ABSTRACT: Thin films of rhenium trioxide (ReO₃) were produced by reactive DC magnetron sputtering from a metallic rhenium target, followed by annealing in the air in a range of temperatures from 200 to 350 °C. Nanocrystalline single-phase ReO₃ films were obtained after being annealed at about 250 °C. The thin films appeared bright red in reflected light and blue-green in transmitted light, thus showing an optical transparency window in the spectral range of 475−525 nm. The film exhibited a high conductivity as evidenced by macro- and nanoscale conductivity measurements. The long-range and local atomic structures of the films were studied in detail by structural methods, such as X-ray diffraction and X-ray absorption spectroscopy. The oxidation state (6+) of rhenium was confirmed by X-ray photoemission and X-ray absorption spectroscopy. The nanocrystalline morphology of the annealed films was evidenced by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The obtained results allowed us to propose a mechanism of rhenium oxide conversion from the initially amorphous ReO_x phase to cubic ReO₃.

ABSTRACT: CdTe and CdO nanocrystals were synthesized by chemical deposition into a-SiO₂/n-Si ion track template formed by 200 MeV Xe ion irradiation with the fluence of 10⁸ ions/cm². Depending on the temperature of the solution CdTe + CdO and single-phase CdO with a hexagonal crystal structure were obtained, respectively. The study of the current – voltage characteristics of the obtained structure with the single-phase CdO allows us to estimate the number of grain boundaries and the height of the potential barrier, as well as the n-type conductivity.

ABSTRACT: In this study, we demonstrated for the first time the growth of ZnO nanowires (NWs) decorated with highly crystalline few-layer PbI₂ and fabricated two-terminal single-nanowire photodetector devices to investigate the photoelectric properties of the hybrid nanostructures. We developed a novel two-step growth process for uniform crystalline PbI₂ nanosheets via reactive magnetron deposition of a lead oxide film followed by subsequent iodination to PbI₂ on a ZnO NW substrate, and we compared as-grown hybrid nanostructures with ones prepared via thermal evaporation method. ZnO–PbI₂ NWs were characterized by scanning and transmission electron microscopy, X-ray diffraction analysis and photoluminescence measurements. By fabricating two-terminal single-nanowire photodetectors of the as-grown ZnO–PbI₂ nanostructures, we showed that they exhibit reduced dark current and decreased photoresponse time in comparison to pure ZnO NWs and have responsivity up to 0.6 A/W. Ab initio calculations of the electronic structure of both PbI₂ nanosheets and ZnO NWs have been performed, and show potential for photoelectrocatalytic hydrogen production. The obtained results show the benefits of combining layered van der Waals materials with semiconducting NWs to create novel nanostructures with enhanced properties for applications in optoelectronics or X-ray detectors. © 2020 Elsevier B.V.

ABSTRACT: Pulsed laser ablation is used to form high-quality silicon-doped β-Ga₂O₃ films on sapphire by alternatively depositing Ga₂O₃ and Si from two separate sources. X-ray analysis reveals a single crystallinity with a full width at half maximum for the rocking curve around the (−201) reflection peak of 1.6°. Silicon doping concentration is determined by elastic recoil detection analysis (ERDA), and the best electrical performance is reached at a Si concentration of about 1 × 10²⁰ cm⁻³, using optimized deposition parameters. It is found that a high crystalline quality and a mobility of about 2.9 cm2 (V s)⁻¹ can be achieved by depositing Si and Ga₂O₃ from two separate sources. Two types of Schottky contacts are fabricated: one with a pure Pt and one with a PtO_x composition. Electrical results from these structures are also presented.

ABSTRACT: Crystalline wurtzite zinc oxide (w-ZnO) can be used as a wide bandgap semiconductor for light emitting devices and transparent or high temperature electronics. The use of amorphous zinc oxide (a-ZnO) can be an advantage in these applications. In this paper, we report on x-ray amorphous a-ZnO_x thin films (∼500 nm) deposited at cryogenic temperatures by reactive magnetron sputtering. The substrates were cooled by a nitrogen flow through the copper substrate holder during the deposition. The films were characterized by x-ray diffraction, Raman, infrared, UV–Vis-near-infrared spectroscopies, and ellipsometry. The a-ZnO_x films on glass and Ti substrates were obtained at the substrate holder temperature of approximately −100 °C. New vibration bands at 201, 372, and 473 cm⁻¹ as well as O–H stretch and bend absorption bands in the a-ZnO_x films were detected by Fourier transform infrared spectroscopy. Raman spectra showed characteristic ZnO₂ peaks at 386 and 858 cm⁻¹ attributed to the peroxide ion O₂²⁻ stretching and libration modes, respectively. In addition, the films contain neutral and ionized O₂ and O₂− species. The a-ZnO_x films are highly transparent in the visible light range (≈87%) and exhibit a refractive index of 1.68 at 2.25 eV (550 nm). An optical bandgap is 4.65 eV with an additional band edge absorption feature at 3.50 eV. It has been shown that the deposition on actively cooled substrates can be a suitable technique to obtain low temperature phases that cannot be deposited at room temperature.

