A bond-fluctuation mechanism for stochastic switching in wired molecules

Stochastic on-off conductivity switching observed in phenylene-ethynylene oligomers has been explained in terms of changes in ring conformations, or electron localization, or both. We report the observation of stochastic on-off switching in the simplest of wired molecules: octanedithiol, decanedithiol, and dodecanedithiol bonded on an Au(111) surface. Stochastic switching was observed even when a top gold contact was pressed on by a conducting atomic force microscope tip at constant force. The rate of switching increased substantially at 60 degrees C, a temperature at which these films are commonly annealed. Because such switching in alkanethiols is unlikely to be caused by internal molecular electronic changes and cannot be fully accounted for by breaking of the top contact, we argue that the cause is the well-known mobility of molecules tethered to gold via a thiol linkage.     

Molecular positional order in langmuir-blodgett films by atomic force microscopy

Langmuir-Blodgett films of barium arachidate have been studied on both macroscopic and microscopic scales by atomic force microscopy. As prepared, the films exhibit a disordered hexagonal structure; molecularly resolved images in direct space establish a connection between the extent of the positional order and the presence of defects such as dislocations. Upon heating, the films reorganize into a more condensed state with a centered rectangular crystallographic arrangement; in this new state the films exhibit long-range positional order and unusual structural features, such as a height modulation of the arachidic acid molecules.     

Piezoelectric nanogenerators based on zinc oxide nanowire arrays

We have converted nanoscale mechanical energy into electrical energy by means of piezoelectric zinc oxide nanowire (NW) arrays. The aligned NWs are deflected with a conductive atomic force microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zinc oxide creates a strain field and charge separation across the NW as a result of its bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. The efficiency of the NW-based piezoelectric power generator is estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices.     

基于线性互补算法的叠梁接触规律研究

确定叠梁层间发生接触的区间及接触力分布规律是叠梁计算的关键.为克服以往解析解答得到的结论不合实际的现象，将叠梁接触问题构造为线性互补问题，并给出其一般求解过程.为验证该法的合理性，以梁与刚体接触问题的Timoshenko解答与胡海昌解答为例进行对比分析.在此基础上，对典型荷载作用下的叠梁接触问题进行了计算，并对其影响因素进行了深入研究.最后得到如下主要结论：刚度比、荷载作用形式以及剪切挠度对叠梁层间接触规律影响均非常明显；考虑剪切变形影响时，叠梁接触力由集中力变为分布力；集中力作用下，叠梁接触趋于点接触，均布力作用下，叠梁接触趋于线接触；上下梁刚度相等是叠梁接触力分布规律的一个转折点，不同的上下梁刚度比会得到不同的接触规律.     

Three-dimensional Anderson localization of ultracold matter

Anderson localization (AL) is a ubiquitous interference phenomenon in which waves fail to propagate in a disordered medium. We observe three-dimensional AL of noninteracting ultracold matter by allowing a spin-polarized atomic Fermi gas to expand into a disordered potential. A two-component density distribution emerges consisting of an expanding mobile component and a nondiffusing localized component. We extract a mobility edge that increases with the disorder strength, whereas the thermally averaged localization length is shown to decrease with disorder strength and increase with particle energy. These measurements provide a benchmark for more sophisticated theories of AL.     

Atomistic mechanisms and dynamics of adhesion, nanoindentation, and fracture

Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.     

Nanoscale imaging of molecular adsorption

In situ atomic force microscope observations were made of the adsorption of anions (1- or 2-) of the organic diacid 5-benzoyl-4-hydroxy-2-methoxybenzenesulfonic acid from aqueous solution onto the (0001) surface of hydrotalcite (HT), a layered clay. This adsorption process is believed to mimic the ion-exchange reactions that occur within the layers of HT and other layered clays. Atomic force microscope images of the (0001) surfaces of HT, acquired in aqueous solutions, reveal an ordered structure with respect to magnesium and aluminum atoms. In the presence of the anions, atomic force microscopy indicates pH-dependent adsorption onto the formally cationic HT surface. The anion coverage is governed by electroneutrality and steric interactions between the bulky anions within the adsorbed layer, whereas the orientation of the anions with respect to the HT surface is dictated by coulombic interactions and hydrogen bonding between the anion's sulfonate moiety and clay hydroxyl triads. These observations reveal that the reversible adsorption of molecular species can be examined directly by in situ atomic force microscopy, providing details of surface stoichiometry and adlayer symmetry on the local, molecular level.     

