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We probe the room temperature photoluminescence of $N$-layer molybdenumditelluride (MoTe$_2$) in the continuous wave (cw) regime. Thephotoluminescence quantum yield of monolayer MoTe$_2$ is three times largerthan in bilayer MoTe$_2$ and forty times greater than in the bulk limit. Mono-and bilayer MoTe$_2$ display almost symmetric emission lines at $1.10~\rm eV$and $1.07~\rm eV$, respectively, which predominantly arise from directradiative recombination of the A exciton. In contrast, $N\geq3-$layer MoTe$_2$exhibits a much reduced photoluminescence quantum yield and a broader,redshifted and seemingly bimodal photoluminescence spectrum. The low- andhigh-energy contributions are attributed to emission from the indirect anddirect optical band gaps, respectively. Bulk MoTe$_2$ displays a broad emissionline with a dominant contribution at 0.94~eV that is assigned to emission fromthe indirect optical band gap. As compared to related systems (such as MoS$_2$,MoSe$_2$, WS$_2$ and WSe$_2$), the smaller energy difference between themonolayer direct optical band gap and the bulk indirect optical band gap leadsto a smoother increase of the photoluminescence quantum yield as $N$ decreases.In addition, we study the evolution of the photoluminescence intensity inmonolayer MoTe$_2$ as a function of the exciton formation rate $W_\mathrm{abs}$up to $3.6\times 10^{22}~\rm{cm}^{-2} s^{-1}$. The lineshape of thephotoluminescence spectrum remains largely independent of $W_\mathrm{abs}$,whereas the photoluminescence intensity grows sub-linearly above$W_\mathrm{abs}\sim 10^{21}~\rm cm^{-2} s^{-1}$. This behavior is assigned toexciton-exciton annihilation and is well-captured by an elementary rateequation model.

Direct versus indirect band gap emission and exciton-exciton annihilation in atomically thin molybdenum ditelluride (MoTe$_2$)