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.     

Experimental validation of atomic force microscopy-based cell elasticity measurements

Atomic force microscopy (AFM) is widely used for measuring the elasticity of living cells yielding values ranging from 100 Pa to 100 kPa, much larger than those obtained using bead-tracking microrheology or micropipette aspiration (100-500 Pa). AFM elasticity measurements appear dependent on tip geometry with pyramidal tips yielding elasticities 2-3 fold larger than spherical tips, an effect generally attributed to the larger contact area of spherical tips. In AFM elasticity measurements, experimental force-indentation curves are analyzed using contact mechanics models that infer the tip-cell contact area from the tip geometry and indentation depth. The validity of these assumptions has never been verified. Here we utilize combined AFM-confocal microscopy of epithelial cells expressing a GFP-tagged membrane marker to directly characterize the indentation geometry and measure the indentation depth. Comparison with data derived from AFM force-indentation curves showed that the experimentally measured contact area for spherical tips agrees well with predicted values, whereas for pyramidal tips, the contact area can be grossly underestimated at forces larger than ～0.2 nN leading to a greater than two-fold overestimation of elasticity. These data suggest that a re-examination of absolute cellular elasticities reported in the literature may be necessary and we suggest guidelines for avoiding elasticity measurement artefacts introduced by extraneous cantilever-cell contact.     

Augmented reality user interface for an atomic force microscope-based nanorobotic system

A real-time augmented reality (AR) user interface for nanoscale interaction and manipulation applications using an atomic force microscope (AFM) is presented. Nanoscale three-dimensional (3-D) topography and force information sensed by an AFM probe are fed back to a user through a simulated AR system. The sample surface is modeled with a B-spline-based geometry model, upon which a collision detection algorithm determines whether and how the spherical AFM tip penetrates the surface. Based on these results, the induced surface deformations are simulated using continuum micro/nanoforce and Maugis-Dugdale elastic contact mechanics models, and 3-D decoupled force feedback information is obtained in real time. The simulated information is then blended in real time with the force measurements of the AFM in an AR human machine interface, comprising a computer graphics environment and a haptic interface. Accuracy, usability, and reliability of the proposed AR user interface is tested by experiments for three tasks: positioning the AFM probe tip close to a surface, just in contact with a surface, or below a surface by elastically indenting. Results of these tests showed the performance of the proposed user interface. This user interface would be critical for many nanorobotic applications in biotechnology, nanodevice prototyping, and nanotechnology education.     

High spatial resolution Kelvin probe force microscopy with coaxial probes

Kelvin probe force microscopy (KPFM) is a widely used technique to measure the local contact potential difference (CPD) between an AFM probe and the sample surface via the electrostatic force. The spatial resolution of KPFM is intrinsically limited by the long range of the electrostatic interaction, which includes contributions from the macroscopic cantilever and the conical tip. Here, we present coaxial AFM probes in which the cantilever and cone are shielded by a conducting shell, confining the tip-sample electrostatic interaction to a small region near the end of the tip. We have developed a technique to measure the true CPD despite the presence of the shell electrode. We find that the behavior of these probes agrees with an electrostatic model of the force, and we observe a factor of five improvement in spatial resolution relative to unshielded probes. Our discussion centers on KPFM, but the field confinement offered by these probes may improve any variant of electrostatic force microscopy.     

Batch fabrication of atomic force microscopy probes with recessed integrated ring microelectrodes at a wafer level

A batch fabrication process at the wafer-level integrating ring microelectrodes into atomic force microscopy (AFM) tips is presented. The fabrication process results in bifunctional scanning probes combining atomic force microscopy with scanning electrochemical microscopy (AFM-SECM) with a ring microelectrode integrated at a defined distance above the apex of the AFM tip. Silicon carbide is used as AFM tip material, resulting in reduced mechanical tip wear for extended usage. The presented approach for the probe fabrication is based on batch processing using standard microfabrication techniques, which provides bifunctional scanning probes at a wafer scale and at low cost. Additional benefits of batch fabrication include the high processing reproducibility, uniformity, and tuning of the physical properties of the cantilever for optimized AFM dynamic mode operation. The performance of batch-fabricated bifunctional probes was demonstrated by simultaneous imaging micropatterned platinum structures at a silicon dioxide substrate in intermittent (dynamic) and contact mode, respectively, and feedback mode SECM. In both, intermittent and contact mode, the bifunctional probes provided reliable correlated electrochemical and topographical data. In addition, simulations of the diffusion-limited steady-state currents at the integrated electrode using finite element methods were performed for characterizing the developed probes.     

