Transient Absorption Microscopy of Monolayer and Bulk WSe₂

Abstract

We present an experimental investigation on the exciton dynamics of monolayer and bulk WSe2 samples, both of which are studied by femtosecond transient absorption microscopy. Under the excitation of a 405 nm pump pulse, the differential reflection signal of a probe pulse (tuned to the A-exciton resonance) reaches a peak rapidly that indicates an ultrafast formation process of excitons. By resolving the differential reflection signal in both time and space, we directly determine the exciton lifetimes of 18±1 and 160±10 ps and the exciton diffusion coefficients of 15±5 and 9±3 cm2/s in the monolayer and bulk samples, respectively. From these values, we deduce other parameters characterizing the exciton dynamics such as the diffusion length, the mobility, the mean free path, and the mean free length. These fundamental parameters are useful for understanding the excitons in monolayer and bulk WSe2 and are important for applications in optoelectronics, photonics, and electronics.

Vibrational and optical properties of MoS2: From monolayer to bulk

“…Although theory predicts a long lifetime for the valley polarization, recent studies observe a non-negligible decoherence [200], which points towards other reasons than magnetic impurities as the origin of the spin-flip. Experimental studies have been undertaken to identify the reasons of decoherence of valley polarizations [201,170,202,203,199]. The analysis of the time-dependent optical response of single-layer MoS 2 supported in several substrates or suspended in vacuum can give a hint [170] of how the environment can alter the electronic structure and hence the valley polarization.…”

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

“…Reflection contrast [34][35][36], transient absorption [35], time-integrated photoluminescence (PL) [37,38] and time-resolved photoluminescence (TRPL) [39,40] experiments, have been performed to study the emission properties of WSe 2 . In prior studInfluence of the substrate material on the optical properties of tungsten diselenide monolayers 2 S Lippert et al ies on WSe 2 , layers were deposited on SiO 2 /Si substrates [41,42], sapphire [43,44], graphene [45], and fused silica (quartz) [34,46] or sandwiched between layers of hBN [47].…”

Valley Trion Dynamics in Monolayer MoSe2

“…2 A number of exciton decay mechanisms have been suggested as possible underlying processes in previously published studies [31][32][33][34][35][36][37][38][39][40][41][42][43] . For instance, some experiments 36,40 show that the fast decay dynamics (< 50 ps) are dependent on the exciton density and exciton-exciton annihilation is the dominating decay channel, while other studies 33,34,41 show no significant variation in the decay dynamics with the excitation density and that trapping by surface defect states is the responsible process.…”

“…The studies on exciton dynamics [31][32][33][34][35][36][37][38][39][40][41][42][43] have shown considerable discrepancy in the interpretation of the exciton dynamics and the specific time scales due to the differences in samples and measurement conditions such as the excitation energy, excitation density, temperature and polarizations. Particularly, the dependence on the excitation density can provide crucial evidence in determining the decay mechanism.…”