Surface-enhanced Raman spectral study of Au nano-particles/alkanethiol self-assembled monolayers/Au(1 1 1 ) heterostructures

This paper describes Raman spectroscopic investigations of octadecane-thiol (C₁₈H₃₇-SH; OD-mT) and octane-dithiol (HS-C₈H₁₆-SH; O-dT) self-assembled monolayers (SAMs) on an epitaxial Au(1 1 1 )/cleaved-mica substrate. The epitaxial Au(1 1 1 ) substrate having nanometer-scale domains increases the enhanced Raman signals of SAMs, whereas the Raman bands are much less intense on the micrometer-scale domains of Au(1 1 1 ). The Raman intensity of the ν_s(CH₂) band is increased for depositions above 4 nm, and takes a maximum at a thickness of 9 nm when gold is vacuum-deposited on the OD-mT SAMs that are formed on micrometer-scale domains. For the O-dT SAMs, Raman band intensity profiles show two features at gold thicknesses of 2 and 9 nm; the latter intensity is about three times greater than that of the former. Atomic force microprobe (AFM) images reveal that 20-30-nanometer-scale islands are formed by 9 nm gold deposition for both SAMs. In contrast, 2 nm gold deposition onto the O-dT SAMs creates striped nanostructures (ca. 7 nm in spacing). The results suggest that the specific nanostructures formed by post-deposition of gold serve to enhance the SAMs Raman signals.