Studies And Modulations On The Photocarrier Dynamics In Transitional Metal Dichalcogenides And Their Heterostructures

Transitional metal dichalcogenides(TMDs),one of the most widely studied two-dimensional(2D)semiconductors,have attracted significant attention and are deemed to be an ideal platform to construct next generation nanodevices.In particular,superior optical absorption and high quantum yield make TMDs appealing for a range of optoelectronic applications,such as diode emitter,photodetector and optical modulator.For these applications,several important figures-of-merit are closely linked to the photocarrier dynamics.Hence investigations on the photocarrier dynamics in TMDs and their heterostructures will not only promote the fundamental understanding on the photo-physics in 2D materials,but also benefit the development of related devices.In this dissertation,the dynamic evolution of photocarriers in monolayer TMDs and their heterostructures has been investigated by ultrafast spectroscopy techniques,with an emphasis on exploring methods to modulate the photocarrier dynamics.It was discovered that the exciton lifetimes in monolayer TMDs can be effectively tuned by nearly one order of magnitude through interfacial engineering method,owing to the tailoring of interfacial electron-phonon coupling.The interlayer recombination of photocarriers in a type-II TMD-based heterostructure was found to be well described by the Langevin model,and the unusually fast interlayer recombination suggested the possibility to modulate such processes by constructing specific heterostructures.The contents of this thesis are as follows:1.The lack of techniques to control the photocarrier dynamics of 2D materials limits their practical application of in optoelectronic devices.Although a number of techniques have been developed for conventional semiconductors to control the photocarrier dynamics,those techniques for bulk materials cannot be straightforwardly applied to 2D systems as hindered by their atomically-thin nature.To find a new control method,the transient absorption spectroscopy was performed on the monolayer TMDs on different oxide substrates(SiO2,Al2O3,HfO2).Surprisingly the interfacial engineering was found to strongly influence the exciton dynamics.The exciton liftime tuning range reaches one order of magnitude.Combined with simulation methods,the underlying physics behind the tuning effect from substrates was revealed to be the interfacial electron-phonon coupling.The phononic environments,created by different oxide substrates,determined different interfacial electron-phonon coupling strength and then changed the non-radiative recombination process of the excitons.This finding indicates the interfacial engineering as a promising method to modulate photocarrier dynamics and a new route for customizing devices functions.2.The 2D van der waals(vdW)heterostructures can offer new phenomena and properties that cannot be provided by individual materials,such as efficient separation of photocarriers in type-? heterostructures.Through transient absorption spectroscopy,the author measured the dynamics of photo-excited free carrier in a vdW heterostructure,composed of black phosphorus(BP)film and molybdenum disulfide(MoS2)bilayer,which had a type-II band alignment.In addition to observing the ultrafast interlayer transfer of photo-generated electrons from BP to MoS2,an unusually short lifetime of interlayer electron-hole pairs was identified,much shorter than those of individual constituent materials.Interestingly,it was found that this interlayer free carrier recombination could be well accounted for by Langevin model.According to the Langevin model,the ultrafast interlayer recombination could be attributed to carriers'high density and mobility,which suggested two potential ways to tune the photocarriers'lifetime.3.Conventional pump-probe techniques mainly rely on transient absorption change to characterize dynamic evolution of photocarriers,however the change in optical absorption cannot distinguish transient between behaviors of carriers in different momentum spaces.Therefore,a direct observation technique is needed to uncover the dynamic evolution of photocarriers.Time-resolved Angle-resolved photoemission spectroscopy(TR-ARPES)can directly detect the transient distribution and motion of electrons in momentum space.A femtosecond exteme ultraviolet(EUV)beamline for TR-ARPES has been designed,constructed and chararterized successfully,so that the evolution of surface electronic states could be probed with femtosecond time resolution.The EUV source is generated by high harmonic generation(HHG),produced by a kHz-repetition-rate Ti:sapphire laser with a pulse duration of 50 fs.The photon energy of the EUV source ranged from 10 to 70 eV and the maximum photon flux measured is 5.5 X 1010 ph/s.The specific EUV monochromator not only selects wavelength,but also preservs the EUV pulse duration.Based on this femtosecond EUV beamline,ultrafast surface photovoltaic effect in p-type GaAs and transient distribution of excited electrons in conducting band of InSe crystal have been observed though TR-ARPES.