Abnormal adsorption behavior of dimethyl disulfide on gold surfaces

Adsorption of dimethyl disulfide (DMDS) on gold colloidal nanoparticle surfaces has been examined to check its binding mechanism. Differently from previous results, DMDS molecules adsorbed on the gold surface at high concentration showed the S–S stretching band at 500 cm⁻¹ in surface-enhanced Raman scattering (SERS) spectra, which indicates the presence of intact adsorption of DMDS molecules. However, it was found that the S–S bond of disulfides was easily cleaved on the gold surface at low concentration. These behaviors were not observed for diethyl disulfide (DEDS) or diphenyl disulfide (DPDS). Our results indicate that DMDS molecules with the shortest alkyl chains on the gold surface can be inserted into self-assembled monolayers (SAMs) without the S–S bond cleavage during self-assembly due to insufficient lateral van der Waals interaction and the low adsorption activity of disulfides, whereas DEDS with longer alkyl chains or DPDS with the weak disulfide bond dissociation energy would not. These unusual DMDS adsorption behaviors were examined by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). We also compared the bonding dissociation energy of the S–S bonds of various disulfides by means of a density functional theory (DFT) calculation.     

一种自支撑金纳米薄膜的制备、结构和氮吸附特性

以一种新的硅微米/纳米结构复合体系——硅纳米孔柱阵列作为还原性衬底，采用浸渍技术制备出一种自支撑的金纳米薄膜，并对其表面形貌和结构进行了表征.实验表明，金纳米薄膜的制备过程是一个自终止过程.当硅纳米孔柱阵列被耗尽后，浸渍溶液中Au³⁺的还原反应将自行终止；同时，所形成的金纳米薄膜自动与衬底脱离并成为一种自支撑薄膜.薄膜的形成机理被归因于硅纳米孔柱阵列所具有的高的表面活性和还原性.用能量弥散x射线谱对薄膜表面化学成分分析的结果表明，如此制备的金纳米薄膜具有很强的氮吸附和氮储存能力.这一特性有可能在气体传感器、空气分离和氮纯化以及氮化合物的膜合成器等技术领域得到应用. 关键词： 自支撑金纳米薄膜 硅纳米孔柱阵列 浸渍技术     

Superhydrophobic elastomer surfaces with nanostructured micronails

New approaches to the fabrication of microstructures of special shape were developed for polymers. Unusual superhydrophobic surface structures were achieved with the use of flexible polymers and hierarchical molds.Flexible polyurethane?acrylate coatings were patterned with microstructures with use of microstructured aluminum mold in a controlled UV-curing process. Electron microscope images of the UV-cured coatings on polymethylmethacrylate (PMMA) substrates revealed micropillars that were significantly higher than the corresponding depressions of the mold (even 47 vs. 35 μm). The elongation was achieved by detaching the mold from the flexible, partially cured acrylate surface and then further curing the separated microstructure. The modified acrylate surface is superhydrophobic with a water contact angle of 156° and sliding angle of < 10°.Acrylic thermoplastic elastomers (TPE) were patterned with micro?nanostructured aluminum oxide molds through injection molding. The hierarchical surface of the elastomer showed elongated micropillars (57 μm) with nail-head tops covered with nanograss. Comparison with a reference microstructure of the same material (35 μm) indicated that the nanopores of the micro?nanomold assisted the formation of the nail-shaped micropillars. The elasticity of the TPE materials evidently plays a role in the elongation because similar elongation has not been found in hierarchically structured thermoplastic surfaces. The hierarchical micronail structure supports a high water contact angle (164°), representing an increase of 88° relative to the smooth TPE surface. The sliding angle was close to zero degrees, indicating the Cassie–Baxter state.     

DFT study of CO and NO adsorption on low index and stepped surfaces of gold

Adsorption energies and vibrational frequencies of CO and NO adsorbed on gold (1 1 1), (1 0 0), (1 1 0) and (3 1 0) surfaces, as well as on adatoms on Au(1 0 0) have been calculated using density functional theory. The results clearly show that the adsorption energy of the molecules increases considerably with increasing the degree of coordinative unsaturation of the gold atoms to which the molecules bind, and thus support the view that defects, steps and kinks on the surface determine the activity of gold catalysts.     

Directed assembly of nanostructured carbon materials on to patterned polymer surfaces

Directed assembly of single-walled carbon nanomaterials on to polymer surfaces has been achieved. The approach relies on selective interactions of the polymer functionalities with the surface structures present on the carbon materials. The successful immobilization of the carbon structures was confirmed by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. By generating patterned polymer surfaces with chemically distinct components through the control of polymer–polymer or polymer–substrate interactions, directed assembly of single-walled carbon nanohorns and single-walled nanotubes was demonstrated. This new type of carbon assembly might open up new avenues in the construction of functional polymer/carbon composites and flexible nanocarbon nano-electronics.     