ABSTRACT: Scandium fluoride (ScF₃) belongs to a class of negative thermal expansion (NTE) materials. It shows a strong lattice contraction up to about 1000 K switching to expansion at higher temperatures. Here the NTE effect in ScF₃ is studied in the temperature range from 300 K to 1600 K using ab initio molecular dynamics (AIMD) simulations in the isothermal-isobaric (NpT) ensemble. The temperature dependence of the lattice constant, inter-atomic Sc–F–Sc bond angle distributions and the Sc–F and Sc–Sc radial distribution functions is obtained as a function of supercell size from 2a × 2a × 2a to 5a × 5a × 5a where a is the lattice parameter of ScF₃. A comparison with the experimental Sc K-edge EXAFS data at 600 K is used to validate the accuracy of the AIMD simulations. Our results suggest that the AIMD calculations are able to reproduce qualitatively the NTE effect in ScF₃, however a supercell size larger than 2a × 2a × 2a should be used to account accurately for dynamic disorder. The origin of the NTE in ScF₃ is explained by the interplay between expansion and rotation of ScF₆ octahedra.

ABSTRACT: We performed, for first time, ab initio calculations for the ReO₂-terminated ReO₃ (001) surface and analyzed systematic trends in the ReO₃, SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surfaces using first-principles calculations. According to the ab initio calculation results, all ReO₃, SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surface upper-layer atoms relax inwards towards the crystal bulk, all second-layer atoms relax upwards and all third-layer atoms, again, relax inwards. The ReO₂-terminated ReO₃ and ZrO₂-terminated SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surface band gaps at the Γ–Γ point are always reduced in comparison to their bulk band gap values. The Zr–O chemical bond populations in the SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ perovskite bulk are always smaller than those near the ZrO₂-terminated (001) surfaces. In contrast, the Re–O chemical bond population in the ReO₃ bulk (0.212e) is larger than that near the ReO₂-terminated ReO₃ (001) surface (0.170e). Nevertheless, the Re–O chemical bond population between the Re atom located on the ReO₂-terminated ReO₃ (001) surface upper layer and the O atom located on the ReO₂-terminated ReO₃ (001) surface second layer (0.262e) is the largest.

ABSTRACT: By means of the CRYSTAL computer program package, first-principles calculations of polar ZrO-, Ca- and O-terminated CaZrO₃ (011) surfaces were performed. Our calculation results for polar CaZrO₃ (011) surfaces are compared with the previous ab initio calculation results for ABO₃ perovskite (011) and (001) surfaces. From the results of our hybrid B3LYP calculations, all upper-layer atoms on the ZrO-, Ca- and O-terminated CaZrO₃ (011) surfaces relax inwards. The only exception from this systematic trend is outward relaxation of the oxygen atom on the ZrO-terminated CaZrO₃ (011) surface. Different ZrO, Ca and O terminations of the CaZrO₃ (011) surface lead to a quite different surface energies of 3.46, 1.49, and 2.08 eV. Our calculations predict a considerable increase in the Zr–O chemical bond covalency near the CaZrO₃ (011) surface, both in the directions perpendicular to the surface (0.240e) as well as in the plane (0.138e), as compared to the CaZrO₃ (001) surface (0.102e) and to the bulk (0.086e). Such increase in the B–O chemical bond population from the bulk towards the (001) and especially (011) surfaces is a systematic trend in all our eight calculated ABO₃ perovskites.

