Self-assembly patterns formed upon solvent evaporation of aqueous cetyltrimethylammonium bromide-coated gold nanoparticles of various shapes.

Gold nanocrystals of various shapes, which were produced in high yield in the presence of cetyltrimethylammonium bromide (CTAB), showed a range of two-dimensional self-assembly patterns upon drying from aqueous solution. The interparticle spacings were independent of the size and shape of the gold nanocrystals. Energy-dispersive X-ray analysis (EDAX) and Fourier-transform infrared (FTIR) spectroscopic studies revealed that the CTAB molecules adsorb onto surfaces of the gold nanocrystals in a bilayer or multilayer fashion, consistent with other groups' results. Zeta potential measurements showed that CTAB-coated nanocrystals were positively charged and the zeta potential remained almost the same upon two centrifugations and redispersion of the nanocrystals in deionized water, confirming the high stability of the surfactant-nanoparticle interaction. The nanocrystal shape strongly influenced the nature of the self-assembly patterns, in some cases in accord with theoretical predictions. CTAB is proposed as the medium for self-assembly, via interdigitation of its hydrophobic chains from adjacent nanocrystals for close contact, or via sharing a layer of counterions for larger inter-nanocrystal spacings.