Could Porosity Induce Gaps in the Vibrational Density of States of Nanoporous Silicon?

Abstract

As in our previous work [1] nanoporous silicon periodic supercells with 1000 atoms but now with 80 % porosity were constructed using the Tersoff potential and our novel approach [2]. The approach consists first in constructing a crystalline diamond-like supercell with a density (volume) close to the real value, and then lowering the density by increasing the volume, subjecting the resulting periodic supercell to Tersoff-based molecular dynamics processes at a temperature of 300 K, followed by geometry relaxation [1]. As in the ab initio approach [2] the resulting samples are also essentially amorphous and display pores along some of the crystallographic directions. We report the radial (pair) distribution function (RDF), g(r), the bond angle distribution, the pore structure where prominent and a computational prediction for the vibrational density of states for this structure. We then compare it to the 50 % porous sample presented in Ref [1]. The soft acoustic phonons are displaced towards lower energy in the 80 % porosity sample whereas the optical modes are displaced towards higher energies. The pseudo gap, existing in the 50 % porous sample, is depleted even more in the 80 % sample indicating a tendency towards the creation of a phonon gap for higher porosity materials. Some conjectures that point to the possible engineering of porous materials to produce predetermined phonon properties are discussed.