Excitonic Transport and Intervalley Scattering Dynamics in Large‐Size Exfoliated MoSe₂ Monolayer Investigated by Heterodyned Transient Grating Spectroscopy

Abstract

Exciton intervalley scattering, annihilation, relaxation dynamics, and diffusive transport in monolayer transition metal dichalcogenides are central to the functionality of devices based on them. Here, these properties in a large-size exfoliated high-quality monolayer MoSe 2 are addressed directly using heterodyned transient grating spectroscopy at room temperature. While the free exciton population is found to be long-lived (≈230 ps), an extremely fast intervalley scattering (≤170 fs) is observed, leading to a negligible valley polarization, consistent with steady state photoluminescence measurements and theoretical calculation. The exciton population decay shows an appreciable contribution from the exciton-exciton annihilation, with an annihilation rate of ≈0.01 cm 2 s −1. The annihilation process also leads to a significant distortion of the transient grating evolution. Taking this distortion into account, consistent exciton diffusion constants D ≈ 1.4 cm 2 s −1 are found by a model simulation in the excitation density range of 10 11-10 12 cm −2. The presented results highlight the importance of correctly considering the many-body annihilation processes to obtain a pronounced understanding of the excitonic properties of monolayer transition metal dichalcogenides.

Sub-10 fs Intervalley Exciton Coupling in Monolayer MoS2 Revealed by Helicity-Resolved Two-Dimensional Electronic Spectroscopy

“…The sub-100 fs fluence-independent intervalley coupling in our work is therefore likely distinct from this exchange-mediated or other exciton scattering mechanisms. Kuhn et al observed fluence-independent ∼170 fs intervalley depolarization (equal to the autocorrelation width of their 120 fs pulse) in monolayer MoSe 2 , consistent with Rashba-induced mixing of dark and bright exciton states . However, this coupling is predicted to be negligible in MoS 2 and is therefore likely not responsible for the observed coupling features in our experiments.…”

Bright and Efficient Light‐Emitting Devices Based on 2D Transition Metal Dichalcogenides

“…[ 61,144,173 ] At higher generation rates, the PL QY of MoS 2 (and other TMDCs) drops as recombination at high exciton densities is dominated by the annihilation of two adjacent excitons, a process so‐called exciton–exciton annihilation (EEA). [ 99,217–220 ] EEA is a process in which neutral excitons recombine nonradiatively in the course of colliding with other excitons, causing a reduction in the luminescent efficiency of the material at higher generation rates. [ 99,217–220 ] To eliminate the efficiency droop in 2D semiconductors, a detailed investigation on the origin of the EEA process is required.…”

“…[ 99,217–220 ] EEA is a process in which neutral excitons recombine nonradiatively in the course of colliding with other excitons, causing a reduction in the luminescent efficiency of the material at higher generation rates. [ 99,217–220 ] To eliminate the efficiency droop in 2D semiconductors, a detailed investigation on the origin of the EEA process is required. It is found that the lifetime of excitons is inversely related to the EEA coefficient ( C EAA ), a parameter that measures the rate of annihilation.…”

Spatiotemporal Microscopy: Shining Light on Transport Phenomena

“…Transient grating based approaches have been extensively used for the measurement of the diffusion properties of charge carriers. Among the successful examples are applications to studying exciton diffusion [16] and, more recently, 2D materials such as WSe 2 [25] and MoSe 2 [26] monolayers. Another example are ambipolar mobilities, e.g., the combined electron and hole mobility, in semiconductors [27][28][29] including the recent observation of high ambipolar mobility and ultra-high thermal conductivity in cubic boron arsenide [30] by the Chen group, a finding that was in parallel reported by Yue et al, using widefield imaging-based transient reflection SPTM: [31] This is a great example of how compatible information can be obtained using different SPTM techniques.…”

“…More recently, the same approach was used to study rapid valley depolarization in 2D materials, occurring within 170 femtoseconds for MoSe 2 . [26,34] Point-scanning SPTM has been used to study the (quasi-) particle transport in a host of different materials. Earlier work focused on nanowires, from ensembles of carbon nanotubes, [35] to experiments resolving single nanowires of gold [36] and silicon, [37] to more complex systems and geometries, such as p-i-n junction nanowires, [38] as well as suspended nanowires.…”