Surface Reconstruction and Layer-Dependent Semiconductor-to-Metal Transition of Zinc-Blende CdSe.

In this work, CdSe was taken as the representation to systematically investigate the (111) and (110) surface reconstructions, the electronic properties transition related to the layer size, and the corresponding physical mechanism through the density functional theory (DFT) calculation. For the (111) surface slab structure, the bulk truncated relaxation (BTR) surface and the honeycomb (HC) surface were carefully examined. The HC surface configuration, ignored by previous studies, is an energetically preferred surface compared to both the as-truncated and BTR configurations. Based on the HC surface, the band structure of the (111) surface shows a semiconductor character below four layers (4L). Surprisingly, the (111) CdSe turns metallic in the 4L system. In a higher-layer (>4L) system, the two side surfaces and internal regions show metallic and semiconductivity features, respectively. Such an abundant electronic properties transition should be attributed to the electron transfer under the intrinsic polarization perpendicular to the asymmetrical (111) plane. Different from the (111) surface, drastic structural reconstructions were not observed in the (110) surface and the band gap gradually decreased with the increasing number of layers until it approached the value in the bulk. Our results not only revealed the additional possible surface structure but also clarified the underlying mechanism of semiconductor-to-metal (even the edge metallic) transition related to the number of layers. All these findings could be extended to other II-VI group MX compounds for further development of electronic devices.