Browsing by Author "Yang, Hong"

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Engineering Ultrafast Carrier Dynamics at the Graphene/GaAs Interface by Bulk Doping Level
(2019-10-01)
Yang, Jinghuan
;
Sun, Quan
;
Liu, Wei
;
Zhang, Zhibin
;
Hu, Xiaoyong
Carrier dynamics, the most fundamental process in electronics and optoelectronics, has drawn great attentions owing to its crucial role in property engineering of materials. Exploration and regulation of carrier dynamics are essential for designing devices with specific functions and optimizing their performances. However, the lack of conventional tools with simultaneous ultrafast temporal and ultrasmall spatial resolution has impeded direct observation and manipulation of carrier dynamics at both the femtosecond and nanometer scale. In this study, the direct observation and modulation of ultrafast carrier dynamics at the graphene/gallium arsenide (GaAs) interface is achieved by tuning the doping level of bulk GaAs. This successful characterization is performed using advanced in situ photoemission electron microscopy combined with the ultrafast pump-probe technique. It is found that a change in the doping level in GaAs can change its band bending and switch the hot-carrier transfer direction at the graphene/GaAs interface with a lifetime reduction of nearly six times. This work paves the way of engineering ultrafast carrier dynamics at 2D interfaces by modifying the 3D bulk properties, and also provides a platform for fundamental studies of ultrafast physics with high spatial resolution.
Ultrafast Electron Cooling and Decay in Monolayer WS2 Revealed by Time- and Energy-Resolved Photoemission Electron Microscopy
(2020-05-13)
Li, Yaolong
;
Liu, Wei
;
Wang, Yunkun
;
Xue, Zhaohang
;
Leng, Yu-Chen
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.