Surface-enhanced Raman scattering (SERS) from different azobenzene self-assembled monolayers and sandwiches

Self-assembled monolayers (SAMs) of functionalized azobenzene thiols (RAzoCnSH, n=3-6 for R=H, abbreviated as AzoCnSH; and n=4 for R=CH ₃CONH, abbreviated as aaAzoC4SH) on different substrates RAzoCnS-|S (S represents substrates of vacuum-deposited gold (Au), silver foil (Ag), HNO₃ etched silver foil (EAg), and silver mirror (mAg)) have been studied by SERS in the near-infrared region. SERS of the SAMs on EAg and/or mAg exhibit SERS effects that vary with etching time and/or deposition time. The most appropriate time is 5 s for etching in 1:1 HNO₃ and 40 s for deposition in 0.1 M Ag(NH₃)₂NO₃. Further, a layer of Ag mirror was conveniently deposited on the top of the SAMs on different substrates, yielding a more efficient SERS-active system possessing a "sandwiched" structure of mAg|RAzoCnS-|S. An appropriate surface roughness is required for the strongest SERS effect. Scanning electron microscopy (SEM) indicates that there exist a large number of projects around 100 nm on the surface showing the strongest SERS effect. When the surface roughness is decreased or increased, the SERS effect decreases sharply. The relationship between the SERS effect and the structural nature was investigated and showed that the enhancement factor decays exponentially with increasing in distances of the azobenzene group from the underlying substrate or the overlying silver mirror. This result reveals that the SERS effect may be the result of the electromagnetic coupling effect between two metal layers.