Recently, careful stacking of 2D materials and heterostructures has shown that new interlayer electronics state can have lifetimes that are orders of magnitude longer than present in the monolayer.[1,2] Using time-space resolved ultrafast microscopy on individual 2D crystal grains, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or by twisting the layer stacking orientation. Considering first twisted bilayer graphene (tBLG), we discovered how stacking-angle tunable absorption resonances form a strongly-bound exciton state as a consequence of the symmetrized rehybridization of constrained interlayer 2p orbitals.[3,4] Using two-photon photoluminescence and intraband-transient absorption microscopies, we have recently imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly-bound (450-550 meV), stable interlayer exciton states.

Unlike stacked graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) have diffuse interlayer d-orbital overlap. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer directed E-field, inducing electron-hole dissociation. Our lab [5] and others [6] measure that stacked WSe₂ TMD devices can have both IQE >50% and fast (<50 ps) picosecond electron escape times. Using a first-principle kinetic master equation, our methods analytically extracts both the E-field-dependent interlayer escape velocity and rate-limiting exciton dissociation time. Remarkably our photocurrent response function produces the same E-field-dependent electronic escape and dissociation rates for both the optical and PC addressed ultrafast measurements. As confirmation, the resulting ratio of the electronic rates accurately matches our overall WSe₂ device IQE in the intensity limit of zero Auger recombination. Thus through time-space resolved microscopy, we now obtain a timeline selective to the interlayer electronic dynamics of TMDCs and tBLG van der Waals materials. We show how this novel scanning microscopy approach, combines ultrafast photocurrent and transient absorption to identify new long-lived and metastable interlayer electronic states in emerging twisted and stacked 2D materials and devices.

References

[1] Kim, J., Jin, C., Chen, B., Cai, H., Zhao, T., Lee, P., Kahn, S., Watanabe, K., Taniguchi, T., Tongay, S., Crommie, M.F., Wang, F., (2017). Science Advances 3, e1700518. doi:10.1126/sciadv.1700518
[2] Ceballos, F., Ju, M.-G., Lane, S.D., Zeng, X.C., Zhao, H., (2017) Nano Lett. 17, 1623–1628. doi:10.1021/acs.nanolett.6b04815
[3] H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham, Nano Letters, 15, 5932-7 (2015)
[4] Y. Liang, R. Soklaski, S. Huang, M.W. Graham, R. Havener, J. Park, L. Yang, Phys Rev B, 90, 115418 (2014)
[5] K. Vogt, S. Shi, F. Wang, M. W. Graham, OSA Technical Digest, Ultrafast Phenomena, DOI:10.1364/UP.2016.UW2A.3 (2016)
[6] M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, K.-J. Tielrooij, F. H. L. Koppens, Nature Nanotechnology. 11, 42–46 (2016)