Lett. 2016, 1 (1), 315?322. [2] Zhang, M.; Lihter, M.; Chen, T.-H.; Macha, M.; Rayabharam, A.; Banjac, K.; Zhao, Y.; Wang, Z.; Zhang, J.; Comtet, J.; Aluru, N. R.; Lingenfelder, M.; Kis, A.; Radenovic, ACS Nano 2021, 15 (4), 7168?7178. [3] Lazi?, I.; Bosch, E. G. T.; Lazar, S. Ultramicroscopy 2016, 160, 265?280.

Resume : Periodic arrays of nanoholes are patterned by electron beam lithography upon a 170 µm thick glass coverslip, covered by thin films of Indium Tin Oxide and PMMA resist. Colloidal semiconductor giant core/shell Cd/CdS nanocrystals, shielded by an additional silica layer, are trapped with remarkable efficiency into the PMMA holes by a capillary assembly method based on a finely tuned dropcasting process. The resulting arrays of semiconductor quantum dots demonstrate single photon emission properties studied in detail by photoluminesce and second order photon correlation measurements.

Resume : Due to high emission efficiencies and size-controlled emission wavelengths, colloidal quantum dots (QDs) are attractive materials for the realization of solution-processable laser diodes [1, 2]. In addition to facile spectral tunability, QD gain media benefit from a wide separation between their atomic-like states, which inhibits thermal depopulation of the band-edge ?emitting? levels and thereby reduces lasing thresholds and improves temperature stability [3]. Despite these advantageous features, colloidal QD lasers are yet to reach the stage of technologically viable devices. A primary obstacle is nonradiative Auger recombination, which leads to very fast relaxation of optical gain [4]. This represents an especially serious challenge in the case of inherently slow electrical pumping when multiexciton states, required for optical gain, are generated via step-by-step injection of individual carriers. Another complication is poor stability of QD solids under high current densities needed to enact the lasing effect. Here we are able to resolve these challenges and achieve broad-band optical gain spanning the band-edge (1S) and the higher-energy (1P) transitions. This demonstration is enabled by continuously graded QDs with strongly suppressed Auger recombination [5], incorporated into a current-focusing electroluminescent device driven by short electrical pulses. Using this approach, we achieve ultra-high current densities of >1000 A cm?2 and, as a result, boost device brightness to ~10 million cd m?2. Furthermore, we realize unusual two-band electroluminescence, in which the intensity of the higher-energy 1P feature exceeds that of the lowest-energy 1S band. This observation provides direct evidence for extremely high excitonic occupancies realized in our devices (up to ~8 excitons per dot) that are sufficient to achieve optical gain saturation for both 1S and 1P transitions. [1] Y.-S. Park, J. Roh, B.T. Diroll, R.D. Schaller, V.I. Klimov, Colloidal quantum dot lasers, Nature Reviews Materials 6, 382 (2021). [2] H. Jung, N. Ahn, V.I. Klimov, Prospects and challenges of colloidal quantum dot laser diodes, Nature Phot. 15, 643 (2021). [3] Y. Arakawa, H. Sakaki, Multidimensional Quantum Well Laser and Temperature-Dependence of Its Threshold Current, Appl. Phys. Lett. 40, 939 (1982). [4] V.I. Klimov, A.A. Mikhailovsky, A. Malko, J.A. Hollingsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Optical gain and stimulated emission in nanocrystal quantum dots, Science 290, 314 (2000). [5] J. Lim, Y.-S. Park, V.I. Klimov, Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection, Nature Mater. 17, 42 (2018).

Resume : CdSe nanoplatelets emitting in the blue region of the visible spectrum are promising candidates for light-amplification and light-emitting diode applications. For this reason, recently an improved synthesis protocol for 3.5 monolayer CdSe NPLs was put forward, leading to photoluminescence (PL) quantum yields up to 30%. [1] However, due to the high surface-to-volume ratio, blue emitting core-only nanoplatelets still suffer from charge trapping that results in intra-gap radiative emission from the defect states. As such, it remains an open question to which extent these defects affect their ultrafast properties as well, including the development of net stimulated emission. Here, we first show that optimized 3.5 ML CdSe nanoplatelets show optical gain between 480-520 nm due to stimulated emission along the biexciton-to-exciton transition.[2] Next, we compare the gain characteristic of core-only CdSe with core/crown CdSe/CdS 3.5 ML NPLs of increasing crown volume. The crowning procedure results in both a faster exciton radiative recombination rate and an improvement of the PL quantum yield up to 60%. Our results show that crowned samples exhibit overall a lower gain threshold with compared to core-only CdSe nanoplatelets. On the other hand, we observe comparable gain lifetime regardless of the crowning procedure due to residual ultrafast charge trapping not alleviated by the crown growth. Our results pave the way towards accurate design of ultra-thin quasi two-dimensional systems for blue spectrum light amplifiers and lasers based on. [1] Di Giacomo, A.; Rodà, C.; Khan, A. H.; Moreels, I. Colloidal Synthesis of Laterally Confined Blue-Emitting 3.5 Monolayer CdSe Nanoplatelets. Chem. Mater. 2020, 32, 9260-9267. [2] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644.

Resume : In this work we show that thin polycrystalline films of spin-coated formamidinium tin triiodide (FASnI3) perovskite demonstrate efficient amplification of the spontaneous emission (ASE) and random lasing (RL). ASE is achieved under relatively low excitation fluence threshold (? 2 and ? 25 microJ/cm2 for 15 and 300 K, respectively) in backscattering geometry. When a thin film of FASnI3 is integrated forming optical waveguides (rigid: Si/SiO2/FASnI3/PMMA or flexible: PET/FASnI3/PMMA), this threshold can be further reduced by one order of magnitude, because of the strong electromagnetic field confinement and high gain of the active material. Just above the ASE threshold we simultaneously observed RL effect, which is characterized by a high mode stability and very high-quality factor, up to 10000. This mode stability is a unique property, which is observed here for the first time, up to our knowledge, in a polycrystalline semiconductor system. The origin of RL in FASnI3 can be the high efficiency of light scattering by grains of the film, which is also a result of the high refractive index of the material (higher than in the case of Pb-based halide perovskites), as well as an optimized grain size distribution. In fact, the average size of grains is in the order of the emission wavelength, 890 nm at room temperature. We will also show the different excitation conditions required for generation of narrow RL lines and broader ASE emission. Finally, the operation of flexible (PET substrate) optical waveguides is demonstrated exhibiting ASE and RL lines that disappear under bending conditions. This work would be the base of future flexible and ecofriendly photonic devices, within the objectives of H2020 FET-OPEN project DROP-IT (contract no. 862656).

Resume : The precise control of doping and hetero-interface in colloidal semiconductor nanocrystals (CNCs) or quantum dots (QDs), is very important for the efficient energy or charge transfer through hetero-interface and then their new energy, biological, and new photocatalysis applications. Due to the self-purification effect, deep-position hetero-valent doping in CNCs (or QDs), the growth of monocrystalline semiconductor based metal/semiconductor hybrid nanocrystals (core/shell and heterodimer) under large lattice mismatch with modulated composition, morphology and interface strain are prerequisites for flexible control of electronic impurities, plasmon-exciton coupling, and then efficient electron/hole separation. By putting forward new reverse cation exchange strategy, we realized the heterovalent doping and nanoscale monocrystalline growth of the semiconductor shell on metal building blocks. These controls enable the fine tuning of doped level, plasmon-exciton coupling. Then revolutionary plasmon enhanced photocatalytic, PEC performance and also enhanced photo thermal, photoelectrical applications have been realized.

Resume : Si-nanocrystals (Si-NCs) have gained an increasing interest during the last decades due to their potential applications in various fields such as solar cells, plasmonics and optoelectronics. Doping Si-NCs with either positively or negatively charged impurities opens up a new road to further tailor their electrical and optoelectronic properties. Although high formation energies were predicted for substitutional impurities in semiconductor nanocrystals, phosphorus atoms are nevertheless found to be preferentially localized in Si-NCs rather than in the surrounding silicon dioxide matrix. Despite several published works, many questions still remain open in particular regarding the correlation between the localization of dopants and the optoelectronic properties of doped Si-NCs. In this contribution, we discuss the influence of phosphorus on the optical properties of Si-NCs formed in P-doped SiO/SiO2 multilayers in relation with the localization of the P atoms. The multilayers were obtained by alternating evaporation of SiO and SiO2 from e-beam guns. P atoms were supplied by a GaP decomposition source. After growth at room temperature, the samples were annealed at various temperatures up to 1100°C. The localization of P atoms at the nanoscale was investigated by means of scanning transmission electron microscopy (STEM), STEM-electron energy loss spectroscopy (STEM-EELS) and atom probe tomography (APT). Steady state photoluminescence (PL) measurements were performed both at room temperature and at low temperatures by using a 325 nm He-Cd laser as excitation. The insertion of P atoms is found to promote the phase separation during the post-growth annealing thus leading to nanocrystal formation at lower temperatures as compared to undoped Si-NCs [1]. Consequently, the Si-NC related PL intensity reaches a maximum for annealing temperatures lower than 900°C. Moreover, in the presence of P atoms, the Si-NC size increases thus leading to a redshift of the Si-NC related PL. For low P contents (< 0.1 at. %), the Si-NC related PL signal increased significantly, which is explained by the P-induced reduction of the number of defects at the interface between Si-NCs and the surrounding SiO2 matrix. For larger P contents, the Si-NC related PL decreases rapidly. This observation is interpreted by the formation of Si-NCs with sizes exceeding the critical size for quantum confinement and by the localization of P atoms in the Si-NCs, which act as non-radiative centers. Our work, which is important for practical applications, helps to get a better understanding of the doping at the nanometer scale. [1] F. Trad, A. E. Giba, X. Devaux, M. Stoffel, D. Zhigunov, A. Bouché, S. Geiskopf, R. Demoulin, P. Pareige, E. Talbot, M. Vergnat, H. Rinnert, Nanoscale 13, 19617 (2021)

Resume : Arising from the past decade, increasing interests have been devoted to synthetize and study nanoparticles exhibiting localized surface plasmon resonance (LSPR) at their surface, thus responsible for unique optoelectronic properties. At halfway through semiconductors and metals, doped Zinc Oxide nanoparticles exhibit a high surface density of free-charge-carriers specifically responsible for tunable infrared (IR) and visible absorption properties. In this work, we present an easy, fast and reliable one-pot synthesis of three Doped Zinc Oxide nanoparticles (NP) with either Al, Ga or In as doping atoms. The synthesis process is adapted from the method originally described by E. D. Gaspera and coworkers [1] and is performed in a non-polar media. After a quick purification step, the stability of NPs remained for several months in diverse apolar mediums. Each doping atom (or even mixtures) involved diverse shapes, sizes and finally led to various optoelectronic behaviors. We identified that, while the NPs exhibit the same crystalline structure in hexagonal-Wurtzite configuration, they also shown preferred crystal growth orientations, mostly depending on the doping atom introduced. Indeed, various shapes have been identified through SEM/TEM measurements such as reverse-bowties, mushrooms, spheroids, or even heart-like structures. Thus, the specific IR absorption from each of these structures can vary due many parameters. Either intrinsic, such as the different free-charge-carrier densities generated at the particle?s surface, or extrinsic like the solvent surrounding. Further analysis has also been performed to understand deeply the final NPs shapes and the relationship between doping levels measured by ICP-AES (up to 20%) and growth mechanisms identified by TEM imaging analysis. We also report a typical decreasing in size tendency (respectively from Al to In doping atoms), arising from ionic radius compatibility. Such effect, previously described in the literature [2], has been confirmed by XRD and TEM-SEM imaging analysis. Regarding applicative purposes, their colloidal stability in non-polar mediums allow good processability to build IR optoelectronic devices. In fact, liquid-liquid transfers can easily be achieved in typical electrophoretic ink formulations while maintaining a good NPs stability. By formulating such e-ink embedded in dedicated electrophoretic pixels, binary switches can be achieved between reflective and absorbent states. Thus leading to build IR sensitive screens for final applications, such as smart windows, satellite thermo-regulation, war theater applications and much more. References: [1] E. Della Gaspera, A. S. R. Chesman, J. van Embden, J. J. Jasieniak, Non-injection Synthesis of Doped Zinc Oxide Plasmonic Nanocrystals, ACS Nano, vol. 8, pp. 9154?9163, (2014) [2] R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A, vol. 32, pp. 751?767, (1976)

Resume : Temperature-dependent photoluminescence (PL) measurements under different excitation densities were performed on self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy. Non-monotonic evolutions in the PL peak energy and integrated PL intensity curves were observed in the low-temperature region. In fact, the PL peak energy evolution of the QDs emission shows a sigmoidal feature with increasing temperature. This component is accompanied by an unexpected anomalous increase in the integrated intensity over the temperature range of 8 to 90K. These behaviors were attributed to the emission from deep states that exist in the potential of the dots. A very simple rate-equations model, which describes the thermally activated emission and trapping of photo-injected carriers, is proposed to illustrate the made interpretation for the evolution of the integrated PL intensity as a function of the temperature. A good agreement between the model simulation of the integrated PL intensity and the experimental results was obtained for temperatures ranging from 8 to 300K. It was found that the intra-dot thermal transport of excitons is responsible for the abnormal changes encountered in the dots' photoluminescence when the temperature is increased.

Resume : The detection of high-energy radiation has gained relevant interest thanks to its application in technologic-relevant fields, such as particle physics, astronomy, geology, medical diagnostics, nuclear monitoring and space exploration. In all these areas, the most widely used detectors are scintillating materials that convert the energy deposited by incoming ionizing radiation into visible photons which is finally revealed by coupled photodiodes. Beside typical inorganic materials based on high-Z elements, an alternative class of scintillators which further widens their applicability is plastic scintillators, which exhibit the advantages of a large sizes production, low weight and affordable costs making them particularly adapt for radiation monitors in border and industrial control. Despite these advantages, the low density of plastic materials compared to inorganic crystals limits their interaction with ionizing radiation and typically requires doping these systems with high-Z components, such as organometallic complexes, perovskite or heavy metal chalcogenides nanocrystals (NCs). However, the intrinsic small Stokes shift of these materials represents an issue when used as nanoscintillators in highly dense or large volume detectors because of the strong reabsorption of the scintillation light along its path to the waveguide edges. We aim to takle this issue by developing a new strategy of scintillator consisting in a polymeric scintillating matrix incorporating reabsorption free Cd0.5Zn0.5S/ZnS core/shell NCs doped with manganese ions. Doping NCs with Mn is an established approach to activate efficient luminescence at intragap energy arising from the 4T1 ?6A1 optical transition, yielding the characteristic Mn-emission at ~ 580-600 nm. Crucially, since such a transition is spin-forbidden, the corresponding optical absorption features negligible oscillation strength, resulting in an apparent Stokes shift between the band-edge absorption of the NCs host and the Mn-related luminescence. In our approach, we further adopted a synergic strategy in which both the plastic matrix waveguide and the NCs interact with incoming ionizing radiation, while the propagating emission is generated by the sole NCs, whose optical properties have been properly engineered to efficiently down-convert matrix emission. The emission efficiency and compatibility of the NCs with the polymer host have been optimized resulting in high optical quality nanocomposites completely transparent in the spectral region of their own emission, with scintillation efficiency comparable to commercial plastic scintillators.

