Browsing by Author "Cocemasov, Alexandr"

Now showing 1 - 8 of 8

Energetic, structural and electronic features of Sn-, Ga-, O-based defect complexes in cubic In2O3 [Articol]
(IOP Publishing Ltd, 2020) Cocemasov, Alexandr; Brinzari, Vladimir; Nika, Denis
Defect energy formation, lattice distortions and electronic structure of cubic In2O3 with Sn, Ga and O impurities were theoretically investigated using density functional theory. Different types of point defects, consisting of 1–4 atoms of Sn, Ga and O in both substitutional and interstitial (structural vacancy) positions, were examined. It was demonstrated, that formation of substitutional Ga and Sn defects are spontaneous, while formation of interstitial defects requires an activation energy. The donor-like behavior of interstitial Ga defects with splitting of conduction band into two subbands with light and heavy electrons, respectively, was revealed. Contrarily, interstitial O defects demonstrate acceptor-like behavior with the formation of acceptor levels or subbands inside the band gap. The obtained results are important for an accurate description of transport phenomena in In2O3 with substitutional and interstitial defects.
Force constant matrices from keating interatomic potential: application to graphene [Articol]
(CEP USM, 2014) Cocemasov, Alexandr
Based on the Keating interatomic potential an analytical derivation of force constant matrices of two- and three body interactions in crystals is performed. Using the derive d force constant matrices the in-plane phonon energy spectra of graphene was calculated in the framework of a lattice dynamics theory. A reasonable agreement with experimental data was obtained. The results can extend the applicability of the current force constant models for investigation of crystal dynamics .
Phonon thermal conductivity inhibition in cross-section-modulated SI/GE nanowires [Articol]
(CEP USM, 2013-09-26) Nika, Denis; Cocemasov, Alexandr; Crîşmari, Dumitru; Balandin, Alexander A.
Phonon thermal transport suppression in Si/SiO 2 crystalline/amorphous superlattices [Articol]
(Belgrad: Institute of Technical Sciences of SASA, 2019-12) Cocemasov, Alexandr; Nika, Denis
Clear understanding of phonon properties in nanostructures is instrumental in design of novel thermoelectric materials [1-3]. Crystalline/amorphous superlattices (CASLs) consisting of alternated crystalline and amorphous layers are promising in tailoring the thermoelectric properties because they can be made of materials with distinctly different physical and chemical properties. Here we report on theoretical study of phonon thermal transport in c- Si/a-SiO2 CASLs, which remain less explored in the literature [4]. Comparing equations for lattice thermal conductivity within linearized Boltzmann transport equation [5-6] and Allen- Feldman theory of diffusive thermal transport [7-8] we have obtained an expression for the rate of diffusion of lattice vibrations in a-SiO2 layers. The obtained expression is parameter- free and can be used for a wide range of amorphous materials. The performed theoretical study have showed, that heat transport in c-Si/a-SiO2 CASLs is strongly suppressed as compared to purely crystalline c-Si/c-SiO2 superlattices. Owing to the reduced lattice thermal conductivity, we conclude that c-Si/a-SiO2 CASLs show promise for thermoelectric applications.
Phonons and thermal transport in Si/SiO2 multishell nanotubes: Atomistic study [Articol]
(MDPI, 2021) Isacova, Calina; Cocemasov, Alexandr; Nika, Denis; Fomin, Vladimir
Thermal transport in the Si/SiO2 multishell nanotubes is investigated theoretically. The phonon energy spectra are obtained using the atomistic lattice dynamics approach. Thermal conduc- tivity is calculated using the Boltzmann transport equation within the relaxation time approximation. Redistribution of the vibrational spectra in multishell nanotubes leads to a decrease of the phonon group velocity and the thermal conductivity as compared to homogeneous Si nanowires. Phonon scattering on the Si/SiO2 interfaces is another key factor of strong reduction of the thermal conduc- tivity in these structures (down to 0.2 Wm−1K−1 at room temperature). We demonstrate that phonon thermal transport in Si/SiO2 nanotubes can be efficiently suppressed by a proper choice of nanotube geometrical parameters: lateral cross section, thickness and number of shells. We argue that such nanotubes have prospective applications in modern electronics, in cases when low heat conduction is required.
Rattling-induced suppression of phonon transport in In2O3 with Sn and Ga [Articol]
(CEP USM, 2022-11-10) Cocemasov, Alexandr; Brînzari, Vladimir; Nika, Denis
Thermal transport evolution due to nanostructural transformations in Ga-doped indium-tin-oxide thin films [Articol]
(MDPI, 2021) Brinzari, Vladimir; Jeong, Do-Gyeom; Korotcenkov, Ghenadii; Vatavu, Sergiu; Lee, Jong S.; Nika, Denis; Cocemasov, Alexandr
We report on a comprehensive theoretical and experimental investigation of thermal conductivity in indium-tin-oxide (ITO) thin films with various Ga concentrations (0–30 at. %) deposited by spray pyrolysis technique. X-ray diffraction (XRD) and scanning electron microscopy have shown a structural transformation in the range 15–20 at. % Ga from the nanocrystalline to the amorphous phase. Room temperature femtosecond time domain thermoreflectance measurements showed nonlinear decrease of thermal conductivity in the range 2.0–0.5 Wm−1 K−1 depending on Ga doping level. It was found from a comparison between density functional theory calculations and XRD data that Ga atoms substitute In atoms in the ITO nanocrystals retaining Ia-3 space group symmetry. The calculated phonon dispersion relations revealed that Ga doping leads to the appearance of hybridized metal atom vibrations with avoided-crossing behavior. These hybridized vibrations possess shortened mean free paths and are the main reason behind the thermal conductivity drop in nanocrystalline phase. An evolution from propagative to diffusive phonon thermal transport in ITO:Ga with 15–20 at. % of Ga was established. The suppressed thermal conductivity of ITO:Ga thin films deposited by spray pyrolysis may be crucial for their thermoelectric applications.
Thermal transport evolution due to nanostructural transformations in ga-doped indium-tin-oxide thin films [Articol]
(2021) Cocemasov, Alexandr; Brinzari, Vladimir; Jeong, Do-Gyeom; Korotcenkov, Ghenadii; Lee, Jong S.; Nika, Denis L.
We report on a comprehensive theoretical and experimental investigation of thermal con- ductivity in indium-tin-oxide (ITO) thin films with various Ga concentrations (0–30 at. %) deposited by spray pyrolysis technique. X-ray diffraction (XRD) and scanning electron microscopy have shown a structural transformation in the range 15–20 at. % Ga from the nanocrystalline to the amorphous phase. Room temperature femtosecond time domain thermoreflectance measurements showed nonlinear decrease of thermal conductivity in the range 2.0–0.5 Wm−1 K−1 depending on Ga doping level. It was found from a comparison between density functional theory calculations and XRD data that Ga atoms substitute In atoms in the ITO nanocrystals retaining Ia-3 space group symmetry. The calculated phonon dispersion relations revealed that Ga doping leads to the appearance of hybridized metal atom vibrations with avoided-crossing behavior. These hybridized vibrations possess shortened mean free paths and are the main reason behind the thermal conductivity drop in nanocrystalline phase. An evolution from propagative to diffusive phonon thermal transport in ITO:Ga with 15–20 at. % of Ga was established. The suppressed thermal conductivity of ITO:Ga thin films deposited by spray pyrolysis may be crucial for their thermoelectric applications.