ABSTRACT: Pulsed laser ablation is used to form high-quality silicon-doped β-Ga₂O₃ films on sapphire by alternatively depositing Ga₂O₃ and Si from two separate sources. X-ray analysis reveals a single crystallinity with a full width at half maximum for the rocking curve around the (−201) reflection peak of 1.6°. Silicon doping concentration is determined by elastic recoil detection analysis (ERDA), and the best electrical performance is reached at a Si concentration of about 1 × 1020 cm⁻³, using optimized deposition parameters. It is found that a high crystalline quality and a mobility of about 2.9 cm² (V s)⁻¹ can be achieved by depositing Si and Ga₂O₃ from two separate sources. Two types of Schottky contacts are fabricated: one with a pure Pt and one with a PtOx composition. Electrical results from these structures are also presented.

ABSTRACT: The ferroelectric distortions in perovskites were a subject of numerous investigations for a long time. However, some controversial results still exist, coming from the analysis of diffraction (X-ray, neutron or electron) data and X-ray absorption spectra. In this study, our goal was to revisit these classical materials using recently developed methods without imposing any predefined structural model. Local environment around A-type atom in ABO₃ perovskites (SrTiO₃, BaTiO₃, EuTiO₃) was studied by X-ray absorption spectroscopy (XAS) in a wide range of temperatures (20–400 K). Using reverse Monte Carlo method enhanced by evolutionary algorithm, the 3D structure was extracted from the extended X-ray absorption fine structure (EXAFS) and interpreted in terms of the radial distribution functions (RDFs). Our findings show that both diffraction and XAS data are consistent, but reflect the structure of the material from different points of view. In particular, when strong correlations in the motion of certain atoms are present, the information obtained by XAS might lead to a different from expected shape of the RDF. At the same time, the average positions of all atoms are in good agreement with those given by diffraction. This makes XAS an important technique for studying interatomic correlations and lattice dynamics.

ABSTRACT: Photoluminescence and excitation spectra of ScF₃ single crystals have been measured under vacuum ultraviolet excitations utilizing undulator synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron. The emission peak at 280 nm is explained as emission band of self-trapped excitons in ScF₃. This emission is quenched at 50 K and activation energy of thermal quenching was obtained. The excitation spectrum in vacuum ultraviolet spectral range exhibits that the luminescence of self-trapped excitons effectively occurs under direct excitation in the excitonic absorption band, whereas under higher energies this excitation is strongly suppressed, however, multiplication of electronic excitation processes have been successfully identified.

ABSTRACT: We report the results of ab initio calculations and analysis of systematic trends for the F centers in the bulk and on the (001) surface in oxide perovskites, such as BaTiO₃, SrTiO₃, SrZrO₃, and PbZrO₃, with a corresponding comparison of the F centers in perovskites with those in alkaline earth metal fluorides (CaF₂, BaF₂, and SrF₂). It was found that in perovskites in both bulk F centers and those on their (001) surfaces, two nearest to the vacancy Ti or Zr atoms repel each other, while the next nearest O atoms relax towards the oxygen vacancy. It was also found that the obtained relaxations of atoms in the nearest neighborhood around the F center in ABO₃ perovskites are generally larger than in alkaline earth metal fluorides. The bulk and (001)-terminated surface F center ground states in BaTiO₃, SrTiO₃, and SrZrO₃ perovskites are located 0.23, 0.69, 1.12 eV, and 0.07, 0.25, 0.93 eV, respectively, below the conduction band bottom, indicating that the F center is a shallow donor. The vacancies in BaTiO₃, SrZrO₃, and PbZrO₃ are occupied with 1.103e, 1.25e, and 0.68e, respectively, whereas slightly smaller charges, only 1.052e, 1.10e, and 0.3e are localized inside the F center on the perovskite (001) surface. In contrast to the partly covalent ABO₃ perovskites, charge is well localized (around 80%) inside the ionic CaF₂, BaF₂, and SrF₂ fluorine vacancy.

ABSTRACT: In this paper, we have performed comparative analysis of EPR, optical absorption (OA) and luminescence spectra for a series of Gd₃Ga₅O₁₂ (GGG) single crystals irradiated with fast neutrons with fluencies varied from 10¹⁶ to 10²⁰n/cm². In a crystal irradiated with the maximum neutron fluence, the EPR spectra demonstrated the formation of several paramagnetic defects. In particular, EPR spectrum shows a strong resonance at (effective) g ≈ 1.4 with practically isotropic behavior in the crystal rotation around the [1 1 1] direction (magnetic field being perpendicular to [1 1 1]) and several weaker lines in the g ≈ 1.1–2.6 region, which show more pronounced angular dependences. While the photoluminescence of non-irradiated GGG is characterized by uncontrolled impurities, in the case of neutron-irradiated GGG, a broad asymmetric luminescence band was observed with a peak at 725–733 nm, which increased with fluence. Consequently, this band can be associated with the formation of radiation defects.

