Metal/Si Nanowire Dimers and Oligomers for Surface-Enhanced Raman Scattering and Photocatalysis

Metal nanoparticles (NPs) can sustain localized surface plasmon resonances that can
significantly enhance the surface electric-field (E-field). This large E-field enhancement has
been used for sensing via surface-enhanced Raman spectroscopy (SERS), and for increasing
reaction rates and selectivity under light irradiation due to the generation of energetic “hot”
charge carriers. Both the SERS and plasmonic catalysis effect scale as the E-field intensity
inside and/or at the metal surface; Thus, controlling the field enhancement at/in the metal is
essential. This can be best achieved by coupling the metal nanoparticles with high refractive
index dielectric (such as Si) nanostructures that are separated with nanoscale gaps. Such
structures are challenging to synthesize and have not yet been investigated in this context.
Objectives:
This proposal will investigate the synthesis of macroscale arrays of Si nanowire (NW)
oligomers composed of aggregates with two or more Si NWs separated by small distances,
which will provide different E-field enhancements. These arrays will be decorated with Ru and
Rh NPs, where the SERS effect is used as an analytical method to quantify the field-
enhancement at Ru and Rh photocatalysts for CO2 photomethanation. This proposal will focus
in particular on i) replacing plasmonic absorbers with resonant dielectric nanostructures for
field-enhanced photocatalysis at Ru and Rh NPs, and ii) probing the field-enhancement on the
Ru and Rh NPs on the different Si NW array morphologies via SERS, and iii) correlating this
field-enhancement with the measured photoactivities.
Approaches:
Confocal Raman microscopy will be used to quantify the field at the Rh and Ru catalysts. CO2
photomethanation at Rh and Ru catalysts will be used as a testbed because of their reported
high photoactivities. Rh and Ru are lousy plasmonic materials, thus providing room for
improvement by locating the Rh and the Ru catalyst within regions of large fields.
Photocatalytic experiments will be performed using a home-made high vacuum cell equipped
with on-line mass-spectrometry and compatible with in-situ IR spectroscopy. Si NW oligomers
will be synthesized using non-conventional colloidal templates and metal-assisted chemical
etching (MACE).
Level of originality (high):
This proposal will provide a bench-top chemical approach to synthesize novel nanostructured
metal/Si substrates with direct applications for SERS sensing and photocatalysis. Key
questions that are difficult to address experimentally will be investigated, providing a clear
picture of the effect of field-enhancement in plasmonic catalysis.