Intermittency in amplitude modulated dynamic atomic force microscopy

From a mathematical point of view, the atomic force microscope (AFM) belongs to a special class of continuous time dynamical systems with intermittent impact collisions. Discontinuities of the velocity result from the collisions of the tip with the surface. Transition to chaos in non-linear systems can occur via the following four routes: bifurcation cascade, crisis, quasi-periodicity, and intermittency. For the AFM period doubling and period-adding cascades are well established. Other routes into chaos, however, also may play an important role. Time series data of a dynamic AFM experiment indicates a chaotic mode that is related to the intermittency route into chaos. The observed intermittency is characterized as a type III intermittency. Understanding the dynamics of the system will help improve the overall system performance by keeping the operation parameters of dynamic AFM in a range, where chaos can be avoided or at least controlled.     

Studies of tip wear processes in tapping mode atomic force microscopy

Tip integrity is crucial to atomic force microscope image quality. Tip wear not only compromises image resolution but also introduces artifacts. However, the factors that govern wearing have not been systematically studied. The results presented here of tip wearing on a rough titanium surface were determined by monitoring changes in tip shape and the evolution of histograms of complex surface curvatures under different control parameters. In contrast with the common assumption that operating at a low set point (the ratio of tapping amplitude to free oscillation amplitude) wears the tip quickly, we observed that a low set point actually minimizes tip wear on a hard surface regardless of the free amplitude. The results can be interpreted qualitatively with theoretical calculations based on momentum exchange at tapping impact. Operating at a low set point allows more robust scanning than with a high set point (tapping near free amplitude), providing a method to slow down tip wear. Another advantage of a low set point is that amplitude error grows faster than with a high set point by nearly an order of magnitude, permitting an increase in scanning speed.     

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.     

Analysis of dynamic cantilever behavior in tapping mode atomic force microscopy

Tapping mode atomic force microscopy (AFM) provides phase images in addition to height and amplitude images. Although the behavior of tapping mode AFM has been investigated using mathematical modeling, comprehensive understanding of the behavior of tapping mode AFM still poses a significant challenge to the AFM community, involving issues such as the correct interpretation of the phase images. In this paper, the cantilever's dynamic behavior in tapping mode AFM is studied through a three dimensional finite element method. The cantilever's dynamic displacement responses are firstly obtained via simulation under different tip‐sample separations, and for different tip‐sample interaction forces, such as elastic force, adhesion force, viscosity force, and the van der Waals force, which correspond to the cantilever's action upon various different representative computer‐generated test samples. Simulated results show that the dynamic cantilever displacement response can be divided into three zones: a free vibration zone, a transition zone, and a contact vibration zone. Phase trajectory, phase shift, transition time, pseudo stable amplitude, and frequency changes are then analyzed from the dynamic displacement responses that are obtained. Finally, experiments are carried out on a real AFM system to support the findings of the simulations. Microsc. Res. Tech. 78:935–946, 2015. © 2015 Wiley Periodicals, Inc.     

A complementary-metal-oxide-semiconductor-field-effect-transistor-compatible atomic force microscopy tip fabrication process and integrated atomic force microscopy cantilevers fabricated with this process

A complementary-metal-oxide-semiconductor-field-effect-transistor-compatible process for the fabrication of atomic force microscopy cantilevers with integrated tips has been developed. For the first time, the tips are fabricated after the completion of the regular complementary metal-oxide-semiconductor-field-effect-transistor fabrication process sequence. On-chip circuit components, such as piezoresistive deflection sensors, deflection actuators, and amplifiers, are fabricated on the mirror-polished surface of the wafer, ensuring optimal performance. The tip fabrication process is based on anisotropic silicon etching at low temperature using a tetramethylammonium hydroxide solution. The anisotropic etching process has been optimized to ensure process controllability. Using the described process, complementary-metal-oxide-semiconductor-field-effect-transistor-based cantilevers with piezoresistive deflection sensors and integrated tips have been successfully fabricated. Force-distance curves and scanning images in constant-force mode have been recorded.     