Electron spectroscopic characterization of oxygen adsorption on gold surfaces: II. Production of gold oxide in oxygen DC reactive sputtering

Gold oxide is produced by oxygen DC reactive sputtering in a UHV compatible chamber. It is subsequently characterized by High Resolution Electron Energy Loss, Auger and X-ray Photoelectron Spectroscopies. It is demonstrated that the oxide is of the Au₂O₃ type (auric oxide) and that it decomposes under thermal treatment. Au₂O (aurous oxide) is a possible intermediate of this reduction.     

Photoelectric properties of CdS films with nanostructured surfaces

The results from studying the photoelectric properties of CdS films with nanostructured surfaces and synthesized via pyrolysis of thiourea coordination compounds are presented. It is shown that the photocurrent limiting effect is observed in the studied structures at higher intensities of the excitation light. This effect, which is attributable to the optical smoothing of nonuniform barrier relief, could find practical application in optoelectronics.     

HREELS study of formic acid adsorption on gold (110) and (111) surfaces.

The adsorption at 100 K and the temperature decomposition of formic acid were investigated on (110) and (111) gold single crystal surfaces by high resolution electron energy loss spectroscopy. A multilayer build-up of physisorbed HCOOH with intense hydrogen bondings was observed at increasing coverages for the two gold surface orientations. Above room temperature, formic acid decomposed and desorbed from the (110) crystal, whereas it evolved into an intermediate formic anhydride on the (111) face. Further heating produced on the surfaces species similar to those observed on oxygen treated metals.     

Quasi-total omnidirectional light absorption in nanostructured gold films

A series of experiments and simulations have been performed to evidence the omnidirectional light absorption of planar gold structures containing a two-dimensional lattice of spherical beads or pores. We show that more than 90 % of incident light is absorbed at angles of incidence up to 65° for optimum values of the gold film thickness. We also report the tunability of the absorption wavelength by varying the size of the beads/pores.     

Anisotropically microstructured and micro/nanostructured polypropylene surfaces

Anisotropically microstructured and hierarchically micro/nanostructured surfaces were fabricated on polypropylene by injection moulding. Microstructured mould inserts were obtained by structuring electropolished aluminium foils with a micro-working robot, and hierarchically structured mould inserts by anodizing the microstructured aluminium foils. On both types of inserts, the microstructures were anisotropic, consisting of alternating smooth and microstructured zones. Anisotropy, and other properties of microstructures, can be controlled by adjusting the parameters of the micro-working robot. The mould inserts were used to prepare micro- and hierarchically structured polypropylene discs by injection moulding. Replication accuracy at both structure levels can be controlled through the moulding conditions. The behaviour of water on the structures was characterized by measuring the contact and sliding angles parallel and perpendicular to the microstructured zones. Surfaces with microstructures alone were highly hydrophobic, where water droplets adopted the Wenzel state and had clearly different parallel and perpendicular contact angles. Surfaces with dual structures had contact angles near 170° and sliding angles near 0°, and again the angles in parallel and perpendicular directions differed. Superhydrophobic, anisotropic Cassie-Baxter state was achieved.     

Electron spectroscopic characterization of oxygen adsorption on gold surfaces: I. Substrate impurity effects on molecular oxygen adsorption in ultra high vacuum

Tentative adsorption on clean gold (110) and (111) crystals of molecular oxygen in the pressure range 10 ^?10 to 10 ^?5 Torr, at a temperature varying between 100 and 800 K is reported together with the subsequent characterization of the surfaces by High Resolution Electron Energy Loss, Auger and X-ray Photoelectron Spectroscopies. It is found that oxygen does not adsorb in these UHV conditions, except when a contaminant is present on the surface. Such an interaction with a low level silicon impurity is described.     

Polymer adsorption on heterogeneous surfaces

The adsorption of a single ideal polymer chain on energetically heterogeneous and rough surfaces is investigated using a variational procedure introduced by Garel and Orland (Phys. Rev. B 55, 226 (1997)). The mean polymer size is calculated perpendicular and parallel to the surface and is compared to the Gaussian conformation and to the results for polymers at flat and energetically homogeneous surfaces. The disorder-induced enhancement of adsorption is confirmed and is shown to be much more significant for a heterogeneous interaction strength than for spatial roughness. This difference also applies to the localization transition, where the polymer size becomes independent of the chain length. The localization criterion can be quantified, depending on an effective interaction strength and the length of the polymer chain. Received: 29 October 1997 / Revised: 19 December 1997 / Accepted: 6 March 1998     

Hydrogen adsorption on metal surfaces

Extensive calculations of the ground state properties of hydrogen chemisorbed on transition metal surfaces are presented. The calculations are performed using the effective medium theory. The results for the chemisorption energies on all the 3d, 4d and 5d metals presented are in good agreement with experiment. The trends along a particular row are shown to be dominated by the degree of filling of the d band. The full adiabatic potential energy surface is presented for a number of experimentally interesting systems, including H/Ni(111), H/Ni(110), H/W(100) and H/W(110). Equilibrium sites, bond lengths, vibrational frequencies and surface diffusion energies are deduced and compared with experiment. Again, agreement is good. The surface and adsorbate parameters determining those observables are discussed. It is shown that a simple canonical relationship exists between the perpendicular vibrational frequency and the metal-hydrogen bond length. This formulation, which is not based on pair potentials, should be useful as a first estimate of bond lengths from measured vibrational data.     

