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

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).

5 pages, 2 figures