Computer Simulation of the Formation of Hollow Nanocrystals

Experiments on the formation of hollow nanocrystals of cobalt sulphide by way of the Kirkendall effect have been reported recently by Yin et al. We perform a set of Monte Carlo simulations of the process to explore the phenomenon. 1. Introduction A recent experiment on cobalt nanocrystals heated in sulphur at 180 ºC showed the fascinating formation of hollow nanospheres of cobalt sulphide by the process of interdiffusion with the formation of Kirkendall porosity (at the centre of the original cobalt nanocrystal) [1]. We have undertaken a computer simulation/analytical study to explore this phenomenon. 2. Computer simulation We considered a diffusion couple prepared as two concentric layers or shells with a total width d of about a hundred lattice planes. The internal core (r 1 ≥ r ≥ r 0) consists of A which is a much faster diffuser than the external shell, which is of material B (r 2 ≥ r > r 1 , r 2 – r 1 = d). For definiteness, we assume that the ratio of atom-vacancy exchange frequencies w A /w B = 100. The atomistic simulation employed a new simulation procedure that employed moving boundaries. The experiments were done for the case of a spherical geometry [1]. Assuming that at the initial time there is already a small pore at r = 0 the general behaviour of the interdiffusion couple in a spherical geometry can be satisfactorily modeled by a planar geometry. The boundary conditions for the vacancy site fraction were as follows: c