Resume : Creating a p-n junction is an essential topic in material science for most semiconductor devices fabrication, which has excellent prospective in applications for next-generation integrated circuits and numerous optoelectronic devices. Therefore, a facile and straightforward methodology for building a p-n interface is particularly promising. After discovering graphene, layered two-dimensional materials (2D) family, and molybdenum sulfide (MoS2) in specific, have attracted a great deal of research interest and showed emerging physical properties. The top surface of MoS2 is free of dangling bonds that can artificially allow forming a heterojunction with other 2D crystals. A unique integration of 2D materials facilitates novel device structures, particularly in optoelectronics such as photodetectors, photovoltaics, light-emitting diodes, and lasers. In this work, we focus on developing a reliable approach to form a p-n junction on MoS2 via a combination of a two-step approach and an oxide deposition. A continuous MoS2 layer was fabricated through a laser-assisted chemical vapor deposition process followed by patterned deposition of molybdenum oxide (MoOx) on top of MoS2. The MoOx-MoS2 heterostructure exhibits enhanced photoluminescence confirming p-type doping due to an efficient electron transfer from MoS2 to MoOx. The p-type doping is also confirmed using X-ray photoelectron spectroscopy (XPS). Moreover, Raman spectroscopy reveals a substantial shift in-plane (A1g) and out-of-plane (E2g) Raman peaks within the MoOx-MoS2 heterostructure compared to PLD-CVD grown MoS2. Surprisingly, our data indicate both doping and stress/strain induced in the heterostructure region. Moreover, Fermi level pinning in the interface region of MoOx-MoS2 heterostructure will be analyzed under a varying gate and source-drain voltage in fabricated thin-film field-effect transistors with different thicknesses of MoOx layer deposited. Additionally, the photovoltaic and photodetector performances will be explored. This p-n heterostructure with a large surface area creates a solid optical absorption due to modulated band structure in the p-n junction area by band bending. This allows strong absorption of visible-light photons and numerous photo-generated electron-hole pairs with an efficient transfer of charge carriers. This offers a new path to ease the integration of p-n junctions on a larger area for enhanced optoelectronic device performances in both rigid and flexible substrates.

Resume : The radiative recombination mechanisms of the Green InGaN/GaN light-emitting Diode (LEDs) with regarded silicon doping short-period InGaN/GaN GSL superlattice were studied by using the photoluminescence spectroscopy. The exciton-localization effect (ELE) and quantum-confined Stark effect (QSCE) on the performance have been investigated by means of temperature-dependent and excitation power-dependent photoluminescence measurements. The ELE and QSCE should be highly considered, QSCE should be further reduced and ELE should be an enhancement. The power dependence of the PL peak energy and linewidth indicates that the emission process of the MQW is dominated by the Coulomb screening effect and then by localized states at low temperatures. On the other hand, the nonradiative centers are thermally active in low excitation power at room temperature. The peak energy and linewidth show that the radiative recombination is dominated at low temperature and nonradiative recombination at room temperature. The IQE results also confirmed the emission mechanism at low and room temperature. The measured HRXRD shows the low strain and electric field in the structure, also, the power dependence peak energy as exciton power shows the low QSCE. The GSL in the structure was claimed to be a result of reduced strain-induced electric fields across the QWs, meaning that less charge carrier screening of these electric fields occurred. The temperature dependence results show the anomalous behavior for the peak energy and linewidth. The S and W shape temperature dependences of the emission energy and linewidth indicate the conversion of the carrier transferring mechanism from nonthermalized to the thermalized distribution of localized carriers, and finally to the regular thermalization of the carriers. And for very high power, the localization effect will completely disappear and the emission peak energy follows the Varshini law. This means that by increasing the power, the localization effect is reduced. Increment of the W-shaped temperature-dependent means the density of states exponentially increased with energy in the band tail. For achieving a high localization degree, high thermal energy and low excitation power are needed. Under low excitation power, the QSCE will be high, while the GSL layer decreased that and enhance the localization degree and output of the LED. For achieving a higher degree of localization, more thermal energy is required.

Resume : Nanostructured Si on silicon nitride films has attracted a significant research attention due to its potential application in tunable emission of photons, photoconductivity and photovoltaic applications. Silicon-rich silicon nitride (SRSN) films having two different compositions were irradiated with 100 MeV Ni7+ ions at fluences 5 X 1012 ions/cm2 and 1X1014 ions/cm2. The films, despite having different compositions, show similar microstructural evolution as evidenced in cross sectional transmission electron microscopy (XTEM). Discontinuous tracks, (~2 nm wide) appear at lower fluence which overlap and dissolve at an increased fluence. The corresponding changes in photoluminescence (PL) of the films is investigated with three lasers with the aim of tracking the evolution of different radiative processes in SRSN with ion fluence. The evolution of PL with fluence is well correlated with the microstructural evolution. Results are understood on the basis of thermal spike model of ion-materials interaction. The photovoltaic applications of these SRSN layers are highlighted at the end.

Resume : Solution Processed CMOS compatible Infrared Optoelectronics is a key enabling technology to revolutionize consumer electronic markets offering unprecedented opportunities for low-cost food quality inspection, environmental monitoring, 3D imaging, automotive safety and night vision applications just to name a few. Recent progress in CMOS compatible CQD photodetectors have addressed the InGaAs image sensor challenge and the next step would be the development of the correspondingly low-cost tunable light sources. In this talk, I will be presenting recent results from my lab at ICFO on highly performant infrared CQD LEDs and downconverting light emitters. I will present our device architecture approach that allowed us to achieve very high PLQY in conductive solid state QD films that when implemented in a LED stack led to 8% EQE [1]. I will then discuss the optimization of the matrix supply dots that improved charge balance and allowed to reach 8% EQE at high radiance along with a stark improvement in operational stability [2]. I will then elaborate on fine-tuning the energetic potential landscape in the matrix which taken together with optimized optical out-coupling schemes reached a QE of 18% at 1550 nm. The possibility to tune the light spectrum of such QD films by stacking layers of QDs with different bandgaps offers exquisite control over the emission spectrum offering the opportunity to develop solid-state thin film broadband emitters in the SWIR, either optically or electrically excited with implications in SWIR spectroscopy [3]. The second part of my talk will discuss recent progress on infrared CQD lasers comprising doped PbS CQDs integrated in a DFB cavity [4]. Besides this I will present our approach of elongating Auger lifetime by engineering QD solids at the supra-nanocrystalline level offering lasers with improved optical linewidths, reduced thresholds and drastically improved stability [5]. The last part of my talk, if time allows, will be devoted to a different line of research on how to tune the optical properties of nanocrystals by engineering their atomic configuration. I will show an example of AgBiS2 NCs whereby cation disorder has been homogenized yielding a material with very high optical absorption coefficient. This material has further been optimized in a solar cell stack leading to a record high PCE of 9% for an extremely thin absorber cell of only 35 nm. [6] References: [1] High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level, Nature nanotechnology 14 (1), 72-79, 2019 [2] Highly Efficient, Bright, and Stable Colloidal Quantum Dot Short?Wave Infrared Light?Emitting Diodes, Advanced Functional Materials 30 (39), 2004445, 2020 [3] Solid?State Thin?Film Broadband Short?Wave Infrared Light Emitters, Advanced Materials 32 (45), 2003830, 2020 [4] Solution-processed PbS quantum dot infrared laser with room-temperature tunable emission in the optical telecommunications window, Nature Photonics 15 (10), 738-742, 2021 [5] Low?Threshold, Highly Stable Colloidal Quantum Dot Short?Wave Infrared Laser enabled by Suppression of Trap?Assisted Auger Recombination, Advanced Materials 34 (3), 2107532, 2022 [6] Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells. Nat. Photon. (2022). https://doi.org/10.1038/s41566-021-00950-4

Resume : Realization of electrically pumped solution-processed infrared lasers will be a major breakthrough in a broad range of applications including optical communication, LIDAR and silicon photonics. For attaining this goal, Pb-chalcogenide colloidal semiconductor quantum dots (CQDs) are attractive material owing to their size-controllable excitonic properties and CMOS technology compatibility. Recently solution-processed infrared lasers have been shown in undoped and doped PbS CQDs which however so far optically pumping. [1-3] Demonstration of real lasing action under electrical pumping entails resolving several fundamental challenges including (1) realization of stimulated emission with ultrathin CQD active medium to enable efficient electrical injection, (2) overcoming the issue of optical gain suppression by conductive layer and (3) demonstration of stimulated emission in a real LED device. [4] In this work, we successfully address the aforementioned challenges by reporting stimulated emission in extremely thin film of PbS CQDs within a thickness of 16 nm (two monolayers of CQDs) at a wavelength of 1675 nm. We achieved stimulated emission beyond the slab waveguide theoretical limit by introducing ZnO nanocrystals (NCs) as nanoscatterers which mixed with PbS CQDs as a gain material. The formed nano-sized clusters allow us to trap the stimulated emission at the surface of the devices. [5] In addition, employing PbS/ZnO blend as a gain media leads to suppress Auger process significantly as well as lowering lasing thresholds. [3] Finally, we utilized such thin active media in a full stack LED structure operating as an electroluminescence (EL) device, in which an engineered ITO (E-ITO) transparent conductive oxide and a single layer of graphene act as cathode and anode, respectively. Employing E-ITO and graphene as the electrical contacts led us to overcome the existing challenge underpinned by the optical losses of the conventional metal contacts that have inhibited the realization of stimulated emission in a real LED device. The present study suggests that the demonstrated dual function (EL/lasing) devices pave the way to achieve the ?Holly Grail? in the CQDs field, which is the realization of CQDs laser diodes. [5] References: [1] S. Christodoulou et al., Nano Letters 2020, 20, 5909-5915. [2] G. L. Whitworth et al., Nature Photonics 2021, 15, 738-742. [3] N. Taghipour et al., Advanced Materials 2021, 2107532. [4] J. Roh et al., Nature Communication 2020, 11:271. [5] N. Taghipour et al., under review.

Resume : Colloidal quantum dots (CQD) have attracted lot of interest in the fabrication of optoelectronic devices including light sources, solar cells and photodetectors, due to their strong quantum confinement that provides unique optical properties such as increased absorption and emission as well as size tunability. In addition, thanks to their large effective surface-to-volume ratio and outstanding surface reactivity, combined with surface chemistry and processability, CQD have been recently proposed for gas sensing applications, as well. In this work we report on a compact optical sensor for nitroaromatic explosive (NE) detection based on PbS CQD. The trace detection of explosives is still a very challenging issue due to their typical low vapor pressure. Existing instruments and procedures are expensive and require trained personnel, therefore, there is a strong demand for portable devices suitable for both security and military applications. Here we propose and demonstrate a sensor for NE based on the measurement of the photoluminescence quenching produced by exposing PbS nanoparticles. The sensor consists of a silicon substrate with both surfaces provided with a quantum dot film. The upper suitably functionalized CQD layer acts as photoluminescent probe, pumped by a pulsed blue LED. The change of photoluminescent intensity associated to the interaction between the PbS CQD and the analyte is measured by the PbS CQD photodetector fabricated on the bottom. The silicon substrate blocks the pump while it is transparent to the light emitted by the upper CQD layer whose nanoparticles have been sized to emit in the near infrared. The sensor is mounted into a small chamber provided with the light emitting diode and the front-end electronics. We report on the sensor fabrication and characterization in terms of sensitivity, response time and selectivity, and discuss the proposed sensing mechanism. Due to its sensitivity, simple architecture, and small footprint, we believe the proposed device may have great potential for practical applications as either portable device or element of sensor networks.

Resume : Group IV nanocrystals (NCs) are of high interest for photonics applications, the ultimate goal being the integrated group IV photonics. In particular, intense research has been focused on photocurrent enhancement of nanocrystals / quantum dots from the Si?Ge system, and photodetector applications [1?5]. One well known issue related to Si?Ge system is to improve the light absorption-emission efficiency. This is achievable by nanostructuring (quantum confinement) combined with strain or by crystallization in metastable hexagonal phase instead of diamond one. Here, we combine alloying of Si with higher proportion Ge and nanostructuring by Ge-rich SiGe NCs formation with strain induced by embedding SiGe NCs in nanocrystallized HfO2 matrix at the same time ensuring high-quality SiGe NC/HfO2 matrix interface [6]. This leads to obtaining a high efficiency of photocurrent in a broad spectral range of 600?2000 nm exceeding the sensitivity limit for Ge. For this, we deposit 3-layer stacks of cap HfO2/ SiGeHfO2 active layer/ buffer HfO2/ on Si by magnetron sputtering. The thick intermediate layer with photoactive role is sandwiched by thin adjacent HfO2 layers that play together with Si alloying the role of preventing the fast diffusion of Ge from the active layer. After deposition, rapid thermal annealing at 600 °C was performed for films nanostructuring, i.e. formation of Ge-rich SiGe NCs with 3?7 nm diameter in nanocrystallized matrix of HfO2 with tetragonal/orthorhombic structure. HRTEM, XRD and Raman spectroscopy were employed for investigating crystalline structure, morphology and composition, and the results were correlated with the spectral photocurrent efficiency measured at 100 and 300 K. The obtained high efficiency photocurrent with SWIR-extended cut-off wavelength (~2000 nm for 100 K cooled structures) is due to photocarrier generation in the heterojunction of embedded Ge-rich SiGe NCs and Si substrate. The strain of ~0.6% and the high-quality of the interface with HfO2 of the Ge-rich SiGe NCs can explain the IR extension and enhancement of the photosensitivity. The promising results can have multiple applications such as monitoring of slippery road conditions and biomedical sensing applications. [1] S. Shi, A. Zaslavsky, D. Pacifici, Appl. Phys. Lett. 117, 251105 (2020) [2] I. Stavarache, C. Logofatu, M.T. Sultan, A. Manolescu, H.G. Svavarsson, V.S. Teodorescu, M.L. Ciurea, Sci. Rep. 10, 3252 (2020) [3] V. Dhyani, G. Ahmad, N. Kumar, S. Das, IEEE T. Electron. Dev. 67, 558 (2020) [4] A.M. Lepadatu, C. Palade, A. Slav, O. Cojocaru, V.A. Maraloiu, S. Iftimie, F. Comanescu, A. Dinescu, V.S. Teodorescu, T. Stoica, M.L. Ciurea, J. Phys. Chem. C 124, 25043 (2020) [5] A.M. Lepadatu, A. Slav, C. Palade, I. Dascalescu, M. Enculescu, S. Iftimie, S. Lazanu, V.S. Teodorescu, M.L. Ciurea, T. Stoica, Sci. Rep. 8, 4898 (2018) [6] C. Palade, A.M. Lepadatu, A. Slav, V.S. Teodorescu, T. Stoica, M.L. Ciurea, D. Ursutiu, C. Samoila, Materials 14, 7040 (2021)

Resume : Nowadays, modern types of semiconductor solar cells (SC) like silicon single-junction one and multi-junction ones based on the III-V compounds have almost reached their theoretical limit of efficiency. Therefore, new devices based on 1D materials (nanowires, nanorods etc.) are considered as perspective concepts for the production of SC with higher efficiency. Its performance depends on the technology of nanowire fabrication, but epitaxial growth methods are expensive and complicated for industry, and metal-assisted chemical etching requires catalysts leading to arising of defects (non-radiative recombination centers). Recently, a novel way of dry etching was successfully applied to the fabrication of a homogeneous array of silicon nanowires (SiNWs), so-called ?black silicon?. However, the radiation defects created in SiNWs and at the interface due to plasma influence are important issues for the photovoltaic performance of SCs. Therefore, in this work, photoelectrical and capacitance properties of heterojunction a-Si:H/c-Si grown on SiNWs array will be explored for different dry etching processes. Conventional RIE and cryogenic [1] processes will be considered. Arrays of SiNWs with a diameter of 1.3-1.7 µm and height of 5-6 µm were fabricated by cryogenic dry etching of silicon wafer with a conductivity of 0.06 and 0.2 ??cm. Then, solar cells were created based on the silicon heterojunction solar cell structure: thin layers of (i) a-Si:H and (p) a-Si:H layers were deposited by the PECVD technique, an ohmic contact was fabricated on the rear side of the wafer, and ITO layer was sputtered on the front side as a transparent and conductive layer. In result, the sample on 0.2 ??cm wafer has better performance resulting Jsc = 30 mA/cm2 and Voc= 0.50 V than with 0.06 ??cm - 27 mA/cm2 and 0.48 V at a temperature of 25 º? under the AM1.5G condition. The difference is in the properties of the (i) a-Si:H/(n) Si interfaces, which were studied by trap density analysis with capacitance methods. Responses from interface states at the (i) a-Si:H/(n) Si heterointerface structures on both wafers were detected by admittance spectroscopy. It is associated with the exchange of charge carriers between the Fermi level and the valence band, and the energy and concentration are higher in the more doped substrate. Measurements by deep-level transient spectroscopy confirmed these conclusions, revealing a response for minority charge carriers with energies of 0.1 eV and 0.18 eV for 0.2 ??cm and 0.06 ??cm, respectively, with higher concentration in the second one. Further, estimates of experimental parameters were added in computer simulation of these structures, and higher interface defect concentration and less band bending near the heterointerface in 0.06 ??cm Si wafer led to lower photoelectric parameters. The results for a-Si:H/c-Si heterojunctions based on SiNWs formed by conventional RIE will be presented and discussed in detail. [1] Morozov I, Gudovskikh A, Uvarov A, Baranov A, Sivakov V and Kudryashov D 2020 Phys. status solidi 217 1900535

