Computational alternatives to generate amorphous nanoporous structures using ab initio molecular dynamics

We present two approaches to generate amorphous nanoporous structures based on the geometrical handling of crystalline supercells and ab initio molecular dynamics. The first approach has been partially reported previously and consists in expanding a crystalline supercell (the expanding lattice approach) by lengthening the edges (proportionally increasing the interatomic distance) to obtain the desired porosity (density). Then, molecular dynamics is applied at diverse constant temperatures to allow the atoms to rearrange and create the corresponding atomic topology. The other one, inspired by the experimental process called dealloying is similar to that process which is used to generate porous metals by etching away one or several of the elements of an alloy. Both processes are applied to semiconducting (carbon, silicon) and metallic (copper, silver) materials and the results compared. Pair distribution functions are obtained in order to show that although both techniques may be similar, the dealloying technique seems to be more adequate when dealing with alloy systems. An interesting byproduct is that pair distribution functions tend to values lower that 1 for intermediate-to-long range distances in a region commensurate with the pore size.