Abstract
Twisted bilayer graphene (tBLG) is a metallic material with two degenerate van Hove singularity transitions that can rehybridize to form interlayer exciton states. Here we report photoluminescence (PL) emission from tBLG after resonant 2-photon excitation, which tunes with the interlayer stacking angle, θ. We spatially image individual tBLG domains at room-temperature and show a five-fold resonant PL-enhancement over the background hot-electron emission. Prior theory predicts that interlayer orbitals mix to create 2-photon-accessible strongly-bound (~0.7 eV) exciton and continuum-edge states, which we observe as two spectral peaks in both PL excitation and excited-state absorption spectra. This peak splitting provides independent estimates of the exciton binding energy which scales from 0.5–0.7 eV with θ = 7.5° to 16.5°. A predicted vanishing exciton-continuum coupling strength helps explain both the weak resonant PL and the slower 1 ps−1 exciton relaxation rate observed. This hybrid metal-exciton behavior electron thermalization and PL emission are tunable with stacking angle for potential enhancements in optoelectronic and fast-photosensing graphene-based applications.
Highlights
Twisted bilayer graphene is a metallic material with two degenerate van Hove singularity transitions that can rehybridize to form interlayer exciton states
To search for such optically dark interlayer exciton states in Twisted bilayer graphene (tBLG), we loosely assume parity-based optical selection rules from the hydrogenic exciton model and detect photoluminescence (PL) and excitedstate absorption (ESA) signals generated via resonant 2-photon excitation[9,10,11]
The van Hove singularity (vHs) model for tBLG predicts only non-resonant hot-electron and blackbody PL emission because interlayer optical excitations thermalize rapidly (~10–20 fs) by the efficient electron-electron scattering in graphene[12]
Summary
Twisted bilayer graphene (tBLG) is a metallic material with two degenerate van Hove singularity transitions that can rehybridize to form interlayer exciton states. A predicted vanishing exciton-continuum coupling strength helps explain both the weak resonant PL and the slower 1 ps−1 exciton relaxation rate observed This hybrid metal-exciton behavior electron thermalization and PL emission are tunable with stacking angle for potential enhancements in optoelectronic and fast-photosensing graphene-based applications. Using Bethe–Salpeter equation (BSE) simulations, Liang et al predicted that the lower-lying exciton state, XA is strongly bound, optically dark with vanishing exciton to graphene continuum coupling strength, HXA;k (see Fig. 1b)[6] To search for such optically dark interlayer exciton states in tBLG, we loosely assume parity-based optical selection rules from the hydrogenic exciton model and detect photoluminescence (PL) and excitedstate absorption (ESA) signals generated via resonant 2-photon excitation[9,10,11]. These figures of merit directly impact recent optoelectronic applications of tBLG such as chiral-light sensitive photosensors and lightinduced metal-insulator phase transitions[13,14]
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