Silver nanoparticle thin films with nanocavities for surface-enhanced Raman scattering.

The formation of nanometer-sized gaps between silver nanoparticles is critically important for optimal enhancement in surface-enhanced Raman scattering (SERS). A simple approach is developed to generate nanometer-sized cavities in a silver nanoparticle thin film for use as a SERS substrate with extremely high enhancement. In this method, a submicroliter volume of concentrated silver colloidal suspension stabilized with cetyltrimethylammonium bromide (CTAB) is spotted on hydrophobic glass surfaces prepared by the exposure of the glass to dichloromethysilane vapors. The use of a hydrophobic surface helps the formation of a more uniform silver nanoparticle thin film, and CTAB acts as a molecular spacer to keep the silver nanoparticles at a distance. A series of CTAB concentrations is investigated to optimize the interparticle distance and aggregation status. The silver nanoparticle thin films prepared on regular and hydrophobic surfaces are compared. Rhodamine 6G is used as a probe to characterize the thin films as SERS substrates. SERS enhancement without the contribution of the resonance of the thin film prepared on the hydrophobic surface is calculated as 2 x 10(7) for rhodamine 6G, which is about one order of magnitude greater than that of the silver nanoparticle aggregates prepared with CTAB on regular glass surfaces and two orders of magnitude greater than that of the silver nanoparticle aggregates prepared without CTAB on regular glass surfaces. A hydrophobic surface and the presence of CTAB have an increased effect on the charge-transfer component of the SERS enhancement mechanism. The limit of detection for rhodamine 6G is estimated as 1.0 x 10(-8) M. Scanning electron microscopy and atomic force microscopy are used for the characterization of the prepared substrate.