Surface Reconstruction\nand Layer-Dependent Semiconductor-to-Metal\nTransition of Zinc-Blende CdSe

In this work, CdSe was taken as the representation to\nsystematically\ninvestigate the (111) and (110) surface reconstructions, the electronic\nproperties transition related to the layer size, and the corresponding\nphysical mechanism through the density functional theory (DFT) calculation.\nFor the (111) surface slab structure, the bulk truncated relaxation\n(BTR) surface and the honeycomb (HC) surface were carefully examined.\nThe HC surface configuration, ignored by previous studies, is an energetically\npreferred surface compared to both the as-truncated and BTR configurations.\nBased on the HC surface, the band structure of the (111) surface shows\na semiconductor character below four layers (4L). Surprisingly, the\n(111) CdSe turns metallic in the 4L system. In a higher-layer (>4L)\nsystem, the two side surfaces and internal regions show metallic and\nsemiconductivity features, respectively. Such an abundant electronic\nproperties transition should be attributed to the electron transfer\nunder the intrinsic polarization perpendicular to the asymmetrical\n(111) plane. Different from the (111) surface, drastic structural\nreconstructions were not observed in the (110) surface and the band\ngap gradually decreased with the increasing number of layers until\nit approached the value in the bulk. Our results not only revealed\nthe additional possible surface structure but also clarified the underlying\nmechanism of semiconductor-to-metal (even the edge metallic) transition\nrelated to the number of layers. All these findings could be extended\nto other II–VI group MX compounds for further development of\nelectronic devices.