Spectral asymmetry of phonon sideband luminescence in monolayer and bilayer WSe2

TL;DR

This study investigates the temperature-dependent spectral lineshapes of phonon side-band emission from dark excitons in monolayer and bilayer WSe2, revealing asymmetries linked to exciton thermal distributions and differences from bright excitons.

Contribution

It provides experimental insights into phonon-assisted luminescence of dark excitons, highlighting spectral asymmetries and thermal behaviors in atomically thin WSe2 layers.

Findings

Spectral asymmetry indicates thermal distribution of excitons with finite momentum.

Exciton reservoirs generally follow lattice temperature, with deviations indicating overheated populations.

Bilayer WSe2 shows more pronounced overheating effects at low temperatures.

Abstract

We report an experimental study of temperature-dependent spectral lineshapes of phonon side-band emission stemming from dark excitons in monolayer and bilayer WSe$_{2}$. Using photoluminescence spectroscopy in the range from 4 to 100K, 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.