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

Abstract

Monolayer transition-metal dichalcogenides (ML-TMDs) have the potential to unlock novel photonic and chemical technologies if their optoelectronic properties can be understood and controlled. Yet, recent work has offered contradictory explanations for how TMD absorption spectra change with carrier concentration, fluence, and time. Here, we test our hypothesis that the large broadening and shifting of the strong band-edge features observed in optical spectra arise from the formation of 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-dependent linear absorption data. We further leverage our model to demonstrate that trion formation explains the nonmonotonic potential dependence of the transient absorption spectra, including through photoinduced derivative line shapes for the trion peak. Our results motivate the continued development of theoretical methods to describe cutting-edge experiments in a physically transparent way.

Quantifying interfacial energetics of 2D semiconductor electrodes using in situ spectroelectrochemistry and many-body theory

“…In the section that follows, we describe an approach to quantitatively link the measured absorbance spectrum to carrier concentration using a many-body model of minimal complexity to describe trion formation in ML transition metal dichalcogenides (TMDs) such as MoS 2 . 37,38 MND model and fitting procedure to determine n…”

“…Our previous work provides complete MND model calculation details. 38 Briefly, the model quantitatively links the experimental observable (absorbance spectrum) to n via the Fermi doping energy parameter e F , defined as an energy level position at or above the conduction band minimum. 37,42 First, we obtain model parameters, the undoped A 0 peak (i.e., energy, width, and height), using data acquired at E = +1.00 V and account for phenomenological peak broadening by convolving the simulated peak with a Gaussian.…”

Interpretations of High-Order Transient Absorption Spectroscopies

“…Linear absorption spectroscopy, in frequency ranges from THz to X-ray, gives information about the ground or thermal states of a material and its optically accessible excited states. Transient absorption (TA) spectroscopy measures the change in a probe pulse’s absorption spectrum when it arrives a time T after a pump pulse, revealing both the absorption and dynamics of excited states. − The recently developed high-order transient absorption (HOTA) spectroscopy extends TA spectroscopy by systematically separating higher orders of nonlinear response, which has not previously been possible using TA methods. − These higher orders of nonlinear response contain information about energies, transition dipoles, and dynamics of multiply excited states.…”