The high-conductivity, low cost and relative ease of fabrication have made graphene well-suited to transform nanoscale electronics.  Unfortunately, as a gapless semimetal, photo-excited electrons in graphene recombine far faster than electron-hole pairs can be extracted.[1]  To make carrier lifetimes in 2D materials more competitive with carrier separation, we demonstrate promising, new electron extraction pathways in graphene-like systems including twisted bilayer graphene (tBLG) and transition metal dichalcogenides (TMD).  In tBLG, we show the emergence of twist angle-tunable excitonic absorption peaks that result from the rehybridization of constrained interlayer 2p orbitals[2]. Specifically, we use one and two-photon transient absorption microscopy to directly image the predicted[3] bright and dark stable, strongly-bound excitons states. This observation of strongly bound excitons is a first for a 2D metallic system, and serves to substantially increase graphene’s carrier lifetime, suggesting new pathways for photocurrent collection.   Unlike graphene, semiconducting 2D TMDs like WSe₂, have long carrier lifetimes, but challenging mobility and exciton dissociation bottlenecks that inhibit photocurrent generation. By combining femtosecond resolved photocurrent microscopy[1,4] with ultrafast transient absorption, we have selectively imaged the dominant kinetics bottlenecks that inhibit photocurrent production in devices made from WSe₂TMD materials.  Using these ultrafast space-time maps of the electron-hole dissociation dynamics, we hope to better engineer 2D material heterostructures devices to avoid efficiency-destroying, carrier recombination bottlenecks and maximize photocurrent yield.

(1) M.W. Graham, S. Shi, D.C. Ralph, J. Park, P.L. McEuen (2013), Nature Physics, 9, 103

(2) H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham (2015),  Nano Letters, 15, 5932-7

(3) Y. Liang, R. Soklaski, S. Huang, M.W. Graham, R. Havener, J. Park, L. Yang (2014), Phys Rev B, 90, 115418

(4) M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, K.-J. Tielrooij, and F. H. L. Koppens (2015), arXiv:1507.06251 [cond-mat]