Quantitative measurement of short-range chemical bonding forces

We report direct force measurements of the formation of a chemical bond. The experiments were performed using a low-temperature atomic force microscope, a silicon tip, and a silicon (111) 7x7 surface. The measured site-dependent attractive short-range force, which attains a maximum value of 2.1 nanonewtons, is in good agreement with first-principles calculations of an incipient covalent bond in an analogous model system. The resolution was sufficient to distinguish differences in the interaction potential between inequivalent adatoms, demonstrating the ability of atomic force microscopy to provide quantitative, atomic-scale information on surface chemical reactivity.     

The force needed to move an atom on a surface

Manipulation of individual atoms and molecules by scanning probe microscopy offers the ability of controlled assembly at the single-atom scale. However, the driving forces behind atomic manipulation have not yet been measured. We used an atomic force microscope to measure the vertical and lateral forces exerted on individual adsorbed atoms or molecules by the probe tip. We found that the force that it takes to move an atom depends strongly on the adsorbate and the surface. Our results indicate that for moving metal atoms on metal surfaces, the lateral force component plays the dominant role. Furthermore, measuring spatial maps of the forces during manipulation yielded the full potential energy landscape of the tip-sample interaction.     

Electronic control of friction in silicon pn junctions

A remarkable dependence of the friction force on carrier concentration was found on doped silicon substrates. The sample was a nearly intrinsic n-type Si(100) wafer patterned with 2-micrometer-wide stripes of highly B-doped p-type material. The counter surface was the tip of an atomic force microscope coated with conductive titanium nitride. The local carrier concentration was controlled through application of forward or reverse bias voltages between the tip and the sample in the p and the n regions. Charge depletion or accumulation resulted in substantial differences in friction force. The results demonstrate the capability to electronically control friction in semiconductor devices, with potential applications in nanoscale machines containing moving parts.     

基于AFM的107杨纤维表面形貌特征研究

以107杨为研究对象,采用过氧化氢和冰醋酸混合液对应拉木和正常木进行处理获得2种离析纤维;采用铬酸对正常木进行处理获得1种离析纤维;采用硫酸盐制备正常木化学浆获得1种化学浆纤维,并取出一半化学浆进行打浆处理获得1种打浆纤维。使用原子力显微镜(AFM)对上述5种不同处理方法获得的纤维表面形貌进行观察比较。结果表明,应拉木纤维比正常木纤维容易观测到清晰初生壁表面;铬酸离析方法比过氧化氢和冰醋酸混合液离析方法容易观测到清晰初生壁表面;相比离析纤维,化学浆纤维次生壁微纤丝的定向排列方式清晰可见;打浆后纤维较化学浆纤维,微纤丝定向排列被干扰,出现分丝帚化。     

Atomic force microscopy of the electrochemical nucleation and growth of molecular crystals

In situ atomic force microscopy reveals the morphology, surface topography, and growth and dissolution characteristics of microscopic single crystals of the low-dimensional organic conductor (tetrathiafulvalene)Br(0.76)' which are grown by electrocrystallization on a highly oriented pyrolytic graphite electrode in an atomic force microscope liquid cell. The growth modes and the distribution and orientation of topographic features on specific crystal faces, whose identity was determined by "atomic force microscope goniometry," can be correlated with the strength and direction of anisotropic solid-state intermolecular bonding. Growth on the (011) face of (tetrathiafulvalene)Br(0.76) crystals involves the formation of oriented self-similar triangular islands ranging in size from 200 to 5000 angstroms along a side. These nuclei eventually transform into rectangular rafts at larger length scales, where bulk intermolecular bonding interactions and surface energies dominate over nuclei-substrate interactions.     

