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Publications in 2021 

ABSTRACT: The discovery of superconductivity above 250 K at high pressure in LaH10 and the prediction of overcoming the room temperature threshold for superconductivity in YH10 urge for a better understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH3 under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH3: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4d yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.

Nat. Commun. 12 (2021) 1765.
DOI: 10.1038/s41467-021-21991-x
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ABSTRACT: We combined the hybrid density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) experiments in the study of the local atomic structure around Ir ions in ZnO thin films with different iridium content. This was then used in the first principles analysis of the thermoelectric properties of material. The emphasis has been put on the conditions for a positive Seebeck coefficient and p-type electrical conductivity as the functions of the Fermi level. We studied both computationally and experimentally several possible IrOx polyhedra (complexes) with a different number of surrounding oxygens and Ir oxidation states, including those with the formation of peroxide ions (O22−). In particular, octahedral coordination of iridium ions was identified by reverse Monte Carlo (RMC) simulations of the Ir L3-edge EXAFS spectra of ZnO:Ir thin films as the predominant complex, which is supported by the calculated lowest interstitial oxygen incorporation energies. All the calculated IrOx (x = 4, 5, 6) complexes, regardless of Ir the oxidation state, demonstrate potential for p-type conduction if the Fermi level (μF) falls in the range of 0–0.8 eV from the valence band maximum (VBM) and the Ir concentration is high enough (12.5% in the present DFT calculations). Even though the corresponding calculated Seebeck coefficient (S) around 80–89 μV K−1 slightly exceeds the experimental values, we emphasise the presence of an important plateau in the dependence of S on μF in this range for two complexes with the formation of peroxide ions (O22−). We predicted also that peroxide ions O22− are characterized by the calculated phonon frequencies of 810–942 cm−1 in agreement with our previous Raman experimental results. In this light, we discuss the high sensitivity of calculated S(μF) dependences to the atomic and electronic structure.

Journal of Materials Chemistry C, 2021, Accepted Manuscript.
DOI: 10.1039/D1TC00223F


ABSTRACT: The natural spinel crystal MgAl2O4 containing the Cr3+ ions was studied experimentally and theoretically in this paper. The absorption and emission spectra at room and low temperatures were recorded. The experimental spectroscopic results were aided with the crystal field calculations, which returned the energy level schemes of the Cr3+ ions at the Al sites with the local D3d symmetry. Effects of the trigonal crystal field were clearly seen in the calculated results, which were compared with the experimental spectra to yield a good agreement between both data sets.

Optical Materials 121 (2021) 111496.
DOI: 10.1016/j.optmat.2021.111496


ABSTRACT: First-principles density functional theory (DFT) is employed to study the electronic structure of oxygen and gallium vacancies in monoclinic bulk β-Ga2O3 crystals. Hybrid exchange–correlation functional B3LYP within the density functional theory and supercell approach were successfully used to simulate isolated point defects in β-Ga2O3. Based on the results of our calculations, we predict that an oxygen vacancy in β-Ga2O3 is a deep donor defect which cannot be an effective source of electrons and, thus, is not responsible for n-type conductivity in β-Ga2O3. On the other hand, all types of charge states of gallium vacancies are sufficiently deep acceptors with transition levels more than 1.5 eV above the valence band of the crystal. Due to high formation energy of above 10 eV, they cannot be considered as a source of p-type conductivity in β-Ga2O3.

Materials. 14, 7384 (2021)
DOI: 10.3390/ma14237384
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ABSTRACT: We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F-center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F-center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F-center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F-center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F-center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F-centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites.

Symmetry 13(10), 1920 (2021)
DOI: 10.3390/sym13101920
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ABSTRACT: The tribovoltaic devices have demonstrated an enormous current density output from friction. This has attracted attention, and thus, the tribovoltaic device research is expected to grow rapidly, providing mechanical energy harvesting from human motion or mechanical vibrations to power the microdevices. Herein, we are demonstrating the novel tribovoltaic device based on the W/WO3 Schottky junction enabled by high-energy electrons as in hot-carrier photovoltaic devices. The hot carrier injection from the metal to the semiconductor has been well demonstrated before in light-driven devices but not demonstrated for tribovoltaic devices. Friction-caused electronic excitations on the W needle provide energy for electrons to overcome the Schottky barrier and generate the unbiased current density up to 1270 A m-2. The amorphous WO3 derived from magnetron sputtering shows high durability and reliability of the tribovoltaic device.

