Stochastic Interface Formation in Nanoscale Heterostructures Visualized by Atomic-Resolution Microscopy

Well-defined heterostructures exhibit emergent properties distinct from their single-phase constituents, enabling advances across diverse technologies. Typically classified as self-assembly and epitaxy, heterointerface formation is generally assumed to proceed unidirectionally and irreversibly at bulk scales. Here we use in situ electron microscopy at 298 K to visualize the heterostructure formation from nanoscale mixtures of intrinsically immiscible salts at ambient conditions, NaCl and NaI. We find that both self-assembly and heteroepitaxy occur reversibly, governed by stochastic equilibria. During self-assembly, the system fluctuates between order and semiordered states, with kinetically favored iodide–graphene interactions and eventual formation of thermodynamically favored charge-neutral planes. The ions in the liquid state move rapidly. In the epitaxial regime, NaI layers repeatedly grow and exfoliate from a buffer layer. The observed motion of atoms driven by high interfacial energy occurs with an energy barrier of 18 kcal mol–1 or less as estimated from the Nyquist frequency of the frame rate of the movie. These observations reveal that heterointerface formation is governed by dynamic and stochastic processes, providing a framework for the understanding of the initiation stage of the heterointerface formation in high interface energy regimes such as surface and structural defects of bulk solids.