Resume : The controlled insertion of electronic states within the band gap of semiconductor nanocrystals (NCs) is a powerful tool which enables to engineer their physical properties. One compelling example is represented by metal chalcogenide NCs incorporating heterovalent p-type impurities such as d10 coinage metals (Cu+, Ag+, or Au+). In this type of nanostructures the interplay between the dopant and the host semiconductor energy levels leads in the ultra-fast capture of the hole by the localized state of the dopant leading to the so-called ?acceptor-bound? excitons. The control of this paradigm unlocked technologically relevant functionalities, such as Stokes-shift between the emission and the absorptionspectra, extended luminescence lifetimes, photomagnetic behaviors, and enhanced electrical transport. To date, although conceptually analogous to hole-management schemes, the opposite ?donor-bound? exciton scheme using aliovalent elements adopted to n-doped NCs (e.g., Al3+ and In3+ in) has not been realized due to the natural propensity of such cations to produce shallow donor states that inject electrons directly in the CB. Here, we exploit the propensity of metal sulfides to present sulfur vacancies (VS) that introduce a localized level pinned about 1 eV below the CB to produce a model system for ?donor-bound? excitons in CdSeS NCs. The investigation of the optical and magneto-optical properties of these NCs revealed that the VS state is responsible for the ultrafast capture of electron in the CB which can then either decay non-radiatively or recombine with the VB photohole leading to long-lived, Stokes-shifted emission with size-tunable energy. Moreover, VS-localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the VS are not magnetically coupled to the NC bands and that the polarization properties are determined by the spin of the valence-band photo-hole, whose spin flip is massively slowed down due to suppressed exchange interaction with the donor localized electron.

Resume : In the last years, there has been an increasing attention on Lead Halide Perovskites (LHPs) because of their potential applications in optoelectronics devices. Nevertheless, Pb is a harmful element, highly toxic, that is desirable to be substitute. Bering in mind this important bearing this strong inconvenience, Sn2+ has been proposed as one of the possible substitutes of Pb because of their similar ionic radius and electronic configuration, allowing keeping the same configuration and stoichiometry than lead-based counterparts (ASnX3). In addition, 2D halide perovskites have become a striking research spotlight. It is noteworthy that low-dimensional Sn2+-based halide perovskites exhibit remarkably enhanced air stability in comparison with their 3D counterparts. In this work, the stability and emission properties of single-layer 2D tin perovskite nanoplates fabricated by inkjet printing with chemical formula TEA2SnI4 (TEA=2-thiophene-ethylammonium) is reported. An ink solution of TEA2SnI4 based on molecular precursors dissolved on DMSO has been prepared for inkjet printing technology. Successful fabrication of thin films of TEA2SnI4 has been achieve by using those inks, by modifying inkjet printing parameters and post fabrication curing processes, Structural characterization performed by SEM and optical microscopy allowed determining the good uniformity of the films (uniform and pin-hole free) and monitoring the influence on the final film on printing different number of layers and curing processes. XPS measurements confirmed the nominal stoichiometry of the compound, while XRD was employed to corroborate their tetragonal structure. Finally, optical analysis through absorption and emission measurements has revealed an optical band gap energy of 1.89 eV and a narrow emission at 1.95 eV.

Resume : Nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices due to its high hole mobility, good environmental stability and low fabrication cost. In this research NiOx thin film and nanoporous layer (NPL) were utilized as the HIL for the fabrication of quantum dot light-emitting diodes (QLEDs). The obtained NiOx NPLs have sponge-like nanostructures that possess larger surface area to enhance carrier injection and to lower turn-on voltage of devices, as compared with the NiOx thin film. The energy levels of NiOx were slightly downshifted by incorporating nanoporous structure from UPS analysis. The amount of Ni2O3 species is higher than NiO in the NiOx NPL from XPS experiments, confirming its good hole transport ability. The best QLED was achieved with 30 nm-thick NiOx NPL, revealing a max brightness of 68,646 cd/m2, a max current efficiency of 7.60 cd/A, and a low turn-on voltage of 3.4 V. More balanced carrier transport from NiOx NPL and ZnO NPs/PEIE is responsible for the improved device performance.

Resume : The development of hole transport layers (HTL) that elevate charge extraction, improve perovskite crystallinity and decrease interfacial recombination is extremely important for performance enhancement of inverted perovskite solar cells (PSCs). In this research nickel oxide (NiOx) nanoporous layer as well as NiOx thin film was prepared via chemical bath deposition to serve as the HTL. The sponge-like nanoporous NiOx helps to grow pinhole-free perovskite film with larger grain size compared to the NiOx thin film. Furthermore, the downshifted valence band of the nanoporous NiOx HTL is beneficial to improve hole extraction from the perovskite absorbing layer. The optimized PSC using nanoporous NiOx HTL showed the highest efficiency of 13.43% and negligible hysteresis that was better than the one using the NiOx thin film as the HTL. Besides, the PSC based on NiOx nanoporous layer sustained 80% of their initial efficiency after 50 days storage in the dark ambient environment. The results provide a powerful strategy to construct PSCs with long-term stability for future production.

Resume : IV-VI semiconductor quantum dots-doped SiO2-P2O5-Al2O3 thin films were synthesized by sol-gel method, spin coating technique. Various planar substrates were used for deposition such as glass, ITO (indium tin oxide) and silicon, respectively. Precursor sols composition, gelation time, substrate rotation rate, number of deposited layers and pH of the precursor sols were changed aiming to adjust the hydrolysis and condensation chemical reactions, specific to the gelification process, leading to uniform and the homogeneous thin films. As precursors, the following reagents were used: tetraethyl orthosilicate for SiO2, triethyl phosphate for P2O5 and aluminium acetylacetonate for Al2O3. The final nanostructured materials were prepared by drying and subsequent annealing of the deposited films, in vacuum atmosphere. The influence of the chemical composition of the precursor solution and the substrate type on UV-VIS-NIR transmittance/absorbance/reflectance was investigated. FTIR (Fourier Transform Infrared) spectroscopy showed optical phonons specific to SiO2, P2O5 and semiconductor dopant. The specific vibration modes evidenced the vitreous network forming role of SiO2 and P2O5. SEM (Scanning Electron Microscopy) measurements in cross section presented the thickness uniformity as well as the morphology of the deposited layers. The thickness and the adhesion of the deposited films on substrates are in direct dependence on the chemical composition of the precursor sols and the substrate type. Excitation spectra revealed the exciton peaks specific to the semiconductor dopant, used as excitation wavelengths. Besides excitonic peaks wavelengths, various excitation wavelengths from the fundamental absorption domain were used aiming to choose the performing materials regarding the photoluminescence characteristics in the NIR domain. Related measurements such as luminescence decay, luminescence lifetime, quantum efficiency were performed showing the potential application of these nanostructured materials for temperature detection.

Resume : Communications within personal working/living spaces are highly demanded. Multi-device connectivity can tell users, from any device, where they are, where they need to be and what they need to do when they get there. In future accurate indoor positioning might not be viable by sole utilizing RF communications. To support people?s wayfinding activities we propose a Visible Light Communication (VLC) cooperative system that supports guidance services and uses an edge/fog based architecture for wayfinding services. A mesh cellular hybrid structure is proposed. The dynamic navigation system is composed of several transmitters (ceiling luminaries) which send the map information and path messages required to wayfinding. The luminaires are equipped with one of two types of nodes: a ?mesh? controller that connects with other nodes in its vicinity and can forward messages to other devices in the mesh, effectively acting like routers nodes in the network and a ?mesh/cellular? hybrid controller, that is also equipped with a modem providing IP base connectivity to the central manager services. These nodes acts as border-router and can be used for edge computing. Mobile optical receivers, using joint transmission, collect the data at high frame rates, extracts theirs location to perform positioning and, concomitantly, the transmitted data from each transmitter. Each luminaire, through VLC, reports its geographic position and specific information to the users, making it available for whatever use. The core element of the receiver is a photodetector that converts the optical power into electrical current. The VLC photosensitive receiver is a double pin/pin photodetector based on a tandem heterostructure, p-i'-n/p-i-n sandwiched between two conductive transparent contacts. Due to its tandem structure, the device is an optical controlled filter able to identify the wavelengths and intensities of the impinging optical signals. Its quick response enables the possibility of high speed communications. Since the photodetector response is insensitive to the frequency, phase, or polarization of the carriers, this kind of receiver is useful for intensity-modulated signals. The generated photocurrent is processed using a transimpedance circuit, obtaining a proportional voltage. This voltage is then processed, by using signal conditioning techniques (adaptive bandpass filtering and amplification, triggering and demultiplexing), until the data signal is reconstructed at the data processing unit (digital conversion, decoding and decision). Bidirectional communication is implemented and the best route to navigate through venue calculated. The results show that the system makes possible not only the self-localization, but also to infer the travel direction and to interact with information received optimizing the route towards a static or dynamic destination.

Resume : Nowadays, Global Positioning Systems (GPS) are used everywhere for positioning and navigation. However, its use is not suitable in indoor environment, due to power budget constraints and strong attenuation inside buildings. Therefore, indoors navigation takes advantage of other technologies to infer position. Visible Light Communication (VLC) is a promising technology for this purpose, as its operation is based on the use of LED lights, currently used in the illumination of most buildings. In this paper, we propose an indoor navigation system based on VLC in an industrial application for automated warehouses, where the navigation of autonomous vehicles (AVG) is supported by VLC. The proposed VLC system establishes bi-directional communication between the infrastructure and the AVGs. LED transmitters support downlink data transmission from the Infrastructure to Vehicle (I2V). This channel provides positioning and navigation, as well as transmission of dedicated messages related to the requested tasks of the management warehouse system to the AVGs. The uplink channel from the Vehicle to the Infrastructure (V2I) is used to acknowledge the requested tasks and transmit updates on the concluded tasks. Optical transmitters are tri-chromatic white LEDs with a wide angle beam. The characterization of the optical transmitter system is done through MatLab simulations for path loss and VLC channel gain prediction, using the Lambertian model for the LED light distribution. Dedicated receivers based on a-SiC:H/a-Si:H photodiodes with selective spectral sensitivity are used to record the transmitted signal. The decoding strategy is based on accurate calibration of the output signal. Characterization of the transmitters and receivers, description of the coding schemes and decoding algorithms will be the focus of discussion in this paper.

Resume : Using the methods of ultraviolet photoelectron spectroscopy, light absorption spectroscopy, and true secondary electrons, the effects of deposition of Cs atoms with a monolayer thickness of   1 on the density of state of electrons in the valence band and the conduction band, energy band parameters, and quantum yield of photoelectrons have been studied for the first time. It has been established that the band gap Eg of the film does not change upon deposition of Cs, the photoelectronic work function Ф sharply decreases, and the quantum yield of photoelectrons increases up to 3 times. The decrease in Ф is mainly due to a decrease in the width of the conduction band χ (electron affinity). It is shown that as χ decreases to 3 eV, the position of the maxima of the density of states of valence electrons practically does not change. It is shown that when Cs is deposited on a CoSi2 surface with a thickness of   1 monolayer, the value of Еg and the position of the maxima of the density of state of valence electrons practically do not change, the work function of photoelectrons decreases to 3 eV, and the quantum yield of photoelectrons increases by a factor of 3 or more. At   0.6, a monolayer appears in the spectrum with a new maximum at Ecv = -7.2 eV, which is characteristic of silicon. For the first time, the positions of the density maxima of free electronic states in the reducibility band of CoSi2 have been experimentally determined. These maxima are located at energies of 0.8 and 1.9 eV below the vacuum level. References 1. Умирзаков Б.Е., Ташмухамедова Д.А., Ташатов А.К., Мустафоева Н.М. // Журнал технической физики. 2019, вып. 5. с. 759-161. Doi: 10.21883/JTF.2019.05.47481.192-18. 2. Умирзаков Б.Е., Ташмухамедова Д.А., Турсунов М.А., Эргашов Ё.С., Аллаярова Г.Х. // Журнал технической физики, 2019, том 89, вып. 7, с.1115-1117.

Resume : The Schottky junction formed by contacting an n-type semiconductor with a sufficiently high work function conductor has been explored over the years for use as a simple photovoltaic cell. Graphene (Gr), benefiting from its popularity in recent years, has been used as an alternative for the conductor material because of its relatively good electrical properties for thin and highly transparent material. Many efforts reported the doping Gr to improve its conductivity and work function, and the engineering at the interface with a passivation layer to improve the power conversion efficiency (PCE) but they have not reached levels necessary for practical use so far. In this study, on-site Gr formation was accomplished by rapid chemical vapor deposition (CVD) synthesis using nickel catalysts deposited on nanoimprint lithography SiNWs. The on-site Gr growth was hypothesized to serve as an advantageous method of producing Schottky junction solar cells which can improve light trapping close to the junction compared to a planar device without sacrificing carrier collection. The Gr growth process was modified for fabricating functional solar cell devices, and samples were tested for their photovoltaic properties compared to equivalent planar controls. Experiments were carried out using n-Si (100) substrates. A nanoimprint lithography technique and Bosch process were used to form the SiNWs. The SiNW diameter and spacing are controlled by the quartz mold. The SiNW length is controlled by the number of plasma etching cycles. Gr formation was performed at a comparatively cool temperature between room temperature and 750 oC for 3 minutes on a 200 nm Ni catalyst layer under an atmosphere of 100 sccm He: 30 sccm CH4 at 400 torr. The 300 nm Au front contacts were deposited on the SiNW sample surface with a finger pattern over a 0.5 cm2 sample area for Schottky junction solar cell fabrication. From the results, the NW-shaped Gr was clearly visible on SiNWs after the underlying Ni catalyst was chemically removed, forming a Gr-Si three-dimensional junction. Complicated Gr transfer procedures could be avoided, and the process is capable of being scaled up. The Raman spectra obtained from the Gr-SiNW samples displayed the typical Gr D, G, and 2D peaks. The SiNWs substrate contributed light trapping qualities while the NW-shaped Gr increased the carrier collecting area compared to flat Gr resting on SiNWs. Simple SiNW-based devices improved the PCE to 2.19 % without doping in Gr, or 3.83% after p-type doping with gold chloride. High recombination losses likely limited device performance. The photovoltaic characteristics with various optimizations will be discussed. Further necessitating improvements in Gr quality and interface engineering is in progress. [1] S. Wallace, et al., Nanoscale Adv., 2 [12] (2020) 5607-5614. [2] S. Wallace, et al., Nanoscale Adv., 2 [4] (2020) 1718-1725.

