J. Kokavecz

Novel amplitude and frequency demodulation algorithm for a virtual dynamic atomic force microscope

Frequency-modulated atomic force microscopy (FM-AFM; also called non-contact atomic force microscopy) is the prevailing operation mode in (sub-)atomic resolution vacuum applications. A major obstacle that prohibits a wider application range is the low frame capture rate. The speed of FM-AFM is limited by the low bandwidth of the automatic gain control (AGC) and frequency demodulation loops. In this work we describe a novel algorithm that can be used to overcome these weaknesses. We analysed the settling times of the proposed loops and that of the complete system, and we found that an approximately 70-fold improvement can be achieved over the existing real and virtual atomic force microscopes. We show that proportional-integral-differential controllers perform better in the frequency demodulation loop than conventional proportional-integral controllers. We demonstrate that the signal to noise ratio of the proposed system is 5.7 × 10(-5), which agrees with that of the conventional systems; thus, the new algorithm would improve the performance of FM-AFMs without compromising the resolution.

Surface energy maps of nanostructures: Atomic force microscopy and numerical simulation study

Topography and surface energy distribution of etched graphite were examined by atomic force microscopy (AFM). AFM images show atomic monolayer deep circular holes (etch pits). At certain imaging conditions, these etch pits appear surrounded by rims. Numerical simulation of AFM images reveals that the rims are formed due to an increased surface energy zone at the edges. The vertical dimension of the rim correlates with the magnitude of the local surface energy. Such a correlation between the imaging features and the surface energy profiles can be used to demarcate local chemical constituents in a composite nanomaterial.

Investigation of fluid cell resonances in intermittent contact mode atomic force microscopy

In fluid, the probe resonance curve of the atomic force microscope contains several apparent resonance peaks whose origin is not well understood. In this work, the authors focus on identifying the cause of these peaks and finding the optimal imaging conditions for acoustic intermittent contact mode in fluid environment. The authors demonstrate that the peaks are also present in the spectrum of the fluid movement and in that of the shaker piezo. These peaks may or may not coincide with the natural resonance of a probe in liquid, thus it is possible to drive the probes off-resonance. Numerical calculations show the feasibility of off-resonance imaging, but predict much higher imaging force.

Dynamical properties of the Q-controlled atomic force microscope

In intermittent contact mode atomic force microscopy (AFM), the quality factor (Q) of the oscillating probe is believed to account for the imaging speed and sensitivity. Q control is a method to artificially modify the quality factor of the probe. Here, we present a comprehensive study of the dynamics of the Q-controlled AFM. By comparing the analytical solutions of the force equations, we prove that the Q-controlled and non-Q-controlled systems are equivalent in the absence of surface forces. We also determine the conditions for the numerical simulation. In order to study the mechanism of contrast enhancement, we simulate the normal AFM operation including the surface forces. We found that there is a maximal probe sensitivity which cannot be exceeded even with Q control. Consistently, Q control enhances sensitivity only when imaging soft samples. Finally, we show that the phase signal of the Q-controlled system is more sensitive to the changes of the sample properties than in case of non-Q-controlled AFMs.

Characterization and modeling of tungsten nanoparticles generated by laser-assisted chemical vapor deposition

Tungsten nanoparticles were generated by photolytical (UV) laser-activated chemical vapor deposition from WF6/H2/Ar gas mixture. Emission spectroscopy of thermal radiation allowed temperature determination of the nanoparticles while varying the laser fluence. A model including known cooling mechanisms was used to calculate the laser-induced temperature as a function of time and laser fluence, where the only fitting parameter was the absorption efficiency of the particles, obtained from measured temperatures. Size decrease of the particles due to evaporation was modeled at different laser fluences, and connected to size-distribution measurements from transmission electron microscopy micrographs, where a maximum geometric mean diameter (for the experimental conditions used) of 10 nm was obseved at a laser fluence of ∼120 mJ/cm2. Measurements and the model calculations showed that the laser-excited particles reached the melting temperature of tungsten at ∼95 mJ/cm2. Above ∼130 mJ/cm2, very high rates of evap...

Investigation of incubation in ArF excimer laser irradiated poly(methyl-methacrylate) using pulsed force mode atomic force microscopy

An atomic force microscopic method to study the incubation states of UV laser irradiated polymer samples is presented. Targets were illuminated by different number of pulses at 5.8 and 8.9mJ∕cm2 fluences. The induced adhesive and morphological changes were investigated simultaneously by an atomic force microscope equipped with a pulsed force mode extension. Importantly, below 100 pulses morphological changes were not observable while significant changes in the adhesion were found as a result of the incubation at 8.9mJ∕cm2 fluence. This method allows the imaging and detection of the whole laser modified area with nanometer resolution.

Effect of step function-like perturbation on intermittent contact mode sensors: a response analysis

Abstract The dynamics of the intermittent contact mode (ICM) probe was investigated. In the experimental study we applied a step function signal to the Z piezo drive and recorded the amplitude signal of the probe while the probe was engaged with the surface. Transient overshoots appear at the edges of the steps. These transients are absent from the control contact force measurements, that is, they are proper to the ICM operation. The phenomenon was investigated by numerical calculations, focused on the effect of change of the drive frequency and the quality factor. We concluded, that the low value of the quality factor results in small transients and short settling time, which are necessary for fast atomic force microscopic operation. Simultaneously, the interaction force increases. Our calculations indicate that the tip–sample force can be lowered by setting the drive frequency slightly below the resonance.

The observability of poorly bound powder-like material on hard surface by atomic force microscopy

Various working modes of atomic force microscopy (AFM) are compared as imaging surfaces with poorly bound, easy-to-displace powder-like material on it. The poorly bound amorphous/turbostratical carbon structures are formed by laser ablation on the top of highly oriented pyrolytic graphite (HOPG) surface. Working in contact mode, these structures can easily be displaced by the AFM tip. Non-contact (low amplitude resonance) mode experiments proved to be adaptable to image the debris-like material, while tapping amplitude and phase detection mode measurements do not yield proper topographic information of it. On the other hand, tapping modes have the advantages of following the topography of a hard surface even when covered with poorly bound materials. Model calculations describing tip motion and the feedback circuit of the piezo driver confirm the observed phenomena.