基底支撑刚度梯度变化对石墨烯层间摩擦力的影响

基于纳米摩擦能耗理论,利用分子动力学方法建立了公度接触下支撑刚度梯度变化的石墨烯层间摩擦力模型,分析了基底质心刚度和支撑刚度梯度变化对基底和薄片各接触区摩擦能耗的贡献.结果表明:软边界区始终贡献驱动力;硬边界区贡献的摩擦力最大,且随着支撑刚度的增大,硬边界区对总摩擦的贡献比也越高.各接触区的摩擦力是薄片和基底之间的褶皱势和接触区产生的法向变形差两部分的共同作用.前者是公度接触下阻碍滑移的界面势垒和刚度梯度方向上不同刚度支撑原子热振动引起的势梯度;后者是接触边界过渡区两侧原子的非对称变形和自由度约束突变引起的非平衡边界势垒相耦合的结果.本文对研究公度接触下刚度梯度支撑的纳米器件的相对运动规律有指导意义.     

Micro-mechanical analysis of dynamic processes of nanomanipulation

In this study, atomic force microscope (AFM) tips are used as tools to cut and manipulate carbon nanotubes on various surfaces. The lateral forces acting on AFM tips during manipulation are also recorded and analyzed from the perspective of micro-mechanics. It is found that differences in surface conditions can lead to obvious increase in micro-friction between nanotube and substrate. And also due to rehybridization, carbon nanotubes present excellent resilience when undergoing different degrees of strain. Finally, carbon nanotubes can complexly deform from elastic stage to plastic stage before complete rupture.     

Micro-mechanical analysis of dynamic processes of nanomanipulation

In this study, atomic force microscope (AFM) tips are used as tools to cut and manipulate carbon nanotubes on various surfaces. The lateral forces acting on AFM tips during manipulation are also recorded and analyzed from the perspective of micro-mechanics. It is found that differences in surface conditions can lead to obvious increase in micro-friction between nanotube and substrate. And also due to rehybridization, carbon nanotubes present excellent resilience when undergoing different degrees of strain. Finally, carbon nanotubes can complexly deform from elastic stage to plastic stage before complete rupture.     

Elastic–plastic cube model for ultrasonic friction reduction via Poisson's effect

Ultrasonic friction reduction has been studied experimentally and theoretically. This paper presents a new elastic–plastic cube model which can be applied to various ultrasonic lubrication cases. A cube is used to represent all the contacting asperities of two surfaces. Friction force is considered as the product of the tangential contact stiffness and the deformation of the cube. Ultrasonic vibrations are projected onto three orthogonal directions, separately changing contact parameters and deformations. Hence, the overall change of friction forces. Experiments are conducted to examine ultrasonic friction reduction using different materials under normal loads that vary from 40 N to 240 N. Ultrasonic vibrations are generated both in longitudinal and vertical (out-of-plane) directions by way of the Poisson effect. The tests show up to 60% friction reduction; model simulations describe the trends observed experimentally.     

Simple microscopic theory of Amontons's laws for static friction

A microscopic theory for the ubiquitous phenomenon of static friction is presented. Interactions between two surfaces are modeled by an energy penalty that increases exponentially with the degree of surface overlap. The resulting static friction is proportional to load, in accordance with Amontons's laws. However, the friction coefficient between bare surfaces vanishes as the area of individual contacts grows, except in the rare case of commensurate surfaces. An area independent friction coefficient is obtained for any surface geometry when an adsorbed layer of mobile atoms is introduced between the surfaces. The predictions from our simple analytic model are confirmed by detailed molecular dynamics simulations.     