Resume : For the demand of multicolor detection and communication, searching for broadband photodetectors with high sensitivity and stability is of great significance. Si-based photodetectors show broad spectral response, high reliability, fast response speed, and well-developed low-cost preparation process, which make it more popular on broadband photodetectors. However, Si-based photodetectors cannot efficiently solve the issues of weak photoresponse in ultraviolet (UV) region and the large dark current, which are still the critical drawbacks for practical applications. Under this circumstance, integration of strong UV absorber with Si material was considered to be an effective solution. In this study, we had successfully synthesized Cs3Cu2I5 nanocrystals (NCs) which have strong absorption in UV region and decorated that on Si nanowire arrays (NWAs) to form a hybrid structure to overcome the above two issues. Because of the wide and direct bandgap of Cs3Cu2I5 with intrinsic UV light absorption, the photodetection ability of the hybrid detector in UV region was substantially enhanced owing to the efficient down-conversion of the UV incident light. Moreover, driven by a designed Schottky junction from asymmetric electrodes, the hybrid detector can be operated without external power supply. Typically, at 265 nm light excitation, the Si NWA-Cs3Cu2I5 NCs hybrid device achieved a high photoresponsivity of 83.6 mA W 1, a specific detectivity of 2.1×1012 Jones, and a large on/off ratio of 3.72×103 at 0 V, nearly 350 times higher than the bare Si NWA device. More importantly, the unencapsulated photodetector demonstrates an outstanding operational stability over the aging test for 10 h, and can endure a high humidity (75%, 7 days) and a long-term storage for 300 days in air ambient. Since the different deep-UV light sensitivity of the devices modified with/without Cs3Cu2I5 NCs, a monolithically deep-UV light recognition system was therefore fabricated. It is anticipated that the present strategy provides a new avenue for the preparation of UV-enhanced broadband photodetectors, opening up opportunities for development of integrated optoelectronic systems in the future.

Resume : Heterostructures (HS) of two different semiconducting transition metal dichalcogenide monolayers (1L-TMDCs) have attracted great research interests owing to their high potential for diverse device applications. The MoS2/WSe2 HSs lead to various physical processes such as ultrafast charge transfer and the emergence of interlayer exciton states. The interlayer coupling strength should be strongly related to the separation between the two monolayers, depending on a relative twist angle. In this study, we report the temperature evolution of interlayer exciton emission from the MoS2/WSe2 HSs with various twist angles stacked by a standard dry transfer process utilizing exfoliated 1L-TMDCs. It is found that the twist angle of the HS affects a thermal activation energy of the interlayer exciton. Firstly, we measured photoluminescence spectra of 1L-MoS2, 1L-WSe2 and HS areas. The PL peak around 1.03 eV was clearly seen only in the HS area. Photoexcited electrons and holes transfer to the conduction band edge of MoS2 and the valence band edge of WSe2, respectively, due to the type II band alignment of the MoS2/WSe2 HSs. The Coulomb attraction between the spatially separated electrons in MoS2 and holes in WSe2 forms the interlayer exciton, which yields the lowest energy bright exciton in the HS area. Next, we investigated temperature dependence on interlayer exciton emission intensity obtained from the HS area and estimated thermal activation energy for samples with different twist angle of HSs by using a standard exponential decay model. The thermal activation energy depended on the twist angle, and it became large for HS samples with twist angles of 0 and 60 degrees. It has been reported that the separation between two monolayers becomes smaller for the 0 and 60 degree twist angles. It could be therefore concluded that the smaller separation results in the larger activation energy of the interlayer exciton due to the stronger Coulomb interaction between spatially separated electron-hole pairs.

Resume : Thin film transistors (TFTs) are a key electronic component that has been developed expeditiously in terms of device density and are currently widely used in the field of display and pixelated detector backplanes and integrated circuits. Metal oxide TFTs have gained attention benefitting from their transparency, flexibility, high charge carrier mobility and low processing temperature over conventional amorphous and poly-Si TFTs. [1] For realizing a scalable, low-cost fabrication process for oxide TFTs, inkjet-printing method has been utilized earlier to fabricate In2O3-based TFTs on a low-cost flexible substrate [2]. However, like many other oxide semiconductors, In2O3 is only sensitive to short wavelength light like blue (470 nm [3]), because of the large band gap energy (3.87 eV [4]). This limits its further application as the sensing element in photosensor applications. Therefore, a dye-sensitized In2O3 photo-TFT device has been proposed, in which the photoinduced charge carriers can be transferred from organic dye to oxide semiconductor to enhance the photosensing performance of oxide TFTs over selected wavelengths [3]. In this work, we exploited two organic dyes, rhodamine 6G (R6G) and phloxine B (PB), as photosensitizers to improve the photo-response of inkjet-printed In2O3 TFTs to green light (565 nm). The R6G and PB dyes were either printed over the TFT channel or encapsulated within an In2O3 matrix as novel printed composite materials. Electrical characterization of the dye-sensitized devices showed increased light response consisting of an obvious increase of the off-current and a negative shift of the turn-on-voltage when comparing to the pure In2O3-based TFTs. The photosensitivity and responsivity of the dye-sensitized devices reached 5x10^5 and 250 A/W, respectively, larger than the pure In2O3 devices (10^4 and 0.3 A/W). In addition, the presented persistent photoconductivity was successfully removed by applying a high gate voltage pulse (40 V), which renders our In2O3 phototransistors applicable as resettable printed photodetectors. [1] E. Fortunato, P. Barquinha, R. Martins, Adv. Mater. 2012, 24, 2945. [2] J. Leppäniemi, K. Eiroma, H. Majumdar, A. Alastalo, ACS Appl. Mater. Interfaces 2017, 9, 8774. [3] A. D. Mottram, Y. H. Lin, P. Pattanasattayavong, K. Zhao, A. Amassian, T. D. Anthopoulos, ACS Appl. Mater. Interfaces 2016, 8, 4894. [4] Y. H. Lin, H. Faber, J. G. Labram, E. Stratakis, L. Sygellou, E. Kymakis, N. A. Hastas, R. Li, K. Zhao, A. Amassian, N. D. Treat, M. McLachlan, T. D. Anthopoulos, Adv. Sci. 2015, 2, 1500058.

Resume : A semiconducting molybdenum ditelluride (2H-MoTe2) is emerging as a new class of a material with unique properties such as exciton formation at room temperature and a stronger spin-orbit coupling in contrast to other Mo-based transition metal dichalcogenides. In particular, a monolayer of the 2H-MoTe2 is considered to have a high potential as a host material of a light source for optical communication devices since it emits photons with a near-infrared wavelengths around 1150 nm. However, the 2H-MoTe2 has critical disadvantages that crystal defects are formed by thermal treatment at 200 °C and crystals are easily oxidized due to exposure in air for several days. We previously reported that thermal treatment of hBN-encapsulated MoTe2 monolayer enabled the suppression of the crystal quality degradation and the removal of the residues at the heterointerfaces. In this study, we investigated effects of thermal treatment on electrical properties of hBN-encapsulated MoTe2 with graphite source and drain electrodes. We measured the drain current (Id) as a function of a Si back-gate voltage (Vg) for mono- and few-layered MoTe2. For samples without thermal treatment, the Id slightly increases by increasing the Vg in the range from -100 to 100 V, being consistent with n-type operation. In contrast, we succeeded in demonstrating the ambipolar transistor operation by observing hole conductivity for the samples with thermal treatment at 200 °C. Furthermore, it was found that the on/off current ratio (Ion/Ioff) was about 105 in p-type operation. In the case of thermal treatment at 400 °C, the Ion/Ioff was reduced to be about 101, indicating the crystal quality degradation of MoTe2 due to thermal treatment at high temperature. It could be concluded that thermal treatment at 200 °C is effective for improvements of electrical properties of MoTe2-based devices.

Resume : Thermally activated delayed fluorescence (TADF) OLEDs are well-known for their high efficiency due to their triplet harvesting ability by the reverse intersystem crossing (RISC) process. However, TADF OLEDs suffer from low color purity due to their wide emission spectrum. We study the effect of the inclusion of a fluorescent blue dopant material with better color purity in the emissive layer (EML) of a TADF blue OLED to enhance the emission spectrum of the OLED. We show an improvement in the CIE coordinate in the OLED where the fluorescent blue-emitting material is incorporated; from (0.15,0.27) to (0.14, 0.16). However, the inclusion of the fluorescent dopant in the EML reduces the efficiency of the OLED compared to the TADF OLED.

Resume : Material quality is key to high-efficiency optoelectronic devices. Defects in the crystal structure such as dislocations, vacancies and grain boundaries cause carrier recombination in semi-conductors and ohmic losses in metals, degrading the efficiency of devices. For this reason, state-of-the art solar cells rely on single crystal absorbers such as GaAs, GaInP and Silicon (Polman et al., Science, 2016). However, the techniques used to grow defect-free monocrystalline films are in most cases time consuming, expensive (high vacuum and temperature) or require lattice-matched substrate, and finally are restricted to a few materials. It has been demonstrated that single-crystal silver nanocubes, grown in solution phase, could be used as building blocks for the fabrication of continuous monocrystalline nanostructures via epitaxial welding of adjacent units at room temperature (Sciacca et al, Adv Mater., 2017). More recently, our group showed that this approach could be used as a general method to fabricate 1D and 2D monocrystalline gold (Au) nanopatterns (under review), that can be transferred to any substrate via contact printing. This allows the swift integration of high-quality contacts and/or photonic nanostructures in optoelectronic devices. Copper oxide (Cu2O) is recognized as an interesting material integrated as a photovoltaic absorber in a solar cell or a photocathode in a photoelectrochemical cell because of its direct bandgap of ~ 2.2 eV, abundance and low-cost, but unfortunately the growth of smooth monocrystalline thin film of high quality is challenging. Conversely, monocrystalline Cu2O nanocubes can be synthesized in solution at low temperature (Ke et al., Small, 2016). First, by self-assembling these nanocubes into a 2D closely packed configuration, with individual nanocube subunits lying face-to-face, we form a discontinuous thin film on an arbitrary substrate such as polydimethylsiloxane (PDMS). Then, by performing Cu2O overgrowth on those nanocubes we transform the assembly into a high-quality quasi-monocrystalline film in the time scale of a few minutes. Finally, we show that such thin film can be transferred from PDMS to various substrates by contact printing at low temperature, paving the way for integration in complex devices with unexplored geometries. We use high-resolution transmission electron microscopy, electron diffraction (SAED and EBSD), and x-ray diffraction to provide evidence for epitaxy, and optical characterization to probe optoelectronic properties. By correlating electron microscopy images and localized time-resolved photoluminescence measurements, we study the impact of nanocube misalignment and film morphology on the optoelectronic properties, for various film thicknesses. This low-cost bottom-up solution route proves a viable method for the fabrication of monocrystalline metal oxides thin films and paves the way to other material that can be grown as nanocubes in solution such as PbS or halide perovskites.

Resume : Chemical Bath Deposition of ZnO nanowires (NWs) [1] is a low-cost and low temperature hydrothermal growth process of particular interest. It enables homogeneous and oriented growth of vertically aligned ZnO NWs from a polycrystalline ZnO seed layer deposited on a wide range of substrates. During the growth of NWs, the chemical bath reactants introduce crystal defects caused by the presence of numerous impurities such as carbon, nitrogen, and hydrogen [2]. These defects and their related complexes consequently impact the NW properties. On the one hand, interstitial hydrogen in bond-centered sites (HBC) and 3-hydrogen paired zinc vacancy (VZn-3H) are examples of such defect complexes that operate as shallow donors with low formation energies [3] and govern the electrical behavior of ZnO NWs. On the other hand, the zinc vacancy paired with nitrogen and hydrogen (VZn-NO-H) defect complex that also has a low formation energy behaves as a deep acceptor and acts as a compensating defect. Raman and cathodoluminescence spectroscopy revealed that thermal annealing under an oxygen atmosphere allows control of the concentration and nature of these defect complexes [4]. The increase of annealing temperature induces a series of associative and dissociative processes that affects the concentration of defect complexes under an oxidizing atmosphere. The use of oxygen plasma is a different way of tuning the formation and concentration of these defects since it directly impacts their nature and the related electrical properties of ZnO NWs [5-6]. Although some plasma process parameters have been shown to play a significant role on the properties of ZnO NWs, data on local plasma parameters such as ion energy and ion flux near the interaction site is lacking. In this work, we measure those parameters using a retarding field energy analyzer (RFEA) and a Langmuir probe to report their impact on hydrogen-related defects and oxygen vacancies (VO) using Raman spectroscopy and X-ray photoelectron spectroscopy . [1] L. Vayssieres et al. J. Phys. Chem. B. 105 (17), 3350–3352 (2001). [2] C. Van De Walle. Phys. Rev. Lett. 85 (5), 1012–1015 (2000). [3] J. Villafuerte et al. J. Phys. Chem. C, 124 (30), 16652–16662 (2020). [4] J. Villafuerte et al. Phys. Rev. Mater. 5 (5), 1–15 (2021). [5] H. Jiang et al. R. Soc. Chem. 51 (8), 28928 (2018). [6] D. Park et al. Korean J. Met. Mater. 59 (3), 209–216. (2021)

Resume : Ternary copper halide compounds, where copper presents oxidation state 1 (Cu ), have been of great interest because of their optoelectronic properties. Synthesis of cesium copper halide nanostructures, i.e., 0D Cs3Cu2I5 and 1D CsCu2I3, have been successfully achieved by different methods, such as hot-injection or antisolvent vapor-assisted crystallization. The ratio between those two phases can be controlled by either the reaction temperature or the reactant concentration in the solution. [1],[2] Both structures display a bandgap energy in the UV range (3.9-4.3 eV), however Cs3Cu2I5 exhibits emission at 2.8 eV while CsCu2I3 presents emission at 2.2 eV. One-pot synthesis is a simple method to obtain perovskite-based nanostructures, that minimize chemical waste and reduces synthesis time. This method has been employed in this work to synthesize cesium copper iodide (Cs-Cu-I) tuning their stoichiometry from CsCu2I3 to Cs3Cu2I5 by modifying different parameters of the reaction.We hereby present the synthesis and the structural and optical characterization of the resulting materials. Synthesis of Cs-Cu-I was carried out by dissolving CsI and CuI in N,N-dimethylformamide (DMF) and adding oleic acid (OA) to the solution as a surfactant. To control the final stoichiometry of the Cs-Cu-I compound, the volume of OA was modified from 0.2 ml to 1 ml, in intervals of 0.2 ml, while the temperature of the reaction was kept constant at either 25, 60, 80, or 100ºC. After that, thin films were fabricated by spin-coating the solutions on both silicon and fused silica substrates. Finally, samples were annealed in a vacuum oven at 100ºC for 45 mins. Structural characterization by XRD has revealed a crystalline arrangement that is consistent with the presence of either one of the phases or the two of them, with varying ratios. The stoichiometry was determined by XPS measurements, confirming the existence of the two phases, in agreement with XRD results. Finally, the optical properties were assessed by absorption and photoluminescence measurements. Whereas the absorption edge is around ~4.13 eV for all the films, the emission shifts from 2.21 eV to 2.75 eV, in accordance with the stoichiometry dependance of the Stokes shift. Both CsCu2I3 and Cs3Cu2I5 are synthesized following a simple chemical reaction method. The stoichiometry of the final material is tuned between the two by changing the molar ratio between the reactants, the reaction temperature, and the annealing temperature, as corroborated by the structural and optical characterization carried out. REFERENCES: [1] P. Cheng et al., Angew. Chemie - Int. Ed., 58, 16087, 2019. [2] X. Mo et al., Nano Energy, 81, 105570, 2021.

Resume : Perovskite light emitting diodes (PeLED) have become the new generation in organic display applications. Its high flexibility in bandgap tuning, narrow PL emission and high color purity have attracted a lot of research in the field. Promising external quantum efficiencies (EQE) and luminance have been achieved. However, one of the reasons for the delay in its introduction to the market is its low stability. Suppressing non-radiative recombination, reduction of leakage current, improving charge transfer characteristics, having defect-free films and ensuring quantum confinement in the active layer are some of the suggested solutions to enhance the performance and stability in PeLEDs. The usage of small molecules (SMs) has been reported for interface passivation and improvement in the perovskite film quality. However, in majority, SMs are added to the perovskite precursor solution and its influence in the morphology of the perovskite films has been widely reported (DOI: 10.1002/adom.202101361). Albeit having few reports with small molecule as charge transport layers (CTL), no literature shows deeper analysis on the small molecule charge transport layer (SM-CTL) itself. In this work, N,N?Di(1naphthyl)N,N?diphenyl(1,1?biphenyl)-4,4?-diamine (NPB) is used as small molecule hole transport layer (SM-HTL) and 1,3Bis(3,5dipyrid-3-ylphenyl)benzene (BmPyPhB) as small molecule electron transport layer (SM-ETL). Several concentrations of solution-based ambient air processed SM-CTLs are studied. SEM images investigated show longer needle-like structures for higher concentrations. Further, the influence of these morphological changes in the performance of an all-solution processed PeLEDs will be compared with optical and electrical characterization techniques. The charge transport and blocking properties of SM-CTLs will be studied separately and in conjunction with metal oxide CTLs to enhance the PeLED performance.

