Single-Molecule and Single-Particle-Based Correlation
Studies between Localized Surface Plasmons of Dimeric Nanostructures
with ∼1 nm Gap and Surface-Enhanced Raman Scattering
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Abstract
Understanding the detailed electromagnetic field distribution inside
a plasmonically coupled nanostructure, especially for structures with
∼1 nm plasmonic gap, is the fundamental basis for the control
and use of the strong optical properties of plasmonic nanostructures.
Using a multistep AFM tip-matching strategy that enables us to gain
the optical spectra with the optimal signal-to-noise ratio as well
as high reliability in correlation measurement between localized surface
plasmon (LSP) and surface-enhanced Raman scattering (SERS), the coupled
longitudinal dipolar and high-order multipolar LSPs were detected
within a dimeric structure, where a single Raman dye is located via
a single-DNA hybridization between two differently sized Au–Ag
core–shell particles. On the basis of the characterization
of each LSP component, the distinct phase differences, attributed
to different quantities of the excited quadrupolar LSPs, between the
transverse and longitudinal regimes were observed for the first time.
By assessing the relative ratio of dipolar and quadrupolar LSPs, we
found that these LSPs of the dimer with ∼1 nm gap were simultaneously
excited, and large longitudinal bonding dipolar LSP/longitudinal bonding
quadrupolar LSP value is required to generate high SERS signal intensity.
Interestingly, a minor population of the examined dimers exhibited
strong SERS intensities along not only the dimer axis but also the
direction that arises from the interaction between the coupled transverse
dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision
correlation measurement strategy with a plasmonic heterodimer with ∼1
nm gap allows for the observation of the characteristic spectral features
with the optimal signal-to-noise ratio and the subpopulation of plasmonic
dimers with a distinct SERS behavior, hidden by a majority of dimer
population, and the method and results can be useful in understanding
the whole distribution of SERS enhancement factor values and designing
plasmonic nanoantenna structures