Long Yuan - Purdue University
Co-Author(s): Ting-Fung Chung, Yan Wan, Zhi Guo, Yang Xu, Yong P. Chen, and Libai Huang, Purdue University, IN
Efficient interfacial carrier generation and charge separation in two-dimensional (2D) van der Waals heterostructures is a central challenge for their potential electronic and optoelectronic applications. Since the interlayer distance in the 2D heterostructure can be as small as 3 Å, interlayer coupling can significantly modify the optical and electronic properties of 2D heterostructure. A largely unexplored question is how interlayer coupling can be utilized to control interfacial charge generation and separation. However, the inherent spatial heterogeneity at the 2D interfaces presents a major difficulty in elucidating interfacial charge transfer dynamics in relation to interlayer coupling. To elude this difficulty, we employ ultrafast transient absorption microscopy (TAM) with high spatial and temporal resolutions to directly image interfacial charge generation and separation at different interlayer coupling strengths in WS2-graphene heterostructure. An up to 4-fold enhancement in interfacial carrier generation with visible optical excitation is observed in WS2-graphene heterostructures, which we attribute to interfacial charge transfer transitions. Such interlayer states could also promote electrons from the graphene layer to the WS2 layer and allow carrier generation with excitation energy well below WS2 bandgap. The lifetime of the interlayer charge transfer states has been measured to be ~ 1 ps. Our results highlight the largely unexplored potential of heterostructures based on 2D nanostructures with atomically sharp interfaces as a platform for enhancing interfacial charge transfer and separation. Future research will focus on controlling photo-carrier generation at the 2D interface through modulating the electronic property of graphene via electrical or chemical doping.
References: Britnell, L., Ribeiro, R. M., Eckmann, A., Jalil, R., Belle, B. D., Mishchenko, A., Kim, Y. J., Gorbachev, R. V., Georgiou, T., Morozov, S. V., Grigorenko, A. N., Geim, A. K., Casiraghi, C., Castro Neto, A. H. & Novoselov, K. S. 2013. Strong light-matter interactions in heterostructures of atomically thin films. Science, 340: 1311-1314.
Zhu, X., Monahan, N. R., Gong, Z., Zhu, H., Williams, K. W. & Nelson, C. A. 2015. Charge Transfer Excitons at van der Waals Interfaces. J Am Chem Soc, 137: 8313-8320.
Ghorbani-Asl, M., Bristowe, P. D., Koziol, K., Heine, T. & Kuc, A. 2016. Effect of compression on the electronic, optical and transport properties of MoS2/graphene-based junctions. 2D Materials, 3: 025018.
Funder Acknowledgement(s): This study was supported by a grant from NSF under award number 1433490. We thank Steven Louie of UC Berkeley and Wei Hu from Lawrence Berkeley National Laboratory for helpful discussions.
Faculty Advisor: Libai Huang, email@example.com
Role: I contributed to the sample preparation and instrument development. I also carried out the measurements and analyzed the data.