  Raman spectroscopy is a powerful technique to characterize materials. It
probes non-destructively chemical composition, crystallinity, defects, strain
and coupling phenomena. However, the Raman response of surfaces or thin films
is often weak and obscured by dominant bulk signals. Here we overcome this
limitation by placing a transferable porous gold membrane (PAuM) on top of the
surface of interest. Slot-like nanopores in the membrane act as plasmonic slot
antennas and enhance the Raman response of the surface or thin film underneath.
Simultaneously, the PAuM suppresses the penetration of the excitation laser
into the bulk, efficiently blocking the bulk Raman signal. Using graphene as a
model surface, we show that these two simultaneous effects lead to an increase
in the surface-to-bulk Raman signal ratio by three orders of magnitude. We find
that 90% of the Raman enhancement occurs within the top 2.5 of the material,
demonstrating truly surface-sensitive Raman scattering. To validate our
approach, we analyze the surface of a LaNiO3 thin film. We observe a Raman mode
splitting for the LaNiO3 surface-layer, which is spectroscopic evidence that
the surface structure differs from the bulk. This result underpins that PAuM
give direct access to Raman signatures of surfaces and their structural
properties.

Materials Science;Applied Physics