Determination of the Degree of Charge‐Transfer Contributions to Surface‐Enhanced Raman Spectroscopy

Abstract

We explore the application of a previously suggested formula for determining the degree of charge transfer in surface-enhanced Raman scattering (SERS). SERS is often described as a phenomenon which obtains its enhancement from three major sources, namely the surface plasmon resonance, charge-transfer resonances as well as possible molecular resonances. At any chosen excitation wavelength, it is possible to obtain contributions from several sources and this has led to considerable confusion. The formula for the degree of charge transfer enables one to separate these effects, but it requires that spectra be obtained either at two or more different excitation wavelengths or as a function of applied potential. We apply this formula to several examples, which display rather large charge-transfer contributions to the spectrum. These are p-aminothiophenol (PATP), tetracyano-ethylene (TCNE) and piperidine. In PATP we can show that several lines of the same symmetry give the same degree of charge transfer. In TCNE we are able to identify the charge-transfer transition, which contributes to the effect, and are able to independently determine the degree of charge transfer by wavenumber shifts. This enables a comparison of the two techniques of measurement. In piperidine, we present an example of molecule to metal charge transfer and show that our definition of charge transfer is independent of direction.

A Unified View of Surface-Enhanced Raman Scattering

“…For p CT ) 1 / 2 , the charge-transfer and surface plasmon contributions are about equal. This expression has been successfully applied to the spectra of several molecules for which there is a sizable charge-transfer contribution to SERS, including p-amiothiophenol, 18 tetracyanoethylene, piperidine, 38 and crystal violet. 31 To determine the contribution from a molecular resonance, we may calculate p mol (k) using the same formula as eq 3 with I k (mol) substituted for I k (CT).…”

The SERS effect in coordination chemistry

“…The contributions of the chemical mechanisms can lead to enhancement factors from 10 to 10 6 , and are strongly dependent on the nature of the adsorbed molecule, or on the complex formed at the nanoparticle surface [193][194][195][196][197] Lombardi and Birke [198,199] extended the classical Albrecht [200] formalism, in order to deal with the molecule-metal system, considering the Herzberg-Teller coupling between the filled and unfilled levels. In this way, they proposed a unified view of surface-enhanced Raman scattering encompassing the contributions of i) the electromagnetic (EM) surface plasmon resonance in the metal nanoparticle, ii) the charge-transfer (CT) resonance at the metal-molecule interface and iii) the molecular resonance (RR) at the plasmon surface, as expressed by eq.…”

Charge‐transfer contributions in surface‐enhanced Raman scattering from Ag, Ag2S and Ag2Se substrates

“…Use of the ratio also has the advantage of making our measure of charge‐transfer independent of other effects on the total enhancement factor. The degree of charge‐transfer ( p CT ) is then defined as …”