Potential-induced structure transitions in self-assembled monolayers: ethanethiol on Au(1 0 0)

We have characterized the structural behaviour of ethanethiol self-assembled monolayers (SAMs) on Au(1 0 0) in 0.1 M H₂SO₄ as a function of electrode potential, using in-situ scanning tunneling microscopy (STM). After modification of the Au(1 0 0) electrode in an ethanolic solution of ethanethiol, STM images in air reveal a disordered thiol adlayer and a surface that is covered by 25% of monoatomic high gold islands, which originate from lifting of the (hex) reconstruction during thiol adsorption. In contrast to alkanethiol SAMs on Au(1 1 1), no vacancy islands are seen on the Au(1 0 0) surface. After contact of the SAM-covered Au(1 0 0) electrode with 0.1 M H₂SO₄ under potential control, two different structures are observed, depending on the potential range positive or negative of +0.3 V vs. SCE. In both cases the emerging ordered structures are quadratic, their unit cells being rotated by 45° with respect to the main crystallographic axes of the substrate. However, the ordered structure at negative potentials is more densely packed than the one at positive potentials, and in addition the surface reveals an almost 50% coverage of monoatomic high gold islands. The structure of the SAM changes reversibly with the electrode potential, the long range order gradually decreasing with each transition. Concomittant with this structure transition monoatomic deep holes are created when the potential is stepped from the cathodic to the anodic region. The experimental observations are rationalized by a high mobility of the gold thiolate moiety, causing the surface density of the SAM-covered gold to change drastically with potential.     

ATR–SEIR study of anions and water adsorbed on platinum electrode

Adsorbed species on bare Pt, and UPD-Pb or UPD-Cu/Pt electrodes were characterized in HClO₄ or H₂SO₄ solutions at various potentials using attenuated total reflection (ATR)–surface enhanced infrared absorption (SEIRA) spectroscopy. On the bare Pt electrode, anions were observed at 1120–1095 cm⁻¹ at +0.0 < E < +0.6 V, solvated by water molecules with OH stretching absorption at 3600 cm⁻¹ and HOH bending mode at 1610–1620 cm⁻¹. In addition to the S–OH totally symmetric mode at 950 cm⁻¹, adsorbed sulfate species gave two bands at 1230–1100 cm⁻¹ between 0.0 V < E < +0.8 V that are assigned to ν₃ (symmetric stretch of S–O in SO₃) of ions with different coordination based on the peak shift by isotope substitution. At more negative potential, solely water molecules adsorb on the bare Pt surfaces. In contrast, it was found that electrolyte anions such as bisulfate and with hydrating water molecules adsorb onto the UPD-Pb/Pt and UPD-Cu/Pt electrodes even at much negative potentials, e.g. −0.2 V for UPD-Pb.     

Electrode contact study for SiGe thin film operated at high temperature

A study on the electrode contact of the sputtered SiGe thin film is reported for application of devices working at high temperature. Surface morphological characterization with optical microscope and AFM (atomic force microscope) together with the electrical characterization by TLM measurements (transmission line method) were performed before and after aging at 500 °C for 24 h using various sputtered multilayer electrodes, Ti/Au/Ti, Ta/Pt/Ta and Ti/Pt/Ti, on 300-nm B-doped SiGe thin film deposited by magnetron sputtering and furnace crystallisation at high temperature. After aging at 500 °C for 24 h, the Ti/Au/Ti multilayer electrodes seriously degraded to be non-ohmic contact, showing rough surface morphology. The Ti/Pt/Ti metal layers showed the lowest specific contact, resistivity before and after aging, 1.46 × 10⁻³ Ω cm² and 1.68 × 10² Ω cm² respectively.     

