Angular Dependence of Lateral and Levitation Forces in Asymmetric Small Magnet/Superconducting Systems

The dipole-dipole interaction model is used to calculate the angular dependence of lateral and levitation forces on a small permanent magnet and a cylindrical superconductor in the Meissner state lying laterally off the symmetric axis of the cylinder. Under the assumption that the lateral displacement of the magnet is small compared with the physical dimensions of the system, we obtain analytical expressions for the lateral and levitation forces as functions of geometrical parameters of the superconductor as well as the height, the lateral displacement and the orientation of magnetic moment of the magnet. The effect of thickness and radius of the superconductor on the levitation force is similar to that for a symmetric magnet/superconducting cylinder system, but within the range of lateral displacement. The splitting in the levitation force increases with the increasing angle of orientation of the magnetic moment of the magnet. For a given lateral displacement of the magnet, the lateral force vanishes when the magnetic moment is perpendicular to the surface of the superconductor and has a maximum value when the moment is parallel to the surface. For a given orientation of the magnetic moment, the lateral force has a linear relationship with the lateral displacement. The stability of the magnet above the superconducting cylinder is discussed in detail.     

Theory of multifrequency atomic force microscopy

We develop a theory that explains the origin of the high force sensitivity observed in multifrequency force microscopy experiments. The ability of the microscope to extract complementary information on the surface properties is increased by the simultaneous excitation of several flexural cantilever modes. The force sensitivity in multifrequency operation is about 0.2 pN. The analytical model identifies the virial and the energy dissipated by the tip-surface forces as the parameters responsible for the material contrast. The agreement obtained among the theory, experiments and numerical simulations validates the model.     

基于机械-化学的方法在硅表面制造功能化结构

"利用机械与化学结合的方法在芳烃重氮盐溶液中用金刚石刀具切削硅片,使得芳香烃分子和硅之间以共价键连接,实现了对硅片的"成形并功能化"的一步完成．研究了在大气环境如何利用金刚石刀具在硅表面加工出表面质量较好的微结构,为下一步在溶液中"成形并功能化"硅提供好的基底．在溶液中对硅表面进行可控自组装实验,初步研究了切削速度和组装时间对切削处生成自组装膜质量的影响,总结出较适合膜生长的参数．用X射线光电子能谱对自组装膜进行了检测,用扫描电子显微镜和原子力显微镜对自组装膜的表面形貌进行了表征,并用原子力显微镜对自     

Fracture Mechanics Studies of Adhesion in Biological Systems

A fracture mechanics based methodology for quantifying adhesive interactions between soft solids, or between a soft solid and a rigid substrate, is reviewed. An emphasis is placed on the application of these techniques to the characterization of adhesive interactions in biological systems. Results from experiments involving the adhesion of gelatin hydrogels to hydrophilic and hydrophobic substrates are described as an illustration of the application of these methods. In these experiments a hemispherical gelatin cap is brought into contact with a flat surface. Separation of the two materials is described in terms of crack propagation along the gelatin/substrate interface. Simultaneous measurements of the applied load, the resulting displacement, and the contact area between the two materials enable us to determine the elastic modulus of the cap, in addition to the crack driving force, or energy release rate. The adhesive behavior of the interface is quantified by the relationship between the energy release rate and the crack velocity. Analogies are made to information obtained from contact angle measurements, and from measurements made with the Israelachvili surface forces apparatus.     

Planarization process of single crystalline silicon asperity under abrasive rolling effect studied by molecular dynamics simulation

In the chemical mechanical polishing (CMP) process, the complex behaviors of abrasive particles play important roles in the planarization of wafer surface. Particles embedded in the pad remove materials by ploughing, while particles immersed in the slurry by rolling across the wafer surface. In this paper, processes of the particle rolling across a silicon surface with an asperity under various down forces and external driving forces were studied using molecular dynamics (MD) simulation method. The simulations clarified the asperity shape evolution during the rolling process and analyzed the energy changes of the simulation system and the interaction forces acted on the silica particle. It was shown that both the down force and the driving force had important influences on the amount of the material removed. With relatively small down forces and driving forces applied on the particle, the material removal occurred mainly in the front end of the asperity; when the down forces and driving forces were large enough, e.g., 100?nN, the material removal could take place at the whole top part of the asperity. The analysis of energy changes and interaction forces provided favorable explanations to the simulation results.     