Fabricated cantilever for AFM measurements and manipulations: Pre-stress analysis of stress fibers

The atomic force microscope (AFM) is a highly successful instrument for imaging of nanometer-sized samples and measurement of pico- to nano-Newton forces acting between atoms and molecules, especially in liquid. Generally, commercial AFM cantilevers, which have a sharp tip, are used for AFM experiments. In this review, we introduce micro-fabricated AFM cantilevers and show several applications for cell biology. In manipulation of samples on a cellular scale with a force of tens to hundreds of nano-Newtons, attempts have been made to secure the formation of covalent/non-covalent linkages between the AFM probe and the sample surface. However, present chemistry-based modification protocols of cantilevers do not produce strong enough bonds. To measure the tensile strength and other mechanical properties of actin-based thin filaments in both living and semi-intact fibroblast cells, we fabricated a probe with a hooking function by focused ion beam technology and used it to capture, pull and eventually break a chosen thin filament, which was made visible through fusion with fluorescent proteins. Furthermore, we fabricated a microscoop cantilever specifically designed for pulling a microbead attached to a cell. The microscoop cantilevers can realize high-throughput measurements of cell stiffness.     

Investigation of growth, coverage and effectiveness of plasma assisted nano-films of fluorocarbon

Plasma-assisted functional films have significant potential in various engineering applications. They can be tailored to impart desired properties by bonding specific molecular groups to the substrate surface. The aim of this investigation was to develop a fundamental understanding of the atomic level growth, coverage and functional effectiveness of plasma nano-films on flat surfaces and to explore their application-potential for complex and uneven shaped nano-materials. In this paper, results on plasma-assisted nano-scale fluorocarbon films, which are known for imparting inertness or hydrophobicity to the surface, will be discussed. The film deposition was studied as a function of time on flat single crystal surfaces of silicon, sapphire and graphite, using microwave plasma. X-ray photoelectron spectroscopy (XPS) was used for detailed study of composition and chemistry of the substrate and coating atoms, at all stages of deposition. Atomic force microscopy (AFM) was performed in parallel to study the coverage and growth morphology of these films at each stage. Combined XPS and AFM results indicated complete coverage of all the substrates at the nanometer scale. It was also shown that these films grew in a layer-by-layer fashion. The nano-films were also applied to complex and uneven shaped nano-structured and porous materials, such as microcellular porous foam and nano fibers. It was seen that these nano-films can be a viable approach for effective surface modification of complex or uneven shaped nano-materials.     

碳纳米管材料及应用

碳纳米管自发现以来，由于其独特的结构和奇特的物理，化学和力学特性以及其潜在的应用前景而倍受人们的关注。本文介绍近年来这一前沿研究领域所取得的部分重要研究进展，并讨论碳纳米管的几种应用前景。     

基于扫描电子显微镜的碳纳米管拾取操作方法研究

碳纳米管场效应管是未来纳米器件的发展方向,而制造纳米器件的前提是拾取碳纳米管,基于扫描电子显微镜(SEM)的微纳机器人操作系统能够实现碳纳米管拾取操作.本文建立拾取操作中碳纳米管与原子力显微镜(AFM)探针间范德瓦耳斯力力学模型,不同接触状态下范德瓦耳斯力越大越有利于拾取碳纳米管.在SEM视觉反馈图像中建立相对坐标系,首先提出倾角变值方法检测碳纳米管与AFM探针的接触状态,然后运用动态差值方法识别碳纳米管与AFM探针空间位姿并校正碳纳米管位姿,最后自下而上拾取碳纳米管.实验结果表明:拟合直线倾角变值较大时碳纳米管与AFM探针发生接触,动态差值变化为零时碳纳米管与AFM探针为空间线接触,在完全线接触模型下选择合适的接触角度、接触长度和拾取速度能够成功拾取碳纳米管.     