High pyroelectric Ba0.65Sr0.35TiO3 thin films with Ba0.65Sr0.35RuO3 seeding-layer for monolithic ferroelectric bolometer

Ba_0.65Sr_0.35TiO₃ (BST) thin films were deposited by RF sputtering with a very thin Ba_0.65Sr_0.35RuO₃ (BSR) seeding-layer on Pt/Ti/SiO₂/Si substrate. The crystallization of BST thin films and the surface morphology of BSR seeding-layer were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD patterns show that the BSR seeding-layer affected the orientation of BST thin film, which is highly a-axis textured. It was also found that the BSR seeding-layer had a marked influence on the dielectric properties of BST thin films. Comparing with BST thin films directly deposited on Pt electrode, the dielectric relaxation can be suppressed and dielectric constant increased due to a possible reduction of interface oxygen vacancies at BST/BSR interface. Moreover, J–V measurement indicates that the leakage current density of BST thin films on BSR seeding-layer were greatly reduced compared with that of BST thin films directly on Pt electrodes. The pyroelectric coefficient of BST thin films with BSR seeding-layer is 7.57 × 10⁻⁷ C cm⁻² K⁻¹ at 6 V/μm at room temperature (RT). Our results reveal that high pyroelectric property of BST thin film could be achievable using BSR seeding-layer as a special buffer.     

X-ray diffraction evidence of the single solid solution character of bi-metallic Pt-Pd catalyst particles on an amorphous SiO2 substrate

An X-ray diffraction study was carried out on powders of a series of catalysts prepared from aqueous solutions of H₂PtCl₆ and PdCl₄ and amorphous SiO₂ with different concentrations in weight of Pt and Pd at about 4% in overall metallic weight. Measurements of the position of high angle Bragg reflections in the diffractograms show evidence of the fact that the small catalyst particles are bi-metallic Pt-Pd crystals having a face-centred cubic Bravais lattice. The lattice constant of these crystals was found to change with the relative concentration of Pt and Pd by following the Vegard’s rule. This correlation leads to the conclusion that the bi-metallic catalyst particles are made of a single solid solution of Pt and Pd atoms in the whole range of relative concentrations. Relative concentrations of these metals in the samples under study were determined by using energy dispersive X-ray spectrometry and their values were found to be close to the stochiometric relative concentrations in weight of the metals in the precursor aqueous solution. An average size of about 96 Å was estimated for the bi-metallic particles from the full-width at half-maximum value measured for the (2 0 0) diffractometric curve.     

Three-Ni-atom cluster formed by sulfur adsorption on Ni(1 1 1)

Sulfur adsorption on Ni(1 1 1) at room temperature has been studied by scanning tunneling microscopy. Irregularly-shaped islands, which are aggregates of small particles, appear with troughs on (1 1 1) terraces. Estimating the number of small particles in the irregular islands and that of Ni atoms ejected through the formation of troughs, we find that each small particle contains three Ni atoms. A three-Ni-atom cluster is proposed, where three S atoms are adsorbed on 4-fold hollow sites formed on the sides of the cluster. The Ni₃S₃ cluster can assume two orientations on the (1 1 1) surface. We show that the apparently-irregular distribution of small particles is well understood by assuming the alternate orientation of neighboring Ni₃S₃ clusters and the slight energy difference between the two orientations of a single Ni₃S₃ cluster. Real-time observation of a Ni(1 1 1) surface under H₂S ambience reveals that small islands composed of three or four Ni₃S₃ clusters change their positions rapidly, preventing the simple growth of islands at the initially-nucleated positions.     

三维多孔复合碳层对电极的制备及其光伏性能研究

基于TiO_2光阳极、Pt对电极的染料敏化太阳能电池(DSSC)因其优异的光电转换特性受到了广泛的关注,然而Pt昂贵的价格制约了其发展与应用.针对这一问题,本文设计、制备了一种由相对致密且高导电的石墨膜(PC层,底层)及多孔碳纳米颗粒膜(CC层,顶层)构成的低成本、高性能三维多孔复合碳层对电极.基于该CC/PC对电极的DSSC具有优异的光伏性能:在1.5标准太阳光照射下,其填充因子高达65.28%(较Pt对电极高4.1%)、光电转换效率高达5.9%(为Pt对电极的94.2%). CC/PC对电极的优异光伏性能主要归因于其独特的三维多孔导电结构,该结构有极高的比表面积和丰富的催化反应活性位,有利于电子的快速传输及离子的快速转移,在这些因素的协同作用下,其光电转换性能大大改善.     