Assembling 100 nm Scale Particles by an Electrostatic Potential Field

The assembly of particles is one of the many methods for the fabrication of organized structures in the range of micro- to nanometer sizes. These structures have potential applications in the electronic, optical and biochemical fields. Recently, many papers have reported the patterning of particles using patterned SAM (self-assembly monolayer) films and micro molding methods. We have been developing a new technique to assemble particles using an electrostatic field. This paper describes a new technique to fabricate two-dimensional microstructures assembled from 100 nm particles. Spherical silica of 900 nm diameter and aluminum of 100 nm diameter were used as the model particles. An electrostatic image was formed on an insulating substrate by drawing a focused electron beam at 10 keV. Both types of particles were deposited on the electrostatic images. In this process, the dielectrophoretic (DEP) force plays an important role in depositing particles on the electrostatic images. The DEP forces for particles in a suspension were calculated using numerical analysis. The result showed that the DEP force above the electrified region on the substrate is larger than disturbing forces, such as Brownian motion.     

Levitation and lateral forces between a point magnetic dipole and a superconducting sphere

The dipole–dipole interaction model is employed to investigate the angular dependence of the levitation and lateral forces acting on a small magnet in an anti-symmetric magnet/superconducting sphere system. Breaking the symmetry of the system enables us to study the lateral force which is important in the stability of the magnet above a superconducting sphere in the Meissner state. Under the assumption that the lateral displacement of the magnet is small compared to the physical dimensions of our proposed system, analytical expressions are obtained for the levitation and lateral forces as a function of the geometrical parameters of the superconductor as well as the height, the lateral displacement, and the orientation of the magnetic moment of the magnet. The dependence of the levitation force on the height of the levitating magnet is similar to that in the symmetric magnet/superconducting sphere system within the range of proposed lateral displacements. It is found that the levitation force is linearly dependent on the lateral displacement whereas the lateral force is independent of this displacement. A sinusoidal variation of both forces as a function of the polar and azimuthal angles specifying the orientation of the magnetic moment is observed. The relationship between the stability and the orientation of the magnetic moment is discussed for different orientations.     

A semiempirical formula describing the recoil yield in silicon

Krypton ions in the energy range 20–300 keV are used to generate recoiling atoms in silicon from thin layers evaporated on its surface. The recoil yields and the impurity distributions in the substrate have been measured as a function of several parameters (energy, thickness of the layer, incident dose). The results are used to propose a new formulation of the recoil yield based on the possibility, for both projectiles and recoiling atoms, to remove impurities previously introduced in silicon.The calculation fits very well the experimental results using displacement energies close to the generally admitted values     

The effect of substrate roughness on the static friction of CuO nanowires

The dependence of static friction on surface roughness was measured for copper oxide nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chemical etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an atomic-force microscope (AFM) tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calculate the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estimated using a multiple elastic asperity model, which is based on the Derjaguin–Muller–Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.     

湿颗粒堆力学特性的离散元法模拟研究

利用基于线性黏聚接触模型的离散元法对不同颗粒系统的堆积过程进行了数值模拟研究,分析了颗粒形状和湿颗粒间液桥力对颗粒堆积形态的影响机理,获得了球形和块状湿颗粒堆基底表面所受的法向力以及堆中颗粒间的法向力和切向力"中心凹陷"式的分布规律,讨论了颗粒形状和黏聚能量密度对基底表面作用力和颗粒间作用力的影响.研究结果表明,颗粒形状和液桥力对颗粒堆的堆积形态具有显著的影响.堆积角随着黏聚能量密度的增加而增大,并且相同条件下的块状颗粒堆积角大于球形颗粒.颗粒形状和黏聚能量密度对基底表面所受作用力和堆中颗粒间的作用力变化及最大幅值均有影响作用.当黏聚能量密度值逐渐增大时,颗粒堆的作用力最大幅值均逐渐增大,并且块状颗粒堆的作用力最大幅值大于球形颗粒堆.当黏聚能量密度值过大时,颗粒堆力学特性更加复杂,液桥力对颗粒堆积特性的影响作用大于颗粒形状的影响.     

