Zinātniskie raksti (CFI) / Scientific articles

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2d slab models of nanotubes based on tetragonal tio2 structures: Validation over a diameter range
(Multidisciplinary Digital Publishing Institute (MDPI), 2021) Lisovski, Oleg; Piskunov, Sergei; Bocharov, Dmitry; Kenmoe, Stephane
One-dimensional nanomaterials receive much attention thanks to their advantageous properties compared to simple, bulk materials. A particular application of 1D nanomaterials is pho-tocatalytic hydrogen generation from water. Such materials are studied not only experimentally, but also computationally. The bottleneck in computations is insufficient computational power to access realistic systems, especially with water or another adsorbed species, using computationally expensive methods, such as ab initio MD. Still, such calculations are necessary for an in-depth understanding of many processes, while the available approximations and simplifications are either not precise or system-dependent. Two-dimensional models as an approximation for TiO2 nanotubes with (101) and (001) structures were proposed by our group for the first time in Comput. Condens. Matter journal in 2018. They were developed at the inexpensive DFT theory level. The principle was to adopt lattice constants from an NT with a specific diameter and keep them fixed in the 2D model optimization, with geometry modifications for one of the models. Our previous work was limited to studying one configuration of a nanotube per 2D model. In this article one of the models was chosen and tested for four different configurations of TiO2 nanotubes: (101) (n,0), (101) (0,n), (001) (n,0), and (001) (0,n). All of them are 6-layered and have rectangular unit cells of tetragonal anatase form. Results of the current study show that the proposed 2D model is indeed universally applicable for different nanotube configurations so that it can be useful in facilitating computationally costly calculations of large systems with adsorbates. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.
2D slab models of TiO2 nanotubes for simulation of water adsorption: Validation over a diameter range
(Elsevier B.V., 2020) Lisovski, Oleg; Piskunov, Sergei; Bocharov, Dmitry; Kenmoe, Stéphane
Currently a lot of attention is paid to 1D nanomaterials due to their advantages in comparison to bulk materials. They offer broad possibilities of application, including photocatalytic water splitting. Simulations of water adsorption on such materials with computationally costly theoretical methods, such as ab initio molecular dynamics, are needed for improvement of such photocatalysts’ efficiency. Still, it is very problematic to treat a real-size nanotube at available computational power. The existing nanotube surface approximations are not accurate and universal enough. We have already proposed methods for 2D model construction out of TiO2 nanotubes of (101) and (001) configuration at the moderately expensive DFT level. The idea behind was to provide a partial description of nanotubular strain by applying lattice constants from nanotubes to slab models, and preserving geometry motifs. We use water adsorption energy, valence band maximum and conduction band minimum positions, as well as DOS shape as criteria for model validation. Our previous work was limited only to specific variants of nanotubes and water adsorption. In this work we establish these novel approaches along a wide nanotube diameter range, in particular for water adsorption studies. We demonstrate that the 2D models do not impose critical compromises in terms of accuracy, and therefore allow calculations of much larger nanotubes than common approaches do.
Ab initio calculations of CaZrO3 (011) surfaces: systematic trends in polar (011) surface calculations of ABO3 perovskites
(Springer, 2020) Eglitis, Roberts; Kleperis, Janis; Purans, Juris; Popov, Anatoli I.; Jia, Ran
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. This work is licensed under a CC BY license.
AB Initio Calculations of CU N @Graphene (0001) Nanostructures for Electrocatalytic Applications
(Sciendo, 2018) Piskunov, Sergei; Zhukovskii, Y.F.; Sokolov, M.N.; Kleperis, Janis
Substitution of fossil-based chemical processes by the combination of electrochemical reactions driven by sources of renewable energy and parallel use of H 2 O and CO 2 to produce carbon and hydrogen, respectively, can serve as direct synthesis of bulk chemicals and fuels. We plan to design and develop a prototype of electrochemical reactor combining cathodic CO 2 -reduction to ethylene and anodic H 2 O oxidation to hydrogen peroxide. We perform ab initio calculations on the atomistic 2D graphene-based models with attached Cu atoms foreseen for dissociation of CO 2 and H 2 O containing complexes, electronic properties of which are described taking into account elemental electrocatalytical reaction steps. The applicability of the model nanostructures for computer simulation on electrical conductivity of charged Cu n /graphene (0001) surface is also reported.
