The Origin of Surface-Enhanced Raman Scattering of 4,4′-Biphenyldicarboxylate on Silver Substrates

We combine surface-enhanced Raman spectroscopy (SERS) and tools of computational chemistry with scanning electron, atomic force, and photoemission electron microscopy 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 nanoholes featuring an average diameter of 350 nm and a periodicity of 700 nm. The second is a nanojunction formed between a 100 nm silver nanoparticle and a flat silver surface. We find that the underlying background signal in the SERS spectra collected from the former strongly resemble the SEAS spectra of the nanosphere-featuring substrate, engineered to operate at the quantum limit. Our analysis of a series of SERS spectra consecutively collected from one nanocylinder suggests that the optical response of a single molecule can be extracted, with its brightest Raman-active mode enhanced by a factor of 2.2 x 10(6).