Experimental observation of a negative grey trion in an electron-rich WSe2 monolayer

We measure the evolution of low temperature photoluminescence in a WSe 2 monolayer with increasing electron concentration level. By comparing non-resonant and resonant laser excitation, we find that the formation of negative trions is facilitated by very efficient phonon emission. The most prominent line in photolumienscence spectra in the intermediate range of carrier concentrations (below cm −2 ) is found to be 66 meV below the bright negative trion. Its measured properties, including low oscillator strength and the temperature dependence point to an interacting bright intervalley and dark intervalley trion state as the origin of the line.