Low energy electron diffraction studies of chemisorbed gases on stepped surfaces of platinum

The chemisorption of hydrogen, oxygen, carbon, carbon monoxide and ethylene was studied by low-energy electron diffraction on ordered stepped surfaces of platinum which were cut at angles less than 10° from the (111) face. The chemisorption characteristics of stepped platinum surfaces are markedly different from those of low index platinum surfaces and they are also different from each other. Hydrogen and oxygen which do not chemisorb easily on the (111) and (100) crystal faces chemisorb readily and at relatively low temperatures and pressures on the stepped platinum surfaces used in this study. In contrast to the ordered adsorption of carbon monoxide and ethylene on low index faces, the adsorption was disordered on the stepped surfaces and there is evidence for dissociation of the molecule. Carbon formed several ordered surface structures and caused faceting on the stepped surface, which are not observed on low index platinum surfaces. There appears to be a much stronger interaction of chemisorbed gases with stepped surfaces than with low index planes that must be caused by the differing atomic structures at the steps. Evidence for the differing reactivities of the two stepped surfaces are also discussed.     

Chemisorption and ordering of naphthalene and azulene on Pt(s)[7(111) × (100)]: The effect of periodic defects on long range order

Naphthalene and azulene are molecularly adsorbed on the stepped platinum surface Pt(s)[7(111)×(100)] at room temperature. The (111) terraces of this surface are wide enough to accommodate the unit cell of ordered naphthalene on Pt(111). The periodic defects of this surface disrupt the long range ordering seen for both naphthalene and azulene on Pt(111). There appears to be no correlation between ordered islands on neighboring terraces. In the case of naphthalene, a LEED intensity anomaly allows us to place restrictions on the relative orientations of neighboring naphthalenes and the orientation of the naphthalene islands on each terrace. The naphthalene molecules adjacent to the steps have the long axis of the molecule close to perpendicular to the step.     

Calculated photoelectron angular distributions of ω-(n-pyrrolyl)alkanethiol self-assembled monolayers for distinguishing between different arrangements of the pyrrole groups

Photoelectron angular distributions from ω-(n-pyrrolyl)alkanethiol self-assembled monolayers (SAMs) were calculated within a single-scattering approximation of the photoemission process. The calculations were carried out on two different surface structures with face-stacked and herringbone arrangements of the pyrrole groups, which were deduced from molecular dynamics calculations. The characteristic angular patterns calculated for the molecular orbital originating from the pyrrole group involve information on the orientations of the pyrrole groups, which allows distinction between these arrangements. The photoelectron angular distributions from substituted SAMs can be used as a clue for studying the surface structures of the substituent groups.     

Growth kinetics of self-assembled monolayers of thiophene and terthiophene on Au(111): An infrared spectroscopic study

The growth kinetics of self-assembled monolayers (SAMs) of thiophene compounds on Au(111) surfaces was revealed by Fourier-transform infrared reflection absorption spectroscopy (FT-IR-RAS). Thiophene and terthiophene form well-ordered SAMs on Au(111) surfaces by immersing gold substrates into their ethanol solutions for ca. 15 h. Gibbs free energies for the adsorption processes of thiophene and terthiophene were found to be identical. However, the growth and molecular orientation of SAMs are different between two thiophene compounds. Terthiophene in SAMs orients parallel to the surface. The SAM growth of terthiophene obeys a time-dependent Langmuir scheme. On the other hand, the thiophene SAM undergoes a two-step growth process with unique molecular orientations. In the primary phase, thiophene assumes a parallel orientation on the Au(111) surface. In the second phase, thiophene is oriented close to the normal of the surface. The different growth process between thiophene and terthiophene is attributable to the topology of sulfur positions in the molecules. Received 23 May 2001 and Received in final form 11 February 2002     

Cells anchored upon a thin organic film with different nano-mechanical properties

By applying dynamic contact module and particular measurement of phase angles, harmonic contact stiffness (S) along with the measured displacement (D) of different self-assembled monolayers (SAMs) adsorbed on Au can be distinguished. The relatively ordered and hydrophobic ODT and DDT molecules adsorbed on Au form high contact stiffness, which are presumably unfavorable substrates for a cell to adhere upon. Short-chain MUA molecules adsorbed on Au provides a hydrophilic characteristic with a relatively low contact stiffness, which may significantly promote cell adhesion. It is, therefore, estimated that the behavior of a cell adhered on SAMs/Au is correlated not only with their outermost chemical species but also with a proper dS/dD matrix acting as a cushion.     

Exploring the magnetic and organic microstructures with photoemission electron microscope

We present photoemission electron microscopy (PEEM) studies on geometrically constrained ferromagnetic, organic, and organics–ferromagnet hybrid structures. Powered by an elliptically polarized undulator, the PEEM at Taiwan Light Source (TLS) is capable of recording polarization enhanced X-ray images and has been employed to examine the domain configurations in a lithographically patterned permalloy film as well as the orientations of pentacene molecules adsorbed on self-assembled monolayers (SAMs) modified gold surfaces. In addition, magnetic images acquired on cobalt/pentacene and pentacene/cobalt bilayers reveal that in hybrid structures the order of thin film deposition can lead to distinct domain configurations. Spectroscopic evidence further suggests that there is significant orbital hybridization at the interface where metallic cobalt was deposited directly on organic pentacene.     

