Mapping the Subnanometer Interfacial Distance in a van der Waals Junction

The interfacial distance, <i>d</i>, in van der Waals (vdW) junctions reflects the nature of interfacial interaction and plays a crucial role in determining the performance of vdW devices. However, spatially resolved mapping of <i>d</i> remains limited, as existing techniques provide cross-sectional or line-profile data, risk tip-induced damage, or lack quantitative resolution. Here, we introduce an optical interferometry-based method to spatially map <i>d</i>, enabling <i>in situ</i>, real-time, and nondestructive characterization with subnanometer resolution. The method is demonstrated by tracking variations of <i>d</i> during the approaching of a NbSe₂/graphite heterojunction. Also, when applied to a sliding graphene/graphite junction, this method directly visualizes the confined motion of nanoscale third bodies, providing microscopic evidence of anisotropic diffusion during sliding and offering unprecedented experimental insight into the diffusion process. In addition, the interfacial contact states of the sliding junction are resolved, enabling the predictive evaluation of friction, wear, and stress distribution, which are key issues in mechanics and tribology.