Determination of lubricant film thickness on a particulate disksurface by atomic force microscopy

The ability of the atomic force microscope (AFM) to measure the lubricant film thickness on the surface of particulate disks is demonstrated, and experimental results are presented. AFM measure the thickness of the lubricant film at a particular location on the disk surface with a lateral resolution of the order of the AFM tip radius, ~1000 Å. For an unused disk, 50-70 Å of lubricant thickness is found. After the disk has been in use for several years, the lubricant thickness decreases to 35-55 Å. In both cases, the lubricant is uniformly distributed on the disk surface, although somewhat more uniformly on the used disk. For all types of disks studied, most of the lubricant resides below the disk surface, presumably in the medium's porosity, with only a molecular thin film of lubricant on the surface     

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.     

Two-dimensional stick-slip on a soft elastic polymer: pattern generation using atomic force microscopy

It has been demonstrated that it is possible to create laterally differentiated frictional patterning and three-dimensional structures using an atomic force microscope (AFM) probe on the surface of a soft elastic polymer, poly(dimethylsiloxane) (PDMS). The resulting effect of contact mode imaging at low loading forces (<100?nN), observed in the lateral force mode, revealed a homogeneous pattern on the PDMS surface exhibiting higher friction. With higher loading forces ([Formula: see text]?nN) the effect is non-uniform, resulting in structures with depths on the nanometre scale. The topographic and frictional data revealed stick-slip responses in both the fast (orthogonal to the long axis of the lever) and slow (parallel to the long axis of the lever) directions of probe travel from scanning in a raster pattern. The stick-slip events are manifested in the form of a series of shallow channels spaced evenly apart on the polymer surface. Detailed friction loop analysis acquired during the manipulation process showed that the lateral force changed according to the strength of trapping of the tip with the polymer surface exhibiting significant in-plane deformation due to lateral forces being imposed. An incremental increase in the initial loading force resulted in an increase in in-plane displacement and a greater spacing between the stick lines/channels in the slow-scan direction. A decrease in channel length in the fast-scan direction is also observed as a result of an increase in static friction with normal force, resulting in greater surface deformation and shorter track length for sliding friction.     

An atomic force microscope tip designed to measure time-varying nanomechanical forces

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.     

Qualitative characterisation of contact strength between carbon nanotubes and electrodes

Carbon nanotubes (CNTs) were aligned between microelectrodes by dielectrophoresis. Atomic force microscope (AFM) was used to manipulate the deposited CNTs, and the contact strength between CNTs and electrodes was qualitatively characterised by the level of difficulty for manipulation. The results show that unwelded CNTs were moved away easily by AFM tip, while the welded CNTs could resist the tip lateral force even if the CNTs were cut off by the tip. The improved mechanical property of the welded sample is attributed to the embedding of CNTs, which bonds reliably with the metal electrode.     

Gold-coated conducting-atomic force microscopy probes

Some aspects of the performance of gold-coated conductive probes used in conducting atomic force microscopy (C-AFM) technique are discussed. The resistance of the nanocontact between the gold-coated AFM tip and the graphite substrate has been monitored at various applied forces. For small forces (<50 nN), resistance on the order of a few kiloohms was observed. Minimal contact resistance was observed for forces in the range 100-150 nN, beyond which the tip seems to undergo plastic deformation. The resistance of the nanocontact increased when current on the order of 100 microA was allowed to pass through, finally resulting in melting of the gold coating.     

Elastic property of vertically aligned nanowires

An atomic force microscopy (AFM) based technique is demonstrated for measuring the elastic modulus of individual nanowires/nanotubes aligned on a solid substrate without destructing or manipulating the sample. By simultaneously acquiring the topography and lateral force image of the aligned nanowires in the AFM contacting mode, the elastic modulus of the individual nanowires in the image has been derived. The measurement is based on quantifying the lateral force required to induce the maximal deflection of the nanowire where the AFM tip was scanning over the surface in contact mode. For the [0001] ZnO nanowires/nanorods grown on a sapphire surface with an average diameter of 45 nm, the elastic modulus is measured to be 29 +/- 8 GPa.     

