Tilt of atomic force microscope cantilevers: effect on spring constant and adhesion measurements

In atomic force microscopy, the cantilevers are mounted under a certain tilt angle alpha with respect to the sample surface. In this paper, we show that this increases the effective spring constant by typically 10-20%. The effective spring constant of a rectangular cantilever of length L can be obtained by dividing the measured spring constant by cos2 alpha(1 - 2D tan alpha/L). Here, alpha is the tilt angle and D is the size of the tip. In colloidal probe experiments, D has to be replaced by the radius of the attached particle. To determine the effect of tilt experimentally, the adhesion force between spherical borosilicate particles and planar silicon oxide surfaces was measured at tilt angles between 0 degrees and 35 degrees. The experiments revealed a significant decrease of the mean apparent adhesion force with a tilt of typically 20-30% at alpha = 20 degrees. In addition, they demonstrate that the adhesion depends drastically on the precise position of contact on the particle surface.     

Friction coefficient mapping using the atomic force microscope

A friction coefficient map of the surface of an immiscible polymer blend has been constructed using data obtained with the atomic force microscope. Spatially resolved friction coefficients, obtained from gradients of linear plots of frictional force versus applied load, were used to construct the map, with corresponding frictional forces being derived from lateral force data and the lateral spring constant. Values of the friction coefficient were confirmed using an Si₃N₄/Si₃N₄ couple, for which literature values were available. Excellent agreement with the literature was observed through the use of this method. Copyright © 2004 John Wiley & Sons, Ltd.     

Friction force measurements relevant to de-inking by means of atomic force microscope

In the pulping step of the de-inking process, the ink detaches from the fibers due to shear and physical chemical interaction. In order to get a better understanding of the forces involved between cellulose and ink, the atomic force microscope and the colloidal probe technique have been used in the presence of a model chemical dispersant (hexa-ethyleneglycol mono n-dodecyl ether, C12E6). A cellulose bead was used as the colloidal probe and three different lower surfaces have been used, an alkyd resin, mica and a cellulose sphere. The normal and lateral forces have been measured at a range of nonionic concentrations. It was found that the lateral sliding friction forces deceased with increasing surfactant concentration for both the alkyd resin and mica while no differences were observed for the cellulose surface. In addition, only a very small change in normal force could be detected for the alkyd surface as the concentration changed.     

Scanning-induced growth on single crystal calcite with an atomic force microscope

We report observations of localized growth on the (1014) surface of single-crystal CaCO3 in supersaturated solutions while scanning with the tip of an atomic force microscope (AFM). At low contact forces, AFM scanning strongly enhances deposition along preexisting steps. This enhancement increases rapidly with increasing solution supersaturation, and is capable of filling in multilayer etch pits to produce defect-free surfaces at the resolution of the AFM. Attempts to achieve similar deposition rates in the absence of scanning require high supersaturations that produce three-dimensional crystal nuclei, which are important defects. Localized deposition produced by drawing the AFM tip back and forth across step edges can produce monolayer deposits extending well over a micron from the scanned area. These tip-induced deposits provide convincing evidence for the importance of ledge diffusion in calcite crystal growth.     

Generation of amino-terminated surfaces by chemical lithography using atomic force microscopy

Self-assembled monolayers (SAMs) covered with nitroso end groups were reduced using an atomic force microscope. As the bias voltage become more negative (beyond -4 V), the surface potential of the scanned area become closer to that of the amino-terminated SAM. Following this chemical change, however, no change in topographic features was detected, implying retained stability of the underlying SAM layer. We then released carboxylate-modified polystyrene (PS) spheres into a pH 4 solution containing the sample. Subsequent imaging with atomic force microscopy (AFM) revealed that these PS spheres were only selectively immobilized on the regions that were originally scanned at -6 V to form amino termination. In summary, using AFM set to a specific voltage, we were able to selectively generate micropatterned regions of the SAM with amino termination.     

Glass fracture surfaces seen with an atomic force microscope

Fracture surfaces of Suprasil 2, Herasil 2, AR glass and Duran glass rods have been studied by an atomic force microscope (AFM) in the contact mode. They could be characterized in the fracture mirror, the mist and the hackle zones. The RMS (root mean square) roughness in the fracture mirror of all glasses investigated increased with growing distance from the origin of the fracture. On several fracture surfaces of different glasses steps have been observed, due to fracture in shear mode. Furthermore changes in the fracture surfaces during scanning have also been observed. They are thought to stem from reactions of the freshly broken glass surface with the surrounding atmosphere and forces between the scanning tip and the soft surface.     

