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An ab initio computational study was performed to assess the impact of uniaxial strain (along the wurtzite c axis) on the electronic structure of CdSe and CdTe systems of decreasing dimensionality. In bulk situations, the band gap of CdSe under tension behaves as expected, decreasing with increasing c, whereas CdTe under compression is accompanied by an sp 3 to sp 2 transition that results in a decrease in the band gap. Through a series of two-dimensional heterostructure slabs, the impact of strain on the valence band offset (VBO) between CdSe and CdTe was considered and found to decrease with increasing c lattice parameter. The variation of the band gap of single-component CdSe and CdTe nanowires strictly follows the trends obtained in the bulk calculations, although the absolute values of the nanowire band gap were larger due to quantum confinement. The impact of strain on the VBO in core/shell heterostructure nanowires was found to depend on the choice of core material. When CdSe is used as the core material, the VBO increases with increasing c lattice parameter, whereas when CdTe is used as the core material, the VBO decreases with increasing c. Regardless of the choice of core material, the overlap between the electron and hole states was found to be quite low in these structures and depends only weakly on strain.

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 (0001 j ) surfaces) can be successively created only on the opposite side. For growth to occur along either the (0001) or the (0001 j ) directions, both (0001) surfaces or both (0001 j ) 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.

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We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe and of their interface. We employed the local-density approximation-1/2 self-energy correction scheme [L. G. Ferreira, M. Marques, and L. K. Teles, Phys. Rev. B 78, 125116 (2008)] to obtain improved band gaps and band offsets, as well as spin-orbit coupling to further correct the valence band edges. Our results are in good agreement with experimental values for bulk band gaps and reproduce the staggered band alignment characteristic of this system. We found that the spin-orbit effect is of considerable importance for the bulk band gaps, but has little impact on the band offset of this particular system. Moreover, the electronic structure calculated along the 61.4 Å transition region across the CdSe/CdTe interface shows a non-monotonic variation of the bandgap in the range 0.8-1.8 eV. This finding may have important implications to the absorption of light along the interface between these two materials in photovoltaic applications. V