原子力显微镜中微悬臂梁/探针横向力的标定

利用微加工制造的微悬臂梁／探针尖已经广泛应用在微观表面性质测试和微纳米尺度加工等领域，成为微纳米研究领域中不可缺少的重要工具．为了能够定量研究原子力显微镜中探针与表面的相互作用力，需要对微悬臂梁／探针的力学性能进行表征．本文简要地论述了原子力显微镜中微悬臂梁的形变光反射原理和探针与表面的接触刚度理论．阐明了微悬臂梁横向力标定的重要性．综述了目前几种微悬臂梁／探针横向力的标定方法、简单的推倒过程和特点、     

Observation of Si pattern sidewall using inclination atomic force microscope for evaluation of line edge roughness

Inclination atomic force microscope (AFM) imaging has been studied on the possibility to observe a pattern sidewall in contact mode or digital probing (step-in) mode for a line edge roughness (LER) or line width roughness (LWR). Analysis of the AFM tip bending and slipping indicates that it is serious problem to measure and control very fine patterns within an error of less than 1 nm in contact of the tip on the steep slop of the pattern, and it is very important directly to observe the sidewall at inclination angle. In experiments using pyramidal tip and steep Si pattern with about 90 degrees slop, it has demonstrated that the inclination angle is 35-40 degrees for faithful observation of the sidewall. We have observed the etched strip lines on the sidewall with a width of about 100 nm and a depth of about 6.4 nm. We have demonstrated that the inclination AFM is very useful for evaluation of the LER or LWR.     

Studying electrical transport in carbon nanotubes by conductance atomic force microscopy

An introduction to conductance atomic force microscopy in the context of carbon nanotubes is provided where the main problems and performances of this technique are discussed. The conductance measured in SWNT as a function of the loading force applied by an AFM metallized tip is reported. These experiments allow us to study the process of the electrical contact formation between the tip and the nanotube. This will also lead to a study of the electromechanical properties of nanotubes for radial deformations.     

Determining Mechanical Properties of Carbon Microcoils Using Lateral Force Microscopy

Mechanical properties of amorphous carbon microcoil (CMC) synthesized by thermal chemical vapor deposition method were examined in compression and tension tests, using the lateral force mode of atomic force microscope (AFM). The AFM cantilever tip was manipulated by a piezoelectric scanner to contact, pull, and push an individual CMC. The lateral force that was exerted by the CMC deformation causes the twist of the AFM cantilever. It was monitored by the laser and photodetector of the AFM during the experiments. A linear response of the CMC was observed in the range of 25 nm to 5 mum of tension experiments. The results show that the spring constant of the CMC is reasonably proportional to the coil number. The shear modulus of the amorphous CMC is estimated to be 3 plusmn 0.2 GPa. The proposed method is promising to manipulate the compression and tension of the CMC and to measure the lateral force exerted in an ambient environment.     

A nanoscale friction investigation during the manipulation of nanoparticles in controlled environments

Future micro/nanodevices will contain very small features such that liquid lubrication is not practical and inherent lubricity is needed. In this study, a nanoscale friction investigation was carried out during the manipulation of Au and SiO(2) nanoparticles on silicon using atomic force microscopy (AFM). Nanoparticle sliding was characterized by quantifying the lateral force associated with the AFM tip twisting as it hits the particle edge. The friction force varies with particle area and humidity, illustrating how meniscus forces on nanoparticles affect friction. A large tip slid on the nanoparticle-coated surface exhibited friction reduction due to nanoparticle sliding and contact area reduction.     

Atomic force microscopy study of silica nanopowder compacts

Nanometer silica powders compacted at different pressures have been studied by atomic force microscopy (AFM). Local elastic moduli measurements made on the powder compacts yield values smaller than that of bulk silica. Loading force-distance curves measured show break points at some critical pressures. AFM images obtained at constant contact forces above and below the critical force at which a break point occured show the break point was a result of AFM tip plowing into the nanometer powder compacts. The applied force required for break points to occur increases with sample density. Such a behavior has been qualitatively explained in terms of adhesion force between nanoparticle and sample surface morphology.     

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。     

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.