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Thin Films Laboratory website » Publications » 2022
 
Publications in 2022 
 
     
1.

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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2.

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
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3.

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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4.

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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5.

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
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6.

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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7.

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
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8.

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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9.

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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10.

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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11.

ABSTRACT: During the synthesis of heterostructured nanomaterials, unwanted structural and morphological changes in nanostructures may occur, especially when multiple sequential growth steps are involved. In this study, we describe a synthesis strategy of heterostructured ZnS/Al2O3/MoS2 core-shell nanowires (NWs), and explore the role of the Al2O3 interlayer during synthesis. Core-shell NWs were produced via a four-step route: (1) synthesis of ZnO NWs on a silicon wafer, (2) deposition of thin Al2O3 layer by ALD, (3) magnetron deposition of MoO3 layer, and (4) annealing of the sample in the sulphur atmosphere. During sulphurization, ZnO is converted into ZnS, and MoO3 into MoS2 , while the Al2O3 interlayer preserves the smooth surface of an NW required for the growth of a continuous MoS2 shell. The resulting ZnS/Al2O3/MoS2 core-shell NWs were characterized by transmission electron microscopy, X-ray diffraction and photoelectron spectroscopy, Raman spectroscopy, and optical photoluminescence spectroscopy. A reported strategy can be used for the synthesis of other core-shell NWs with a transition metal dichalcogenides (TMDs) shell to protect the NW core material that may otherwise be altered or damaged by the reactive chalcogenides at high temperatures.


Journal of Alloys and Compounds (in press), Volume 918, 15 October 2022, 165648
DOI: 10.1016/j.jallcom.2022.165648
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12.

ABSTRACT: The synthesis of the photochromic YHO films is based on the oxidation of deposited yttrium hydride in ambient conditions. The actual state of the films during the deposition process, which is influenced by the deposition pressure and the oxidation caused by the residual gases, is not completely known. We report on the YHxOy thin films deposited by reactive pulsed-DC magnetron sputtering. Since the visible light transmittance is closely related to the phase and chemical composition of the films, in-situ transmittance measurements during and after deposition are performed. Ex-situ spectroscopic ellipsometry is used to determine the optical constants of YHxOy throughout the film thickness. In order to obtain metallic YH2-x films, the densest possible structure with a high deposition rate is required, otherwise the films could already be partially transparent during the deposition. The transmittance is higher if deposition pressure is increased. This is because of the oxidation promoted by more porous growth of the microstructure that is observed at the surface and cross-section images of the films. The films exhibit a refractive index gradient perpendicular to the substrate surface, which is related to the porosity and variation of the chemical composition.


Vacuum, Available online 3 June 2022, 111218, In Press, Journal Pre-proof
DOI: 10.1016/j.vacuum.2022.111218
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13.

ABSTRACT: Layered two-dimensional (2D) materials, such as p-type WSe2, are potential key materials in the manufacture of the next generation electronic devices. One of the remaining main challenges is the large area growth of high-quality films. A potential large-scale 2D WSe2 synthesis method is conversion (selenization) of a pre-deposited sacrificial precursor coating. However, its use is still limited, mainly due to a lack of understanding the growth mechanisms involved. Here, we have studied and compared properties of thin crystalline WSe2 films prepared via selenization of sputterdeposited sacrificial WO3 and W films. Surface morphology of the as-grown films was studied using a scanning electron microscope complemented with an atomic force microscope. The structure and chemical composition were confirmed by X-ray diffraction and micro-Raman spectroscopy, respectively. On-chip photoconductive devices were made using the standard photolithography process, and their photoresponse was investigated with 405 nm wavelength light. For the electrical characterization, field effect transistors (FETs) were made to measure output and transfer curves. The results obtained give insight into the growth of crystalline WSe2 via sacrificial film selenization.


Vacuum, Available online 3 June 2022, 111218, In Press, Journal Pre-proof
DOI: 10.1016/j.jcrysgro.2022.126764
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14.

ABSTRACT: Bulk rhenium trioxide (ReO3) has an unusually high electrical conductivity and, being nanosized, has promising catalytic properties. However, the production of pure ReO3 thin films is challenging due to the difficulty to stabilize rhenium in a 6+ oxidation state. Here we present a novel approach for the deposition of ReOx (x ≈ 1.6–2.9) thin films using reactive high power impulse magnetron sputtering (r-HiPIMS) from a metallic rhenium target in a mixed Ar/O2 atmosphere. The thin films were deposited in the gas-sustained self-sputtering regime, observed during r-HiPIMS process according to current waveforms. The influence of the substrate temperature, the oxygen-to-argon flow ratio and post-annealing at 250 °C in the air for 3 h on the properties of the films were studied. The as-deposited films have an X-ray amorphous structure (α-ReOx) when deposited at room temperature while a nano-crystalline β-ReOx phase when deposited at elevated temperatures (150 or 250 °C). The amorphous α-ReOx can be converted into the crystalline ReO3 with a lattice parameter of 3.75 Å upon annealing in the air. The surface morphology of the films is dense without detectable voids when elevated substrate temperatures are used. Various Re oxidation states are observed on the surface of the films in different ratios depending on the deposition parameters. All samples exhibit electrical resistivity on the order of 10-3 Ω cm and optical properties typical for thin metallic films.


Materials Chemistry and Physics, Volume 289, 15 September 2022, 126399
DOI: 10.1016/j.matchemphys.2022.126399
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15.

ABSTRACT: Combining defect semiconductors Ga2S3 and Ga2Se3 in Ga2O3-based heterostructured nanowires (NWs) have potential in photonics and optoelectronics applications due to the materials appealing optical properties. In this work, we have developed and studied Ga2O3–Ga2S3 and, for the first time, Ga2O3–Ga2Se3 core-shell NWs. Ga2S3 and Ga2Se3 shell was obtained during high-temperature sulfurization and selenization process of pure Ga2O3 NWs, respectively, in a chemical vapour transport reactor. As-grown nanostructures were characterized with scanning and transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and photoluminescence measurements. Single-nanowire photodetector devices were fabricated in order to demonstrate their electric and photoconductive properties. Such novel core-shell NW heterostructures could potentially be used in next-generation nanoscale electronic and optoelectronic devices.


Optical Materials, Volume 131, September 2022, 112675
DOI: 10.1016/j.optmat.2022.112675
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16.

ABSTRACT: Wurtzite Zn1−xMgxO epilayers (x = 0, 0.26, 0.44, 0.49, 0.66) grown by the plasma-assisted molecular beam epitaxy on ScAlMgO4 substrate were characterized using the methods of optical spectroscopy: spectroscopic ellipsometry (SE), optical absorption (OA), and photoluminescence (PL). The complex dielectric function in the spectral range of 210–1690 nm, band gap width, exciton absorption and emission parameters, and film quality were studied and discussed. Individual characterization of samples was provided by combining SE and OA measurement results. The observed increase of the band gap up to 4.35 eV with the rise of the MgO content allowed the recommendation of the wurtzite Zn1−xMgxO epilayers as material for UV sensors. The origin of defects hampering the practical application of the materials was discussed.


Journal of Alloys and Compounds, Volume 912, 15 August 2022, 165178
DOI: 10.1016/j.jallcom.2022.165178
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Institute of Solid State Physics, University of Latvia, Thin Films Laboratory