J. Phys. Chem. C, 125, 14212–14220 (2021)
DOI: 10.1021/acs.jpcc.1c04312


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.0

Physica Status Solidi (B) Basic Research 258(2),2000362 (2021)
DOI: 10.1002/pssb.202000362
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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 (2021)
DOI: 10.1002/pssa.202100610
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ABSTRACT: The development of nanoscale X-ray sensors is of crucial importance to achieve higher spatial resolution in many X-ray-based techniques playing a key role in materials science, healthcare, and security. Here, we demonstrate X-ray detection using individual CdS, SnO2, and ZnO nanowires (NWs). The NWs were produced via vapor–liquid–solid technique and characterized using X-ray diffraction, scanning, and transmission electron microscopy. Electrical measurements were performed under ambient conditions while exposing two-terminal NW-based devices to X-rays generated by a conventional tungsten anode X-ray tube. Fast and stable nanoampere-range X-ray beam induced current (XBIC) in response to X-ray illumination was observed. The high XBIC measured in the NW devices could be attributed to the efficient transport and collection of generated charge carriers due to the single-crystalline nature of NWs and the short NW length. Such fast-response and high-sensitivity nanoscale X-ray detectors can find applications in sub-micron resolution imaging and nanofocused beam shape measurements.

Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 1014, 165736 (2021)
DOI: 10.1016/j.nima.2021.165736
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ABSTRACT: We carried out comparative first principles calculations for the ReO2-terminated ReO3 as well as TiO2-terminated SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001) surfaces. As follows from our first principles calculations, systematic trend for the ReO2-terminated ReO3 as well as for TiO2-terminated SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001) surfaces is inward relaxation of all upper layer and outward relaxation of all second layer atoms. The only two exceptions from this systematic trend is outward relaxation of the first layer O atom on the TiO2-terminated PbTiO3 (001) surface as well as inward relaxation of the ReO2-terminated ReO3 (001) surface second layer O atom. Our calculated band gaps at the Γ-Γ point point for the ReO2-terminated ReO3 as well as TiO2-terminated SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001) surfaces in all cases are decreased regarding to the respective bulk values. Just opposite to the TiO2-terminated SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001) surfaces, where the Ti-O chemical bond population are larger than in the bulk, near the ReO2-terminated ReO3 (001) surface, the Re-O chemical bond population is reduced in comparison to the bulk value.

Integrated Ferroelectrics, 220(1), pp. 9–17 (2021)
DOI: 10.1080/10584587.2021.1921530


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 (2021)
DOI: 10.1002/pssb.202100238
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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 (2021)
DOI: 10.1002/pssb.202100374
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ABSTRACT: To increase the sensitivity and efficiency of a gas sensor, nanostructured ZnO and Co3O4 layers were obtained by hydrothermal synthesis directly on the electrode surface, eliminating the use of binders. Scanning electron microscope images showed that the resulting nanostructured coatings were characterised by good adhesion to the surface and high porosity, which opened up the possibility of their further use in the process of developing a gas sensor. The efficiency of the obtained nanostructured coatings and their sensitivity at room temperature to various concentrations of CO2 were determined. The resistance curves of the samples were obtained as a function of gas concentration in the chamber, for Co3O4 and ZnO nanostructures.

Latvian journal of physics and technical sciences 2021, N 5
DOI: 10.2478/lpts-2021-0036
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ABSTRACT: The present study highlights the beneficial effects of premixing of highly loaded poly (butylene succinate) (PBS) / nanofibrillated cellulose (NFC) nanocomposites under solution conditions and its use as a masterbatch for melt blending. The proposed masterbatch process strategy is a very promising manufacturing technique for nanocomposites. Herein, we show the preparation of masterbatch with NFC with a very high loading of 50 wt.%. Research is aimed towards understanding the solution and melt processing effects on the structure and exploitation properties. The composites with NFC loadings from 5 up to 15 wt.% have been prepared by diluting the masterbatch and compared to conventional solvent casting. The masterbatch process significantly reduced overall solvent usage and improved the NFC dispersion within the polymer matrix. The samples prepared by solution casting showed excellent mechanical performance with an increase in elastic modulus up to 1.6-fold and storage modulus up to 2-fold at room temperature (20 °C) compared to the neat PBS, while masterbatch processed samples showed even higher overall mechanical properties – 1.8-fold and 2.5-fold increase in elastic modulus and storage modulus, correspondingly. Scanning electron microscopy (SEM) imaging indicated a homogeneous NFC dispersion for masterbatch samples and revealed agglomeration of NFC for the solvent cast ones. Biodegradation studies in the composting conditions were performed to underpin the weight, visual changes, calorimetric properties, while chemical changes were studied using spectroscopy. The NFC significantly accelerated the nanocomposites' biodegradation process from 80 days for the neat PBS to 60 days for the nanocomposites. The calorimetric studies indicate that NFC promoted crystalline phase formation and reduced crystallinity, but thermal stability was not significantly affected. In addition, the reinforcement factor analysis shows that the suitable masterbatch NFC nanocomposite preparation method's choice has a high potential to obtain high-performance materials for PBS films and packaging applications.