Influence of nanometer scale film structure of ZDDP tribofilm on Its mechanical properties: A computational chemistry study

We investigated the influence of a nanometer scale film structure of a tribofilm generated from zinc dialkyldithiophosphate (ZDDP) anti-wear additive on its mechanical properties using a combined molecular dynamics (MD) and finite element (FE) method. The frictional behavior of an interface between a native iron oxide layer on steel surface and zinc metaphosphate - regarded as a model material of ZDDP tribofilm - was firstly studied using the MD method. The results showed that the iron atoms in the oxide layer diffused into the phosphate layer during the friction process. The zinc atoms in the phosphate layer also diffused into the oxide layer. Significant interdiffusion of iron and zinc atoms was observed with increasing simulation time. Thus, metallic phosphate with a gradient composition of iron and zinc atoms was formed on the phosphate/oxide interface. We then constructed an axisymmetric nanoindentation simulation model from the MD-derived structures at a certain simulation time and carried out a FE calculation. As a result, we found that the rubbed ZDDP tribofilm, including the phosphate with the gradient composition of metallic atoms, showed larger contact stiffness and hardness. The combined MD/FE simulation indicates that the tribofilm becomes stiffer and harder due to the interdiffusion of iron and zinc atoms on the tribofilm/oxide interface. We have found that the gradient composition formation in ZDDP tribofilm during friction process influences on its mechanical properties.     

Methods of calculation of a friction coefficient: application to nanotubes

In this Letter we develop theoretical and numerical methods to calculate the dynamic friction coefficient. The theoretical method is based on an adiabatic approximation which allows us to express the dynamic friction coefficient in terms of the time integral of the autocorrelation function of the force between both sliding objects. The motion of the objects and the autocorrelation function can be numerically calculated by molecular-dynamics simulations. We have successfully applied these methods to the evaluation of the dynamic friction coefficient of the relative motion of two concentric carbon nanotubes. The dynamic friction coefficient is shown to increase with the temperature.     

Atomic friction

We show that wearless atomic friction, recently observed by AFM techniques, is well desribed by the mechanism of “plucking of atoms” proposed by Tomlinson in 1929. Our results for a single-asperity contact yield a satisfactory fit of the experimental data. For extended contact areas, the relative orientation of the crystal lattices and the resulting misfit becomes important. The misfit gives rise to the formation of domains where the two surface structures are approximately in registry. The domains are separated by two sets of shift lines, crossing each other in topological defects. During quasistatic sliding, the whole domain pattern moves perpendicular to the driving force. Frictional behaviour occurs if the ratio of the coupling strength of the surface atoms to the bulk of the body to the potential barrier for sliding is less than a critical value. Dissipation and friction hysteresis are caused by irreversible jumps of these topological defects. The friction force depends strongly on both the sliding direction and the misfit angle.     

NANOBUBBLES AT THE LIQUID/SOLID INTERFACE STUDIED BY ATOMIC FORCE MICROSCOPY

Bubbles on the nanometer scale were produced by a special method on solid surfaces. Atomic Force Microscopy (AFM) was used to detect these bubbles. It shows that nanobubbles can be seen clearly in the interfaces of liquid/graphite and liquid/mica. In AFM images, the nanobubbles appeared like bright spheres. Some of the bubbles kept stable for hours during the experiments. The bubbles were disturbed under high load during AFM imaging. The conformation of the bubbles is influenced by the atomic steps on the graphite substrate. In addition, a shadow was found around the bubbles, which was due to the interactions between a bubble adhered to the tip and a bubble on the substrate.     

Force-feedback joystick as a low-cost haptic interface for an atomic-force-microscopy nanomanipulator

