Surface reconstructions and phase transitions on the GaAs(111)B surface

The (111)B surface of GaAs has been investigated using scanning tunneling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1)_LT, ( ) and (1 × 1)_HT with increasing annealing temperature, corresponding to decreasing surface As concentration. The (1 × 1)_LT is of particular interest, since it only occurs in a narrow temperature window between the two more established reconstructions, the (2 × 2) and the ( ). We find the (1 × 1)_LT to take the form of a mixture of the local structures of both the (2 × 2) and ( ) phases, rather than having a distinct structure. This is behaviour consistent with a kinetically limited system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1)_LT transition, atomic resolution images of the ( ) surface reveal only a three-fold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding due to the 23.4° rotation of the surface unit cell relative to the substrate. This is responsible for the disorder found in the ( ) reconstruction, since the structure may form in one of two domains. At lower surface As concentration, the (1 × 1)_HT surface adopts a structure combining small domains of a 19.1° structure and random disorder. There is no apparent similarity between the (1 × 1)_LT and (1 × 1)_HT structures, which may be due to our measurements being conducted at room temperature and without an As flux to control the surface As concentration.     

Construction of a novel multilayer system and its use for oriented immobilization of immunoglobulin G

In this contribution, we have developed a novel multilayer system composed of 3-aminopropyltrimethoxysilane (APTS), biotin, streptavidin and biotinylated protein A on the Si(1 0 0) surface to immobilize of immunoglobulin G (IgG) molecule. Existence of the first APTS layer covalently attached on the silicon surface was revealed by X-ray photoelectron spectroscopy (XPS). Average thickness of the APTS overlayer was estimated by spectroscopic ellipsometry analysis as 6.3 nm. The surface topography of the APTS overlayer observed by atomic force microscopy is found to be considerably different from the hydroxylated silicon surface and many additional islands of various heights are observed over the substrate. The water contact angle of the APTS overlayer was determined as 38°, whereas that of the hydroxylated silicon was obtained as 6°, indicating the difference in their surface hydrophobicility. Furthermore, multilayer studies on the APTS overlayer were carried out by using biotin, streptavidin and fluorescein-labeled biotinylated protein-A molecules. The fluorescence images obtained by fluorescence microscopy showed the formation of the multilayer on the Si(1 0 0) surface. The results indicated that the protein A-terminated surfaces can be used to immobilize IgG molecules in a highly oriented manner and maintain IgG molecular functional configuration on the multilayer system.     

四丁基锡在MCM-41表面的接枝反应及其产物表征

采用XRD,FTIR,UV-Vis DRS和TG等手段对四丁基锡改性的MCM-41及其在高纯氧气流中焙烧后得到的Sn-O-MCM-41介孔分子筛进行了表征,发现焙烧前后样品的水热稳定性、表面酸性以及表面锡物种的配位形式明显不同。焙烧后表面锡原子仍保持四配位形式,水热稳定性提高,表面L酸酸性增强。据此对Sn-O-MCM-41样品催化活性提高的原因进行了讨论。     

Surface structure and energy of B2 type intermetallic compound NiAl

The surface structure and energies for 22 surfaces of NiAl, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (1 1 0) surface belonged to the other kind of the surface consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (1 1 0) texture of NiAl appears mostly in the experiments.     

埋层界面和空气表面和频光谱信号贡献的调控

本文研究表明通过膜厚控制和表面等离激元增强方法可有效区分隐藏界面和空气表面的和频振动光谱信号. 以氟化钙基底支撑的PMMA薄膜为模型,观察到隐藏界面和空气表面对和频信号贡献的变化. 通过监控羰基和甲基伸缩振动基团,发现薄PMMA膜的和频信号来自PMMA/空气表面的化学基团-CH₂、-CH₃、-OCH₃和C=O,而厚PMMA膜的和频信号则来自基底/PMMA埋层界面的-OCH₃和C=O基团. 随制膜浓度增大,埋层界面C=O基团的取向角从65°下降到43°,且浓度大于或等于0.5 wt%时,取向角等于45°±2°. 相比之下,空气表面C=O的取向角落在21°∽38°之间. 在金纳米棒存在条件下,表面等离激元可以极大地增强和频信号,尤其是来自埋层界面信号.     

