Direct versus indirect band gap emission and exciton-exciton annihilation in atomically thin molybdenum ditelluride(MoTe2)

We probe the room temperature photoluminescence of $N$-layer molybdenum ditelluride $({\mathrm{MoTe}}_{2})$ in the continuous wave (cw) regime. The photoluminescence quantum yield of monolayer ${\mathrm{MoTe}}_{2}$ is three times larger than in bilayer ${\mathrm{MoTe}}_{2}$ and 40 times greater than in the bulk limit. Mono- and bilayer ${\mathrm{MoTe}}_{2}$ display almost symmetric emission lines at 1.10 and 1.07 eV, respectively, which predominantly arise from direct radiative recombination of the A exciton. In contrast, $N\ensuremath{\ge}3\ensuremath{-}\mathrm{layer}\phantom{\rule{4pt}{0ex}}{\mathrm{MoTe}}_{2}$ 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 ${\mathrm{MoTe}}_{2}$ 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 ${\mathrm{MoS}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{MoSe}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$), 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 ${\mathrm{MoTe}}_{2}$ as a function of the exciton formation rate ${W}_{\mathrm{abs}}$ up to $3.6\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The line shape of the photoluminescence spectrum remains largely independent of ${W}_{\mathrm{abs}}$, whereas the photoluminescence intensity grows sublinearly above ${W}_{\mathrm{abs}}\ensuremath{\sim}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. This behavior is assigned to exciton-exciton annihilation and is well captured by an elementary rate equation model.