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.