(color online) (a) 2D hexagonal lattice of monolayer TMDs of x–y plane. It has a structure that is lacking of inversion symmetry. (b) Optical selection rules in monolayer MoS₂ for the A and B exciton states at K (K′) valley coupled with left (right) circularly polarized light. (c) Left (σ) and right (σ⁺) circularly polarized resolved photoluminescence spectra for monolayer MoS₂ on h-BN substrates. (d) Corresponding photoluminescence helicity ρ. Panels (b)–(d) are reproduced from Ref. [2].