异代酯基枝状分子表面组装及结构转化机理的STM研究

利用扫描隧道显微技术(STM)研究了3种异代酯基Janus枝状分子(G₁G₂COOCH₃, G₀G₃COOCH₃, G₁G₃COOCH₃)在高定向裂解石墨(HOPG)表面的组装结构. 室温下, G₁G₂COOCH₃, G₀G₃COOCH₃和G₁G₃COOCH₃分别在表面形成有序、局部有序及无序结构. 将样品加热至60~80 ℃后, 表面无序结构可以转化为有序的二聚体密堆积结构. 实验结果表明, 随着枝状分子代数的增加, 分子之间及分子与基底之间作用力增强, 室温下分子组装行为更依赖于分子吸附动力学过程, 组装结构趋于无序化, 而加热可以增强枝状分子在表面的活动性, 调整分子的位置关系, 使表面无序结构转化为热力学稳定的有序二聚体密堆积组装结构.     

Structure of Reconstructed Cu(100) Surface Induced by Dissociative Adsorption of Gaseous Oxygen

The reconstructed structures of Cu(100) surface induced by O₂ dissociative adsorption wereinvestigated by low energy electron diffraction and scanning tunneling microscopy. At lower oxygen coverage, it was found that two reconstructed structures, i.e. c(2×2)-O and (√2×2√2)R45°-O are coexistent. The domain size of the c(2×2)-O structure decreased with the increasing of O₂ exposure. The reconstructed structure at very small coverage was also investigated and a “zigzag” structure was observed at this stage. The “zigzag” structure was identified as boundaries of local c(2×2) domains. It was found that the strip region shows much stronger molecule-substrate interaction than that of oxygen covered regions, making it a proper template for patterned organic films. The sequence of the thermal stability was found as zigzag structure>c(2×2)>(√2×2√2)R45°-O.     

Effect of electric field on the partitioning behavior of solutes in entropic interaction chromatography

In this study, a novel column design with a round cross‐section was proposed to be suitable for a transverse electric field (EF). Additionally, two beads for entropic interaction chromatography (EIC) were prepared by grafting glycidyl methacrylate onto Toyopearl HW‐65F (T65F) beads. Solute partitioning was then investigated to elucidate the role of graft polymerization with and without an EF. In a T65F column, solute partitioning was attributed to the distinct pore structure in the beads and was governed by pore flow. Under EF, partition coefficients (K_p) for solutes decreased with increasing EF strength. In the two EIC columns, a decrease of K_p was also observed without an EF while the fractionation windows were extended. It was more pronounced in the EIC column with a high grafting density (T65F‐H). This was explained by the decrease in the effective pore size of solutes caused by the steric hindrance of polymer chains. Under an EF, the solutes showed different partitioning behaviours in the T65F‐H column. With increasing EF strength, K_p for vitamin B₁₂ and myoglobin was decreased. In contrast, K_p for large solutes increased as a result of concentration polarization on the bead surface. Both behaviors were related to the modulation of graft polymerization to residual charge on the matrix and the pore size of the solutes.     

Investigating electric field induced molecular distortions in polypropylene using Raman spectroscopy

Polymeric electric insulators are an integral part of many electronic circuits and systems. Changes induced by an electric field can affect various mechanisms; including electrical polarisation and electromechanical properties. Changes in the dielectric material can be tracked using spectroscopic methods. This study has shown that analysing polypropylene under electric field stress using Raman spectroscopy in combination with principal component analysis allows small changes in the non-crystalline phase to be identified. We have observed that for polypropylene, vibrational motion and changes in conformation occur mostly within the tie molecules connecting the overall cluster network. Amorphous molecular chains in the spherulites were also found to orient and form into a smectic mesophase. These electromechanical changes at the micro- and macromolecular level were found to be generally reversible once the stress is removed. However, with increased aging, these changes may lead to adverse structural changes and thus, in the future, this information may be used to inform faults and defect detection within polymeric dielectric materials.     

