Rough Au Research Articles

SERS spectra of poly(3-hexylthiophene) in oxidized and unoxidized states

Surface enhanced Raman scattering (SERS) is being increasingly used for the study of the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin, arising from the excitation of surface plasmons in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, for which available experimental data are scarce. The chemical process originates from the increase in the polarizability of the molecules at the metal surface under the action of the incident radiation, which leads to the formation of new chemical bonds with the atoms of the metal support. The present work was devoted to the study of the SERS spectra of poly(3-hexylthiophene) deposited on rough Ag and Au supports by evaporating the solvent from a solution of known concentration. The experiments revealed the existence of a chemical surface effect. The results obtained show that the SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds of the type MeX (Me=Ag or Au, X=Cl or O). The SERS measurements reported here reveal an increase in the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. It was observed for the first time by SERS spectroscopy that the doping of 3-PHT with FeCl3 leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. © 1998 John Wiley & Sons, Ltd.

Biofuel cell based on glucose oxidase and microperoxidase-11 monolayer-functionalized electrodes

Apoglucose oxidase was reconstituted onto a pyrroloquinoline quinone and flavin adenine dinucleotide phosphate, PQQ–FAD, monolayer associated with a rough Au electrode to yield a bioelectrocatalytically active glucose oxidase, GOx. Electrically contacted PQQ–FAD/GOx monolayer was applied to the biocatalytic oxidation of glucose. A microperoxidase-11, MP-11, monolayer was assembled onto a rough Au electrode and used for the biocatalytic reduction of H2O2. Both biocatalytic electrodes, Au/PQQ–FAD/GOx and Au/MP-11, were assembled into a biofuel cell using glucose and H2O2 as the fuel substrate and the oxidizer, respectively. The biofuel cell generates an open-circuit voltage, Voc, of ca. 310 mV and a short-circuit current density, isc, of ca. 114 µA cm–2. The maximum electrical power, Pmax, extracted from the cell is 32 µW at an external optimal load of 3 kΩ. The fill factor of the biofuel cell, f = PmaxIsc–1Voc–1, is ca. 25%.

Potential Dependence of the Conformations of Nicotinamide Adenine Dinucleotide on Gold Electrode Determined by FT−Near-IR−SERS

High-quality Fourier transform surface-enhanced Raman scattering (FT−SERS) of a dilute solution of nicotinamide adenine dinucleotide (NAD) was recorded on a roughened Au electrode. It was found that the detection limit could be lower as much as 3 orders of magnitude by using a near-infrared laser as the exciting source and an in-situ roughening technique. The SERS of NAD shows a strong potential dependence in the non-Faradaic regions. In regions of positive electrode potential, only the bands responsible for the adenine and nicotinamide moieties can be observed. In contrast, with a negative shift of the potential, several additional strong bands representing the ribose and phosphate moieties are also evident. Based upon the SERS behaviors of NAD in the regions of different potential, an adsorption mechanism for dilute NAD on the gold electrode was proposed. With this mechanism, the stacked NAD molecule is considered to be opened to some extent on an electric charged electrode. Specifically, under sufficie...

Glucose and Acetylcholine Sensing Multilayer Enzyme Electrodes of Controlled Enzyme Layer Thickness

Multilayer enzyme networks assembled by a stepwise synthesis onto Au electrodes provide the basis for glucose, choline, and acetylcholine amperometric biosensors. Glucose oxidase is assembled as a multilayer array on smooth and rough Au electrodes. Electrical communication of the GOx layers with the electrode is established by the presence of ferrocenecarboxylic acid as diffusional electron mediator or by covalent linkage of [(ferrocenylmethyl)amino]caproic acid to the enzyme components. The resulting enzyme electrodes are applied as amperometric biosensors for glucose. The sensitivity of the resulting enzyme electrode is controlled by the number of enzyme layers assembled onto the electrode and by the roughness factor of the electrode surface. A multilayer enzyme array of choline oxidase (ChO) immobilized onto a rough Au electrode provides an amperometric biosensor for the amperometric detection of choline in the presence of 2,6-dichloroindophenol (DIP) as diffusional electron mediator. Stepwise organization of a multilayer biocatalytic array consisting of four layers of ChO and three layers of acetylcholine esterase (AChE) provides a bifunctional enzyme electrode for the amperometric detection of acetylcholine. In this system, choline generated by AChE hydrolysis of acetylcholine is amperometrically detected by the ChO layers in the presence of DIP acting as diffusional electron mediator. Here we report on novel means to enhance the sensitivity of multilayer enzyme electrodes by the application of rough Au electrodes as the surface for assembling the enzyme network.

