What phenomena is responsible for the shift of the nature of the band gap of transition metal dichalcogenides from being indirect band gaps to direct band gap material once it is slimmed down from bulk to its respective monolayers? (I had asked this question in physics stack exchange and nobody bothered with an answer, so I'm here)
From "The indirect to direct band gap transition in multilayered MoS2 as predicted by screened hybrid density functional theory":
First, in the bilayer, there is strong splitting in the conduction band along the $K$ $\rightarrow$ $\Gamma$ line, which shifts the minimum down relative to the $K$ point minimum, leading to an indirect transition. Adding additional layers increases this splitting, reducing the indirect gap as more layers are added, eventually converging toward the calculated bulk value as seen in Figure 1.
In summary, splitting of energy levels is responsible. The atoms in a monolayer are in a different environment than those in the bulk. As a result, the energy levels are split differently. Most of the time, the important splitting occurs in the K point and $\Gamma$ points. In most metal chalcogenides, the valence band of the $K$ point energy decreases in energy as you add more layers. This consistency in behavior is due to the crystal structure of metal chalcogenides. The majority of them form hexagonal structures.
- Applied Physics Letters 99 , 261908 (2011); doi: 10.1063/1.3672219