Probing the origin of excitonic states in monolayer WSe2

Abstract

Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications.

Figure 1. Optical and AFM images of a WSe₂ sample.

(a) Optical microscopy image taken with a 100× objective. (b) AFM image of the area in (a) indicated by the dashed lines. (c) Height profile taken along the dashed line in (b) confirming the presence of monolayer WSe₂.

Figure 2. Excitation power dependence of emission dynamics.

(a) PL intensity as a function of the excitation power for peaks 1, 2 and 5. Inset: PL spectra from monolayer and bulk WSe₂ at T = 10 K. Five different emission peaks were observed for monolayer WSe₂, whereas only one broad peak was observed for the bulk. A 30 μW, 488 nm cw excitation laser was used in the measurements. (b) PL intensity as a function of the excitation power for peaks 3 and 4. The solid lines are fits to a power law.

Figure 3. Temperature dependence of emission dynamics.

(a) PL spectra of monolayer WSe₂ at different temperatures. The top curve is bulk WSe₂ at T = 300 K for comparison. The dashed gray lines are guides to the eye. (b) Photon energies of the peaks labeled as 1, 2, 4 in (a) as a function of temperature. The solid lines are fits to the data using Eq. (1). (c) PL intensities of peaks 1, 2 and 4 as a function of 1/T. The solid lines are fits to the data using Eq. (2). A 30 μW, 632 nm fs-laser was used for excitation in the measurements.

Figure 4. Temperature dependence of the PL polarization of monolayer WSe₂.

Degree of circular polarization as defined in Eq. (3) as a function of temperature for peaks 1, 2 and 4 as labeled in the inset. Inset: Polarization-resolved PL spectra for σ⁺ and σ⁻ detection for a 30 μW, 632 nm (1.96 eV) laser excitation at T = 10 K.