In order to manipulate materials at the nanometer scale, new methods and devices have to be developed. A nanomanipulator interface was designed and implemented in a commercial atomic-force-microscope (AFM) system. With the aid of a positioning joystick, direct positioning of the AFM probe with nanometer precision was possible. A commercial force-feedback joystick served as a haptic interface and provided the user with real-time feeling of the tip–sample interactions. Due to the open design, the manipulator interface could be used with other microscopes of the SPM family. In addition, the nanomanipulator and an UV-laser microbeam for photoablation were combined on an inverted optical microscope. To test the nanomanipulator, human metaphase chromosomes were dissected using both photoablation and mechanical AFM manipulation. The experimental results show that by combining both methods, biological material can be manipulated on different size scales in one integrated instrument. The effects of manipulation on the chromosome were studied in detail by AFM. Sub-400 nm cuts were achieved by photonic ablation. Chromosomal fragments of a size less than of 500 nm could be isolated. By means of mechanical microdissection, different cut sizes ranging from 80 nm to 500 nm could be easily obtained by applying different load forces. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Current affiliation: Nanotechnology Group, ETH Zurich, Switzerland RID="**" ID="**"Corresponding author. Fax: +49-89/2180-4331, E-mail: heckl@lmu.de     

Lattice-resolution imaging of the sapphire (0 0 0 1) surface in air by AFM

Lattice-resolution images of single-crystal α-alumina (sapphire) (0 0 0 1) surfaces have been obtained using contact-mode AFM under ambient conditions. It was found that the hexagonal surface lattice has a periodicity of 0.47 ± 0.11 nm, which is identical to that reported previously when the same surface was imaged in water. Large lattice corrugations (as high as 1 nm) were observed, but were concluded to be imaging artifacts because of the strong friction which causes additional deflection of the cantilever. The additional deflection of the cantilever is registered by the detector of the optical beam-deflection AFM resulting in an overestimation of the height at each lattice point. Abrupt changes were also resolved in the topography including honeycomb patterns and a transition from 2D lattices to 1D parallel stripes, with scanning direction. These phenomena can be explained by the commensurate sliding between the tip and sapphire surface due to the strong contact force.     

Influence of a lubricant modified with fine-dispersed β-sialon on a steel surface structure under friction loading

Steel sample surfaces subjected to different friction loads with lubricants modified with fine-dispersed β-sialon are studied via electron spectroscopy and x-ray diffraction analysis. It is revealed that friction loading in β-sialon-containing lubricants stimulates the ordering of the crystalline structure of surfaces (a decrease in the relative root-mean-square microstrain and dislocation density). The assumption is made that, in the presence of heavy loads, the friction surface is formed from a complex α-Fe compound with nitrogen and carbon atoms embedded into its crystal lattice and silicon atoms in the positions of iron atoms.     

Polarity effects on ZnO films grown along the nonpolar [1120] direction

The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces.     

缺陷对碳纳米管摩擦与运动行为的影响

本文采用分子动力学方法研究了公度、无公度情况下含空位、Stone-Thrower-Wales(STW)型缺陷的单壁碳纳米管(SWCNT)在石墨基底上的摩擦与运动行为.结果表明,公度时缺陷的存在导致了界面局部无公度,减小了摩擦.随着碳纳米管底部STW缺陷的增多,碳纳米管变形增大,侧向力波动的幅值减小,局部无公度性增强,摩擦减小.含空位缺陷的碳纳米管所受的摩擦力明显大于含STW缺陷的碳纳米管,原因在于含空位缺陷的碳纳米管在运动的后期出现了明显的翻转现象,增大了能量耗散.无公度时,碳纳米管与石墨基底间的摩擦力很小,缺陷对其摩擦力影响不大,原因在于无论是否含有缺陷,碳纳米管与石墨组成的界面的无公度性差别不大.     

Measurements of metal-polymer adhesion properties with dynamic force spectroscopy

We investigate nanoscale interface properties using dynamic mode atomic force microscopy (AFM) operated in the frequency modulation mode in ultrahigh vacuum. The AFM tip was was functionalised by a thin layer of aluminium and the polymer was treated by plasma-etching. In the spectroscopy mode we could measure the adhesion properties between the metal and the polymer surfaces. We found that plasma-etching of the polymer resulted in strongly enhanced force interactions, indicating a chemical activation of the polymer surface. Values for the adhesion force and work of adhesion were measured on the nanometer scale and are compared to conventional macroscopic adhesion failure tests.