Cantilever Oscillation Research Articles

Single-electron tunneling to insulator surfaces detected by electrostatic force

The detection of single-electron tunneling events between a metallic scanning probe tip and an insulating surface is demonstrated by an electrostatic force method. When a voltage-biased oscillating atomic force microscopy tip is placed within tunneling range of the surface of an insulator, single-electron tunneling events are observed between the tip and electronic states at the surface. The events cause an abrupt reduction in cantilever oscillation amplitude, due to the instantaneous reduction of the force gradient at the tip. In most cases, only a single electron tunnels to or from the surface. Experimental data show that no physical contact is made during the tunneling events.

Capillary forces in tapping mode atomic force microscopy

We investigated the influence of the relative humidity on amplitude and phase of the cantilever oscillation while operating an atomic force microscope (AFM) in the tapping mode. If the free oscillation amplitude A0 exceeds a certain critical amplitude Ac, the amplitude- and phase-distance curves show a transition from a regime with a net attractive force between tip and sample to a net repulsive regime. For hydrophilic tip and sample this critical amplitude Ac is found to increase with increasing relative humidity. In contrast, no such dependence was found for hydrophobic samples. Numerical simulations show that this behavior can be explained by assuming the intermittent formation and rupture of a capillary neck in each oscillation cycle of the AFM cantilever.

Influence of volume and surface properties on phase contrast in tapping mode atomic force microscopy

Phase imaging angle of the cantilever oscillation in tapping mode scanning force microscopy can be related to the energy dissipated per oscillation. Nevertheless, in many cases, the physical properties (adhesion, viscosity…) of the sample that are sources of this energy dissipation are not clearly identified. We propose a simple method to distinguish the influence of surface and volume properties on phase imaging. This method is based on equalizing of surface properties by covering the sample surface with thin gold layers. As an example, in our specific working conditions (moderate tapping on glass–polyester composite), we show that volume properties have an important influence on the phase signal in comparison to surface properties.

Cantilever Oscillations in MAC-Mode AFM: Inertiality and Viscosity Effects of Cantilever - Sample Interaction

In MAC mode AFM the cantilever is inclined at some angle (∼15°) relative to the sample surface. For this reason the viscous resistance of the liquid between the cantilever and the sample grows in direction from the cantilever base to its tip. Simultaneously, the density of the joint mass decreases in that direction. A model for viscous forces and joint mass distribution along the cantilever is suggested on the basis of liquid flow analysis. A boundary value problem for the shape of the oscillating cantilever is solved numerically for different driving frequencies and distances of the cantilever tip from the surface of the sample. This results in the dependency of the phases and amplitudes for the cantilever tip and laser beam (cantilever slope at the tip end) oscillations on the distance from the sample and on driving frequency. Fitting these results to experimental data allows to find out the parameters for viscous resistance and joint mass distributions. Altogether, the method represents a necessary tuning for each specifical experimental setup oriented towards noncontact MAC-mode studies of surface properties.

Scanning force microscopy three-dimensional modesapplied to the study of the dielectric response of adsorbed DNAmolecules

We have developed a set of working modes for scanning probemicroscopy (SPM), which generalizes the usual method of acquiringdata. We call these modes three-dimensional (3D) modes. Using thesemodes it is possible to measure typical SPM magnitudes, such as, forexample, the tunnel current, the normal force and the amplitude orfrequency of the cantilever oscillation, as a function of any othertwo magnitudes of the system: f(x1,x2). In this paper wepresent different examples of 3D modes. In particular, we haveapplied 3D modes to the study of the electrostatic interaction ofco-adsorbed single walled carbon nanotubes and individual DNAmolecules with a metallic scanning force microscopy tip. The dataindicate that adsorbed DNA has a dielectric constant similar to thatof the glass substrate.

Scanned probe imaging of single-electron charge states in nanotube quantum dots.

An atomic force microscope was used to study single-electron motion in nanotube quantum dots. By applying a voltage to the microscope tip, the number of electrons occupying the quantum dot could be changed, causing Coulomb oscillations in the nanotube conductance. Spatial maps of these oscillations were used to locate individual dots and to study the electrostatic coupling between the dot and the tip. The electrostatic forces associated with single electrons hopping on and off the quantum dot were also measured. These forces changed the amplitude, frequency, and quality factor of the cantilever oscillation, demonstrating how single-electron motion can interact with a mechanical oscillator.

Self-oscillation mode induced in an atomic force microscope cantilever

We developed a technique to excite the self-oscillation of a commercially available metal-coated cantilever and detect the deflection simultaneously with only a single optical fiber. The technique is based on the fact that the light transmitted to a cantilever in the fiber-optic interferometer induces the photothermal positive feedback to the cantilever oscillation. The model analysis on the cantilever oscillation shows that the product of the slope of an intensity curve and the thermal coefficient of deflection of the cantilever should be greater than the product of the damping parameter and the dynamic heat capacity to excite the self-oscillation, which was confirmed by experiment. It can be used either to excite the self-oscillation in the frequency modulation atomic force microscopy or to avoid it in normal force microscopy.

