Direct Observation of Locally Modified Excitonic Effects within a Moiré Unit Cell in Twisted Bilayer Graphene

Abstract

Bilayer graphene, which forms moiré superlattices, possesses distinct electronic and optical properties owing to its hybridized energy band and the emergence of van Hove singularities depending on its twist angle. Extensive research has been conducted on the global characteristics of moiré superlattices induced by their long-range periodicity. However, the local properties, which differ owing to the variations in the three-dimensional atomic arrangement, within a moiré unit cell have been rarely explored. In this study, we demonstrate the highly localized excitation of carbon 1s electrons to unoccupied van Hove singularities in twisted bilayer graphene by electron energy loss spectroscopy using a monochromated transmission electron microscope. The core-level excitations associated with the van Hove singularities exhibit a systematic twist-angle dependence analogous to optical excitations. Furthermore, local variations in the core-level van Hove singularity peaks, which can originate from the core-exciton lifetimes and band modifications corresponding to the local stacking geometry within a moiré unit cell, are unambiguously corroborated.

Angle-Resolved Optical Imaging of Interlayer Rotations in Twisted Bilayer Graphene

“…When the energy difference of the vHs ( E ab in Figure c) coincides with the energy of incident light at which the allowed optical transitions take place, indicated by green arrows in Figure c, an absorption maximum in each optical absorption spectrum occurs for TBG with a given θ. Moreover, twisted angle-dependent optical conductivity in TBG has been well-established by a number of theoretical works via the two-mode approximation, tight binding calculations, , and DFT calculations , and has been further verified by a series of experimental studies. ,− For 8° < θ < 30°, the correlation between the maximum absorption energy E ab and the twist angle θ can be expressed using the following empirical formula E ab = 1740 − false( 41.4 − θ false) 2 399 Figure d plots the twist angle θ-dependent E ab curve in the black solid line. First, the maximum absorption energy induced by vHs covers the entire visible-light spectrum.…”