Browsing by Author "Nika, Denis"

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ENERGETIC, STRUCTURAL AND ELECTRONIC FEATURES OF Sn-, Ga-, O-BASED DEFECT COMPLEXES IN CUBIC In2O3
(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.
ENGINEERING OF THERMAL FLUXES IN PHONON MISMATCHED HETEROSTRUCTURES
(American Scientific Publishers, 2009) Nika, Denis; Zincenco, N.D.; Pokatilov, E.P.
The authors investigated theoretically the lattice thermal fluxes in planar three-layered heterostructures with inner silicon or germanium layer using the face-centered cubic cell dynamic lattice model. It has been established that the acoustically mismatched claddings significantly influence on the phonon spectra and thermal flux of heterostructures. The claddings with small sound velocity reduce average phonon velocity and decrease thermal flux in the nanostructure. The claddings with high sound velocity have an opposite effect. The predicted effects can be used for the engineering of the thermal properties of acoustically mismatched heterostructures and for the improving of the thermal management and thermoelectric properties of nanodevices.
MICROSCOPIC FERMI-LIQUID APPROACH TO THE RESONANT EFFECTS OF SPIN-ORBIT INTERACTION IN SOLIDS
(CEP USM, 2017) Klyukanov, Alexandr; Nika, Denis
Kondo effect, saturation magnetization and heat capacity of ferromagnetic are calculated from the first principles in the spirit of Landau’s Fermi-liquid theory. Temperature dependence of resistivity of metal with magnetic impurity is obtained in a good agreement with existing experimental data. Resistance curves demonstrate a minimum due to the resonance character of the interaction between spins of the localized and conduction electrons. It has been demonstrated that both temperature dependence of magnetic momentum and internal energy of ferromagnetic are in a good agreement with those predicted by the Heisenberg’s model.
NANODIMENSIONAL CHALCOGENIDE FILM-METAL STRUCTURE: NUMERICAL MODELING OF THE GaS SENSITIVE PROPERTIES
(American Scientific Publishers, 2009) Dmitriev, Serghei V.; Nika, Denis; Pokatilov, Evghenii P.
Paper presents results of the numerical simulation of the gas sensitive device on the basis of chalcogenide (Ch) semiconducting nanodimensional film (NDF). Considered model device had been included also underlying electrode and two top coplanar electrodes. The developed theoretical model has been applied for the consideration of chalcogenide (As2S3, As2Se3 and their solid solutions) NDF surface interaction with gaseous species. Performed numerical modeling has allowed to estimating the hole's potential distribution in the NDF and film's gas sensitivity as function of its basic electrophysical parameters (thickness, hole concentration, dielectric permittivity) and gas molecules surface concentration. It was shown, first of all, that considered materials can possess sufficient gas sensitivity already at room temperature. The second interesting result is connected with the presence of the maximum on the gas sensitivity versus thickness dependence and its position strong dependence on the hole concentration in NDF. Also modeling results have shown the significant influence of dielectric permittivity value on the magnitude of gas sensitivity. The proposed model is expected to be successfully used for estimation and optimization of gas sensitive devices on the basis of chalcogenide NDF.
PHONON THERMAL CONDUCTIVITY INHIBITION IN CROSS-SECTION-MODULATED SI/GE NANOWIRES
(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
(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.
PHONON TRANSPORT IN AMORPHOUS SILICON NANOWIRES
(CEP USM, 2013-09-26) Crîşmari, Dmitrii; Nika, Denis
PHONONS AND THERMAL TRANSPORT IN Si/SiO2 MULTISHELL NANOTUBES: ATOMISTIC STUDY
(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.
PHONONS IN TWISTED GRAPHENE
(2019) Nika, Denis
When two graphene layers are placed on top of each other, they can form a moire pattern – twisted graphene. In this case, one layer of carbon atoms is rotated relative to another layer by an arbitrary angle. In the talk, a brief review of recent theoretical and experimental results [1-3] on the engineering of phonon properties of twisted graphene by atomic plane rotations are provided. Angle-dependent specific heat and thermal conductivity of twisted graphene are discussed.
QUANTUM OSCILLATIONS OF MAGNETORESISTANCE AND SIZE EFFECTS IN Bi1−xSbx TOPOLOGICAL INSULATOR SEMICONDUCTOR WIRES
(CEP USM, 2022-11-10) Nikolaeva, Albina; Konopko, Leonid; Huber, Tito; Nika, Denis; Popov, Ivan
RATTLING-INDUCED SUPPRESSION OF PHONON TRANSPORT IN In 2O3 WITH Sn AND Ga
(CEP USM, 2022-11-10) Cocemasov, Alexandr; Brînzari, Vladimir; Nika, Denis
RESONANT TERAHERTZ LIGHT ABSORPTION BY VIRTUE OF TUNABLE HYBRID INTERFACE PHONON–PLASMON MODES IN SEMICONDUCTOR NANOSHELLSN
(MDPI, 2019) Nika, Denis; Fomin, Vladimir; Devreese, Josef; Pokatilov, Evghenii; Tempere, Jacques
Metallic nanoshells have proven to be particularly versatile, with applications in biomedical imaging and surface-enhanced Raman spectroscopy. Here, we theoretically demonstrate that hybrid phonon-plasmon modes in semiconductor nanoshells offer similar advantages in the terahertz regime. We show that, depending on tm,n,nhe doping of the semiconductor shells, terahertz light absorption in these nanostructures can be resonantly enhanced due to the strong coupling between interface plasmons and phonons. A threefold to fourfold increase in the absorption peak intensity was achieved at specific values of electron concentration. Doping, as well as adapting the nanoshell radius, allowed for fine-tuning of the absorption peak frequencies.
THERMAL TRANSPORT EVOLUTION DUE TO NANOSTRUCTURAL TRANSFORMATIONS IN Ga-DOPED INDIUM-TIN-OXIDE THIN FILMS
(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 IN SEMICONDUCTOR NANOSTRUCTURES, GRAPHENE, AND RELATED TWO-DIMENSIONAL MATERIALS
(IOP Publishing Ltd, 2018) Cocemasov, Alexander.; Isacova, Calina; Nika, Denis
We review experimental and theoretical results on thermal transport in semiconductor nanostructures (multilayer thin films, core/shell and segmented nanowires), single- and few-layer graphene, hexagonal boron nitride, molybdenum disulfide, and black phosphorus. Different possibilities of phonon engineering for optimization of electrical and heat conductions are discussed. The role of the phonon energy spectra modification on the thermal conductivity in semiconductor nanostructures is revealed. The dependence of thermal conductivity in graphene and related two-dimensional (2D) materials on temperature, flake size, defect concentration, edge roughness, and strain is analyzed.