TDDFT Study of Charge-Transfer Raman Spectra of 4‑Mercaptopyridine on Various ZnSe Nanoclusters as a Model for the SERS of 4‑Mpy on Semiconductors

We have used DFT and TDDFT calculations mostly at the B3LYP/6-31+G­(d) level to investigate the optimized geometry and the normal (static) Raman and preresonance Raman (RR) spectra of Zn_<i>n</i>Se_<i>m</i> nanoclusters with several forms of 4-mercaptopyridine, 4-Mpy, ligated to Zn surface atoms on the nanocluster. Both symmetrical nanoclusters with <i>n</i> = <i>m</i>, Zn₃Se₃, Zn₁₃Se₁₃, and Zn₃₃Se₃₃, and unsymmetrical nanoclusters with <i>m</i> = <i>n</i> – 1, Zn₇Se₆ and Zn₁₃Se₁₂, were studied as the bare cluster and the cluster–ligand complex. The optimized structures show two types of surface bonds are formed for the 4-Mpy anion bound through the thiol end of the molecule. Binding energy calculations and the structures demonstrate that a bridged structure involving a Zn–S–Zn bond forms with 4-Mpy anion on the unsymmetrical clusters and a single Zn–S bonded anion forms on the symmetrical clusters. A pyridine protonated form of 4-Mpy and the disulfide dimer of 4-Mpy were also studied as a Zn₁₃Se₁₃–ligand complex. Normal mode assignments are given for all these molecular forms on the various nanoclusters. Charge-transfer (CT) states of the Zn₁₃Se₁₃–ligand complexes were examined with both B3LYP and CAM-B3LYP and it was concluded that B3LYP is adequate to study pre-RR simulations. All the complexes studied showed several CT states in the first 20 or more excited states and excitation near these CT states gave CT enhancements for the strong bands in the spectra as high as 10⁴ comparable to experimental SERS spectra of 4-Mpy on semiconductor nanoparticles. Both Franck–Condon and Herzberg–Teller types of scattering were found depending on surface geometry and the preresonant CT state. The spectra also show features related to the type of surface bond formed

Size and Wavelength Dependence of the Charge-Transfer Contributions to Surface-Enhanced Raman Spectroscopy in Ag/PATP/ZnO Junctions

By use of surface-enhanced Raman spectroscopy, we observe the degree of charge-transfer for Ag/PATP/ZnO sandwich compounds as a function of both ZnO nanoparticle size and as a function of excitation wavelength. We show that there are several likely charge-transfer resonances. The most obvious is the resonance at particle diameter of 27.7 nm for all wavelengths. In a theoretical study it has been suggested that when there is an electron acceptor on the nanoparticle surface it may form a complex with the semiconductor exciton and that this is most likely the origin of the size-dependent resonance. At the smallest size (18.2 nm) studied here, there is an increase in degree of charge-transfer (relative to adjacent sizes), indicating the possibility of another, lower-lying charge-transfer state, which also could be caused by the acceptor−exciton complex. The other resonance suggested by our data is to higher excitation energy for all particle sizes. It can be seen that the degree of charge-transfer is rising as the excitation wavelength is shortened, indicating an additional charge-transfer resonance in the ultraviolet

Coupled Exciton and Charge-Transfer Resonances in the Raman Enhancement of Phonon Modes of CdSe Quantum Dots (QDs)

We report the observation of the enhancement of a transverse optical (TO) phonon mode of e1 symmetry and a normally forbidden surface optical (SO) phonon mode of b1 symmetry in CdSe quantum dots (QDs) due to adsorption of 4-mercaptopyridine molecules. The former is observed in 3, 4, and 5 nm diameter particles, while the latter is observed only in the 2 nm particles. Maximum enhancement of the phonon modes is obtained through a coupling of the charge-transfer transition and the exciton transition which are in resonance with the laser energy. Selection rules using Herzberg–Teller (vibronic) coupling are invoked to explain the observed enhancements

Sample Treatment Considerations in the Analysis of Organic Colorants by Surface-Enhanced Raman Scattering

The introduction of surface-enhanced Raman spectroscopy (SERS) in the field of cultural heritage has significantly improved the analysis of the organic dyes and their complexes that have been used as textile dyes and pigments in paintings and other polychrome works of art since antiquity. Over the last five years, a number of different procedures have been developed by various research groups. In this Article, we evaluate the effect of pretreating samples by exposing them to hydrofluoric acid (HF) vapor prior to SERS analysis, a step designed to hydrolyze the dye–metal complexes and increase analyte adsorption on the nanosized metallic support, thus enhancing the SERS signal. Materials studied include pure colorants, commercial lake pigments, and fibers from dyed textiles, as well as actual aged samples, such as microscopic fragments of lakes on paper and ancient pigments and glazes from several works of art, covering a wide range of time, from the second century B.C. to the early 20th century. In each case, SERS spectra obtained with or without HF hydrolysis were critically evaluated. The pretreatment with HF vapor resulted in faster analysis and increased sensitivity in most cases, with the exception of dyed silk fibers, where silk protein hydrolyzates were found to interfere with SERS analysis. As a final point, a two-step procedure including SERS on untreated and treated samples is proposed as a standard approach: by analyzing a sample first without hydrolysis, and then, following removal of the colloid, upon HF treatment, the best and most reliable results for a great number of dyes and substrates are assured

Surface-Enhanced Raman Scattering Due to Charge-Transfer Resonances: A Time-Dependent Density Functional Theory Study of Ag₁₃-4-Mercaptopyridine

