(color online) (a) The band structure of monolayer MoS₂. The arrows indicate the transitions in A, B and the band nesting C. (b) PL spectra (red) from excitation at the C peak and differential reflectance spectra (grey) of monolayer MoS₂ flake on quartz substrate. The scale bar indicates 20% absorption based on the differential reflectance spectra. The PL intensity is normalized by the A exciton peak of the differential reflectance spectra. (c) Gate-dependent trion and exciton quasi-particles and transitions. (d) PL of MoSe₂ plotted as a function of back-gate voltage. Neutral and impurity-trapped excitons are observed mostly at near zero doping. Charge excitons dominate the spectrum with large electron (hole) doping, negatively (positively) charged excitons. (e) Schematic diagram of biexcitons formation from excitons as four-body quasi-particles with increasing exciton density. (f) PL spectra at 50 K for pulsed excitation under applied fluence of 0.8 μJ·cm⁻² and 12 μJ·cm⁻². The spectra are normalized to yield the same emission strength for the neutral exciton. Panels (a) and (b) are reproduced from Ref. [63]; panels (c) and (d) are reproduced from Ref. [10]; and panels (e) and (f) are reproduced from Ref. [11]