Zinātniskie raksti (CFI) / Scientific articles

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Quantitative optical analysis of filler dispersion degree in MWCNT – epoxy nanocomposite
(Elsevier, 2012) Glaskova, Tatjana; Zarrelli, Mauro; Aniskevich, Andrey; Giordano, Michele; Trinkler, Laima; Berzina, Baiba
Influence of uniaxial pressure on dielectric properties of (1-x)Na0.5Bi0.5TiO3–xSrTiO3 for x = 0.01, 0.04, and 0.1 ceramics
(Estonian Academy Publishers, 2017) Czaja, Piotr; Suchanicz, Jan; Kluczewska, Kamila; Sitko, Dorota; Dutkiewicz, Erazm Maria; Konieczny, Krzysztof; Węgrzyn, Adrian; Antonova, Maija; Sternberg, Andris
The conventional solid-state sintering was applied to synthesized (1-x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0.01, 0.04, and 0.1) ceramics. Dielectric measurements of these ceramics were taken in the temperature range from 20 to 600 °C, in the frequency range from 1 kHz to 2 MHz and under uniaxial pressure ranging from 10 to 1100 bar. The study of the dielectric behaviour showed that the influence of uniaxial pressure on the investigated properties was considerable. The peaks εm gradually decreased and shifted towards lower temperatures with an increase of uniaxial pressure for all samples. The first effect developed with an increase of the strontium ion concentration. Experimental results revealed most interesting properties of the material in the context of its potential applications.
Electric relaxation and Mn3+/Mn4+ charge transfer in Fe-doped Bi12MnO20–BiMn2O5 structural self-composite
(Springer New York LLC, 2017) Leonarska, A.; Kadziołka-Gaweł, M.; Szeremeta, A. Z.; Bujakiewicz-Koronska, R.; Molak, A.; Kalvane, Anna
Fe-doped Bi12MnO20–BiMn2O5 ceramics was sintered at 1130 K for 6 h in ambient air. Two centro-symmetric phases formed thermodynamically stable self-composite material that was deduced from X-ray pattern analysis. The lattice parameters were a = 10.147(8) Å—for the cubic I23 Bi12MnO20 phase; and a = 7.545(4) Å, b = 8.538(1) Å, c = 5.758(3) Å—for the orthorhombic Pbam BiMn2O5 phase. The 57Fe Mössbauer spectrum, recorded at room temperature, has shown pure electronic quadrupolar split. The major doublets reflected the occurrence of Fe3+ ions distributed in two sites, i.e., octahedral Fe4+O6 and square pyramidal Fe3+O5, with preferential occupation of the pyramidal sites, that was consistent with the Pbam phase symmetry. The third doublet resulted from the presence of iron Fe3+ in tetrahedral Fe3+O4 coordination and corresponded to a small admixture of the I23 phase. The DC resistivity ρDC(T) dependence on temperature has shown thermally activated features, and the value of Ea,DC varied in the range of 0.22–0.37 eV. The electric impedance was measured in the f = 20 Hz–1 MHz and 100–690 K range. Two electrical relaxations were determined using the electric modulus formalism M″(T). Low-temperature relaxation has shown the temperature-dependent activation energy EA,1 = 0.14–0.20 eV and characteristic time values of τ01 = 10−10–10−12 s in 100–200 K range. It was attributed to the charge transfer between Mn4+/Mn3+ sites. The other relaxation occurred in the 170–220 K range, and it exhibited the following values: τ02 = 10−11 s, and EA,2 = 0.27 eV. A disorder-related VRH polaron model was proposed for ρDC(T) and for electric relaxation processes.