Effect of micro-dimple patterns on capillary pull-off force and friction force of silicon surface

A microtribometer is used to measure and compare pull-off forces and friction forces exerted on (a) micro-dimpled silicon surfaces, (b) bare silicon surfaces, and (c) octadecyltrichlorosilane (OTS) treated silicon surfaces at different relative humidity (RH) levels separately. It is found that above a critical RH level, the capillary pull-off force increases abruptly and that the micro-dimple textured surface has a lower critical RH value as well as a higher pull-off force value than the other two surfaces. A micro topography parameter, namely sidewall area ratio, is found to play a major role in controlling the capillary pull-off force. Furthermore, micro-dimpled silicon surface is also proved to be not sensitive to variation in RH level, and can realize a stable and decreased friction coefficient compared with un-textured silicon surfaces. The reservoir-like function of micro dimples is considered to weaken or avoid the breakage effect of liquid bridges at different RH levels, thereby maintaining a stable frictional behaviour.     

Massively parallel low-cost pick-and-place of optoelectronic devices by electrochemical fluidic processing

We describe a novel electrochemical technique for the nonlithographic, fluidic pick-and-place assembly of optoelectronic devices by electrical and optical addressing. An electrochemical cell was developed that consists of indium tin oxide (ITO) and n -type silicon substrates as the two electrode materials and deionized water (R = 18 MOmega) as the electrolytic medium between the two electrodes. 0.8-20-microm-diameter negatively charged polystyrene beads, 50-100-microm-diameter SiO(2) pucks, and 50-microm LED's were successfully integrated upon a patterned silicon substrate by electrical addressing. In addition, 0.8-microm-diameter beads were integrated upon a homogeneous silicon substrate by optical addressing. This method can be applied to massively parallel assembly (>1000 x 1000 arrays) of multiple types of devices (of a wide size range) with very fast (a few seconds) and accurate positioning.     

Slip on the surface of silicon wafers under laser irradiation:Scale effect

The slip mechanism on the surface of silicon wafers under laser irradiation was studied by numerical simulations and experiments. Firstly, the slip was explained by an analysis of the generalized stacking fault energy and the associated restoring forces. Activation of unexpected {110} slip planes was predicted to be a surface phenomenon. Experimentally,{110} slip planes were activated by changing doping concentrations of wafers and laser parameters respectively. Slip planes were {110} when slipping started within several atomic layers under the surface and turned into {111} with deeper slip.The scale effect was shown to be an intrinsic property of silicon.     

单脉冲纳秒激光诱导硅表面微结构

利用Nd:YAG纳秒脉冲激光(波长532nm)在空气中对单晶硅表面进行单脉冲辐照,研究了激光能量密度和光斑面积变化对微结构的影响。通过场发射扫描电子显微镜和原子力显微镜(AFM)对样品表征,并对纳秒激光辐照硅的热力学过程进行分析。结果显示:当脉冲激光的能量密度接近硅的熔融阈值且光斑直径小于8μm时,形成尖峰微结构;随着能量密度或光斑面积增大,尖峰结构消失,形成边缘隆起和弹坑微结构。通过流体动力学模型得到微结构形貌的解析解,模拟得到的微结构形貌与实验测得的AFM数据一致。研究表明微结构的形成主要是由于表面张力引起的熔融硅流动。表面张力与表面温度和表面活性剂的质量浓度有关。温度梯度引起的热毛细流作用和表面活性剂浓度引起的毛细作用共同影响下形成尖峰、边缘隆起和弹坑微结构。     

Surface adhesion between hexagonal boron nitride nanotubes and silicon based on lateral force microscopy

