  MoS2 and WS2 layered transition-metal dichalcogenides are indirect band gap
semiconductors in their bulk forms. Thinned to a monolayer, they undergo a
transition and become direct band gap materials. Layered structures of that
kind can be folded to form nanotubes. We present here the electronic structure
comparison between bulk, monolayered and tubular forms of transition metal
disulfides using first-principle calculations. Our results show that armchair
nanotubes remain indirect gap semiconductors, similar to the bulk system, while
the zigzag nanotubes, like a monolayer, are direct gap materials, what suggests
interesting potential applications in optoelectronics.
