Plasmonic nanostructures are often
used in surface-enhanced infrared
absorption (SEIRA) spectroscopy to probe surface assembled molecules
or the dielectric environment surrounding the metallic nanostructures.
Here we fabricate metallic nanogap structures using self-aligned techniques
on an intrinsic silicon substrate and correlate resulting SEIRA spectra
with the choice of metal nanostructure geometry. A motivation is to
compare the enhancement from hybridization of bright plasmon modes
with the effect of hybridization between bright and dark plasmon modes.
These structures provide a gap size below 10 nm and support strong
field enhancements. The structures demonstrate their sensitivity through
the enhanced absorption signature of the Si–O stretch in the
native silicon oxide layer of nanometer thickness beneath the metal.
Simulations reveal this thin layer plays a critical role in determining
the plasmon modes of the nanostructures. Numerical simulations of
the optical properties are consistent with the observations that stronger
Si–O stretch signals are detected on self-aligned nanogap structures
than nanorod arrays, highlighting the enhanced electromagnetic fields
in the underlying native oxide
