Exploring the chemical enhancement for surface-enhanced Raman scattering with Au bowtie nanoantennas

Abstract

Single metallic bowtie nanoantennas provide a controllable environment for surface-enhanced Raman scattering (SERS) of adsorbed molecules. Bowties have experimentally measured electromagnetic enhancements, enabling estimation of chemical enhancement for both the bulk and the few-molecule regime. Strong fluctuations of selected Raman lines imply that a small number of p-mercaptoaniline molecules on a single bowtie show chemical enhancement >10 7 , much larger than previously believed, likely due to charge transfer between the Au surface and the molecule. This chemical sensitivity of SERS has significant implications for ultra-sensitive detection of single molecules.Rapid and accurate detection and identification of trace amounts of chemical species is of the utmost importance in biology, chemistry, medicine, and defense. Recent advances in fluorescence spectroscopy methods offer exquisite sensitivity, enabling the ultimate in analytical detection: single molecules. 1 However, fluorescence studies require specially engineered labels, a limitation that places constraints on potential applications. Raman spectroscopy does not suffer from this limitation, since most molecules display a unique set of molecular vibrations that give rise to a distinctive chemical fingerprint, especially attractive for ultra-selective analysis.Because Raman transitions are incredibly weak, this technique was not generally believed to offer the potential sensitivity afforded by fluorescence. However, over 30 years ago, it was first observed that the Raman signal of pyridine dramatically increases when adsorbed on a roughened Ag electrode, 2,3 and the detailed origins of surface-enhanced Raman scattering (SERS) arising from nanostructured metals have remained a topic of debate. Researchers linked SERS signals to a combination of two effects, 4 electromagnetic (EM) enhancement, where illumination intensity is enhanced due to sharp metal edges or plasmon effects, and chemical enhancement (CE), where the Raman cross-section of adsorbed molecules is increased above the solution value, 5 with EM enhancement dominant. Detailed SERS experiments performed on roughened metal films in electrochemical cells revealed the importance of CE due to the applied potential dependence of SERS spectra, 6,7 but measured values of either enhancement were not available.Interest in the SERS mechanism blossomed with the recent observation of Raman lines apparently arising from single molecules adsorbed onto colloidal Ag and Au particles. 4,8,9 To obtain the 14-order of magnitude enhancement required to make Raman signals competitive

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

“…The Becke-Perdew (BP86) XC-potential [91,92] and a triple-polarized slater type (TZP) basis set from the ADF basis set library were used in our simulation. The validity of using DFT and ADF for quantum mechanical SERS calculation has already been proven by many researchers [37][38][39][40][41][42][43][44][45][46][47][48].…”

Modeling Coherent Anti-Stokes Raman Scattering with Time-Dependent Density Functional Theory: Vacuum and Surface Enhancement.

“…Metal surfaces largely enhance (spontaneous) Raman spectra via the increased fields which exist in the vicinity of a surface plasmon excitation, [29][30][31] although there is also a chemical effect [32][33][34][35] which arises from the metal perturbing the electronic structure of an attached molecule. Classical electrodynamics can account for the former, 36 and TDDFT calculations can be used to describe the latter.…”

Extraordinary Light-Induced Local Angular Momentum near Metallic Nanoparticles

“…To understand why it is difficult to induce quadrupolar transitions using standard optical sources, consider the amplitude of the field gradient of a plane wave (PW), given by |∇ | = kE E PW 0 (4) where E PW = E 0 e −ik·r e −iωt is the associated field. Clearly, the gradient of the PW is in the direction of wave propagation, and its magnitude is proportional to the wave vector modulus k = 2π/λ.…”