Evidence for Ag cluster vibrations in enhanced Raman scattering from the Ag/electrolyte interface

Fast fabrication and judgement of tip-enhanced Raman spectroscopy-active tips

The quality of the scanning tip is crucial for tip-enhanced Raman spectroscopy (TERS) experiments towards large signal enhancement and high spatial resolution. In this work, we report a controllable fabrication method to prepare TERS-active tips by modifying the tip apex at the atomic scale, and propose two important criteria to in-situ judge the tip's TERS activity for tip-enhanced Raman measurements. One criterion is based on the downshift of the first image potential state to monitor the coupling between the far-field incident laser and near-field plasmon; the other is based on the appearance of the low-wavenumber Raman peaks associated with an atomistic protrusion at the tip apex to judge the coupling efficiency of emissions from the near field to the far field. This work provides an effective method to quickly fabricate and judge TERS-active tips before real TERS experiments on target molecules and other materials, which is believed to be instrumental for the development of TERS and other tip-enhanced spectroscopic techniques.

Silver(I) and Copper(I) Complexation with Decachloro-Closo-Decaborate Anion

A series of complexation reactions of silver(I) and copper(I) in the presence of a polyhedral weakly coordinating [B10Cl10]2− anion has been carried out. The effect of the solvent and the presence of Ph3P on the composition and structure of the reaction product were studied. Eight novel complexes were obtained and characterized by 11B Nuclear magnetic resonance, Infra-Red, and Raman spectroscopies as well as powder and single-crystal X-ray diffraction techniques. The [B10Cl10]2− anion demonstrated weaker coordinating ability towards coinage metals than [B10H10]2− at similar reaction conditions. The [B10Cl10]2− anion remains unreacted in the copper(I) complexation reaction, while in the absence of competing ligands, we obtained the first complexes containing decachloro-closo-decaborate anion directly coordinated by the metal atom. The bonding between metal atoms and the boron cluster anions was studied using the atomic Hirshfeld surfaces. Besides edge and face coordination of the polyhedral anion, this method allowed us to reveal the Ag–Ag bond in crystal of {Ag2(DMF)2[B10Cl10]}n, the presence of which was additionally supported by the Raman spectroscopy data.

Aerobic Conditions Enhance the Photocatalytic Stability of CdS/CdOx Quantum Dots

Photocatalytic H₂ production through water splitting represents an attractive route to generate a renewable fuel. These systems are typically limited to anaerobic conditions due to the inhibiting effects of O₂ . Here, we report that sacrificial H₂ evolution with CdS quantum dots does not necessarily suffer from O₂ inhibition and can even be stabilised under aerobic conditions. The introduction of O₂ prevents a key inactivation pathway of CdS (over-accumulation of metallic Cd and particle agglomeration) and thereby affords particles with higher stability. These findings represent a possibility to exploit the O₂ reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilise photocatalysts that suffer from similar degradation reactions.

Electromagnetic theories of surface-enhanced Raman spectroscopy

A fundamental theoretical understanding of SERS, and SERS hotspots, leads to new design principles for SERS substrates and new applications in nanomaterials and chemical analysis.

The effects of Au aggregate morphology on surface-enhanced Raman scattering enhancement

We have identified empirically a relationship between the surface morphology of small individual aggregates (<100 Au nanoparticles) and surface-enhanced Raman scattering (SERS) enhancement. We have found that multilayer aggregates generated greater SERS enhancement than aggregates limited to two-dimensional (2D) or one-dimensional structures, independent of the number of particles. SERS intensity was measured using the 730 cm(-1) vibrational mode of the adsorbed adenine molecule on 75 nm Au particles, at an excitation wavelength of 632.8 nm. To gain insight into these relationships and its mechanism, we developed a qualitative model that considers the collections of interacting Au nanoparticles of an individual aggregate as a continuous single entity that retains its salient features. We found the dimensions of the modeled surface features to be comparable with those found in rough metal surfaces, known to sustain surface plasmon resonance and generate strong SERS enhancement. Among the aggregates that we have characterized, a three 75 nm nanoparticle system was the smallest to generate strong SERS enhancement. However, we also identified single individual Au nanoparticles as SERS active at the same wavelength, but with a diameter twice in size. For example, we observed a symmetric SERS-active particle of 180 nm in diameter. Such individual nanoparticles generated SERS enhancement on the same order of magnitude as the small monolayer Au aggregates, an intensity value significantly stronger than predicted in recent theoretical studies. We also found that an aspect of our model that relates the dimensions of its features to SERS enhancement is also applicable to single individual Au particles. We conclude that the size of the nanoparticle itself, or the size of a protrusion of an irregularly shaped single Au particle, will contribute to SERS enhancement provided that its dimensions satisfy the conditions for plasmon resonance. In addition, by considering the ratio of the generated intensities of typical 2D Au aggregates to the enhancement of individual SERS-active particles, a value of approximately 2 is determined. Its moderate value suggests that it is not the aggregation effect that is responsible for much of the observed SERS enhancement but the surface region associated with the SERS-active site.

