Photocarrier Dynamics In WS₂ And Its Heterostructures Studied By Exciton Intraband Absorption

Since 2010,many kinds of 2D materials,especially transition metal dichalcogenides,have attracted wide attention because of their interesting properties.Due to their large binding energies,on the order of hundreds of meV,excitons in 2D transition metal dichalcogenides are stable at elevated temperatures,and play a key role in determining the optical and electronic properties of these materials.Hence,understanding excitonic dynamics is important for developing applications based on 2D transition metal dichalcogenides.Recently,transient absorption techniques based on resonant excitonic absorption has been used to study various aspects of excitonic dynamics in these materials.The transient absorption in such measurements originates from phase-space state filling,bandgap renormalization,and screening effects.Here we report a new method to probe exciton dynamics based on exciton intraband absorption.In this drude-like process,probe photons are absorbed by excitons through intraband excitation,causing a transient absorption signal.Although the magnitude of the transient absorption is lower than the resonant techniques,the new method is less restrictive on the selection of probe wavelength,has a much larger linear range,and can provide complementary information on photocarrier dynamics.Using WS2 monolayer and bulk samples as examples,we show that the new method can probe exciton-exciton annihilation at high densities and reveal exciton formation processes.We also found that the exciton intraband absorption cross section of WS2 monolayer is on the order of 10-18 cm-2.Monolayer tungsten disulfide(WS2)possess high optical absorption coefficient and high photoluminescence yield,with a very narrow spectral width;monolayer and multilayer black phosphorus(BP)possess high room-temperature mobilities on the order of 104 cm2V-1s-1.Therefore,the heterostructures of WS2 and BP can be a kind of material with both excellent optical and electrical property.Theoretical studies indicated that the band alignments of monolayer WS2 and phosphorene heterostructures belong to type-I.We have studied photocarrier dynamics in monolayer WS2,7-layer BP,and a heterostructure formed by them by time-resolved differential reflection measurements.We obtained multiple pieces of evidence that this heterostructure has type-I band alignment,with the lowest energy states in the conduction and the highest energy states in the valance bands both located in BP.We found that photocarriers injected in WS2 transfer to BP with a transfer time of about 0.6 ps.That indicates the WS2-BP heterostructures can be used for novel optoelectronic devices,by effectively combining novel optical absorption properties of monolayer WS2 and high charge transport performance of BP.