Theory of SERS enhancement: general discussion

Abstract

Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency? Richard Van Duyne answered: The extra enhancement is not due to an increase in the Raman cross section, as that is ratioed out in the calculation of the enhancement factor. The charge transfer (CT) process does not transfer a complete electron, it is a fractional degree of CT. Thus the change in vibrational frequency is small. DFT calculations that provide eigenvectors allow one to reference the vibrational modes of the free molecule with those of the adsorbed molecule. Sylwester Gawinkowski asked: You have shown that the enhancement factor curve is redshifted relative to the plasmon resonance band and has a maximum at about 800 nm. This means that the SERS signal should be strongest for excitations in the near infrared spectral region. Why do most SERS reports, particularly related to single molecule SERS, have the excitation in the green or red spectral range and not in the near infrared? Richard Van Duyne replied: The SERS excitation spectrum for isolated nanoparticles (e.g. the NSL nanotriangles that I showed in Fig. 1 of the introductory lecture 1 ) is redshifted with respect to the localized surface plasmon resonance (LSPR) by half the Stokes frequency of the vibrational mode. As the nanoparticle size is decreased the LSPR shifts to the blue so it is only for a specific size that one gets an LSPR maximum at 800 nm. Essentially all single molecule SERS experiments are done with dye molecules and the laser excitation wavelength is chosen to get maximum resonance Raman (RR) as well as SERS enhancement. For Rhodamine 6G (R6G) the laser excitation wavelength of 532 nm is close to the absorption maximum of R6G. SMSERS should be possible in the NIR for a wide range of dye molecules with absorption maxima in that spectra region. 1 A.-I. Henry, T. W. Ueltschi, M. O. McAnally and R. P. Van Duyne, Faraday Discuss., 2017, DOI: 10.1039/c7fd00181a. Marc Porter asked: Why is the oxidized form of nitrobenzene (I may not have the name of the reactant correct; my notes are a bit fuzzy, which I blame on jet lag) more sensitive to the local environment than its reduced from. Does the supporting electrolyte play a role here? Richard Van Duyne replied: The redox system you are referring to is the dye Nile Blue. The oxidized form is positively charged and the adsorption has electrostatic character. Hence it is more sensitive to the electrostatics of the local environment than the neutral reduced form. Sumeet Mahajan commented: In your work on surface-enhanced FSRS with a high rep rate laser why does the signal to noise not increase when there are 10× more pulses with the 1 MHz setup compared to the 100 kHz...

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