Surface enhanced Raman scattering (SERS) has been extensively investigated for rough metal surfaces fabricated by chemical
etching and particles fabricated by different methods of nanolithography. Surface roughness is assumed to be necessary
for SERS. Sharp edges of nanoparticles are desired for near-field enhancements and, consequently, contribute to the enhanced
Raman effect. However, high porosity and roughness decreases the optical near-field strength due to losses by
radiation. Arrays of nanoparticles fabricated by nanosphere lithography, are able to generate strong near-fields and do not
have reduced roughness. Despite its advantages this fabrication method has an undesired effect: contaminant carbonaceous
species attached to the nanoparticles generate strong Raman spectra and prevent their use for other molecules.
We have investigated the effect of the particle roughness and preparation method on the characteristics of SERS spectra
and how the contamination can be avoided. Samples were prepared by nanosphere lithography and using silver and gold
nanoparticles in solution. Dye molecules and alkanethiols were used to decorate the nanoparticles in order to obtain
enhanced Raman scattering. SERS spectra were acquired using a scanning confocal Raman microscope. Concentrated
solution of alkanethiols reduce or remove completely the SERS spectra typical of the contaminants.
Surface enhanced Raman scattering (SERS) has been investigated for different molecules adsorbed on metallic nanostructures.
The local field enhancements due to the confinement and resonances of surface plasmons, can reach many orders of
magnitude. These field enhancements allow molecules to produce strong Raman spectra, although they have tiny Raman
scattering cross sections. The high sensitivity demonstrated was relevant for sensing applications of single molecules.
We have investigated experimentally the SERS effect on rhodamine 6G molecules, adsorbed on triangular silver particles
and photonic metallo-dielectric structures based on polymers. These structures were fabricated by evaporation of a
thin metallic film on colloidal crystals followed by casting in PDMS and epoxy resin. In the later, the polystyrene spheres
were removed by sonication in organic solvents. The remaining structure allows molecules to be adsorbed at its metallic
surface, on top of the triangular particles or inside the spherical holes. The SERS spectra were measured by a scanning
confocal Raman microscope.
The location of the SERS active centers (hot spots) in arrays of triangular particles (corners and edges) is correlated
with the optical near-field enhancements obtained by numerical simulations. In metallo-dielectric photonic structures made
of PDMS the Raman images show regions of stable SERS spectra (several pixels wide) and many isolated bright pixels.The
isolated pixels are instable in time, i.e. show spectral blinking.
The photonic structures we propose can be fabricated in a reproducible way. The field enhancements depend mainly on
the size and shape of the arrays, which is not the case for etched silver films and for clusters prepared by colloidal silver.
Thus, they are more suitable to investigate the electromagnetic contribution to SERS.
The development of new photonic and plasmonic devices rely on the new pioneering techniques of micro- and nanofabrication,
combining both standard lithography techniques and self-assembly. The combination of colloidal crystals, projection
patterning and soft-lithography are examples of fabrication techniques which allow to obtain complex structures of sizes
smaller than the wavelength of visible light. In many cases, the structures fabricated by this way are not possible to obtain
using standard lithography techniques, like electron-beam, UV-VIS lithography and focused ion beam (FIB).
We have used two-dimensional colloidal crystals as templates to fabricate arrays of isolated metallic particles of triangular
shape on surfaces and two-dimensional gratings. Either dielectric or metallic structures can be obtained. In the later
case the coupling between light and the locally confined surface plasmon-polaritons leads to resonances, field enhancements
and other related phenomena.
The scattering properties of the particles and gratings have been investigated experimentally, using a confocal, a near-
field optical microscope and a spectrometer, and theoretically, using FDTD methods.
We show that triangular particles of noble metals are highly sensitive to the relative direction of incidence of light and
its polarization. On the other hand, the light scattered in the direction perpendicular to the plane of the particles reveals
strong spectral dependency. This dependency can be exploited to fabricate photonics devices sensitive to the direction of
incidence of light.