Competition between Free Carriers and Excitons Mediated by Defects Observed in Layered WSe_2 Crystal with Time-Resolved Terahertz Spectroscopy

摘要

The dynamics of photoexcited species is quite important for the developmentrnof next-generation ultrafast optoelectronic devices based on transition metalrndichalcogenides (TMDs). Herein, time-resolved optical pump terahertz (THz)rnprobe spectroscopy, which is sensitive to both bounded excitons and freernelectrons/holes, is employed to study the dynamics of photo-induced carriersrnin the typical layered TMDs crystal tungsten diselenide (Wse_2). Initial photoexcitationrncould generate both free carriers and excitons. The free carriersrndecay followed by phonon-assistance (≈30 ps) and defect-assistancern(≈200–280 ps). The excitons decay followed by the phonon-assisted recombinationrn(≈100 ps) and the defect-induced exciton separation (≈40–200 ps).rnWith the increasing of pump fluence, more free electrons and holes will bindrnto form excitons by many-body effect, while with the decay of time, morernexcitons will dissociate into free carriers by defects. The frequency-dependentrntransient complex THz photoconductivity of layered Wse_2 crystal can be wellrndescribed by Drude–Smith–Lorentz model, which suggests preferential freerncarriers backscattering due to the defects. The ratio of free carriers to excitonsrnsuggests that free carriers dominate after the photoexcitation, which is importantrnfor the optoelectronic devices such as solar cells and photodetectors.