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GaN nanowires (NWs) grown by molecular beam epitaxy on Ti films sputtered on Al2O3 are studied by X‐ray diffraction (XRD) and grazing‐incidence small‐angle X‐ray scattering (GISAXS). XRD, performed both in symmetric Bragg reflection mode and at grazing incidence, reveals Ti, TiN, Ti3O, Ti3Al and Ga2O3 crystallites with in‐plane and out‐of‐plane lattice parameters intermediate between those of Al2O3 and GaN. These topotaxial crystallites in the Ti film, formed as a result of interfacial reactions and N exposure, possess little misorientation with respect to Al2O3. As a result, GaN NWs grow on the top TiN layer, possessing a high degree of epitaxial orientation with respect to the substrate. The measured GISAXS intensity distributions are modelled by the Monte Carlo method, taking into account the orientational distributions of NWs, the variety of their cross‐sectional shapes and sizes, and the roughness of their side facets. The cross‐sectional size distributions of the NWs and the relative fractions of the {1100} and {1120} side facets are determined. [ABSTRACT FROM AUTHOR]

The quality and the performance of hybrid perovskite (HP)'s films strongly depend on the complete conversion into MAPbI3 of a spin-coated solution of methylammonium iodide (MAI) and PbI2. Highly crystalline PbI2 on a substrate limits such a conversion and, consequently, the HP's solar cell performances. We investigate for the first time the use of short-chain organic acids as additives in a non-complexing solvent like γ-butyrolactone (GBL), that can retard retard the crystallization of PbI2. Based on XRD analyses of the spin coated films, the acetic acid is the most effective additive in retarding the PbI2 crystallization, making Pb2+ available for a subsequent reaction with MAI. These results open a new experimental path for fabricating perovskite films by single or sequential step methods involving acid additives. [ABSTRACT FROM AUTHOR]

Organic thin films based on naphthalenediimides (NDIs) bearing alkyl substituents have shown interesting properties for application in OLEDs, thermoelectrics, solar cells, sensors and organic electronics. However, the polymorphic versatility attributed to the flexibility of alkyl chains remains a challenging issue, with detrimental implications on the performances. Aryl analogues containing C6H6N2 constitutional isomers are herein investigated as one of the possible way-out strategies. The synthesis of molecular and polymeric species is described, starting from naphthaleneteracarboxyldianhydride with isomeric aromatic amines and hydrazine. The materials are fully characterized by spectroscopy, thermal and structural X-ray diffraction methods, both as bulk powders and thin films, revealing a rich structural landscape. Depending on the stereochemistry of the branching aryls, the compounds show a variety of parallel stacking of the NDI cores, and high structural stability upon heating, up to 560 °C in the polymeric form. Thin films prepared by spin coating from organic solvent solutions and studied by grazing-incidence X-ray diffraction exhibit a high degree of crystallinity indicating the intrinsic tendency of these molecules to self-assemble in an ordered fashion without the need for any post-processing technique. In line with other NDI-based diimides, UV-vis spectroscopy indicates the optical band gaps falling in the visible region (2.87–3.02 eV). DFT calculations reveal a significant lowering of the frontier orbital energies of the hydrazido derivative. Beyond solution processing, the high thermal stability and the absence of polymorphic forms of these materials suggest that sublimation-based routes for films and device preparation can also be followed. [ABSTRACT FROM AUTHOR]

1/f noise represents the dominant source of noise in the low‐frequency range in several physical systems, including field‐effect transistors. Its investigation can provide very important information on the fabrication process, highlighting the steps that are more prone to the introduction of defects. Here, 1/f noise in bilayer MoS2 transistors on paper with inkjet‐printed Ag contacts and hBN dielectric is investigated. These devices are promising building blocks for future low‐cost, flexible, and easily recyclable disposable electronics. The analysis of 1/f noise, performed following Hooge's empirical approach, results in a Hooge parameter ≈1–10, which is comparable to those reported for bilayer MoS2 transistors on SiO2. The present results indicate that the noise properties of the investigated devices are stable against substrate bending and are mainly determined by the printing of the dielectric, while not being sensibly affected by the use of the paper substrate. These results are promising for the further development of low noise 2D material‐based flexible electronics on paper. [ABSTRACT FROM AUTHOR]