Sonochemical immobilization of noble metal nanoparticles on the surface of maghemite: mechanism and morphological control of the products

Aqueous sample solutions containing noble metal ions (HAuCl₄, Na₂PdCl₄, H₂PtCl₆), polyethyleneglycol monostearate, and magnetic maghemite nanoparticles were irradiated with high power ultrasound. Analyses of the products showed that noble metal ions were reduced by the effects of ultrasound, and the formed noble metal nanoparticles were uniformly immobilized on the surface of the maghemite. The present “one pot process” significantly simplifies the immobilization of noble metal nanoparticles on the surface of supports, compared with the conventional impregnation method. The average diameter of immobilized Au was 7–13 nm, and the diameters of Pd and Pt were several nm. The diameters depended upon the concentration of polyethyleneglycol monostearate and the concentration of noble metal ions, but not upon the maghemite concentration, indicating the possibility of the morphological controls of the products by adjusting these preparation conditions. The measurements of the average diameters and the numbers of immobilized Au nanoparticles obtained under various conditions suggest that the nucleation of Au does not occur on the surface of maghemite, but it might occur in the homogeneous bulk solution.     

In situ synchrotron far infrared micro-spectroelectrochemistry with a grazing angle objective

We demonstrate the use of a grazing angle objective attachment to carry out in situ far infrared micro-spectroelectrochemistry at a copper electrode on a nano-scale. A thin-layer spectrochemical cell made out of Teflon was used, fitted with a 20-μm thick Mylar window; the working electrode was 500 μm in diameter. Measurements were carried out in 0.1 M NaOH solution as a function of applied potential between –1.4 and 0 V vs a Hg/Hg₂SO₄ reference electrode. Spectra were obtained with excellent signal to noise ratio for the surface oxide film, formed on copper electrochemically with less than 1 nL of active solution volume. The surface film at 0 V was about 130 nm thick and consisted mainly of CuO, with possibly some Cu(OH)₂ also present. This interpretation is consistent with previous works and thermodynamic calculations. The technique should be useful in other investigations and the further development of electrochemical surface science.     

The effect of thickness and annealing condition on the microstructure and magnetic properties of Fe/Pt multilayer thin films

[Fe(0.5 nm)/Pt(0.5 nm)]₄₀, [Fe(1 nm)/Pt(1.5 nm)]₂₀ and [Fe(3 nm)/Pt(3 nm)]₁₀ multilayer were prepared by DC magnetron sputtering. By conventional furnace annealing (CA) at 270–600 °C for various time, all of the films still remained the disordered structure with the soft magnetic phase. By rapid thermal annealing (RTA) at 500 °C for various time, we obtained the [Fe(1 nm)/Pt(1.5 nm)]₂₀ and [Fe(3 nm)/Pt(3 nm)]₁₀ films with L1₂ ordered FePt₃ phase which was almost ferromagnetic at room temperature. However, the [Fe(0.5 nm)/Pt(0.5 nm)]₄₀ films was still disordered state even under RTA. Compared with CA, RTA exposed an outstanding effect on accelerating the phase transition when the film thickness is over [Fe(0.5 nm)/Pt(0.5 nm)]₄₀.     

Studies of sol–gel TiO2 and Pt/TiO2 catalysts for NO reduction by CO in an oxygen-rich condition

The textural properties, morphological features, surface basicity and oxygen reduction behaviours of titania and Pt supported titania catalysts synthesized via a sol–gel method were studied by means of N₂ physisorption, SEM, TEM, CO₂-TPD and H₂-TPR techniques. Mesostructured TiO₂ shows a very narrow pore size distribution that uniformly centred at about 4 nm. High resolution TEM images confirmed that most of Pt particles on Pt/TiO₂-SG had a size smaller than 2 nm. Both the titania support and Pt loaded catalysts chiefly contained weak basic sites with small amount of strong basic sites. Loading Pt did not significantly alter the surface reduction characters of titania, indicating a weak interaction between Pt metals and titania support. Catalytic evaluation revealed that the selectivity of NO reduction over titania was insensitive to variation of textural property. On the bare titania, low NO conversion but high selectivity to N₂O was obtained. However, the Pt/TiO₂-SG catalysts exhibited high NO conversion and high selectivity to N₂, which is assumed to relate to NO dissociation catalysed by the metallic Pt clusters. In addition, when the reaction temperature was above 200 °C, 3–11% NO₂ was yielded over the Pt/TiO₂-SG catalysts, which was discussed on a basis of reaction competition, metal-support interaction and NO dissociation.     