The adsorption of oxygen on gold

The oxidation of gold has been studied under UHV conditions by AES, XPS, and TDS. The previously reported adsorbed oxygen state, which formed by heating the sample above 600 K in 10^?5 Torr of oxygen and which remained after subsequent heating to 1100 K in vacuo, has been shown to result from the reaction of oxygen with silicon diffusing from the bulk. No oxygen adsorption was detected on a clean sample for oxygen pressures up to 10^?4 Torr and sample temperatures between 300–600 K. Chemisorption of oxygen atoms could be induced by placing a hot platinum filament close to the sample during exposure to oxygen. The activation energy for desorption of this oxygen state was estimated from the thermal desorption spectra to be about 163 kJ mol^?1. The chemisorbed oxygen atoms and the oxygen associated with silicon were distinguished by different O(1s) binding energies (529.2 and 532.3 eV respectively) and by different O(KVV) Auger fine structure.     

Dehydrogenation of oligo-phenylenes on gold surfaces

Adsorption of oligo-phenylenes (p-quaterphenyl (p-4P), p-sexiphenyl (p-6P)) on various gold surfaces has been investigated under ultrahigh vacuum conditions. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS) have been applied to study the adsorbates. We have focused on the dehydrogenation of the organic molecules during thermal desorption. Whereas p-4P exhibits only a small tendency to dehydrogenate on the flat Au(1 1 1) surface, the degree of dehydrogenation on a stepped Au(4 3 3) surface or on a polycrystalline Au surface is considerable. Interestingly, p-6P shows already a strong dehydrogenation even on the flat Au(1 1 1) surface. Dehydrogenation takes place in two successive steps. The first partial dehydrogenation appears at the trailing edge of the oligo-phenyl desorption spectrum. Parallel to that stable polycyclic aromatic hydrocarbons (PAH) are formed on the surface, which eventually dehydrogenate completely at higher temperatures around 850 K. Pure carbon remains on the surface in form of a graphite like layer.     

High-temperature morphology of stepped gold surfaces

Molecular dynamics simulations with a classical many-body potential are used to study the high-temperature stability of stepped non-melting metal surfaces. We have studied in particular the Au(111) vicinal surfaces in the (M + 1,M− 1,M) family and the Au(100) vicinals in the (M,1,1) family. Some vicinal orientations close to the non-melting Au(111) surface become unstable close to the bulk melting temperature and facet into a mixture of crystalline (111) regions and localized surface-melted regions. On the contrary, we do not find high-temperature faceting for vicinals close to Au(100), also a non-melting surface. These (100) vicinal surfaces gradually disorder with disappearance of individual steps well below the bulk melting temperature. We have also studied the high-temperature stability of ledges formed by pairs of monatomic steps of opposite sign on the Au(111) surface. It is found that these ledges attract each other, so that several of them merge into one larger ledge, whose edge steps then act as a nucleation site for surface melting.     

Self‐assembled monolayers of mercaptosuccinic acid monolayers on silver and gold surfaces designed for protein binding. Part II: vibrational spectroscopy studies on cytochrome c immobilization

An attempt has been made for using MSA‐modified electrodes as linkage monolayers for electrostatic and covalent binding of cytochrome c (Cc). For MSA monolayers grown from an aqueous solution on Ag, attachment of Cc in its native state is proved in the case of covalent bonding. Electrostatic immobilization of Cc at pH 7 results in presence of at least some amount of Fe²⁺ high‐spin configuration and/or Fe³⁺ oxidation state. Native protein Fe²⁺ low‐spin state of Cc is observed after applying a negative potential to the Ag electrode. The influence of the solvent used for the preparation of the MSA monolayer and thiol surface coverage of the Ag surface was studied. It was shown that the key factor to obtain the native structure of Cc is the successful blocking of the metal surface by the MSA linking layer. IRRAS measurements of MSA on monocrystalline gold (111) at neutral pH confirm the successful electrostatic Cc immobilization, which preserves the Fe²⁺ oxidation state of the chromophore on this substrate. Copyright © 2007 John Wiley & Sons, Ltd.     

Charge‐dependent adsorption of rhodamine 6G on gold nanoparticle surfaces: fluorescence and Raman study

We study the adsorption behaviors of rhodamine dyes on gold nanoparticles (Au NPs) depending on their surface charges. Rhodamine 6G (Rh6G) dye is tested comparatively for positively and negatively charged Au NPs prepared by the reduction of chitosan and citric acid, respectively. The adsorption of Rh6G is found to be weaker on the positively charged Au NPs, whereas more substantial aggregation is found on negatively charged Au NPs. An increase in the concentration of Au NPs enhances the surface‐enhanced Raman scattering (SERS) intensities only for the Au(−) NPs, whereas the Au(+) NPs do not exhibit any strong SERS signals. Our findings suggest that SERS and reciprocal fluorescence measurements of Rh6G can be used to estimate the surface charges and atomic percentages of Au NPs less than ∼5 ppm. Copyright © 2011 John Wiley & Sons, Ltd.