Temperature-Dependent Coalescence of Individual Nonpolar Gold Nanoparticles in Liquid

Self-assembled nanoparticles (NPs) in superlattices are in close contact. Their dense packing and the proximity of aligned facets can facilitate coalescence and enable crystal lattices to fuse at temperatures below the bulk melting point. This phenomenon could be applied in nanodevice manufacture. We study NP fusion in superlattices in liquid and dry environments at controlled temperatures using electron microscopy at minimized electron doses. We found that coalescence of self-assembled gold NPs (AuNPs, diameter 8.1 +/- 0.4 nm) depended on their arrangement. A double layer of AuNPs in a hexagonally close packed superlattice started to coalesce within 2 min at a temperature of 70 degrees C in cyclohexane but remained stable for 30 min at 98 degrees C when it was dry. AuNPs assembled in hexagonal monolayers coalesced after 5 min at 75 degrees C in cyclohexane. The mobility of the ligand shells and the interfacial gold atoms and the sparse ligand coverage on (111) facets likely facilitated this AuNP coalescence at low temperatures.