Ab initio calculations of Li2(Co, Mn)O8 solid solutions for rechargeable batteries
(World Scientific Publishing Co. Pte Ltd, 2019) Eglitis, Roberts
Current commercially available rechargeable Li-ion batteries, for example LiCoO2, are working mostly in the 4 V regime. One often suggested possibility to improve the effectivity of Li-ion batteries are the creation of the 5 V cathode materials. We performed quantum mechanical calculations on the average battery voltage for the Li2CoxMn4−xO8 (x = 0, 1, 2, 3 and 4) cathode materials by means of the WIEN2k computer program package. The calculated average battery voltages for x = 0, 1, 2, 3 and 4 are equal to 3.95, 5, 4.47, 4.19 and 3.99 V. Our ab initio calculation results are compared with the available experimental data for x = 0, 1, 2 and 4 which are equal to 4, 5, 5 and 4 V. Thereby, for the Li2Co1Mn3O8 battery cathode material, our calculated average battery voltage around 5 V is in perfect agreement with the experimentally available battery voltage value of 5 Volt. Nevertheless, our calculated average battery voltage is underestimated (4.47 V) for the Li2Co2Mn2O8 cathode material, which also experimentally exhibits the 5 V voltage.
Ab initio calculations of pure and Co+2-doped MgF2 crystals
(Elsevier B.V., 2020) Usseinov, Abay; Gryaznov, Denis; Popov, Anatoli; Kotomin, Eugene A.; Seitov, D. D.; Abuova, Fatima; Nekrasov, Kirill A.; Akilbekov, Abdirash T.
Ab initio calculations of the atomic, electronic and vibrational structure of a pure and Co+2 doped MgF2 crystals were performed and discussed. We demonstrate that Co+2 (3d7) ions substituting for Mg is in the high spin state. In particular, the role of exact non-local exchange is emphasized for a proper reproduction of not only the band gap but also other MgF2 bulk properties. It allows us for reliable estimate of the dopant energy levels position in the band gap, and its comparison with the experimental data. Thus, the present ab initio calculations and experiment data demonstrate that the Co+2 ground state level lies at ≈2 eV above the valence band top.
Ab Initio Computations of O and AO as well as ReO2, WO2 and BO2-Terminated ReO3, WO3, BaTiO3, SrTiO3 and BaZrO3 (001) Surfaces
(MDPI, 2022) Eglitis, Roberts I.; Purans, Juris; Popov, Anatoli I.; Bocharov, Dmitry; Chekhovska, Anastasiia; Jia, Ran
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). © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
Ab Initio Modeling of Y and O Solute Atom Interaction in Small Clusters within the bcc Iron Lattice
(Wiley-VCH GmbH, 2018) Mastrikov, Yuri A.; Sokolov, Maksim N.; Kotomin, Eugene A.; Gopejenko, Aleksejs; Zhukovskii, Yuri F.
Small yttrium and oxygen complexes in the body‐centred cubic (bcc) iron matrix are modeled by performing density functional theory (DFT) calculations. The interaction between Y and O in isolated molecule, rock‐salt crystal is compared with that in Y/O, Y/2O, 2Y/O clusters within the bcc iron matrix. Interaction energies and electron charge redistribution are also analyzed. Among the clusters, the most stable ones are analyzed further. It is shown that chemical bonding in YO molecule and crystal is significantly stronger than in the host matrix and the main interaction in the matrix occurs with nearby Fe atoms.
Ab initio molecular dynamics simulations of negative thermal expansion in ScF3: The effect of the supercell size
(Elsevier B.V., 2020) Krack, M.; Bocharov, Dmitry; Rafalskij, Yuri; Kuzmin, Alexei; Purans, Juris
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
Ab initio simulation of (Ba,Sr)TiO3 and (Ba,Ca)TiO3 perovskite solid solutions
(Elsevier B.V., 2019) Rusevich, Leonid L.; Zvejnieks, Guntars; Kotomin, Eugene A.
