Experimental and theoretical results are presented in Chapters 2, 3, and 4 on the effects of dielectric medium, particle size, and interparticle distance, respectively, on plasmon coupling in two-dimensional arrays of silver nanoparticles. The arrays were fabricated via the self-assembly of single crystal particles from c.a. 46 nm to 287 nm on PVP-modified glass substrates. Spin-coated poly(methylmethacrylate) layers were used to immobilize the particles and prevent their surface aggregation. Varying the thickness of the PMMA layer made it possible to change the average dielectric medium between the particles. UV-Visible spectra of different arrays were compared to the corresponding electron microscope images. It was found that increasing the dielectric function of the medium surrounding the particles promoted the coherent plasmon coupling of the particles. It was also observed that increasing the size of the particles in arrays resulted in red shifting and broadening of the coupled plasmon peak. The peak position against the particle size exhibited a linear trend, providing the possibility to adjust a lambda maximum by selecting an appropriate particle size. Decreasing the interparticle distance resulted in spectral shifting and broadening of the plasmon peak and affected the intensity and sharpness of the peaks. Smaller particles shifted to the red spectral range, where as larger particles shifted in the opposite direction upon decreasing the interparticle distance.
The phenomenon known as Surface Enhanced Raman Spectroscopy (SERS) is presented in Chapter 5. An analysis of spectra from 20 different molecules adsorbed on SERS substrates revealed competitive Raman enhancement from different types of molecules simultaneously present on the surface. The observed SERS behavior could not be explained using local field enhancement or charge-transfer models. A different SERS mechanism is proposed based on plasmon-induced electronic coupling between the oscillating electrons in the metal and the electronic system of adsorbed molecules.
Chapter 6 continues a discussion of SERS but introduces a new substrate based on the low-pressure air plasma reduction of silver compounds to produce porous nanostructured surfaces. This method is advantageous because substrates are easy to prepare and the silver surface is inherently clean for the adsorption of molecules. Silver chloride was found to be the best compound to make reproducible and stable SERS substrates. SERS activity of the substrates was tested using L-tryptophan, 4-mercaptobenzoic acid, and adenine.