Disorder,solvent effects and substitutional self-assembly of alkane dithiols from alkane thiol SAMs

Substitutional self-assembly of nonanedithiol from a hexanethiol self-assembled monolayer and the effects of use of ethanol and n-hexane as solvents were investigated. It was found that presence of original solvents in the HT SAM allowed easy replacement, while dried SAMs were more resilient to substitution and much longer times are necessary for substitution to occur. In general substitution in ethanol leads to production of a disordered dithiol SAM, with existence of extra molecules bonded to the SAM. Well ordered substituted SAMs were obtained with degassed n-hexane solutions and in absence of ambient light.     

氮分子固体中配位环境对其分子振动的影响

研究了孤立氮分子与处于氮分子固体中氮分子之间的振动频率差异.基于α-N_2晶体结构建立了5种不同氮分子数的氮分子固体团簇模型,采用密度泛函理论计算了孤立自由氮分子及各固体模型中氮分子的振动频率,并对它们的频率进行了比较和讨论.比较发现:受集体效应的影响,处于分子固体模型中的所有氮分子的键长较孤立自由氮分子的键长更短,振动频率更高;就固体模型本身而言,分子数越多,平均振动频率越大,而且,内部氮分子的振动频率总是大于表面氮分子的振动频率,整体来说,频率大小关系为v_(内部)v_(表面)v_(孤立).讨论分析认为这种频率差异主要是由于孤立自由氮分子、固体表面和内部分子的配位关系不同引起的;表面分子存在大量配位缺陷,与其相互作用的分子相对较少,氮分子键力较弱,从而频率更低.     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.     

Adsorption properties of decyl thiocyanate and decanethiol on platinum substrates studied by sum-frequency generation spectroscopy

Recently, thiocyanate groups were successfully used as precursors for thiolate assemblies. Indeed, they provide a useful alternative to thiol groups for self-assembly on metallic substrates. In order to check the adsorption properties and the quality of the thiocyanate-based self-assembled monolayers (SAMs), we use sum-frequency generation spectroscopy (SFG) and apply it to characterize the ad-layers of two similar molecules: decanethiol (DT) and decyl thiocyanate (DTCN) adsorbed on platinum surfaces. By comparing the SFG signals of the methyl and methylene vibration modes, we show that DTCN SAMs are less ordered than DT ones. These effects are related to the SAMs quality which depends on the molecular packing as highlighted by scanning tunnelling microscopy and X-ray photoelectron spectroscopy.     

Self-assembly of particles for densest packing by mechanical vibration

It is shown that by properly controlling vibrational and charging conditions, the transition from disordered to ordered, densest packing of particles can be obtained consistently. The method applies to both spherical and nonspherical particles. For spheres, face centered cubic packing with different orientations can be achieved by monitoring the vibration amplitude and frequency, and the structure of the bottom layer, in particular. The resultant force structures are ordered but do not necessarily correspond to the packing structures fully. The implications of the findings are also discussed.     

Silver ions adsorbed to self-assembled monolayers of alkanedithiols on gold surfaces form Ag–dithiol–Au multilayer structures

Double-ended alkanedithiols, 1,9-nonanedithiol and 1,5-pentanedithiol, formed self-assembled monolayers (SAMs) on Au(l11) substrates and were used to adsorb silver ions from an ethanolic solution of silver nitrate and formed Ag–dithiol–Au multilayer structures. Ellipsometry, contact angle measurement and X-ray photoelectron spectroscopy (XPS) confirmed that the alkanedithiol molecules formed SAMs with only one-ended thiol groups attached to the Au substrates, which was supported by molecular mechanics calculation. XPS and X-ray Auger electron spectroscopy (XAES) indicated that silver ions were deposited onto the SAMs from the solution by the chemical reaction of silver nitrate with another-ended thiol groups of the SAMs. Atomic force microscopy (AFM) was used to observe SAMs and multilayer structures. Received: 20 January 2000 / Accepted: 18 April 2000 / Published online: 9 August 2000     

Low energy electron diffraction studies of high index crystal surfaces of platinum

Several high Miller Index crystal surfaces of platinum have been examined by low energy electron diffraction (LEED) and have been shown to consist of low index (111) or (100) terraces of constant width, linked by steps of monatomic height. The ordered stepped surfaces were found to be stable in ultra-high vacuum up to 1500 K, and the reasons for this remarkable stability are discussed. The presence of ordered atomic steps may be considered as a general structural property of high index surfaces regardless of the chemical bonding in the crystal. A nomenclature is suggested to identify the surface structures of stepped surfaces.     

