Effect of laser illumination oxidation-reduction cycles upon surface-enhanced Raman scaterring from silver electrodes

The intensity of surface-enhanced Raman scattering (SERS) from thiocyanate and chloride adsorbed on silver electrodes is shown to depend critically on whether the electrode is illuminated during the oxidation-reduction cycle used to pretreat the electrode. The value and magnitude of the photoeffect is dependent upon the type of surface vibrational mode, the adsorbate and the wavelength of the radiation during the ORC. The effects are attributed to the production of SERS active clusters of Ag atoms by photoreduction of the Ag(I) phase films during the ORC.     

拓宽银电极上SERS活性的研究电位范围

电极表面的粗糙化处理是进行表面增强拉曼光谱（ＳＥＲＳ）研究的重要前提，通过研究两种截然不同的氧化还原循环（ＯＲＣ）粗粗糙电极的方法，分析其ＳＥＲＳ活性稳定电位区间与ＯＲＣ还原电位之间的关系，发现高活性的ＳＥＲＳ位皆处于亚稳状态，易随电极电位趋近零电位（ＰＺＣ）而发生表面原子重排，以至推动活性，引入强吸附物种，可以使特殊ＯＲＣ得到的ＳＥＲＳ活性在ＰＺＣ以正电位区稳定存在，并可在ＰＺＣ以负一得到常规Ｏ     

Ag|kCl|六氢吡啶体系表面增强喇曼散射效应的猝灭和恢复

在简述近年来电化学体系中表面增强喇曼散射(SERS)活性位置研究的实验结果和有关论述后, 着重报道作者在Ag/KCl/六氢吡啶体系中, Ag-Cl振动峰和六氢吡啶诸振动峰的强度, 在一定电位范围内电极被施加负脉冲电位后的同步下降, 以及采用氧化还原循环(ORC)处理得到同步恢复等新的实验结果, 并对上述体系中表面络合物的构成及其在SERS中的作用作了进一步的论证。     

Roughness assessment and wetting behavior of fluorocarbon surfaces

The wetting behavior of fluorocarbon materials has been studied with the aim of assessing the influence of the surface chemical composition and surface roughness on the water advancing and receding contact angles. Diamond like carbon and two fluorocarbon materials with different fluorine content have been prepared by plasma enhanced chemical vapor deposition and characterized by X-ray photoemission, Raman and FT-IR spectroscopies. Very rough surfaces have been obtained by deposition of thin films of these materials on polymer substrates previously subjected to plasma etching to increase their roughness. A direct correlation has been found between roughness and water contact angles while a superhydrophobic behavior (i.e., water contact angles higher than 150° and relatively low adhesion energy) was found for the films with the highest fluorine content deposited on very rough substrates. A critical evaluation of the methods currently used to assess the roughness of these surfaces by atomic force microscopy (AFM) has evidenced that calculated RMS roughness values and actual surface areas are quite dependent on both the scale of observation and image resolution. A critical discussion is carried out about the application of the Wenzel model to account for the wetting behavior of this type of surfaces.     

Influence of compaction and surface roughness on low‐energy ion scattering signals

Investigation of the surface composition of powders often requires compaction. To study the effect of compaction on surface analysis, samples have been compacted at various pressures ranging from 0 Pa (i.e. no compaction) up to 2000 MPa (2 × 10⁴ kg cm^?2) Low‐energy ion scattering (LEIS) was used to determine the composition of the outermost atomic surface layer. Using scanning electron microscopy, changes in the morphology due to compaction have been detected in the SiO₂ test samples. The LEIS yield of a compacted silica powder is found to be independent of the applied pressure during compaction between 2 MPa and 2000 MPa (2 × 10⁴ kg cm^?2). Analysis of a submonolayer of Ta₂O₅ on a silica support shows that the composition of the outermost atomic layer is not changed after compaction up to a pressure of at least 300 MPa. When compaction is applied, the absolute LEIS yield appears to be independent of the specific surface area of silica supports in the range 50–380 m² g^?1. A minor difference in LEIS signals is observed between compacted silica supports and flat quartz samples. In order to determine the surface roughness factor independently, and to study the material dependence of the surface roughness factor, angle‐dependent LEIS measurements have been carried out on oxidized silicon, gallium and gold surfaces. The results on the oxidized silicon confirm the small influence of surface roughness for silica particles, whereas measurements on the more closely packed metallic gallium and gold surfaces indicate a significant surface roughness effect. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigation of chemically modified barium titanate beads as surface-enhanced Raman scattering (SERS) active substrates for the detection of benzene thiol, 1,2-benzene dithiol, and rhodamine 6G

