Vitaly V. Porsev

Application of zone-folding approach to the first-principles estimation of thermodynamic properties of carbon and ZrS2 -based nanotubes.

A zone‐folding (ZF) approach is applied for the estimation of the phonon contributions to thermodynamic properties of carbon‐and ZrS2‐based nanotubes (NTs) of hexagonal morphology with different chiralities. The results obtained are compared with those from the direct calculation of the thermodynamic properties of NTs using PBE0 hybrid exchange‐correlation functional. The phonon contribution to the stability of NTs proved to be negligible for the internal energy and small for the Helmholtz free energy. It is found that the ZF approach allows us an accurate estimation of phonon contributions to internal energy, but slightly overestimates the phonon contributions to entropy.

Phonon spectra, electronic, and thermodynamic properties of WS2 nanotubes

Hybrid density functional theory calculations are performed for the first time on the phonon dispersion and thermodynamic properties of WS2‐based single‐wall nanotubes. Symmetry analysis is presented for phonon modes in nanotubes using the standard (crystallographic) factorization for line groups. Symmetry and the number of infra‐red and Raman active modes in achiral WS2 nanotubes are given for armchair and zigzag chiralities. It is demonstrated that a number of infrared and Raman active modes is independent on the nanotube diameter. The zone‐folding approach is applied to find out an impact of curvature on electron and phonon band structure of nanotubes rolled up from the monolayer. Phonon frequencies obtained both for layers and nanotubes are used to compute the thermal contributions to their thermodynamic functions. The temperature dependences of energy, entropy, and heat capacity of nanotubes are estimated with respect to those of the monolayer. The role of phonons in the stability estimation of nanotubes is discussed based on Helmholtz free energy calculations.

First‐principles modeling of hafnia‐based nanotubes

Hybrid density functional theory calculations were performed for the first time on structure, stability, phonon frequencies, and thermodynamic functions of hafnia‐based single‐wall nanotubes. The nanotubes were rolled up from the thin free layers of cubic and tetragonal phases of HfO2. It was shown that the most stable HfO2 single‐wall nanotubes can be obtained from hexagonal (111) layer of the cubic phase. Phonon frequencies have been calculated for different HfO2 nanolayers and nanotubes to prove the local stability and to find the thermal contributions to their thermodynamic functions. The role of phonons in stability of nanotubes seems to be negligible for the internal energy and noticeable for the Helmholtz free energy. Zone folding approach has been applied to estimate the connection between phonon modes of the layer and nanotubes and to approximate the nanotube thermodynamic properties. It is found that the zone‐folding approximation is sufficiently accurate for heat capacity, but less accurate for entropy. The comparison has been done between the properties of TiO2, ZrO2, and HfO2.

Temperature dependence of strain energy and thermodynamic properties of V2 O5 -based single-walled nanotubes: Zone-folding approach.

A zone‐folding approach is applied to estimate the thermodynamic properties of V2O5‐based nanotubes. The results obtained are compared with those from the direct calculations. It is shown that the zone‐folding approximation allows an accurate estimation of nanotube thermodynamic properties and gives a gain in computation time compared to their direct calculations. Both approaches show that temperature effects do not change the relative stability of V2O5 free layers and nanotubes derived from the α‐ and γ‐phase. The internal energy thermal contributions into the strain energy of nanotubes are small and can be ignored.

Ab initio modeling of single wall nanotubes folded from α- and γ-V2O5 monolayers: structural, electronic and vibrational properties

We have performed first-principles calculations to study the atomic and electronic structures of single wall nanotubes (NTs) of two possible chirality types rolled up from monolayers of α- and γ-V2O5 phases. We have used a hybrid exchange–correlation PBE0 functional within density functional theory and a basis set of localized atomic orbitals. A dispersion correction has been taken into account. All the lattice parameters and atomic positions have been totally optimized. The strain energies calculated for the nanotubes folded from the layers of both phases along the [100] direction are close to zero. This reflects the unique flexibility of the layers for folding in the [100] direction. The electronic structure of the nanotubes of both phases appeared to be similar to that of the parent layer. It was found that for both considered phases, the nanotubes of the same chirality are energetically equivalent but the shape of γ-NTs is closer to the cylindrical form than that of α-NTs. Youngs moduli calculated for (6,0) α- and γ-NTs were found to be 172 GPa and 148 GPa, respectively. The phonon mode frequencies of (6,0) α- and γ-NTs have been calculated and compared with those of α- and γ-V2O5 free layers.

Theoretical Study of α-V2O5 -Based Double-Wall Nanotubes.

First-principles calculations of the atomic and electronic structure of double-wall nanotubes (DWNTs) of α-V2 O5 are performed. Relaxation of the DWNT structure leads to the formation of two types of local regions: 1) bulk-type regions and 2) puckering regions. Calculated total density of states (DOS) of DWNTs considerably differ from that of single-wall nanotubes and the single layer, as well as from the DOS of the bulk and double layer. Small shoulders that appear on edges of valence and conduction bands result in a considerable decrease in the band gaps of the DWNTs (up to 1 eV relative to the single-layer gaps). The main reason for this effect is the shift of the inner- and outer-wall DOS in opposite directions on the energetic scale. The electron density corresponding to shoulders at the conduction-band edges is localized on vanadium atoms of the bulk-type regions, whereas the electron density corresponding to shoulders at the valence-band edges belongs to oxygen atoms of both regions.