Layer Rotation-Angle-Dependent Excitonic Absorption in van der Waals Heterostructures Revealed by Electron Energy Loss Spectroscopy.

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. The optoelectronic functionality of 2D heterostructure devices is critically dependent on the relative rotation angle of the layers. However, the role of the relative rotation angle of the constituent layers on intralayer absorption is not clear yet. Here, we investigate MoS2/WSe2 vdW heterostructures using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy and report that momentum conservation is a critical factor in the intralayer absorption of TMDC vdW heterostructures. The evolution of the intralayer excitonic low-loss EEL spectroscopy peak broadenings as a function of the rotation angle reveals that the interlayer charge transfer rate can be about an order of magnitude faster in the aligned (or anti-aligned) case than in the misaligned cases. These results provide a deeper insight into the role of momentum conservation, one of the fundamental principles governing charge transfer dynamics in 2D vdW heterostructures.

Citation

Gogoi, P. K., Lin, Y.-C., Senga, R., Komsa, H.-P., Wong, S. L., Chi, D., … Suenaga, K. (2019). Layer Rotation-Angle-Dependent Excitonic Absorption in van der Waals Heterostructures Revealed by Electron Energy Loss Spectroscopy. ACS Nano. doi:10.1021/acsnano.9b04530

Acknowledgements

This work is (partially) supported by JSPS KAKENHI (16H06333, 17H04797, 18K14119, and P18350). P.K.G., M.B.H.B., and S.J.P. acknowledge MOE, Singapore grant number R-144-000-389-114. S.L.W. and D.C. acknowledge support from the Institute of Materials Research and Engineering (IMRE) under the Agency for Science, Technology, and Research (ASTAR) under ASTAR Science and Engineering Research Council Pharos 2D Program (SERC Grant No 152-70-00012). H.P.K. and A.V.K. thank CSC-IT Center for Science Ltd. for the computing resources and the Academy of Finland for the support under Project Nos. 286279 and 311058