Investigation of the role of oxygen in NO reduction by C2H4 on the surface of stepped Pt(3 3 2)

The influence of pre-dosed oxygen on NO–C₂H₄ interactions on the surface of stepped Pt(3 3 2) has been investigated using Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). The presence of oxygen significantly suppresses the adsorption of NO on the steps of Pt(3 3 2), leading to a very specific adsorption state for NO molecules when oxygen–NO co-adlayers are annealed to 350 K (assigned as atop NO on step edges). An oxygen-exchange reaction also takes place between these two kinds of adsorbed molecules, but there appears to be no other chemical reaction, which can result in the formation of higher-valence NOx.

C₂H₄ molecules which are post-dosed at 250 K to adlayers consisting of ¹⁸O and NO do not have strong interactions with either the NO or the ¹⁸O atoms. In particular, interactions which may result in the formation of new surface species that are intermediates for N₂ production appear to be absent. However, C₂H₄ is oxidized to C¹⁸O₂ by ¹⁸O atoms at higher annealing temperature. This reaction scavenges surface ¹⁸O atoms quickly, and the adsorption of NO molecules on step sites is therefore quickly restored. As a consequence, NO dissociation on steps proceeds very effectively, giving rise to N₂ desorption which closely resembles that following only NO exposure on a clean Pt(3 3 2), both in peak intensity and desorption temperature. It is concluded that the presence of ¹⁸O₂ in the selective catalytic reduction (SCR) of NO with C₂H₄ on the surface of Pt(3 3 2) does not play a role of activating reactants.     

Annealing effect on InAs islands on GaAs(0 0 1) substrates studied by scanning tunneling microscopy

Post-annealing effects on InAs islands grown on GaAs(0 0 1) surfaces have been investigated by scanning tunneling microscopy (STM) connected to molecular beam epitaxy (MBE). It is found that for islands grown by 1.6 ML InAs deposition at 450 °C, post-annealing at 450 °C in an As₄ atmosphere causes dissolving of the InAs islands. In contrast, for larger islands obtained by 2.0 ML InAs deposition at 450 °C, the post-annealing leads to coarsening of the islands. The result can be explained in terms of a critical nucleus in heterogeneous nucleation.     

Overpotential deposition of Ag monolayer and bilayer on Au(1 1 1) mediated by Pb adlayer underpotential deposition/stripping cycles

Ultra-thin Ag films on the Au(1 1 1) surface were prepared via overpotential deposition (OPD) in the presence of Pb²⁺ ions. By carrying out repetitive Pb adlayer underpotential deposition (UPD) and stripping cycles during Ag bulk deposition, the two-dimensional growth of Ag films was significantly enhanced in high OPD. The Ag monolayer sample was made by comparing the voltammetry curves, in which the signatures for Pb adlayer UPD on Au(1 1 1) changed to that on Ag(1 1 1). As demonstrated by the X-ray specular reflectivity measurements, nearly complete monolayer and bilayer films can be made with optimized deposition procedures. On subatomic scale, however, we found that these films have significant higher root-mean-square displacement amplitudes than those underpotentially deposited Ag monolayer and bilayer on either Au(1 1 1) or Pt(1 1 1).     

Vanadium oxide nanostructures on Rh(1 1 1): Promotion effect of CO adsorption and oxidation

The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 × 1)ORh(1 1 1) surface and on vanadium oxideRh(1 1 1) “inverse model catalyst” surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V₃O₉Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of 50 nm, type I model catalyst) and small (mean size <15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 × 1)ORh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 × 2)O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I “inverse model catalyst” surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary.     

Improved dielectric and electrical properties of (Ba,Sr)TiO3 thin films using Pt/LaNiO3 as the top-electrode material

