Raksti konferenču krājumā un tēzes (CFI) / Conference Papers and Abstracts

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Determination of Kerr and two-photon absorption coefficients of indandione derivatives
(SPIE, 2017) Bundulis, Arturs; Mihailovs, Igors; Nitiss, Edgars; Busenbergs, Janis; Rutkis, Martins
We studied nonlinear optical properties of two different aminobenziliden-1,3-indandione derivatives - DDMABI and DMABI-OH by employing the Z-scan method. Through this we described how different donor and acceptor groups influence third-order nonlinear optical properties such as Kerr effect and two-photon absorption. During experimental measurements we used 1064 nm Nd:YAG laser with 30 ps pulse duration and 10 Hz repetition rate. From acquired values of Kerr and two-photon absorption coefficients we calculated values for real and imaginary parts of third-order susceptibility, as well as second-order hyperpolarizability. Quantum chemical calculations were carried out for secondorder hyperpolarizability to study how well calculations correlate with experimental values. Acquired data for DDMABI and DMABI-OH were compared with data for other ABI derivatives studied previously.
Characterization of crystalline structure and morphology of Ga2O3 thin film grown by MOCVD technique
(Trans Tech Publications Ltd., 2017) Kjapsna, Alvars; Dimitrocenko, Lauris; Tale, Ivars; Trukhin, Anatoly; Ignatans, Reinis; Grants, Rolands
Growth of gallium oxide thin film was realized with MOCVD on (0001) sapphire substrate. Structural and compositional properties of thin film were studied employing trimethylgallium and water as precursors, carrier gases were H2 and N2. Obtained film is polycrystalline and predominantly consisted of (201) oriented β-Ga2O3. Sample exhibited blue luminescence which is attributed to oxygen vacancies. H2 gas proved to have beneficial effect on film quality and overall growth process.
Synthesis of Eu2+ and Dy3+ doped strontium aluminates and their properties
(Trans Tech Publications Ltd., 2017) Rozenberga-Voska, Laura; Grabis, Janis; Zolotarjovs, Aleksejs
Strontium aluminate phosphors were synthesized by the solution combustion method using citric acid, urea or glycine as reducing agent and europium and dysprosium as dopants. The content of both dopants was in the range of 1-2 mol%. Dependence of phase composition, crystallite size and specific surface area on calcinations temperature, used reducing agents and dopants were determined. Luminescent properties of the calcinated at 1300 °C powders contained SrAl2O4 (90 %) and Sr4Al24O25 (10%) phases with crystallite size of 80 nm were determined.
Energy level determination of purine containing blue light emitting organic compounds
(SPIE - International Society for Optics and Photonics, 2018) Grzibovskis, Raitis; Vembris, Aivars; Sebris, Armands; Kapilinskis, Zigfrids; Turks, Maris
Organic light emitting diodes (OLED) have found their applications in the mobile and TV screens. Till now the commercially available diodes are made by expensive thermal evaporation in a vacuum. The costs of OLED fabrication could be decreased by applying low-cost wet casting methods, for example, spin-coating. In this work, we have studied a group of blue light emitting purine derivatives which could potentially be used in OLEDs. The advantage of these compounds is their ability to form amorphous thin films from solutions. All the thin films were prepared by the spincoating method from chloroform solution on ITO glass. The position of hole and electron transport energy levels is important for efficient OLED fabrication. Ionization energy was determined using photoelectron yield spectroscopy. The gap between ionization energy and electron affinity was determined using photoconductivity measurements. Electron affinity (Ea) then was calculated as a difference between ionization energy (I) and photoconductivity threshold value (Eth). Changes in the energy level values depending on the molecule structure were investigated. The position of electron acceptor group strongly affects the gap between ionization energy and electron affinity, while with the help of the attached substitute groups it is possible to alter the ionization energy. Fine “tuning” of the ionization energy values can be achieved by altering the length of the “tail” where the inactive bulky group is attached.
Transition levels of acceptor impurities in ZnO crystals by DFT-LCAO calculations
(Institute of Physics Publishing, 2018) Usseinov, A. B.; Zhukovskii, Yu. F.; Kotomin, Eugene A.; Akilbekov, A. T.; Zdorovets, M. V.; Baubekova, G. M.; Karipbayev, Zh. T.