Localized electric field induced transition and miniaturization of two‐phase flow patterns inside microchannels

Strategic application of external electrostatic field on a pressure‐driven two‐phase flow inside a microchannel can transform the stratified or slug flow patterns into droplets. The localized electrohydrodynamic stress at the interface of the immiscible liquids can engender a liquid‐dielectrophoretic deformation, which disrupts the balance of the viscous, capillary, and inertial forces of a pressure‐driven flow to engender such flow morphologies. Interestingly, the size, shape, and frequency of the droplets can be tuned by varying the field intensity, location of the electric field, surface properties of the channel or fluids, viscosity ratio of the fluids, and the flow ratio of the phases. Higher field intensity with lower interfacial tension is found to facilitate the oil droplet formation with a higher throughput inside the hydrophilic microchannels. The method is successful in breaking down the regular pressure‐driven flow patterns even when the fluid inlets are exchanged in the microchannel. The simulations identify the conditions to develop interesting flow morphologies, such as (i) an array of miniaturized spherical or hemispherical or elongated oil drops in continuous water phase, (ii) “oil‐in‐water” microemulsion with varying size and shape of oil droplets. The results reported can be of significance in improving the efficiency of multiphase microreactors where the flow patterns composed of droplets are preferred because of the availability of higher interfacial area for reactions or heat and mass exchange.     

Expression of Chirality by Achiral Coadsorbed Molecules in Chiral Monolayers Observed by STM

The supramolecular packing mode of physisorbed monolayers built up by chiral isophthalic acid derivatives and coadsorbed achiral solvent molecules was imaged at the liquid/graphite interface with scanning tunneling microscopy (STM). The picture on the right shows the submolecularly resolved STM image of an enantiomorphous domain composed of the R enantiomer of the isophthalic acid derivative studied and 1-heptanol molecules; the latter express the chirality of the monolayer. Upon adsorption a racemic mixture is separated into enantiomorphous domains.     

STM and STS Studies of One-Dimensional Row Growth: Ag on Si(5 5 12)

We have studied the growth of Ag on Si(5 5 12) using scanning tunneling microscopy and spectroscopy (STM/STS). At metal coverages as low as 0.05 monolayer (ML), Ag forms well-ordered overlayer rows, or one-dimensional clusters, on the underlying silicon surface. To produce these ordered structures, it is necessary to anneal the surface to 450°C. As the coverage is increased above 0.05 ML, the rows grow in length and number until the surface forms a periodic array of such structures at 0.25 ML. A statistical analysis of the rows reveals a linear increase in median row length as a function of coverage. With regard to their electronic behavior, STS measurements show a significantly narrower band gap along the Ag rows than is found on the underlying silicon structures. Therefore, the deposited Ag atoms do retain some metallic behavior.     

Structure formation and dynamics of water in strong external electric fields

We have performed molecular dynamics computer simulations of water in homogeneous external electric fields which were varied in a wide range of field strengths. The dielectric response is found to be linear up to fields E₀≈0.01 V/Å from where dielectric saturation effects become important. At fields of E₀≈3 V/Å a phase transition into an ordered, ice-like structure is observed, which is stabilized through hydrogen-bonds. With an increasing external electric field, the frequency spectrum of the water dynamics shows a remarkable red shift of the intramolecular modes and a blue shift of the librational motions, where the frequency varies quadratically with the field strength. A simple analytical model is discussed which reproduces the observed behavior.     