SERS active surfaces were prepared by depositing silver films using Tollen's reaction on to barium titanate beads. The SERS activity of the resulting surfaces was probed using two thiols (benzene thiol and 1,2-benzene dithiol) and rhodamine 6G. The intensity of the SERS signal for the three analytes was investigated as a function of silver deposition time. The results indicate that the SERS intensity increased with increasing thickness of the silver film until a maximum signal intensity was achieved; additional silver deposition resulted in a decrease in the SERS intensity for all of the studied molecules. SEM measurement of the Ag coated barium titanate beads, as a function of silver deposition time, indicate that maximum SERS intensity corresponded with the formation of atomic scale islands of silver nanoparticles. Complete silver coverage of the beads resulted in a decreased SERS signal and the most intense SERS signals were observed at deposition times of 30 min for the thiols and 20 min for rhodamine 6G.     

Quantification of surface roughness effect on elastically backscattered electrons

Electron inelastic mean free path can be obtained from a measured elastic peak electron spectroscopy spectrum combined with a Monte Carlo simulation. It is thus necessary to know the influence of various experimental factors to the measured and calculated results. This work investigates the effect of the surface roughness or the surface topography on the intensity of the elastic peak. A Monte Carlo simulation, by taking into account of realistic surface roughness for both Gaussian and non‐Gaussian type rough surfaces experimentally prepared, has been employed to study the surface topography effect. The simulations of elastic peak electron spectroscopy were performed for both planar and rough Al and Cu surfaces and for varied primary energies ranging from 200 to 2000 eV. To quantify the surface roughness effect, the surface roughness parameter is introduced according to the ratio of elastic peak intensities between a rough surface and an ideal planar surface. Simulation results have shown that surface roughness parameter is important in a certain range of emission angle and particularly for large emission angles. For grazing emission, the elastic peak intensity can be largely enhanced by roughness even at nanometer scale. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of electrolytes used in roughening gold substrates by oxidation–reduction cycles on surface-enhanced Raman scattering

In this work, the effects of electrolytes used in roughening gold substrates by electrochemical methods on surface-enhanced Raman scattering (SERS) were first investigated. First, gold substrates were roughened by triangular-wave oxidation–reduction cycles (ORC) in aqueous solutions containing different kinds of 0.1 M electrolytes. Then Rhodamine 6G (R6G) was used as Raman probe to examine this effect of electrolytes used on the SERS observed. The result indicates that the highest intensity of SERS of R6G was obtained on the roughened Au substrate prepared in 0.1 M NaCl, which was less used in the literature. Meanwhile, it was also found that the rougher surface morphology observed, which is contributive to the higher SERS obtained, is corresponding to the smaller cathodic peak area shown in the cyclic voltammograms for roughening the Au substrate.     

Wetting of heterogeneous surfaces of block copolymers containing fluorinated segments

 The wetting of well-characterized heterogeneous surfaces of block copolymers has been studied by low-rate dynamic contact angle measurements using axisymmetric drop-shape analysis. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the roughness, the heterogeneity and the chemical composition of the surfaces. By changing the block length of polysulfone and semifluorinated polyester segments in the block copolymers, the surface heterogeneity of thin films prepared on silicon wafers could be controlled. Tapping-mode AFM measurements showed that soft, hydrophobic domains of varying size on the submicrometer length scale were obtained on these surfaces (60–250 nm). The mean roughness was of the order of several nanometers. The results of the contact angle measurements showed that neither roughness nor heterogeneity had a significant effect on the advancing contact angle of water, at the scale of the features present; however, the contact angle hysteresis increased with increasing percentage of the soft domains. We assume that liquid retention by the solid upon retraction of the three-phase line is the main cause for the observed increase in contact angle hysteresis. Concerning the molecular composition of these block copolymer surfaces, angle-resolved XPS analysis showed a surface segregation of fluorine within the surface region. A direct correlation was found between the fluorine content of the block copolymer surfaces and the advancing contact angle of water. Received: 26 May 2000 Accepted: 3 January 2001     

