Metal nanocrystal superlattice nucleation and growth

Thin films of dodecanethiol-passivated Au and Ag nanocrystals drop cast from different solvents were examined by high-resolution scanning electron microscopy (HRSEM). C12-coated Au and Ag nanocrystals, 5-7 nm in diameter, form face-centered cubic (fcc) superlattices oriented with the (111)s planes (subscript s denoting superlattice) parallel to the substrate when deposited from good solvents, such as hexane, chloroform, and toluene. The gross morphology of the films depended on the solvent: hexane produced rough superlattice films whereas chloroform deposited smooth films. The difference in interparticle attraction, which is approximately 20% higher in hexane, appears to give rise to the difference in film morphology. Addition of a poor solvent to the dispersion prior to drop casting led to superlattices with decreased order. Although the superlattices always orient with (111)s as the basal plane on the substrate, superlattices deposited from chloroform grow preferentially in the [110]s direction, whereas hexane deposits superlattices that grow primarily in the [111]s direction.