Hexagonal arrays of plasmonic gold nanopyramids on flexible substrates for surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) with pyramidal nanostructures increases the signal of Raman active analytes, since hotspots form at the edges and tip of a nano-pyramid under illumination. 2D hexagonal arrays of pyramidal nanostructures with a quadratic base are fabricated through cost-effective nano-sphere lithography and transferred onto elastomeric polydimethylsiloxane (PDMS). By making use of the {111} crystal plane of a silicon (100) wafer, an inverted pyramidal array is etched, which serves as the complementary negative for the gold nanostructures. Either a continuous gold thin-film with protruding pyramids or separate isolated nano-pyramids are produced. Three main fabrication strategies are presented, in which a linker molecule between the PDMS and the gold is mandatory to increase the weak Au-PDMS adhesion. 3-Mercaptopropyltriethoxysilane (MPTS) is able to bind to both PDMS and to the gold structures, thus strongly increasing stability under mechanical strain. The SERS enhancement is verified by Raman mapping of 4-mercaptobenzoic acid (4-MBA) molecules. Fabrication on a flexible substrate paves the way for future applications on curved surfaces or insitu tunable resonances.