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.     

Microtribological studies of unlubricated sliding Si/Si contact in air using AFM/FFM

Tribological properties of Si/Si contacts were measured on a microscale by using an atomic force/friction force microscope. Friction forces and pull-off forces between a Si tip and a polished surface of a Si(100) wafer were studied as a function of applied normal load and relative humidity of the surrounding air. The results show that pull-off forces and friction coefficients increased and were strongly influenced by capillary forces with increasing humidity. Tribological interactions during 20 passes of overlapping sliding contact at 50% relative humidity and very small loads of 70 nN were confined to the layer of adsorbates and chemical reactions, without measurable solid damage on the Si(100) wafer.     

Effects of higher oscillation modes on TM-AFM measurements

The finite element method and molecular dynamics simulations are used for modeling the AFM microcantilever dynamics and the tip-sample interaction forces, respectively. Molecular dynamics simulations are conducted to calculate the tip-sample force data as a function of tip height at different lateral positions of the tip with respect to the sample. The results demonstrate that in the presence of nonlinear interaction forces, higher eigenmodes of the microcantilever are excited and play a significant role in the tip and sample elastic deformations. Using comparisons between the results of FEM and lumped models, how some aspects of the system behavior can be hidden when the point-mass model is used is illustrated.     

Surface force microscope observations of corrosive tribological wear on single crystal NaNO3 exposed to moist air

Simultaneous tribological loading and exposure to a chemically reactive environment can yield wear processes not produced by either stimulus alone. We report surface force microscopic (SFM) observations of cleaved, single crystal NaNO₃ in air, where tribological loading was provided by the SFM tip itself and chemical exposure was due to controlled introduction of water vapor at relative humidities from 10 to 65%. Scanning in the contact mode with nN loads at 30–50% relative humidity produces readily visible surface modifications, including preferential removal of material along steps. Material transfer along the surface can yield parallel ridges and depressions tens of nanometer high. In contrast, scanning in the tapping mode under certain humidity conditions produces localized deposition, possibly reflecting the dehydration of solvated ions and subsequent incorporation into the solid phase. We discuss the influence of contact force, tip velocity, relative humidity, and possible mechanochemistry on the rates of wear and deposition.Department of Mechanical Engineering, Kansai University, Osaka 564, Japan.     

Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.     

Imaging the substructure of antibodies with tapping-mode AFM in air: the importance of a water layer on mica

Monoclonal antibodies (immunoglobulin G; IgG) against the N-terminal domain of the A subunit of DNA gyrase have been imaged using tapping-mode atomic force microscopy under ambient conditions on hydrophilic mica surfaces. The familiar tri-nodal submolecular resolution of IgG (i.e. 50-kDa resolution) has been achieved when operating the microscope with the tip predominantly in the attractive force regime. Under common laboratory conditions of about 40% relative humidity, the resolution of this substructure was not achieved owing to motion of the antibodies on the surface and/or image distortion from tip–sample instabilities. Reproducible imaging of the tri-nodal antibody substructure was achieved by desiccating the samples for extended periods of time (1 week or more) before imaging. This effect is attributed to the presence of a humidity-dependent thin water layer (a few molecules or nanometres thick), which has been observed previously using the surface force apparatus and scanning polarization force microscopy. Desiccation of the mica surfaces allowed enough water to be removed from the mica surface to prevent this effect. Degradation in the image quality over the imaging period of an hour or two was observed, owing to re-adsorption of water under the ambient laboratory conditions.     

DNA height in scanning force microscopy

The measured height of DNA molecules adsorbed on a mica substrate by scanning probe microscopy is always less than the theoretical diameter. In this paper we show that, when imaged in ambient conditions, the molecules are usually immersed in the salt layer used to adsorb them to the substrate. This layer distorts the measurement of DNA height and is the main source of error but not the only one. We have performed different experiments to study this problem using two scanning force techniques: non-contact tapping mode in air and jumping mode in aqueous solution, where the dehydration phenomena is minimized. Height measurements of DNA in air using tapping mode reveal a height of 0.7+/-0.2nm. This value increases up to 1.5+/-0.2nm when the salt layer, in which the molecules are embedded, is removed. Jumping experiments in water give a value of 1.4+/-0.3nm when the maximum applied force is 300pN and 1.8+/-0.2nm at very low forces, which confirms the removal of the salt layer. Still, in all our experiments, the measured height of the DNA is less than the theoretical value. Our results show that although the salt layer present is important, some sample deformation due to either the loading force of the tip or the interaction with the substrate is also present.     

