Electronic structure and band gap engineering of CdTe semiconductor nanowires

The structural and electronic properties of (10 (1) over bar0) faceted CdTe nanowires with hexagonal or triangular cross sections were investigated using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. The formation energies and band gap of CdTe nanowires are studied as a function of both nanowire size and surface atom ratio. The atomic relaxations of the surface of the (10 (1) over bar0) CdTe nanowires are compared with the corresponding (10 (1) over bar0) CdTe surface. The surface strain was eliminated by passivating the dangling bonds with hydrogen atoms. The passivation of the dangling bonds has only little influence on the band gap resulting only in an increase of about 0.06 eV as compared to unpassivated nanowires. However, it had a significant influence on the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO). We also investigated the effect of the adsorption of dicarboxylic acid derivatives on the (10 (1) over bar0) surface of the hexagonal unpassivated CdTe nanowire with a goal to engineer the band gap. From the band alignment we conclude that the hybrid systems NW-DCDC (di-cyano di-carboxylic acid) and NW-DNDC (di-nitro di-carboxylic acid) represent a type II surface characterized by the presence of molecular states in the gap which reduce the optical gap and may be suitable for use in nanowire-dye sensitized solar cells.