DNA imaging. Getting a feel for genetic variations

A team of researchers has come up with a novel atomic imaging microscope that may dramatically speed the task of linking genetic variants to diseases. The microscope is a modification of the popular atomic force microscope (AFM), which uses an ultrasharp tip to map surfaces of everything from computer chips to DNA at the atomic level. By using this molecule-sized tip, the researchers were able get their AFM to march down a strand of DNA and identify uniquely shaped reporter molecules engineered to tag the genetic variations.     

柔性物体的虚拟力觉再现

为研究柔性物体的虚拟力觉再现,提出了一种以正三角形面片构成的弹簧-质点变形模型,并提出根据连续变形公式构造面片各顶点的变形.该变形模型的特点是可适用于较复杂的接触变形--如多点接触变形、面(线)接触变形.在此模型的基础上,利用DELTA手控器,对柔性物体接触变形以及实时虚拟力反馈进行半物理仿真.     

Revealing the angular symmetry of chemical bonds by atomic force microscopy

We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.     

番木瓜摘取的接触力学模型构建与试验

【目的】实现番木瓜稳定无损伤摘取。【方法】采用三指对称夹持后扭断的摘取方案,构建摘取接触力平衡方程,依据三指内力汇交原理进行抓取稳定性分析,制定摘取接触力学模型求解方案,并对番木瓜进行摘取试验。【结果】样本表面无明显变形、压痕与裂纹,夹持处果肉室温静置24 h后无明显的颜色变化和伤痕,最大夹持力远小于成熟番木瓜横径方向受压弹性变形阶段压力极限值;质量和摘取扭转力矩与横径、纵径、果柄长度、果柄扭断直径有密切依存关系,质量多元线性回归达极显著水平,扭转力矩多元线性回归达显著水平;依据接触力学模型和回归模型计算的理论夹持力与测量夹持力对比,测量夹持力均高于理论夹持力,两者最大偏差小于20%,两者在趋势上具有较好一致性。【结论】摘取方案能稳定无损伤摘取番木瓜,摘取接触力学模型具有正确性与实用性,可为番木瓜摘取末端执行机构设计与力度控制提供依据。     

Atomic-scale visualization of inertial dynamics

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.     

孔销式少齿差行星减速器的多目标优化及动力学仿真分析

为了达到减小减速器体积同时提高承载能力、降低运转冲击的目的,基于Hertz接触理论,引用销轴和销孔外廓参数和材料参数建立两者法向接触力的数学模型;选取减速器的5个尺寸参数为设计变量,构造8个非线性约束方程,建立起优化法向接触力和减速器体积的多目标优化模型.运用评价函数法和罚函数法求解计算实例,对优化模型进行验证,对比优化前后结果表明,减速器体积和法向接触力分别得到了13.1%和24.0%的优化率;利用ADAMS对法向接触力进行动力学仿真,得到优化前后的时域和频域仿真图,结果表明其平均优化率达到29.4%,与理论结果基本吻合.     

Atomic resolution of the silicon (111)-(7x7) surface by atomic force microscopy

Achieving high resolution under ultrahigh-vacuum conditions with the force microscope can be difficult for reactive surfaces, where the interaction forces between the tip and the samples can be relatively large. A force detection scheme that makes use of a modified cantilever beam and senses the force gradient through frequency modulation is described. The reconstructed silicon (111)-(7x7) surface was imaged in a noncontact mode by force microscopy with atomic resolution (6 angstroms lateral, 0.1 angstrom vertical).     

库仑摩擦接触问题的位移─力混合接触单元

针对Coulomb摩擦接触问题的位移-力混合接触单元,为了防止接触面嵌入量太大,对于粘接状态在修正接触边界法向位移的同时,提出了一种修正切向位移的方法;为了利用FEM软件包的求解器,将接触单元滑动接触状态的单元刚度矩阵作了对称化处理,从而达到整个问题的总刚对称：这就很好地解决了有间隙接触问题。实例验证结果表明,所提出的方法是有效、可行的。