Trion Formation Resolves Observed Peak Shifts in the Optical Spectra of Transition-Metal Dichalcogenides

Monolayer transition-metal dichalcogenides (ML-TMDs)have the potentialto unlock novel photonic and chemical technologies if their optoelectronicproperties can be understood and controlled. Yet, recent work hasoffered contradictory explanations for how TMD absorption spectrachange with carrier concentration, fluence, and time. Here, we testour hypothesis that the large broadening and shifting of the strongband-edge features observed in optical spectra arise from the formationof negative trions. We do this by fitting an ab initio based, many-body model to our experimental electrochemical data.Our approach provides an excellent, global description of the potential-dependentlinear absorption data. We further leverage our model to demonstratethat trion formation explains the nonmonotonic potential dependenceof the transient absorption spectra, including through photoinducedderivative line shapes for the trion peak. Our results motivate thecontinued development of theoretical methods to describe cutting-edgeexperiments in a physically transparent way.