Metal–adsorbate hybridized electronic states and their impact on surface enhanced Raman scattering

Electromagnetic interactions of dye molecules surrounding a nanosphere

Enhanced interaction between light and molecules adsorbed on metallic nanoparticles is a cornerstone of plasmonics and surface-enhanced spectroscopies. Recent experimental access to the electronic absorption spectrum of dye molecules on silver colloids at low molecular coverage has revealed subtle changes in the spectral shape that may be attributed to a combination of factors, from a chemical modification of the molecule in contact with a metal surface to electromagnetic dye-dye and dye-metal interactions. Here we develop an original model to rigorously address the electromagnetic effects. The dye molecules are described as coupled anisotropic polarisable dipoles and their interaction with the core metal particle is described using a generalised Mie theory. The theory is readily amenable to numerical implementation and yields far-field optical cross-sections that can be compared to experimental results. We apply this model to specific adsorption geometries of practical interest to highlight the effect of molecular orientation on predicted spectral shifts and enhancement factors, as a function of surface coverage. These are compared to experimental results and reproduce the measured spectral changes as a function of concentration. These results have direct implications for the interpretation of surface selection rules and enhancement factors in surface-enhanced spectroscopies, and of orientation and coverage effects in molecular/plasmonic resonance coupling experiments.

Photocatalytic Activity and Fluorescence of Gold/Zinc Oxide Nanoparticles Formed by Dithiol Linking

Monolayer-protected gold nanoparticles (AuNPs) with average diameters of 2-4 nm have been covalently attached to zinc oxide nanorods using dithiol ligands. Electron microscopy and Raman spectroscopy show that ozone treatment or annealing at 300 or 450 °C increases the average diameter of the AuNPs to 6, 8, and 14 (±1) nm, respectively, and decomposes the organic layers to various degrees. These treatments locate the AuNPs closer to the nanorods. Heating and subsequent ozone exposure changes the color of the as-prepared nanocomposite powder from blue to purple due to oxidation of the outer layer of the AuNPs, and heating to 300 °C changes it to pink due to oxygen desorption. ZnO nanorods have a bimodal photoluminescence spectrum that consists of an ultraviolet excitonic peak and a visible, surface defect-related peak. Ozone treatment and annealing of the nanocomposite decreases the intensities of both peaks due to quenching by the AuNPs, but the visible peak is affected less. The photocatalytic efficiency of the nanocomposites toward oxidative degradation of rhodamine B has been measured and follows the order 300 °C > 450 °C > ozone treated ≈ as-prepared ≈ bare ZnO. The greater efficiency of the annealed samples likely arises from decreased electron-hole pair recombination rates.

Small-molecule detection in thiol-yne nanocomposites via surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is generally performed on planar surfaces, which can be difficult to prepare and may limit the interaction of the sensing surface with targets in large volume samples. We propose that nanocomposite materials can be configured that both include SERS probes and provide a high surface area-to-volume format, i.e., fibers. Thiol-yne nanocomposite films and fibers were fabricated using exposure to long-wave ultraviolet light after the inclusion of gold nanoparticles (AuNPs) functionalized with thiophenol. A SERS response was observed that was proportional to the aggregation of the AuNPs within the polymers and the amount of thiophenol present. Overall, this proof-of-concept fabrication of SERS active polymers indicated that thiol-yne nanocomposites may be useful as durable film or fiber SERS probes. Properties of the nanocomposites were evaluated using various techniques including UV-vis spectroscopy, μ-Raman spectroscopy, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and transmission electron microscopy.

A metal-dielectric-graphene sandwich for surface enhanced Raman spectroscopy

The Raman intensity of Rhodamine B (RhB) is enhanced by inserting a thin high κ dielectric layer which reduces the surface plasmon damping at the gold-graphene interface. The results indicate that the Raman intensity increases sharply by plasmonic resonance enhancement while maintaining efficient fluorescence quenching with optimized dielectric layer thickness.

Tuning and maximizing the single-molecule surface-enhanced Raman scattering from DNA-tethered nanodumbbells

We extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, interparticle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. Two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various interparticle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed interparticle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm interparticle gap, an EF value of as high as 5.9 × 10(13) (average value = 1.8 × 10(13)) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 10(12) and 5.9 × 10(13). In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with < 1 nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed.