Effect of Mg2+ ions co-doping on luminescence and defects formation processes in Gd3(Ga,Al)5O12:Ce single crystals
(Elsevier B.V., 2017) Babin, V.; Boháček, Pavel; Grigorjeva, Larisa; Kucera, Miroslav; Nikl, M.; Zazubovich, S.; Zolotarjovs, Aleksejs
Photo- and radioluminescence and thermally stimulated luminescence characteristics of Ce³⁺ - doped and Ce³⁺, Mg²⁺ co-doped Gd3(Ga,Al)5O12 (GAGG) single crystals of similar composition are investigated in the 9–500 K temperature range. The Ce³⁺ - related luminescence spectra and the photoluminescence decay kinetics in these crystals are found to be similar. Under photoexcitation in the Ce³⁺ - and Gd³⁺ - related absorption bands, no prominent rise of the photoluminescence intensity in time is observed neither in GAGG:Ce,Mg nor in GAGG:Ce crystals. The afterglow is strongly reduced in GAGG:Ce,Mg as compared to GAGG:Ce, and the afterglow decay kinetics is much faster. Co-doping with Mg²⁺ results in a drastic decrease of the thermally stimulated luminescence (TSL) intensity in the whole investigated temperature range and in the appearance of a new complex Mg²⁺ - related TSL glow curve peak around 285 K. After irradiation in the Ce³⁺ - related 3.6 eV absorption band, the TSL intensity in GAGG:Ce,Mg is found to be comparable with that in the GAGG:Ce epitaxial film of similar composition. The Mg²⁺ - induced changes in the concentration, origin and structure of the crystal lattice defects and their influence on the scintillation characteristics of GAGG:Ce,Mg are discussed.
Luminescence properties of chlorine molecules in glassy SiO2 and optical fibre waveguides
(Estonian Academy Publishers, 2017) Skuja, Linards; Kajihara, Koichi; Smits, Krisjanis; Alps, Kalvis; Silins, Andrejs; Teteris, Janis
Glassy SiO2 is the basic material for optical fibre waveguides and manufacturing-induced Cl impurities reduce their transparency in UV spectral range. This work reports in-depth study/spectroscopic parameters of the near-infrared (1.23 eV) low-temperature photo-luminescence (PL) of interstitial Cl2 molecules in SiO2. The zero-phonon line position was estimated at 2.075 eV on the basis of anharmonicity of Cl2 PL vibronic data. The vibronic sub-bands are broadened by coupling to phonons and by an additional contribution from the glassy disorder. The Huang‒Rhys factor is ≈13. The PL decay time is between 1 and 10 ms in the temperature range 100 K‒13 K and can be reproduced by 3 exponents. Cl2 PL retains relatively high quantum yield and its characteristic structured shape, when the temperature is increased from 13 K to the liquid nitrogen temperature. This allows using it conveniently as a high-sensitivity diagnostic tool for detecting Cl2 impurities in optical fibre waveguides. Time-resolved measurements of optical fibre waveguides indicate that the lower detection limit is below 1010 Cl2/cm3.
Behaviour of thermal expansion of (1-x)Pb(Ni1/3Nb2/3)O3–xPbTiO3 solid solutions
(Estonian Academy Publishers, 2017) Bormanis, Karlis; Gorev, Mikhail G.; Flerov, Igor N.; Sternberg, Andris; Lace, Lelde; Ignatans, Reinis; Kalvane, Anna; Antonova, Maija
The features of the dependence of the bulk coefficient of thermal expansion of the (1-x)Pb(Ni1/3Nb2/3)O3-xPbTiO3 solid solution studied in the series of x = 0-0.8 in the temperature range from 100 to 750 K are reported. The obtained data and observed anomalies of thermal expansion are compared with dielectric properties, heat capacity, and the phase diagram of concentration. The anomalous and lattice constituents of deformation and of the coefficient of thermal expansion are analysed and the data on deformation are used to estimate the Burns temperature and the mean square polarization of the samples found to decrease with the decrease of the PbTiO3 concentration.
