  Integrating graphene with plasmonic nanostructures results in multifunctional
hybrid systems with enhanced performance for numerous applications. In this
work, we take advantage of the remarkable mechanical properties of graphene to
combine it with scalable 3D plasmonic nanostructured silicon substrates, which
enhance the interaction of graphene with electromagnetic radiation. Large areas
of femtosecond laser-structured arrays of silicon nanopillars, decorated with
gold nanoparticles, are integrated with graphene, which conforms to the
substrate nanotopography. We obtain Raman spectra at 488, 514, 633, and 785 nm
excitation wavelengths, spanning the entire visible range. For all excitation
wavelengths, the Raman signal of graphene is enhanced by 2-3 orders of
magnitude, similarly to the highest enhancements measured to date, concerning
surface-enhanced Raman Spectroscopy (SERS) of graphene on plasmonic substrates.
Moreover, in contrast to traditional deposition and lithographic methods, the
fabrication method employed here relies on single-step, maskless,
cost-effective, rapid laser processing of silicon in water, amenable to
large-scale fabrication. Finite-difference time-domain simulations elucidate
the advantages of the 3D topography of the substrate. Conformation of graphene
to the Au-decorated silicon nanopillars enables graphene to sample near fields
from an increased number of nanoparticles. Due to synergistic effects with the
nanopillars, different nanoparticles become more active for different
wavelengths and locations on the pillars, providing broadband enhancement.
Nanostructured plasmonic silicon is a promising platform for integration with
graphene and other 2D materials, for next-generation applications of large-area
hybrid nanomaterials in the fields of sensing, photonics, optoelectronics, and
medical diagnostics.