Mo/Si multilayer-coated ruled blazed gratings for the soft-x-ray region

Two Mo/Si multilayer-coated blazed gratings have been fabricated for operation at soft-x-ray wavelengths above the Si L edge, λ ≥ 12.4 nm, at (near) normal incidence. The sawtooth profile of the grating structure was mechanically ruled into a 200-nm Au film that was deposited onto a plane glass substrate. To smooth the rough Au surface and to prevent interdiffusion of the Au film with the upper Mo/Si multilayer, a carbon film was evaporated onto the Au grating surface of one of the gratings before the deposition of the multilayer coating. We matched the multilayer grating, working on blaze in the third diffraction order, in which an absolute diffraction efficiency of 3.4% at a wavelength of 14 nm was measured, whereas only 1.1% was achieved for a similar grating (without a carbon interlayer). These efficiencies are higher than those obtained for other ruled blazed gratings reported in the literature. As a result of the multilayer and grating periodicity, the wavelength of diffraction can be tuned bya rotation of the grating, which is important for application in a soft-x-ray monochromator.

Surface Raman scattering of the butanol isomers on emersed silver and gold electrodes

Surface Raman scattering in the ν(C-H) region of 1-butanol, 2-butanol, and isobutyl alcohol on emersed rough and smooth Ag and Au electrodes is reported and is compared to the corresponding in-situ surface-enhanced Raman scattering results. Identical spectral behavior is observed for the rough and smooth emersed electrodes compared to the in-situ electrodes, suggesting that the solvent structure and orientation at the electrode is retained upon emersion. The behavior of the ν(Au-Br) band and the ν(O-H) band from interfacial water solvating ions suggests that the electrolyte structure is also retained upon emersion. The stability of the emersed layer is extremely dependent on solvent evaporation, atmospheric purity, and electrode contamination

Examination of the multilevel diffraction model for interface roughness characterization by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to investigate rough Au:Pd thin films. The Au:Pd surface is best described by the self-affine scaling model, with the roughness exponent α=0.79±0.06 and the interface width w=15.0±0.5 Å measured directly from the STM images. We further show how α and w may be extracted from diffraction techniques by utilizing the multilevel diffraction theory, using the STM images as a model self-affine surface.

Surface Raman scattering of self-assembled monolayers formed from 1-alkanethiols: behavior of films at gold and comparison to films at silver

Surface Raman scattering is used to study self-assembled monolayers formed from a series of 1-alkanethiol s, CH3(CH2)^SH, where n = 3-5, 7, 8,11, and 17, at mechanically polished and electrochemically roughened Au surfaces. Defect structure in these films is investigated by use of the relative intensities of peaks due to trans and gauche conformations in the v(C-S) and P(C-C) frequency regions. Surface selection rules for Raman spectroscopy are used to estimate orientation of the alkanethiol layers at Au. The orientation proposed on the basis of the Raman spectral data is consistent with those previously reported on the basis of other measurements at Au surfaces. This orientation is compared to that previously determined for these films at Ag. Alkanethiols at Ag are found to have a chain tilt from the surface normal less than the 30° previously reported for Au. The C-S bond is found to be perpendicular to the Ag surface while it is largely parallel to the surface at Au. Differences in the spectra of short-chain alkanethiols from smooth and rough surfaces are attributed to disordering of the film at the roughened Au surface which occurs predominantly near the S end of the molecule on rough Au surfaces.