Core–Shell Engineering to Enhance the Spectral Stability of Heterogeneous Luminescent Nanofluids
(Wiley-VCH Verlag, 2017) Labrador-Páez, Lucía; Pedroni, Marco; Šmits, Krišjānis; Speghini, Adolfo; Jaqué, Francisco H.; García-Solé, José Antonio; Jaque, Daniel; Haro-González, Patricia
The tendency to the miniaturization of devices and the peculiar properties of the nanoparticles have raised the interest of the scientific community in nanoscience. In particular, those systems consisting of nanoparticles dispersed in fluids, known as nanofluids, have made it possible to overcome many technological and scientific challenges, as they show extraordinary properties. In this work, the loss of the spectral stability in heterogeneous luminescent nanofluids is studied revealing the critical role played by the exchange of ions between different nanoparticles. Such ion exchange is favored by changes in the molecular properties of the solvent, making heterogeneous luminescent nanofluids highly unstable against temperature changes. This work demonstrates how both temporal and thermal stabilities of heterogeneous luminescent nanofluids can be substantially improved by core–shell engineering. This simultaneously avoids the leakage of luminescent ions and the effects of the solvent molecular changes.
Analysis of the U L3-edge X-ray absorption spectra in UO2 using molecular dynamics simulations
(Elsevier Ltd, 2017) Bocharov, Dmitry; Chollet, Mélanie; Krack, Matthias; Bertsch, Johannes Johannes; Grolimund, Daniel; Martin, Matthias; Kuzmin, Alexei; Purans, Juris; kotomin, Eugene A.
Uranium L3-edge X-ray absorption spectroscopy was used to study the atomic structure of uranium dioxide (UO2). The extended X-ray absorption fine structure (EXAFS) was interpreted within the ab initio multiple-scattering approach combined with classical molecular dynamics to account for thermal disorder effects. Nine force-field models were validated, and the role of multiple-scattering contributions was evaluated.
Tunneling luminescence in long lasting afterglow of SrAl2O4:Eu,Dy
(Elsevier B.V., 2017) Liepina, Virginija; Millers, Donats; Smits, Krisjanis
A long lasting (afterglowing) luminescent material SrAl2O4: Eu2+, Dy3+ was prepared by high temperature solid-state reaction method. The processes responsible for long lasting luminescence were studied by means of luminescence spectra, thermally stimulated luminescence and afterglow kinetics. Two processes are found to contribute in excited Eu 2+ creation – the thermally released electrons recombination and electron tunneling from trap to Eu3+. The possible scheme of long lasting luminescence mechanism is proposed.
Nitrogen adsorption on graphene sponges synthesized by annealing a mixture of nickel and carbon powders
(De Gruyter Open Ltd, 2017) Grehov, V.; Kalnacs, J.; Mishnev, A.; Kundzins, Karlis
Adsorption by graphene sponge (GS) manufactured by annealing nickel-carbon powder mixture in inert atmosphere has been studied. By determining the specific surface area (SSA) for the GS sample, it has been found that Brunauer, Emmett, Teller method (BET) of approximation of experimental isotherms gives wrong results in the pressure range of 0.025-0.12 because adsorption in this pressure region is affected by walls of ampoule. Real SSA value has been found by subtracting pore effect method (SPE) or by BET approximation in a low range of relative pressure of 0.0004-0.002.
Complex multilayer carbon structures for green energetics
(Estonian Academy Publishers, 2017) Lesnicenoks, Peteris; Berzina, Astrida; Lukoševičš, Ingars; Grinberga, Liga; Jekabsons, Laimonis; Kleperis, Janis; Knite, Maris; Taurins, Gatis
We investigated a promising material for hydrogen storage and sensing. The material was obtained by exfoliating recycled graphite waste and simultaneously modifying the product with metal impurities (Bi, V, Cu). As a result, graphene sheet stack (GSS) powder was obtained. The material was further processed by hydrothermal annealing and reduction. Raman spectra of the GSS materials are provided to show the presence of graphene-like structures and defects in the exfoliated material. The synthesized graphene material has good semiconductor properties with a low electrical resistance for hydrogen sensing applications.
