Investigation on the self-assembly of gold nanoparticles into bidisperse nanoparticle superlattices

Crystallization of nanoparticles with different sizes into an ordered assembly provides an important means to construct nanostructure that may possess novel collective properties from nanoparticle interactions. In this study, we report the formation of bidisperse Au nanoparticle superlattices that are self-assembled from 4.3-nm and 7.3-nm monodisperse Au nanoparticles. Various self-assembly parameters including particle concentration, type of solvent and capping ligand and assembly temperature are investigated. Hexagonal AlB2-type structure is observed as the dominant type of superlattice structure while NaCl-type structure and assembly containing defects are also present. The optical properties of the formed nanoparticle superlattices deposited on Si substrate are also investigated. In comparison with the case of Au nanoparticle suspension, the reflection spectra of superlattices show distinctively red-shifted surface plasmon resonance (SPR) bands. The contributions from both the intralayer and interlayer plasmonic couplings due to the formation of superlattice structure are assumed to be responsible for the observed optical spectra. (C) 2015 Elsevier B.V. All rights reserved.