Amplitude Of Cantilever Research Articles

A highly sensitive atomic force microscope for linear measurements of molecular forces in liquids

We describe a highly improved atomic force microscope for quantitative nanomechanical measurements in liquids. The main feature of this microscope is a modified fiber interferometer mounted on a five axis inertial slider which provides a deflection sensitivity that is significantly better than conventional laser deflection based systems. The measured low noise floor of 572.0fm∕Hz provides excellent cantilever amplitude resolution. This allows us to operate the instrument far below resonance at extremely small cantilever amplitudes of less than 1 Å. Thus linear measurements of nanomechanical properties of liquid systems can be performed. In particular, we present measurements of solvation forces in confined octamethylcyclotetrasiloxane and water with amplitudes smaller than the size of the respective molecules. In general, the development of the instrument is important in the context of quantitative nanomechanical measurements in liquid environments.

Piezoelectric self-sensing of adsorption-induced microcantilever bending

A self-sensing, self-actuating, piezoelectric microcantilever is used to detect adsorption-induced bending. Bending is measured by bringing the resonating cantilever into intermittent contact with a surface, the elevation of which is maintained by a piezotube in feedback. When the cantilever bends, its oscillatory amplitude changes and the piezotube is adjusted to compensate. Cantilever amplitude is detected using the voltage output of the piezoelectric film. Bending in this test was caused by mercury adsorption onto gold. A 50 ppb mercury concentration in nitrogen gas was detected.

Tapping mode atomic force microscopy in liquid with an insulated piezoelectric microactuator

Tapping mode atomic force microscopy in liquids is enhanced using an insulated cantilever with an integrated piezoelectric microactuator. When vibrating the cantilever via direct force modulation by the actuator, a single resonance peak appears in the plot of rms cantilever amplitude versus excitation frequency, eliminating the spurious resonances typical of acoustic excitation in a liquid medium. This simplifies selection of the cantilever’s natural resonance frequency for improved tuning accuracy and speed. Acoustic excitation can excite cantilever modes that do not displace the tip of the cantilever but vibrate the microscope’s detection system and create unwanted liquid-coupled acoustic waves between the liquid-cell and the sample. These modes are eliminated by directly forcing the cantilever. Insulated microactuated probes offer a simple and more direct alternative solution to recently presented magnetic tuning methods.

Atomic force microscope detector drift compensation by correlation of similar traces acquired at different setpoints

The atomic force microscope measures surface topography by maintaining a certain cantilever deflection or vibration amplitude as the cantilever is scanned over a sample surface. The desired cantilever deflection or amplitude is referred to as the setpoint, and is maintained by moving the sample toward or away from the cantilever. The signal from the cantilever deflection detector has a real component, due to cantilever deflection, and a drift component due to various sources of drift. We present a method of eliminating the drift component by sensing and correcting it in real time. Our method involves automatically changing the setpoint so as to maintain a certain set difference in the relative feature richness of two traces taken with slightly offset setpoints. We show how the system maintains a setpoint only 70 mV above minimum, perturb it with a gentle blow of air that causes 200 mV of detector drift, and observe its recovery within 13+−6 s.

Reproducibility of Nano- and Micro-Scale Multi-Point Probe Sheet Resistance Measurements

ABSTRACTWe have investigated the reproducibility of micro- and nano-scale measurements of sheet resistance performed with micro-fabricated multi-point probes. The probes consisted of Au coated SiO2 cantilevers extending from a Si base. The measurements were done with a four-point probe technique on thin Au films, the probe electrode spacing ranging from 18 μm to 1.5 μm. We find that the standard deviation of repeated sheet resistance measurements ranges from 0.2% at 18 μm spacing to 2.6% at 1.5 μm spacing. It is inversely proportional to the probe electrode spacing. This behaviour is expected if the resolution of the measurements is governed by the positional errors of the probe electrode tips. The corresponding standard deviation of the probe tip positions (in both lateral directions) is calculated to be approximately 20 nm. We argue that these positional errors depend on the probe cantilever amplitude at the time of contacting the surface. The amplitude is inversely proportional to the square root of the cantilever spring constant, indicating that stiff cantilevers give the best reproducibility. We estimate the limiting reproducibility of multi-point probes with nano-scale electrode spacing.

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]

Anomalous Changes in Tip Height in Tapping AFM

Abstract This effect can be understood by considering what happens to the cantilever and tip as the base of the vibrating cantilever is brought closer to the sample (like in a force curve). When the vibrating cantilever first approaches the surface, the tip senses attractive forces. In standard imaging conditions, these are usually due to meniscus forces between a layer of adsorbed liquid (often water) on the surface of the sample and the tip. However, even in clean conditions, other attractive forces like Van der Waals will generally be present. These attractive forces move the resonance of the cantilever downward in frequency. Since the cantilever is being driven at the original resonance frequency of the cantilever, this will cause the cantilever amplitude to decrease as the attractive forces puli the resonance peak out from under the drive frequency. This decrease is often quite linear with distance, but doesn't have to be. In this case the tip is actually turning around some distance (this could typically be a few nanometers) before it touches the surface. This regime is commonly called the “attractive” regime because attractive forces dominate the behavior of the cantilever.