Misfit-Dislocation Induced Surface Morphology of InGaAs/GaAs Heterostructures

The correlation between the surface cross-hatched morphology and the interfacial misfit dislocations in partially relaxed InGaAs/GaAs heterostructures was studied by means of atomic force microscopy and electron-beam induced current mode in a scanning electron microscope. A close correspondence between the misfit-dislocation network at the interface and the surface morphology shows that the cross-hatch development results primarily from the misfit-dislocation generation. Statistical analysis of the surface roughness reveals an anisotropy in strain relaxation of the epitaxial layers, which results from an asymmetry in the misfit-dislocation formation.     

Silver Nanoparticle Thin Films with Nanocavities for Surface‐Enhanced Raman Scattering

The formation of nanometer‐sized gaps between silver nanoparticles is critically important for optimal enhancement in surface‐enhanced Raman scattering (SERS). A simple approach is developed to generate nanometer‐sized cavities in a silver nanoparticle thin film for use as a SERS substrate with extremely high enhancement. In this method, a submicroliter volume of concentrated silver colloidal suspension stabilized with cetyltrimethylammonium bromide (CTAB) is spotted on hydrophobic glass surfaces prepared by the exposure of the glass to dichloromethysilane vapors. The use of a hydrophobic surface helps the formation of a more uniform silver nanoparticle thin film, and CTAB acts as a molecular spacer to keep the silver nanoparticles at a distance. A series of CTAB concentrations is investigated to optimize the interparticle distance and aggregation status. The silver nanoparticle thin films prepared on regular and hydrophobic surfaces are compared. Rhodamine 6G is used as a probe to characterize the thin films as SERS substrates. SERS enhancement without the contribution of the resonance of the thin film prepared on the hydrophobic surface is calculated as 2×10⁷ for rhodamine 6G, which is about one order of magnitude greater than that of the silver nanoparticle aggregates prepared with CTAB on regular glass surfaces and two orders of magnitude greater than that of the silver nanoparticle aggregates prepared without CTAB on regular glass surfaces. A hydrophobic surface and the presence of CTAB have an increased effect on the charge‐transfer component of the SERS enhancement mechanism. The limit of detection for rhodamine 6G is estimated as 1.0×10^?8 M . Scanning electron microscopy and atomic force microscopy are used for the characterization of the prepared substrate.     

Contact of elastic solids with rough surfaces

The effect of surface roughness on the adhesion of elastic solids was examined with artificially roughened surfaces and crosslinked poly(dimethylsiloxane). The amplitudes (σ) and lateral correlation lengths (ξ) of the surface roughness were determined with the height–height correlation function calculated from atomic force microscopy images. The work of adhesion (W) did not change significantly for surfaces where σ ≤ 6 nm. However, with increasing σ and (ξ²/σ)^1/3, W increased. Maximal adhesion was found for surfaces that produced the greatest indentation. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1848–1854, 2001     

Self-assembled metal colloid films: two approaches for preparing new SERS active substrates

In this paper, we propose two new approaches for preparing active substrates for surface-enhanced Raman scattering (SERS). In the first approach (method 1), one transfers AgI nanoparticles capped by negatively charged mercaptoacetic acid from a AgI colloid solution onto a quartz slide and then deoxidizes AgI to Ag nanoparticles on the substrate. The second approach (method 2) deoxidizes AgI to Ag nanoparticles in a colloid solution and then transfers the Ag nanoparticles capped by negatively charged mercaptoacetic acid onto a quartz slide. By transfer of the AgI/Ag nanoparticles from the colloid solutions to the solid substrates, the problem of instability of the colloid solutions can largely be overcome. The films thus prepared by both approaches retain the merits of metal colloid solutions while they discharge their shortcomings. Accordingly, the obtained Ag particle films are very suitable as SERS active substrates. SERS active substrates with different coverages can be formed in a layer-by-layer electrostatic assembly by exposing positively charged surfaces to the colloid solutions containing oppositely charged AgI/Ag nanoparticles. The SERS active substrates fabricated by the two novel methods have been characterized by means of atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) spectroscopy. The results of AFM and UV-vis spectroscopy show that the Ag nanoparticles grow with the increase in the number of coverage and that most of them remain isolated even at high coverages. Consequently, the surface optical properties are dominated by the absorption due to the isolated Ag nanoparticles. The relationship between SERS intensity and surface morphology of the new active substrates has been investigated for Rhodamine 6G (R6G) adsorbed on them. It has been found that the SERS enhancement depends on the size and aggregation of the Ag particles on the substrates. Especially, we can obtain a stronger SERS signal from the substrate prepared by method 1, implying that for the metal nanoparticles capped with stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization in the film is of great use in preparing SERS active substrates. Furthermore, we have found that the addition of Cl- into the AgI colloid solution changes the surface morphology of the SERS active substrates and favors stronger SERS enhancement.     

