Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS₂ heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS₂ following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS₂ interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS₂ to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection.

由石墨烯和过渡金属二卤化碳组成的范德华异质结构在光电应用中显示出了广阔的前景。但是，到目前为止，对于器件操作的关键过程（即界面电荷转移（CT）和重组）的深入了解仍然难以捉摸。在这里，我们通过互补地探测石墨烯中的超快太赫兹光电导和WS _2中的瞬态吸收动力学，研究了石墨烯-WS ₂异质结构中的这些过程。光激发后。我们观察到，CT后异质结构中的分离电荷寿命非常长：超过1 ns，而之前的研究报道约为1 ps的电荷分离。这导致石墨烯的有效光选。此外，对于跨石墨烯-WS ₂界面的CT过程，我们发现它通过亚-A激子激发的光热电子发射和从WS ₂到高于A-激子激发的石墨烯价带的直接空穴转移而发生。这些发现为进一步优化光电器件的性能，特别是光电检测提供了见识。