Atomic-Scale Observation of Moiré Potential in Twisted Hexagonal Boron Nitride Layers by Electron Microscopy

Moiré superlattices (MSLs) are an emerging class of two-dimensional functional materials whose electronic states can be tuned by the twist angle between two van der Waals layers and/or the relative placement of the layers. The intriguing properties of MSLs are closely correlated to the moiré potential, which is the electrostatic potential induced by interlayer coupling. Intensive efforts have been made to understand the nature and distribution of the moiré potential by using various experimental and theoretical techniques. However, the experimental observation of the moiré potential is still challenging because of the possible presence of the surface and/or interlayer contaminants. In this work, we develop a method to obtain hexagonal boron nitride (hBN) nanolayers (with or without twist) using a specially designed chemical exfoliation technique. The resulting hBN nanolayers are atomically clean and strain free, hence providing ideal MSLs for the investigation of their moiré potential. Aberration-corrected high resolution transmission electron microscopy measurements on the twisted hBN nanolayers allow us to observe moiré diffraction spots in Fourier space. Then, the moiré potential is reconstructed by the inverse fast Fourier transform of the moiré diffraction spots. It has been revealed that the local interlayer atomic overlap plays a decisive role in determining the periodicity and distribution of the moiré potential, as supported by density functional theory calculations. This work not only provides a general strategy to observe the moiré potential in MSLs, but it also expands the application of electron microscopy to the further study of MSLs with atomic resolution.