  Trisoctahedral (TOH) shaped Au nanocrystals (NCs) have emerged as a new class
of metal nanoparticles (MNP) due to its superior catalystic and SERS activities
caused by the presence of high density atomic steps and dangling bonds on their
high index facets. We examine the radiative localized surface plasmon resonance
(LSPR) modes of an isolated single TOH Au NC using cathodoluminescence (CL),
with high resolution spatial information of the local density of optical states
(LDOS) across the visible spectral range. Further we show pronounced
enhancement factor in the Raman scattering by performing Raman spectroscopic
measurements on Rhodamine 6G (R6G) covered TOH Au NPs aggregates on a Si
substrate. We believe that the hot spots between two adjacent MNP surfaces
(nanogaps) can be significantly stronger than single particle LSPRs. Such
nanogaps hotspots may have crucial role on the substantial SERS enhancement
observed in this report. Consequently, the present study indicates that MNPs
aggregates are highly desirable than individual plasmonic nanoparticles for
possible applications in SERS based biosensing.