A method to provide rapid in situ determination of tip radius in dynamic atomic force microscopy

We provide a method to characterize the tip radius of an atomic force microscopy in situ by monitoring the dynamics of the cantilever in ambient conditions. The key concept is that the value of free amplitude for which transitions from the attractive to repulsive force regimes are observed, strongly depends on the curvature of the tip. In practice, the smaller the value of free amplitude required to observe a transition, the sharper the tip. This general behavior is remarkably independent of the properties of the sample and cantilever characteristics and shows the strong dependence of the transitions on the tip radius. The main advantage of this method is rapid in situ characterization. Rapid in situ characterization enables one to continuously monitor the tip size during experiments. Further, we show how to reproducibly shape the tip from a given initial size to any chosen larger size. This approach combined with the in situ tip size monitoring enables quantitative comparison of materials measurements between samples. These methods are set to allow quantitative data acquisition and make direct data comparison readily available in the community.     

Imaging cobalt nanoparticles by amplitude modulation atomic force microscopy: comparison between low and high amplitude solutions

In many situations of interest amplitude modulation AFM is characterized by the coexistence of two solutions with different physical properties. Here, we compare the performance of those solutions in the imaging of cobalt nanoparticles. We show that imaging with the high amplitude solution implies an irreversible deformation of the nanoparticles while repeated imaging with the low solution does not produce noticeable changes in the nanoparticles. Theoretical simulations show that the maximum tip-surface force in the high amplitude solution is about 14nN while in the low amplitude solution is about -4nN. We attribute the differences in the high and low amplitude images to the differences in the exerted forces on the sample.     

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.     

Amplitude curves and operating regimes in dynamic atomic force microscopy

The experimental dependence of the amplitude on the average tip-sample distance has been studied to understand the operation of an atomic force microscope with an amplitude modulation feedback. The amplitude curves can be classified in three major groups according to the existence or not of a local maximum and how the maximum is reached (steplike discontinuities vs. smooth transitions). A model describing the cantilever motion as a forced nonlinear oscillator allows to associate the features observed in the amplitude curves with the tip-sample interaction force. The model also allows to define two elemental tip-sample interaction regimes, attractive and repulsive. The presence of a local maximum in the amplitude curves is related to a transition between the attractive and the repulsive regime.     

Monitoring of glass derivatization with pulsed force mode atomic force microscopy

Non-specific adsorption of proteins at solid/liquid interfaces is a major problem in the use of synthetic biomaterials and in ultrasensitive detection methods. Grafting surfaces with a dense layer of poly(ethylene glycol) (PEG) or other polymers is a most widely used strategy to solve this task. While such modified surfaces have been characterized by their ability to resist protein adsorption, the polymer layers themselves have rarely been studied in fine detail. Atomic force microscopy (AFM) using the pulsed force mode (PFM), is an ideal technique to investigate structural features and physiochemical properties of surfaces because topology and adhesion are simultaneously detected with high lateral resolution. In the present study, PFM-AFM was applied to thoroughly characterize different stages of glass derivatization, up to the formation of a dense PEG layer. Lateral inhomogeneities in topology and/or adhesion were observed at all stages before PEG attachment. The covalently bound PEG, however, was seen to form a densely packed monolayer with maximal thickness, smooth surface, and weak adhesion. Thus, PFM-AFM appears to be a valuable tool for the characterization of protein-repelling surfaces in solution.     

Velocity dependent friction laws in contact mode atomic force microscopy

Friction forces in the tip–sample contact govern the dynamics of contact mode atomic force microscopy. In ambient conditions typical contact radii between tip and sample are in the order of a few nanometers. In order to account for the large interaction area the dynamics of contact mode atomic force microscope (AFM) is investigated under the assumption of a multi-asperity contact interface between tip and sample. Thus, the kinetic friction force between tip and sample is the product of the real contact area between both solids and the interfacial shear strength. The velocity strengthening of the lateral force is modeled assuming a logarithmic relationship between shear-strength and velocity. Numerical simulations of the system dynamics with this empirical model show the existence of two different regimes in contact mode AFM: steady sliding and stick–slip where the tip undergoes periodically stiction and kinetic friction. The state of the system depends on the scan velocity as well as on the velocity dependence of the interfacial friction force between tip and sample. Already small viscous damping contributions in the tip–sample contact are sufficient to suppress stick–slip oscillations.     