Mildly reduced graphene oxide-Ag nanoparticle hybrid films for surface-enhanced Raman scattering

Large-area mildly reduced graphene oxide (MR-GO) monolayer films were self-assembled on SiO2/Si surfaces via an amidation reaction strategy. With the MR-GO as templates, MR-GO-Ag nanoparticle (MR-GO-Ag NP) hybrid films were synthesized by immersing the MR-GO monolayer into a silver salt solution with sodium citrate as a reducing agent under UV illumination. SEM image indicated that Ag NPs with small interparticle gap are uniformly distributed on the MR-GO monolayer. Raman spectra demonstrated that the MR-GO monolayer beneath the Ag NPs can effectively quench the fluorescence signal emitted from the Ag films and dye molecules under laser excitation, resulting in a chemical enhancement (CM). The Ag NPs with narrow gap provided numerous hot spots, which are closely related with electromagnetic mechanism (EM), and were believed to remarkably enhance the Raman signal of the molecules. Due to the co-contribution of the CM and EM effects as well as the coordination mechanism between the MR-GO and Ag NPs, the MR-GO-Ag NP hybrid films showed more excellent Raman signal enhancement performance than that of either Ag films or MR-GO monolayer alone. This will further enrich the application of surface-enhanced Raman scattering in molecule detection.

Large-area plasmonic hot-spot arrays: sub-2 nm interparticle separations with plasma-enhanced atomic layer deposition of Ag on periodic arrays of Si nanopillars

Initial reports of plasmonic 'hot-spots' enabled the detection of single molecules via surface-enhanced Raman scattering (SERS) from random distributions of plasmonic nanoparticles. Investigations of systems with near-field plasmonically coupled nanoparticles began, however, the ability to fabricate reproducible arrays of such particles has been lacking. We report on the fabrication of large-area, periodic arrays of plasmonic 'hot-spots' using Ag atomic layer deposition to overcoat Si nanopillar templates leading to reproducible interpillar gaps down to <2 nm. These plasmonic 'hot-spots' arrays exhibited over an order of magnitude increase in the SERS response in comparison to similar arrays with larger interpillar separations.

Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors

Efforts to create reproducible surface-enhanced Raman scattering (SERS)-based chemical and biological sensors has been hindered by difficulties in fabricating large-area SERS-active substrates with a uniform, reproducible SERS response that still provides sufficient enhancement for easy detection. Here we report on periodic arrays of Au-capped, vertically aligned silicon nanopillars that are embedded in a Au plane upon a Si substrate. We illustrate that these arrays are ideal for use as SERS sensor templates, in that they provide large, uniform and reproducible average enhancement factors up to ∼1.2 × 10(8) over the structure surface area. We discuss the impact of the overall geometry of the structures upon the SERS response at 532, 633, and 785 nm incident laser wavelengths. Calculations of the electromagnetic field distributions and intensities within such structures were performed and both the wavelength dependence of the predicted SERS response and the field distribution within the nanopillar structure are discussed and support the experimental results we report.

Studying surface chemistry beyond the diffraction limit: 10 years of TERS

The use of an illuminated scanning probe tip to greatly enhance Raman scattering from the sample underneath the tip is one of the most intriguing developments in optical spectroscopy, and the steeply increasing number of publications per year shows that chemists, physicists and biologists alike recognize the importance and great potential of this technique. With tip-enhanced Raman spectroscopy (TERS), one of the main goals in surface science has been achieved, namely the combination of scanning probe microscopy and optical spectroscopy such as Raman spectroscopy. Important here is the use of the tip as an optical antenna to substantially increase the emitted radiation and to simultaneously improve the optical resolution much beyond the Abbe diffraction limit. This permits the correlation of topographic and chemical information of the same surface region. The synergy of detailed insight in morphology and the chemical nature of the target species facilitates data interpretation significantly and enables characterization of interfaces at the nanometer scale. A wide variety of substrates and sample molecules have been studied with TERS since the first publication of tip-enhanced Raman spectra, and the technique has reached a first level of maturity on its 10th birthday, with TERS applications extending into various research fields from surface chemistry over biology to nanoscale physics.