Ultrafast Electron Cooling and Decay in Monolayer WS₂ Revealed by Time- and Energy-Resolved Photoemission Electron Microscopy

Abstract

A comprehensive understanding of the ultrafast electron dynamics in two-dimensional transition metal dichalcogenides (TMDs) is necessary for their applications in optoelectronic devices. In this work, we contribute a study of ultrafast electron cooling and decay dynamics in the supported and suspended monolayer WS2 by time- and energy-resolved photoemission electron microscopy (PEEM). Electron cooling in the Q valley of the conduction band is clearly resolved in energy and time, on a time scale of 0.3 ps. Electron decay is mainly via a defect trapping process on a time scale of several picoseconds. We observed that the trap states can be produced and increased by laser illumination under an ultrahigh vacuum, and the higher local optical-field intensity led to the faster increase of trap states. The enhanced defect trapping could significantly modify the carrier dynamics and should be paid attention to in photoemission experiments for two-dimensional materials.

Real-time GW: Toward an ab initio description of the ultrafast carrier and exciton dynamics in two-dimensional materials

“…In Fig. 4b we see that for n P = 4 × 10 11 cm −2 the polarization damps in about 100 fs, and after few femtoseconds (t 150 fs) the occupations f k reach steady-state values describing a Fermi-Dirac distribution at temperature ∼ 2000 K (not shown), consistently with recent data [60]. We have systematically studied the lifetime of the polarization p(t) by varying the excitation density.…”

How defects influence the photoluminescence of TMDCs

“…According to the results of TA decay in monolayer TMDCs, carrier cooling is significantly accelerated by defect-assisted scattering processes. These defect-assisted carrier cooling processes are on the order of 300-500 fs, which is much faster than those assisted by carrier−carrier and carrier−phonon scattering processes (tens of picoseconds) [10,63,64]. As a result, the intraband relaxation rate is more than 40-fold higher in the monolayer, causing excited electrons and holes to lose most of their energies in the first few hundred femtoseconds [10].…”

Pathways of Exciton Triggered Hot‐Carrier Injection at Plasmonic Metal−Transition Metal Dichalcogenide Interface

“…[ 3 ] Dynamical optical techniques based on time‐resolved characterizations of the light‐matter interactions, e.g. time‐dependent luminescence, photoelectron emission, [ 20–22 ] and ultrafast pump‐probe spectroscopy, [ 23 ] are thus believed to be more suitable to study the kinetics and to differentiate the contributions by emphasizing charge/energy transfer process. [ 23–25 ]…”