基于原子力显微镜的碳纳米管焊接

在利用碳纳米管（CNT）制作纳米电子器件时,碳纳米管与金属电极的接触特性将决定器件性能.为此本文提出了一种利用原子力显微镜（AFM）进行碳纳米管焊接的新方法.仿真研究了探针电场的强度与分布,解释了焊接中电场产生的机理,进一步分析了偏压、探针-样品距离与探针悬臂梁偏转位移之间的关系;并通过这些优选的相关实验参数进行了焊接实验验证.实验结果表明,碳纳米管与电极间的接触电阻由2.86×106Ω减小至7.14×105Ω,并可实现碳纳米管在电极上的良好固定.     

Carbon nanotube synthesis from propane decomposition on a pre-treated Ni overlayer

Growth of carbon nanotubes (CNTs) was performed by atmospheric pressure chemical vapour deposition (APCVD) of propane on Si(111) with a pre-treated Ni overlayer acting as a catalyst. Prior to the growth of CNTs, a thin film of Ni was deposited on Si(111) substrate by evaporation and heat treated at 900°C. The growth of nanotubes was carried out at 850°C using propane as a source of carbon. Distribution of the catalyst particles over the Si substrate was analysed before and after heat treatment by atomic force microscopy (AFM). The X-ray diffraction (XRD) pattern of the grown material revealed that they are graphitic in nature. Field emission scanning electron microscopy (FESEM) was used to investigate the growth process and it was found that a catalytic particle was always situated at the tip of the tube thus implying a tip growth mechanism. Evidence for the presence of radial breathing mode from multi-wall nanotubes (MWNTs) in the grown sample was obtained from micro-Raman analysis. Finally, high-resolution transmission electron microscopic (HRTEM) analysis confirmed that the graphene layers of the CNTs are well ordered with typical 0·34 nm spacing.     

Influence of sodium chromate and dodecylamine on enhanced dissolution behavior of Cu—Ni alloy induced by AFM tip scratching

The influence of two inhibitors sodium chromate and dodecylamine on enhanced dissolution of Cu–Ni alloy initiated by atomic force microscopy (AFM) tip scratching in 1.5 M NaCl and 0.01 M HCl was investigated. The lateral force traces and force versus distance curves were measured by AFM in distilled water without or with inhibitors to investigate the influence of inhibitors on physical characters of sample surfaces. The results indicated that enhanced dissolution caused by AFM tip scratching was inhibited by adding sodium chromate or dodecylamine into the corrosive solutions, but their inhibition mechanisms are different. The inhibition effect of sodium chromate is due to its oxidation ability to repair the destroyed protection film and the increase of rigidity of metal surface resulted from the formation of oxide film containing Cr elements. On the other hand, the inhibition effect of dodecylamine is due to the organic adsorption film on metal surface to weaken the friction forces between the tip and the sample and to elevate the ionization energy of metal.     

Variable-force microscopy for advanced characterization of horizontally aligned carbon nanotubes

Atomic force microscopy (AFM) performed with variable-force imaging was recently demonstrated to be an accurate method of determining the diameter and number of sidewalls of a carbon nanotube (CNT). This AFM technique provides an alternative to transmission electron microscopy (TEM) when TEM imaging is not possible due to substrate thickness. We have used variable-force AFM to characterize horizontally aligned CNTs grown on ST-cut quartz. Our measurements reveal new aspects of horizontally aligned growth that are essential for enhancing the performance of CNT-based devices as well as understanding the growth mechanism. First, previously reported optimal growth conditions produce a large spread in CNT diameters and a significant fraction of double-walled CNTs. Second, monodispersity is significantly improved when growth temperature is reduced. Third, CNTs with diameters up to 5?nm align to the substrate, suggesting the interaction between CNTs and the quartz lattice is more robust than previously reported.     

Estimation of AFM tip shape and status in linewidth and profile measurement

An atomic force microscopy image is a dilation of the specimen surface with the probe tip. Tips wear or are damaged as they are used. And AFM tip shape and position status make AFM images distorted. So it is necessary to characterize AFM tip shape and position parameters so as to reconstruct AFM images. A geometric model-based approach is presented to estimate AFM tip shape and position status by AFM images of test specimens and scanning electron microscope (SEM) images of AFM tip. In this model, the AFM tip is characterized by using a dynamic cone model. The geometric relationship between AFM tip and the sample structure is revealed in linewidth and profile measurement. The method can easily calculate the tip parameters including half-cone angle, installation angle, scanning tilting angle and curvature radius, and easily estimate the position status of AFM tip when AFM tip moves on the specimen. The results of linewidth and profile measurement are amended accurately through this approach.     

