LAUSR

Two dimensional (2D) nanocrystal functional superlattices with a well controlled structure are of significant importance in photonic, plasmonic and optoelectronic applications and have been well studied, but it remains challenging to understand the formation mechanism and development pathway of the superlattice. In this study, we employed in-situ liquid cell transmission electron microscopy to study the formation of 2D superlattice and its local phase transition from hexagonal-to-square nanocrystal ordering. When colloidal nanocrystals flowed in the solution, long-range ordered hexagonal superlattice could be formed either through shrinking and rearrangement of nanocrystal aggregates or via nanocrystal attachment. As the nanocrystals’ shape transformed from truncated octahedral to cube, the local superlattice rearranged to square geometry. Moreover, our observations and quantitative analyses reveal that the phase transition from hexagonal to square mainly originates from the strong van der Waals interactions between the vertical (100) facets. The tracking of 2D cube superlattice formation in real-time could provide unique insights on the governing force of superlattice assembling and stabilization.