Efficient phonon cascades in hot photoluminescence of WSe${}_2$ monolayers

Abstract

Energy relaxation of photo-excited charge carriers is of significant fundamental interest and crucial for the performance of monolayer (1L) transition metal dichaclogenides (TMDs) in optoelectronics. We measure light scattering and emission in 1L-WSe${}_2$ close to the laser excitation energy (down to~$\sim$0.6meV). We detect a series of periodic maxima in the hot photoluminescence intensity, stemming from energy states higher than the A-exciton state, in addition to sharp, non-periodic Raman lines related to the phonon modes. We find a period $\sim$15meV for peaks both below (Stokes) and above (anti-Stokes) the laser excitation energy. We detect 7 maxima from 78K to room temperature in the Stokes signal and 5 in the anti-Stokes, of increasing intensity with temperature. We assign these to phonon cascades, whereby carriers undergo phonon-induced transitions between real states in the free-carrier gap with a probability of radiative recombination at each step. We infer that intermediate states in the conduction band at the $\Lambda$-valley of the Brillouin zone participate in the cascade process of 1L-WSe${}_2$. The observations explain the primary stages of carrier relaxation, not accessible so far in time-resolved experiments. This is important for optoelectronic applications, such as photodetectors and lasers, because these determine the recovery rate and, as a consequence, the devices' speed and efficiency.

13 pages, 10 figures

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• The ultrafast onset of exciton formation in 2D semiconductors
• Niobium diselenide superconducting photodetectors
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• Nonlinear Dispersion Relation and Out-of-Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers
• Valley Polarization-Electric Dipole Interference and Nonlinear Chiral Selection Rules in Monolayer WSe${}_2$
• Control of the exciton valley dynamics in van der Waals heterostructures
• Monolayer WS${}_2$ electro- and photo-luminescence enhancement by TFSI treatment
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• Surface Effects on Anisotropic Photoluminescence in One-Dimensional Organic Metal Halide Hybrids
• Phenomenological model for long wavelength optical modes in transition-metal dichalcogenide monolayer
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