Direct observation of uncoated spectrin with atomic force microscope

Spectrin molecules extracted from human blood ceil membrane have been examined by atomic force microscopy (AFM) without using shadowing or staining procedures. A drop of the solution containing spectrin molecules was deposited on the freshly deaved mica substrate. After about 1 min, the residual solution was removed with a piece of filter paper. Afterwards the sample was imaged with a home-made atomic force microscope (AFM) in air in a constant force mode. The obtained AFM images revealed that the spectrin molecules prepared from the above procedures exhibit several kinds of structures as follows: (i) the compact rod-like spectrin heterodimers with a length of around 100 nm; (ii) bent or curved linear tetramers with a length of around 200 nm; (iii) somewhat curved spectrin hexamers, octomers or decamers with lengths of about 300, 400, or 500 nm; and (iv) high oligomers with a length above 1 000 nm.     

Imaging nanometer-size metallic clusters with the atomic force microscope

The atomic force microscope has been used in the attractive (non-contact) force mode to produce images of individual nanometer-size clusters pre-formed in the gas phase and deposited on a wide variety of atomically-flat substrates. Using this technique, it is now possible to reliably image pre-formed clusters in their as-deposited positions. Studies of nanometer-size Au clusters supported on highly oriented pyrolitic graphite clearly show how the clusters are distributed across the scanned region. Cluster coverages inferred from atomic force studies are compared to those obtained from TEM studies of amorphous carbon grids simultaneously exposed to the same cluster beam.     

High resolution imaging of functional group distributions on carbon surfaces by tapping mode atomic force microscopy

Here we report, for the first time, the high resolution imaging of hydrophilic, polar functional group distributions on flat carbon surfaces by phase contrast in noncontact tapping mode AFM.     

Atomic-scale roughness effect on capillary force in atomic force microscopy

We study the capillary force in atomic force microscopy by using Monte Carlo simulations. Adopting a lattice gas model for water, we simulated water menisci that form between a rough silicon-nitride tip and a mica surface. Unlike its macroscopic counterpart, the water meniscus at the nanoscale gives rise to a capillary force that responds sensitively to the tip roughness. With only a slight change in tip shape, the pull-off force significantly changes its qualitative variation with humidity.     

Nanoscale patterning of alkyl monolayers on silicon using the atomic force microscope

Self-assembled monolayers (SAMs) of 1-alkenes on hydrogen-passivated silicon substrates were successfully patterned on the nanometer scale using an atomic force microscope (AFM) probe tip. Nanoshaving experiments on alkyl monolayers formed on H-Si(111) not only demonstrate the flexibility of this technique but also show that patterning with an AFM probe is a viable method for creating well-defined, nanoscale features in a monolayer matrix in a reproducible and controlled manner. Features of varying depths (2-15 nm) were created in the alkyl monolayers by controlling the applied load and the number of etching scans made at high applied loads. The patterning on these SAM films is compared with the patterning of alkyl siloxane monolayers on silicon and mica.     

Influence of attractive force on imaging micro‐droplets of water by atomic force microscope

The shape of micro‐droplets of water on a pure copper surface was investigated using the a.c. non‐contact mode of an atomic force microscope (AFM) by applying different attractive forces between the cantilever tip and the liquid surface. The forces largely influenced the observed radii of micro‐droplets; the influence can be reduced significantly by reducing the force. The same attractive force between the cantilever tip and the micro‐droplets is necessary when comparing the contact angles of micro‐droplets on different surfaces. Furthermore, the values of the contact angles of the micro‐droplets should be the average of those on at least four sides of the droplets. Copyright © 2005 John Wiley & Sons, Ltd.     

An atomic force microscope study of thermal behavior of phospholipid monolayers on mica

We observed by using atomic force microscope (AFM) phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers on mica being annealed and cooled to a selection of temperatures through steps of 2-4 degrees C/min. The annealed phospholipid monolayers started to disappear at 45-50 degrees C and disappeared completely above 60-63 degrees C under AFM observation. The phospholipid monolayers reformed when the samples were cooled below 60 degrees C and developed from fractal into compact monolayer films with decreasing temperatures. Simultaneously the height of the reformed phospholipid films also increased with decreasing temperatures from 0.4 nm to the value before annealing. The observed thermal features are attributed to a phase-transition process that upon heating to above 45-50 degrees C, the lipids condensed in the monolayers transform into a low-density expanded phase in which the lipids are invisible to AFM, and the transformation continues and completes at 60-63 degrees C. The lipid densities of the expanded phase inferred from the dissociated area of the condensed phase are observed to be a function of the temperature. The behavior contrasts with a conventional first-order phase transition commonly seen in the Langmuir films. The temperature-dependent height and shape of the reformed phospholipid films during cooling are argued to arise from the adjustment of the packing and molecular tilting (with respect to the mica surface) of the phospholipids in order to accommodate more condensed phospholipids.     

