Raman scattering from Pt(CN)42− adsorbed on Pt colloids

Raman scattering from Pt(CN)₄^2? adsorbed on Pt colloids (average diameter of 16 A) is compared with the Raman signal from the same amount of Pt(CN)₄^2? in solution. The wavelength dependence of the adsorbate Raman intensity is measured from 308 to 647 nm and compared with a Lorenz/Mie calculation of the enhancement factor for the electromagnetic field intensity on the surface of a Pt sphere as a function of sphere radius and incident wavelength.     

Enhancement of Raman scattering for an atom or molecule near a metal nanocylinder: quantum theory of spontaneous emission and coupling to surface plasmon modes

An analytic expression for the electromagnetic enhancement of the spontaneous emission rate and Raman scattering cross-section for an excited atom or molecule in close proximity to a metal nanocylinder has been derived by quantum theory. Coupling of the atomic or molecular optical radiation into the TM0 surface plasmon mode of the nanocylinder results in reradiation by the cylinder, a process that is most efficient when the incident radiation is linearly polarized, with the electric field oriented parallel to the axis of the nanocylinder. For a silver cylinder having a radius and length of 5 and 20 nm, respectively, the enhancement in the spontaneous emission rate is >10(7) for variant Planck's over 2pi omega0 approximately 2.4 eV (lambda=514 nm), which corresponds to an increase of approximately 10(14) in the Raman scattering cross section. This result, as well as the prediction that the atomic dipole generates broadband, femtosecond pulses, are in qualitative agreement with previously reported experiments involving metal nanoparticle aggregates. The theoretical results described here are expected to be of value in guiding future nonlinear optical experiments in which carbon nanotubes or metal nanowires with controllable physical and electrical characteristics are patterned onto a substrate and coupled with emitting atoms or molecules.     

Internal electric field densities of metal nanoshells

The internal field patterns for gold shells filled with the same material as the surrounding medium are calculated with Mie theory and in the quasistatic approximation and their properties compared to the response of homogeneous spheres and metallic rings. One major difference between the sphere and shell case is that the areas of highest field enhancement in metallic shells are located perpendicular to the incident polarization, whereas for metallic spheres they are along the polarization direction. Recent results based on the discrete dipole approximation (DDA) are shown to be misleading, which might be due to the use of a too coarse grid size. We also show that the type of resonance and the associated internal field pattern (low or high energy) has a strong impact on the external fields.     

Probing the Raman scattering tensors of individual molecules

Single-molecule experiments provide new views into the mechanisms behind surface-enhanced Raman scattering. It was shown previously that spectra of individual rhodamine 6G molecules adsorbed on silver nanocrystal aggregates present stronger fluctuations in two low-frequency bending modes, at 614 and 773 cm(-1). Here we use polarization spectroscopy to show that these bands are enhanced by a resonant process whose transition dipole is rotated by 15+/-10 degrees with respect to the molecular transition dipole. We also show that the polarization function remains stable over the whole time scale of a measurement, indicating that molecular reorientation with respect to the surface is unlikely. Together these findings provide further support to the involvement of a charge-transfer resonance in the enhancement of the low-frequency bands and allow us to suggest a model for the orientation of rhodamine 6G molecules at Raman hot spots.     

Effect of sample and substrate electric properties on the electric field enhancement at the apex of SPM nanotips

Finite element (FE) models were built to define the optimal experimental conditions for tip-enhanced Raman spectroscopy (TERS) of thin samples. TERS experimental conditions were mimicked by including in the FE models dielectric or metallic substrates with thin dielectric samples and by considering the wavelength dependence of the dielectric properties for the metallic materials. Electromagnetic coupling between the substrate/sample and the SPM tips led to dramatic changes of both the spatial distribution and magnitude of the scattered electric field which depended on the substrate dielectric permittivity and excitation wavelength. Raman scattering as high as 10(8) with a spatial resolution of approximately 8 nm was estimated for gold SPM tips and gold substrate when excitation is performed at 532 nm (near-resonance wavelength). For dielectric samples (approximately 4 nm thick), the enhancement of Raman scattering intensity is estimated at approximately 10(5); this does not depend significantly on the sample dielectric permittivity for dielectric samples. These results suggest that TERS experimental conditions should be estimated and optimized for every individual application considering the geometric factors and electric properties of the materials involved. Such optimizations could enlarge the range of applications for TERS to samples eliciting weaker intrinsic Raman scattering, such as biological samples.     

