Closing the band gap in 2D semiconductors
Relativistic periodic DFT calculations show that the band gap of single-layer transition-metal dichalcogenides (MoWSeS) are reduced by high external electric fields. MoS2 and MoSe2 become metallic at fields of 6.5 V Å−1 and 4.5 V Å−1.
With proper 2D periodic boundary conditions in BAND, a homogeneous electric field can be applied, polarizing the monolayers and reducing the band gap. 
The band gaps of MoWSeS monolayers are unaffected by field strengths upto 2 V Å−1(left). 2D periodic boundary conditions, without artificial repetition of the surface in the third dimension, allows for proper polarization across the MoS2 surface (right).
See also this advanced exercise: electric field effect on 2D semiconductors.
N. Zibouche, P. Philipsen, T. Heine, A. Kuc, Electron transport in MoWSeS monolayers in the presence of an external electric field, Phys. Chem. Chem. Phys. 16, 11251-11255 (2014).
Key conceptsBAND materials science periodic DFT Relativistic DFT