Accurate height and volume measurements on soft samples with the atomic force microscope

The suitability of three common atomic force microscope (AFM) imaging modes for quantitative height and volume measurements on soft samples was investigated. The height and volume of rehydrated human metaphase chromosomes in liquid were measured using the contact mode, the tapping mode, and the force mapping mode. In both the contact and tapping modes, the measured height and volume strongly depended on the imaging setpoint that sets the imaging force. Measurement deviations up to 50% were observed. The force mapping mode, on the other hand, yielded reproducible height and volume measurements independent of the imaging force. It is therefore suggested that the force mapping mode should be used whenever the height or volume of soft samples need to be accurately determined.     

Quantitative thermodynamic analysis of sublimation rates using an atomic force microscope

Atomic force microscopy (AFM) has been successfully used to study the activation energy for evaporation of pentaerythritol tetranitrate (PETN) nanoislands formed by spin coating. These islands are annealed isothermally in the temperature range of 30-70 degrees C for a given time and are scanned with AFM in contact mode at room temperature. The volume of these islands does not change significantly up to about 35-40 degrees C indicating that sublimation is not significant below 40 degrees C. Above 40 degrees C, the islands start shrinking, and the rate of weight loss is analyzed as a function of temperature. The activation energy of evaporation using AFM was found to be similar to that for bulk PETN crystals using thermogravimetric analysis (TGA) at higher temperatures (110-135 degrees C). These results demonstrate that AFM is a useful tool to measure thermodynamic properties with a nanoscale probe.     

Tapping mode imaging and measurements with an inverted atomic force microscope

This report demonstrates the successful use of the inverted atomic force microscope (i-AFM) for tapping mode AFM imaging of cantilever-supported samples. i-AFM is a mode of AFM operation in which a sample supported on a tipless cantilever is imaged by one of many tips in a microfabricated tip array. Tapping mode is an intermittent contact mode whereby the cantilever is oscillated at or near its resonance frequency, and the amplitude and/or phase are used to image the sample. In the process of demonstrating that tapping mode images could be obtained in the i-AFM design, it was observed that the amplitude of the cantilever oscillation decreased markedly as the cantilever and tip array were approached. The source of this damping of the cantilever oscillations was identified to be the well-known "squeeze film damping", and the extent of damping was a direct consequence of the relatively shorter tip heights for the tip arrays, as compared to those of commercially available tapping mode cantilevers with integrated tips. The functional form for the distance dependence of the damping coefficient is in excellent agreement with previously published models for squeeze film damping, and the values for the fitting parameters make physical sense. Although the severe damping reduces the cantilever free amplitude substantially, we found that we were still able to access the low-amplitude regime of oscillation necessary for attractive tapping mode imaging of fragile molecules.     

Microdrops on atomic force microscope cantilevers: evaporation of water and spring constant calibration

The evaporation of water drops with radii approximately 20 microm was investigated experimentally by depositing them onto atomic force microscope (AFM) cantilevers and measuring the deflection versus time. Because of the surface tension of the liquid, the Laplace pressure inside the drop, and the change of interfacial stress at the solid-liquid interface, the cantilever is deflected by typically a few hundred nanometers. The experimental results are in accordance with an analytic theory developed. The evaporation process could be monitored with high accuracy even at the last stage of evaporation because (1) cantilever deflections can be measured with nanometer resolution and (2) the time resolution, given by the inverse of the resonance frequency of the cantilever of approximately 0.3 ms, is much faster than the typical evaporation time of 1 s. Experimental results indicate that evaporation of the last thin layer of water is significantly slower than the rest of the drop, which can be due to surface forces. This drop-on-cantilever system can also be used to analyze the drop impact dynamics on a surface and to determine the spring constant of cantilevers.     

Force measurements of bacterial adhesion on metals using a cell probe atomic force microscope

The adhesion of microbial cells to metal surfaces in aqueous media is an important phenomenon in both the natural environment and engineering systems. The adhesion of two anaerobic sulfate-reducing bacteria (Desulfovibrio desulfuricans and a local marine isolate) and an aerobe (Pseudomonas sp.) to four polished metal surfaces (i.e., stainless steel 316, mild steel, aluminum, and copper) was examined using a force spectroscopy technique with an atomic force microscope (AFM). Using a modified bacterial tip, the attraction and repulsion forces (in the nano-Newton range) between the bacterial cell and the metal surface in aqueous media were quantified. Results show that the bacterial adhesion force to aluminum is the highest among the metals investigated, whereas the one to copper is the lowest. The bacterial adhesion forces to metals are influenced by both the electrostatic force and metal surface hydrophobicity. It is also found that the physiological properties of the bacterium, namely the bacterial surface charges and hydrophobicity, also have influence on the bacteria-metal interaction. The adhesion to the metals by Pseudomonas sp. and D. desulfuricans was greater than by the marine SRB isolate. The cell-cell interactions show that there are strong electrostatic repulsion forces between bacterial cells. Cell probe atomic force microscopy has provided some useful insight into the interactions of bacterial cells with the metal surfaces.