Chemical effects in surface-enhanced raman scattering: pyridine chemisorbed on silver adatoms on Rh (100)

Surface-enhanced Raman scattering (SERS) has been observed from pyridine chemisorbed on silver adatoms which had been deposited at submonolayer coverage on a Rh (100) substrate under ultrahigh vacuum. The vibrational frequencies measured are characteristic of pyridine chemically adsorbed on the silver adatoms and an enhancement factor of ≈ 15–65 is calculated from the intensity of the scattering. This enhancement factor is in addition to the factor of four electromagnetic enhancement found for flat metal surfaces and therefore provides unambiguous evidence for the existence and magnitude of chemical enhancement in SERS.     

Raman spectra of pyridine adsorbed on a copper electrode

The surface enhanced Raman scattering (SERS) of pyridine adsorbed on a copper electrode was observed with 647.1 nm excitation and the dependence of the Raman intensity as a function of electrode potential was obtained for the most intense bands. The results are compared with reported intensity and frequency values for the silver/pyridine system.     

Enhancement in Raman intensity by use of surface plasmons

The excitation of surface plasmons at silver electrodes by attenuated total reflection allows one to intensify the Raman signal from adsorbed molecules. The enhancement of the Raman intensity as a result of the amplification of the surface electromagnetic field at the metal-electrolyte interface is demonstrated for pyridine on silver.     

Resonance-enhanced Raman scattering by aggregated 2,2′-cyanine on colloidal silver

The Raman scattering spectrum of 2,2′-cyanine on colloidal silver metal particles is discussed. Preliminary assignments of some of the vibrational Raman bands to the motions of specific chromophoric units are presented and multiplet character of some bands is discussed. Enhanced Raman scattering of 2,2′-cyanine occurs when the laser radiation is tuned to the J-aggregate absorption feature at 575 nm. The enhancement in Raman intensity is the result of a diminution of fluorescence intensity, as well as a quantitative increase in Raman scattering intensity, and is distinct from other types of enhancement phenomena (e.g., resonance Raman of monomeric solution dye, and surface-enhanced Raman scattering (SERS)). The resonance Raman enhancement, due to excitation at the frequency corresponding to the J-aggregate absorption, is found to be 2 × 10⁺³.     

Assembly of Multilayered Films on Core Substrate:Preparation and Characterization

Certain colloidal metals such as.silver (Ag), gold (Au) and copper, (Cu), when properly or assembled, display remarkable enhancement effect to the Raman scattering cross section of adsorbed molecules. This surface-enhanced Raman scattering (SERS) phenomenon has found wide applications in the study of interfacial chemical processes and is a potentially non-invasive technique in molecule-specific analysis. However, the SERS activity of metal colloids depends sensitively on both the synthetic method and the aggregation and assembly procedure, making it difficult to develop SERS into a reliable and quantitative analytical technique. To solve this problem, one needs to develop a substrate with a well-defined adsorption area and SERS activity. One approach to achieve this goal is to assemble a monolayer of uniform colloidal metals onto a well-defined secondary substrate. Here we report our effort in assembling monolayers of uniform Au nanoparticles on the well-defined optical-inactive microparticles in a layer-by-layer (LbL) manner and the use of such assembly as SERS-active substrate.     

Chemical reactivity and surface-enhanced raman scattering

A mechanism is proposed for surface-enhanced Raman scattering for pyridine which explains the enhancement, the anodizalion “activation”, the significance of silver, the photographitization of coordinated pyridine and formate, the participation of surface roughness, and the irreversibly held yet liquid-like nature of surface pyridine.     