We have used time-dependent density functional theory in conjunction with the CAM-B3LYP functional and MWB28/aug-cc-pVDZ basis set to determine non-, near-, and on-resonance Raman spectra for a complex formed by 4-mercaptopyridine (4-Mpy) binding with a Ag₁₃ cluster via the thiolate Ag–S bond. Geometry optimizations of the Ag₁₃-4-Mpy complex showed an on-top structure directly bound to one Ag atom with the ring of the molecule almost flat with respect to two Ag atoms of the complex. The corresponding B3LYP/MWB28/aug-cc-pVDZ geometry is also an on-top structure directly bound to one Ag atom, but the molecule is directed away from the surface. The near-resonance Raman calculations were carried out in the infinite lifetime approximation, while the on-resonant Raman excitation profiles were calculated with the complex polarization propagator (CPP) approach, introducing a half width at half-maximum spectral broadening of 0.2 eV. Calculation of the UV–vis spectra of the isolated 4-Mpy and of the Ag₁₃-4-Mpy complex showed that binding shifts the spectra from deep in the UV to the visible region. Calculation of the near-resonance Raman spectra of the two structures of the complex at 410 (3.025 eV) and 425 nm (2.918 eV) showed a strong enhancement. A very large variation across vibrational modes by a factor of at least 10³ was found for both the static chemical enhancement and charge-transfer (CT) enhancement mechanisms. This large variation in enhancement factor indicates that B-term Herzberg–Teller scattering is occurring because inactive or very low intensity modes in the static spectra of the molecule are much stronger in both the static and near-resonance spectra of the complex. From the excitation profile using the CPP method, an overall surface enhancement on the order 10³ or higher was found for individual modes on excitation into a CT excited state

Potential Induced Changes in Neuromedin B Adsorption on Ag, Au, and Cu Electrodes Monitored by Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS), electrochemistry, and generalized two-dimensional correlation analysis (G2DCA) methods were used to define neuromedin B (NMB) ordered superstructures on Ag, Au, and Cu electrode surfaces at different applied electrode potentials in an aqueous solution at physiological pH. The orientation of NMB and the adsorption mechanism were determined based on the analysis of enhancement, broadness, and shift in wavenumber of particular bands, which allow drawing some conclusions about NMB geometry and changes in this geometry upon change of the electrode type and applied electrode potential. The presented data demonstrated that NMB deposited onto the Ag, Au, and Cu electrode surfaces showed bands due to vibrations of the moieties that were in contact/close proximity to the electrode surfaces and thus were located on the same side of the polypeptide backbone. These included the Phe9 and Trp4 rings, the sulfur atom of Met10, and the −CCN– and −CO units of Asn2. However, some subtle variations in the arrangement of these fragments upon changes in the applied electrode potential were distinguished. The Amide-III vibrations exhibited an electrochemical Stark effect (potential dependent frequencies) with Stark tuning slope sensitive to the electrode material. Potential-difference spectrum revealed that the imidazole ring of His8 was bonded to the Cu electrode surface at relatively positive potentials

A Long-Range Surface Plasmon Resonance/Probe/Silver Nanoparticle (LRSPR-P-NP) Nanoantenna Configuration for Surface-Enhanced Raman Scattering

The purpose of this paper is to enhance Raman signals with a plasmonic nanoantenna based on a long-range surface plasmon resonance/probe/silver nanoparticle (LRSPR-P-NP) sandwich configuration. The finite-difference time-domain simulation shows that the electromagnetic field at the gap between the silver film and a silver nanoparticle increases by a factor of about 2.1 × 10⁴. The resonance condition of this plasmonic nanoantenna was optimized by incident angle-dependent surface-enhanced Raman scattering (SERS) spectroscopy under an evanescent field excitation mode. The SERS signal obtained under the LRSPR-P-NP configuration at the LRSPR angle was 40 times higher than that collected on the planar film plasmonic nanoantenna. The enhancement factor of the LRSPR-P-NP configuration was 9.2 × 10⁸. This plasmonic nanoantenna was also applied for pH sensing

Active-Tuned Plasmonic Angle Modulator of Light Beams for Potential Application of 3D Display

We propose a plasmonic angle modulator device based on the extraordinary optical transmission (EOT) phenomenon combined with the liquid crystal (LC)-tuned surface plasmons (SPs). The configuration of this angle modulator mainly involves an Ag nanograting film for the SP coupling and a LC layer for continuously tuning SPs via voltage signals. Accordingly, the directions of the transmission light through the Ag nanograting film can be tuned continuously, realizing a light beam scanning in a 5° range at an operation rate of 60 Hz. We expect this active-tuned plasmonic angle modulator would have potential applications in three-dimensional (3D) display techniques that strictly require the elaborate and rapid angle modulation of light beams. In addition, this active-tuned plasmonic angle modulator can also be applied in other fields, such as photocommunication, optical detection, beam steering, and so on

Active-Tuned Plasmonic Angle Modulator of Light Beams for Potential Application of 3D Display

We propose a plasmonic angle modulator device based on the extraordinary optical transmission (EOT) phenomenon combined with the liquid crystal (LC)-tuned surface plasmons (SPs). The configuration of this angle modulator mainly involves an Ag nanograting film for the SP coupling and a LC layer for continuously tuning SPs via voltage signals. Accordingly, the directions of the transmission light through the Ag nanograting film can be tuned continuously, realizing a light beam scanning in a 5° range at an operation rate of 60 Hz. We expect this active-tuned plasmonic angle modulator would have potential applications in three-dimensional (3D) display techniques that strictly require the elaborate and rapid angle modulation of light beams. In addition, this active-tuned plasmonic angle modulator can also be applied in other fields, such as photocommunication, optical detection, beam steering, and so on