Surface enhanced Raman scattering (SERS) of adsorbed molecule on colloidal gold nanoparticles of different shapes, namely
nanospheres (NSs), nanorods (NRs), and nanoprisms (NPs) as well as the three NPs arrays of different interstice prepared by NS
lithography, are studied with incident wavenumbers in the near-dipole and near-quadrpole regions of the nanoparticles. In
the colloidal gold nanoparticles, the SERS enhancement is the largest for the sharp tip followed by the truncated tip NPs, then
the NRs and least enhancement for the NSs. This decreasing order of enhancement occurs although the incident wavenumber
was near the dipole resonance of NSs and the quadrupole resonance for the NPs. These varied enhancements are explained
in part as due to the binding energies of the nanocrystal facets, but the larger contribution results from the plasmon
electromagnetic fields. A parallel finite difference time domain (FDTD) calculations were carried out, which corporate the
experimental results and show agreement with ratios of the SERS enhancement for the different shapes. The normalized SERS
intensity for NPs of different interstice distances show a sharp rise with the decrease of the interstice distances because of
interparticle dipolar and quadrupolar coupling as evidenced also by FDTD calculations. Furthermore, these calculations show
that the enhancement is polarization independent for an incident wavelength near quadrupole resonance but polarization
dependent for an incident wavelength near the plasmon dipole transition. In the last case, the enhancement is larger by an
order of magnitude for a polarization parallel to the NPs bisector than for polarization normal to the bisector with no hot
spots for the relatively large interstice dimensions used. Copyright © 2012 John Wiley & Sons, Ltd. |