Effects of tip-nanotube interactions on atomic force microscopy imaging of carbon nanotubes

We examine the effect of van der Waals (vdW) interactions between atomic force microscope tips and individual carbon nanotubes (CNTs) supported on SiO₂. Molecular dynamics (MD) simulations reveal how CNTs deform during atomic force microscopy (AFM) measurement, irrespective of the AFM tip material. The apparent height of a single- (double-) walled CNT can be used to estimate its diameter up to ～2 nm (～3 nm), but for larger diameters the CNT cross-section is no longer circular. Our simulations were compared against CNT dimensions obtained from AFM measurements and resonant Raman spectroscopy, with good agreement for the smaller CNT diameters. In general, AFM measurements of large-diameter CNTs must be interpreted with care, but the reliability of the approach is improved if knowledge of the number of CNT walls is available, or if additional verification (e.g., by optical techniques) can be obtained.

Open image in new window

     

Atomic force microscope: a tool for studying ionophores

The aim of the investigations was to show the analytical use of an atomic force microscopy (AFM) tip coated with an ion-selective membrane and to show that the ion-selective boundary potential is detectable as a force induced by ion-selective electrostatic interactions, which are more pronounced than double-layer forces. This new technique allows the area-specific ion exchange over boundaries to be displayed with a destruction-free technique by AFM in an aqueous buffer. From experiments with ISEs (ion-selective electrodes), a boundary potential for valinomycin was assumed to be established in close vicinity to a K+-releasing surface. To trace this boundary potential, an AFM tip was coated with a potassium-selective polymer film containing valinomycin as used in preparing ISEs. The K+-releasing substrate was prepared by incorporating a lipophilic potassium salt into a plasticized PVC layer. In contact with an electrolyte such as sodium chloride solution, an ion exchange takes place. Analogue experiments were performed using a sodium-selective ionophore, DD16C5, incorporated into the coating of the AFM tip, with a Na+-releasing substrate. The boundary potential was traced and investigated with the help of force vs distance curves. The resulting adhesion forces for a valinomycin-coated tip in a 150 mM NaCl solution were 9.8+/-3.275 nN using a blank PVC substrate and 330.15+/-113.0 nN using a substrate containing 10 wt % potassium tetrakis(4-chlorophenyl) borate. The selectivity of the ion-selective AFM tips was measured on sodium relative to potassium-releasing substrates and studied in different salt solutions with concentrations between 10 mmol L(-1) and 1 mol L(-1). For valinomycin, a force selectivity coefficient log Kf(K,Na) of -2.5+/-0.5 for K+ against Na+ and a selectivity coefficient log Kf(Na,K) of -4 +/- -0.5 for DD16C5 were measured. In addition, the surface of the polymer substrate was imaged by AFM in contact mode with and without lipophilic potassium salt. The modulation of the force-distance curves induced by the ion exchange was compared to the experimental change in elasticity of the blank and ion-exchanging substrate. The Young's moduli measured with strain force analysis on a potassium-containing substrate were 5 times smaller than the ones registered with nanoindentation and did not explain the modulation of the force vs distance curves.     

Titration force microscopy on supported lipid bilayers

The use of chemically modified atomic force microscopy (AFM) probes allows us to measure the surface charges of supported planar lipid bilayers with high sensitivity through the force spectroscopy operation mode. By controlling the chemistry of the tip, we can perform a classical analytical chemistry titration where the titration agent is a weak acid (attached to the AFM tip) with the particularity of being performed in surface rather than in solution and, especially, at the nanometric scale. Thus, the AFM tip acts as a real "nanosensor". The approaching curves of the force plots reveal that electrostatic interactions between the tip and the supported membrane play a key role. Besides, the plot of the adhesion force (measured from the retracting curve of the force plots) versus pH displays a nonsigmoidal shape with a peak in the adhesion force attributed to high-energy hydrogen bonds. One of these peaks corresponds to the pKa of the surface under study and the other to the pKa of the titrating probe attached to the tip.     

原子力显微镜探针原位有效参数对线宽测量的修正

针对原子力显微镜（AFM）的线宽和轮廓的精确测量，对AFM探针的原位有效参数进行了定义和表征，提出使用AFM探针的原位有效参数对AFM的线宽测量结果进行修正的模型。采用有效半径和半内角表征AFM探针的复合形状，悬臂轴倾角表征探针的安装状态，设计了具有不同梯形截面的两个表征样板，通过对表征样板进行AFM和扫描电子显微镜（SEM）的比对测量获得了探针的原位有效参数。提出了在线宽测量中，当AFM的扫描轮廓线具有不同的斜度时分别采用的不同的修正公式。采用此公式和探针的原位参数对掩膜板的AFM线宽测量结果进行了修正。     

Investigation of sisal fibers by atomic force microscopy: morphological and adhesive characteristics