Surface-enhanced raman scattering from poly(4-vinyl pyridine)

Surface-enhanced Raman scattering (SERS) has been observed for poly(4-vinyl pyridine) absorbed onto silver island films. Bands near 1219 and 1613 cm^?1, which are weak in normal Raman spectra of PVP, are strong in SERS spectra, and the band near 1020 cm^?1, which is the strongest band in the normal spectra, is relatively weak in SERS. The strongest bands in the SERS spectra all belong to the same symmetry species as α_ZZ, implying that the pyridine moieties are adsorbed through the nitrogen atoms with a vertical conformation. The ring breathing mode of the pyridine rings is observed near 1020 cm^?1, a frequency characteristic of pyridinium ions or coordinated pyridine, providing further evidence for adsorption through the nitrogen atoms. Silver catalyzed photooxidation, which can lead to the appearance of artifacts in SERS spectra, particularly of polymers, can be reduced by overcoating SERS samples with thin films of polymers such as poly(methyl methacrylate) that have low Raman scattering cross sections.     

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendon?a et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.     

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.     

Enhanced and normal Raman scattering from pyridine adsorbed on rough and smooth silver electrodes

A multiplex spectrograph has been used to record potential difference and modulation Raman spectra of pyridine adsorbed on silver electrodes in an electrochemical cell. Spectra have been obtained from rough silver surfaces which give SERS and from surfaces where SERS has been diminished by prolonged cathodic polarisation (DSERS). Raman scattering from pyridine at smooth silver surfaces in potassium perchlorate and fluoride solutions has been distinguished from solution scatter by a potential modulation technique. The results show that the enhanced scattering caused by silver atom or cluster sites is respresentative of the surface as a whole as similar Raman spectra are obtained on smooth surfaces at a count rate as low as ?1.4 photons s^?1 (incident laser power 500 mW).Correlation of simultaneous differential capacitance data and “snapshot” SER spectra indicate that pyridine molecules in aqueous chloride ion solutions adsorb on silver in a flat π-bonded configuration at potentials markedly positive to the point of zero charge and exhibit specific reorientation at ?0.3 V and ?0.45 V (vs. SCE) to become N-bonded, perpendicular to the surface. Results also show that the adsorption behaviour of pyridine in chloride and fluoride ion solutions is largely similar.     

Electron-enhanced Raman scattering: a history of its discovery and spectroscopic applications to solution and interfacial chemistry

Raman scattering spectroscopy can be used to distinguish highly similar molecules and obtain useful information on local physical and chemical environments at their functional group levels. However, obtaining a high-quality Raman spectrum requires high-power excitation and a long acquisition time owing to the inherently small Raman scattering cross section, which is problematic in the analyses of living cells and real-time environmental monitoring. Herein, a new Raman enhancement technique, electron-enhanced Raman scattering (EERS), is described in which artificially generated electrons affect the polarizability of target molecular systems and enhance their inherent Raman cross sections. The EERS technique stands in contrast to the well-known SERS technique, which requires roughened metal surfaces. The history of EERS and its spectroscopic applications to aqueous solutions are presented.     

Surface-enhanced Raman scattering measurements on silver nanoparticles covered with differently formed platinum films

Gold and silver electromagnetic nanoresonators covered by a thin layer of platinum are often used to study adsorption of various molecules on “model platinum surfaces” with surface-enhanced Raman scattering (SERS) spectroscopy. In this contribution spectra of pyridine adsorbed on films formed from core–shell Ag@Pt and Ag@Ag–Pt nanoparticles and pure Pt or Ag nanoparticles were measured using a confocal Raman microscope. The SERS spectra of pyridine adsorbed on alloy Ag@Ag–Pt nanoparticles could not be obtained as a linear combination of spectra measured on pure Ag and Pt surfaces. In other words, for silver electromagnetic nanoresonators covered by platinum there is no simple correlation between the “quality” of the deposited Pt layer and the relative intensity of SERS bands characteristic for adsorbate interacting with silver. The SERS spectra accumulated from various places of a film formed from Ag@Pt or Ag@Ag–Pt nanoclusters may differ significantly. Using Ag@Pt nanoparticles with practically negligible amount of Ag on the surface (as per the stripping measurement), it is possible to record SERS spectrum in which the contribution characteristic for pyridine adsorbed on the Ag surface is well visible. It means that, even for macroscopic samples of core–shell Ag–Pt nanoparticles, averaging of many spectra measured at various locations of the sample should be carried out to characterize reliably their properties.     

