The Surface Enhanced Raman Scattering And Fluorescence Decay Rate From Bimetallic Core-shell Nanoparticles

In comparison with single metal nanoparticles, bimetallic core-shell nanoparticleshave the potential applications in biological sensor, optical and many other fields withspecial electronic structure and surface properties. It is found that, in the vicinity of thesurface enhanced Raman scattering (SERS) active substrate surface, the Ramanscattering is enhanced while the fluorescence is quenched. In this paper, we will mainlyinvestigate the SERS effect and molecular fluorescence decay rate for a moleculeadsorbed on bimetallic core-shell nanoparticles. This thesis includes two parts:1. The SERS effect for a molecule adsorbed on bimetallic core-shellnanoparticles.SERS effect from the system of a molecule adsorbed on bimetallic nanoparticleswith core-shell structure is considered. Firstly, we present a first-principles approach toderive the multipolar moments and polarizability analytically in the quasi-static limit.Then, the formula of SERS enhancement ratio (R) is given on the basis of Gersten-Nitzan model. Due to the interfacial interaction between the core and the shell forbimetallic core-shell nanoparticles, we can observe two resonant peaks obviously. Andby the adjustment of the core-shell radii ratio of bimetallic nanoparticles, the surfaceplasmon resonant wavelength can be tuned in a wide wavelength region, exhibiting atunable Raman scattering.2. Molecular fluorescence decay rate for a molecule adsorbed on bimetalliccore-shell nanoparticles.The fluorescence decay rate from bimetallic core-shell nanoparticles is considered.We start from the Mie theory to deduce the formula of molecular decay rate. And then based on the dipolar particles model, we derive the molecular nonradiative decay rate andradiative decay rate with the optical theorem. For the finite size effect of the particles, thepolarizability can be corrected. In the end, we obtain the analytical expression of themodified molecular radiative decay rate, nonradiative decay rate and the quenchingefficiency. Numerical results show that this wavelength-controlled quenching efficiencyis attributed to the SPR absorption. When the distance between the molecule andparticles is fixed, by adjusting the core-shell radii, two resonant peaks exhibit red shift,hence leading to a tunable quenching efficiency. Our results will have some practicalsignificance on the development of chemical sensing and detection of biological.