Atomic force microscopy (AFM) was used to study the nanoscale surface chemistry and morphological changes caused by chemical treatment of sisal fibers. Scanning Electron Microscopy (SEM) micrographs indicated that sisal in natura (bundle of fibers) is formed by fibers with diameters of approximately 10 microm. AFM images showed that these fibers consist of microfibrils with diameters varying from 250 to 600 nm, which are made up of nanofibrils of ca. 20 nm in diameter. The adhesion force (pull-off force) between the AFM tip and the fibers surface increased after benzylation, pointing to a decrease in the polar groups on the sisal fiber. The adhesion map measured over a scan range of 3 microm was heterogeneous in samples treated with 40% NaOH and the low adhesion sites disappeared after benzylation. Using an established mathematical model, it was possible to evaluate the increase in adhesion work and consequently in the interaction between the AFM tip and sisal fibers. These results indicated that AFM can detect heterogeneity in the wettability of sisal fibers with nanometer resolution and can be applied in the study of fiber-matrix adhesion in polymer composites.     

The interaction of an atomic force microscope tip with a nano-object: a model for determining the lateral force

A calculation of the lateral force interaction between an atomic force microscope (AFM) tip and a nano-object on a substrate is presented. In particular, the case where the AFM tip is used to manipulate the nano-object is considered; i.e., the tip is displaced across the nano-object with the feedback off. The Hamaker equations are used to calculate the force when the tip and sample are not in contact and the Johnson, Kendall and Roberts (JKR) or Derjaguin, Muller and Toporov (DMT) formalisms are used for the contact force. The effect of the material parameters, the choice of contact theory and the shape of the nano-object on the resulting lateral forces are explored. The calculation is applied to an experimental system consisting of a cadmium selenide nanorod on graphite.     

The interaction of serum proteins with carbon nanotubes depend on the physicochemical properties of nanotubes

With more and more potential applications of carbon nanotubes (CNTs) in different fields, the risk of exposure to CNTs is increasing. The interaction between CNTs and protein in biological media can affect the way cells interact with, recognize and process the nanoparticles, and this has important implications for safety considerations. In this study, the interaction of single-walled and multiwall CNTs with various serum proteins was investigated. The adsorption kinetics of protein to CNTs was investigated and a semi-qualitative analysis was provided by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Matrix assisted laser desorption ionization/time of flight mass spectrometry (MALDI-TOF MS) was used to identify the protein species binding to CNTs and atomic force microscopy (AFM) was used to vividly demonstrate the adsorption model of protein on CNTs. All the experimental results showed that the adsorption capacity of CNTs for protein was highly dependent on the type, arrangement model, size and surface modification of CNTs. Significant quantity of proteins in serum could be quickly adsorbed by CNTs, mainly including albumin, prealbumin, transferrin, and immunoglobulin. Noncovalent functionalization of CNTs by polyethylene glycol (PEG) could decrease the protein adsorption on CNTs. These results provide crucial insights into human serum proteins binding to different kinds of CNTs, which is important for understanding the safe application of carbon nanotubes.     

CNT–CdSe QDs nanocomposites: synthesis and photoluminescence studies

In this study, synthesis of carbon nanotube (CNT)–CdSe Quantum dots (QDs) nanocomposites has been investigated. CdSe QDs were synthesized via hydrothermal process. The chemical tendency of CNT and QDS was increased by precipitation after surface functionalization of CNTs (by carboxylated groups) and CdSe QDs (by silane groups), separately. The structure of nanocomposites was amorphous with a little amount of nanocrystalline cubic CdSe. The Fourier-transform infrared (FTIR) spectra and Raman spectrum revealed the strong chemical tendency of linkage between CNTs and QDs after functionalization on the surface of them. The morphology of nanocomposites depended on the QDs concentration and changed from aggregates of CNTs to the marvelous decoration of quantum dots on the ropes of CNTs. Transmission electron microscope (TEM) and atomic force microscope (AFM) images confirmed the adorable coatings of CNTs with CdSe QDs. The nanocomposites emitted in blue–green region with a maximum peak at 490 nm under the exposure of Ultraviolet (UV) light. Below 50 wt% QDs, the emission was quenched completely.     

Accuracy of the spring constant of atomic force microscopy cantilevers by finite element method

Atomic force microscopy (AFM) probe with different functions can be used to measure the bonding force between atoms or molecules. In order to have accurate results, AFM cantilevers must be calibrated precisely before use. The AFM cantilever's spring constant is usually provided by the manufacturer, and it is calculated from simple equations or some other calibration methods. The spring constant may have some uncertainty, which may cause large errors in force measurement. In this paper, finite element analysis was used to obtain the deformation behavior of the AFM cantilever and to calculate its spring constant. The influence of prestress, ignored by other methods, is discussed in this paper. The variations of Young's modulus, Poisson's ratio, cantilever geometries, tilt angle, and the influence of image tip mass were evaluated to find their effects on the cantilever's characteristics. The results were compared with those obtained from other methods.     