The results of ab initio (first-principles) computations of structural, elastic and piezoelectric properties of Ba(1−x)SrxTiO3 (BSTO) and Ba(1−x)CaxTiO3 (BCTO) perovskite solid solutions are presented, discussed and compared. Calculations are performed with the CRYSTAL14 computer code within the linear combination of atomic orbitals (LCAO) approximation, using advanced hybrid functionals of the density-functional-theory (DFT). Supercell model allows us to simulate solid solutions with different chemical compositions (x = 0, 0.125 and 0.25) within ferroelectric tetragonal phases (x < 0.3) of both solid solutions. It is shown that configurational disorder has to be taken into account in simulations of BCTO solid solutions, while for BSTO this effect is rather small. Both BSTO and BCTO show significantly enhanced piezoelectric properties, in a comparison with pure BaTiO3. However, these solid solutions demonstrate opposite behaviour of a tetragonal ratio c/a and elastic constants as the functions of chemical composition. It is predicted that due to decrease of the elastic constants in BCTO, it has much higher converse piezoelectric constants than BSTO.
Ab initio simulations on charged interstitial oxygen migration in corundum
(Elsevier B.V., 2018) Platonenko, Alexander; Gryaznov, Denis; Zhukovskii, Yuri F.; Kotomin, Eugene A.
We have calculated possible migration trajectories for single-charged interstitial Oi− anion using large-scale hybrid density functional theory within linear combination of atomic orbitals approach to defective α-Al2O3 crystals. The most energetically favorable configuration for charged Oi− anion is formation of pseudo-dumbbell (split interstitial) with a regular Oreg ion. For charged interstitial oxygen migration, the energy barrier turns out to be ∼0.8–1.0 eV. This is considerably smaller than that for a neutral interstitial atoms (1.3 eV), in agreement with experimental data.
Ab-Initio Calculations of Oxygen Vacancy in Ga2O3 Crystals
(Sciendo, 2021) Usseinov, A.; Koishybayeva, Zh.; Platonenko, Aleksandrs; Akilbekov, A.; Purans, Juris; Pankratov, Vladimir; Suchikova, Y.; Popov, Anatoli I.
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.--//-- Licenced under CC BY 4.0.
Abrupt elastic-to-plastic transition in pentagonal nanowires under bending
(Beilstein-Institut Zur Forderung der Chemischen Wissenschaften, 2019) Vlassov, Sergei; Mets, Magnus; Polyakov, Boris; Bian, Jianjun; Dorogin, Leonid; Zadin, Vahur
In this study, pentagonal Ag and Au nanowires (NWs) were bent in cantilever beam configuration inside a scanning electron microscope. We demonstrated an unusual, abrupt elastic-to-plastic transition, observed as a sudden change of the NW profile from smooth arc-shaped to angled knee-like during the bending in the narrow range of bending angles. In contrast to the behavior of NWs in the tensile and three-point bending tests, where extensive elastic deformation was followed by brittle fracture, in our case, after the abrupt plastic event, the NW was still far from fracture and enabled further bending without breaking. A possible explanation is that the five-fold twinned structure prevents propagation of critical defects, leading to dislocation pile up that may lead to sudden stress release, which is observed as an abrupt plastic event. Moreover, we found that if the NWs are coated with alumina, the abrupt plastic event is not observed and the NWs can withstand severe deformation in the elastic regime without fracture. The coating may possibly prevent formation of dislocations. Mechanical durability under high and inhomogeneous strain fields is an important aspect of exploiting Ag and Au NWs in applications like waveguiding or conductive networks in flexible polymer composite materials.