Large scale ab-initio calculations are carried out to study the charge state transition levels of nitrogen and phosphorus impurity defects in zinc oxide crystals using the DFT-LCAO approximation as implemented into the CRYSTAL computer code. It is shown that at a high concentration of defects (close location of defects) their formation energy is underestimated due to a significant delocalization of the charge within the supercell. After inclusion the energy offset correction and defect-defective interaction, the formation energy is improved, in a comparison with that calculated in a large supercell. The optical transition levels obtained by a direct calculation confirm the experimental observation: nitrogen and phosphorus impurities are deep acceptor centers with large formation energy in a charged state and, therefore, cannot serve as the effective source of hole charge. The obtained results are in good agreement with the previous theoretical work, in which other calculation methods were used, and are capable of qualitatively describing the energy characteristics of the charged defects.
Theoretical investigations of nitrogen doping on Co 3 O 4 for water dissociation catalytically activity
(Institute of Physics Publishing, 2018) Kaptagay, G. A.; Mastrikov, Yuri A.; Kotomin, Eugene A.; Sandibaeva, S. A.; Kopenbaeva, A. S.; Baitasheva, G. O.; Baikadamova, L. S.
We report the results of theoretical investigations of nitrogen doping on Co 3 O 4 (100) bulk by means of the plane-wave periodic density functional theory (DFT) calculations combined with the Hubbard-U approach and statistical thermodynamics.Using accurate DFT+U calculations, we have shown that bulk Co ions denote charges in value 0.017 - 0.57e. For further water adsorption process studies was selected structures with most changes in charges in Co ions: (3) structure for n = 25%, (2) structure forn = 12.5% and structure for (1) structure.
Luminescence of F2 and F3 + centres in LiF crystals irradiated with 12 MeV 12C ions
(Institute of Physics Publishing, 2018) Akilbekov, A.; Dauletbekova, A.; Kirilkin, N.; Baizhumanov, M.; Seitbayev, A.; Karipbayev, Zh.; Giniyatova, Sh.; Zabels, Roberts
Dependences of the nanohardness and photoluminescence of F 2 and F 3 + centers on the depth in LiF crystals irradiated with 12 MeV 12 C ions to fluences 10 10 -10 15 ions/cm 2 were studied using laser scanning confocal microscopy, luminescent spectroscopy, and the nanoindentation method. The nanohardness measurements showed a significant hardening effect at the end of the ion run with the dominant contribution of defects formed by the mechanism of elastic collisions. The observed attenuation of the luminescence intensity at high fluences is associated with the intense nucleation of dislocations as traps for aggregate color centers.
Determination of Kerr and two-photon absorption coefficients of ABI thin films
(SPIE, 2018) Bundulis, Arturs; Nitiss, Edgars; Rutkis, Martins
In this paper we report the results of studied nonlinear optical properties of DMABI-Ph6 in form of solutions with chloroform as solvent and guest-host thin films with poly(methyl methacrylate) as host material. We implemented the Zscan method for studies of Kerr and two-photon absorption of selected material. During experimental measurements we used 1064 nm Nd:YAG laser with 30 ps pulse duration and 10 Hz repetition rate. From acquired values of Kerr coefficients we calculated values for real part of third-order susceptibility, as well as second-order hyperpolarizability. Acquired data for DMABI-Ph6 were compared with data for other ABI derivatives studied previously to describe how different donor and acceptor groups influence third-order nonlinear optical properties.
Photovoltaic effect in bulk heterojunction system with glass forming indandione derivative DMABI-6Ph
(Elsevier Ltd, 2018) Latvels, Janis; Grzibovskis, Raitis; Pudzs, Kaspars; Vembris, Aivars; Blumberga, Dagnija
The aim of the work is to evaluate possible use of 2-[[4-(bis(2-trityloxyethyl)amino)phenyl]methylene]indane-1,3-dione (DMABI-6Ph) as light absorbing material for solar cells. DMABI-6Ph is a perspective material due to its good photoelectrical, thermal and chemical properties. The main advantage of DMABI-6Ph is its ability to form amorphous films by wet-casting methods thus allowing using the compound in organic solar cells made from solution. For now most popular materials for solution processable solar cells are polymer P3HT and fullerene derivative PCBM, but lot of investigations are in the field of new low molecular weight materials to replace the polymer. Photoelectrical measurements were made to determine molecule ionization and electron affinity levels of DMABI-6Ph. Difference of 2.06 eV between DMABI-6Ph ionization level and PCBM affinity level was obtained. Accordingly open circuit voltage of system DMABI-6Ph:PCBM was measured up to 0.78 V. The best power conversation efficiency was 0.11 % for the DMABI-6Ph:PCBM mass fraction 2:1. Limiting factor for high efficiency could be low charge carrier mobility which can be increase by additional DMABI-6Ph modification.
