Observation of three-particle complexes in WS$_2$ monolayers

TL;DR

This study investigates three-particle complexes in WS$_2$ monolayers, revealing temperature-dependent behaviors, dissociation energies, and environmental effects on photoluminescence, advancing understanding of many-body effects in 2D semiconductors.

Contribution

It provides the first detailed temperature-dependent analysis of excitons and trions in WS$_2$ monolayers, including dissociation energies and environmental sensitivity of photoluminescence.

Findings

X$^-$ persists up to 240 K, X$^D$ disappears above 80 K

Dissociation energy of X$^-$ is 2.5 times that of X$^D$

Room-temperature PL is sensitive to ambient gases

Abstract

Atomically thin semiconducting transition-metal dichalcogenides provide novel insights into the physics of many-body effects mediated by Coulomb interactions. Here, we report on temperature-dependent ($T$ = 7-295 K) reflectance contrast and photoluminescence studies of three-particle complexes in n-doped WS$_2$ monolayers. In low-temperature reflectance contrast spectra we observe distinct resonances of the neutral exciton, negative trion and exciton bound to a donor (X, X$^-$ and X$^D$). For temperatures above 80 K, reflectance contrast signatures of the X$^D$ disappear, whereas the X$^-$ remains detectable up to 240 K, despite the fact that the X$^D$ signal is more red-shifted from the neutral exciton than that of the X$^-$. This experimental observation underlines that in WS$_2$ the dissociation energy of X$^-$ considerably exceeds (factor of 2.5) that of X$^D$. In the laser-power dependent photoluminescence experiments, performed at room temperature, we demonstrate the control of the intensity ratio and energy position of the X and X$^-$ lines, which allows us to evaluate the trion binding energy. Moreover, we demonstrate that the room-temperature PL is sensitive to the environmental gas (ambient, N$_2$, He).