金属配合物分子纳米结构构筑与调控的STM研究进展

金属配合物分子具有结构多样且可控以及功能丰富等特点,在催化、传感、分子识别、纳米器件等领域得到广泛应用, 对金属配合物分子的研究已是分子科学研究中的热点之一.同时, 利用配合物分子构筑表面分子纳米结构以及对配合物单分子性质的研究也日趋活跃. 近年来, 本研究组发展了配合物分子在固体表面的自组装技术, 并结合扫描隧道显微技术(STM)开展了一系列有关金属配合物分子表面纳米结构的研究工作, 在固体表面成功实现了对配体、配合物分子的高分辨STM成像、原位配合以及分子识别, 设计和构筑了多种功能配合物分子纳米结构,并系统研究了结构形成规律. 本文以本研究组近年来有关金属配合物分子组装的研究结果为主, 结合国内外相关研究小组的研究结果,综述有关金属配合物分子纳米结构的构筑与调控的STM研究进展, 介绍该类分子在固体表面的组装和分散规律, 为表面分子纳米结构的构筑和调控提供理论和实验基础.     

Ca ion permeation through liposome membranes with heat generation by square-wave electric field

Phospholipid liposomes that contain Ca ions in inside compartment were subjected to external alternating electric fields of square wave form at several frequencies: 10, 100 Hz, 1, 10, 100 kHz. The leakage of Ca ions from inside to bulk solution caused by the electric fields was detected by the fluorescence change due to Ca-Quin 2 (fluorescent dye) complex formation in the bulk aqueous solution. The temperature increments of the sample solution in the chamber were also measured. The amplitude of the electric field and time interval between application of the electric field were varied. The frequency dependency of both the leakage of Ca ion and the temperature increments was observed. It was observed that the efficiency of the leakage had a minimum at 1 kHz. The origin of the frequency dependency of the leakage is discussed, and it is concluded that the energy consumption on a microscopic scale in the liposome membrane zone can be the reason for membrane permeation in this experimental system.     

Reproducibility in the unfolding process of protein induced by an external electric field

The dynamics of proteins are crucial for their function. However, commonly used techniques for studying protein structures are limited in monitoring time-resolved dynamics at high resolution. Combining electric fields with existing techniques to study gas-phase proteins, such as single particle imaging using free-electron lasers and gas-phase small angle X-ray scattering, has the potential to open up a new era in time-resolved studies of gas-phase protein dynamics. Using molecular dynamics simulations, we identify well-defined unfolding pathways of a protein, induced by experimentally achievable external electric fields. Our simulations show that strong electric fields in conjunction with short-pulsed X-ray sources such as free-electron lasers can be a new path for imaging dynamics of gas-phase proteins at high spatial and temporal resolution.

Controlled unfolding of proteins can reveal structural properties and give insights of the proteins'' dynamics. We show the feasibility of unfolding proteins in the gas phase using electric fields, with a well-defined path at high field strengths.     

Separation of a Racemic Mixture of Two-Dimensional Molecular Clusters by Scanning Tunneling Microscopy

Adsorption of sub-monolayer amounts of 1-nitronaphthalene (NN) onto Au(111) leads to the aggregation of NN decamers, which exhibit two-dimensional chirality and represent a racemic mixture. In analogy to Pasteur's experiment of 1848 a scanning tunneling microscope can be used to discriminate and separate the enantiomers on a molecular scale.     

Polyelectrolyte layer-by-layer self-assembly enhanced by electric field and their multilayer membranes for separating isopropanol–water mixtures

An electric field enhanced method is developed for fabricating layer-by-layer (LbL) self-assembly polyelectrolyte multilayer membranes. Three kinds of electric field enhanced polyelectrolyte multilayer membranes (EPEMs), poly(diallyl dimethylammonium chloride)/poly(styrenesulfonate sodium salt) (PDDA/PSS), poly(diallyldimethylammonium chloride)/poly(acrylic acid sodium salt) (PDDA/PAA) and polyethylenimine/poly(acrylic acid sodium salt) (PEI/PAA), were self-assembled on a reverse osmosis membrane (ROM). The pervaporation performances of EPEMs for separating isopropanol–water mixtures (90/10, w/w) are all superior to those of corresponding normal self-assembled polyelectrolytes membranes (PEMs), and the selectivity increases with PDDA/PSS, PDDA/PAA and PEI/PAA in order. For (PEI/PAA)₄PEI EPEM, the separation factor is 1075 and permeation flux is 4.05 kg m⁻² h⁻¹ at 70 °C. This novel method speeds up the LbL process, which makes it promising for the practical application of the LbL multilayer membrane.     

