Capturing the Crystalline Phase of Two-dimensional Nanocrystal Superlattices in Action*

Critical photonic, electronic, and magnetic applications of two-dimensional nanocrystal superlattices often require nanostructures in perfect single-crystal phases with long-range order and limited defects. Here we discovered a crystalline phase with quasi-long-range positional order for the two-dimensional nanocrystal superlattice domains self-assembled at the liquid-air interface during droplet evaporation, using in situ time-resolved x-ray scattering along with rigorous two-dimensional crystal theories. It was observed that drying these superlattice domains resulted in only orientational order but not long-range positional order, also supported by quantitative analysis of transmission electron microscopy images.