Rapid prototyping of nanotube-based devices using topology-optimized microgrippers

Nanorobotic handling of carbon nanotubes (CNTs) using microgrippers is one of the most promising approaches for the rapid characterization of the CNTs and also for the assembly of prototypic nanotube-based devices. In this paper, we present pick-and-place nanomanipulation of multi-walled CNTs in a rapid and a reproducible manner. We placed CNTs on copper TEM grids for structural analysis and on AFM probes for the assembly of AFM super-tips. We used electrothermally actuated polysilicon microgrippers designed using topology optimization in the experiments. The microgrippers are able to open as well as close. Topology optimization leads to a 10-100 times improvement of the gripping force compared to conventional designs of similar size. Furthermore, we improved our nanorobotic system to offer more degrees of freedom. TEM investigation of the CNTs shows that the multi-walled tubes are coated with an amorphous carbon layer, which is locally removed at the contact points with the microgripper. The assembled AFM super-tips are used for AFM measurements of microstructures with high aspect ratios.     

Single- and few-walled carbon nanotubes grown at temperatures as low as 450 degrees c: electrical and field emission characterization

Single-wall (SW-) and few-walled (FW-) carbon nanotubes (CNTs) were synthesized on aluminum/ cobalt coated silicon at temperatures as low as 450 degrees C by plasma enhanced chemical vapor deposition technique (PECVD). The SWCNTs and FWCNTs grow vertically oriented and well separated from each other. The cold field emission studies of as-grown SWCNTs and FWCNTs showed low turn-on field emission threshold voltages, strongly dependent of the nanotubes morphology. Current-voltage curves of individual CNTs, measured by conductive atomic force microscopy (CAFM), showed an electrical resistance of about 90 Komega, that is attributed mainly to the resistance of the contact between the CNTs and the conductive CAFM tip (Au and Pt).     

纳米碳管-(Ni-P)复合材料制备及性能

采用化学复合镀的方法,在45#钢衬底上制备纳米碳管(CNTs)-(Ni-P)复合材料。探讨了该复合材料的制备技术及工艺条件,通过对实验结果的观察和分析,确定了制备CNTs-(Ni-P)复合材料的最佳工艺条件。利用透射电镜(TEM)观察纳米碳管的结构;用扫描电镜(SEM)观察纳米碳管形貌及其在复合材料中的分布;利用原子力显微镜(AFM)观察复合材料表面的粗糙度。同时还对纳米碳管复合材料的耐磨性进行了初步的测试。实验结果表明,该复合材料的耐磨性明显好于未镀及单纯镀镍材料。     

Monitoring DNA immobilization and hybridization on surfaces by atomic force microscopy force measurements

DNA immobilization and hybridization was carried out on Au substrates that were modified with mercaptopropanoic acid and then treated with aluminum(III) solution. The positively charged AI(III) film can be used to immobilize both ds-DNA and ss-DNA. Atomic force microscopy (AFM) was used to monitor the process by force measurements between a negatively charged silica tip and the substrates while immersed in dilute electrolyte. Surface hybridization of ss-DNA produces an increase in the surface charge and surface potential of the substrates, which is reflected by the increasing repulsive force as determined from AFM force-separation curves. A single-base mismatch was detectable in surface hybridization. The AFM force measuring technique was also employed to investigate the interaction of Ru(phen)3(2+) with ss-DNA and ds-DNA. The force measurement results showed that there is a small interaction between Ru(phen)3(2+) and ss-DNA, which was ascribed to the electrostatic binding of Ru(phen)3(2+) to the ss-DNA surface. For ds-DNA, there is a strong interaction which is believed to be due to the association or intercalation of Ru(phen)3(2+) with ds-DNA.     

摩擦力显微镜及几种材料的微摩擦特性

介绍了自制的摩擦力显微镜基本原理，结构及关键技术，该仪器能够同时或分别采集横向力和法向力信号，进行多种微观力函数的程控采集，例如法向力与信号电流的对应关系，针尖与样品间的粘附力，还介绍了ｎＮ级载荷定量设定方法等。利用该仪器研究了探针与金膜、光盘之间的接触式滑动的微观摩擦行为，对微观接触状态进行了理论分析，提出了计算摩擦系数的方法。     

Effects of welding head on the carbon nanotube field emission in ultrasonic nanowelding

Ultrasonic nanowelding technique was used to improve the field emission properties of carbon nanotube (CNT) cathodes. Two kinds of welding heads were used and the effects of the head on the emission properties were studied. The results show that cathodes welded by Al₂O₃ flat head demonstrate excellent field emission properties with high emission current density and good current stability. The improved field emission performance is attributed to the reliable and low resistance contact between CNTs and metal substrates. Cathodes welded by steel matrix head show a lower turn-on electric field due to the protruding CNTs at the edge of the welded pits.