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Publications in 2022 

ABSTRACT: Nanocrystalline zinc peroxide (nano-ZnO2) was synthesized through a hydrothermal process and comprehensively studied using several experimental techniques. Its crystal structure was characterized by X-ray diffraction, and the average crystallite size of 22 nm was estimated by Rietveld refinement. The temperature-dependent local environment around zinc atoms was reconstructed using reverse Monte Carlo (RMC) analysis from the Zn K-edge X-ray absorption spectra. The indirect band gap of about 4.6 eV was found using optical absorption spectroscopy. Lattice dynamics of nano-ZnO2 was studied by infrared and Raman spectroscopy. In situ Raman measurements indicate the stability of nano-ZnO2 up to 250 °C above which it decomposes into ZnO and O2. The obtained experimental results were supported by first-principles density functional theory (DFT) calculations.

Journal of Physics and Chemistry of Solids, Volume 160, January 2022, 110318
DOI: 10.1016/j.jpcs.2021.110318
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ABSTRACT: Strong electroluminescence (EL) of reverse-biased Er-doped β-Ga2O3 Schottky barrier diodes is demonstrated. The devices are prepared by pulsed laser deposition featuring co-doping of n-type dopant Si and isovalent Er, while Schottky contacts are formed by Pt-sputtering. The diodes display a rectification ratio of more than nine orders of magnitude at ±3 V in the virgin state, but under a reverse bias that yields a leakage current density of 0.2–0.4 A cm-2, clearly visible multiband EL emerges. The EL is homogeneously distributed across the diode area, and the peak wavelengths compare well with the reported transition for Er3+.

Physica Status Solidi (A) Applications and Materials Science, 2022, 219(4), 2100610
DOI: 10.1002/pssa.202100610


ABSTRACT: Herein, the concept of point of darkness based on polarized light phase difference and absorption of light is demonstrated by simulations using low refractive index and extinction coefficient semiconductor and dielectric, and high refractive index nonoxidizing metal multilayer thin film structures. Several multilayer sensor configurations show great sensitivity to thickness and refractive index variation of the detectable material by measuring the reflectivity ratio Ψ and phase shift Δ. Focus is on such multilayers, which have sensitivity to both parameters (Ψ, Δ) in the visible spectral range, thus opening the possibility for further research on a new biomedical sensor development with enhanced double parameter sensing.

Physica Status Solidi (A) Applications and Materials Science, 2022, 219(4), 2100424
DOI: 10.1002/pssa.202100424
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ABSTRACT: Zinc–iridium oxide (Zn–Ir–O) thin films have been demonstrated as a p-type conducting material. However, the stability of p-type conductivity with respect to chemical composition or temperature is still unclear. In this study we discuss the local atomic structure and the electrical properties of Zn–Ir–O films in the large Ir concentration range. The films are deposited by reactive DC magnetron co-sputtering at two different substrate temperatures—without intentional heating and at 300 °C. Extended X-ray absorption fine structure (EXAFS) analysis reveals that strongly disordered ZnO4 tetrahedra are the main Zn complexes in Zn–Ir–O films with up to 67.4 at% Ir. As the Ir concentration increases, an effective increase of Ir oxidation state is observed. Reverse Monte Carlo analysis of EXAFS at Zn K-edge shows that the average Zn–O interatomic distance and disorder factor increase with the Ir concentration. We observed that the nano-crystalline w-ZnO structure is preserved in a wider Ir concentration range if the substrate is heated during deposition. At low Ir concentration, the transition from n- to p-type conductivity is observed regardless of the temperature of the substrates. Electrical resistivity decreases exponentially with the Ir concentration in the Zn–Ir–O films.

Physica Status Solidi (B) Basic Research, 2022, 259(2), 2100374
DOI: 10.1002/pssb.202100374
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ABSTRACT: Tungsten trioxide (WO3) is a well-known electrochromic material with a wide band gap, while rhenium trioxide (ReO3) is a “covalent metal” with an electrical conductivity comparable to that of pure metals. Since both WO3 and ReO3 oxides have perovskite-type structures, the formation of their solid solutions (ReO3–WO3 or RexW1–xO3) can be expected, which may be of significant academic and industrial interest. In this study, layered WO3/ReO3, ReO3/WO3, and mixed ReO3–WO3 thin films were produced by reactive DC magnetron sputtering and subsequent annealing in air at 450 °C. The structure and properties of the films were characterized by X-ray diffraction, optical spectroscopy, Hall conductivity measurements, conductive atomic force microscopy, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoemission spectroscopy. First-principles density functional theory calculations were performed for selected compositions of RexW1–xO3 solid solutions to model their crystallographic structure and electronic properties. The calculations predict metallic conductivity and tetragonal distortion of solid solutions in agreement with the experimental results. In contrast to previously reported methods, our approach allows us to produce the WO3–ReO3 alloy with a high Re content (>50%) at moderate temperatures and without the use of high pressures.