Internal structure of C60 fullerence molecules as revealed by low-temperature STM

We have performed Scanning Tunneling Microscopy (STM) in ultra-high vacuum at low temperatures (5 and 50K) of unordered and ordered C₆₀ layers adsorbed on a Au(110) surface. STM topographs of the frozen C₆₀ molecules reveal four symmetric patterns within single molecules, which may be associated with different orientations of the fullerenes on a highly corrugated gold substrate.     

Probing into adsorption behavior of human plasma fibrinogen on self-assembled monolayers with different chemical properties by scanning probe microscopy

The adsorption behaviors of fibrinogen on the self-assembled monolayers (SAMs) with different chemical properties were investigated using an atomic force microscopy (AFM). AFM images indicated that the adsorption amounts of fibrinogen molecules increased with an increase of the surface hydrophobicity. High-resolution AFM imaging revealed that the fibrinogen conformations adsorbed on the SAM surface changed with dependent on the surface chemistry. The adsorption models of fibrinogen molecules adsorbed on SAM surfaces with different chemical properties were proposed based on the high-resolution AFM images.     

一种测量吸附分子列向的方法——利用近阈结构谱和选择定则

局域分子轨道不易受环境影响,因此当分子被吸附且未被破坏时,跃迁到局域分子轨道产生的近阈结构谱基本不受环境影响.对于固定列向的吸附分子,当用偏振X射线测量其近阈结构谱时,相对强度将与偏振方向和分子列向有关.根据这一特性,基于选择定则可以判断吸附分子的列向. 关键词： 量子数亏损理论 近阈结构 选择定则 分子列向     

Morphology of pentacene films deposited on Cu(1 1 9) vicinal surface

We investigate the morphology of a pentacene (C₂₂H₁₄) film adsorbed on the Cu(1 1 9) vicinal surface by scanning tunnelling microscopy (STM). Thermal treatment of a thick film of molecules generates a long-range ordered structure. Series of molecular rows are alternated with areas where the molecules assume two equivalent orientations. STM data analysis suggests that the ordered structure can be described by a rippled morphology. The behaviour of the film at different annealing temperatures suggests a possible explanation of the film structure as due to an adsorbate-induced modification of the substrate.     

并五苯分子在Ag(110)表面成膜过程中的结构研究

采用低能电子衍射原位研究了并五苯分子（C₂₂H₁₄）在Ag(110)单晶表面的生长，观察了在初期沉积过程及随后基底变温过程中分子层结构的形成和变化.室温下并五苯分子在Ag(110)基底表面具有高的迁移性，从开始无序的亚单层膜结构逐渐形成有序的单层膜结构．实验和理论分析表明：并五苯分子平铺在Ag(110)表面，形成了与基底构成有关的有序结构，存在沿基底晶向成镜面对称的两种畴界取向，在基底温度从室温升高到接近并五苯升华温度的过程中，基底上的有序分子层结构保持稳定不变 关键词： 并五苯 低能电子衍射图案 分子层结构 Ag(110)     

Phase transitions in lattice gas models for water

Water-like lattice gases on the triangular and body-centered cubic lattices are investigated. Molecules may reside on the lattice sites in either of two possible orientations, a hydrogen bond being formed between molecules on neighboring sites if they have the proper orientation with respect to one another. For a range of chemical potential at sufficiently low temperatures, the models are shown to have an ordered phase consisting of an open, hydrogen-bonded, icelike structure. The models are shown to be transitionfree at sufficiently high temperature, indicating the existence of a critical point.Research supported in part by the U.S. National Science Foundation, Grant CHE-7726177, and by The Robert A. Welch Foundation, Grant P-446.     

Surface modification of cobalt-chromium-tungsten-nickel alloy using octadecyltrichlorosilanes

Cobalt-chromium (Co-Cr) alloys have been extensively used for medical implants because of their excellent mechanical properties, corrosion resistance, and biocompatibility. This first time study reports the formation and stability of self-assembled monolayers (SAMs) on a Co-Cr-W-Ni alloy. SAMs of octadecyltrichlorosilanes (OTS) were coated on sputtered Co-Cr-W-Ni alloy thin film and bulk Co-Cr-W-Ni alloy. OTS SAM coated alloy specimens were characterized using contact angle goniometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Contact angle analysis and FTIR suggested that ordered monolayers were coated on both sputtered and bulk alloy. XPS suggested the selective dissolution of cobalt from the alloy during the formation of OTS SAM. The bonding between the alloy and the OTS SAM was mainly attributed to Si-O-Cr and Si-O-W covalent bonds and a smaller contribution from Si-O-Co bonds. AFM images showed the distribution of islands of monolayers coated on the alloy. The height of monolayers in majority of the islands was closer to the theoretical length of fully extended OTS molecules oriented perpendicular to the surface. The stability of OTS SAM was investigated in tris-buffered saline at 37 °C for up to 7 days. Contact angle, FTIR, and XPS collectively confirmed that the monolayers remain ordered and bound to the alloy surface under this condition. This study shows that Co-Cr alloys can be surface modified using SAMs for potential biomedical applications.