Accumulation of radiation defects and modification of micromechanical properties under MgO crystal irradiation with swift 132Xe ions
(Elsevier B.V., 2020) Baubekova, G.; Akilbekov, A.; Feldbach, E.; Grants, Rolands; Manika, Ilze P.; Popov, Anatoli I.; Schwartz, K.; Vasil'Chenko, Evgeni A.; Zdorovets, Maxim V.; Lushchik, Aleksandr Ch
Accumulation of F-type defects under irradiation of MgO crystals by 0.23-GeV 132Xe ions with fluence varying by three orders of magnitude has been investigated via the spectra of optical absorption and low-temperature cathodoluminescence. The number of single centers continuously increases with fluence without any marks of saturation. At the highest fluence, a mean volume concentration of 3.1 × 1019 and 3.35 × 1019 cm−3 is reached for F and F+ centers, respectively. The F+ emission strongly dominates in the cathodoluminescence of irradiated MgO and its enhancement with fluence is detected. However, the creation efficiency of the F2 aggregate centers is very low and fluence dependence has a complicated shape. Radiation-induced changes of micro-mechanical properties of the same samples have been analysed; the depth profiles of hardening correlate with the ion energy loss. A joint contribution of ionization and impact mechanisms in the formation of structural defects under MgO irradiation with Xe ions is considered.
Adhesion and mechanical properties of PDMS-based materials probed with AFM: A review
(Institute of Problems of Mechanical Engineering, 2018) Vlassov, Sergei; Oras, Sven; Antsov, Mikk; Sosnin, Ilya M.; Polyakov, B.; Sutka, Andris; Krauchanka, M. Yu; Dorogin, Leonid M.
Polydimethylsiloxane (PDMS) is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological properties. PDMS has found extensive usage in various fields ranging from microfluidics and flexible electronics to cosmetics and food industry. In certain applications, like e.g. dry adhesives or dry transfer of 2D materials, adhesive properties of PDMS play crucial role. In this review we focus on probing the mechanical and adhesive properties of PDMS by means of atomic force microscopy (AFM). Main advantages and limitations of AFM-based measurements in comparison to macroscopic tests are discussed.
Advanced approach to the local structure reconstruction and theory validation on the example of the W L3-edge extended X-ray absorption ﬁne structure of tungsten
(IOP Publishing, 2018) Jonane, Inga; Anspoks, Andris; Kuzmin, Alexei
Atomistic simulations of the experimental W L3-edge extended x-ray absorption fine structure (EXAFS) of bcc tungsten at T = 300 K were performed using classical molecular dynamics (MD) and reverse Monte Carlo (RMC) methods. The MD-EXAFS method based on the results of MD simulations allowed us to access the structural information, encoded in EXAFS, beyond the first coordination shell and to validate the accuracy of two interaction potential models—the embedded atom model potential and the second nearest-neighbor modified embedded atom method potential. The RMC-EXAFS method was used for more elaborate analysis of the EXAFS data giving access to thermal disorder effects. The results of both methods suggest that the correlation in atomic motion in bcc tungsten becomes negligible above 8 Å. This fact allowed us to use the EXAFS data to determine not only mean-square relative displacements of atomic W–W pair motion but also mean-square displacements of individual tungsten atoms, which are usually accessible from diffraction data only.
Afterglow, TL and OSL properties of Mn2+-doped ZnGa2O4 phosphor
(Nature Publishing Group, 2019) Luchechko, Andriy; Zhydachevskyy, Yaroslav; Ubizskii, Sergii; Kravets, Oleh; Popov, Anatoli I.; Rogulis, Uldis; Elsts, Edgars; Suchocki, Andrzej; Bulur, Enver
Zinc gallate (ZnGa2O4) spinel ceramics doped with Mn2+ ions was prepared by a solid-state reaction at 1200 °C in air. Manganese concentration was equal to 0.05 mol.% of MnO with respect to ZnO. Ceramics produced in this way show an efficient green emission at about 505 nm under UV or X-ray excitations, which is caused by Mn2+ ions. This green emission is observed also as a relatively long afterglow (visible to the naked eye in the dark for about one hour) after switching-off the X-ray excitation. Time profiles of the beginning of glow and afterglow have been studied together with thermally stimulated (TSL) and optically stimulated (OSL) luminescence. Experimental results demonstrate a presence of few types of shallow and deep traps responsible for the observed afterglow and TSL/OSL emission of the material. The possibility of pulsed optical stimulation and time-resolved OSL characteristics of ZnGa2 O4: Mn2+ has been reported for the first time. The presented results suggest the ZnGa2O4: Mn2+ spinel as a promising material for further fundamental research and possibility of application as a green long-lasting phosphor or storage phosphor for TSL/OSL radiation dosimetry.
