Direct versus indirect band gap emission and exciton-exciton annihilation in atomically thin molybdenum ditelluride (MoTe2)
Abstract
We probe the room temperature photoluminescence of N-layer molybdenum ditelluride (MoTe2) in the continuous wave (cw) regime. The photoluminescence quantum yield of monolayer MoTe2 is three times larger than in bilayer MoTe2 and forty times greater than in the bulk limit. Mono- and bilayer MoTe2 display almost symmetric emission lines at 1.10~ eV and 1.07~ eV, respectively, which predominantly arise from direct radiative recombination of the A exciton. In contrast, N≥3-layer MoTe2 exhibits a much reduced photoluminescence quantum yield and a broader, redshifted and seemingly bimodal photoluminescence spectrum. The low- and high-energy contributions are attributed to emission from the indirect and direct optical band gaps, respectively. Bulk MoTe2 displays a broad emission line with a dominant contribution at 0.94~eV that is assigned to emission from the indirect optical band gap. As compared to related systems (such as MoS2, MoSe2, WS2 and WSe2), the smaller energy difference between the monolayer direct optical band gap and the bulk indirect optical band gap leads to a smoother increase of the photoluminescence quantum yield as N decreases. In addition, we study the evolution of the photoluminescence intensity in monolayer MoTe2 as a function of the exciton formation rate Wabs up to 3.6× 1022~cm-2 s-1. The lineshape of the photoluminescence spectrum remains largely independent of Wabs, whereas the photoluminescence intensity grows sub-linearly above Wabs 1021~ cm-2 s-1. This behavior is assigned to exciton-exciton annihilation and is well-captured by an elementary rate equation model.