The Role of O2 in SERS-Active Thin Metal Film Photodynamics

Optical emission from thin Ag films excited by visible light under a non-oxygen atmosphere was analyzed as a function of time and excitation power. The emission behavior under anaerobic conditions was strikingly different from that obtained under an oxygen-containing atmosphere. Specifically, emission intensity increased as a function of time with no photoinduced signal decay. A simple mechanistic model was developed and demonstrated to accurately predict the photodynamic trends observed under both conditions. The model involves oxygen-mediated cyclic production and destruction of photoactive silver clusters on the thin film. The evidence presented here strongly supports the hypothesis that oxygen plays a key role in the "blinking" phenomenon observed on SERS-active Ag films.

Preparation and Characterization of Cr(CO)4dpp (Chromium Tetracarbonyl 2,3-Bis(2‘-pyridyl)pyrazine) Adsorbed on Silver Nanoparticles

A transition metal carbonyl species, Cr(CO)(4)dpp, has been successfully attached to bare silver nanoparticles prepared by laser ablation of a metal foil in ethanol. Transmission electron microscopy (TEM) images have shown that at least a portion of the silver nanoparticles have been capped by the chromium species, and ligand shells corresponding to Cr(CO)(4)dpp multilayer adsorption onto the silver nanoparticles of 30-50 nm diameter have been observed. The detection of the strongest Raman-active nu(CO) band of Cr(CO)(4)dpp at 2004 cm(-1) revealed that the species has been adsorbed without decomposition. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) signals recorded of the chromium-capped silver nanoparticles were also consistent with the nondecomposition adsorption process. Density functional calculations have been used to reproduce the Raman spectrum using Ag(7)(+) as a model surface. A large binding energy of about 122 kJ/mol has also been computed between silver and nitrogen atoms thus lending support to Cr(CO)(4)dpp being chemisorbed onto the silver surface.

Potential-dependent surface-enhanced Raman scattering from adsorbed thiocyanate for characterizing silver surfaces with improved reproducibility

Surface-enhanced Raman scattering (SERS) spectroelectrochemistry is used to characterize electrochemically roughened and highly polished polycrystalline silver SERS-active substrates. Changes in the nitrile stretching vibrational mode of adsorbed thiocyanate are used as an in situ spectroscopic probe: the potential dependence of band position (Stark tuning), shape, and scattering intensity of this mode are measured in order to investigate differences between SERS-active sites found on smooth and roughened electrode surfaces. Results obtained from thiocyanate adsorbed onto two different types of highly polished Ag surfaces (alumina and diamond polishing) show discrete populations of SERS-active adsorption sites that remain stable over a wide potential range. This behavior stands in contrast to that observed on electrochemically roughened surfaces, where very strong Stark tuning, large vibrational bandwidths, and irreversible loss of SERS enhancement upon negative potential excursions can be attributed to a diverse population of labile SERSactive sites that exhibit strong charge-transfer interactions with the adsorbate and large chemical SERS enhancement.

Determination of the orientation of azathioprine adsorbed on a silver electrode by SERS and ab initio calculations

We report the SERS spectrum of azthioprine (AZA) on a silver electrode surface and the results of normal mode calculations using empirical and ab initio calculations of the 6-mercaptopurione (6-MP) component of AZA. The empirical calculations were done with a Urey-Bradley force field (UBFF) and the ab initio calculations with the STO-3G basis set using the UHF, MP2 and BLYP methods. From the difference between the SERS and solid spectra, we determined that AZA attaches edge-on to the surface through the N3 site on the 6-MP component of the molecule. The UBFF calculation on an Ag adatom-molecule model reproduced most of the main observed frequency shifts in the SERS spectrum. With a similar model, the ab initio calculations yielded frequency shifts in the same direction as the one observed for the in-plane normal modes, but they yielded opposite shifts for the out-of-plane normal modes. This phenomenon may be attributed to a face-on interaction of the 6-MP component with a neighboring adatom made possible by an inclination of the molecule on the surface.

Fluorescence and surface-enhanced Raman study of 9-aminoacridine in relation to its aggregation and excimer emission in aqueous solution and on silver surface

Fluorescence spectroscopy and surface-enhanced Raman spectroscopy (SERS) have been applied to study the aggregation and excimer emission of 9-aminoacridine (9AA) and 9-aminoacridine hydrochloride (9AA-HCl) in aqueous solution and on silver colloids. The effect of the drug concentration, pH, and chloride concentration on these processes has been investigated. The excimer emission of 9AA is connected to the dimerization of this drug in solution: the formation of 9AA dimers is greatly favored when the drug is under the amino form at neutral and acidic pH, while at alkaline pH the imino 9AA form tends to form large-size aggregates which cannot be excited to render excimer emission. 9AA is adsorbed on the silver surface under two different forms: strongly and weakly attached 9AA, each one corresponding to the different drug tautomers: imino and amino. The interaction of 9AA with silver induces a charge transfer from the adsorbate to the metal leading to a remarkable fluorescence quenching, a basicity decrease of the adsorbed drug and a considerable weakening of the dimer-excimer emission. Furthermore, an attribution of the main Raman features appearing in the SERS spectra has been proposed, providing marker bands for the imino and amino 9AA tautomers, and a mechanism for the molecular dimerization is also suggested.