Attachment of carbon nanotubes to atomic force microscope probes

In atomic force microscopy (AFM) the accuracy of data is often limited by the tip geometry and the effect on this geometry of wear. One way to improve the tip geometry is to attach carbon nanotubes (CNT) to AFM tips. CNTs are ideal because they have a small diameter (typically between 1 and 20nm), high aspect ratio, high strength, good conductivity, and almost no wear. A number of methods for CNT attachment have been proposed and explored including chemical vapour deposition (CVD), dielectrophoresis, arc discharge and mechanical attachment. In this work we will use CVD to deposit nanotubes onto a silicon surface and then investigate improved methods to pick-up and attach CNTs to tapping mode probes. Conventional pick-up methods involve using standard tapping mode or non-contact mode so as to attach only those CNTs that are aligned vertically on the surface. We have developed improved methods to attach CNTs using contact mode and reduced set-point tapping mode imaging. Using these techniques the AFM tip is in contact with a greater number of CNTs and the rate and stability of CNT pick-up is improved. The presence of CNTs on the modified AFM tips was confirmed by high-resolution AFM imaging, analysis of the tips dynamic force curves and scanning electron microscopy (SEM).     

Correlation Between Mechanical Properties and Nanofriction of [Ti–Cr/Ti–Cr–N] n and [Ti–Al/Ti–Al–N] n Multilayers

In this work, thin films deposited by pulsed DC magnetron sputtering of [Ti–Al/Ti–Al–N] _n and [Ti–Cr/Ti–Cr–N] _n multilayers of nanometric periods were analyzed by AFM in contact mode to measure values of lateral and normal forces. From these measurements, the coefficient of friction (COF) of these materials in contact with the AFM tip was calculated. Measurements were made with three types of silicon tips, diamond-coated, Pt–Cr-coated, and bare silicon. Significant differences between the tip materials in contact with the samples, which affected the COF, were observed. The effect of the environmental layer of water covering the surface sample and the tip appears as the most important factor affecting the tribology behavior of the tip-sample contact. For diamond-coated and bare silicon tips there is an additional adherence force increasing the normal load. But for tips platinum–chromium-coated there is a repulsive force due to this water layer, which behaves as a lubricant layer before a threshold load.     

Lateral force microscopy of multiwalled carbon nanotubes

Carbon nanotubes are usually imaged with the atomic force microscope (AFM) in non-contact mode. However, in many applications, such as mechanical manipulation or elasticity measurements, contact mode is used. The forces affecting the nanotube are then considerable and not fully understood. In this work lateral forces were measured during contact mode imaging with an AFM across a carbon nanotube. We found that, qualitatively, both magnitude and sign of the lateral forces to the AFM tip were independent of scan direction and can be concluded to arise from the tip slipping on the round edges of the nanotube. The dependence on the normal force applied to the tip and on the ratio between nanotube diameter and tip radius was studied. We show that for small values of this ratio, the lateral force signal can be explained with a simple geometrical model.     

Method of electrochemical etching of tungsten tips with controllable profiles

We demonstrate a new and simple process to fabricate tungsten tips with good control of the tip profile. In this process, we use a commercial function generator without any electronic cutoff circuit or complex mechanical setup. The tip length can be varied from 160 μm to 10 mm, corresponding to an aspect ratio of 1.6-100. The radius of curvature of the tip apex can be controlled to a size <10 nm. Surface roughness and the taper angle can be controlled independently. Through control of the etching parameters, the tip length, the radius of curvature, surface roughness, and the taper angle can be controlled to suit different requirements of various applications. The possible etching mechanisms are also discussed.     

Fabrication of sharp tungsten-coated tip for atomic force microscopy by ion-beam sputter deposition

Tungsten (W) is significantly suitable as a tip material for atomic force microscopy (AFM) because its high mechanical stiffness enables the stable detection of tip-sample interaction forces. We have developed W sputter-coating equipment to compensate the drawbacks of conventional Si cantilever tips used in AFM measurements. By employing an ion gun commonly used for sputter cleaning of a cantilever tip, the equipment is capable of depositing conductive W films in the preparation chamber of a general ultrahigh vacuum (UHV)-AFM system without the need for an additional chamber or transfer system. This enables W coating of a cantilever tip immediately after sputter cleaning of the tip apex and just before the use in AFM observations. The W film consists of grain structures, which prevent tip dulling and provide sharpness (<3 nm in radius of curvature at the apex) comparable to that of the original Si tip apex. We demonstrate that in non-contact (NC)-AFM measurement, a W-coated Si tip can clearly resolve the atomic structures of a Ge(001) surface without any artifacts, indicating that, as a force sensor, the fabricated W-coated Si tip is superior to a bare Si tip.     

Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips

Dynamic electrochemical etching technique is optimized to produce tungsten tips with controllable shape and radius of curvature of less than 10 nm. Nascent features such as "dynamic electrochemical etching" and reverse biasing after "drop-off" are utilized, and "two-step dynamic electrochemical etching" is introduced to produce extremely sharp tips with controllable aspect ratio. Electronic current shut-off time for conventional dc "drop-off" technique is reduced to ～36 ns using high speed analog electronics. Undesirable variability in tip shape, which is innate to static dc electrochemical etching, is mitigated with novel "dynamic electrochemical etching." Overall, we present a facile and robust approach, whereby using a novel etchant level adjustment mechanism, 30° variability in cone angle and 1.5 mm controllability in cone length were achieved, while routinely producing ultra-sharp probes.     

Characterization of the cutting edge of glass knives for ultramicrotomy by scanning force microscopy using cantilevers with a defined tip geometry

The geometry of glass knife edges for ultramicrotomy was studied with nanoscale resolution using scanning force microscopy (SFM) in the contact mode. The local shape of the cutting edge was estimated from single line profiles of the SFM topographic images by taking into account the exact radius of the ultrasharp silicon tip. The tip radius was estimated from secondary electron micrographs recorded at low voltage by field emission scanning electron microscopy (FESEM). The radius of the investigated cutting edges was found to be in range 5–20 nm. The results obtained illustrate that the combination of SFM and high resolution FESEM provides a unique means to determine precisely the radius of glass knives.     

Design of characteristic parameters for controlling tungsten tip profile during electrochemical etching

Micro/nano-scale tungsten tips fabricated by electrochemical etching have many diverse industrial applications. The characteristic parameters of the tungsten tip profile include apex radius, taper angle, and aspect ratio. These parameters are governed by many factors including applied voltage, concentration of the electrolyte (potassium hydroxide) solution, and diameter of the inner gold ring. However, a systematic investigation with the aim of determining the best conditions for fabricating micro/nano-scale tips with desired profiles has not been carried out yet. This study is aimed at obtaining controllable tungsten tip profiles—particularly with respect to the radius of curvature and aspect ratio of tips (taper angle)—by altering the experimental conditions. A series of experiments were executed and the results were aggregated and analyzed using response surface methodology in order to identify the relationships between the tungsten tip characteristics and input parameters. The method proposed herein would prove to be suitable for a variety of applications in industries that require tungsten tips with a specific profile.     

Effect of tip shape on capacitance determination accuracy in scanning capacitance microscopy

We present an analysis of the measurement error caused by the stray field of scanning capacitance microscope probes of various shapes. Cylindrical islands and wells of varying radius and height or depth, in both conducting surfaces and structures containing dielectric films, were used as test features for modelling. The results show that high accuracy and good contrast of small details are contradictory requirements. Probes with small radius of curvature of the tip apex yield smaller errors on features with small diameter but larger ones on features with large diameter than tips with large radius of curvature. The stray electrostatic field causes large errors, which are exceptionally severe with microfabricated probes. Contrary to general belief, differential measurements, based on modulation of the probe/sample separation or of the width of depletion layer in semiconductors, do not reduce the effect of the stray field significantly. For best results, the probe should be shielded as close to the tip apex as possible. In the case of microfabricated probes, at least the side of the cantilever facing the sample should be shielded.     

Mesoscale scanning probe tips with subnanometer rms roughness

Surface smoothness of probe tips is critical for applications, such as measuring surface tension of various liquids, oscillatory hydration forces, and interfacial shear strengths from friction experiments. In this study we establish conditions for fabricating tips with smooth surfaces by controlling the electrochemical polishing process throughout the tip evolution rather than following the current practice of producing tips by the drop-off method. Polishing is conducted under a constant voltage, with the wire immersed below the nominal air/electrolyte interface by no more than one-half of the wire diameter and stopping the etching at different current levels. This process provides a tip radius range of approximately 100 nm to 5 microm for a tungsten wire with a 0.2 mm diameter. Alternatively, the wire can be placed above the nominal air/electrolyte interface but within the meniscus until the current drops to zero. In this case, the tip radii range from 5 to 50 microm. In both cases, atomic force microscopy scans of these tips show that the surface rms roughness is about 0.3 nm.     

Using defined structures on very thin foils for characterizing AFM tips

Knowledge of tip geometry is necessary for reproducible atomic force microscope (AFM) measurements. This is particularly important for measurements in contact mode, in which a certain wear of the tip will always occur. For small or flat structures or for structures of larger dimensions, knowledge of the tip radius and the entire tip geometry is important. Additionally, the tilt of the tip in relation to the sample is of importance. Normally, very complicated lithographically manufactured structures for tip characterization are used. In contrast, the structures shown in this work are very simple. For measuring the tip geometry very thin foils patterned by focused ion beam (FIB) were used. In this work we demonstrate the possibility of determining the AFM tip geometry and the tilt based on several different large structures. A proven algorithm was developed for the reconstruction of the tips. The shape of FIB-structured foils was determined by electron microscopy prior to AFM measurements. This new method for determining tip shape is also presented as it compares to other current methods. In this case a discussion on the stability and advantages of the new method is presented.     