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

Surface modification of polyacrylonitrile-based carbon fiber and its interaction with imide

In this work, sized polyacrylonitrile (PAN)-based carbon fibers were chemically modified with nitric acid and maleic anhydride (MA) in order to improve the interaction between carbon fiber surface and polyimide matrix. Bismaleimide (BMI) was selected as a model compound of polyimide to react with modified carbon fiber. The surface characteristic changing after modification and surface reaction was investigated by element analysis (EA), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and surface enhanced Raman scattering (SERS). The results indicated that the modification of carbon fiber surface with MA might follow the Diels Alder reaction mechanism. In the surface reaction between modified fibers and BMI, among the various surface functional groups, the hydroxyl group provided from phenolic hydroxyl group and bridged structure on carbon fiber may be the most effective group reacted with imide structure. The results may shed some light on the design of the appropriate surface structure, which could react with polyimide, and the manufacture of the carbon fiber-reinforced polyimide matrix composites.     

Surface structure and composition of flat titanium thin films as a function of film thickness and evaporation rate

To correlate flat titanium film surface properties with deposition parameters, titanium flat thin films were systematically deposited on glass substrates with various thicknesses and evaporation rates by electron-beam evaporation. The chemical compositions, crystal structure, surface topographies as well as wettability were investigated by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM) and water contact angle measurement, respectively. The films consisted mainly of TiO₂. Small percentages of Ti₂O₃ and metallic Ti were also found at the film surface using high-resolution XPS analysis. Quantitative XPS showed little differences regarding elemental compositions among different groups of films. The films were obtained by varying the deposition rate and the film thickness, respectively. XRD data showed consistent reflection patterns of the different titanium samples deposited using different film thicknesses. Without exception measurements of all samples exhibited contact angles of 80° ± 5°. Quantitative AFM characterization demonstrated good correlation tendency between surface roughness and film thickness or evaporation rate, respectively. It is important to notice that titanium films with different sizes of grains on their surfaces but having the same chemistry and film bulk structure can be obtained in a controllable way. By increasing the film thickness and evaporation rate, the surface roughness increased. The surface morphology and grain size growth displayed a corresponding trend. Therefore, the control of these parameters allows us to prepare titanium films with desired surface properties in a controllable and reproducible way for further biological investigations of these materials.     

Surface elasticity and size effect on the vibrational behavior of silicon nanoresonators

Predominance of nano-scale effects observed in material behavior at small scales requires implementation of new simulation methods which are not merely based on classical continuum mechanic. On the other hand, although the atomistic modeling methods are capable of modeling nano-scale effects, due to the computational cost, they are not suitable for dynamic analysis of nano-structures. In this research, we aim to develop a continuum-based model for nano-beam vibrations which is capable of predicting the results of molecular dynamics (MD) simulations with considerably lower computational effort. In this classical-based modeling, the surface and core regions are taken to have different mechanical properties, where core atoms are assumed to have macroscale properties whereas surface layer is showing a different elastic modulus from the core components. By estimating physical parameters of proposed classical model using molecular dynamics results and the genetic algorithm, calibrated classical Euler–Bernoulli and Timoshenko beam models are developed. The results demonstrates that a Timoshenko beam model incorporating surface effects and having calibrated parameters, is able to provide almost the same results as molecular dynamics method which can be used to predict the vibrational behavior of nano-beams at different scales from nano to macro.     

平面光栅支撑结构设计及其面形分析计算

为满足二氧化碳探测仪平面光栅高精度面形要求,对光栅及其支撑结构进行了设计和分析.首先,对光栅结构组件各材料的选取和支撑结构形式进行了研究;然后,以坐标变换法和最小二乘法为理论依据,对镜面刚体位移和畸变的计算求解进行了探讨;接着,采用有限元法对光栅结构组件在不同热力耦合状态下的面形精度和结构强度进行仿真分析.运用有限元软件本身提供的二次开发功能,开发了能内部调用MATLAB编写的面形误差计算程序.经计算,各工况下面形误差PV≤63nm,RMS≤12.6nm.最后,为了验证结构组件的动态特性,进行了模态分析和谐响应分析,三个方向的基频分别达到1 046.3Hz,1 640Hz,1 147.5Hz.这些结果表明,光栅结构组件设计合理可行,能够满足各项设计要求.     