Enhanced Raman Scattering by ZnO Superstructures: Synergistic Effect of Charge Transfer and Mie Resonances

A remarkable enhancement of Raman scattering is achieved by submicrometer‐sized spherical ZnO superstructures. The secondary superstructures of ZnO particles with a uniform diameter in the range of 220–490 nm was formed by aggregating ca. 13 nm primary single crystallites. By engineering the superstructure size to induce Mie resonances, leading to an electromagnetic contribution to the SERS enhancement. Meanwhile, a highly efficient charge‐transfer (CT) contribution derived from the primary structure of the ZnO nanocrystallites was able to enhance the SERS signals as well. The highest Raman enhancement factor of 10⁵ was achieved for a non‐resonant molecule by the synergistic effect of CT and Mie resonances. The Mie resonances scattered near‐field effect investigated in the present study provides not only an important guide for designing novel SERS‐active semiconductor substrates, but also a coherent framework for modelling the electromagnetic mechanism of SERS on semiconductors.     

A quantum mechanical theory for single molecule-single nanoparticle surface enhanced Raman scattering

Single molecule-single nanoparticle surface enhanced Raman scattering event is analyzed using a quantum mechanical approach, resulting in an analytical expression for the electromagnetic enhancement factor that succinctly elucidates the fundamental aspects of SERS. The nanoparticle is treated as a dielectric spherical cavity, and the resulting increase in the spontaneous emission rate of a molecule adsorbed onto the surface of the nanoparticle is examined. The overall enhancement in Raman scattering is due to both the increased local electromagnetic field and the Purcell effect. The predictions of the present model are in agreement with the simulation results of the classical model.     

Surface enhanced Raman optical activity of molecules on orientationally averaged substrates: theory of electromagnetic effects

We present a model for electromagnetic enhancements in surface enhanced Raman optical activity (SEROA) spectroscopy. The model extends previous treatments of SEROA to substrates, such as metal nanoparticles in solution, that are orientationally averaged with respect to the laboratory frame. Our theoretical treatment combines analytical expressions for unenhanced Raman optical activity with molecular polarizability tensors that are dressed by the substrate's electromagnetic enhancements. We evaluate enhancements from model substrates to determine preliminary scaling laws and selection rules for SEROA. We find that dipolar substrates enhance Raman optical activity (ROA) scattering less than Raman scattering. Evanescent gradient contributions to orientationally averaged ROA scale to first or higher orders in the gradient of the incident plane-wave field. These evanescent gradient contributions may be large for substrates with quadrupolar responses to the plane-wave field gradient. Some substrates may also show a ROA contribution that depends only on the molecular electric dipole-electric dipole polarizability. These conclusions are illustrated via numerical calculations of surface enhanced Raman and ROA spectra from (R)-(-)-bromochlorofluoromethane on various model substrates.     

One-dimensional arrays of nanoshell dimers for single molecule spectroscopy via surface-enhanced Raman scattering

The optical properties of one-dimensional arrays of metal nanoshell dimers are studied systematically using the T-matrix method based on Mie theory, within the context of surface enhanced Raman scattering (SERS). It is shown that the local electromagnetic enhancement can be as high as approximately 4.5 x 10(13) for nanoshell dimer arrays with optimal geometry, and sensitive tunability in the resonant frequency can be gained by varying the geometrical parameters, making such structures appealing templates for SERS measurements with single molecule sensitivity. The extraordinarily high enhancement is attributed to a collective photonic effect constructively superposed onto the intrinsic enhancement associated with an isolated nanoshell dimer.     

密度泛函理论研究黄曲霉素B1的表面增强拉曼散射效应

用密度泛函理论B3LYP方法和6—311G（d,p）／Lan12DZ优化得到黄曲霉素B1（AFBI）分子及其复合物AFB1-Ag的稳定结构,并计算了复合物的表面增强拉曼光谱和预共振拉曼光谱．结果表明,AFB1分子的拉曼光谱很大程度依赖于吸附位点以及入射光的激发波长．与分子的常规拉曼光谱相比,复合物表面增强拉曼光谱中C=O伸缩振动模的增强因子约为10^2—10^3,是由于复合物的极化率增强而导致的静态化学增强,并分析了振动模式的振动方向与其拉曼强度的关系．选择复合物最大吸收峰附近激发光266和482nm以及远离共振吸收波长785和1064nm作为入射光,计算得到不同入射光激发下复合物的预共振拉曼光谱．结果表明其增强因子最大达N100量级,主要是由电荷转移产生的共振增强引起的．     

