Strong Optical Absorption and Photocarrier Relaxation in 2-D Semiconductors

Strongly bound excitons in the 2-D crystals of transition metal dichalcogenides give rise to characteristic optical absorption and emission features. Here, we review and provide additional experimental evidence on the unusually strong optical absorption features of monolayer molybdenum disulfide (MoS₂) arising from band nesting and corresponding divergence of joint density of states, which is a distinct signature of 2-D systems. We study low-temperature photoluminescence excitation spectra of suspended MoS₄ and discuss spontaneous formation of indirect excitons for excitation at the band nesting point.