Temperature dependent surface enhanced Raman spectroscopy of piperidine in AgBr sol

The temperature dependent surface enhanced Raman (SER) spectra of piperidine in AgBr sols are presented with emphasis on the study of the intensity variation as the temperature increases. Most of the SER intensities decrease as the temperature increases. This is interpreted as due to the increase of the adsorption distance from the sol particle surface. It is also inferred from the various slopes of the decrease in intensity of various modes that there is variation of the SER effect in the dimension of a single bond, i.e. in 1.5 Å. Moreover, from the temperature dependent SER spectra, it is concluded that due to thermal agitation, both axial and equatorial piperidine molecules are adsorbed on the sol particle surface as the temperature increases. The energy difference between these two forms are calculated to be around 20 kcal/mol which is believed to be larger than that in solution due to the adsorption effect. Besides, two peaks at 1390 and 1231 cm⁻¹ show an anomalous positive thermal effect which could be due to the very complicated SERS mechanism. Finally, it is discussed that the temperature dependent SERS study possesses potentiality in revealing the chemical structure near the sol particle and noble metal electrode surfaces.     

Scaling analysis of polyacrylamide gel surfaces synthesized in the presence of surfactants

Surfaces of polyacrylamide hydrogels synthesized in the presence of surfactants were imaged by atomic force microscopy (AFM), and the surface morphology was studied by numerical scaling analysis. The gels were formed by polymerizing acrylamide plus a cross-linker in the presence of surfactants, which were then removed by soaking in distilled water. Gels formed in the presence of over 20% surfactant (by weight) formed clear, but became opaque upon removal of the surfactants. Other gels formed and remained clear. The surface morphology of the gels was studied by several one- and two-dimensional numerical scaling methods. The surfaces were found to be self-affine on short length scales, with a roughness (Hurst) exponent in the range from 0.85 to 1, crossing over to a constant root-mean-square surface width at long scales. Both the crossover length between these two regimes and the saturation value of the surface width increased significantly with increasing surfactant concentration, coincident with the increase in opacity. We propose that the changes in the surface morphology are due to a percolation transition in the system of voids formed upon removal of the surfactants from the bulk.     

Rationalization of the behavior of solid-liquid surface free energy of water in Cassie and Wenzel wetting states on rugged solid surfaces at the nanometer scale

The present work aims to contribute to the understanding at a molecular level of the origin of the hydrophobic nature of surfaces exhibiting roughness at the nanometer scale. Graphite-based smooth and model surfaces whose roughness dimension stretches from a few angstroms to a few nanometers were used in order to generate Cassie and Wenzel wetting states of water. The corresponding solid-liquid surface free energies were computed by means of molecular dynamics simulations. The solid-liquid surface free energy of water-smooth graphite was found to be -12.7 ± 3.3 mJ/m(2), which is in reasonable agreement with a value estimated from experiments and fully consistent with the features of the employed model. All the rugged surfaces yielded higher surface free energy. In both Cassie and Wenzel states, the maximum variation of the surface free energy with respect to the smooth surface was observed to represent up to 50% of the water model surface tension. The solid-liquid surface free energy of Cassie states could be well predicted from the Cassie-Baxter equation where the surface free energies replace contact angles. The origin of the hydrophobic nature of surfaces yielding Cassie states was therefore found to be the reduction of the number of interactions between water and the solid surface where atomic defects were implemented. Wenzel's theory was found to fail to predict even qualitatively the variation of the solid-liquid surface free energy with respect to the roughness pattern. While graphite was found to be slightly hydrophilic, Wenzel states were found to be dominated by an unfavorable effect that overcame the favorable enthalpic effect induced by the implementation of roughness. From the quantitative point of view, the solid-liquid surface free energy of Wenzel states was found to vary linearly with the roughness contour length.     