Al-driven peculiarities of local coordination and magnetic properties in single-phase Al x-CrFeCoNi high-entropy alloys
(Tsinghua University Press, 2021) Smekhova, Alevtina; Kuzmin, Alexei; Siemensmeyer, Konrad; Luo, Chen; Chen, Kai; Radu, Florin; Weschke, Eugen; Reinholz, Uwe; Buzanich, Ana Guilherme; Yusenko, Kirill V.
Modern design of superior multi-functional alloys composed of several principal components requires in-depth studies of their local structure for developing desired macroscopic properties. Herein, peculiarities of atomic arrangements on the local scale and electronic states of constituent elements in the single-phase face-centered cubic (fcc)- and body-centered cubic (bcc)-structured high-entropy Alx-CrFeCoNi alloys (x = 0.3 and 3, respectively) are explored by element-specific X-ray absorption spectroscopy in hard and soft X-ray energy ranges. Simulations based on the reverse Monte Carlo approach allow to perform a simultaneous fit of extended X-ray absorption fine structure spectra recorded at K absorption edges of each 3d constituent and to reconstruct the local environment within the first coordination shells of absorbers with high precision. The revealed unimodal and bimodal distributions of all five elements are in agreement with structure-dependent magnetic properties of studied alloys probed by magnetometry. A degree of surface atoms oxidation uncovered by soft X-rays suggests different kinetics of oxide formation for each type of constituents and has to be taken into account. X-ray magnetic circular dichroism technique employed at L2.3 absorption edges of transition metals demonstrates reduced magnetic moments of 3d metal constituents in the sub-surface region of in situ cleaned fcc-structured Al0.3-CrFeCoNi compared to their bulk values. Extended to nanostructured versions of multicomponent alloys, such studies would bring new insights related to effects of high entropy mixing on low dimensions. [Figure not available: see fulltext.] © 2021, The Author(s). --//-- This is the accepted version of the article: Smekhova, A., Kuzmin, A., Siemensmeyer, K. et al. Al-driven peculiarities of local coordination and magnetic properties in single-phase Alx-CrFeCoNi high-entropy alloys. Nano Res. (2021). https://doi.org/10.1007/s12274-021-3704-5. Accepted manuscript's terms of use: https://www.springernature.com/gp/open-research/policies/accepted-manuscript-terms
All-organic electro-optic waveguide modulator comprising SU-8 and nonlinear optical polymer
(OSA - The Optical Society, 2017) Nitiss, Edgars; Tokmakovs, Andrejs; Pudzs, Kaspars; Busenbergs, Janis; Rutkis, Martins
In this paper we describe the principles of operation as well as the fabrication and testing steps of an all-organic waveguide modulator. The modulator comprises an SU-8 core and an electro-optic host-guest polymer cladding. The polymer properties are tuned in order to achieve single mode operation. We used direct-write laser lithography in two steps for the preparation of the devices. The electro-optic coefficient of the polymer is estimated from observing the modulation of the device operated in push-pull mode.
All-Organic Waveguide Sensor for Volatile Solvent Sensing
(Springer Verlag, 2019) Nitiss, Edgars; Bundulis, Arturs; Tokmakovs, Andrejs; Busenbergs, Janis; Rutkis, Martins
An all-organic Mach-Zehnder waveguide device for volatile solvent sensing is presented. Optical waveguide devices offer a great potential for various applications in sensing and communications due to multiple advantageous properties such as immunity to electromagnetic interference, high efficiency, and low cost and size. One of the most promising areas for applications of photonic systems would be real-time monitoring of various hazardous organic vapor concentrations harmful to human being. The optical waveguide volatile solvent sensor presented here comprises a novel organic material applied as a cladding on an SU-8 waveguide core and can be used for sensing of different vapors such as isopropanol, acetone, and water. It is shown that the reason for the chemical sensing in device is the absorption of vapor into the waveguide cladding which in turn changes the waveguide effective refractive index. The presented waveguide device has small footprint and high sensitivity of the mentioned solvent vapor, particularly that of water. The preparation steps of the device as well as the sensing characteristics are presented and discussed.

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