Photoluminescence in two-dimensional crystals

Two-dimensional (2D) crystals derived from layered structures exhibit a unique set of properties as elegantly demonstrated for graphene. Semiconducting 2D structures such as MoS2 sheets are attractive building blocks for novel electronic and optoelectronic devices. In this talk, I will report photoluminescence properties of group 6 transition metal dichalcogenide (TMD) 2D crystals and discuss how their spectral features provide insight into the evolution of chemical, structural, and electronic properties of these materials. A single layer MoS2 is a direct gap semiconductor in striking contrast to its indirect gap bulk counterpart [2]. As a result, single layer MoS2 exhibits distinct band gap photoluminescence. We find that photoluminescence spectra of monoto few-layer WS2 and WSe2 indicate that their band structure undergoes a similar indirect-to-direct gap transition when thinned to a single monolayer (Fig. 1) [3]. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS2 and WSe2, respectively. We demonstrate that indirect gap emission and direct gap hot electron emission is pronounced in few-layer WSe2 due to small energy difference between the two transitions. At sufficiently high temperatures, short-range interlayer interactions in multilayer sheets weaken such that each layer behaves like an individual monolayer, giving rise to enhanced photoluminescence similar to the case of MoSe2 [4].