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.

具有良好控制的结构的二维（2D）纳米晶体功能超晶格在光子，等离子体和光电应用中具有重要意义，并且已经进行了充分的研究，但是了解超晶格的形成机理和发展途径仍然具有挑战性。在这项研究中，我们采用原位液体细胞透射电子显微镜研究了二维超晶格的形成及其从六边形到正方形纳米晶有序的局部相变。当胶体纳米晶体在溶液中流动时，可以通过收缩和重排纳米晶体聚集体或通过纳米晶形成长程有序六边形超晶格。纳米晶体的附件。随着纳米晶体的形状从截断的八面体转变为立方体，局部超晶格重新排列为正方形几何形状。此外，我们的观察和定量分析表明，从六边形到正方形的相变主要源自垂直（100）面之间的强范德华相互作用。实时跟踪二维立方体超晶格的形成可以提供有关超晶格组装和稳定控制力的独特见解。