Near-Infrared Surface-Enhanced Raman Scattering Study of Ultrathin Films of Azobenzene-Containing Long-Chain Fatty Acids on a Silver Surface Prepared by Silver Mirror and Nitric Acid Etched Silver Foil Methods

Near-infrared (NIR) Fourier transform (FT) surface-enhanced Raman scattering (SERS) spectra were measured for Langmuir-Blodgett (LB) and dipping films of azobenzene-containing long-chain fatty acids (nAmH, n ) 8, 12, m ) 3, 5) on silver substrates prepared by silver mirror and nitric acid etched silver foil methods. FT-IR spectra were also obtained for some of the films on silver mirror, silver foil, and CaF2 plate. The dipping films on the silver mirror always show much greater SERS effect than those on the silver foils, revealing that the silver mirror method is more powerful than the silver foil method in preparing the substrates suitable for the SERS measurements. The fatty acids assume the COOform in the dipping films on the silver mirrors, while they mainly take the COOH form in the LB films on the silver mirror and CaF2 plate. Probably, silver reacts with the COOH groups during the dipping of the silver mirrors into the chloroform solutions of the azobenzene fatty acids. Bands at 1603 and 853 cm-1 due to the phenyl ring modes are enhanced markedly in the NIR-SERS spectra of the dipping films of 8A3H and 12A3H on the silver mirrors. Based upon the SERS “surface selection rule”, it may be concluded that the phenyl rings are nearly perpendicular to the silver mirror surfaces. Such enhancement is much weaker for the dipping film of 8A5H. Therefore, it seems that the length of the spacer (m) of the fatty acid is an important factor for the great SERS enhancement.