Electronic structures and band alignment transition in double-wall MoS2/WS2 nanotubes for optoelectronic applications

Band alignment caused by discontinuous band structures of two different materials plays a central role in semiconductor heterojunctions and interface physics. We investigate the electronic structures and band alignments of double-wall MoS2/WS2 nanotubes with different chirality and geometry. We find that the band gap of both armchair and zigzag nanotubes obeys E-G=E-G(in)-exp-[-delta>Delta R-alpha)], wutg E-G(in) being the band gap of the inner nanotube and Delta R the spacing distance between the inner and outer nanotubes. When placing MoS2 in the inner and WS2 in the outer of the nanotubes, the band alignment belongs to type-I because the conduction band minimum (CBM) of the inner MoS2 nanotube is lower than that of the outer WS2 nanotube, and the valence band maximum (VBM) of MoS2 nanotube is higher than that of WS2 nanotube. While putting the WS2 nanotube in the inner and MoS2 nanotube in the outer, the CBM and VBM of MoS2 nanotube are both lower than the corresponding band extreme points of WS2 nanotube when their diameter is small. Therefore, type-II to type-I band alignment transition occurs, originating from charge transfer with increasing spacing distance. Due to the adjustable band gap and electronic structures, we expect that these double-wall hetero-structured nanotubes will have great potential in optoelectronic devices.