Thermally induced fading of Mn-doped YAP nanoceramic
(Institute of Physics Publishing, 2018) Martynyuk, Natalya; Popov, Anatoli I.; Zhydachevskyy, Yaroslav; Sugak, D.; Bilski, Paweł; Ubizskii, Sergii; Berkowski, Marek; Suchocki, Andrzej B.
Investigation of photoluminescence and amplified spontaneous emission properties of cyanoacetic acid derivative (KTB) in PVK amorphous thin films
(SPIE, 2018) Pervenecka, Julija; Vembris, Aivars; Zarins, Elmars; Kokars, Valdis
In this work photoluminescence and amplified spontaneous emission properties of new original 2-cyanoacetic acid derivative in different concentration mixed in polyvinyl carbazole (PVK) matrix were investigated. Ethyl 2-(2-(4-(bis(2- (trityloxy)ethyl)amino)styryl)-6-tert-butyl-4H-pyran-4-ylidene)-2-cyanoacetate (KTB) is recently synthesised nonsymmetric red light emitting laser dye, that in previous experiments with neat thin films showed low amplified spontaneous emission (ASE) threshold value. Based on PVK high refractive index it has been used as a polymer to ensure the preparation of good planar waveguide. Luminescence quenching is expected in neat amorphous thin films according to previous experiments which reduces photoluminescence quantum yield and increases ASE excitation threshold energy. It could be overcome by a decrease of the intermolecular interactions between laser active molecules by doping them in polymer matrix thereby decreasing photoluminescence quenching effect in the system by increasing distance between organic molecules which in turn results in lowering ASE excitation threshold energy. The lowest threshold value of ASE was achieved at 20wt% of KTB molecule in PVK matrix. Ability to significantly decrease intermolecular interactions and excitation threshold energy of investigated compound in host-guest systems makes it promising to be used as a laser dye in preparation of organic solid state lasers.
Nafion® and polyaniline composite modification with Li and Mg ions
(Institute of Physics Publishing, 2019) Lozkins, Stanislavs; Hodakovska, Julija; Kleperis, Janis; Merijs - Meri, Remo
Modification of Nafion®-based samples with polyaniline (PANI) and positive charged ions could potentially allow achieving good proton conductivity at higher working temperatures what is observed in typical polymer electrolyte membrane fuel cells. In this work Nafion® polymer was modified with 1% polyaniline emeraldine salt and Li and Mg ions containing solutions. The influence of activation process and the type of ions on composites' proton conductivity was investigated. To define the best composition the physiochemical properties of synthesized materials were determined by the means of thermogravimetric analysis, impedance spectroscopy, determination of water absorption and its kinetics, and FTIR. From the original samples the best results of σ=155 mS/cm were shown by Nafion®/PANI membrane what was modified by Li+ reduction solution. This membrane also had better thermal characteristic compared to other researched samples: membrane degradation starts on higher temperature interval.
Structural investigation of crystallized Ge-Ga-Se chalcogenide glasses
(Institute of Physics Publishing, 2019) Klym, H.; Shpotyuk, O.; Karbovnyk, I.; Calvez, L.; Popov, Anatoli I.
Crystallization transformation in the 80GeSe2-20Ga2Se3 chalcogenide glasses caused by annealing at 380 °C during different duration (25, 50, 80 and 100 hours) are studied using X-ray diffraction and atomic force microscopy methods. It is established that GeGa4Se phase of low- and high-temperature modification, Ga2Se3 phase (α- and γ-modification) and GeSe2 phases are crystallized during this process. It is shown that annealing duration over 50 h does not lead to further internal structural crystallization, while annealing for 80 h result in processes of surface crystallization.