Resume : SnO2 and NiO are wide bandgap semiconducting metal oxides with applications in a variety of fields, including UV light sensing, gas sensing, and as a transparent conductor for LEDs and/or solar cells. Thanks to their wide bandgap, SnO2 and NiO are almost completely transparent to visible light, and thus ideal for UV detection. Metal oxides have traditionally been fabricated by physical and chemical deposition methods. Nevertheless, solution-processing approaches have gained prominence in the last years because of their versatility and relative simplicity in layer deposition. In particular, technologies such as inkjet printing or spin coating enable controlled deposition of wide range of materials. Inkjet printing is a digital deposition technique based on controlled ink ejection from a series of nozzles placed in printing head. As the printing head travels above the substrate, the ink is deposited in the form of a matrix of droplets in a specified pattern with high precision (reaching < 10?m depending on conditions) without the use of masks or photolithography. Despite these advantages, inkjet-printed layers are prone to surface irregularitiy. In contrast, spin coating achieves thin layers by spreading a certain amount of solution through high-speed substrate rotation. Even though better surface regularity is achieved through spin coating, it cannot control the deposition pattern. In this work, we compare the performance of high-quality inkjet-printed and spin-coated SnO2/NiO heterojunctions. Single layers of SnO2 and NiO deposited by both approaches are thoroughly studied by a variety of structural and optical techniques. SEM and profilometry show low surface roughness and a lack of pinholes. Optical absorption and conductivity measurements display layers with good transparency and electrical conductivity. Heterojunctions combining these two metal oxides (ITO/SnO2/NiO/Ag) were fabricated using both techniques, with a large current ratio between negative and positive polarizations and notable UV photodetection. When compared to the the spin-coated ones, our inkjet-printed devices show results comparable. These promising results showed that our inkjet-printed metal oxides are applicable to UV photodetection as well as for use as transparent charge conductors for either LEDs or solar cells.

Resume : The terrestrial photovoltaic industry is still dominated by silicon-based solar cells. It is due to high abundance, nontoxicity of silicon and the well-developed silicon technology. Nowadays, there is movement from traditional Si based solar cells to nanoscale Si structures such as silicon nanowires (SiNWs). The SiNWs, for example, can provide new opportunities for developing an amorphous Si radial p-i-n junction solar cells. The SiNWs provides broadband antireflection and have a good light-trapping effect, which can increase the optical absorption of solar radiation in a:Si-H active layers and overall performance of solar cell. However, at the moment there is no definite idea about the required geometry (diameter, periodicity and height) of SiNWs for the deposition of (i)a-Si:H/(p)a-Si: H layer stack with precise thickness control. In this paper, we have fabricated of five types of SiNWs structures that differ geometry for subsequent deposition of a-Si:H p-i-n structures. Vertically aligned structures were prepared using nanosphere lithography and dry etching in SF6/O2 mixture at cryogenic temperatures. Just at low temperatures > -100 °C, the side surface is passivated due to the formation of a non-volatile SiOxFy compound. This compound blocks the etching mechanism and thus it provides deep and anisotropic etching of silicon. We used the 100 mm polished wafers n-type (100) silicon substrates. First, 500 nm layer of SiO2 was grown on Si substrate by PECVD using Oxford Instruments PlasmaLab 100 PECVD setup. Second, template during 2 µm diameter latex spheres was formed on the substrate surface by spin coating. Third, latex spheres were etched an oxygen plasma to reduce their diameter. Further, through the formed sphere template, silicon oxide was removed in CHF3 plasma forming thus a SiO2 hard mask for the subsequent silicon cryogenic etching. And then, the array of SiNWs was obtained by deep cryogenic etching of Si in a SF6/O2 plasma using Oxford PlasmaLab System100 ICP380. Finally, the spheres were removed by boiling in carbon tetrachloride (CCl4) and then isopropyl alcohol. Then the SiO2 mask was removed in a HF/H2O solution. It is shown that specific nanowire diameter and periodicity are obtained by adjusting the diameter of the latex spheres during the etching time in O2 plasma. The length of the nanowires is regulated by the etching time in the SF6/O2 mixture at cryogenic process. Next, a-Si:H p-i-n structures are deposited on n-type SiNWs using PECVD. Photovoltaic properties as function of SiNWs geometry are explored. We also managed to create a flexible SiNWs structure with a bending radius of 25 mm. For this a technology for filling SiNWs (6 µm in height and 1.7 µm in diameter) with the Su-8 photoresist has been developed using spin-coating. Then the substrate with back side was thinned first mechanically, and then in plasma.

Resume : Nitrides are promising functional materials for a variety of applications. Despite their technological importance many promising theoretically predicted metal nitrides are yet to be discovered.[1] This is partly rooted in their challenging synthesis as compared to oxides. Non-equilibrium PVD in ultra-high vacuum conditions, such as reactive RF-magnetron sputtering, provides ideal prerequisites for the formation of novel metastable nitride thin films.[2] Furthermore, combinatorial sputtering approaches are extremely effective for a rapid screening of the associated complex synthesis phase-spaces.[3] In this study, Zn-V-N was selected as a promising material system for a comprehensive experimental phase screening mainly due to the theoretically predicted Zn2VN3 semiconducting phase with properties suitable for optoelectronic applications.[4,5] We carried out a combinatorial PVD screening of the entire Zn-V-N phase space.[6] RF co-sputtering of Zn and V targets was carried out in mixed plasma of Ar and N2 to grow combinatorial libraries on plain glass substrates. Automated mapping characterization of the sample libraries was performed using X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and four-point probe techniques. In addition, selected samples were studied using hard X-ray photoelectron spectroscopy (HAXPES), Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), photoluminescence (PL) and optical spectrophotometry. The study revealed the presence of a disordered wurtzite Zn1-xVxN phase over a large compositional range with a narrow process window for single-phase Zn2VN3. A combined RBS/ERDA analysis on these samples demonstrates that the material is stoichiometric with only trace amounts of O, well below 1 at. %. Further, XPS/HAXPES analysis confirms this result with Zn and V being present in oxidation states of +2 and +5, respectively, as expected for Zn2VN3. The Fermi level is located ~ 1 eV above valence band maximum indicating a weak p-type doping. Zn2VN3 thin films exhibit a Hall mobility of ~80 V?s/cm2. Moreover, room-temperature PL exhibits broad band emission in the range from 2 eV to 3 eV three distinctive peaks and a tail towards 2 eV. The latter is consistent with a significant amount of sub-band gap absorption measured using optical spectrophotometry and cation disorder. In conclusion, we performed an experimental screening of the Zn-V-N phase space and isolated the novel semiconducting Zn2VN3 phase. The experimental results are consistent with DFT calculations. However, further studies are required to improve optoelectronic properties of the compound. [1] W. Sun et al. Nat. Mater. 18 (2019) 732?739. [2] A. Zakutayev, J. Mater. Chem. A 4 (2016) 6742?6754. [3] K. Alberi et al. J. Phys. D. Appl. Phys. 52 (2019) 013001. [4] A. Jain et al. APL Mater. 1 (2013) 011002. [5] S. Lany, Phys. Rev. B 87 (2013) 085112. [6] S. Zhuk et al. Chem. Mater. 33 (2021) 9306 - 9316.

Resume : Nanowires represent a hot research topic that gained prominence in many application areas. Featured by a high ratio of surface area to volume and tunable physico-chemical properties, the nanowires present enhanced performance over their bulk counterparts. To maximize these assets in various application domains such as optoelectronics, sensing, biomedicine, etc., it is essential to be able to control their morphologies and properties by identifying and using the most adequate preparation methods. In this study, ZnO-CuO core-shell nanowires decorated with Ag nanoparticles were prepared in order to combine the advantages of different components. Thus, the metal oxide nanowires decorated with metallic nanoparticles were fabricated by combining two simple dry preparation methods: thermal oxidation in air for ZnO core and RF magnetron sputtering for CuO shell and Ag decoration process. The obtained nanowire arrays were characterized from morphological, structural, compositional, surface chemistry, optical and antibacterial point of view. Joining ZnO (n-type semiconductor) and CuO (p-type semiconductor), an axial p-n junction is formed, leading to a better separation of the charge carrier pairs generated under visible light irradiation. Thus, the charge carriers are implicated in the generation of the reactive oxygen species, which are responsible for the antibacterial activity. Moreover, through a synergetic effect, the decoration with Ag nanoparticles of the surface of the core-shell nanowires enhances the bactericide effect under visible light. Consequently, multi-component nanowires with tailored architecture and specific properties can be fabricated by straightforward, cost-effective and scalable techniques.

Resume : Gallium oxide (Ga2O3) is an n-type semiconductor with a wide band gap (4.5-4.9) eV, with higher transparency than In, Sn and Zn oxides; in form of nanowires, nanoribbons, nanobars, etc., it shows great promise for nanoelectronics applications. In this work, both the micro- and nano-structured β-Ga2O3 layers up to several hundred micrometers in length, and β-Ga2O3/Ga2Se3 structures were obtained by thermal annealing in air of single- and polycrystalline Ga2Se3 plates. Ga2Se3 is a layered group III-VI semiconductor compound with a band gap of ~ 2 eV, high photosensitivity and intense luminescence in visible and near-IR ranges. As revealed by X-ray diffraction and microscopy studies, by heat treatment in air of Ga2Se3 crystals at temperatures above 500 ℃, their surface is covered with a homogeneous gallium oxide layer; for temperatures 700-900 ℃, presence of homogeneous phase β-Ga2O3 was found. Elemental and chemical composition of the surface layer of Ga2Se3 crystals, subjected to heat treatment in air at 500-800 ℃, was determined from X-ray Photon Spectroscopy and Raman spectral analyses. Under the assumption that Ga atoms in the surface layer do form the compound Ga2O3, the excess oxygen contained is of ~ 7%. Raman spectra were excited by means of a Nd:YAG laser at a wavelength of 532 nm. In the wavelength range of 100-800 cm-1, Raman spectra contain ~ 10-12 bands determined by the vibrations of the Ga-O bonds inside the GaO4 tetrahedra and the vibrational modes of GaO4 tetrahedra. As revealed by Scanning Electron Microscopy study, the surface of β-Ga2O3 layers consists of ensembles of nanowires and nanobars, with length up to tens of micrometers. The fundamental absorption edge of the material from Ga2Se3 surface layer was studied using diffuse reflectance measurements. As determined by spectral distribution of the Kubelka-Munk function, the band gap of β-Ga2O3 layers obtained at 500-800 ℃ was found in the range of 4.58-4.71 eV. The photoluminescence spectra of the β-Ga2O3 layer were excited by laser radiation of wavelength 337.4 nm and Hg-vapor emission line (254 nm). The emission spectrum of Ga2Se3 crystals, under 532 nm laser excitation, contains an intense band with a maximum centered at ~ 700 nm, interpreted as a donor-acceptor (D-A) type band. The emission spectra of β-Ga2O3 surface layers contain a series of bands located between 370 nm and ~ 850 nm, the structure and intensity of which depend on the preparation temperature and surface morphology of β-Ga2O3 layer. A band energy diagram for the nanostructured β-Ga2O3 compound is proposed based on the distribution and concentration of oxygen and Ga vacancies in β-Ga2O3 layers, which interprets adequately the results of optical and luminescence studies. The present study recommends as obtained nanosized Ga2O3/Ga2Se3 structures and β-Ga2O3 layers for nanoelectronics applications.

Resume : Graphitic carbon nitrides (g-CN) and heptazine-based polymers like melon are materials of high interest for photocatalytic hydrogen evolution and water splitting applications due to their suitable bandgap and outstanding chemical stability.[1,2] Over the last decade, significant efforts have been made to develop and investigate nanostructured g-CNs.[3] Apart from these covalently bonded 2D polymers, supramolecular chemistry allows to form well-defined nanostructures via directed secondary interactions. Such nanostructures provide the opportunity to realize unprecedented and outstanding properties, which cannot be observed in the individual molecular building blocks. Here we demonstrate the synthesis, self-assembly and functionality of novel building blocks based on heptazine as core. This electron-deficient C3-symmetric core is substituted with amide groups, which are linked either to hydrophobic or hydrophilic peripheries. Both building blocks can be conveniently self-assembled into supramolecular nanofibers or ribbons via H-bonding and ?-? interaction, which feature widths below 20 nm and lengths of multiple micrometers. These nanostructures were successfully used for the photodegradation of model compounds including an organic dye and an antibiotic, outperforming the photocatalytic activity of melon and even of the benchmark molecule TiO2 up to a factor of 10. These findings demonstrate that supramolecular heptazine-based nanostructures provide the potential to be used as effective photocatalysts for water splitting. References [1] X. Wang et al, Nature Mater. 8., 76-80 (2009). [2] W.-h. Lau et al. Nat. Commun., 7, 12165 (2016) [3] L. Chen et al., Adv. Funct. Mater., 27, 1702695 (2017)

Resume : The incorporation of N into GaP induces an indirect-to-direct band gap transition, as described by the band anticrossing model [1]. Furthermore, GaP1-xNx is lattice matched to Si for x = 0.021, with a band gap close to 1.95 eV at 300K. This makes it a good candidate to integrate light-emitting devices with the standard Si-based photovoltaic technology. In this work, we report a comprehensive study on the chemical beam epitaxy (CBE) growth of GaP1-xNx thin films on (001)-oriented GaP-on-Si pseudo-substrates. Specifically, we systematically investigate the impact of the growth parameters on the incorporation of N, the structural quality, and the morphological properties, using a wide variety of experimental techniques such as reflection high-energy electron diffraction (RHEED), high-resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The analysis of the chemical composition by HRXRD as a function of the growth parameters reveals that the average N content increases linearly with the flux of the N precursor and, decreases exponentially with the growth temperature according to an Arrhenius? law, with an activation energy of 0.83 eV. Our study also evidences the existence of two different growth regimes: a single-phase regime characterized by a well-defined chemical composition, and a phase-separated growth regime characterized by the presence of two regions with different N contents. The monitoring of the growing surface, in situ by RHEED and ex situ by AFM, demonstrates that single-phase samples exhibit a smooth surface topography, while phase separated ones exhibit a corrugated surface due to the formation of 3 dimensional (3D) islands elongated along the [11 ?0] direction. Regardless of the growth regime, the observation of RHEED intensity oscillations at the onset of the growth indicates that all samples initially grow in a layer-by-layer fashion. However, for the phase-separated samples, the layer-by-layer growth mode is only maintained up to reaching a critical thickness of about 5 nm, then the surface becomes faceted. All these results are illustrated in a growth diagram that depicts how the average chemical composition and the boundary between both different growth regimes depend on the growth parameters [2]. Last, to assess the material quality at atomic scale as well as to unveil the origin of the two different chemical phases, the samples were studied by TEM-related techniques. This study shows that single-phase samples present excellent crystal quality and compositional uniformity. In contrast, phase-separated samples show abundant extended defects and a highly inhomogeneous N concentration, being the N content higher at the interface with the GaP buffer layer. They also exhibit strong lateral compositional modulations perpendicular to the [11 ?0] direction with the same periodicity as the elongated island observed by AFM. [1] W. Shan et al. Appl. Phys. Lett. 76, 3251 (2000). [2] K. Ben Saddik et al. APL Mater. 9, 121101 (2021).