Non-contact atomic force microscope with a PZT cantilever used for deflection sensing, direct oscillation and feedback actuation

A non-contact atomic force microscope (NC-AFM) based on a microfabricated piezoelectric cantilever is presented. A single piezoelectric lead zirconate titanate thin film layer on the cantilever serves as deflection sensing, cantilever oscillation and feedback actuation. Since such an AFM requires neither external oscillator nor external deflection sensor, considerably simple instrumentation becomes possible even for extreme environments such as low temperature or ultra-high vacuum. Also feedback control by the integrated actuator in the cantilever makes faster scanning possible. Images of atomic steps on annealed sapphire (0001) surfaces have been observed in air atmosphere in frequency modulation mode.

Simulation of fluctuation and dissipation in dynamic force microscopy

We have simulated three possible effects of the driving force fluctuation of the cantilever, fluctuation of the surface location and contribution of an additional dissipation due to the sample, with the hope to identify effective methods of improvement of the achievable resolution during a dynamic force microscopy experiment. This study is performed through numerical simulations of the approach–retract curves. We find that in the case of soft materials, the driving force fluctuation has only a small effect on the amplitude of the cantilever oscillations, while surface fluctuation can markedly decrease the maximum amplitude. We also show that dissipation has a larger effect on the phase of oscillator than on its amplitude.

Modified atomic force microscope applied to the measurement of elastic modulus for a single peptide molecule

A modified atomic force microscopy (AFM) system, based on a force modulation technique, has been used to find an approximate value for the elastic modulus of a single peptide molecule directly from a mechanical test. For this purpose a self-assembled monolayer built from two kinds of peptides, reactive (able to anchor to the AFM tip) and nonreactive, was synthesized. In a typical experiment a single C3K30C (C=cysteine, K=lysine) peptide molecule was stretched between a Au(111) substrate and the gold-coated tip of an AFM cantilever to which it was attached via gold–sulfur bonds. The amplitude of the cantilever oscillations, due to an external force applied via a magnetic particle to the cantilever, was recorded by a lock-in amplifier and recalculated into stiffness of the stretched molecule. A longitudinal Young’s modulus for the α-helix of a single peptide molecule and for the elongated state of this molecule has been estimated. The obtained values; 1.2±0.3 and 50±15 GPa, for the peptide α-helix and elongated peptide backbone, respectively, seem to be reasonable comparing them to the Young’s modulus of protein crystals and linear organic polymers. We believe this research opens up a means by which scientists can perform quantitative studies of the elastic properties of single molecule, especially of biologically important polymers like peptides or DNA.

Investigation of crossed SAW fields by scanning acoustic force microscopy.

We used multimode scanning acoustic force microscopy (SAFM) for studying noncollinearly propagating Rayleigh and Love wave fields. By analyzing torsion and bending movement of SAFM cantilever, normal and in-plane wave oscillation components are accessible. The SAFM principle is the down-conversion of surface oscillations into cantilever vibrations caused by the nonlinearity of the tip-sample interaction. Through mixing of complementary oscillation components, phase velocities of crossed Rayleigh waves on GaAs and crossed Rayleigh and Love waves on the layered system SiO2/ST-cut quartz were obtained simultaneously. Now, it is possible to investigate elastic properties of submicron areas through multimode SAFM measurements. Finally, we present mixing experiments of four SAWs on GaAs and discuss the various influences on the measured SAFM amplitude and phase contrast.

A chemical sensor based on a microfabricated cantilever array with simultaneous resonance-frequency and bending readout

We present a chemical sensor based on a microfabricated array of eight silicon cantilevers actuated at their resonance-frequency and functionalized by polymer coatings. The operating principle relies on transduction of chemical or physical processes into a mechanical response. After exposure to analyte vapor, analyte molecules diffuse into the cantilever coating, which begins to swell. Jointly with the mass increase, a change of interfacial stress between coating and cantilever occurs, resulting in a bending of the cantilevers. Our setup allows the simultaneous detection of cantilever oscillation and bending of eight cantilevers by time-multiplexed optical beam deflection readout. The ac component of the cantilever response is demodulated, and the cantilever resonance-frequency is tracked by a custom-built phase-locked loop. By filtering out the ac component (oscillation), the dc signal (bending) is extracted, yielding information on mass as well as surface stress changes simultaneously. Detection results of water, primary alcohols, alkanes and perfumes are presented.

Influence of surface acoustic waves on lateral forces in scanning force microscopies

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever’s torsion and bending. The experiments show that friction can locally be suppressed by Rayleigh-type SAWs. Through the mapping of crossed standing wave fields, the wave amplitude dependence of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, friction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction effect. Thus, we conclude that the mechanical diode effect, i.e., the effective shift of the cantilever off of the oscillating surface, is responsible for the SAW-induced lubrication. This explanation is supported by vertical and lateral SAFM measurements: in areas with completely vanishing friction, low frequency vertical cantilever oscillations are still observable, whereas lateral (torsional) cantilever oscillations are no longer excited. Additionally, at very high Rayleigh wave amplitudes an effect of lateral force rectification was observed. It results in a scan direction-independent appearance of the LFM traces.

