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

Bilayer graphene, which forms moire ' 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 moire ' 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 moire ' 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 corelevel 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 coreexciton lifetimes and band modifications corresponding to the local stacking geometry within a moire ' unit cell, are unambiguously corroborated.

Automated summary

Bilayer graphene exhibits both local and global properties, and through electron energy loss spectroscopy, we have discovered its internal local properties and variations in the core-level van Hove singularity peaks.