Ag clusters electrochemically reduced in stearic acid Langmuir-Blodgett films and their structural characterizations

Silver ions confined in the silver stearate Langmuir-Blodgett (L-B) films of 8-14 layers were electrochemically reduced into Ag clusters, which made the observation of well-ordered scanning tunneling microscope (STM) images of the films with molecular resolution. A hexagonal packing of the hydrophobic chains was obtained, which was superimposed on the two-dimensional Ag clusters of 2-3 nm in diameter. Furthermore, a (2 x 1) structure of the hexagonal packing was observed. It was attributed to a self-assembled structure of the hydrophilic surface of the L-B films through hydrogen bonding. The strong surface enhanced Raman scattering (SERS) effect of Ag clusters made it possible to record Raman spectra of two-layer stearic acid L-B films in the region of C-C stretching vibration frequencies. The appearance of the bands at 1 128.0 and 1 100.7 cm^-1, which correspond to the antisymmetric C-C stretching model of the acyl chains in the all-trans conformation and the C-C stretching model of the acyl chains in the gauche conformations, respectively, means that the methylene chains are mainly in all-trans conformation, but some of them become disordered.