which recently were synthesized. We find that two-dimensional MoS 2 is a semiconductor
with a gap which is rather close to that of the three-dimensional analog, and that NbSe 2 is a
metal, which is similar to the three-dimensional analog of this compound. We further
computed the electronic structure of the two-dimensional hexagonal (graphene-like) lattices
of Si and Ge and compared them with the electronic structure of graphene. It is found that the …