Oxygen Adsorption on CdSe Surfaces: Case Study of Asymmetric Anisotropic Growth through ab Initio Computations

Asymmetric anisotropic growth in wurtzite based II-VI semiconductors has been exploited in the past to create a diversity of nanocrystal shapes and topologies. Here, we present a methodology, based on ab initio calculations, to assess the circumstances (i.e., chemical environments) under which anisotropic and asymmetric growth could occur in terms of the ordering and magnitude of the surface energies. This methodology is applied to wurtzite CdSe systems exposed to oxygen atmospheres. Our results show that oxygen adsorption, in the most favored binding mode, is exothermic on all polar and nonpolar CdSe facets. On nonpolar facets, which contain equal numbers of Cd and Se atoms, adsorption of oxygen takes place preferentially on Se resulting in very stable surface configurations with large drops in surface energy (relative to the clean surfaces). This renders all nonpolar facets passive toward growth in the presence of oxygen. Among the four major inequivalent polar surface facets, two (the Cd-terminated and Se-terminated (0001) surfaces) can be successively created on one side of the nanocrystal and two other facets (the Cd-terminated and Se-terminated (0001j) surfaces) can be successively created only on the opposite side. For growth to occur along either the (0001) or the (0001j) directions, both (0001) surfaces or both (0001j) surfaces, respectively, should display high surface energies relative to all other surfaces. We find that, for appropriate choices of the Cd chemical potential and oxygen coverage, the surface energies of the two (0001) facets are far higher than any of the other surfaces, thereby making (0001) facets relatively unstable and prone to rapid growth along only that direction (resulting in asymmetric anisotropic growth). Thus by controlling the ordering of the surface energies (e.g., through proper choices of precursor concentration and surfactants), control of directional growth can be achieved, as has been done before empirically.