Surface-enhanced Raman scattering-active silver nanostructures with two domains

Generally, a controllable and reproduced surface roughness for surface-enhanced Raman scattering (SERS) studies can be generated through control of the electrochemical oxidation–reduction cycles (ORC) procedure. In this work, we propose a new sonoelectrochemical approach to prepare SERS-active substrates with two domain-Ag nanostructures. The method is based on a strategy of deposition–dissolution cycles (DDCs) by using a cathodic overpotential and an anodic overpotential from open circuit potential (OCP) in turn under sonication. The prepared SERS-active substrate demonstrates large Raman scattering enhancement for adsorbed Rhodamine 6G (R6G) with an enhancement factor of 2.3 × 10⁸ and a limit of detection of 2 × 10⁻¹³ M. The improved SERS performances can be successfully explained from the viewpoints of electromagnetic (EM) and chemical (CHEM) enhancements.     

金纳米粒子的二步电解合成及表面增强拉曼散射活性研究

在超声条件下采用二步电解方法在十六烷基三甲基溴化铵/丙酮/水三组分体系中合成金纳米粒子. 首先采用恒电流或电位的阶跃方法, 使体系中生成较小的金纳米粒子并作为晶种; 接着采用电位双阶跃方法, 使金纳米粒子在原来基础上继续生长, 控制电解电量可获得不同大小的金纳米粒子. 通过静电作用在洁净的单晶硅片表面组装金纳米粒子, 获得具有不同形貌的硅片, 并以此作为表面增强拉曼散射基底, 以吡啶为探针分子, 研究了不同基底的表面增强拉曼散射活性, 结果表明吡啶谱峰强弱与纳米粒子在硅片表面的排列形貌有关.     

An Electrochemical Approach to and the Physical Consequences of Preparing Nanostructures from Gold Nanorods with Smooth Ends

We have developed a method to smooth the end sections of nanowires and nanograps generated via the On-Wire Lithography process and studied these rods with optical spectroscopies and theoretical modeling (Discrete Dipole Approximation). The first step of the smoothing process is a reductive one aimed at controlling the diffusion and migration of metal ions to the growing nanorod surface by adjusting the applied potential and concentration of the metal ions in the growth solution. A second oxidative smoothing step, based in part on the energetic differences between topologically rough and smooth surfaces, is used to further smooth the nanorod. The RMS roughness can be reduced over five fold to approximately 5 nm. The properties of these smoothed rods were investigated by empirical and theoretical methods, where it was found the smoothed rods have sharper plasmon resonances and decreased SERS intensity.     

Processing of nanoporous Ag layers by potential-controlled displacement (PCD) of Cu

A cementation-like process taking place under potential control and introduced in this work as a "potential-controlled displacement" (PCD) is developed as a new method for processing of nanoporous Ag structures with controlled roughness (porosity) length scales. Most of the development work is done in a deoxygenated electrolyte containing 1 x 10(-3) M AgClO(4 )+ 5 x 10(-2) M CuSO(4) + 1 x 10(-1) M HClO(4) using a copper rotating disk electrode at 50 rpm. At this electrolyte concentration, the Ag deposition is under diffusion limitations whereas the Cu dissolution displays a typical Butler-Volmer anodic behavior. Thus, a careful choice of the operational current density enables strict control of the ratio between the dissolving and depositing metals as ascertained independently by atomic absorption spectrometry (AAS). The roughness length scale of the resulting surfaces is controlled by a careful selection of the current density applied. The highest surface area and finest morphology is obtained when the atomic ratio of Ag deposition and Cu dissolution becomes 1:1. Preseeding of uniform Ag clusters on the Cu surface made by pulse plating of Ag along with complementary plating and stripping of Pb monolayer is found to yield finer length scale resulting in up to a 67% higher surface area. An electrochemical technique using as a reference value the charge of an underpotentially deposited Pb layer on a flat Ag surface is used for measuring the real surface area. Scanning electron microscopy (SEM) studies are conducted to examine and characterize the deposit morphology of Ag grown by PCD on Cu substrates.