Probing Photoinduced Charge Transfer at Atomically Smooth Metal Surfaces Using Surface Enhanced Raman Scattering

Recent advances and perspectives in photo-induced enhanced Raman spectroscopy

Phototreatment is at the leading edge of a research hot topic as a driving force for structural transformation, spectral and electromagnetism improvements, and the functional performance of nanomaterials. Light irradiation can excite surface plasmons in noble metal nanoparticles, create electron-hole pairs, and produce charge transfer in semiconductor substrates, which have led to it being widely used in surface-enhanced Raman spectroscopy (SERS) for life sciences, environmental protection, and biological analysis. Photo-induced enhanced Raman spectroscopy (PIERS) is a new technology developed on the basis of traditional SERS and has proven to be an efficient way to resolve several critical challenges thanks to its incomparable superiority for incontiguous operation, efficient charge separation and enrichment, and a large signal enhancement for a wide range of biomolecules at the trace level. This makes PIERS a powerful technique with very appealing and promising applications in various branches of analytical science. In this review, the enhancement mechanisms of PIERS are analyzed in comparison with SERS. Afterward, the parameters influencing the enhancement of PIERS, including the substrate, light irradiation, and relaxation are discussed in detail. Finally, some perspectives on further developments of PIERS are exemplified. The PIERS technique will continue to evolve and grow with new developments and its successful application in bioanalysis and life sciences.

Invited review article: combining scanning probe microscopy with optical spectroscopy for applications in biology and materials science

This is a comprehensive review of the combination of scanning probe microscopy (SPM) with various optical spectroscopies, with a particular focus on Raman spectroscopy. Efforts to combine SPM with optical spectroscopy will be described, and the technical difficulties encountered will be examined. These efforts have so far focused mainly on the development of tip-enhanced Raman spectroscopy, a powerful technique to detect and image chemical signatures with single molecule sensitivity, which will be reviewed. Beyond tip-enhanced Raman spectroscopy and/or topography measurements, combinations of SPM with optical spectroscopy have a great potential in the characterization of structure and quantitative measurements of physical properties, such as mechanical, optical, or electrical properties, in delicate biological samples and nanomaterials. The different approaches to improve the spatial resolution, the chemical sensitivity, and the accuracy of physical properties measurements will be discussed. Applications of such combinations for the characterization of structure, defects, and physical properties in biology and materials science will be reviewed. Due to the versatility of SPM probes for the manipulation and characterization of small and/or delicate samples, this review will mainly focus on the apertureless techniques based on SPM probes.

Raman enhancement on graphene: adsorbed and intercalated molecular species

Strong Raman scattering is observed from iodine anions adsorbed at ca. 3% coverage on single layer graphene. In addition, the Raman signal from just one bromine intercalation layer inside three and four layer thick graphenes is observed. We analyze and model the intramolecular electronic, charge-transfer, and multiple reflection electromagnetic mechanisms responsible for this unusual sensitivity. Graphene is an excellent Raman substrate for adsorbed species showing intramolecular electronic resonance, because graphene efficiently quenches interfering excited-state luminescence. The Raman sensitivity for adsorbed and intercalated molecular species is highest for single layer graphene and decreases with increasing thickness. These phenomena are compared with surface enhanced Raman spectroscopy field enhancement and "chemical" Raman processes in aggregated Ag particles and on flat, highly reflective metal surfaces. The Raman spectra of adsorbed bromine layers are not observed, despite significant charge transfer to graphene. Charge transfer from adsorbed bromine is about one-half of charge transfer from intercalated bromine. We attribute the large Raman signal for both adsorbed iodine and intercalated bromine species to intramolecular electronic resonance enhancement. The signal evolution with varying graphene thickness is explained by multiple reflection electromagnetic calculations.

Surface-Enhanced Raman Scattering ofp-Aminothiophenol on a Au(core)/Cu(shell) Nanoparticle Assembly

Surface-enhanced Raman scattering (SERS) of p-aminothiophenol (PATP) molecules adsorbed onto assemblies of Au(core)/Cu(shell) nanoparticles is reported. We compare it with the SERS spectrum of PATP adsorbed onto gold nanoparticles: both the absolute and relative scattered intensities of various bands in the two spectra are very different. The difference in relative intensity can be ascribed to chemical effects; the chemical enhancement ratio of the two substrates is approximately 3-5. A theoretical analysis based on a charge-transfer model is carried out, which yields a consistent result and shows that the difference in chemical enhancement is mainly due to the state densities and Fermi levels of the substrates. The difference in absolute intensity originates from electromagnetic (EM) enhancement. EM enhancement of Au(core)/Cu(shell) nanoparticles is unlike that of single-component gold or copper SERS-active substrates. The core/shell particle size for optimal enhancement is about 20 nm in the case of a 632.8 nm incident laser (the size ratio of the core and shell layers is about 0.6).