Resume : Metal oxide plasmonic nanocrystals (NCs) synthesis and physical properties are already widely described. Their interaction with the visible light is subject to extensive studies both for fundamental research and industrial applications. Among them, transparent conductive oxide (TCO) such as tin-doped indium oxide or aluminum doped zinc oxide (Al:ZnO) showed promising results. Indeed, their plasmonic band is shifted to the infrared wave lengths which made it interesting for a variety of applications from solar cells, smart glazing technologies to satellites thermoregulation. Moreover, visible transparency is retained. The synthesis of aluminum doped zinc oxide (Al:ZnO) is performed in organic media at high temperature. Aluminum and zinc precursors, alcohol and surfactant are mixed together in the solvent and then heated up. The reactivity of the ligands, the temperature program and relative concentration between metallic precursors and surfactant lead to different geometry of nanoparticles (NPs). The size is mainly controlled by the temperature and reaction time. These NPs have been characterized by X-Ray diffraction (XRD) to identify the crystalline structures, by atomic absorption spectroscopy (AAS) to obtain the Al-doping level and by attenuated total reflectance (ATR-FTIR) to analyze the infrared properties of the plasmonic NCs. ATR-FTIR confirms that the Al:ZnO is an excellent IR absorbing material that can be used in a wide range of technologies. Cells with rear IR-mirror are filled up with dispersions containing different concentration of obtained NPs (up to 40 mg/ml) and then observed with an IR-camera. This experiment allows observing the contrast between emissive and reflective states. Measurements are performed in thermally controlled environment with a black body emitting at high temperature, which allow obtaining the value of the emissivity. The emissivity varied from 0.5 for the solvent to 0.76 for the solvents with NPs, which correspond to a maximal contrast around 26%. Several optimizations are still possible to achieve higher values of contrast. By adjusting the synthesis parameters we could prepare NPs with optimal geometries and Al-doping levels while ideal NPs concentration could be reached by colloidal formulation tuning.

Resume : Ternary copper halide compounds with monovalent Cu have recently attracted great research interest due to their optoelectronic performances. Han et al. [1] first reported the colloidal synthesis of low dimensional cesium copper halide NCs (including 0D Cs3Cu2I5 and 1D CsCu2I3) and demonstrated that the reaction temperature is crucial for the morphological structure. To this end, Cs3CuI5 nanocrystals were synthesized following the hot injection synthesis approach. An ink solution for inkjet printing technology was prepared by harvesting the nanocrystals and dissolving them in hexane and dodecane. Thin films were fabricated by inkjet-printing, using different amounts of layers and submitting them to different curing processes, for ensuring homogeneous and pin-hole free layers. The morphology of inkjet-printed films was characterized via SEM and optical microscopies. These results were combined with XPS measurements that allow characterizing the composition of the layers and XRD to determine crystalline structure and micro/nanocrystal size. Finally, optical analysis done by absorbance and PL (photoluminescence) allows measuring band gap energy and confirm that the layers are optically active with a broad and strong blue emission at ~450 nm (E=2.76 eV), that exhibits a near unity PL quantum yield under excitation at 290 nm, and a large Stokes shift (optical band gap energy around 4.43 eV) as a consequence of self-trapped excitons. [1] Han et al., Angewandte Chemie International Edition, vol. 58, no. 45, pp. 16087?16091, 2019.

Resume : Semiconductor nanoplatelets have attracted interest due to their exceptionally narrow optical features. However, the direct synthesis of semiconductor nanoplatelets is limited to nanoparticles with thicknesses from 2 to 6 monolayers (N monolayers NPLs present N+1 planes of cations alternated with N planes of chalcogens in the [001] direction of the zinc blende crystal structure). It thus limits the wavelength range reached by such materials. Here, we show that it is possible to synthesize thicker NPLs by carefully tuning the NPLs surface energy. The native carboxylate ligands are exchanged with halides ligands which induce a further dissolution of the NPLs and a recrystallization of the released monomers on the top and bottom wide facets. This versatile method is applied to all three cadmium chalcogenide semiconductor. Thus, in NPLs, where the surface represents a high ratio of the particles, the surface chemistry is a key factor. In the second part of the presentation, we will show that the control of surface chemistry is also a way to tune the shape of the NPLs.

Resume : The synthesis of indium phosphide nanoparticles has been an active research topic for thirty years ?1?. The most common synthesis route involves the reaction of indium carboxylate and tris(trimethylsilyl)phosphine at high temperature (>230°C). However, the use of high temperatures results on one hand, in the partial oxidation of the surface nanoparticles and on the other hand, in a lack of growth control and broader emission [2]. In this presentation, I will show that the replacement of indium carboxylate by a novel indium precursor allows the formation of InP nanoparticle at record low temperature (150°C) and therefore prevent the surface oxidation even in the presence of O-containing ligands [3]. The study of the mechanism will be first presented and the influence of the second sphere of coordination on the growth processes will be highlighted through the addition of hydrogen bond donor compound. On the bases of this comprehensive study, we have developed better controlled synthesis, leading to an improvement of the optical properties, as well as a simplification of the protocol. These results will also be detailed here. References: [1] Chemistry of InP Nanocrystal Syntheses. Tamang, S ; Lincheneau, C ; Hermans, Y ; Jeong, S ; and Reiss, P. Chemistry of Materials 2016 28 (8), 2491-2506.DOI: 10.1021/acs.chemmater.5b05044 [2] Interfacial Oxidation and Photoluminescence of InP-Based Core/Shell Quantum Dots. M.D. Tessier, E.A. Baquero, D. Dupont, V. Grigel, E. Bladt, S. Bals, Y. Coppel, Z. Hens, C. Nayral, F. Delpech, Chem. Mater., 2018, 30, 6877-6883. DOI : 10.1021/acs.chemmater.8b03117 [3] Synthesis of Oxide-Free InP Quantum Dots: Surface Control and H2-Assisted Growth. Baquero, E.; Virieux, H.; Swain, R.; Gillet, A.; Cros-Gagneux, A.; Coppel, Y.; Chaudret, B.; Nayral, C.; and Delpech, F. Chemistry of Materials 2017 29 (22), 9623-9627. DOI: 10.1021/acs.chemmater.7b04069

Resume : Scintillators are materials able to efficiently convert ionizing radiation into ultraviolet or visible light, thus allowing the detection of highly energetic photons and particles by conventional optical sensors. Scintillating materials are widely used in several applications, ranging from high-energy physics calorimetry and particle discrimination to medical devices, homeland security, and industrial controls. In general, scintillator materials must have a high atomic number (Z) to ensure high radiation cross-section, high radiation hardness for long-term operation, and they cannot contain radioactive isotopes in order to minimize background noise. The scintillation performances mostly depend on the efficiency of radiation-to-light conversion and of light outcoupling towards photodetectors coupled to the scintillator. Therefore, de-coupling of the emission and absorption features is required to eliminate otherwise dramatic reabsorption losses. In this framework, lead halide perovskites are attracting growing attention as affordable scintillating materials alternative to inorganic single crystals, because of their scalable low temperature synthesis, high average Z, defect tolerance and tunable, near unity, efficient excitonic luminescence. More recently, in the attempt to reduce possible harmful effects of toxic lead, lead free low dimensional halides have been introduced as effective scintillators, also prized for their characteristic intragap luminescence originating from the radiative recombination of excitons localized in isolated structural units or dopant sites. Among these systems, antimony halides combine a relatively high average atomic number (ZSb=51) and highly efficient intrinsically Stokes-shifted visible luminescence that ensures reabsorption free emission without the need for further doping. Despite this potential, as of today, the very few examples of Sb halide-based scintillators have been focused only on X-ray detection and no study has tackled the scintillation process in detail. Nearly nothing is known about the suitability of these systems for γ detection as well as on the details of trapping and detrapping mechanisms involved in the scintillation process. Here, we aim at contributing to the advancement of this field by synthesizing and investigating the scintillation properties of the hybrid organic-inorganic zero-dimensional antimony chloride system, Gua3SbCl6, consisting of alternating layers of SbCl6 octahedra separated by guanidinium (Gua) moieties. Side-by-side optical and radiometric experiments indicate that the optical properties of Gua3SbCl6 are determined by the absorption by isolated [SbCl6]3- whereas the photo as well as the radio-luminescence are due to the essentially purely radiative transition of Sb. Thermally stimulated luminescence experiments, performed here for the first time on Sb halides, reveal the presence of thermally activated delayed recombination mechanisms of carriers released by deep trap states. Finally, we conducted scintillation experiments using 57Co as 120 keV γ source to directly evaluate a light yield of about 2000 ph/MeV and assessed the radiation resistance by exposing Gua3SbCl6 perovskites to γ radiation doses as high as 1 MGy.

Resume : CdSe/CdS core-crown nanoplatelets display minimal shifts in optical features relative to their respective CdSe core, but yield higher stability and photoluminescence quantum efficiencies,(1) imperative for cutting-edge photonic applications including lasing. In the present work, using femtosecond pump-probe spectroscopy we assessed the influence of crown (CdS) lateral area on the optical gain threshold, gain lifetime and gain bandwidth of CdSe/CdS core-crown nanoplatelets. Our results demonstrate that thin CdS crowns lowers the gain threshold twofold compared to the CdSe core, achieving gain close to 1 exciton per nanoplatelet. The lower gain thresholds for the thin-crown nanoplatelets is likely a consequence of efficient surface trap passivation, as we also observed an increasing gain lifetime of 700 ps, nearly threefold longer than CdSe cores for similar exciton densities. Further increase of the crown lateral area increased the gain threshold to an exciton density similar to the core nanoplatelets, yet the threshold is obtained at a fourfold lower photon flux, as a result of higher absorption cross section. The gain lifetime of 400 ps is also still twofold longer than core nanoplatelets. Furthermore, the thick crown broadened the gain bandwidth to 105 nm. The maximum material gain obtained from the core and core-crown nanoplatelets are similar and reach 15000 cm-1. Our studies confirm that core-crown nanoplatelets are promising materials for light amplification. (1) Leemans, J.; Singh, S.; Li, C.; Ten Brinck, S.; Bals, S.; Infante, I.; Moreels, I.; Hens, Z. Near-Edge Ligand Stripping and Robust Radiative Exciton Recombination in CdSe/CdS Core/Crown Nanoplatelets. J. Phys. Chem. Lett. 2020, 11 (9), 3339?3344

Resume : The transition metal dichalcogenides (TMDCs), layered and two-dimensional (2D) semiconductors, have received great interest during the last decade because of their high chemical stability and good electrocatalytic properties [1-2]. The exfoliation of layered materials into 2D semiconductors creates new structural domains, for example defect-rich in-planes and edges sites, creating surface states within the bandgap, which in turn act as recombination centers for the photogenerated charge carriers [3-4]. Pt nanoparticles (NPs) on 2D surfaces are considered either in a catalytic context for the photoelectrochemical (PEC) hydrogen evolution reaction (HER), or as passivating agent of defect states [3-4]. We need to understand the impact of defect states on the PEC properties of 2D TMDCs and the role of Pt NPs in defect healing. The WSe2 nanosheets were mechanically exfoliated to get bulk, few-layered, and monolayer specimens. The WSe2 nanoflakes had well-defined thicknesses as measured by atomic force microscopy (between ca. 0.9 nm and 250 nm) and the Pt nanoparticles (NPs) were deposited by a variable number of atomic layer deposition (ALD) cycles. I will present the use of reversible model redox species to mimic photoelectrocatalytic processes proving the differences between basal and edges planes. Additionally, I show a record high photocurrent and photon-to-electron conversion efficiency values for Pt-decorated WSe2 nanoflake photocathodes applied in PEC HER. The effect of WSe2 nanosheet thickness, as well as Pt surface loading was carefully quantified. Finally, I demonstrate the effect of structural domains on the PEC performance of 2D materials. i) The photocurrent losses with growing the fraction of the edge site, and the parallel rise of dark currents. ii) The LPE produced WSe2 bulk and few-layer nanoflakes in water reduction and oxidation, achieving only µA cm?2 current densities. This decrease in the PEC performance can be explained by the growing of defect densities, because of the number of edge sites is increased and the area is decreased of LPE prepared nanoflakes, which means the main issue in the larger-scale application of these materials. [5-6]. References [1] K. S. Novoselov, et al., PNAS, 2005, 102, 10451-10453. [2] M. Velický, and P. S. Toth, Appl. Mater. Today, 2017, 8, 68-103. [3] W. Kautek, et al., Ber. Bunsen Ges. Phys. Chem., 1980, 84, 1034. [4] J. M. Velazquez, et al., Energy Environ. Sci., 2016, 9, 164-175. [5] P. S. Toth, G. Szabo, C, Janaky, J. Phys. Chem. C, 2021, 125, 7701-7710. [6] R. A. Wells, et al., ACS Appl. Nano Mater., 2019, 2, 7705?7712.

Resume : Transition Metal Dichalcogenides (TMDs) represent very attractive materials for next generation opto-electronic applications as they are two-dimensional semiconductors with thickness-dependent bandgap, high optical absorption coefficient, good transport properties and mechanical properties such as to allow for flexible devices. [1] Although the best performances are recorded for monocrystalline flakes fabricated by exfoliation methods, large area growth techniques are actively investigated in view for real world applications. [2-4] Here we describe a large area nanofabrication approach based on Laser Interference Lithography for the fabrication of ultra-thin 2D-TMD van der Waals heterostructures supported on transparent nanogrooved silica which feature enhanced optical absorption. Our maskless nanofabrication approach exploits an original physical deposition process at glancing angles to obtain WS2 nanostripes confined on the side of the nanogrooved silica templates. The latter are subsequently vertically capped by a MoS2 layer to form van der Waals heterostructures, extending over cm2 areas. The periodic modulation of refractive index induced by the template is engineered and amplified by insertion of WS2 nanostripes which act as sensitizers, dramatically boosting light extinction in the heterostructure (95% at 480nm wavelength) compared to a flat reference film, for which extinction amounts to 30%. These results show the potential impact of large area 2D heterostructures in the field of nanophotonics. [1] Adv. Mater. 2018, 30, 1705615 [2] Adv. Mater. 2017, 1605785 [3] Nanoscale, 2020, 12, s24385-24393 [4] ACS Appl. Mater. Interfaces 2021, 13, 13508?13516

Resume : Bandgap tunable quantum dots are very interesting in solar cell technology as they can be used to potentially make high-efficiency multi-junction solar cells. One of the more well studied materials in this field is PbS, however, these particles are not environmentally friendly at all as they contain lead. An interesting candidate to replace PbS, and potentially supersede it as well, is Ge1-xSnx nanoparticles. In Ge1-xSnx the bandgap changes with the Sn percent and size due to quantum confinement. If control over the composition and size of the Ge1-xSnx particles is obtained, the bandgap size and type can also be controlled. Looking into the literature shows that these particles can have bandgaps with a wide range of energies from about 0.41 eV to 1.88 eV and potentially more. Previous methods to synthesize these particles are inert syntheses where the used precursors are easily oxidized, and the resulting batch of Ge1-xSnx particles after the synthesis will often contain GeO2 due to the highly reactive precursors used in these syntheses. The requirements for the inert syntheses needed to make these particles makes it a large challenge to succeed in producing oxide free Ge1-xSnx particles. We have instead attempted to make a new synthesis method that is easier and less prone to oxidation, by developing an autoclave synthesis that can be handled in air. The syntheses have mainly been studied with PXRD to determine the crystallite size, the Sn percent through the lattice parameter, and the phases that were present in the resulting particles. Furthermore, the synthesis has also been studied in-situ at the DESY P21.1 beamline, to see how the particles develop throughout the synthesis. The currently resulting particles have sizes ranging from approximately 15-80 nm in size and Sn percent of approximately 1-4 %, which shows successful synthesis results, although still in a limited parameter space.