Improving tapping mode atomic force microscopy with piezoelectric cantilevers

This article summarizes improvements to the speed, simplicity and versatility of tapping mode atomic force microscopy (AFM). Improvements are enabled by a piezoelectric microcantilever with a sharp silicon tip and a thin, low-stress zinc oxide (ZnO) film to both actuate and sense deflection. First, we demonstrate self-sensing tapping mode without laser detection. Similar previous work has been limited by unoptimized probe tips, cantilever thicknesses, and stress in the piezoelectric films. Tests indicate self-sensing amplitude resolution is as good or better than optical detection, with double the sensitivity, using the same type of cantilever. Second, we demonstrate self-oscillating tapping mode AFM. The cantilever's integrated piezoelectric film serves as the frequency-determining component of an oscillator circuit. The circuit oscillates the cantilever near its resonant frequency by applying positive feedback to the film. We present images and force-distance curves using both self-sensing and self-oscillating techniques. Finally, high-speed tapping mode imaging in liquid, where electric components of the cantilever require insulation, is demonstrated. Three cantilever coating schemes are tested. The insulated microactuator is used to simultaneously vibrate and actuate the cantilever over topographical features. Preliminary images in water and saline are presented, including one taken at 75.5 μm/s—a threefold improvement in bandwidth versus conventional piezotube actuators.     

Near-grain-boundary characterization by atomic force microscopy

Characterization of near-grain boundary is carried out by atomic force microscopy (AFM). It has been observed to be the most suitable technique owing to its capability to investigate the surface at high resolution. Commercial purity-grade nickel processed under different conditions, viz., (i) cold-rolled and annealed and (ii) thermally etched condition without cold rolling, is considered in the present study. AFM crystallographic data match well with the standard data. Hence, it establishes two grain-boundary relations viz., plane matching and coincidence site lattice (CSL Σ=9) relation for the two different sample conditions.     

C-banding visualized by atomic force microscopy

C-banding is a method used for studying chromosome rearrangements near centromeres and for investigating polymorphisms. In human chromosomes, the C-bands are located at the centromere of all the chromosomes and the distal long arm of the Y chromosome. In this study, we aimed to detect the structural changes in chromosomes during the stages of C-banding by atomic force microscopy. We observed crater-like structures in the chromosomes after 2xSSC (saline sodium citrate) treatment and measured the relative difference between the heights of chromatid and centromere of the chromosomes. Results showed that the relative difference was 3 nm in chromosomes 1, 9, 16, and Y, whereas in the other chromosomes this value was 11.6 nm. After Giemsa staining, the relative difference increased by a factor of 16 in chromosomes 1, 9, 16, and Y. The other chromosomes showed no such increase, which is in accordance with our suggestion that nonhiston proteins associated with DNA in constitutive heterochromatin can make the constitutive heterochromatin resistant to C-banding.     

一种新颖的点衍射干涉轻敲模式原子力显微镜

论述了一种新颖的原子力显微镜,它利用硅微探针的特殊结构和相关光学系统所引起的点衍射干涉现象[1]来扫描成像,因为硅微探针被用作反射型点衍射板,故光路完全共路,再结合锁相检测技术,使得该仪器抗干扰力极强且结构精巧紧凑,可适用于测试软硬不同材料样品,对软质高分子膜材料检测得到了实际的链状结构。     

Saponaria officinalis karyology and karyotype by means of image analyzer and atomic force microscopy

The aim of this work was to offer a contribution to the characterization of taxonomic entity of Saponaria officinalis (2n = 28; an herbaceous perennial species; saporin, a type 1 Ribosome Inactivating Protein, is present in leaves and seeds) by a cytogenetic and karyomorphological approach. We investigated the karyotype's morphometry correlated with Stebbin's symmetric index; the same information has been used for computing the indices resemblance between chromosomes (REC), symmetric indices (SYI), and total form (TF%) which allow the comparison between species and evaluation of karyological evolution. Fluorescence intensities of the stained nuclei were measured by a flow cytometer and, for the first time, values for nuclear DNA content were estimated by comparing nuclei fluorescence intensities of the test population with those of appropriate internal DNA standards. Our study is also aimed to introduce chromosomal volumes, which were determined by atomic force microscopy (AFM), as novel karyomorphological parameter which could allow for chromosome discrimination especially when tiny ones are present.     

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions prepared by microcontact printing (microCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.