(Invited) Ultrafast Photocurrents from Interlayer Excitons in Twisted and Stacked 2D Materials

Abstract

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. 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 (up to 690 meV), quasi-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. Time-resolved photocurrents show that stacked WSe2 devices can have both IQE >50% and fast (<50 ps) picosecond electron escape times. 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. To confirm the fast electronic escape rates, we show the ratio of the electronic rates accurately matches our overall TMDC device IQE in the limit of zero Auger recombination. Collectively, we show how optical and photocurrent-based ultrafast microscopies provide an accurate timeline of photocurrent generating dynanamics in both TMDCs and tBLG van der Waals materials.