Computer simulations of the effect of atomic structure and coordination on the stabilities and melting behaviour of copper surfaces and nano-particles

We have studied the structures and stabilities of copper nano-particles and the melting properties of copper surfaces using interatomic potential-based molecular dynamics simulations, where the (1 1 1) surface has been shown to be the most stable in terms of surface energy and melting behaviour. Low energy shapes of nano-particles are influenced by the surfaces present and therefore have a higher proportion of (1 1 1) surface. The effect of surface structure on stability becomes less marked as the size of the nano-particle is increased. Melting is observed to occur below the bulk melting temperature in all the surfaces investigated, at increasingly lower temperatures from the (1 1 1), (1 0 0), (1 1 0) down to the (2 1 0) surface, confirming their order of decreasing stability. The melting processes of defective close-packed copper surfaces were also simulated. Steps, kinks, and facets were all shown to accelerate the melting of the surfaces. The melting is shown to initiate at the site of the defect and the results demonstrate that it is the low-coordinated atoms, at the step edge or kink, that are more mobile at lower temperatures. These features facilitate surface melting even further below the melting temperature than was observed for the perfect surfaces. Furthermore, facets of (1 0 0) surface were shown to be unstable even at moderate temperatures on the close-packed surface.     

Surface morphology and crystalline structure of high-stable polycrystalline transparent conductive zinc oxide films

Surface morphology and crystalline structure of high-stable zinc oxide films were evaluated by atomic force microscopy, scanning electron microscopy and X-ray diffraction (XRD) measurements. AFM measurement revealed that the higher stable samples have smaller roughness (average roughness and root mean square) parameters than the lower stable samples. Furthermore, in-plane XRD measurement showed that the crystallite size of high stable samples is smaller than that of the low stable samples. These results indicate that the larger surface area and lower film density deteriorates the stability of zinc oxide films through the adsorption and reaction of water or oxygen molecules. They also suggest that we can prepare the high stable zinc oxide transparent electrode films by controlling the surface morphology.     

Molecular dynamics study on surface structure and surface energy of rutile TiO2 (1 1 0)

The formula for surface energy was modified in accordance with the slab model of molecular dynamics (MDs) simulations, and MD simulations were performed to investigate the relaxed structure and surface energy of perfect and pit rutile TiO₂(1 1 0). Simulation results indicate that the slab with a surface more than four layers away from the fixed layer expresses well the surface characteristics of rutile TiO₂ (1 1 0) surface; and the surface energy of perfect rutile TiO₂ (1 1 0) surface converges to 1.801±0.001 J m⁻². The study on perfect and pit slab models proves the effectiveness of the modified formula for surface energy. Moreover, the surface energy of pit surface is higher than that of perfect surface and exhibits an upper-concave parabolic increase and a step-like increase with increasing the number of units deleted along [0 0 1] and [1 1 0], respectively. Therefore, in order to obtain a higher surface energy, the direction along which atoms are cut out should be chosen in accordance with the pit sizes: [] direction for a small pit size and [0 0 1] direction for a big pit size; or alternatively the odd units of atoms along [1 1 0] direction are removed.     

Surface and interfacial properties of PVDF/acrylic copolymer blends before and after UV exposure

Morphological and chemical properties of both the surface and interface of poly(vinylidene fluoride)/poly(methyl methacrylate)-co-poly(ethyl acrylate) (PVDF/PMMA-co-PEA) blend films have been investigated before and after the samples were exposed to ultraviolet (UV) irradiation using a xenon arc lamp at 50 °C and 9% relative humidity (RH) for 7 months. Surface and interfacial morphologies were studied by atomic force microscopy (AFM). Chemical composition information was obtained by confocal Raman microscopy, attenuated total reflection-FTIR spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Results show an enrichment of the PVDF material at the air surface, while the acrylic copolymer enriches the interface. Blends having greater than 50% mass fraction of PVDF show little change in the surface morphology after UV exposure for 7 months. However, for a lower PVDF content, blends exhibit significant degradation of PMMA-co-PEA copolymer and a much rougher surface after UV exposure. Microstructural changes in the PVDF spherulites are also observed after UV degradation. It is found that the surface and interfacial morphologies are correlated with the chemical properties.     