Chemical interactions of pyridine and cyanide with silver

From a quantitative determination of pyridine and cyanide adsorbed on a silver electrode, by a radiochemical technique, we have shown that the two adsorbate - silver systems are different. After a dissolution - redeposition electrochemical cycle the quantity of pyridine adsorbed depends on the charge transfer. For low charge transfer (<50 mC cm^?2) the quantity increases from three to nine monolayers and depends on the nature of the supporting electrolyte, which suggests the formation of new bonds between pyridine, Ag and the anion of the supporting electrolyte. For high charge transfers the quantity of pyridine increases, the rate of increase depending on the supporting electrolyte (KI>KCl>KClO₄); in our opinion this is due to a trapping of pyridine in the salt formed between the support electrolyte anion and silver. The quantity of pyridine adsorbed at the silver electrode which can be as large as 100 equivalent monolayers can explain part of the enhancement of the Raman signal observed for this system.After a dissolution - redeposition electrochemical cycle the quantity of cyanide adsorbed remains constant, the cyanide - silver system is reversible and the Raman enhancement observed at the rest potential, is due only to Ag-CN interactions.     

Signal limitations in tip-enhanced Raman scattering: the challenge to become a routine analytical technique

The fundamental parameters and limitations that determine the signal strength in tip-enhanced Raman scattering (TERS) are discussed. A semiquantitative analysis of the Raman signal expected in different experimental geometries and with different sample systems is presented, taking into account experimental parameters including Fresnel factor, numerical aperture of the illumination and collection optics, detection efficiency, and the Raman scattering cross section of the material. A top/side-on illumination geometry is essential for the study of nontransparent samples. It can yield the highest signal levels when strong tip–sample coupling using a metallic substrate provides large field enhancement. In contrast, axial/through-sample illumination is limited to transparent sample materials. Although conceptually simpler in experimental implementation and despite high numerical aperture signal collection efficiency, signals are generally weaker due to limited field enhancement. Crystalline solids with small Raman cross sections and dense molecular/biological systems with unavoidable far-field background provide the biggest challenge for TERS analysis yet at the same time hold the most exciting outstanding scientific questions TERS has the potential to answer.

Laser damage depths,carbon film thickness and possible zero metal enhancement in silver/pyridine sers: A scanning electron microscopy study

Surface-enhanced Raman scattering (SERS) by pyridine on anodized silver electrodes emanates from faint but visible carbon-overlayered regions within the laser damage microzone. Scanning electron microscopy reveals that initial silver surface damage consists of a central zone (0.03 mm diameter) of increased roughness surrounded by a halo (0.06 nm diameter) of intermediate roughness. Laser perforation studies on thin (500 nm) film electrodes reveal that the depth of laser damage under typical SERS conditions is ≈1250 silver-atom layers. Optical skin-depth measurements and surface tone comparisons indicate a carbon film depth of ≈300 layers. The implications for possible zero metal enhancement are considered.     

Influence of electric field on SERS: frequency effects, intensity changes, and susceptible bonds

The fundamental mechanism proposed to explain surface-enhanced Raman scattering (SERS) relies on electromagnetic field enhancement at optical frequencies. In this work, we demonstrate the use of microfabricated, silver nanotextured electrode pairs to study, in situ, the influence of low frequency (5 mHz to 1 kHz) oscillating electric fields on the SERS spectra of thiophenol. This applied electric field is shown to affect SERS peak intensities and influence specific vibrational modes of the analyte. The applied electric field perturbs the polar analyte, thereby altering the scattering cross section. Peaks related to the sulfurous bond which binds the molecule to the silver nanotexture exhibit strong and distinguishable responses to the applied field, due to varying bending and stretching mechanics. Density functional theory simulations are used to qualitatively verify the experimental observations. Our experimental and simulation results demonstrate that the SERS spectral changes relate to electric field induced molecular reorientation, with dependence on applied field strength and frequency. This demonstration creates new opportunities for external dynamic tuning and multivariate control of SERS measurements.     

