Trion formation in monolayer MoS 2 observed via femtosecond time-resolved photoluminescence measurements

An ultrathin two-dimensional (2D) semiconductor, monolayer ${\mathrm{MoS}}₂$, holds stable excitons and charged excitons (trions) even at ambient temperature. Trion formation time is of interest to the physics of 2D electron systems because it relates to strengths of interactions between excitons and charges, exciton size, and exciton diffusion, which depend on dimensionality. In this paper, we investigate the trion formation via exciton-to-trion relaxation using femtosecond time-resolved photoluminescence (PL) measurements for monolayer ${\mathrm{MoS}}₂$ on a sapphire substrate. The number of layers and defect concentration were evaluated by measuring Raman scattering spectra. The electron concentration was estimated from the PL peak-energy difference between A excitons and trions. The time evolutions of exciton and trion PL in the femtosecond and picosecond regions were obtained. The rate-equation analysis provided the time constant of the relaxation from exciton to trion, i.e., the time constant of trion formation. This time constant was compared with that of other 2D electron systems and discussed in terms of the 2D confinement determined by the thickness. In this paper, we advance our understanding of exciton physics in low-dimensional systems.