Doped zirconia phase and luminescence dependence on the nature of charge compensation
(Nature Publishing Group, 2017) Smits, Krisjanis; Olsteins, Dags; Zolotarjovs, Aleksejs; Laganovska, Katrina; Millers, Donats; Ignatans, Reinis; Grabis, Janis
Zirconia is a relatively new material with many promising practical applications in medical imaging, biolabeling, sensors, and other fields. In this study we have investigated lanthanide and niobium doped zirconia by luminescence and XRD methods. It was proven that charge compensation in different zirconia phases determines the incorporation of intrinsic defects and activators. Thus, the structure of zirconia does not affect the Er luminescence directly; however, it strongly affects the defect distribution around lanthanide ions and the way in which activator ions are incorporated in the lattice. Our results demonstrate the correlation between the crystalline phase of zirconia and charge compensation, as well as the contribution of different nanocrystal grain sizes. In addition, our experimental results verify the theoretical studies of metastable (tetragonal, cubic) phase stabilization determined using only oxygen vacancies. Moreover, it was found that adding niobium drastically increases activator luminescence intensity, which makes Ln 3+ doped zirconia even more attractive for various practical applications. Although this study was based on the luminescence of the Er ion, the phase stabilization, charge compensation, and luminescence properties described in our results are expected to be similar for other lanthanide elements. Our results suggest that the luminescence intensity of other oxide matrices where lanthanides incorporate in place of tetravalent cations could be increased by addition of Nb ions.
A comparative study of heterostructured CuO/CuWO4 nanowires and thin films
(Elsevier B.V., 2017) Polyakov, Boris; Kuzmin, Alexei; Vlassov, Sergei; Butanovs, Edgars; Zideluns, Janis; Butikova, Jelena; Kalendarev, Robert; Zubkins, Martins
A comparative study of heterostructured CuO/CuWO4 core/shell nanowires and double-layer thin films was performed through X-ray diffraction, confocal micro-Raman spectroscopy and electron (SEM and TEM) microscopies. The heterostructures were produced using a two-step process, starting from a deposition of amorphous WO3 layer on top of CuO nanowires and thin films by reactive DC magnetron sputtering and followed by annealing at 650 °C in air. The second step induced a solid-state reaction between CuO and WO3 oxides through a thermal diffusion process, revealed by SEM-EDX analysis. Morphology evolution of core/shell nanowires and double-layer thin films upon heating was studied by electron (SEM and TEM) microscopies. A formation of CuWO4 phase was confirmed by X-ray diffraction and confocal micro-Raman spectroscopy.
Certain doping concentrations caused half-metallic graphene
(Elsevier B.V., 2017) Miao, Lu; Jia, Ran; Wang, Yu; Kong, Chui-Peng; Wang, Jian; Eglitis, Roberts I.; Zhang, Hong-Xing
The singly B and N doped graphene systems are carefully studied. The highly concentrated dopants cause a spin polarization effect in the systems. The spin polarization limits are affirmed in the singly B and N doped graphene systems through periodic hybrid density functional theory studies. The spin polarization effects must be considered indeed in the B and N doped graphene systems if the dopant concentration is above 3.1% and 1.4%, respectively. The system symmetry cooperating with the presence of the spin polarization brings half-metallic properties into the doping systems. The semiconducting channels in the half-metallic systems are in two different spin directions due to the different electron configurations of the B and N dopants in graphene.