pH-Dependent Surface-Enhanced Raman Scattering of 8-Hydroxy Quinoline Adsorbed on Silver Hydrosol

Surface-enhanced Raman scattering (SERS) of 8-hydroxy quinoline (HQ) adsorbed on silver hydrosols are compared with the FTIR and normal Raman spectrum in the bulk and in solution. Definite evidence of the charge transfer interaction to the overall contribution in the SER enhancement has been reported. The excitation profile study also supports the evidence of a charge transfer interaction. The effect of pH variation on the SER band intensity is explained in terms of chemisorption of the molecule on bare and chlorinated silver surfaces. The apparent enhancement factor calculations of the principal Raman bands indicate that in the surface-adsorbed state, an HQ molecule is oriented neither flat nor vertical to the silver surface but is tilted. Copyright 2000 Academic Press.     

Silver–platinum core–shell nanoparticles for surface-enhanced Raman spectroscopy

Core–shell Ag@Pt nanoparticles have been synthesised by the means of seed-growth reaction including reduction of PtCl₄²⁻ with silver and replacing Ag atoms with Pt. Surface-enhanced Raman scattering (SERS) spectra of pyridine (which gives slightly different spectra when interacting with various metals) adsorbed on synthesised Ag@Pt clusters were measured. SERS measurements have revealed that deposition of the platinum layer causes near elimination of the spectral interferences from pyridine directly interacting with the silver core. The average SERS enhancement factor for pyridine adsorbed on the Ag@Pt clusters was estimated as equal to about 10³–10⁴, significantly higher than the SERS enhancement factor achievable on the pure platinum nanostructures. Using the silver core (instead of the previously used gold cores) allows for measurement of strong SERS spectra on the Pt covered nanostructures for the wider range of the excitation radiation. This procedure of platinum deposition was tested with various silver nanoparticles – produced with borohydride, citrate and citrate/borohydride methods – which substantially differ in size distribution. The application of formed Ag@Pt structures for obtaining intense Raman spectra for molecules adsorbed on only slightly modified platinum surfaces is discussed.     

Enhancement mechanism of SERS from cyanine dyes adsorbed on Ag2O colloids

Surface enhancement mechanism of Raman scattering from molecules adsorbed on silver oxide colloids is reported. Absorption spectra and Raman spectra of the cyanine dye D266 and pyridine molecules adsorbed on Ag₂O colloids, and the influences of S₂O₃²⁻ and OH⁻ on the SERS are studied respectively. The results indicate that ‘chemical' enhancement is dominant in Ag₂O colloidal solution. Surface complexes of adsorbed molecules and small silver ion clusters Ag_n⁺ as the SERS active sites make an important contribution to surface enhanced Raman scattering (SERS). At these active sites, charge transfer between the adsorbed molecules and the small silver ion clusters is the main enhancement origin. The enhancement factor of D266 adsorbed on Ag₂O colloids is theoretically estimated with the excited-state charge transfer model, which is roughly in accordance with the experiments.     

Surface-enhanced Raman scattering in the ultraviolet spectral region: UV-SERS on rhodium and ruthenium electrodes

We report the first observation of surface-enhanced Raman scattering (SERS) excited with ultraviolet (UV) light from transition metal electrodes. Adsorbed pyridine and SCN- on rough rhodium (Rh) and ruthenium (Ru) electrodes, respectively, have been studied using 325 nm laser excitation. In contrast, the best enhancers in the visible and near infrared, silver and gold, do not produce UV-SERS. The experimental data of UV-SERS are in agreement with our preliminary theoretical calculation based on the electromagnetic enhancement mechanism. The enhancement factor is about 2 orders of magnitude for the Rh and Ru electrodes when they are excited at 325 nm.