Industrial Crops and Products, Volume 169, 1 October 2021, 113669.
DOI: 10.1016/j.indcrop.2021.113669


ABSTRACT: Glyphosate, commonly known by its original trade name Roundup™, is the world's most widely used herbicide. Glyphosate and its metabolites have a profound negative environmental impact and long-term toxicity risk, including carcinogenicity, genotoxicity, and endocrine disruption, even at concentration levels too low to have a herbicidal effect. Therefore, the detection of these pollutants at low concentrations is an important task.
To increase the sensitivity of the sensor, nanostructures were used.
To analyze the presence of glyphosate and its metabolites in rye juice, two groups of samples were selected. In the first case, glyphosate at different concentrations was added to the water for irrigation on the first day, and then the rye samples were watered with pure water for 7 days. In the second case, the samples were watered with pure water for all 8 days, and glyphosate was artificially added just before the measurement. The obtained samples were studied by the Different Pulse Voltammetry (DPV) employing nanostructured working electrodes. To analyze changes in the DNA sequence, a Polymerase chain reaction (PCR) product obtained from samples of the first group was electrochemically studied. To confirm the results obtained, an electrophoresis method was also applied. The results indicate that the DPV signal obtained from samples with artificially added glyphosate has significant differences compared to the signal obtained from the juice of plants absorbing glyphosate in a natural way during growth. However, in both cases, the CuO nanostructure based sensor detects the presence of glyphosate or its metabolites compared to the control sample. The experiment also found significant changes in the DNA caused by exposure with glyphosate during the growth process of rye sprouts.
A nanostructured electrochemical sensor was used for the detection of glyphosate residuals and genetic changes caused by glyphosate in untreated rye juice.

Surfaces and Interfaces, Volume 26, October 2021, 101332
DOI: 10.1016/j.surfin.2021.101332


ABSTRACT: We performed, to the best of our knowledge, the world’s first first-principles calculations for the WO2-terminated cubic WO3 (001) surface and analyzed the systematic trends in the WO3, SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001) surface ab initio calculations. According to our first principles calculations, all WO2 or TiO2-terminated WO3, SrTiO3, BaTiO3, PbTiO3 and CaZrO3 (001) surface upper-layer atoms relax inwards towards the crystal bulk, while all second-layer atoms relax upwards. The only two exceptions are outward relaxations of first layer WO2 and TiO2-terminated WO3 and PbTiO3 (001) surface O atoms. The WO2 or TiO2-terminated WO3, SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001) surface-band gaps at the Γ–Γ point are smaller than their respective bulk-band gaps. The Ti–O chemical bond populations in the SrTiO3, BaTiO3, PbTiO3 and CaTiO3 bulk are smaller than those near the TiO2-terminated (001) surfaces. Conversely, the W–O chemical bond population in the WO3 bulk is larger than near the WO2-terminated WO3 (001) surface.

Crystals 2021, 11(4), 455.
DOI: 10.3390/cryst11040455
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ABSTRACT: Gallium oxide β-Ga2O3 is an important wide-band gap semiconductor. In this study, we have calculated the formation energy and transition levels of oxygen vacancies in β-Ga2O3 crystal using the B3LYP hybrid exchange-correlation functional within the LCAO-DFT approach. The obtained electronic charge redistribution in perfect Ga2O3 shows notable covalency of the Ga-O bonds. The formation of the neutral oxygen vacancy in β-Ga2O3 leads to the presence of deep donor defects with quite low concentration. This is a clear reason why oxygen vacancies can be hardly responsible for n-type conductivity in β-Ga2O3.

Latv. J. Phys. Tech. Sci., 2021, 58, n2, pp. 3-10.
DOI: doi.org/10.2478/lpts-2021-0007
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ABSTRACT: Layered 2D van der Waals (vdW) materials such as graphene and transition metal dichalcogenides have recently gained a great deal of scientific attention due to their unique properties and prospective applications in various fields such as electronics and optoelectronics, sensors and energy. As a direct bandgap semiconductor in both bulk and monolayer forms, ReS2 stands out for its unique distorted octahedral structure that results in distinctive anisotropic physical properties; however, only a few scalable synthesis methods for few-layer ReS2 have been proposed thus far. Here, the growth of high-quality few-layer ReS2 is demonstrated via sulfurization of a pre-deposited rhenium oxide coating on different semiconductor material nanowires (GaN, ZnS, ZnO). As-produced core-shell heterostructures were characterized by X-ray diffraction, scanning and transmission electron microscopy, micro-Raman spectroscopy and X-ray absorption spectroscopy. Experimental characterizations were supported by total energy calculations of the electronic structure of ReS2 nanosheets and GaN, ZnS, and ZnO substrates. Our results demonstrate the potential of using nanowires as a template for the growth of layered vdW materials to create novel core-shell heterostructures for energy applications involving photocatalytic and electrocatalytic hydrogen evolution.

Applied Surface Science, Volume 536, 15 January 2021, 147841.
DOI: 10.1016/j.apsusc.2020.147841
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Institute of Solid State Physics, University of Latvia, Thin Films Laboratory