(Ba,Sr)TiO₃ (BST) capacitors grown on a LaNiO₃ (LNO) bottom electrode, with Pt (80 nm), LNO (100 nm), and double-layer Pt/LNO (80/10 nm) top electrodes have been investigated. It was found that the dielectric behavior is improved by a decrease of the electrical properties for the BST capacitor using double-layer Pt/LNO top electrodes. The dielectric constant of 100 nm-thick BST films with a Pt electrode was only 165 at 100 kHz, while that with a double-layer electrode was about 242. Correspondingly, the tunability was greatly improved from 26% to 41% with an electric field of 600 kV/cm. These have been attributed to increased interfacial capacitance density, which resulted from an improved interface, between the films and the top electrode. The dielectric loss was also reduced by using a double-layer electrode. Furthermore, the leakage current of a capacitor with a double-layer Pt/LNO electrode was one order of magnitude lower than that with a single LNO electrode. It can be explained by the fact that the weak chemical interaction between LNO (10 nm) and BST causes a high potential barrier at the interface. PACS 81.15.-z; 81.15.Cd; 77.55.+f; 77.84.Dy; 77.22.-d     

Effects of pH and citric acid contents on characteristics of ester-derived BaFe12O19 powder

The single domain size of BaFe₁₂O₁₉ powder with crystallite sizes less than 200 nm was produced using a citric acid precursor method. Fe³⁺ and Ba²⁺, in a molar ratio of 12, were chelated by COOH⁻ in an aqueous solution. After ethylene glycol additions, esterification, dehydration, and calcination led to the formation of ester-derived BaFe₁₂O₁₉ powder. High pH and/or high citric acid contents in the starting solution are required to complete chelate metallic ions in the solution and to form pure barium ferrite powder at 1073 K. Pure single magnetic domain BaFe₁₂O₁₉ particles of M(30 kOe)≈54 emu/g, M_r≈28 emu/g, and H_c≈3.7 kOe were produced using [citric acid]/[metallic ions]=1.5 and pH7.     

Time evolution of interface roughness during thermal oxidation on Si(0 0 1)

The surface morphological change at an initial stage of thermal oxidation on Si(0 0 1) surface with O₂ was investigated as a function of oxide coverage by a real-time monitoring method of Auger electron spectroscopy (AES) combined with reflection high energy electron diffraction (RHEED). At 653 °C where oxide islands grow laterally, protrusions were observed to develop under the oxide islands as a consequence of concurrent etching of the surface. The rate of etching was measured from a periodic oscillation of RHEED half-order spot intensity I_(1/2,0) and I_(0,1/2). At 549 °C where Langmuir-type adsorption proceeds, it was observed that both I_(1/2,0) and I_(0,1/2) decrease more rapidly in comparison with an increase of oxide coverage and the intensity ratio between them decreases gradually with O₂ exposure time. These suggest that Langmuir-type adsorption occurs at sites where O₂ adsorbs randomly, leading to subdivision of the 2×1 and 1×2 domains by oxidized regions, and that Si atoms are ejected due to volume expansion in oxidation to change the ratio between 2×1 and 1×2 domains.     

真空中固液界面的离子束分析研究

原位实时地高精度测量固液界面的元素或离子(电荷)组成和动态变化对于界面反应和相互作用研究非常重要,但是传统的高分辨离子束分析实验在真空环境中不能直接测量液体样品。本文研制了一种固体-液体界面探针,该探针使用氮化硅-铝纳米复合膜作为真空密封窗和电化学电极,利用复旦大学核微探针成功开展了真空中固体-液体界面探针0.01 mol/L氯化钡和1 mol/L氯化镧溶液样品固体-液体界面的卢瑟福背散射(RBS)分析和粒子激发X射线(PIXE)分析。实验结果表明,真空环境下,固液界面探针纳米薄窗可承受2 MeV He+离子注量为1.0×1018 ions/cm2的辐照。微区PIXE分析成功获得了固液界面探针结构的元素分布。通过对卢瑟福背散射能谱进行分析,获取了20 nm分辨的电极界面微米深溶液中的La, Cl元素浓度。在1 mol/L的LaCl3固液界面电极表面,负电压(–2.3 V)时电解质离子在电极表面高浓度聚集,正电压(+2.3 V)时电解质在电极表面呈低浓度分布,在约1 250 nm深处电解质溶液趋向于体浓度。     

Elemental steps in the growth of thin β-Ga2O3 films on CoGa(1 0 0)

The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered β-Ga₂O₃ films grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide films exhibit Fuchs–Kliewer phonons at 305, 455, 645, and 785 cm⁻¹. For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide films have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of β-Ga₂O₃ are found also on the terraces. After an exposure of 200 L O₂ at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin film of β-Ga₂O₃.