Tip bluntness transition measured with atomic force microscopy and the effect on hardness variation with depth in silicon dioxide nanoindentation

Depth-sensing indentation measurements of surfaces and structures with indentation depths less than 100 nm necessitate the use of accurate area functions for correct property evaluation. Here, the effect of a blunt nanoindenter tip geometry is characterized using atomic force microscopy to measure the direct tip geometry and modeled by a power law profile shape. Direct measurement of tip geometry is a method to observe changes in the tip curvature and transition from the blunt tip region to an ideal tip geometry. The tip shape, curvature, and transition to ideal geometry is found to correspond with the increase in hardness observed experimentally in SiO2 using a self-similar contact model. For a Berkovich indenter, tip bluntness was found to have a power law degree of 1.5 near the tip apex with a continuously varying degree of bluntness until an ideal pyramidal shape was reached at a depth of 160 nm.     

Scaling effects between micro- and macro-tribology for a Ti–MoS2 coating

The tribological properties of a Ti–MoS₂ coating (9 at% Ti) were studied at macroscopic length scales with an in situ tribometer and at microscopic length scales with a nanoindentation instrument equipped for microsliding experiments. Measurements were conducted in controlled environments at both low and high humidity (i.e. ～4%RH and ～35%RH). Reciprocating micro- and macro-sliding tests were performed with spherical diamond tip with a 50 μm radius and a sapphire tip with a radius of 3.175 mm, respectively. For both scales, the range of Hertzian contact pressures was between 0.41 GPa and 1.2 GPa. In situ video microscopy observations identified that the dominant velocity accommodation mode at macro-scale was interfacial sliding. However, an additional velocity accommodation mode, transfer film shearing, was also observed with higher humidity. Overall higher friction was observed with microtribology compared to macrotribology. The higher coefficient of friction was attributed to three different stages during the sliding process, which were identified with respect to different contact pressures, contact areas, tip shapes, and environmental conditions. The first two stages exhibited a solid lubrication behavior with some combination of interfacial sliding, transfer film shearing and microplowing. The transfer film thicknesses for these stages, normalized to the initial Hertzian contact radius, fell in a range of 0.001–0.1. For the third stage, the dominant VAM was plowing and the normalized transfer film thickness fell below this range. Comparisons between the two scales demonstrated that for dry sliding, microscopic contacts on Ti–MoS₂ deviate slightly from macroscopic behavior, showing higher limiting friction and microplowing. For humid sliding, microscopic contacts deviate significantly from macroscopic behavior, showing plowing behavior and absence of transfer films.     

An attempt to determine the true stress-strain curves of amorphous polymers by nanoindentation

We have made an attempt to identify the true material stress-strain curves of some amorphous polymers (PC, PMMA, CR39 (diethylene glycol bis(allyl carbonate)) from nanoindentation experiments by using non-self similar tip shapes, i.e. tips promoting an increasing strain with an increasing indentation depth. We firstly found that, for a defined strain rate, the relationship linking the contact pressure to the ratio between the contact depth and the contact radius is intrinsic to the material, i.e. it doesn’t depend on the tip shape used. From this relationship, for each material, we calculated the evolution of the average stress with the average strain beneath the tip, by using the theoretical background developed for elasto-perfectly plastic materials. By comparing to compression test results, we concluded that this approach works well for PC and PMMA only for an average strain below 12%. This approach is inapplicable for the CR39 material even at low strains. We assumed that this is mainly due to the occurrence of a significant strain hardening, that doesn’t allow us to neglect the strain gradient existing beneath the tip.     

Scanning (atomic) force microscopy imaging of earthworm haemoglobin calibrated with spherical colloidal gold particles

