We have theoretically demonstrated that phonon heat flux can be significantly suppressed in Si and Si/SiO 2 nanowires with the periodically modulated cross-section area-referred to as the cross-section-modulated nanowires-in comparison with the generic uniform cross-section nanowires. The phonon energy spectra were obtained using the five-parameter Born-von Karman-type model and the face-centered-cubic cell model for description of the lattice dynamics. The thermal flux and thermal conductivity in Si and Si/SiO 2 cross-section-modulated nanowires were calculated from the Boltzmann transport equation within the relaxation time approximation. Redistribution of the phonon energy spectra in the cross-section-modulated nanowires leads to a strong decrease of the average phonon group velocities and a corresponding suppression of the phonon thermal flux in these nanowires as compared to the generic nanowires. This effect is explained by the exclusion of the phonon modes trapped in cross-section-modulated nanowires segments from the heat flow. As a result, a three-to sevenfold drop of the phonon heat flux in the 50-to 400-K temperature range is predicted for Si and Si/SiO 2 cross-section-modulated nanowires under consideration. The obtained results indicate that cross-section-modulated nanowires are promising candidates for thermoelectric applications.

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. PACS: 63.22.Rc, 63.22-m, 65.80.Ck, 65.80.-g 1 Corresponding author (DLN): dlnika@yahoo.com 2 Thermal transport in semiconductor nanostructuresRapid miniaturization of electronic devices to nanoscale range requires new approaches for efficient management of their heat and electrical conductions. One of these approaches, referred to as phonon engineering [1], is related to optimization of thermal and electronic properties of nanodimensional structures due to modification of their phonon properties [1][2][3]. At the end of the previous century several research groups have demonstrated that many phonon confined branches appear in energy spectra of homogeneous semiconductor thin films and nanowires [4][5][6][7][8][9], leading to change in phonon density of states and reduction of average phonon group velocity in comparison with corresponding bulk materials [7][8][9]. The latter together with enhanced phonon boundary scattering results in decreasing of lattice thermal conductivity (TC). Balandin and Wang [7] have theoretically predicted that lattice thermal conductivity of 10-nm-wide silicon film is by an order of magnitude smaller than that in bulk silicon at room temperature (RT). Fivetimes drop of lattice thermal conductivity was also theoretically predicted for Si nanowire with a diameter of 20 nm [10]. Subsequent independent theoretical studies [11][12][13][14][15][16][17] and experimental measurements of thermal conductivity in several nm-thick free-standing Si films and nanowires [18][19][20][21] confirmed the initial predictions: strong reduction of lattice thermal conductivity as compared with bulk material was revealed.More precise tuning of phonon properties and heat conduction at nanoscale can be realized in multilayer films (MFs) and core/shell nanowires (NWs) [22][23][24][25][26][27][28][29][30][31][32][33]. The evolution of phonon energies in homogeneous silicon films and silicon films covered by diamond claddings is illustrated in Figure 1, where we show the dispersion relations for the dilatation (SA) phonon modes in the freestanding Si film (a); Diamond/Si/Diamond heterostructures with the different thickness of the diamond (D) barrier layer (b-c); and Si film with the clamped external surfaces (d), which correspond to a film embedded in the "absolutely" rigid material. The thickness of the Si layer in all cases is 2 nm to insure the ...

Thermal transport in the Si/SiO2 multishell nanotubes is investigated theoretically. The phonon energy spectra are obtained using the atomistic lattice dynamics approach. Thermal conductivity 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 conductivity 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.

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