Epitaxial growth of organic heterostructures: Morphology, structure, and growth mode

For organic molecular materials the definition of the condition of epitaxial growth has been subjected to several misconceptions and a great debate, leading in any case to less restrictive requirements than for inorganic materials. Here, the deposition of oligothiophene films by molecular beam epitaxy on properly grown organic single crystals is discussed and all-organic rigorous epitaxy demonstrated for both sub-monolayer and several-monolayer thick films. The possibility of growing all-organic nanostructures directly follows from these results.     

Surface roughness and color characteristics of wood treated with preservatives after accelerated weathering test

Wood samples treated with ammonium copper quat (ACQ 1900 and ACQ 2200), chromated copper arsenate (CCA), Tanalith E 3491 and Wolmanit CX-8 have been studied in accelerated weathering experiments. The weathering experiment was performed by cycles of 2 h UV-light irradiation followed by water spray for 18 min. The changes on the surface of the weathered samples were characterized by roughness and color measurements on the samples with 0, 200, 400 and 600 h of total weathering.

The objective of this study was to investigate the changes created by weathering on impregnated wood with several different wood preservatives. This study was performed on the accelerated weathering test cycle, using UV irradiation and water spray in order to simulate natural weathering.

Surface roughness and color measurement was used to investigate the changes after several intervals (0–200–400–600 h) in artificial weathering of treated and untreated wood.     

Surface structure analysis based on the exclusive use of the specular LEED spot – a theoretical study

The exclusive use of the specularly reflected beam (the (0,0) spot) may be a more practical way of collecting data for a LEED I–V structure analysis under certain experimental conditions. In this paper we discuss the special properties of the (0,0) spot intensity and test its sensitivity towards structural changes for the model system CO/Ni(1 1 1) within the framework of a R factor analysis. It is found that the (0,0) spot can, indeed, be used for a reliable structure determination if the energy range is increased by collecting data at different polar and azimuthal angles of incidence. The R factor contrast is, however, reduced with respect to a conventional LEED I–V analysis.     

IR Absorption,Raman Scattering,and IR-Vis Sum-Frequency Generation Spectroscopy as Quantitative Probes of Surface Structure

Abstract:Vibrational spectroscopy is a valuable quantitative tool for the determination of structure at surfaces. Various techniques may be applicable and useful, depending on what is available, the transparency of the substrates, the need for in situ probes, and the degree of interfacial specificity required. We examine and compare signals in infrared absorption, Raman scattering, and vibrational sum-frequency generation spectroscopy to the underlying molecular response. In all of these experiments, varying the beam polarizations enables the orientation of specific chemical functional groups to be determined. However, the sensitivity of each technique is directly connected to the manner in which the molecular response manifests itself in the measured signal. Starting with simple distributions of a single vibrational mode, leading up to multiple vibrational bands in more complex orientation distributions, we compare these three techniques in terms of their sensitivity to features of the molecular orientation distribution. This review is aimed at guiding planned experiments when multiple techniques are available for surface structural analysis.     

Surface morphology of InGaAs on GaAs(100) by chemical beam epitaxy using unprecracked monoethylarsine, triethylgallium and trimethylindium

Temperature-dependent evolution of surface corrugation and the interface dislocation in In_0.15Ga_0.85As epilayer on GaAs(100) substrate grown by chemical beam epitaxy using unprecracked monoethylarsine have been investigated by atomic force microscope (AFM) and transmission electron microscopy (TEM). AFM images showed that the line direction of surface ridge changes from [011] to [0 1] with increasing temperature. However, TEM micrographs showed that dislocation networks are formed along both [011] and [0 1] directions at the interface. These results indicate that growth kinetics on the terrace and at surface steps generated by the dislocations play an important role in determining the direction of surface corrugation. We suggest that the temperature-dependent change of surface corrugation is caused by an anisotropic surface diffusion on the terrace and different sticking probability of adsorbates on the surface steps which were produced by interface misfit dislocation along the two orthogonal surface directions.