Changes in structure and conduction type upon addition of Ir to ZnO thin films
(Elsevier B.V., 2017) Zubkins, Martins; Kalendarev, Robert; Gabrusenoks, Jevgenijs; Plaude, A. V.; Zitolo, Andrea; Anspoks, Andris; Pudzs, Kaspars; Vilnis, Kaspars; Azens, Andris; Purans, Juris
Zn-Ir-O (Zn/Ir ≈ 1/1) thin films have been reported to be a potential p-type TCO material. It is, however, unknown whether it is possible to achieve p-type conductivity at low Ir content, and how the type and the magnitude of conductivity are affected by the film structure. To investigate the changes in properties taking place at low and moderate Ir content, this study focuses on the structure, electrical and optical properties of ZnO:Ir films with iridium concentration varying between 0.0 and 16.4 at.%. ZnO:Ir thin films were deposited on glass, Si, and Ti substrates by DC reactive magnetron co-sputtering at room temperature. Low Ir content (up to 5.1 at.%) films contain both a nano-crystalline wurtzite-type ZnO phase and an X-ray amorphous phase. The size of the crystallites is below 10 nm and the lattice parameters a and c are larger than those of pure ZnO crystal. Structural investigation showed that the film's crystallinity declines with the iridium concentration and films become completely amorphous at iridium concentrations between 7.0 and 16.0 at.%. An intense Raman band at approximately 720 cm− 1 appears upon Ir incorporation and can be ascribed to peroxide O22– ions. Measurable electrical conductivity appears together with a complete disappearance of the wurtzite-type ZnO phase. The conduction type undergoes a transition from n- to p-type in the Ir concentration range between 12.4 and 16.4 at.%. Absorption in the visible range increases linearly with the iridium concentration.
Orientation of the electric field gradient and ellipticity of the magnetic cycloid in multiferroic BiFeO3
(Taylor and Francis Ltd., 2017) Pierzga, Angelika; Blachowski, A.; Komędera, Kamila; Ruebenbauer, Krzysztof; Kalvane, Anna I.; Bujakiewicz-Korońska, Renata
The paper deals with the hyperfine interactions observed on the 57Fe nucleus in multiferroic BiFeO3 by means of the 14.41-keV resonant transition in 57Fe and for transmission geometry applied to the random powder sample. Spectra were obtained at 80 K, 190 K and at room temperature. It was found that iron occurs in the high-spin trivalent state. Hyperfine magnetic field follows distribution due to the elliptic-like distortion of the magnetic cycloid. The long axis of the ellipse is oriented along 〈1 1 1〉 direction of the rhombohedral unit cell. The hyperfine magnetic field in this direction is about 1.013 of the field in the perpendicular direction at room temperature. This ratio diminishes to 1.010 at 80 K. Axially symmetric electric field gradient (EFG) on the iron atoms has the principal axis oriented in the same direction and the main component of the EFG is positive. Our results are consistent with the finding that iron magnetic moments are confined to the (Formula presented.) crystal plane.
Facile synthesis of magnetically separable CoFe2O4/Ag2O/Ag2CO3 nanoheterostructures with high photocatalytic performance under visible light and enhanced stability against photodegradation
(Elsevier Sci Ltd., 2017) Šutka, Andris; Döbelin, Nicola; Joost, Urmas; Smits, Krisjanis; Kisand, Vambola; Maiorov, Mihael; Kooser, Kuno; Kook, Mati; Duarte, Roberto Felix; Käämbre, Tanel
We have developed magnetically separable and reasonably stable visible light active photocatalysts containing CoFe2O4 and mixture of Ag2O/Ag2CO3 nanoheterostructures. Obtained ternary nanoheterostructures outperform previously reported magnetically separable visible light photocatalysts, showing one of the highest visible light photocatalytic dye degradation activities in water by a magnetically separable photocatalyst. Photocatalytically active part is Ag2O/Ag2CO3 whereas the CoFe2O4 mainly has stabilizing and magnetic separation functions. The Ag2CO3 phase junction on Ag2O nanoparticle surface were obtained by straightforward phase transformation from silver oxide to silver carbonate in air due to ambient CO2. The phase transformation was followed using X-ray diffraction (XRD), and hard X-ray photoelectron spectroscopy (HAXPES) measurements.
