Study of SERS chemical enhancement factors using buffer layer assisted growth of metal nanoparticles on self-assembled monolayers.

Highly controlled morphology Au nanoparticle films can be formed on the surfaces of self-assembled monolayers (SAMs) by vapor deposition at cryogenic temperatures (approximately 10 K) with intervening condensed Xe layers on the SAMs serving as a buffer to reduce the kinetic energy of the Au atoms impinging on the surface (buffer layer assisted growth or BLAG). Under these conditions pristine Au nanoparticles (AuNp) of a uniform shape and size were deposited onto two SAMs differing only by their terminal groups, 4-benzenedithiol (BDT) and 4-methylbenzenethiol (MBT), to form -S/Au and -CH(3)/Au interfaces with essentially identical AuNp overlayer morphologies. A surface enhanced Raman (SERS) enhancement factor ratio EF(BDT)/EF(MBT) of approximately 130 was observed uniformly across the surfaces (approximately <10% variation). Since equal electromagnetic contributions to the SERS enhancements are expected from the two identically structured Au overlayer films, the observed SERS intensity ratio accordingly reflects a pure chemical enhancement (CE) contribution arising from the -S/Au relative to the -CH(3)/Au interface and thereby provides the first quantitative experimental data for the magnitude of the SERS CE for well-defined Au-molecule contacts.