SERS on Silver-coated Carbon nanotubes

In a collaboration with two research groups in the United Kingdom under the leadership of Dr. Paul Dawson, we have studied in detail the enhancement mechanisms of surface enhanced Raman scattering (SERS) for molecules deposited on Silver-coated multiwalled Carbon nanotubes (MWNTs).

This figure shows SEM images taken at (a) 45° and (b) normal incidence of multiple-walls Carbon nanotubes coated with Silver. (e) The Raman spectrum from crystal violet molecules deposited on the sample taken with Raman microscope using input laser of wavelength 632.8 nm and power is also shown. The spectral intensity has been normalized to k-counts mW-1 s-1 (inset in e).

In order to better understand the enhancement measured on the structures, detailed calculations were carried out with the durface integral equation method. This figure shows the electric field intensity for nanoposts with granular Ag surface structure illuminated by light of wavelength 632.8 nm with polarization and geometry the same as for the top figure and different geometries: (a) Granular nanopost comprised of pure Ag (i.e., noMWNTcore) in ambient environment of refractive index = 1.0 (air). (b) Granular nanopost comprised of pure Ag in ambient environment of refractive index = 1.5, corresponding approximately to case of multilayer CV coverage. (c) Nanopost comprised ofMWNT core of 60 nmdiameter with granular Ag coating in ambient environment of refractive index =1.0 (air). (d) Nanopost comprised of MWNT core of 60 nm diameter with granular Ag coating in ambient environment of refractive index = 1.5.

In conclusion, we have modeled the electromagnetic response of a complex nanostructured substrate, comprising highly granular, MWNT-supported Ag antenna in the context of a SERS study to reveal not only (and not surprisingly) the need for hot-spots to explain the SERS enhancement factor, but a remarkable contrast in the behavior of different types of hot-spots. That contrast is between amplification, or lack of it, of the localized by the delocalized plasmonic response of the lumpy nanopost antennae, that is, between fieldenhanced regions at the outer edges of the Ag grains that are entirely antenna fed (no such regions occur in the antenna nodal regions) and intergrain plasmon resonances that are largely independent of the antenna response with a significant proportion occurring in antenna nodal regions. In addition, hotspots between the tops of coleaning nanoposts also seem to benefit from additional antenna enhancement. The understanding gained from these intriguing structures informs the route to forming lumpy antenna substrates for improved SERS performance. Primary factors are shorter antennae supporting a fundamental resonance (a node at the antenna base and a single antinode at the top) in the region of the excitation and scattering wavelengths, the use of a support structure that is optically less dissipative than MWNTs (pure Ag structures would clearly be superior) and, more challengingly, better control over the granular Ag coating with a view to optimizing the nanometer-scale intergrain regions both in terms of density and uniformity.

Check the corresponding publication: PDF External link: doi: 10.1021/nl102838w