John R. Lombardi

Charge‐transfer theory of surface enhanced Raman spectroscopy: Herzberg–Teller contributions

A comprehensive development of the charge‐transfer theory of surface enhanced Raman scattering (SERS) is presented. We incorporate the Herzberg–Teller mixing of zero‐order Born–Oppenheimer electronic states by means of vibronic interaction terms in the Hamiltonian. This is similar to the theory of Tang and Albrecht12 except that we include metal states as part of a molecule–metal system. When this is done we may no longer discard a term involving mixing of ground‐state vibrations. The theory is comprehensive in that both molecule‐to‐metal and metal‐to‐molecule transfer is considered. Furthermore, both Franck–Condon and Herzberg–Teller contributions to the intensity are obtained. The former, however, contribute only to the intensity of totally symmetric vibrations, while the latter contribute to nontotally symmetric vibrations as well. Since overtones are observed in SERS only weakly if at all, the Herzberg–Teller terms are most consistent with experimental findings. The resulting formulas may be interpreted as a type of resonance Raman effect in which intensity for the charge transfer transitions is borrowed from an allowed molecular transition. We may also carry out the sum over metal states. This procedure predicts a logarithmic resonance at the Fermi level of the metal. We thus predict intensity vs voltage profiles such as I ∝ ‖ln(ωFI−ω+iΓ)‖2 which fits the experimental curves quite well.

A Unified View of Surface-Enhanced Raman Scattering

In the late 1970s, signal intensity in Raman spectroscopy was found to be enormously enhanced, by a factor of 10(6) and more recently by as much as 10(14), when an analyte was placed in the vicinity of a metal nanoparticle (particularly Ag). The underlying source of this huge increase in signal in surface-enhanced Raman scattering (SERS) spectroscopy has since been characterized by considerable controversy. Three possible contributions to the enhancement factor have been identified: (i) the surface plasmon resonance in the metal nanoparticle, (ii) a charge-transfer resonance involving transfer of electrons between the molecule and the conduction band of the metal, and (iii) resonances within the molecule itself. These three components are often treated as independently contributing to the overall effect, with the implication that by properly choosing the experimental parameters, one or more can be ignored. Although varying experimental conditions can influence the relative degree to which each resonance influences the total enhancement, higher enhancements can often be obtained by combining two or more resonances. Each resonance has a somewhat different effect on the appearance of the resulting Raman spectrum, and it is necessary to invoke one or more of these resonances to completely describe a particular experiment. However, it is impossible to completely describe all observations of the SERS phenomenon without consideration of all three of these contributions. Furthermore, the relative enhancements of individual spectral lines, and therefore the appearance of the spectrum, depend crucially on the exact extent to which each resonance makes a contribution. In this Account, by examining breakdowns in the Born-Oppenheimer approximation, we have used Herzberg-Teller coupling to derive a single expression for SERS, which includes contributions from all three resonances. Moreover, we show that these three types of resonances are intimately linked by Herzberg-Teller vibronic coupling terms and cannot be considered separately. We also examine the differences between SERS and normal Raman spectra. Because of the various resonant contributions, SERS spectra vary with excitation wavelength considerably more than normal Raman spectra. The relative contributions of totally symmetric and non-totally symmetric lines are also quite different; these differences are due to several effects. The orientation of the molecule with respect to the surface and the inclusion of the metal Fermi level in the list of contributors to the accessible states of the molecule-metal system have a strong influence on the observed changes in the Raman spectrum.

Identification of organic colorants in fibers, paints, and glazes by surface enhanced Raman spectroscopy.

Organic dyes extracted from plants, insects, and shellfish have been used for millennia in dyeing textiles and manufacturing colorants for painting. The economic push for dyes with high tinting strength, directly related to high extinction coefficients in the visible range, historically led to the selection of substances that could be used at low concentrations. But a desirable property for the colorist is a major problem for the analytical chemist; the identification of dyes in cultural heritage objects is extremely difficult. Techniques routinely used in the identification of inorganic pigments are generally not applicable to dyes: X-ray fluorescence because of the lack of an elemental signature, Raman spectroscopy because of the generally intense luminescence of dyes, and Fourier transform infrared spectroscopy because of the interference of binders and extenders. Traditionally, the identification of dyes has required relatively large samples (0.5-5 mm in diameter) for analysis by high-performance liquid chromatography. In this Account, we describe our efforts to develop practical approaches in identifying dyes in works of art from samples as small as 25 microm in diameter with surface-enhanced Raman scattering (SERS). In SERS, the Raman scattering signal is greatly enhanced when organic molecules with large delocalized electron systems are adsorbed on atomically rough metallic substrates; fluorescence is concomitantly quenched. Recent nanotechnological advances in preparing and manipulating metallic particles have afforded staggering enhancement factors of up to 10(14). SERS is thus an ideal technique for the analysis of dyes. Indeed, rhodamine 6G and crystal violet, two organic compounds used to demonstrate the sensitivity of SERS at the single-molecule level, were first synthesized as textile dyes in the second half of the 19th century. In this Account, we examine the practical application of SERS to cultural heritage studies, including the selection of appropriate substrates, the development of analytical protocols, and the building of SERS spectral databases. We also consider theoretical studies on dyes of artistic interest. Using SERS, we have successfully documented the earliest use of a madder lake pigment and the earliest occurrence of lac dye in European art. We have also found several examples of kermes and cochineal glazes, as well as madder, cochineal, methyl violet, and eosin lakes, from eras ranging from ancient Egypt to the 19th century. The ability to rapidly analyze very small samples with SERS makes it a particularly valuable tool in a museum context.