We analyze the temporal variation of the substrate optical reflectance during the formation of GaN nanowires in molecular beam epitaxy using transverse electric and transverse magnetic polarized light. The time dependence of the reflectance signal is simulated using an effective medium approach that takes into account the spatial fluctuations in the length and density of the nanowire ensemble within the area probed by the laser beam. Our study demonstrates that optical reflectometry is a powerful technique to detect the onset of nanowire nucleation, assess the deposition rate, and estimate the fluctuations in the nanowire length and covered area fraction at the μm scale. [ABSTRACT FROM AUTHOR]

We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The approach is based on the fact that these nanostructures do not form in plasma-assisted molecular beam epitaxy on structurally and chemically uniform cation-polar substrates. By in situ depositing and nitridating Si on a Ga-polar GaN film, we locally reverse the polarity to induce the selective area epitaxy of N-polar GaN nanowires. We show that the nanowire number density can be controlled over several orders of magnitude by varying the amount of predeposited Si. Using this growth approach, we demonstrate the synthesis of single-crystalline and uncoalesced nanowires with diameters as small as 20 nm. The achievement of nanowire number densities low enough to prevent the shadowing of the nanowire sidewalls from the impinging fluxes paves the way for the realization of homogeneous core-shell heterostructures without the need of using ex situ prepatterned substrates. [ABSTRACT FROM AUTHOR]

In this study, the structure and morphology, as well as time, ultraviolet radiation, and humidity stability of thin films based on newly developed 1D (PRSH)PbX3 (X = Br, I) pseudo-perovskite materials, containing 1D chains of face-sharing haloplumbate octahedra, are investigated. All films are strongly crystalline already at room temperature, and annealing does not promote further crystallization or film reorganization. The film microstructure is found to be strongly influenced by the anion type and, to a lesser extent, by the DMF/DMSO solvent volume ratio used during film deposition by spin-coating. Comparison of specular X-ray diffraction and complementary grazing incidence X-ray diffraction analysis indicates that the use of DMF/DMSO mixed solvents promotes the strengthening of a dominant 100 or 210 texturing, as compared the case of pure DMF, and that the haloplumbate chains always lie in a plane parallel to the substrate. Under specific DMF/DMSO solvent volume ratios, the prepared films are found to be highly stable in time (up to seven months under fluxing N2 and in the dark) and to highly moist conditions (up to 25 days at 78% relative humidity). Furthermore, for representative (PRSH)PbX3 films, resistance against ultraviolet exposure (λ = 380 nm) is investigated, showing complete stability after irradiation for up to 15 h at a power density of 600 mW/cm2. These results make such thin films interesting for highly stable perovskite-based (opto)electronic devices. [ABSTRACT FROM AUTHOR]

GaN nanowires (NWs) grown by molecular beam epitaxy on Ti films sputtered on Al 2 O 3 are studied by X-ray diffraction (XRD) and grazing-incidence small-angle X-ray scattering (GISAXS). XRD, performed both in symmetric Bragg reflection mode and at grazing incidence, reveals Ti, TiN, Ti 3 O, Ti 3 Al and Ga 2 O 3 crystallites with in-plane and out-of-plane lattice parameters intermediate between those of Al 2 O 3 and GaN. These topotaxial crystallites in the Ti film, formed as a result of interfacial reactions and N exposure, possess little misorientation with respect to Al 2 O 3 . As a result, GaN NWs grow on the top TiN layer, possessing a high degree of epitaxial orientation with respect to the substrate. The measured GISAXS intensity distributions are modelled by the Monte Carlo method, taking into account the orientational distributions of NWs, the variety of their cross-sectional shapes and sizes, and the roughness of their side facets. The cross-sectional size distributions of the NWs and the relative fractions of the {1100} and {1120} side facets are determined., (© Vladimir M. Kaganer et al. 2023.)

Methylammonium lead tribromide (MAPbBr 3 ) perovskite single crystals demonstrate to be excellent direct X-ray and gamma-ray detectors with outstanding sensitivity and low limit of detection. Despite this, thorough studies on the photophysical effects of exposure to high doses of ionizing radiation on this material are still lacking. In this work, we present our findings regarding the effects of controlled X-ray irradiation on the optoelectronic properties of MAPbBr 3 single crystals. Irradiation is carried out in air with an imaging X-ray tube, simulating real-life application in a medical facility. By means of surface photovoltage spectroscopy, we find that X-ray exposure quenches free excitons in the material and introduces new bound excitonic species. Despite this drastic effect, the crystals recover after 1 week of storage in dark and low humidity conditions. By means of X-ray photoelectron spectroscopy, we find that the origin of the new bound excitonic species is the formation of bromine vacancies, leading to local changes in the dielectric response of the material. The recovery effect is attributed to vacancy filling by atmospheric oxygen and water.