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Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures

Abstract

Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional layers, including graphene, hexagonal-boron nitride and transition-metal dichalcogenides (MX2), give rise to fascinating new phenomena1,2,3,4,5,6,7,8,9,10. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because two-dimensional MX2 monolayers can have an optical bandgap in the near-infrared to visible spectral range and exhibit extremely strong light–matter interactions2,3,11. Theory predicts that many stacked MX2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron–hole separation for light detection and harvesting12,13,14,15,16. Here, we report the first experimental observation of ultrafast charge transfer in photoexcited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond pump–probe spectroscopy. We show that hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled two-dimensional layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel two-dimensional devices for optoelectronics and light harvesting.

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Figure 1: Band alignment and structure of MoS2/WS2 heterostructures.
Figure 2: Photoluminescence spectra and mapping of MoS2/WS2 heterostructures at 77 K.
Figure 3: Transient absorption spectra of MoS2/WS2 heterostructures.
Figure 4: Ultrafast hole transfer dynamics from vertical cuts in Fig. 3a,b.

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Acknowledgements

Optical measurements and MoS2 growth were supported by the Office of Basic Energy Science, Department of Energy (contract no. DE-SC0003949, Early Career Award; contract no. DE-AC02-05CH11231, Materials Science Division). The WS2 growth part was supported financially by the National Natural Science Foundation of China (grants nos. 51222201, 51290272) and the Ministry of Science and Technology of China (grant no. 2011CB921903). F.W. acknowledges support from a David and Lucile Packard fellowship. The authors thank K. Liu and Y. Chen for help in sample characterization and L. Ju for providing the evaporation mask.

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F.W. conceived and supervised the experiment. X.H., J.K. and S-F.S. carried out photoluminescence and pump–probe measurements. Y.S., S.T. and J.W. grew CVD monolayer MoS2. Y.Z. and Y.F.Z. grew CVD monolayer WS2. J.K., X.H. and S-F.S prepared the heterostructure sample. X.H., J.K., S-F.S. and C.J. performed data analysis. All authors discussed the results and wrote the manuscript.

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Correspondence to Feng Wang.

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The authors declare no competing financial interests.

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Hong, X., Kim, J., Shi, SF. et al. Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures. Nature Nanotech 9, 682–686 (2014). https://doi.org/10.1038/nnano.2014.167

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