Unveiling defect-mediated carrier dynamics in monolayer semiconductors by spatiotemporal microwave imaging

Zhaodong Chu, Chun-Yuan Wang, Jiamin Quan, Chenhui Zhang, Chao Lei, Ali Han, Xuejian Ma, Hao-Ling Tang, Dishan Abeysinghe, Matthew Staab, Xixiang Zhang, Allan H. MacDonald, Vincent Tung, Xiaoqin Li, Chih-Kang Shih, Keji Lai

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals, respectively. Time-resolved experiments indicate that the critical process for photoexcited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the long-lived photoconductivity signal is higher in chemical-vapor deposited (CVD) samples than exfoliated monolayers due to the presence of traps that inhibits recombination. Our work reveals the intrinsic time and length scales of electrical response to photoexcitation in van der Waals materials, which is essential for their applications in optoelectronic devices.
Original languageEnglish (US)
Pages (from-to)202004106
JournalProceedings of the National Academy of Sciences
Issue number25
StatePublished - Jun 9 2020


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