Highly Raman‐Enhancing Substrates Based on Silver Nanoparticle Arrays with Tunable Sub‐10 nm Gaps

Abstract

Raman spectroscopy, which is based on the inelastic scattering of photons by chemical entities, has been successfully utilized for the investigation of adsorbed molecules on surfaces, although the low cross section limits its applications. Surface-enhanced Raman scattering (SERS) has drawn a lot of attention since its discovery in 1974, primarily because it can greatly enhance the normally weak Raman signal and thereby facilitate the convenient identification of the vibrational signatures of molecules in chemical and biological systems. Recently, the observation of single-molecule Raman scattering has further enhanced the Raman detection sensitivity limit and widened the scope of SERS for sensor applications. Although SERS effects can be achieved simply by exploiting the electromagnetic resonance properties of roughened surfaces or nanoparticles of Au or Ag, the fabrication of reliable SERS substrates with uniformly high enhancement factors remains the focus of much research. Spraying Au or Ag colloids on a substrate leads to an extremely high SERS signal at some local ‘hot-junctions’; however, it is not easy to achieve a reliable, stable, and uniform SERS signal spanning a wide dynamical range using this method. Van Duyne and coworkers have used nanosphere lithography, while Liu and Lee exploited soft lithography, in order to fabricate Ag nanoparticle arrays with high SERS activity and improved uniformity. Kall and co-workers have shown theoretically that the effective Raman cross section of a molecule placed between two metal nanoparticles can be enhanced by more than 12 orders of magnitude. Such enhancement is likely to be related to the ‘hot-junctions’ observed in some SERS experiments. Several theoretical groups have also investigated field enhancement for SERS from metal nanoparticle arrays. Specifically, Garcia–Vidal and Pendry proposed that very localized plasmon modes, created by strong electromagnetic coupling between two adjacent metallic objects, dominate the SERS response in an array of nanostructures. The interparticle-coupling-induced enhancement was attributed to the broadening of the plasmon resonance peak because the probability of the resonance covering both the excitation wavelength and the Raman peak increases with its width. They calculated the average enhancement factor over the surfaces of an array of infinitely long Ag nanorods with semicircular cross sections, and showed that significant near-field interaction occurs between adjacent nanorods when the gap between the nanorods reaches half the value of their diameter. Other groups have studied the dependence of the enhancement factor on the gap between adjacent nanoparticles on a SERS active substrate. For example, Gunnarsson et al. investigated SERS on ordered Ag nanoparticle arrays with an interparticle gap above 75 nm. Lee and co-workers were able to achieve the temperature-controlled variation of interparticle gaps between Ag nanoparticles embedded in a polymer membrane. Wei et al. performed SERS on self-organized Au nanoparticle arrays with narrow interparticle gaps, although they have not carried out a detailed investigation of the dependence of the SERS signal on the interparticle gap. Sauer et al. investigated SERS from nanowire arrays embedded in an alumina matrix with interparticle gaps of ∼ 110 nm, but no gap-related enhancement was observed in their experiment. These theoretical and experimental studies indicate that the precise control of gaps between nanostructures on a SERS-active substrate in the sub-10 nm regime, which is extremely difficult to obtain by existing nanofabrication methods, is likely to be critical for the fabrication of substrates with uniformly high enhancement factors, and for understanding collective surface plasmons existing inside the gaps. C O M M U N IC A IO N S