This study presents the surface adhesion between hexagonal boron nitride nanotube (BNNT) and silicon based on lateral manipulation in an atomic force microscope (AFM). The BNNT was mechanically manipulated by the lateral force of an AFM pyramidal silicon probe using the scan mechanism in the imaging mode. With a controlled normal force of the AFM probe and the lateral motion, the lateral force applied to the BNNT could overcome the surface adhesion between BNNT and silicon surface. The individual BNNT is forced to slide and rotate on the silicon surface. Based on the recorded force curve, the calculated shear stress due to surface adhesion is 0.5 GPa. And the specific sliding energy loss is 0.2 J/m². Comparing BNNTs and carbon nanotube (CNT), the shear stress and specific sliding energy loss of BNNT are an order of magnitude larger than that of CNT. Therefore, the results show that the surface adhesion between BNNT and silicon surface is higher than that of CNT.     

Quantitative measurement of tip-sample forces by dynamic force spectroscopy in ambient conditions

We introduce a dynamic force spectroscopy technique enabling the quantitative measurement of conservative and dissipative tip-sample forces in ambient conditions. In difference to the commonly detected force-vs-distance curves dynamic force microscopy allows to measure the full range of tip-sample forces without hysteresis effects caused by a jump-to-contact. The approach is based on the specific behavior of a self-driven cantilever (frequency-modulation technique). Experimental applications on different samples (Fischer-sample, silicon wafer) are presented.     

基于能量强度测量的多声源目标位置线性估计方法

基于能量强度的多声源定位模型,本文提出了一种声源发射能量强度未知下的多声源目标位置线性估计方法。将多声源定位模型转化为线性最小二乘估计问题,估计方法以代数解形式表示多声源目标位置初始值。对初始估计值进一步优化,得到了精确的多声源目标位置估计值。该定位计算方法将定位结果以代数解形式表示,避免了数值计算过程中因初始解选择不当而导致的局部最优问题。仿真测试了所设计算法的定位精度,并由此分析了噪声及声音能量强度增益对定位误差的影响。结果表明优化后的计算结果较初始估计值有较大改进,在一定噪声范围内其定位精度可以接近于克拉美罗（CRLB）下界值。     

飞秒激光改性6H-碳化硅晶体表面光电导增益现象研究

半绝缘6H型碳化硅(6H-SiC)具有高电阻率性质,在可见光照射下进行光电导测量时,通常光生电流很小;然而经过飞秒激光辐照改性之后,发现在可见光波段的光电导有明显的增益.本文利用紫外-可见-近红外吸收谱、X射线光电子能谱和发光光谱测量分析了激光改性之后碳化硅样品的光谱吸收、发射和晶体元素比例变化情况.分析认为碳化硅光电导增益的原因是飞秒激光辐照过程改变了碳化硅表面的硅碳元素的原子浓度比,形成新的物质结构形式,从而导致了表面光电导性能的提高.     

Forces that Drive Nanoscale Self-assembly on Solid Surfaces

Experimental evidence has accumulated in the recent decade that nanoscale patterns can self-assemble on solid surfaces. A two-component monolayer grown on a solid surface may separate into distinct phases. Sometimes the phases select sizes about 10 nm, and order into an array of stripes or disks. This paper reviews a model that accounts for these behaviors. Attention is focused on thermodynamic forces that drive the self-assembly. A double-welled, composition-dependent free energy drives phase separation. The phase boundary energy drives phase coarsening. The concentration-dependent surface stress drives phase refining. It is the competition between the coarsening and the refining that leads to size selection and spatial ordering. These thermodynamic forces are embodied in a nonlinear diffusion equation. Numerical simulations reveal rich dynamics of the pattern formation process. It is relatively fast for the phases to separate and select a uniform size, but exceedingly slow to order over a long distance, unless the symmetry is suitably broken.     

Atomistic wear in a single asperity sliding contact

Abrasive wear of sharp silicon tips sliding distances of up to 750 m on a polymeric surface is studied using atomic force microscopy. The data cannot be explained by conventional macroscopic wear models. We present a new model in which the barrier for breaking an atomic bond is lowered by the frictional stress acting on the contact. Quantitative agreement is obtained between the model and wear data for all load forces and sliding distances studied.