Study On Assembly And Surface-enhanced Raman Scattering Of Metal Nanoparticles

Gold and silver colloid dispersions are prepared by chemical reduction from corresponding metal compounds. Ordered mono- and double-layer structures of silver nanoparticles on smooth silver substrates are fabricated by self-assembly technique, where 1,4-benzenedithio] (1,4-BDT) and p-aminothiophenol (PATP) are used as the coupling molecules, respectively. Surface-enhanced Raman spectroscopic (SERS) characterization shows that Raman scattering of the coupling molecules in the assemblies is greatly enhanced. The Raman spectrum of PATP in the assemblies shows several differences from that obtained on a roughened silver electrode, including frequency shifts and changes in relative intensity of the characteristic bands. The results indicate that the enhancement of the spectrum of PATP in the assemblies results from an electromagnetic coupling between the silver particles and the surface of the silver metal. The fact that only the SERS spectrum of PATP is observed from the assembly, where PATP is partially replaced with N,N'-diphenylthiourea, demonstrates that the electromagnetic field is mainly localized in the region between the silver particles and the surface of the silver metal. In the double-layer assemblies, the Raman enhancement of the upper coupling molecules arises from the coupling interaction of two layers of silver nanoparticles. The enhancement effects in both cases are comparable to each other.