Electro-catalytic and photo-catalytic reformation of CO2–reactions and efficiencies processes (Review)
(Institute of Physics Publishing, 2019) Knoks, Ainars; Lesnicenoks, Peteris; Kleperis, Janis; Grinberga, Liga; Hodakovska, Julia; Klavins, Janis; Cikvaidze, Georgijs; Lukosevics, Ingars
Energy harvesting with lowest environmental impact is one of key elements for cleaner future. Photocatalytic as well as electrocatalytic CO2 reformation processes are considered as prominent methods. Thus, extensive research of CO2 reformation is being done to find the right materials that holds crucial qualities. For photocatalysis that includes pronounced separation of light-generated opposite sign charge carriers, sensitivity to visible light, high quantum yield. In electrocatalysis high CO2 adsorption, chemical stability, multielectron reaction catalysts are necessary. Additionally, materials participating in the reaction process must be provided with charge carriers at proper reduction and oxidation potentials. To meet the set goal of lowering environmental impact and lower CO2 amounts exhausted into the atmosphere by human activities, it is necessary to find right technology for capturing, storing, and reusing carbon dioxide. Various technologies and materials in different levels of readiness are available and under development, such as CuO loaded TiO2 nanotubes for photocatalytic reformation or electrocatalytic reduction on copper. Not only the proof of concept is necessary but estimation and more importantly determination of the efficiency of both electro and photo catalytic reformation of CO2. In this work review of reactions and efficiency of both processes based on existing established technological methods is done.
Optimization of the enzyme power source for a nano drug delivery system fuelled by glucose in blood plasma
(Institute of Physics Publishing, 2019) Naidoo, S.; Thage, L.; Ying, Q.; Vallie, S.; Vaivars, Guntars
A unique in vivo electrical pulse generator to improve membrane permeability for drugs and simultaneously facilitate self-powered nano devices for nano drug delivery systems (NDDS) was identified. The use of an unsupported biological catalyst component of the power supply was aimed at the NDDS instead of a conventional membrane electrode assembly (MEA). Self-powered carriers of drugs and prodrugs with improved controlled release capability to target areas using substrate available in biological matrices such as glucose in blood is envisaged. The experimental application implemented prototype designed chambers allowing the entry of premixed precursors and low ohm resistance due the absence of diffusion layers and optimised open circuit voltage (OCV). This would also minimise poisoning and rupturing of the proton exchange membrane (PEM). The model uses the isothermal experimental design (37°C) parameter and the glucose is partly oxidised prior to entry and mostly oxidised at the surface of the proton exchange membrane (PEM). The experimental model used a residence time instead of the usual flow rate. The power was notably high for short periods due to the absence of carbon supported diffusion layers. The findings included low levels of glucose and glucose oxidase (GOx) are needed for OCV optimisation.
Pressure-induced structural changes in α-MoO3 probed by X-ray absorption spectroscopy
(Institute of Physics Publishing, 2019) Jonane, Inga; Anspoks, Andris; Nataf, Lucie; Baudelet, Francois; Irifune, Tetsuo; Kuzmin, Alexei
Energy-dispersive X-ray absorption spectroscopy at the Mo K-edge was used to study pressure-induced (up to 36 GPa) changes in the local atomic structure of 2D layered oxide α-MoO3. A linear combination analysis based on the low and high-pressure X-ray absorption near edge structure (XANES) spectra shows clear evidence of two high-pressure phases, existing at 18-25 GPa and above 32 GPa. The first transition is due to gradual decrease of the interlayer gap, whereas the second one - to its collapse and oxide structure reconstruction. The local atomic structure around molybdenum atoms at 0.2, 18.5 and 35.6 GPa was determined from the extended X-ray absorption fine structure (EXAFS) using reverse Monte Carlo calculations.
Negative thermal expansion of ScF3: First principles vs empirical molecular dynamics
(Institute of Physics Publishing, 2019) Bocharov, Dmitry; Rafalskij, Yuri; Krack, Matthias; Putnina, Mara; Kuzmin, Alexei
The crystal lattice of cubic scandium fluorine (ScF3) exhibits negative thermal expansion (NTE) over a wide range of temperatures from 10 K to 1100 K. Here the NTE effect in ScF3 is studied using atomistic simulations based on empirical and ab initio molecular dynamics (AIMD) in the isobaric-isothermal (NpT) ensemble. The temperature dependence of the average lattice constant, the Sc-F-Sc bond angle distribution and the radial distribution functions were obtained. Crossover from the NTE to positive thermal expansion occurring at about 1100 K is reproduced by AIMD simulations in agreement with the known experiment data. At the same time, empirical MD model fails to reproduce the NTE behaviour and suggests an expansion of the ScF3 lattice with increasing temperature. However, both MD models predict strong anisotropy of fluorine atom thermal vibration amplitude, being larger in the direction orthogonal to the Sc-F-Sc atom chain.