Shear force imaging of DNA in a near-field scanning optical microscope (NSOM)

A near-field scanning optical module has been constructed as an accessory for a Nanoscope IIIa commercial scanning probe microscope. Distance feedback and topographic registration are accomplished with an uncoated optical fibre scanning tip by implementation of the shear force technique. The tip is driven by a piezoelectric actuator at a resonance frequency of 8–80 kHz. A laser diode beam is scattered by the tip and detected by a split photodiode, with lock-in detection of the difference signal. The amplitude ( r ) and phase (τ) responses were characterized as a function of the calibrated tip–sample separation. Using an r cos τ feedback signal, imaging of pUC18 relaxed circular plasmid DNA spread on mica precoated with cetylpyridinium chloride was achieved. The apparent width (28 ± 5 nm) was approximately four times that achieved by scanning force measurements with the same instrument; the apparent height of the DNA (0.6 ± 0.3 nm) was similar with the two techniques. These results demonstrate the applicability of the shear force signal for imaging biological macromolecules according to topography and in conjunction with the optical signals of a near-field scanning optical microscope (NSOM).     

Energy dissipation in scanning force microscopy-friction on an atomic scale

Stick-slip behaviour for a typical scanning force microscope setup operated in the wearless friction regime is modelled. Not only the deflection of the cantilever but also the local elastic deformation of tip and sample are taken into account. The combined effect of macroscopic spring and microscopic elastic deformation is a key feature to the scanning motion of the tip. Within this model, energy dissipation arises naturally due to mechanical instabilities either of the macroscopic cantilever or of the microscopic tip-sample contact. Our model reproduces all features of atomically resolved friction loops, which can be calculated from interatomic potentials. Moreover, a general scheme is introduced which allows the exact response of the tip-sample system to be calculated from the different interacting potentials.     

Improvement of DNA-visualization in dynamic mode atomic force microscopy in air

Near-contact mode atomic force microscopy (AFM) imaging leads to sharper representations of DNA double strands on mica imaged at ambient conditions compared with noncontact mode AFM. Phase shift was used for feedback control yielding height information using a simple model calculation. No contact between tip and sample occurs. Measured DNA widths were up to four times smaller than measured with the same AFM tip in noncontact mode at ambient condition.     

Influence of the tip mass on the tip-sample interactions in TM-AFM

This paper focuses on the influences of the tip mass ratio (the ratio of the tip mass to the cantilever mass), on the excitation of higher oscillation eigenmodes and also on the tip-sample interaction forces in tapping mode atomic force microscopy (TM-AFM). A precise model for the cantilever dynamics capable of accurate simulations is essential for the investigation of the tip mass effects on the interaction forces. In the present work, the finite element method (FEM) is used for modeling the AFM cantilever to consider the oscillations of higher eigenmodes oscillations. In addition, molecular dynamics (MD) is used to calculate precise data for the tip-sample force as a function of tip vertical position with respect to the sample. The results demonstrate that in the presence of nonlinear tip-sample interaction forces, the tip mass ratio plays a significant role in the excitations of higher eigenmodes and also in the normal force applied on the surface. Furthermore, it has been shown that the difference between responses of the FEM and point-mass models in different system operational conditions is highly affected by the tip mass ratio.     

How dry are dried samples? Water adsorption measured by STM

When operating scanning probe microscopes, like STM or AFM, under ambient conditions, the presence of water on the sample and the tip always plays an important role. The water not only influences the structure of the sample itself, but also the imaging process; in the case of the STM using a wet etched w-tip, by interfering with the electron transfer process, and in the case of the AFM, due to the capillary forces in the micro Newton range that dominate the tip surface interaction forces. In this paper, the distribution and the amount of adsorbed water on different surfaces is investigated with the help of the STM, which can provide information by imaging and by current/distance spectroscopy. Hydrophilic and hydrophobic surfaces like titanium, gold, and graphite were studied at a relative humidity between 10 and 90%. Under very dry conditions with relative humidity below 15%, the presence of water was only detectable by the longer decay length of the measured current with distance compared to samples prepared in UHV completely free of water. At less dry conditions on gold surfaces, water was found as droplets. With increasing humidity, the quantity and the size of these droplets increased until the whole surface became covered with water. Above 55% humidity, the thickness of the water film increased with increasing humidity up to several 10 nm. On titanium and graphite, water was always present in the form of closed layers growing in thickness with increasing humidity.     

