Surface-enhanced Raman scattering on silvered porous alumina templates: role of multipolar surface plasmon resonant modes
S. N. Terekhov, S. M. Kachan, A. Yu Panarin, and P. Mojzes,
Phys. Chem. Chem. Phys. 17 (47), 31780-31789 (2015).
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Nanostructured silver films with different thicknesses were prepared by vapor deposition onto the surface
of the anodic aluminum oxide (AAO) template to be used as surface-enhanced Raman scattering (SERS)
active substrates. Both the peak position of the surface plasmon resonance (SPR) band and SERS
enhancement of silvered AAO samples displayed non-monotonous dependence on Ag layer thickness.
Using 441.6 nm excitation and a water-soluble cationic porphyrin, Cu(II)-tetrakis (4-N-methylpyridyl)
(CuTMPyP4), as a SERS-reporting analyte, two maxima of the SERS enhancement were obtained for Ag
layers of 15 and 120 nm thickness. Thickness dependencies have been analyzed taking into account the
type of SPR modes identified by means of quasicrystalline approximation (QCA) of statistical theory of
multiple scattering of waves and multi-Lorentzian deconvolution. The analysis revealed that SERS
enhancement is related to the absolute magnitude of the distance between excitation wavelength and
spectral position of collective SPR mode. It was shown that matching of excitation wavelength and the
most intensive SPR modes with non-radiative decay, generated mainly by coherent interaction of higher-order plasmon resonant modes (quadrupole and octupole), plays a dominate role in SERS performance.
Besides, it has been observed that more intense SERS signal can be obtained when the analyte deposited
on the Ag/AAO substrate was excited through the AAO template rather than from the silvered side. Our
results demonstrate that appropriate excitation geometry and fine-tuning of the optical properties of the
Ag/AAO substrate by adjusting the thickness of the Ag layer with respect to particular excitation
wavelength can contribute to more effective SERS enhancement.
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