Phase Contrast Imaging in Atomic Force Microscopy

Phase detection in TappingMode™ enhances capabilities of Atomic Force Microscopy (AFM) for soft samples (polymers and biological materials). Changes of amplitude and phase changes of a fast oscillating probe are caused by tip-sample force interactions. Height images reflect the amplitude changes, and in most cases they present a sample topography. Phase images show local differences between phases of free-oscillating probe and of probe interacting with a sample surface. These differences are related to the change of the resonance frequency of the probe either by attractive or repulsive tip-sample forces. Therefore phase detection helps to choose attractive or repulsive force regime for surface imaging and to minimize tip-sample force. For heterogeneous materials the phase imaging allows to distinguish individual components and to visualize their distribution due to differences in phase contrast. This is typically achieved in moderate tapping, when set-point amplitude, Asp, is about half of the amplitude of free-oscillating cantilever, Ao. In contrast, light tapping with Asp close to Ao is best suited for recording a true topography of the topmost surface layer of soft samples. Examples of phase imaging of polymers obtained with a scanning probe microscope Nanoscope® IIIa (Digital Instruments). Si probes (225 μk long, resonance frequencies 150-200 kHz) were used.

How does a tip tap?

There is a demand for good theoretical understanding of the response of an atomic force microscope cantilever to the extremely nonlinear impacts received while tapping a sample. A model and numerical simulations are presented in this paper which provide a very pleasing comparison with experimental results. The dependence of the cantilever amplitude and phase upon the sample stiffness, adhesion and damping are investigated using these simulations, and it is found that `topographic' tapping images are not independent of sample properties, nor will it be trivial to measure materials' properties from the tapping data. The simulation can be applied to other probe microscope configurations as well.

Ac-mode atomic force microscope imaging in air and solutions with a thermally driven bimetallic cantilever probe

An ac imaging mode for atomic force microscopy (AFM) has been developed that employs a thermally driven bimetallic cantilever to sense surface topography. Oscillations are induced in a composite cantilever, comprising a Si3N4 layer and Au overcoat, by local heating with a resistive wire heater placed in close proximity to the cantilever. Cantilever bending occurs upon heating due to the difference in thermal expansion coefficients of the Si3N4 and Au layers. The magnitude of this bending is a function of the heat input, the cantilever geometry, the frequency of the excitation, and the thermal properties of the surrounding medium. A commercially available contact mode AFM has been modified to perform ac mode imaging by driving the cantilever with a peak-to-peak amplitude of 5–15 nm using resistive heating. The heating frequency was typically fixed near the cantilever’s resonance frequency, which was in the range of 15–50 kHz in the air and 2–15 kHz in solution for the cantilevers used here. Simultaneous cantilever deflection and amplitude measurements during sample approach indicate that the cantilever free amplitude is damped upon surface contact. While imaging, a fixed damping of the cantilever oscillation is used as a feedback signal to maintain a constant tip-sample separation. Images with this ac imaging mode were obtained in both air and liquid environments. Results show an improvement in image quality in the ac mode over the corresponding contact images, which is the result of a decrease in lateral forces with an oscillating tip. This provides a simple and robust method for ac-mode AFM imaging in air and solutions that can be achieved with only slight modification to a commercially available contact-mode microscope.

Force gradient mapping of domain wall structures in magnetite (abstract)

We have used a magnetic force microscope (MFM) in a new imaging mode to study domain walls in magnetite. The oscillation amplitude and phase of a vibrating cantilever were recorded versus cantilever tip-sample separation at each point in an x-y raster scan of a conventional MFM image. Amplitude and phase changes of the MFM cantilever depend on the topographic, interferometric, damping, and magnetic force gradients effects the cantilever experiences. For small scan areas, the magnetic force gradient acting between the tip and sample could be separated from the other interactions. This allowed quantitative extraction of the magnetic force gradients into a three-dimensional dataset called a force gradient map (FGM). FGMs were made over a number of samples; the work reported here focuses on domain wall structures in single crystal magnetite. The results of previous conventional MFM images and FGM images will be compared. One result was that the apparent resolution of the MFM tip was strongly dependent on the oscillation amplitude of the cantilever tip, implying that this must be considered when modelling MFM response. The data also suggest that FGMs may provide more sensitive tests of micromagnetic models than conventional MFM imaging of magnetite. This is for two reasons: (i) as mentioned above, FGMs allow the magnetic signal to be separated from the topographic, damping and interferometric effects and (ii) because FGMs sample the interactions between the tip and sample at a variety of well defined separations, it is possible to identify critical micromagnetic structural length scales.

Transient response of tapping scanning force microscopy in liquids

Tapping-mode scanning force microscopy in liquids is usually accomplished by acoustic excitation of the cantilever because of the strong viscous damping. Contact of the tip with the sample surface results in a damping of the cantilever amplitude with an anharmonic response. This interaction is modeled as a viscous-damped, one-dimensional harmonic oscillator periodically perturbed by an exponential surface potential. Experimental results verify the validity of the model.