Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS 2 Films.

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS ₂ ) films. To this end, we deposit a model heterostructure of thin amorphous MoS ₂ films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS ₂ into crystalline MoS ₂ domains, thereby emulating widely used elevated temperature MoS ₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS ₂ films in situ and at the atomic scale. Our observations suggest that during MoS ₂ processing, various MoS ₂ polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS ₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS ₂ crystallization can be influenced by a weak MoS ₂ /graphene support epitaxy. Our atomic-scale in situ approach thereby visualizes multiple fundamental processes that underlie the varied MoS ₂ morphologies observed in previous ex situ growth and processing work. Our work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.