ABSTRACT: Original results on the EPR and photoluminescence Cr³⁺ ions in natural magnesium aluminum spinel (MgAl₂O₄) are presented. The photoluminescence spectra of Cr³⁺ ions in natural MgAl₂O₄ have been measured before and after irradiated by fast neutrons.

ABSTRACT: The synthesis of CdTe nanocrystals was studied by electrochemical (ECD) and chemical (CD) deposition in SiO2/Si-n structures irradiated with 200 MeV ¹³²Xe ions. Both types of electrolyte (sulfate and chloride) used in ECD led to the formation of CdTe nanocrystals in single wurtzite phase. On the other hand, in the case of CD in a sulfate solution, CdTe nanocrystals with a zinc blende structure (ZBS) are formed that coexist with CdO nanocrystals in the hexagonal structure. Furthermore, an increase in the temperature of the CD solution led only to the formation of hexagonal phase of CdO nanocrystals. A model is proposed for the formation of a CdTe nanocrystal inside the ion track, taking into account the influence of a weak external electric field on the region of the inner surface of the ion tracks.

ABSTRACT: The paper discusses the results of temperature studies of polymer-based nanocomposites obtained by incorporating multi-walled carbon nanotubes in thin layers of poly(3,4-ethylenedioxythophene):poly(4-styrenesulfonate), specifically focusing on interesting features in the dependencies of electrical properties across the wide range of temperatures from ambient one down to 10 K.

ABSTRACT: The paper discusses the results of temperature studies of polymer-based nanocomposites obtained by incorporating multi-walled carbon nanotubes in thin layers of poly(3,4-ethylenedioxythophene):poly(4-styrenesulfonate), specifically focusing on interesting features in the dependencies of electrical properties across the wide range of temperatures from ambient one down to 10 K.

ABSTRACT: In the present paper, we investigate the effect of heat treatment on the geometry and mobility of Au nanoparticles (NPs) on a Si substrate. Chemically synthesized Au NPs of diameter ranging from 5 to 27 nm were annealed at 200, 400, 600 and 800 °C for 1 h. A change in the geometry from faceted to more rounded shapes were observed with increasing annealing temperature. Kinetic Monte Carlo simulations indicate that the NPs become rounded due to the minimization of the surface area and the transition to lower energy surface types {111} and {100}. The NPs were manipulated on a silica substrate with an atomic force microscope (AFM) in tapping mode. Initially, the NPs were immovable by AFM energy dissipation. However, annealed NPs became movable, and less energy was required to displace the NPs annealed at higher temperature. However, after annealing at 800 °C, the particles became immovable again. This effect was attributed to the diffusion of Au into the Si substrate and to the growth of the SiO₂ layer.

ABSTRACT: The paper presents and discusses the results of performed calculations for YAlO₃ (111) surfaces using a hybrid B3LYP description of exchange and correlation. Calculation results for SrTiO₃, BaTiO₃ and BaZrO₃ (111) as well as YAlO₃, SrTiO₃, BaTiO₃ and BaZrO₃ (001) surfaces are listed for comparison purposes in order to point out systematic trends common for these four ABO₃ perovskite (001) and (111) surfaces. According to performed ab initio calculations, the displacement of (001) and (111) surface metal atoms of YAlO₃, SrTiO₃, BaTiO₃ and BaZrO₃ perovskite, upper three surface layers for both AO and BO₂ (001) as well as AO₃ and B (111) surface terminations, in most cases, are considerably larger than that of oxygen atoms. The YAlO₃, SrTiO₃, BaTiO₃ and BaZrO₃ (001) surface energies for both calculated terminations, in most cases, are almost equal. In contrast, the (111) surface energies for both AO₃ and B-terminations are quite different. Calculated (111) surface energies always are much larger than the (001) surface energies. As follows from performed ab initio calculations for YAlO₃, SrTiO₃, BaTiO₃ and BaZrO₃ perovskites, the AO- and BO₂-terminated (001) as well as AO₃- and B-terminated (111) surface bandgaps are almost always reduced with respect to their bulk bandgap values.