隧道结电场辅助下叔丁胺分子在Cu(111)表面跳跃(英文)

采用扫描隧道显微镜(STM)于78 K研究了单个叔丁胺分子在Cu(111)表面的横向跃迁现象.研究发现叔丁胺分子的跳跃几率随隧道电流的增加而线性增加,这表明该过程是单电子激发过程;在不同极性的隧道结电场作用下,叔丁基胺分子跳跃行为发生的几率不同,这种现象可以用电场辅助的扩散过程解释.在不同极性电场作用下叔丁胺分子在Cu(111)表面的吸附能和扩散势垒不同,从而表现出不同的跳跃几率.     

Mechanisms behind concentration profiles illustrated by charge and concentration distributions around ions in double layers

The mechanisms behind the behaviour of concentration and charge density profiles in diffuse electric double layers are investigated quantitatively for 1:1 and 2:2 electrolytes. This is done by analysing various contributions to the mean force that acts on each ion. The forces are obtained from the calculated ionic charge and concentration distributions around individual ions at various positions in the double layer. These distributions are presented graphically which allows an immediate visual illustration of the mechanisms in action. Some features studied are charge inversion in double layers for divalent aqueous electrolytes, overcompensation of surface charge due to large amounts of physisorbed counterions, ion size effects in the double layer structure and various mechanisms that cause deviations from the predictions of the Poisson–Boltzmann approximation. A major objective of the paper is to present the results in a visual form and explain aspects of modern double layer theory in a simple manner.     

STM Images of Individual Porphyrin Molecules on Cu(100) and Cu(111) Surfaces

Simply sublime! Samples of monomeric and dimeric zinc porphyrins were sublimed onto a Cu surface under ultrahigh vacuum conditions. Images obtained by scanning tunneling microscopy at room and low temperature (98 K) show features attributed to individual porphyrin molecules with excellent resolution. In the case of the (relatively large) linear dimer shown, two distinct conformations were detected on a surface with low coverage area. R=CH₂CH₂COOCH₃.     

Calculation of the deuterium electric field gradients in HD and LiD using a variation-perturbation method with a Gaussian basis set

Summary We present calculations of the deuterium electric field gradients in the HD and LiD molecules obtained with a variation-perturbation method using Gaussian atomic orbitals. The differences between our theoretical values and the corresponding experimental or best calculated values are 2%. We conclude that high accuracy can be obtained with the variation-perturbation method using either Gaussian or Slater orbitals.     

表面分子自组装结构的外界调控及STM研究进展

结合近年来国内外的研究工作, 概述了利用外界调控方式包括溶剂调控、热调控、光激发调控及电场/电位调控表面分子自组装结构的STM研究进展, 展望了该领域今后的研究方向.     

Effect of electric field during gel-layer controlled ultrafiltration of synthetic and fruit juice

Electric field enhanced ultrafiltration of pectin–sucrose mixture (synthetic juice) and mosambi (Citrus sinensis (L.) Osbeck) fruit juice using 50,000 (MWCO) polyerthersulfon membrane is studied in a cross-flow cell. Pectin, completely rejected by the membrane, forms a gel type layer over the membrane surface. Under the application of an external dc electric field across the membrane, gel-layer formation is restricted leading to an enhancement of permeate flux. During ultrafiltration of synthetic juice, application of dc electric field (800 V/m) increases the permeate flux to almost threefold compared to that with zero electric field. A theoretical model based on integral method assuming suitable concentration profile in the boundary layer is developed. The proposed model is used to predict the permeate flux in gel-layer governed electric field enhanced ultrafiltration. Predictions of the model are successfully compared with the experimental results under a wide range of operating conditions. Experiments with fruit juice also demonstrated significant increase in flux with the application of a suitable electric field.