Raman spectroscopy of single molecules on noble metal nanoparticles

This thesis presents theoretical calculations and experimental results on the mechanism of surface enhanced single molecule Raman spectroscopy. Raman scattering from single Rhodamine 6G molecule adsorbed large Ag nanoparticles was studied. A superlinear power dependence of the SERS signal was observed. The result is discussed in terms of model where single R6G molecule that yields surface enhanced Raman signal is located at the junction of two Ag nanoparticles. By increasing the incident light intensity, the huge gradient force can pull the molecule further into the junction. The dynamics of molecular motion and junction geometry were investigated by Raman depolarization studies. Results show depolarization ratio is wavelength independent for both the R6G Raman lines and the underlying continuum. In addition, depolarization ratio can usually be changed by changing the laser polarization on the sample, and varies as a radiating dipole of extreme anisotropic Raman tensor. This is explained by the local field properties at the junction sites of particle aggregates. The origin of the Raman continuum is proposed as the Ag electronic Raman triggered by chemisorbed R6G.; Classical electromagnetic (EM) field calculations are presented to discuss the EM field enhancement for a single sphere and dimer of spheres as a function of separation. For a Ag particle dimer, as separation gets smaller, the EM field at the junction is further enhanced at least 4–6 orders of magnitude compared to the single sphere case. The density distribution of coherent oscillating electron-hole pairs in the sphere is also calculated as a function of particle distance. It changes from a volume excitation in an isolated sphere to a surface excitation in a closely spaced dimer.; Controlled metal nanoparticle dimers have been self-assembled using rigid conjugated di-thiol bridging molecules. Preliminary results show that weak Raman signal from single optically transparent molecule at particle junctions is observed.