Transition metal dichalcogenide (TMD) monolayers (MLs) exhibit rich photoluminescence spectra associated with interband optical transitions of direct-gap semiconductors. Upon absorption of photons, direct excitons with zero center-of-mass momentum are formed by photo-excited electrons in the conduction band and the respective unoccupied states in the valence band of the same valley. Different spin configurations of such momentum-direct excitons as well as their charged counterparts provide a powerful platform for spin-valley and microcavity physics in two-dimensional materials. The corresponding spectral signatures, however, are insufficient to explain the main characteristic peaks observed in the photoluminescence spectra of ML TMDs on the basis of momentum-direct excitons alone. Here, we show that the notion of momentum-indirect excitons is important for the understanding of the versatile photoluminescence features. Taking into account phonon-assisted radiative recombination pathways for electrons and holes from dissimilar valleys, we interpret unidentified peaks in the emission spectra as acoustic and optical phonon sidebands of momentum-dark excitons. Our approach will facilitate the interpretation of optical, valley and spin phenomena in TMDs arising from bright and dark exciton manifolds.
We report an experimental study of temperature-dependent spectral lineshapes of phonon sideband emission stemming from dark excitons in monolayer and bilayer WSe2. Using photoluminescence spectroscopy in the range from 4 to 100 K, we observe a pronounced asymmetry in the phonon-assisted luminescence from momentum-indirect exciton reservoirs. We demonstrate that the corresponding spectral profiles are distinct from those of bright excitons with direct radiative decay pathways. The lineshape asymmetry reflects thermal distribution of exciton states with finite center-of-mass momenta, characteristic for phonon sideband emission. The extracted temperature of the exciton reservoirs is found to generally follow that of the crystal lattice, with deviations reflecting overheated populations. The latter are most pronounced in the bilayer case and at lowest temperatures. Our results add to the understanding of phonon-assisted recombination of momentum-dark excitons and, more generally, establish means to access the thermal distribution of finite-momentum excitons in atomically thin semiconductors with indirect bandgaps.
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