Au nanoparticle monolayers: preparation, structural conversion and their surface-enhancedRaman scattering effects

Abstract

An environment-friendly method is developed to fabricate close-packed Au nanoparticle(AuNP) monolayers with sub-10 nm interparticle spacing simply by covering n-butanol onthe surface of an Au aqueous colloid. The close-packed nanostructure can further transforminto two-dimensional (2D) aggregates with different aggregation degrees upon aging forseveral days. This structural evolution process was disclosed by transition electronmicroscopy (TEM) and UV–vis spectroscopy and its influence on the ensemble opticalproperties was further demonstrated by surface-enhanced Raman scattering (SERS). It wasrevealed that creating sub-10 nm interparticle spacing and particle dimers are highlydesirable for engendering strong SERS activity under a 632.8 nm excitation. Further agingthe film leads to the formation of larger aggregates, which moves the surfaceplasmon resonance of the aggregates gradually ‘off-resonance’ from the 632.8 nmexcitation line and costs some numbers of sub-10 nm interparticle spacings. Thetwo parameters together decrease the SERS activity of the close-packed AuNPmonolayers. The present strategy thus provides an easy way to finely tune theSERS properties of thin nanoparticle films and other ensemble properties, whichcan easily be realized by creating sub-10 interparticle spacing, controlling theparticle aggregation degree and by adopting suitable particle sizes and shapes.