Figure 1. Structure of Monolayer MoS₂.

(a) Crystal structure of a single-layer transition-metal dichalcogenide. A central layer of transition metal atoms is sandwiched by layers of chalcogens. (b) Brillouin zone for a reduced dimensional hexagonal lattice. Some relevant high symmetry points are indicated. (c) Schematic of the single-particle band structure at K and K’ valleys. The arrows denote the A (solid line) and B (dashed line) excitons. Each valley can only be excited with a specific helicity, σ⁺ or σ⁻.