Scanning (atomic) force microscopy (SFM) permits high-resolution imaging of a biological specimen in physiological solutions. Untreated extracellular haemoglobin molecules of the common North American earthworm, Lumbricus terrestris, were imaged in NH₄Ac solution using calibrated SFM. Individual molecules and their top and side views were clearly identified and were comparable with the images of the same molecule obtained by scanning transmission electron microscopy (STEM). A central depression, the presumed mouth of the hole, was detected. We analysed 75 individual molecules for their lateral dimensions. Compression varied for different molecules, presumably because of the variation of the interaction between the SFM tip and the protein molecule. Two effective heights which correspond to the heights of the points of the haemoglobin molecules first and last touched by the tip, h₁ and h₂, respectively, were measured for each protein and ranged between 1.58 and 16.2 nm for h₁ and 1.23 and 13.6 nm for h₂. The apparent diameter was measured and ranged from 44.9 to 86.6 nm (63.2±10.5 nm, n =75), which is about twice the diameter of the molecule reported by STEM for the top view orientation. The higher the measured effective heights, the worse was the tip convolution effect. In order to determine the tip parameters (semivertical angle, curvature of radius and the cut-off height) and to calibrate images of earthworm haemoglobin molecules, spherical gold particles were scanned as standards. The tip sectional radii at distances of h₁ and h₂ above the tip apex were subtracted from the apparent diameter of the protein. The calibrated lateral dimension was 29.1 ±3.85 nm, which is close to the reported scanning transmission electron microscopy data 30.0 ±0.8 nm. The results presented here demonstrate that the calibration approach of imaging gold particles is practical and relatively accurate. Calibrated SFM imaging can be applied to the study of other biomacromolecules.     

An experimental study on the adhesion at a nano-contact

Nano-adhesion characteristics between scanning probe microscope (SPM) tips of various radius of curvature and flats of different materials were experimentally studied. Adhesion and friction forces between Si-wafer (1 0 0) and Si₃N₄ tips were measured under various applied normal loads, and the results were compared to those of diamond-like carbon (DLC), tungsten incorporated diamond-like carbon (W-DLC) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) formed on Si-wafer surfaces. Also in order to study the effect of capillary force, tests were performed in various relative humidity. Results showed that the adhesion increased with the tip radius. When the applied normal load increased from 0 to 40 nN, the adhesion did not change, but the friction increased linearly. Results generally showed that surfaces of the more hydrophobic property revealed the lower adhesion. The adhesion forces increased with the relative humidity. The nano-adhesion phenomenon was discussed on the basis of JKR model and capillary force exerted by meniscus.     

Lubrication of sliding point contacts of AISI 52100 steel — the influence of curvature

The lubrication mechanism of fully submerged sliding point contacts of non-oxidized AISI 52100 steel has been studied as a function of radius of curvature in the contact area. The results of experiments, performed with hemispherically tipped pins with radii of curvature R₁ ranging from 1 mm to 23 mm in contact with the curved surfaces of rings of 76 mm diameter, corroborate the existence of three well-defined lubrication regimes, i.e. a regime of thin oil film lubrication, a regime of boundary lubrication and a regime of virtually unlubricated contact. At small radii of curvature (i.e.R₁ = 1 mm and 5 mm), the contacts derive their load-carrying capacity almost entirely from the presence of a boundary lubricant film. At R₁ = 23 mm, lubrication is mainly due to the presence of a thin oil film.     

In situ metal tip sharpening of scanning probe microscopes

This article demonstrated a new approach for fabrication and sharpening of metal tips of scanning probe microscopes. Experimentally, a metal tip was heated and melted by a focused laser light. The tip was then sharpened by a strong electric field and consolidated as the laser was turned off. With a low‐vacuum and a high‐voltage source, a 25‐µm indium‐coated platinum wire was sharpened to a tip with diameters below 50 nm. The minimal tip radius by this method is estimated to be below 1 nm. With this technique, in situ tip sharpening for SPM would be possible. SCANNING 34: 76–79, 2012. © 2011 Wiley Periodicals, Inc.     

基于电化学研磨的SPM钨探针制备方法研究

钨探针是扫描隧道显微镜(STM)常用探针之一。为了将钨探针应用于扫描探针显微镜(SPM)，根据钨探针的受力，通过理论分析确定了钨探针的理想轮廓：钨探针的直径变化应具有指数曲线。为了获得指数曲线轮廓、尖锐测头等良好特性的钨探针，分别提出并研究了改进的钨探针液面直流电化学研磨法和薄膜直流电化学研磨法。通过这两种电化学研磨法获得的探针以及通过传统的交流电化学研磨法获得的钨探针，分别在探针的形状、探针尖端曲率半径、表面质量、研磨速度、可复现性等多个方面进行了观察和比较。通过实验发现，除了研磨速度外，改进的液面直流电化学研磨法和薄膜直流电化学研磨法在其他各方面都优于交流电化学研磨法。     

Editorial

From both simple estimates and a 'blind' reconstruction based on cryo-AFM images of filamentous actin, we find that the radius of curvature at the apex of Si₃N₄ tips can be as small as 1 nm with a conical angle in the range 30~40°, revealing a relatively high aspect ratio that is much greater than previously anticipated. Our results show that commercially available cantilevers are often sharp enough for routine high resolution imaging of biological materials, and suggest that factors other than an inherent blunt tip are probably responsible for frequent occurrences of poor resolution.