Characterization of the surface enhanced raman scattering (SERS) of bacteria

The surface enhanced Raman scattering (SERS) of a number of species and strains of bacteria obtained on novel gold nanoparticle (approximately 80 nm) covered SiO(2) substrates excited at 785 nm is reported. Raman cross-section enhancements of >10(4) per bacterium are found for both Gram-positive and Gram-negative bacteria on these SERS active substrates. The SERS spectra of bacteria are spectrally less congested and exhibit greater species differentiation than their corresponding non-SERS (bulk) Raman spectra at this excitation wavelength. Fluorescence observed in the bulk Raman emission of Bacillus species is not apparent in the corresponding SERS spectra. Despite the field enhancement effects arising from the nanostructured metal surface, this fluorescence component appears "quenched" due to an energy transfer process which does not diminish the Raman emission. The surface enhancement effect allows the observation of Raman spectra of single bacterial cells excited at low incident powers and short data acquisition times. SERS spectra of B. anthracis Sterne illustrate this single cell level capability. Comparison with previous SERS studies reveals how the SERS vibrational signatures are strongly dependent on the morphology and nature of the SERS active substrates. The potential of SERS for detection and identification of bacterial pathogens with species and strain specificity on these gold particle covered glassy substrates is demonstrated by these results.     

Surface-enhanced Raman scattering from silver-coated opals

We describe surface-enhanced Raman scattering measurements from a benzenethiol monolayer adsorbed on a silver-coated film that is, in turn, deposited on an artificial opal, where the latter is a close-packed three-dimensional dielectric lattice formed from polystyrene spheres. Data for a range of sphere sizes, silver film thicknesses, and laser excitation wavelengths are obtained. Enhancement factors can be in the range of 10(7). To partially explain these large enhancements, we have performed model finite-difference time domain simulations of the position-dependent electric fields generated at the opal surfaces for several experimentally studied laser wavelengths and sphere diameters.     

Surface-enhanced Raman scattering on nanoshells with tunable surface plasmon resonance

Fabrication, characterization, and optical enhancement applications of bimetallic AgAu nanoparticles and nanoshells are reported. Nanoparticles with tunable surface plasmon resonances are synthesized at room temperature and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and photon correlation spectroscopy. The collective electron oscillation of the nanoparticles shows a controllable tunability in the 400-990 nm spectral range, in agreement with plasmon absorption calculated using Mie theory, providing an optimum substrate for surface plasmon-assisted enhanced spectroscopy. Surface-enhanced Raman scattering experiments show that the average enhancement factor obtained with nanoshells could be higher than those obtained with silver sols.     

Electromagnetic model and calculations of the surface-enhanced Raman-shifted emission from Langmuir-Blodgett films on metal nanostructures

We present a theoretical study of the electromagnetic contribution to surface-enhanced Raman scattering (SERS) from a Langmuir-Blodgett film close to a metal surface. This macroscopic dipolar model fully accounts for the Raman-shifted emission so that meaningful SERS (electromagnetic) enhancement factors that do not depend only on the local electromagnetic field enhancement at the pump frequency are defined. For a plane metal surface, analytical SERS enhancement factors that are consistent for all pump beam polarization and molecular orientation are obtained. In order to investigate SERS on complex nanostructured metal surfaces, we introduce this model into the formally exact, Green's theorem surface integral equation formulation of the scattered electromagnetic field. This formulation is thus employed to calculate numerically the near-field and far-field emissions at the Raman-shifted frequency for very rough, random nanostructured surfaces, with emphasis on the impact of collective processes for varying pump frequency and Raman shift. Our results reveal that the widely used |E|4 approximation tends to overestimate average SERS enhancement factors.     

