On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

https://iopscience.iop.org/article/10.1088/0953-8984/4/5/001/pdf

Surface-enhanced Raman scattering

Since 2000, there has been an explosion of activity in the field of plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this Review, we explore the mechanism of PERS and discuss PERS hotspots — nanoscale regions with a strongly enhanced local electromagnetic field — that allow trace-molecule detection, biomolecule analysis and surface characterization of various materials. In particular, we discuss a new generation of hotspots that are generated from hybrid structures combining PERS-active nanostructures and probe materials, which feature a strong local electromagnetic field on the surface of the probe material. Enhancement of surface Raman signals up to five orders of magnitude can be obtained from materials that are weakly SERS active or SERS inactive. We provide a detailed overview of future research directions in the field of PERS, focusing on new PERS-active nanomaterials and nanostructures and the broad application prospect for materials science and technology. Assisted by rationally designed novel plasmonic nanostructures, surface-enhanced Raman spectroscopy has presented a new generation of analytical tools (that is, tip-enhanced Raman spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy) with an extremely high surface sensitivity, spatial resolution and broad application for materials science and technology.

Nanostructure-based plasmon-enhanced Raman spectroscopy for surface analysis of materials

Surface-enhanced Raman spectroscopy (SERS) and related spectroscopies are powered primarily by the concentration of the electromagnetic (EM) fields associated with light in or near appropriately nanostructured electrically-conducting materials, most prominently, but not exclusively high-conductivity metals such as silver and gold. This field concentration takes place on account of the excitation of surface-plasmon (SP) resonances in the nanostructured conductor. Optimizing nanostructures for SERS, therefore, implies optimizing the ability of plasmonic nanostructures to concentrate EM optical fields at locations where molecules of interest reside, and to enhance the radiation efficiency of the oscillating dipoles associated with these molecules and nanostructures. This review summarizes the development of theories over the past four decades pertinent to SERS, especially those contributing to our current understanding of SP-related SERS. Special emphasis is given to the salient strategies and theoretical approaches for optimizing nanostructures with hotspots as efficient EM near-field concentrating and far-field radiating substrates for SERS. A simple model is described in terms of which the upper limit of the SERS enhancement can be estimated. Several experimental strategies that may allow one to approach, or possibly exceed this limit, such as cascading the enhancement of the local and radiated EM field by the multiscale EM coupling of hierarchical structures, and generating hotspots by hybridizing an antenna mode with a plasmonic waveguide cavity mode, which would result in an increased local field enhancement, are discussed. Aiming to significantly broaden the application of SERS to other fields, and especially to material science, we consider hybrid structures of plasmonic nanostructures and other material phases and strategies for producing strong local EM fields at desired locations in such hybrid structures. In this vein, we consider some of the numerical strategies for simulating the optical properties and consequential SERS performance of particle-on-substrate systems that might guide the design of SERS-active systems. Finally, some current theoretical attempts are briefly discussed for unifying EM and non-EM contribution to SERS.

Electromagnetic theories of surface-enhanced Raman spectroscopy

The National Institute of Standards and Technology (NIST) and the Pacific Northwest National Laboratory (PNNL) are each creating quantitative databases containing the vapor-phase infrared spectra of pure chemicals. The digital databases have been created with both laboratory and remote-sensing applications in mind. A spectral resolution of approximate, equals 0.1 cm(-1) was selected to avoid degrading sharp spectral features, while also realizing that atmospheric broadening typically limits line widths to 0.1 cm(-1). Calculated positional (wave- number, cm(-1)) uncertainty is /=9) path length-concentration burdens and fitting a weighted Beer's law plot to each wavenumber channel. The two databases include different classes of compounds and were compared using 12 samples. Though these 12 samples span a range of polarities, absorption strengths, and vapor pressures, the data agree to within experimental uncertainties with only one exception.

Gas-Phase Databases for Quantitative Infrared Spectroscopy

Molecules adsorbed on evaporated thin metal films exhibit enormously strong infrared absorption. The thin metal films that exhibit the surface-enhanced infrared absorption (SEIRA) consist of metal particles much smaller than the wavelength of light. Electric field associated with the incident infrared radiation is enhanced via the excitation of localized plasmon of the particles, yielding the absorption enhancement. Preferential orientation and the change in absorption coefficient of molecules caused by chemisorption onto the metal surface provide additional enhancement. Most characteristic observations in SEIRA experiments are well explained by a simple electromagnetic theory. The infrared spectroscopy utilizing the SEIRA effect is promising as a new surface analytical tool. In particular, it is very useful for in situ studies of electrode/electrolyte interfaces. By the combined use of the attenuated-total-reflection technique, reactions and adsorption/desorption of molecules at the interfaces can be inv...

Dynamic Processes in Electrochemical Reactions Studied by Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS)

The problem of nonlinear detector response in FT-IR spectroscopy is discussed. The effects of nonlinearity are demonstrated with the use of synthetic data, and a convenient method for detection of the nonlinear response is given. The interaction between phase correction and nonlinear response is also shown with the use of synthetic data.

Nonlinear Detector Response in FT-IR

Near-infrared Raman spectroscopy has been used to obtain Surface-Enhanced Raman Spectra (SERS) of pyridine on both silver and gold electrodes. The enhancement factor is higher than that found for visible excitation, and the band intensities are somewhat different. There is no indication of photochemical damage or change of the sample when near-infrared radiation is used, even at relatively high power levels.

Surface-Enhanced Raman Spectroscopy in the Near-Infrared

The basic problems of condensed phase infrared emission spectroscopy are discussed. Conditions which cause instrumental background and the method for background removal are reviewed. The sensitivity of the technique is illustrated with spectra of microgram quantities. Spectra are shown for samples near room temperature (30°C) and for a sample below room temperature. The major limitations in infrared emission spectroscopy are reviewed and the problems associated with multiple passing of radiation through the modulator are outlined.

The Sensitivity and Limitations of Condensed Phase Infrared Emission Spectroscopy

Two approaches to phase correction in FT-IR spectroscopy are discussed. Using synthetic data, both the Mertz method and the symmetrization-convolution method are evaluated with respect to the accurate reproduction of a single-beam spectrum. The variation in photometric accuracy with (1) the number of points used in the phase calculation, and (2) apodization in the phase calculation is examined.

Phase Correction in FT-IR

Diffuse reflectance FT-IR spectroscopy is used to obtain infrared spectra of paints directly on paper panels. The binder contribution to the spectrum can be effectively eliminated by spectral subtraction and the spectra of photodecomposition products are obtained. Comparison with reference spectra allows the determination of the photodecomposition mechanism.

D. B. Chase

Papers

Nonlinear Detector Response in FT-IR

Surface-Enhanced Raman Spectroscopy in the Near-Infrared

The Sensitivity and Limitations of Condensed Phase Infrared Emission Spectroscopy

Phase Correction in FT-IR

Applications of Diffuse Reflectance FT-IR to Pigment Photodecomposition in Paint