Revisit of the plasmon-mediated chemical transformation of para-aminothiophenol.

Fingerprint Raman features of para-aminothiophenol (pATP) in surface-enhanced Raman scattering (SERS) spectra have been widely used to measure plasmon-driven catalytic activities because the appearance of characteristic spectral features is purported to be due to plasmon-induced chemical transformation of pATP to trans-p,p'-dimercaptoazobenzene (trans-DMAB). Here, we present a thorough comparison of SERS spectra for pATP and trans-DMAB in the extended range of frequencies covering group vibrations, skeletal vibrations, and external vibrations under various conditions. Although the fingerprint vibration modes of pATP could be almost mistaken with those of trans-DMAB, the low-frequency vibrations revealed distinct differences between pATP and DMAB. Photo-induced spectral changes of pATP in the fingerprint region were explained well by photo-thermal variation of the Au-S bond configuration, which affects the degree of the metal-to-molecule charge transfer resonance. This finding indicates that a large number of reports in the field of plasmon-mediated photochemistry must be reconsidered.

In situ surface-enhanced electronic and vibrational Raman scattering spectroscopy at metal/molecule interfaces.

SERS signals from nanostructured surfaces of Au, covered with thiol monolayers, were monitored under application of various electrochemical potentials over a wide Raman-shift range of both the Stokes and anti-Stokes branches. The background continuum in the SERS spectra varied in intensity with apparent correlations with breaking of Au-S bond or evolution of Au-O. This clearly indicates that the origin of the background can be ascribed to non-resonant electronic Raman scattering, which is sensitive to the electronic density at the surface. Using the property of the electronic Raman scattering, full information on the electric double layer at both sides of the metal/dielectric interface was analysed. In the low Raman-shift region below 200 cm-1, moreover, the evolution and disappearance of collective motions of thiol assembly was able to be monitored in situ, which is hardly obtainable with other vibrational absorption spectroscopies.

Electronic and vibrational surface-enhanced Raman scattering: from atomically defined Au(111) and (100) to roughened Au.

In surface-enhanced Raman spectra, vibrational peaks are superimposed on a background continuum, which is known as one major experimental anomaly. This is problematic in assessing vibrational information especially in the low Raman-shift region below 200 cm-1, where the background signals dominate. Herein, we present a rigorous comparison of normal Raman and surface-enhanced Raman spectra for atomically defined surfaces of Au(111) or Au(100) with and without molecular adsorbates. It is clearly shown that the origin of the background continuum is well explained by a local field enhancement of electronic Raman scattering in the conduction band of Au. In the low Raman-shift region, electronic Raman scattering gains additional intensity, probably due to a relaxation in the conservation of momentum rule through momentum transfer from surface roughness. Based on the mechanism for generation of the spectral background, we also present a practical method to extract electronic and vibrational information at the metal/dielectric interface from the measured raw spectra by reducing the thermal factor, the scattering efficiency factor and the Purcell factor over wide ranges in both the Stokes and the anti-Stokes branches. This method enables us not only to analyse concealed vibrational features in the low Raman-shift region but also to estimate more reliable local temperatures from surface-enhanced Raman spectra.

Electrochemical THz-SERS Observation of Thiol Monolayers on Au(111) and (100) Using Nanoparticle-assisted Gap-Mode Plasmon Excitation.

Surface-enhanced Raman scattering (SERS) microscopy using nanoparticle-assisted gap-mode plasmon excitation, which enables us to observe an atomically defined planar metal surface, was combined with THz-Raman spectroscopy to observe ultra-low-frequency vibration modes under electrochemical conditions. This combination helps us to gain deeper insights into electrode/electrolyte interfaces via direct observation of extramolecular vibrations including information on intermolecular and substrate/molecule interactions. Electrochemical reductive desorption of benzenethiol derivatives from Au(111) and (100) was monitored to demonstrate the power of this spectroscopy. Structural differences of the monolayers between these surfaces were seen only in the extramolecular vibration modes such as a large-amplitude hinge-bending motion of the phenyl ring. On the Au(111), where hollow-site and bridge-site adsorption coexisted, the electrochemical reductive desorption was preferentially induced at the hollow sites.