Resume : Transition metal dichalcogenide (TMD) nanosheets have become an intensively investigated topic in the field of 2D nanomaterials, due to their semiconductor nature, the direct band gap transition and the broken inversion symmetry going from bulk to monolayer. These properties makes TMDs suitable for different technological applications such as photovoltaics, valleytronics, or hydrogen evolution reactions (HER), or transistors. Among them, MoX2 (X = S, Se) are only direct-gap semiconductors when their thickness is reduced to a monolayer, hence an important effort is devoted to obtain single layer TMDs. Colloidal synthesis of TMDs has been developed in recent years as it provides a cost-efficient and scalable way to produce few-layer TMDs having homogenous size and thickness, yet obtaining a monolayer has proved challenging. Here we present a general method for the colloidal synthesis of mono- and few-layer MoX2 (X = S, Se) using elemental chalcogenide and metal chloride as precursors. Using a synthesis with slow injection of the MoCl5 precursor under nitrogen atmosphere, and optimizing the synthesis parameters with a Design Of Experiments (DOE) approach, we obtained a monolayer MoX2 sample with the required semiconducting (2H) phase, a band gap of 1.96 eV for 2H-MoS2 and 1.67 eV for 2H-MoSe2, respectively, both displaying fluorescence at cryogenic and elevated temperatures. A correlation between the blue shifted absorption spectrum and the spectral difference between the Raman modes was established and confirmed that a single-layer thickness was obtained.

Resume : Neutral ?-radicals are an intriguing class of materials where absorption and emission of light involve a spin doublet. They are inherently magnetic due to the open-shell electronic structure with an unpaired electron in their ground state. ?-Radicals have been utilized in a wide range of applications spanning from molecular conductors and solid-state electronics to molecular and polymer magnetics, spintronics and organic light-emitting diodes (OLEDs), thanks to their much-improved stabilities and tailored design strategies in recent years. Electroluminescence from ?-radicals comes from a spin-allowed doublet?doublet (D1?D0) transition, thus setting the theoretical upper limit of internal quantum efficiency to 100%. This performance limit has been demonstrated for molecular monoradicals, making it attractive to build larger luminescent radical platforms with controlled spin?spin interactions. We present our strategy to stabilize two or more ?-radicals in conjugated backbones, so as to obtain diradicals and polyradicals with high luminescence efficiencies. We focus specifically on enhancing the chemical stability of tris(2,4,6-trichlorophenyl)methyl (TTM) radicals, their selective carbon?carbon coupling reactions and conversion to mono-, di- and polyradicals. Various coupling modes of conjugated ?-radicals deliver stable charge-transfer type emission with photoluminescence quantum efficiencies (PLQEs) near unity in the red spectral region. Moreover, the emission can be largely modulated by tuning the conjugation and intramolecular interactions between the radical centres. Our work aims at providing design protocols for ?-radicals in a wide perspective: for the attainment of strongly emissive (but weakly absorbing) and strongly absorbing (but weakly emissive), non-interacting and interacting, localised and delocalised spin centres, and their utilisation in organic optoelectronic applications.

Resume : CdSe-based nanoplatelets are promising for various optoelectronic applications due to their narrow emission spectra and high quantum yield. In a broad temperature range, from cryogenic to room temperatures, nanoplatelet photoluminescence is dominated by recombination of neutral and charged excitons, which can be distinguished by means of magnetooptics. An overview of our recent studies of bare CdSe and core-shell CdSe/CdS nanoplatelets, including CdSe/CdMnS diluted magnetic semiconductor nanostructures will be given. Exciton parameters, such as recombination rates, binding energies of neutral and charged excitons, fine structure splittings, and g-factors were measured. In particular, exciton binding energy in bare nanoplatelets equals to 195?315 meV depending on nanoplatelet thickness, which is about 20 times greater than that in bulk CdSe. Exchange interaction of excitons with surface spins or magnetic Mn ions introduced into the shells was studied. These interactions affect exciton polarization in applied magnetic field, and spin-dependent dark exciton recombination assisted by the surface spins was discovered in bare CdSe nanoplatelets.

Resume : It is well known that the optoelectronic properties of colloidal nanoplatelets are largely set by the strong electron-hole Coulomb attraction, which is enabled by the dielectric confinement and 2D geometry, and gives rise to large exciton binding energies or giant oscillator strength, to name a few effects. Much less is known about the role of electron-electron or hole-hole repulsions. Naturally, the same conditions that lead to strong attractions in nanoplatelets, should lead to strong repulsions as well. In combination with the weak lateral confinement, such repulsions can trigger severe electronic correlations, which are found neither in  nanocrystals (owing to the strong confinement) nor in bulk (owing to the weak Coulomb interaction). In this work, we explore theoretically the opportunities of exploting Coulomb repulsions to engineer the band structure of colloidal nanoplatelets. We study type-I and type-II CdSe-based nanoplatelets charged with up to 4 electrons or holes. A number of remarkable phenomena are then predicted[1,2]. 1) The giant oscillator strength effect, present for neutral excitons, is largely suppressed for trions. 2) Addition energies exceeding 100 meV are required to introduce extra charges in the nanoplatelets, which implies that carriers can be injected one-by-one at room temperature 3) High electron spin states are populated even at low temperatures, which provides multi-electron platelets with an enhanced magnetic moment and paramagnetic response. 4) In type-II core/crown nanoplatelets, large and reversible changes in the emission intensity and energy (~100 meV) can be achieved when switching from X2+ to X3- excitonic species. It is concluded that charging nanoplatelets with a few interacting electrons or holes  is a promising route to develop novel magnetic and optical functionalities. [1] Macias-Pinilla, D.F. et al. "Comparison between Trion and Exciton Electronic Properties in CdSe and PbS Nanoplatelets", J. Phys. Chem. C 2021, 125, 15614. [2] Llusar, J and Climente, J.I. "Charging of colloidal nanoplatelets: effect of Coulomb repulsion on spin and optoelectronic properties", arxiv:2105.14825.

Resume : Nanostructured semiconductors are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although further work is necessary to reduce gain threshold densities for efficient lasing action, there has been some push toward moving away from the current red gain band region, toward green emission. In this work, we take a look at weakly confined ?giant? CdS Quantum Dots which display disruptive optical gain metric in the green optical region. While showing similar gain thresholds compared to state-of-the-art materials, the gain window (440-600nm), amplitude (up to 50000/cm) and gain lifetime (up to 3ns) vastly outpace other materials in the same optical region. These remarkable results are explained by using a bulk semiconductor gain model, which can be done due to the large size of the CdS Quantum Dots (8-12nm). We can very accurately reconstruct the gain window with this model, by including large bandgap renormalization (up to 70meV). This inclusion helps us to understand the gain mechanism in these particles.

Resume : In the field of telecommunications, the global optical infrastructure has become the central nervous system upon which our modern society and economy heavily rely for cloud-services developments and 5G network deployment. The nearly exponential traffic growth experienced in intra- and inter-Data Center Interconnects and metro networks in recent years [1], in particular, poses increasing challenges towards the development of novel solutions for sustaining the demand for bandwidth. Low cost optoelectronic devices are key elements to the achievement of dense and ultra-dense wavelength-division multiplexing networks. In addition to the need of tunable sources, amplification is a key step in long haul telecommunications. A straightforward solution to support the increase of data rate, additionally to developing high speed devices, consists in increasing the spectral bandwidth actually used in optical fibers. The current erbium-doped fiber amplifiers show typical bandwidths of 40 nm centered in the C-band or L-band, and extended bandwidths of 75 nm have been demonstrated by associating a C-band EDFA with a L-band EDFA at the cost of a complex set-up. [2]. Semiconductor optical amplifiers (SOAs), owing to their discrete nature and electrical pumping capability, present a number of key advantages over rare-earth doped fiber amplifiers, such as: smaller size and weight, low cost, higher electrical-to-optical conversion efficiency, broader gain bandwidth, flexibility in the design of the amplification wavelength and potential monolithic integration with other semiconductor components (e.g. lasers, modulators, detectors) [3]. Moreover, thanks to the possibility of tuning the heterostructure geometry and material band gap, amplification over a large bandwidth can be achieved. In this work, we present our latest achievements relating to the realization of InGaAsP quantum-well-based SOA in the O and C+L bands of optical telecommunications. We present the designs of the active layers, including band structure and gain calculation, and their optical characterizations by photoluminescence and photocurrent spectroscopies. Thanks to accurate design and state-of-the-art technological manufacturing, we have realized optical amplifiers operating in the targeted telecommunications bands, and demonstrated operation over 100nm bandwidth as demonstrated by amplified spontaneous emission measurements. [1] Cisco Annual Internet Report (2018?2023), https://www.cisco.com/c/en/us/solutions/collateral/ executiveperspectives/annual-internet-report/white-paper-c11-741490.pdf [2] Y. Sun et al., ?A Gain-Flattened Ultra Wideband EDFA for High Capacity WDM Optical Communications Systems,? in Europen Conference on Optical Communications (ECOC), (Madrid), 1998 [3] M. J. Connelly, Semiconductor optical amplifiers (Springer Science & Business Media, 2007)

Resume : Use of heteromaterial interactions to modify the properties of electronic materials is an ageless topic with vast implications for optical and electronic materials, catalysis, and the fundamental science of synthetic processes. Here, I will highlight several variations on this broad theme in recent work from my group. In the first portion of my talk, I will discuss how in-situ programmed molecular modification of semiconductor surfaces can allow one to stably and reliably tune materials across the spectrum from hole to electron transporting, which immediate use for p-n logic, electronics, and thermoelectrics. I also introduce use of a photo-Seebeck technique to evaluate carrier mobilities rapidly and accurately. In the middle portion of my talk, I will present recent work on the use of interfacial interactions to modify catalyst properties for hydrogen production from alcohols and methane pyrolysis. In the final part of my talk I will highlight a recent collaboration on the use of machine learning protocols to guide growth over dimensional control of perovskite materials. References: 175. ?Producing ultrastable Ni-ZrO2 nanoshell catalysts for dry reforming of methane by flame synthesis and Ni exsolution?, Shuo Liu, Chaochao Dun, Mihir Shah, Junjie Chem, Satyarit Rao, Jilun Wei, Eleni Kyriakidou, Jeffrey J. Urban, and Mark T. Swihart*, submitted, (2021). DOI: Role in work: Contributed to measurements, and data interpretation 174. ?Iron(III) dopant counterions affect the charge transport properties of poly(thiophene) and poly(dialkoxythiophene) derivatives?, Khaled Al Kurdi^, Shawn A. Gregory^, Madeleine P. Gordon, James F. Ponder Jr., Amalie Atassi, Joshua M. Rinehart, Austin L. Jones, Jeffrey J. Urban, John R. Reynolds, Stephen Barlow, Seth R. Marder*, and Shannon K. Yee*, under revision, Advanced Energy Materials, (2021). DOI: Role in work: Contributed to measurements, and data interpretation 169. ?In-situ n-type doping of a solution processed p-type semiconductor for homojunction fabrication?, Håvard Mølnås, Boris Russ, Steven L. Farrell, Madeleine P. Gordon, Jeffrey J. Urban, and Ayaskanta Sahu, under review (2021) DOI: Role in work: Contributed to experimental design, measurements, and data interpretation ***167. ?Dimensional control over metal halide perovskites guided by active learning?, Zhi Li, Philip W. Nega, Mansoor Ani Najeeb, Chaochao Dun, Matthias Zeller, Jeffrey J. Urban, Wissam A. Saidi, Joshua Schrier, Alexander J. Norquist, and Emory M. Chan*, accepted, Chem. Mater., (2021). DOI: https://doi.org/10.1021/acs.chemmater.1c03564 Role in work: Contributed to measurements, and data interpretation 149. ?Stabilized open metal sites in bimetallic metal-organic framework catalysts for hydrogen production from alcohols?, Jonathan L. Snider^, Ji Su^, Pragya Verma, Farid El Gabaly, Joshua D. Sugar, Luning Chen, Jeffrey M. Chames, A. Alec Talin, Chaochao Dun, Jeffrey J. Urban, Vitalie Stavila, David Prendergast, Gabor A. Somorjai, and Mark D. Allendorf*, J. Mat. Chem. A., 9, 10869, (2021). DOI: 10.1039/d1ta00222h Role in work: Contributed to experimental design, measurements, and data interpretation 143. ?Synergistic modulation of the interfacial electronic structure on defective BN nanosheet-supported ultrasmall Ni nanoclusters toward enhanced hydrogen production from methanol?, Zhuolei Zhang, Su Ji, Ana Sanz Matias, Pragya Verma, Chaochao Dun, Yi-sheng Liu, Jinghua Guo, Chengyu Song, David Prendergast, Gabor A. Somorjai*, and Jeffrey J. Urban*, PNAS, 117 (47), 29442-29452 (2020) +ALS DOI: www.pnas.org/cgi/doi/10.1073/pnas.2015897117 141. ?Evaluating mobilities of both carriers from a single bulk sample using photo-Seebeck effect?, Zhenyu Pan, Zheng Zhu, Jeffrey J. Urban, Fan Yang, Ayaskanta Sahu, and Heng Wang*, Materials Today Physics, 17, 100331, (2020) DOI: https://doi.org/10.1016/j.mtphys.2020.100331 Role in work: Contributed to experimental design, measurements, and data interpretation 131. ?In-situ resonant band engineering of solution-processed semiconductors generates high-performance n-type thermoelectric nano-inks?, Ayaskanta Sahu, Boris Russ, Miao Liu, Fan Yang, Edmond W. Zaia, Madeleine P. Gordon, Jason D. Forster, Ya-Qian Zhang, Mary C. Scott, Kristin A. Persson, Nelson E. Coates, Rachel A. Segalman, and Jeffrey J. Urban*, Nature Communications., 11(1), 2069, (2020). DOI: https://doi.org/10.1038/s41467-020-15933-2

Resume : The development of high-quality single Ge/SiGe quantum wells has put Ge as a frontrunner material platform in the race for a large-scale spin-qubit quantum processor. Breaking into the third dimension by integrating two quantum wells in a Ge/SiGe heterostructure could provide an extra degree of freedom for further design of the electronic properties and new device architectures for the germanium quantum information route. Firstly, quantum devices patterned in multiple layers may provide increased connectivity for larger quantum circuits, similarly to 3D integration in conventional classical electronics. Secondly, the shape of the hole wavefunction may be engineered by shifting it in between quantum wells, allowing for a larger parameter space for tuning the g-factor and the spin-orbit coupling. These are relevant components for advanced spin-qubit control and hybrid semiconductor/superconductor devices. Finally, high mobility bilayers may provide a suitable solid-state test bed for fundamental studies of exotic phenomena such as exciton condensation in the quantum Hall regime and counterflow superconductivity. In this work, we take the first significant step in this direction and demonstrate high-mobility hole bilayers in germanium double quantum wells. Through careful design of the heterostructure and due to the low disorder in both quantum wells, we are able to study in detail the quantum transport properties of the system with magnetotransport measurements. In particular, we are able to observe the signature of a symmetric-antisymmetric gap when we tune the density in the quantum wells to be the same. Nico W. Hendrickx et al. “A four-qubit germanium quantum processor”. In: Nature 591.7851 (2021), pp. 580–585 N. W. Hendrickx et al. “A single-hole spin qubit”. In: Nature Communications 11.1 (2020) J.P. Eisenstein and MacDonald A.H. “Bose Einstein Condensation of Excitons in Bilayer Electron Systems Bose-Einstein Condensation of Excitons in Bilayer”. In: Nature March 2005 (2014), pp. 691–694 Jung Jung Su and A. H. MacDonald. “How to make a bilayer exciton condensate flow”. In: Nature Physics 4.10 (2008), pp. 799–802 Sara Conti et al. “Electron–hole superfluidity in strained Si/Ge type II heterojunctions”. In: npj Quantum Materials 6.1 (Dec. 2021)

Resume : A field-effect transistor with an organic semiconductor in its channel is known as an organic field-effect transistor (OFET). The semiconductors, dielectric layer, and electrodes in an OFET are all made of organic or polymer materials. They've received a great deal of attention because of their ability to make large-area, mechanically flexible, lightweight, and comparatively cheaper devices. In order to investigate the OFET performance, metal-insulator-semiconductor (MIS) structure was used to investigate the behaviour between the insulator and semiconductor. The semiconductor and dielectric layer have a significant impact on the performance of MIS capacitors. The accumulation/depletion behaviour, charge transport and trapping, and interface effects between insulator and semiconductor are all significant properties for MIS [1]. Because the insulator is sandwiched between the semiconductor and ohmic metal contacts, and the bias voltage is delivered across an insulator, the interface between the insulator and the semiconductor layer must be well-matched in order to have acceptable MIS properties. In this work we report the fabrication and Characterization of organic field effect transistor by using nano posited of PMMA:TiO2 At first we synthesized TiO2 nanoparticles through hydrothermal method using titanium tetra isopropoxide. TTIP was kept under continuous stirring for 30 minutes, in a 20 mL ethanol solution. The dispersion medium was then formed by adding a little amount of distilled water approximately 10-12 drops. For 20 minutes, the product was immersed in ultrasonic water. The solution was sonicated before being placed in an autoclave for 3 hours at 150°C. To eliminate the contaminants, the solution was cooled to room temperature before being rinsed and centrifuged with deionized water. Then Filter paper was used to filter it. The filtered sample was dried for 5 hours in a 110°C oven before being annealed for 2 hours at 500°C. The TiO2 NPs that resulted were then collected and characterised further. All the physiochemical characterisation including, XRD analysis, SEM, TEM, Surface Area calculation and Particle size analysis were performed.In case of XRD profile the peaks were matched with JCPDS card no (21-1272). Perfect matching of peak position confirmed the crystalline growth of TiO2 nano materials. All the other characterization results perfectly corroborated with the TiO2 nano materials formation. In the same way composite of TiO2:PMMA was synthesized and similar characterization techniques were used that confirmed the composite formed. Finally an OFET device was fabricated using the above synthesized materials. The electrical properties of this device was measured using Kiethley2450 characterization system. The drain voltage varied at a step of 0.1 from -4V to 0V and gate voltage varied from -10 to +10V.The recorded I-V curve looks same as the theoretical curve for transistor.The fabricated OFET exhibits good saturation current making a promising future material.

