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

Abstract

We measure the evolution of low temperature photoluminescence in a WSe2 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.

Interplay of excitonic complexes in p -doped WSe2 monolayers

“…The number of optically observable excitonic complexes is especially high in W-based TMD-MLs, due to their spin-inverted band gap at the K-points of the hexagonal Brillouin zone 19,20 . The portfolio of identified excitonic complexes in W-based MLs includes neutral bright excitons, charged bright excitons (or trions) 21 , neutral and charged biexcitons [22][23][24][25][26] , grey trions 27,28 , as well as spin-forbidden dark excitons 29,30 and trions 31,32 and momentum-indirect dark excitons 33 . Exciton-polarons have been reported from highly doped MoSe 2 34,35 .…”

“…Due to the relatively low spin-orbit coupling in the conduction band of W-based TMD-MLs, the largest variety of charged excitonic complexes is observed in electrondoped MLs. The formation of negative trions is very efficient since their binding energy equals the A 1 optical phonon energy 28,40 . As a consequence, virtual negative trions appear in the photoluminescence spectra even at low electron concentrations 41 .…”

“…The highest energy emission (1.721 eV), which is present in a wide range of gate voltages, is attributed to the radiative recombination of the neutral bright exciton (X 0 ). The signal from the neutral dark exciton (X 0 d , 1.678 eV) is observable only in a narrow voltage range around 0.9 V. In the electron-doped regime (V g > 0.7 V), the most prominent excitonic features are the fine-structure split negative trion (X − , 1.684 eV and 1.691 eV), the negative dark trion (X − d , 1.662 eV), as well as X − g (1.649 eV), which originates from the recombination of an inter-valley exciton interacting with an extra electron 27,28 . In the holedoped regime (V g < 0.7 V), the spectra contain emission from the positive bright trion (X + , 1.698 eV), the positive dark trion (X + d , 1.663 eV) and a broad band at low energy (1.640 eV).…”

“…In the supplementary information 42 , we present low-energy emission lines, which suggest the presence of charged impurities in the ML. The importance of the hole gas as energy absorber in the formation process of positive bright trions results from the fact that their binding energy lies within an energy gap where no phonons with suitable symmetry are available 56 , which is the case in the n-doped regime 28,40 .…”

How Photoinduced Gate Screening and Leakage Currents Dynamically Change the Fermi Level in 2D Materials

“…[ 23 ] Furthermore, although the feature in the PL measurement exhibits a small increase in amplitude, it nevertheless does not undergo the gradual redshift normally seen toward higher charge carrier densities. [ 24–28 ]…”