Raman enhancement factor of a single tunable nanoplasmonic resonator

We have developed a novel technique to precisely determine the Raman enhancement factor in single nanoplasmonic resonators (TNPRs). TNPRs are lithographically defined metallodielectric nanoparticles composed of two silver disks stacked vertically, separated by a silica layer. At resonance, the local electromagnetic fields are enhanced at the TNPR surface, making it an ideal surface-enhanced Raman scattering (SERS) active substrate. The ability to control the dimensions of the metallic and dielectric layers offers the unique advantage of fine-tuning the plasmon resonance frequency to maximize the enhancement of the Raman signal. Furthermore, by selective shielding of the outer surface of the metallic structure, the efficiency can be further enhanced by guiding the molecular assembly to the locations that exhibit strong electromagnetic fields. We experimentally demonstrate SERS enhancement factors of (6.1+/-0.3)x10(10), with the highest enhancement factor being achieved by using an individual nanoparticle. By using nanofabrication techniques, we eliminate the issues such as large size variations, cluster aggregation, and interparticle effects common in preparing SERS substrates using conventional chemical synthesis or batch fabrication methods. TNPRs produce very controllable and repeatable SERS signals at the desired locations and, thus, make an ideal candidate for device integration.     

The interaction between an oscillating dipole and a metal surface described by a jellium model and the random phase approximation

The electric field acting on an oscillating dipole located near a metal surface is computed. The latter is described by a jellium model and the random phase approximation. The aim is ascertaining to what extent spatial dispersion and variation of the dielectric properties across the interface modify the image dipole. Implications for surface Raman and fluorescence spectroscopy are discussed.     

Detection of Plasmon Coupling between Silver Nanowires Based on Hyperspectral Darkfield and SERS Imaging and Supported by DDA Theoretical Calculations

We report the unprecedented observation of plasmon coupling between silver nanowires, showing how the surface‐enhanced Raman scattering depends upon this interaction and how the spectrum can be shaped by the hot spot. Such observations were accomplished by Raman spectroscopy mapping of silver nanowires modified with rhodamine. The local spectra on the hot spots were measured by darkfield hyperspectral microscopy, a powerful but uncommonly used technique that is capable of determining the location, structure, and spectra of the hot spots. The result obtained by the simulation of two parallel nanowires based on the discrete dipole approximation (DDA) method was in excellent agreement with the results obtained experimentally.     

Highly sensitive Raman spectroscopy using a gap mode plasmon under an attenuated total reflection geometry

We investigated a gap mode plasmon under an attenuated total reflection (ATR) configuration toward realization of near-field Raman spectroscopy with a single molecule sensitivity and spatial resolution. Additional enhancement in Raman scattering at a nanogap was obtained by a coupling of a propagating surface plasmon (PSP) of Ag films on a prism, and a gap mode between Ag films and Ag nanoparticles (AgNPs). Immobilization of AgNPs on Ag films through thiol-SAM slightly up-shifted the resonance angle of a PSP, which broadened the reflectivity dip owing to an increased out-coupling of a PSP. Raman enhancement factor at a nanogap increased with decreasing surface coverage of AgNPs, albeit the enhancement factor averaged over illuminated area in Ag films decreased, ensuring the largest enhancement factor in tip-enhanced Raman scattering. This is due to increased efficiency for a PSP excitation at lower coverage of AgNPs in consistent with that in theoretical evaluation using finite difference time domain calculations. A gap mode under an ATR configuration was applied to elucidate a plausible photochemical reaction of p-amino thiophenol (PATP) adsorbed on Ag films on a prism. Spectral changes in Raman scattering under laser illumination were observed for PATP with a deuterated amino group, but suppressed by a dimethyl amino group owing to steric hindrance, supporting the photochemical dimerization.     

Efficient immobilization of silver nanoparticles on metal substrates through various thiol molecules to utilize a gap mode in surface enhanced Raman scattering

To utilize a gap mode in surface enhanced Raman scattering, we elucidated the interaction between adsorbed species and Ag nanoparticles (AgNPs). Various thiol molecules such as normal alkanethiols, thiols with a phenyl, cyclohexane or naphthalene ring on Ag films immobilized AgNPs through van der Waals force, and electrostatic interaction. Immobilized AgNPs provided enormous Raman enhancement by a factor of 10⁷–10¹⁰ for thiol molecules at a nanogap, in consistent with that anticipated by finite difference time domain calculations. Only alkanethiols with a tert-methyl group and those with a carboxylic group did not immobilize any AgNPs probably owing to steric hindrance. A gap mode is relevant for a variety of metals even with large damping like Pt and Fe, indicating a crucial role of electric multipoles in AgNPs generated by a localized surface plasmon and induced mirror images in metal substrates for markedly enhanced electric field at a nanogap.