PNNL

Abstract

We combine scanning electron microscopy, atomic force microscopy, surface-enhanced Raman scattering (SERS), and tools of computational chemistry to investigate the origin of Raman scattering of 4,4’-biphenyldicarboxylic acid adsorbed as 4,4’-biphenyldicarboxylate on two different silver substrates. The first consists of a 100 nm deep cylindrical aperture embedded in an array of cylindrical nano-holes featuring an average diameter of 350 nm and a periodicity of 700 nm. The second is a nano-junction formed by a 100 nm silver nano-particle coated with the adsorbate and a flat silver surface. We find that the underlying background signal in the SERS spectra collected from the former strongly resemble the SERS spectra of the nano-sphere-featuring substrate, engineered to operate in the charge transfer plasmon limit. Our analysis of a series of SERS spectra consecutively collected from one nano-cylinder suggests that the optical response of a single molecule can be extracted, its brightest Raman active mode enhanced by a factor of 7.4 x 106.