The effect of molecular structure on voltage induced shifts of charge transfer excitation in surface enhanced Raman scattering

The Raman intensity versus electrode voltage behavior of a series of substituted pyridines and saturated nitrogen heterocycUc compounds was studied at two laser excitation energies. The voltage (resonance) maximum shifts to more positive potential with increasing excitation energy for the substituted pyridines (case I) and to more negative potential with increasing excitation energy for the saturated nitrogen heterocyclic compounds (case II). The voltage maxima can be correlated with the Hammett sigma function for substituted pyridines and with pK a for the saturated nitrogen heterocycles. The results are consistent with charge transfer from the metal to molecule (adcluster-molecule complex) in the case I system and from the molecule (adcluster-molecule complex) to the metal in the case II system.

Dipole Moments of the Lowest Singlet π* ← π States in Phenol and Aniline by the Optical Stark Effect

The Stark effect in the rotational fine structure is used to measure the change in dipole moment on excitation in aniline and phenol. The spectra are in the 0–0 bands of the lowest‐lying singlet π* ← π states. The results are | Δμ | = 0.85 ± 0.15 D for aniline and | Δμ | = 0.20 ± 0.20 D for phenol. Evidence is presented which indicates that the sign of Δμ is positive. If this is true, these results correlate well with the structural changes on excitation, namely, that the contraction along the in‐plane long axis is considerably greater in aniline than phenol. This correlation is strong indication of the extent of contribution of quinoidlike resonance structures to the electronic nature of the excited state.

Enhanced Raman Scattering with Dielectrics

Dielectrics represent a new frontier for surface-enhanced Raman scattering. They can serve as either a complement or an alternative to conventional, metal-based SERS, offering key advantages in terms of low invasiveness, reproducibility, versatility, and recyclability. In comparison to metals, dielectric systems and, in particular, semiconductors are characterized by a much greater variety of parameters and properties that can be tailored to achieve enhanced Raman scattering or related effects. Light-trapping and subwavelength-focusing capabilities, morphology-dependent resonances, control of band gap and stoichiometry, size-dependent plasmons and excitons, and charge transfer from semiconductors to molecules and vice versa are a few examples of the manifold opportunities associated with the use of semiconductors as SERS-active materials. This review provides a broad analysis of SERS with dielectrics, encompassing different optical phenomena at the basis of the Raman scattering enhancement and introducing future challenges for light harvesting, vibrational spectroscopy, imaging, and sensing.

Growth of tetrahedral silver nanocrystals in aqueous solution and their SERS enhancement.

Silver nanocrystals with tetrahedral shapes and {111} faces have been synthesized by the light-driven growth method in an aqueous solution. The nanocrystals of T(d) symmetry were formed under the effect of tartrate and citrate as the structural-directing reagents at the appropriate stages of reaction. Further, the nanocrystals may be assembled through electrostatic interaction to develop large-scale particle surfaces with sharp vertexes, which can generate strong localized electromagnetic field for surface-enhanced Raman scattering (SERS) studies. Benzenethiol was used as the probe to evaluate their SERS enhancement, and enhancement factors of up to 10(6) are reached. As a kind of promising material, these novel nanocrystals will be applied in surface enhanced spectroscopy and plasmonics field.

Photochemical Modification of an Optical Fiber Tip with a Silver Nanoparticle Film: A SERS Chemical Sensor

We present a method of photochemical modification of an optical fiber tip with a silver nanoparticle film. The deposited silver nanoparticle film displays alternating light and dark circles, which are similar to a radial diffraction pattern. The modified optical fiber is examined as a chemical sensor for in situ detection. The modified fibers show excellent SERS activity, a low limit of detection (LOD), and good reproducibility. The maximum SERS activity of the sensor was achieved within 5.0 min of deposition. Thus, the method is also quite rapid.

ZnO nanoparticle size-dependent excitation of surface Raman signal from adsorbed molecules : Observation of a charge-transfer resonance

By observing the Raman intensity of several molecules (4-mercaptopyridine and 4-mercaptobenzoic acid) adsorbed on the surface of ZnO nanocrystals as a function of particle size in the range of 18–31nm diameter, we show that a large increase in intensity is observed near 28nm. This is an indication of a size-dependent charge-transfer resonance. We interpret this effect as due to formation of a charge-transfer complex between a surface-bound exciton and the adsorbed molecule.

Time-dependent picture of the charge-transfer contributions to surface enhanced Raman spectroscopy

We reexamine the Herzberg-Teller theory of charge-transfer contributions to the theory of surface enhanced Raman scattering (SERS). In previous work, the Kramers-Heisenberg-Dirac framework was utilized to explain many of the observed features in SERS. However, recent experimental and theoretical developments suggest that we revise the theory to take advantage of the time-dependent picture of Raman scattering. Results are obtained for molecular adsorption on nanoparticles in both the strong confinement limit and the weak confinement limit. We show that the Herzberg-Teller contributions to the charge-transfer effect in SERS display a resonance at the molecule-to-metal or metal-to-molecule transition while retaining the selection rules associated with normal Raman spectroscopy (i.e., harmonic oscillator, as opposed to Franck-Condon overlaps). The charge-transfer contribution to the enhancement factor scales as Gamma(-4), where Gamma is the homogeneous linewidth of the charge-transfer transition, and thus is extremely sensitive to the magnitude of this parameter. We show that the Herzberg-Teller coupling term may be associated with the polaron-coupling constant of the surface phonon-electron interaction. A time-dependent expression for the Raman amplitude is developed, and we discuss the implications of these results for both metal and semiconductor nanoparticle surfaces.