Data analysis of interaction forces measured with the atomic force microscope

The force-distance cycle mode of the atomic force microscope (AFM) allows for detection of interaction forces between the AFM-tip and a substrate (probe). This can either be a direct tip-sample interaction or an interaction between molecules coupled to the tip and probe, respectively. The interaction forces are typically in the range of a few pN to some hundred pN. In this article we describe algorithms for the analysis of force-distance cycles, to quantify interaction forces between tip and probe. Both, the direct tip-probe interaction as well as the interaction between specifically bound molecules are analyzed. The molecules bound to tip and probe have to be either long and flexible or have to be bound via a flexible cross linker. The algorithms are exemplified on direct tip-probe interactions and on unbinding events of cadherins which are bound via PEG-spacers to the AFM-tip and to the probe.     

Spectroscopy of the shear force interaction in scanning near-field optical microscopy

Shear force detection is a common method of tip-sample distance control in scanning near-field optical microscopy. Shear force is the force acting on a laterally oscillating probe tip near a surface. Despite its frequent use, the nature of the interaction between tip and sample surface is a matter of debate. In order to investigate the problem, approach curves, i.e. amplitude and phase of the tip oscillation as a function of the tip-sample distance, are studied in terms of a harmonic oscillator model. The extracted force and damping constants are influenced by the substrate material. The character of the interaction ranges from elastic to dissipative. The interaction range is of atomic dimensions with a sharp onset. Between a metal-coated tip and a Cu sample, a power law for the force-distance curve is observed.     

一种基于剪切力的距离控制方法在扫描近场光学显微镜中的应用

介绍了扫描近场光学显微镜中基于剪切力的样品、探针间距离控制的方法。当受振动激励的光纤探针由远处逐渐接近样品表面时,样品与针尖间的剪切力使针尖的振动振幅减小,通过检测探针振幅的变化从而控制针尖与样品间的距离。此种方法可以方便地将光纤探针导入工作区域内并在扫描过程中保持适当的高度。我们测量了探针系统的幅频特性和力曲线,并用该方法获得4μm×4μm的范围内光盘表面的形貌信息。     

Shear force distance control in a scanning near-field optical microscope: in resonance excitation of the fiber probe versus out of resonance excitation

The experimental results of the direct measurement of the absolute value of interaction force between the fiber probe of a scanning near-field optical microscope (SNOM) operated in shear force mode and a sample, which were performed using combined SNOM-atomic force microscope setup, are discussed for the out-of-resonance fiber probe excitation mode. We demonstrate that the value of the tapping component of the total force for this mode at typical dither amplitudes is of the order of 10 nN and thus is quite comparable with the value of this force for in resonance fiber probe excitation mode. It is also shown that for all modes this force component is essentially smaller than the usually neglected static attraction force, which is of the order of 200 nN. The true contact nature of the tip-sample interaction during the out of resonance mode is proven. From this, we conclude that such a detection mode is very promising for operation in liquids, where other modes encounter great difficulties.     

Calculation of the Bloch wall contrast in magnetic force microscopy

The magnetic force microscope (MFM) is a promising analytical tool for the mapping of magnetic microfield distributions on a nanometre scale. The detailed interpretation of experimentally obtained data requires a rigorous micromagnetic analysis of the underlying magnetostatic tip-sample interactions. We have performed model calculations yielding the MFM image contrast obtained for 180° Bloch walls using ferromagnetic microscope tips. The actually observed wall image is shown to depend critically upon the mesoscopic tip design. The spatial resolution and also values for the resulting interaction forces and tip-sample compliances are derived.     

Local raster scanning for high-speed imaging of biopolymers in atomic force microscopy

A novel algorithm is described and illustrated for high speed imaging of biopolymers and other stringlike samples using atomic force microscopy. The method uses the measurements in real-time to steer the tip of the instrument to localize the scanning area over the sample of interest. Depending on the sample, the scan time can be reduced by an order of magnitude or more while maintaining image resolution. Images are generated by interpolating the non-raster data using a modified Kriging algorithm. The method is demonstrated using physical simulations that include actuator and cantilever dynamics, nonlinear tip-sample interactions, and measurement noise as well as through scanning experiments in which a two-axis nanopositioning stage is steered by the algorithm using simulated height data.     

Salt-dependent chromosome viscoelasticity characterized by scanning force microscopy-based volume measurements

Metaphase chromosomes prepared according to the standard spreading procedure exhibit viscoelastical behavior after rehydration. The salt-dependency of this elasticity was investigated using contact mode scanning force microscopy (SFM). Therefore, chromosomes were imaged in solutions of different ionic strength (0.3 x PBS and water). The elasticity was probed by stepwise increase of the loading force of the scanning tip, resulting in a set of images. The images were used for the determination of the height and the apparent volume of each chromosome, and these values were the base for a characterization of the viscoelastical response of the chromosomes under different salt conditions. Lower ionic strength resulted in a greater response of the chromosome structures to applied loading forces.