Electron paramagnetic resonance and photoluminescence investigation of europium local structure in oxyfluoride glass ceramics containing SrF2 nanocrystals
(Elsevier B.V., 2017) Antuzevics, Andris; Kemere, Meldra; Krieke, Guna; Ignatans, Reinis
Different compositions of europium doped aluminosilicate oxyfluoride glass ceramics prepared in air atmosphere have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy methods. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show presence of homogenously distributed SrF2 nanocrystals after the heat treatment of the precursor glass. Efficient Eu3+ incorporation in the high symmetry environment of glass ceramics is observed from the photoluminescence spectra. EPR spectra indicate Eu3+ → Eu2+ reduction upon precipitation of crystalline phases in the glass matrix. For composition abundant with Eu2+ in the glassy state such behaviour is not detected. Local structure around europium ions is discussed based on differences in chemical compositions. ----/ / /---- This is the preprint of the following article: A. Antuzevics, M. Kemere, G. Krieke, R. Ignatans, Electron paramagnetic resonance and photoluminescence investigation of europium local structure in oxyfluoride glass ceramics containing SrF2 nanocrystals, Optical Materials 72 (2017), which has been published at https://www.sciencedirect.com/science/article/abs/pii/S0925346717304731. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving.
Thin film organic thermoelectric generator based on tetrathiotetracene
(Wiley-VCH Verlag, 2017) Pudzs, Kaspars; Vembris, Aivars; Rutkis, Martins; Woodward, Simon
Thin films of p- and n- type organic semiconductors for thermo-electrical (TE) applications are produced by doping of tetrathiotetracene (TTT). To obtain p-type material TTT is doped with iodine during vacuum deposition of thin films or by post-deposition doping using controlled exposure to iodine vapors. Thermal co-deposition in vacuum of TTT and TCNQ is used to prepare n-type thin films. The attained thin films are characterized by measurements of Seebeck coefficient and electrical conductivity. Seebeck coefficient and conductivity could be varied by altering the doping level. P-type TTT:iodide thin films with a power factor of 0.52 μWm-1K-2, electrical conductivity of 130 S m-1 and Seebeck coefficient of 63 μV K-1 and n-type TCNQ:TTT films with power factor of 0.33 μWm-1K-2, electrical conductivity of 57 S m-1 and Seebeck coefficient of -75 μV K-1 are produced. Engineered deposition of both p- and n-type thermoelectric conducting elements on the same substrate is demonstrated. A proof of concept prototype of planar thin film TE generator based on a single p-n couple from the organic materials is built and its power generation characterized.
Stimulated emission and optical properties of pyranyliden fragment containing compounds in PVK matrix
(Elsevier Ltd, 2017) Vembris, Aivars; Zarins, Elmars; Kokars, Valdis
Organic solid state lasers are thoughtfully investigated due to their potential applications in communication, sensors, biomedicine, etc. Low amplified spontaneous emission (ASE) excitation threshold value is essential for further use of the material in devices. Intramolecular interaction limits high molecule density load in the matrix. It is the case of the well-known red light emitting laser dye - 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). The lowest ASE threshold value of the mentioned laser dye could be obtained within the concentration range between 2 and 4 wt%. At higher concentration threshold energy drastically increases. In this work optical and ASE properties of three original DCM derivatives in poly(N-vinylcarbazole) (PVK) at various concentrations will be discussed. One of the derivatives is modified DCM dye in which the methyl substituents in the electron donor part have been replaced with bulky trityloxyethyl groups (DWK-1). These sterically significant functional groups do not influence electron transitions in the dye but prevent aggregation of the molecules. The chemical structure of the second investigated compound is similar to DWK-1 where the methyl group is replaced with the tert-butyl substituent (DWK-1TB). The third derivative (DWK-2) consists of two N,N-di(trityloxyethyl)amino electron donor groups. All results were compared with DCM:PVK system. Photoluminescence quantum yield (PLQY) is up to ten times larger for DWK-1TB with respect to DCM systems. Bulky trityloxyethyl groups prevent aggregation of the molecules thus decreasing interaction between dyes and amount of non-radiative decays. The red shift of the photoluminescence and amplified spontaneous emission at higher concentrations were observed due to the solid state solvation effect. The increase of the investigated dye density in the matrix with a smaller reduction in PLQY resulted in low ASE threshold energy. The lowest threshold value was obtained around 21 μJ/cm2 (2.1 kW/cm2) in DWK-1TB:PVK films.

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