Resume : The recently growing research field called ?Nanophononics? deals with the investigation and control of vibrations in solids at the nanoscale. Phonon engineering leads to a controlled modification of phonon dispersion, phonon interactions, and transport [1,2]. However, engineering and probing phonons and phonon transport at the nanoscale is a non-trivial problem. In this talk, we discuss how phononic properties can be engineered and measured in nanowires [3-5] and the challenges and progresses in the measurement of the thermal conductivity of nanostructures and low dimensional systems [6,7]. The concept of phonon engineering in NWs is exploited in superlattice (SL) NWs. We experimentally show that a controlled design of the NW phononic properties can be decided à la carte by tuning the SL period. References [1] M. Maldovan, Nature 503, 209 (2013). [2] S. Voltz et al., Eur. Phys. J. B 89, 15 (2016) [3] M. De Luca et al. Nano Lett. 19, 4702 (2019) [4] D. de Matteis et al. ACS Nano 14, 6845-6856 (2020) [5] E. M. T. Fadaly et al. Nano Letters 21, 3619-3625 (2021) [6] D. Vakulov et al. Nano Lett. 20, 2703-2709 (2020) [7] M. Y. Swikels et al. Phys. Rev. Appl. 14, 024045 (2020)

Resume : Monoclinic VO2 (m-VO2) undergoes a Metal-Insulator Transition (MIT) at ~67°C and is therefore labelled as thermochromic. In this work, we first demonstrate how magnetron sputtering of a vanadium target in an Ar/O2 mixture can be optimized to synthesize 200 nm thick films containing m-VO2 crystals. In our synthesis conditions, the thermochromic material is obtained with a precise oxygen flow rate followed and by annealing the films in N2 for 60 minutes at 400 °C. In the second part, we validate numerical results (CAvity Modelling Framework, CAMFR) by comparison with the optical properties of the synthesized films. Through simulations, we validate how the nanostructuration can be tuned in order to improve film properties for application as smart windows. By varying the VO2 nano-ribbon width, the periodicity, and the film thickness, one can generate a better performance in terms of energy saving and a less opaque color as compared to a dense film with the same thickness [Savorianakis G. et al. J. Appl. Phys. 129 (2021)]. In the final part, we experimentally combined the m-VO2 films with gold nanoparticles (AuNPs) to obtain tunable plasmonic signal according to the temperature. We show how AuNPs can be grafted onto the VO2 film surface using (3-aminopropyl) trimethoxysilane (APTMS) linkers. A 10nm-shift in wavelength of the plasmonic peak is evidenced and quantified as a function of the temperature. In parallel, a 2D version of this platform has been simulated in order to understand the underlying physics and provide pathways to improve the optical shift thanks to the CAMFR tool. The here-mentioned work may pave the way towards the elaboration of thin film materials with high optical accordability which can potentially be used in applications as colour display, protection against counterfeiting and opto-electronics chips.

Resume : In order to serve the changing needs of road traffic control, the road space and road structure surrounding an intersection have evolved into complex forms. A proposed innovative treatment for congested urban is the split intersection. It facilitates a smoother flow with less driver delay by reducing the number of vehicle signal phases. We propose a Visible Light Communication system based on Vehicle-to-Vehicle, Vehicle-to-Infrastructure and Infrastructure-to-Vehicle communications able to safely manage vehicles crossing through an intersection leveraging Edge of Things (EoT) facilities. Using the headlights, the street lamps and the traf?c signaling to broadcast the information, the connected vehicles interact with each other and with the infrastructure through visible light. Mobile optical receivers, using joint transmission, collect the data at high frame rates, extracts theirs location to perform positioning and, concomitantly, the transmitted data from each transmitter. In parallel, an intersection manager coordinates the traffic flow and interacts with the vehicles through Driver Agents installed on them. To command the passage of vehicles crossing the intersection safely request/response mechanisms and temporal/space relative pose concepts are used. A communication scenario is stablished and a ?mesh/cellular? hybrid network configuration proposed. Data is encoded, modulated and converted into light signals emitted by the transmitters. As receivers and decoders, optical sensors with light filtering properties, are used. The core element of the receiver is a photodetector that converts the optical power into electrical current. The VLC photosensitive receiver is a double pin/pin photodetector based on a tandem heterostructure, p-i'-n/p-i-n sandwiched between two conductive transparent contacts. Due to its tandem structure, the device is an optical controlled filter able to identify the wavelengths and intensities of the impinging optical signals. Its quick response enables the possibility of high speed communications. Since the photodetector response is insensitive to the frequency, phase, or polarization of the carriers, this kind of receiver is useful for intensity-modulated signals. Bidirectional communication between the infrastructure and the vehicles is tested, using the VLC request/response concept. Results show that the short-range mesh network ensures a secure communication from street lamp controllers to the edge computer through the neighbor traffic light controller with active cellular connection and enables peer-to-peer communication, to exchange information between V-VLC ready connected cars.

Resume : Since the discovery of Graphene, atomically thin semiconductor 2D materials and their van der Waals heterostructures have attracted a lot of interest for electronic and optoelectronic devices [1, 2]. For the growth of 2D materials and heterostructures, Pulsed laser Deposition (PLD) has emerged recently as a suitable growth technique. It allows a homogeneous deposition on the substrate on the cm x cm scale, an efficient control of the number of layers, and stoichiometric transfer of the target material during the growth [3, 4]. In present study, we will present the deposition conditions and characterizations of individual layers of WS2 and MoS2 grown on sapphire and STO substrate by PLD. The presence of two prominent Raman modes E2g which appear at ~359 cm-1 for WS2 and ~386 cm-1 for MoS2 due to in plane vibration and A1g mode which appears ~421 cm-1 and ~407 cm-1 for WS2 and MoS2 respectively due to out of plane vibration mode confirm the formation of 2D materials on those substrates [5]. The transport study confirms that MoS2 exhibits p-type and WS2 show n-type behavior. Finally, PLD grown 2D van der Waals heterojunctions are grown by stacking of two different p-type MoS2 and n-type WS2 layers [6]. The presence of the above Raman modes confirms the fabrication of p-n heterojunctions on sapphire and STO substrate. A detail analysis of PLD grown p-n heterojunctions will be presented and discussed. References: [1] Andrea C. Ferrari et.al., Nanoscale, 7 (2015) 4598?4810 [2] Riccardo Frisenda, day J. Molina-Mendoza, Thomas Mueller, Andres Castellanos-Gomez and Herre S. J. van der Zant, Chem. Soc. Rev., 47 (2018) 3339-3358 [3] Tamie A. J. Loh and Daniel H. C. Chua, ACS Appl. Mater. Interfaces, 6 (2014) 15966?15971 [4] Martha I. Serna et.al., ACS Nano, 10 (2016) 6054?6061 [5] Sangyeon Pak et.al. Nano Lett. 17 (2017) 5634-5640 [6] Victro Zatko et.al. ACS Nano 15 (2021) 7279-7289

Resume : The Template-Assisted Selective Epitaxy (TASE) method, developed at IBM Research Europe ? Zurich, permits to create a homogeneous integration route for various semiconductor materials which is compatible with the CMOS process. With this approach, a III-V nanowire can be grown horizontally inside a silicon oxide template by metal-organic chemical vapor deposition (MOCVD). The confined growth implies that the diffusion mechanisms of group III and group V components are different when compared with both planar epitaxy[1]. This means that TASE allows us to grow complex III-V heterostructures. In this work we have used TASE to create an InGaAs-InP type II superlattice system with dimensions of approximately 200 nm x 200 nm in cross section and 700 nm in length with < 10-nm-thick layers in the superlattice stack. Our goal was to evaluate and optimise heterointerface quality. Based on our fabrication approach, we observed different, facet-dependent, growth rates for the InP and InGaAs layers. In particular, early experiments on < 001> Silicon-on-Insulator (SoI) wafers resulted in a complex facet morphology with different behaviour of the group III and V components. Here we observe staggering of the III component through the interface, leading to alloying and loss of interface sharpness. However, the interfaces were well defined in regards to the group V component. To improve the interface definition even further we have introduced buffer time at the switching of the group III precursor flows. We used a group V precursor overpressure during these buffer steps to avoid temperature-driven desorption of group V atoms from the first surface layers, adjusting the concurrent group V element switching process to minimize group V element substitution at the interface. In order to evaluate the InGaAs/InP type II superlattice heterointerfaces, we employed characterization techniques such as Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive Xray Spectroscopy (EDS). We have investigated both crystalline quality, in terms of defects and facet selection, and composition intermixing for both III and V components. Our results highlight the challenges posed by the complex and varied diffusion dynamics in such complex InGaAs/InP type II superlattice heterostructure and give indications of how to solve those challenges, namely alloying, facet control, and process control. This project has received funding from the European Union?s Horizon 2020 research and innovation programme under the Marie Sk?odowska-Curie grant agreement No 860095. The authors thank the Cleanroom Operations Team of the Binnig and Rohrer Nanotechnology Center (BRNC) for their help and support. References: [1] M. Borg, H. Schmid, K. E. Moselund, D. Cutaia, and H. Riel, ?Mechanisms of template-assisted selective epitaxy of InAs nanowires on Si,? Journal of Applied Physics, vol. 117, no. 14, pp. 1?8, 2015, doi: 10.1063/1.4916984.

Resume : Well-defined III-V material conditions are required for the development of high-performance optoelectronic devices. To achieve such controlled properties, it is necessary to study the electrical behaviour with high spatial resolution. For this purpose, various sophisticated tip-based methods such as AFM or multi-tip scanning tunnelling microscopy (MTSTM) can be employed. Based on our studies on III-V semiconductor nanowire (NW) heterostructures with the MTSTM operated as a four-point nanoprober, we demonstrate that in electrical, tip-based measurement methods, the tip-to-semiconductor contact is essential in interpreting the properties of the sample. In order to study the electrical behaviour of the NW structure, we investigated a variety of NWs covered with native oxide, in particular upright, freestanding GaAs-based axial as well co-axial NWs on the growth substrates. The axial NWs as well as the cores of the co-axial NWs were grown in a low-pressure horizontal metalorganic vapor-phase epitaxy reactor via the vapor-liquid-solid mode, followed, in the case of the co-axial structures, by epitaxial shell growth. We employ a combination of material-selective wet chemical etching of as-grown coaxial NWs to access the pure NW core and to obtain spatially resolved I-V analysis. Based on a core-shell p-GaAs/ n-GaInP NW sample, we show in detail charging currents at the interface between the measuring tip and the semiconductor via the native insulating oxide, which acts as a MIS-capacitor with charging and discharging conditions in the operating voltage range. Only through our special measurement setup with the movable probes that could be placed at defined positions along the nanowire, it was possible to show that, all the samples investigated displayed a strong dependency of the overall electrical behaviour on the condition of the measuring tip-to-semiconductor contact. We analyse in detail the observed I-V characteristics of p-GaAs/n-GaInP as well as the other samples and demonstrate a reduction of the charging and discharging effects. Based on this, we propose a strategy to achieve an optimized tip-to-semiconductor junction which minimizes the influence of the native oxide layer on the overall electrical measurements. Our high-end characterization method enables a direct relation between the tip-to-NW-junction and the electronic properties of as-grown (co-)axial NWs, which provides precise advice for future investigations.

Resume : Field effect transistors (FETs) can be used as fast room temperature detectors of THz radiation [1]. The mechanism of the detection in FETs is based on the excitation of resonant or overdamped plasma waves in the device channel [2-3]. Si-based detectors, operating at room temperature, have an advantage of compatibility with mainstream CMOS technology as compared to other technologies such as the ones based on III-V materials. Strained-Si modulation doped FETs (MODFETs) offer higher mobility than the one achievable in conventional MOSFETs. The ability of n-channel strained-Si MODFETs as room temperature non-resonant detectors of THz electromagnetic radiation (EM) has been experimentally demonstrated. [4]. Strained-Si MODFETs were used in terahertz imaging [5] and, recently, it was shown that using this technology together with the terajet effect the diffraction limit can be overcome allowing to enhance the spatial resolution of terahertz images. The epistructure of the MODFETs in which this work is based was grown by molecular beam epitaxy (MBE) on a thick relaxed SiGe virtual substrate grown by low-energy plasma-enhanced chemical vapour deposition (LEPECVD) over a p-doped conventional Si wafer. The device had a 8 nm tensile strained (in terms of biaxial deformation) Si channel, sandwiched between two heavily doped SiGe electron supply layers to generate a high carrier density in the strained-Si quantum well. Two transistors with different gate lengths (50 nm and 250 nm) were used in measurements. The photovoltaic (drain-to-source measure in open circuit) response of the device was measured in the sub-THz range. A non-resonant sub-THz response was found at room temperature confirming the ability of those devices to be used in THz detection. The maximum sensitivity was obtained when the gate was biased nearly to the threshold voltage. Both the sub-THz responsivity and the NEP (Noise Equivalent Power) of the device were obtained. A 2D numerical analysis of the electron transport in the transistor was carried on using a hydrodynamic model coupled to the Poisson equation. The time-dependent response of the device was obtained assuming a sinusoidal excitation of the gate. Simulations were performed to understand the intense response found in measurements and, subsequently, optimize the structure. Additionally, 3D simulations of the Maxwell equations were used to gain insight into the coupling of the incoming THz EM radiation to the transistor channel. REFERENCES [1] Kachorovskii V Yu and Shur M S, Solid State Electron. vol. 52, 182, 2008. [2] Dyakonov M and Shur M S, Phys. Rev. Lett. vol. 71, 2465, 1993. [3] Dyakonov M and Shur M S, IEEE Trans. Electron Dev., vol. 43, 380, 1996. [4] Delgado-Notario, J.A.; Velazquez-Perez, J.E.; Meziani, Y.M.; Fobelets, K. Sensors, 18, 543, 2018. [5] Minin I.V., Minin O.V., Salvador-Sánchez, J., Delgado-Notario J.A., Calvo-Gallego J., Ferrando-Bataller M., Fobelets K., Velázquez-Pérez J.E., and Meziani, Y.M. Opt. Lett. 46, 3061-3064, 2021.