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Monday, 01 March 2021

Defects to the rescue: Exploiting defects at Van der Waals interfaces between layered materials

A paper recently published in the journal “Science Advances” demonstrates that defects in van der Waals materials, normally detrimental for device applications, can be exploited to capture and store charge carriers and therefore be beneficial for practical applications, such as photodetectors. Dr Klaas-Jan Tielrooij, leader of the Ultrafast Dynamics in Nanoscale Systems group at the ICN2, participated in this study.

Bulk layered materials or van der Waals (vdW) materials, e.g. graphite and transition metal dichalcogenides (TMDs, e.g. WS2), consist of strongly bonded two-dimensional (2D) layers which are weakly bound in the third dimension through vdW forces. Due to the weak interlayer interaction, atomically thin vdW monolayers can be produced via mechanical exfoliation. The most notable example is the discovery of graphene, a monolayer of graphite, produced by peeling off layers from the bulk form simply by scotch tape. Graphene is a semimetal, and exhibits extremely high charge carrier mobility relevant for electrical devices. Other vdW monolayers, e.g. those exfoliated from TMDs, are usually semiconductors which are also useful for many optical and optoelectronic applications. The 2D monolayers can further be stacked vertically to form so called “vdW heterostructure”.

Van der Waals (vdW) heterostructures consisting of graphene and TMDs have recently shown great promise for high-performance optoelectronic applications. For instance, the integration of graphene and monolayer WS2 enables sensitive photodetectors with ultrahigh photoresponsivity, by combining strong light absorption in WS2, efficient charge transfer across the interfaces, and superior charge transporting properties in graphene. Currently, an in-depth understanding of interfacial charge transfer and recombination has so far remained elusive, limiting the optimization of device performance.

Researchers from the Max Planck Institute for Polymer Research, in collaboration with partners from Delft University of Technology, University of Electronic Science and Technology of China, Zhejiang University, and the ICN2 Ultrafast Dynamics in Nanoscale Systems group, led by Dr Klaas-Jan Tielrooij,  now shed light on these processes in graphene-WS2 heterostructures, by complementarily monitoring the dynamics of charge carriers both in graphene (by terahertz spectroscopy) and in WS2 (by transient absorption dynamics) in sub-picosecond (ps, =10-12 s) time scale. The researchers demonstrate that defects, normally detrimental for device applications, can be beneficial for practical applications for graphene-WS2 vdW structures, e.g. photodetectors. These defects at interfaces can be exploited to “capture” and “store” charge carriers, which are transferred across the heterojunction. This results in an extremely long-lived charge separation and an efficient photogating phenomenon in graphene-based vdW heterostructures for photodetection applications. The results are published in the journal “Science Advances”.


News source: Max Plank Institute for Polymer Research


Following photoexcitation and interfacial charge transfer (CT, the dash line), the interfacial defects (e.g. sulfur vacancies) can capture (the red solid line) and store the injected electrons, leading to an extremely long-lived charge separation and thus an interfacial electrical field (E) to effectively gate graphene.

Reference article:

Shuai Fu, Indy du Fossé, Xiaoyu Jia, Jingyin Xu, Xiaoqing Yu, Heng Zhang, Wenhao Zheng, Sven Krasel, Zongping Chen, Zhiming M. Wang, Klaas-Jan Tielrooij, Mischa Bonn, Arjan J. Houtepen, Hai I. Wang, Long-lived charge separation following pump-wavelength–dependent ultrafast charge transfer in graphene/WS2 heterostructures. Science Advances 2021; 7: eabd9061. DOI: 10.1126/sciadv.abd9061