Observation and explanation of light-emission spectra from statistically rough Cu, Ag, and Au tunnel junctions

A detailed description of the experimentally observed light output from statistically rough ${\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$-$M$ ($M=\mathrm{A}\mathrm{g},\mathrm{A}\mathrm{u},\mathrm{C}\mathrm{u}$) tunnel junctions is presented. These data include a comprehensive description of the polarization and angular distribution of the light emitted from ${\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$-Au junctions as well as spectra from reverse-biased ${\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$-Ag junctions. It is argued, principally on the grounds of an examination of surface-plasmon---polariton (SPP) damping, that the bulk of the output from statistically rough tunnel junctions is due to the fast-SPP mode. The idea of fast-SPP mediation is found, in many respects, to be much more consistent with currently available experimental results than that of slow- (or junction) SPP mediation. Extant theoretical models hold slow-SPP mediation to be the dominant means of visible-regime emission. The view of the emission mechanism presented in this paper suggests that the statistically rough tunnel junction could emit light more efficiently (if the scale of the surface roughness were altered) and that it has potential as a spectroscopic tool.

THEORY OF SURFACE ENHANCED RAMAN SCATTERING : A PROSPECTIVE VIEW

Abstract : Theoretical understanding of the origins of strong Raman signals from molecules near roughened Ag, Cu, and Au surfaces has progressed to the point where the action of two very different types of process are recognized. There is chemical enhancement that is apparent only for chemisorbed species, as for example, specifically adsorbed anions on Ag electrodes. The chemical mechanism is poorly understood, however, ingenious ideas and concepts have been advanced, including localization of electron-hole pair states and optically driven charge transfer between adsorbate and metal, to model aspects of the chemical enhancement. The second and best understood process is the enormous enhancement of the incoming and outgoing radiation fields at the surface resulting from lightening rod geometries and the excitation of localized surface plasmon modes. Calculations show that for special geometries, very high (approximately 1,000,000 - 10,000,000) enhancement factors are possible in confined regions. (Author)

The effect of randomly distributed surface bumps on local field enhancements in surface enhanced Raman spectroscopy

A model is presented for determining the enhancement of local electromagnetic fields which occurs close to rough metal surfaces through surface plasmon excitation. The model considers a random distribution of metal hemispheroids on a perfectly conducting flat plane. The electrodynamics of this system is described using the long wavelength approximation and with only the dipole coupling between different hemispheroids included. The square of the local field is averaged over all locations on the surface to provide an estimate of the enhancement in Raman intensity pertinent to surface enhanced Raman spectroscopy (SERS). Application is made to rough Ag, Cu, Au, Hg, and Pt surfaces, simulating the roughness produced by electrochemical anodization. For Ag, the roughness induced SERS enhancement is found to be about 102, with a weak dependence on wavelength down to 350 nm, where a sharp drop is observed. The variation of this result on the parameters which characterize the hemispheroid distributions is studied, including the variation with the distribution of hemispheroid heights and widths, with the number and arrangements of hemispheroids on the surface, with hemispheroid dielectric constant, and with hemispheroid coverage. Most of these parameters do not change the overall enhancement factors significantly, although the wavlength dependence of the enhancement does change significantly with hemispheroid coverage. The polarization of the local electric field close to the surface is studied, and found to be primarily perpendicular to the local surface for globally perpendicular applied fields. The SERS enhancement on Cu is found to be comparable to Ag for λ≳600 nm, with a drop in intensity by a factor of 5 at 600 nm. Au is similar to Cu, except that the drop occurs at 500 nm. Hg is similar to Ag in both the magnitude and frequency dependence of the enhancement, while Pt is uniformly smaller throughout the visible. All of these enhancements are ∼103 smaller than is seen experimentally using SERS, although the observed change in SERS intensity due to anodization does closely match the predictions of our model. Overall, we conclude that surface roughness probably contributes 102 to 103 to the overall enhancement with a frequency dependence which is close to that seen experimentally. This conclusion indicates that some other enhancement mechanism must be responsible for the remaining factor of 103 needed to explain the observed overall enhancement of 106.

The giant Raman Effect from pyridine on a chemically modified gold substrate

Enhanced Raman scattering from adsorbed pyridine has been observed with 5145 nm incident radiation on a slightly rough Au substrate onto which electrochemically controlled monolayer quantities of Ag have been deposited. This result supports enhancement models in which specific chemistry between the adsorbate and the substrate is important.