Atomic force microscopy investigation of filled elastomers and comparison with transmission electron microscopy — application to silica-filled silicone elastomers

Tapping mode atomic force microscopy experiments were conducted on silica-filled silicone elastomers. Phase images provide a good contrast between the silicone matrix and silica structures, and allow the study of silica microdispersions. However, the phase contrast is sensitive to several experimental factors, which affect the magnitude of tip-sample interactions. The dependence of the phase contrast on the free amplitude of cantilever oscillation, on the setpoint ratio, on the cantilever stiffness and on the sample modulus was investigated. The setpoint ratio was shown to be the main parameter controlling the tip-sample force, and consequently the phase contrast. By comparing the results with transmission electron microscopy experiments, the tapping mode atomic force microscopy with phase imaging is established as an easy and powerful technique for the characterization of the silica dispersion in a silicone matrix. This study opens the way for using Atomic force microscopy to get a better understanding of reinforcement in filled rubbers.

Simultaneous imaging of tunneling current and damping energy by noncontact-AFM in ultra-high vacuum

We have simultaneously observed the images of tunneling current and damping energy with the nc-AFM (noncontact atomic force microscopy) image of Si(111)7×7. When inverted contrast is observed in the constant frequency shift nc-AFM image, the current image is not inverted, and vice versa. On the other hand, the damping images show a contrast similar to that of the nc-AFM images; the damping decreases at a narrow separation between the tip and the sample. This possibly indicates that the damping decreases as the attractive interaction increases under a constant oscillation amplitude of the cantilever. To keep the oscillation amplitude constant under an attractive interaction between a tip and a sample, the total energy of the cantilever oscillation is reduced and the oscillation velocity of the cantilever decreases. An explanation is proposed that the change in energy dissipation occurs in the cantilever oscillation, depending on the oscillation velocity of the cantilever, and the value is estimated from a simple model.

Scanning impedance microscopy of electroactive interfaces

A scanning probe technique based on the detection of the phase change of cantilever oscillations induced by a lateral bias applied to the sample is developed. This technique is used to investigate Σ5 grain boundary in Nb-doped SrTiO3 bicrystal. Tip bias, frequency, and driving amplitude dependence of cantilever response to sample ac bias were found to be in excellent agreement with the theoretical model. This technique, further referred to as scanning impedance microscopy, allows mapping of the local phase angle of complex microstructures. This technique is complemented by scanning surface potential microscopy (SSPM). Ramping the lateral dc bias during SSPM measurements allows the voltage characteristics of the grain boundary to be reconstructed and dc transport properties to be obtained by an equivalent circuit method. The combination of scanning impedance microscopy and scanning surface potential microscopy allows independent quantification of interface resistivity and capacitance, thus providing spatially resolved impedance spectra of complex microstructures.

Dynamic plowing nanolithography on polymethylmethacrylate using an atomic force microscope

We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.

Spectroscopy of the anharmonic cantilever oscillations in tapping-mode atomic-force microscopy

By spectroscopic analysis of the cantilever oscillation in tapping-mode atomic-force microscopy (TM–AFM), we demonstrate that the transition from an oscillatory state dominated by a net attractive force to the state dominated by repulsive interaction is accompanied by the enhanced generation of higher harmonics. The higher harmonics are a consequence of the nonlinear interaction and are amplified to significant amplitudes by the eigenmodes of the cantilever. The results show that in a quantitative description of TM–AFM higher eigenmode excitation must be considered to account for internal energy dissipation.

Electrostatic force at mica surfaces proved by frequency-shift spectroscopy

We have investigated force interactions between a conductive silicon tip and a mica surface using frequency-shift spectroscopy with a non-contact atomic force microscope in ultrahigh vacuum conditions. The frequency shift and amplitude damping of the cantilever oscillation were analyzed using the Krylov–Bogoliubov method. The analysis identifies characteristic tip–sample interactions, i.e., van der Waals', adhesion and electrostatic forces, from the total force. Polarization of the water layer on the mica surface was found to affect the electrostatic potential between the tip and the surface.

High Spatial Resolutive Method Observing the Lubricant Distribution on Magnetic Recording Media

A new method to evaluate the distribution of perfluoropolyether lubricant on hard disks was developed. We used a scanning probe microscope (SPM) technique with the dynamic force mode to detect the adhesion between the tip and the lubricant layer. We found that the distribution of phase of the cantilever oscillation varied with the cantilever amplitude when hard disks were observed, and that the behavior of how the phase image varies with the cantilever amplitude gives the distribution of the lubricant. The relative lubricant amount estimated by this method was linearly correlated with the lubricant thickness measured by Fourier transform infrared spectroscopy (FT-IR). This suggested that the method could quantitatively evaluate the lubricant distribution. Visualization of the lubricant distribution showed that the lubricant was thicker at the bottoms and thinner at the bevels around grooves in the mechanical texture. [S0742-4787(00)02104-4]