Nanoporous characterization of modified humidity-sensitive MgO-Al2O3 ceramics by positron annihilation lifetime spectroscopy method
(Institute of Physics Publishing, 2019) Klym, H.; Ingram, A.; Hadzaman, I.; Karbovnyk, I.; Vasylchyshyn, I.; Popov, Anatoli I.
Investigation of nanopores in the humidity-sensitive MgO-Al2O3 ceramics modified at 1100°C for 2 hours was performed using positron annihilation lifetime spectroscopy method. Lifetimes of the third and fourth components of positron annihilation lifetime spectra obtained by four-component fitting procedure were used for calculation of nanopores radii. Transformation in free volumes was studied in ceramics after drying and exposure to water vapor. It is established that the lifetime of the third component decreases and the intensity of this component increases in ceramics exposed to water vapor reflecting ortho-positronium decay in nanopores simultaneously with annihilation in water bubbles. The characteristics of the fourth component (lifetime and intensity) decrease in ceramics exposed to water vapor reflecting ortho-positronium trapping in free-water volume of nanopores. Final drying of the MgO-Al2O3 ceramics results in returning of ortho-positronium lifetimes and intensities to the initial values confirming good desorption of water from nanopores of ceramics.
Optical properties of zinc-iridium oxide thin films
(Institute of Physics Publishing, 2019) Skvorcova, Vera; Zubkins, Martins; Kalendarev, Roberts; Arslan, Halil; Purans, Juris
We present the results of an investigation of ultraviolet, visible, near-infrared (UV-Vis-NIR) and X-ray absorption spectroscopy absorption spectra for zinc-iridium oxide (Zn-Ir- O) thin films with various iridium concentrations deposited by reactive DC magnetron sputtering. It is found that the absorption spectra of zinc-iridium oxide thin films contain a broad band with maxima at 446 nm and 710 nm in the visible region, bands with maxima at about 1100 nm and 3300 nm, and a low-intensity absorption band at 1570 nm in the near-infrared region. The obtained absorption bands are associated with iridium ions at valences of Ir3+, Ir4+ and Ir5+. Changes in the oxidation state of iridium ions from 5+ to 4+ are observed.
First principles evaluation on photocatalytic suitability of 2H structured and [0001] oriented WS2 nanosheets and nanotubes
(Institute of Physics Publishing, 2019) Zhukovskii, Y. F.; Piskunov, Sergei; Evarestov, R. A.
Pristine WS2 multilayer nanosheets (NSs), which thickness h NS varies from 1 to 12 monolayers (MLs), as well as single- and multi-walled nanotubes (SW and MW NTs) of different chirality, which diameter d NT exceeds 1.9 nm, display photocatalytic suitability to split H2O molecules. Obviously, such a phenomenon can occur since the band gap of these nanostructures corresponds to the energy range of visible light between the red and violet edges of spectrum (1.55 eV < Δϵgap < 2.65 eV). For all the studied WS2 NSs and NTs, the levels of the top of the valence band and the bottom of the conduction band must be properly aligned relatively to H2O oxidation and reduction potentials separated by 1.23 eV: ϵ VB < ϵO2/H2O < ϵH+/H2 < ϵ CB. The values of Δϵgap decrease with growth of h NS and increase with enlargement of dNT. 1 ML nanosheet can be considered as a limit of infinite SW NT thickness growth (d NT→∞), which band gap increases up to ∼2.65 eV. First principles calculations have been performed using the hybrid DFT-HF method (HSE06 Hamiltonian) adapted for 2H WS2 bulk. The highest solar energy conversion efficiency (15-18%) expected at Δϵgap = 2.0-2.2 eV (yellow-green range) has been found for 2 ML thick (stoichiometric) WS2 (0001) NS as well as WS2 NTs with diameters 2.7-3.2 nm (irrespectively on morphology and chirality indices n of nanotubes). Moreover, unlike discrete variation of hNS magnitudes, tuning of d NT values provides much higher energy resolution.

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