High-purity gold nanocrystal dimers: scalable synthesis and size-dependent plasmonic and Raman enhancement

We report a method for the solution synthesis of directly-fused dimers of gold nanocrystals, including both homo-dimers (two nanocrystals of the same size) and hetero-dimers (two nanocrystals of significantly different sizes) with high purity and high yield. The first key issue in this seeded solution synthesis was the tight integration of the reduction of the precursors to form tiny nanocrystals in solution and random attachment of these newly formed tiny nanocrystals onto the tips of existing nanocrystals. The second key issue was to decouple the intra-particle ripening from the reduction and random attachment. This new approach was found to be capable of yielding different types of dimers by judiciously tuning the time sequence of reduction, random attachment, and intra-particle ripening in one synthesis. UV-Vis absorption measurements unambiguously demonstrated a two-band feature for the resulting high-purity dimers, indicating strong plasmonic enhancement due to the formation of hot spots within the dimers. The two-band absorption spectra of the dimers were further found to be strongly size dependent and intra-particle distance dependent. Consistent with the observation of the formation of hot spots by absorption measurements, very strong and size-dependent surface-enhanced Raman scattering was observed for these high-purity, directly fused, and stable dimers of gold nanocrystals.