Effect of dimensionality on the charge-density wave in few-layer 2H-NbSe2

We investigate the charge-density wave (CDW) instability in single and double layers, as well as in the bulk 2H-NbSe2. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk 2H-NbSe2. We predict that both monolayer and bilayer NbSe2 undergo a CDW instability. However, while in the bulk the instability occurs at a momentum q(CDW)approximate to 2/3 Gamma M, in freestanding layers it occurs at q(CDW)approximate to 1/2 Gamma M. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at q approximate to q(CDW) plays in forming the CDW ground state.