ACS Omega 2022, 7, 2, 1827–1837
DOI: 10.1021/acsomega.1c05085


ABSTRACT: A comparative study of the isoelectronic CaFeO3 and SrFeO3 perovskites has been performed by means of ab initio quantum chemical calculations and X-ray absorption spectroscopy at the Fe K-edge. EXAFS and XANES measurements are performed and discussed for the first time. The results of simulations are in good agreement with previous findings, supporting a cubic perovskite structure of SrFeO3 and transition from the room-temperature orthorhombic (space group 𝑃𝑏𝑛𝑚 ) charge-delocalized state in CaFeO3 to the low-temperature monoclinic (space group 𝑃21/𝑛) charge-disproportionated state. The local atomic and magnetic structures, as well as electronic properties, are discussed in detail.

Physica Status Solidi (B) Basic Research, 2022, 259(1), 2100238
DOI: 10.1002/pssb.202100238
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ABSTRACT: Transition metal dichalcogenide (TMD) MoS2 and WS2 monolayers (MLs) deposited atop of crystalline zinc oxide (ZnO) and graphene-like ZnO (g-ZnO) substrates have been investigated by means of density functional theory (DFT) using PBE and GLLBSC exchange-correlation functionals. In this work, the electronic structure and optical properties of studied hybrid nanomaterials are described in view of the influence of ZnO substrates thickness on the MoS2@ZnO and WS2@ZnO two-dimensional (2D) nanocomposites. The thicker ZnO substrate not only triggers the decrease of the imaginary part of dielectric function relatively to more thinner g-ZnO but also results in the less accumulated charge density in the vicinity of the Mo and W atoms at the conduction band minimum. Based on the results of our calculations, we predict that MoS2 and WS2 monolayers placed at g-ZnO substrate yield essential enhancement of the photoabsorption in the visible region of solar spectra and, thus, can be used as a promising catalyst for photo-driven water splitting applications.

Energies 2022, 15(1), 150
DOI: 10.3390/en15010150


ABSTRACT: One-dimensional (1D) nanostructures – nanowires (NWs) – exhibit attractive properties for integration in different types of functional devices. Their properties can be enhanced even further or tuned for a specific application by combining different promising materials, such as layered van der Waals materials and conventional semiconductors, into 1D-1D core–shell heterostructures. In this work, we demonstrated growth of GaN-MoS2 and GaN-WS2 core–shell NWs via two different methods: (1) two-step process of sputter-deposition of a sacrificial transition metal oxide coating on GaN NWs followed by sulfurization; (2) pulsed laser deposition of few-layer MoS2 or WS2 on GaN NWs from the respective material targets. As-prepared nanostructures were characterized via scanning and transmission electron microscopies, X-ray diffraction, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. High crystalline quality core–shell NW heterostructures with few-layer MoS2 and WS2 shells can be prepared via both routes. The experimental results were supported by theoretical electronic structure calculations, which demonstrated the potential of the synthesised core–shell NW heterostructures as photocatalysts for efficient hydrogen production from water.

Appl. Surf. Sci. 590, 153106 (2022)
DOI: 10.1016/j.apsusc.2022.153106
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ABSTRACT: The growth direction of nanowires (NWs) can change during synthesis as a result of stochastic processes or modulation of certain growth conditions. This phenomenon is known as kinking. Although deviations from a uniform vertical growth are typically considered to be undesirable, kinking opens a route for additional tweaking of the characteristics and functionalities of NWs in a controllable manner, thus extending the range of potential applications. In the present Review, we give an insight into the kinking mechanisms and summarize the most crucial factors that can lead to kinking of NWs during synthesis. Additionally, the properties and applications of kinked NWs are discussed.

Crystal Growth and Design (2021)
DOI: 10.1021/acs.cgd.1c00802
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ABSTRACT: We present and discuss the results of surface relaxation and rumpling computations for ReO3, WO3, SrTiO3, BaTiO3 and BaZrO3 (001) surfaces employing a hybrid B3LYP or B3PW description of exchange and correlation. In particular, we perform the first B3LYP computations for O-terminated ReO3 and WO3 (001) surfaces. In most cases, according to our B3LYP or B3PW computations for both surface terminations BO2- and O, AO-terminated ReO3, WO3, BaTiO3, SrTiO3 and BaZrO3 (001) surface upper layer atoms shift downwards, towards the bulk, the second layer atoms shift upwards and the third layer atoms, again, shift downwards. Our ab initio computes that ReO3, WO3, BaTiO3, SrTiO3 and BaZrO3 (001) surface Γ-Γ bandgaps are always smaller than their respective bulk Γ-Γ bandgaps. Our first principles compute that B-O atom chemical bond populations in the BaTiO3, SrTiO3 and BaZrO3 perovskite bulk are always smaller than near their BO2-terminated (001) surfaces. Just opposite, the Re-O and W-O chemical bond populations in the ReO3 (0.212e) and WO3 (0.142e) bulk are slightly larger than near the ReO2 and WO2-terminated ReO3 as well as WO3 (001) surfaces (0.170e and 0.108e, respectively).

Symmetry, May 2022, 14(5), 1050
DOI: 10.3390/sym14051050

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Institute of Solid State Physics, University of Latvia, Thin Films Laboratory