arXiv subjects

Valder N. Freire

Publications and source records attributed to Valder N. Freire.

4 recordsLinked to original sources

arXiv2021

Mobility in N-Doped Wurtzite III-Nitrides

A study of the mobility of n-doped wurtzite III-Nitrides is reported. We have determined the nonequilibrium thermodynamic state of the III-Nitrides systems, driven far away from equilibrium by a strong electric field, in the steady state, which follows after a very fast transient. The dependence of the mobility (which depends on the nonequilibrium thermodynamic state of the sample) on the electric field strength is derived, which decreases with the strength of electric field. We analyzed the contributions to the mobility arising out of the different channels of electron scattering, namely, the polar optic, deformation, piezoelectric, interactions with the phonons, and with impurities. The case of n-InN, n-GaN, and n-AlN have been analyzed: as expected the main contribution comes from the polar-optic interactions in these strongly polar semiconductors. The other interactions are in decreasing order, the deformation acoustic, the piezoelectric, and the one due to impurities.

cond-mat.mtrl-sci↗

arXiv2021

Hot Electron Dynamics in Zincblende and Wurtzite GaN

A theoretical investigation of the excess energy dissipation of highly excited photoinjected electrons in both wurtzite and zincblende GaN is presented. The calculations are performed by solving numerically coupled quantum transport equations for the carriers and the acoustic, transversal and longitudinal optical phonon in order to derive the evolution of their nonequilibrium temperatures, dubbed quasi-temperatures. It is shown that the electron energy dissipation is always faster in the wurtzite structure than in zincblende, both occurring in a subpicosecond time scale (< 0.2 ps).

cond-mat.mtrl-sci↗

arXiv2009

Defects in Graphene-Based Twisted Nanoribbons: Structural, Electronic and Optical Properties

We present some computational simulations of graphene-based nanoribbons with a number of half-twists varying from 0 to 4 and two types of defects obtained by removing a single carbon atom from two different sites. Optimized geometries are found by using a mix of classical-quantum semiempirical computations. According with the simulations results, the local curvature of the nanoribbons increases at the defect sites, specially for a higher number of half-twists. The HOMO-LUMO energy gap of the nanostructures has significant variation when the number of half-twists increases for the defective nanoribbons. At the quantum semiempirical level, the first optically active transitions and oscillator strengths are calculated using the full configuration interaction (CI) framework, and the optical absorption in the UV/Visible range (electronic transitions) and in the infrared (vibrational transitions) are achieved. Distinct nanoribbons show unique spectral signatures in the UV/Visible range, with the first absorption peaks in wavelengths ranging from the orange to the violet. Strong absorption is observed in the ultraviolet region, although differences in their infrared spectra are hardly discernible.

cond-mat.mtrl-sci↗

arXiv2009

Möbius and twisted graphene nanoribbons: stability, geometry and electronic properties

Results of classical force field geometry optimizations for twisted graphene nanoribbons with a number of twists $N_t$ varying from 0 to 7 (the case $N_t$=1 corresponds to a half-twist Möbius nanoribbon) are presented in this work. Their structural stability was investigated using the Brenner reactive force field. The best classical molecular geometries were used as input for semiempirical calculations, from which the electronic properties (energy levels, HOMO, LUMO orbitals) were computed for each structure. CI wavefunctions were also calculated in the complete active space framework taking into account eigenstates from HOMO-4 to LUMO+4, as well as the oscillator strengths corresponding to the first optical transitions in the UV-VIS range. The lowest energy molecules were found less symmetric than initial configurations, and the HOMO-LUMO energy gaps are larger than the value found for the nanographene used to build them due to electronic localization effects created by the twisting. A high number of twists leads to a sharp increase of the HOMO $\to$ LUMO transition energy. We suggest that some twisted nanoribbons could form crystals stabilized by dipolar interactions.

cond-mat.mtrl-sci↗