(Invited) Controlling Interlayer Stacking Configuration to Optimize Exciton Extraction Pathways in Van Der Waals Materials

Abstract

Employing ultrafast microscopy methods, we demonstrate how tuning the interlayer coupling by twisting stacking orientations results in different metastable electronic states like Moiré excitons in twisted bilayer graphene (tBLG) and bound triplet pairs (TT) in molecular crystals. Considering first tBLG, we show how stacking-angle tunable absorption resonances form a strongly-bound exciton state due to the interlayer orbitals' symmetric rehybridization. Using two-photon photoluminescence and intraband-transient absorption (TA) microscopies, we have 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), metastable interlayer exciton states.[1] Our observation of resonant PL emission from twisted bilayer graphene materials is best explained by the theoretically predicted coexistence of strongly bound interlayer excitons and metallic graphene continuum states to form Moiré excitons.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 as shown in Fig. 1b. Time-resolved photocurrents show that stacked WSe2 devices shown in Fig. 1a can have both IQE >50% and fast (<60 ps) picosecond electron escape times. Our ultrafast photocurrent rates kinetics give the same E-field-dependent electronic escape and dissociation rates seen from ultrafast optical TA microscopy.[2] To rationalize these fast electronic escape rates, we show the ratio of the electronic rates accurately predicts the actual WSe2 device photocurrent generation efficiency. Lastly, we will show how certain intermolecular twist angle packings of athraditiophene molecular crystals make electron-multiplication by singlet fission of TT states favorable. Singlet fission dynamics are indicated in Fig. 1c by the matching singlet (blue) vs. rising triplet dynamics (red) obtained when the probe polarization aligned along the crystal charge-transfer axis. However, other intermolecular packing angles (at 90o) instead localize and trap excitons as excimers, preventing singlet fission.[3] These interlayer stacked systems collectively demonstrate how remarkably different interlayer electronic states evolve from relatively small changes in interlayer twist angles in van der Waals stacked materials and molecular singlet fission materials.