Signal reversal in Kelvin-probe force microscopy.

Kelvin-probe force microscopy is a measurement mode of atomic force microscopy, which is used to quantitatively map the electrical surface potential of a sample. Inadequate hardware and electronic design can lead to signal cross talk and, in consequence, false results. Here, we show that certain cross talk artifacts not only do manifest themselves in additional noise, reduced resolution, or an offset of the measured surface potential but can also lead to an inverted signal scale and, crucially, cannot be diagnosed with a known reference signal. We show experimental data on an electrically homogeneous sample, describe a method to detect the artifact, and propose simple remedies, which should be well within the reach of most research and industrial laboratories.

Water desorption in Kelvin-probe force microscopy: a generic model.

Nanoparticles or similar, nanoscale objects such as proteins or biological fibrils usually have to be deposited from aqueous suspension onto a solid support surface for further characterization by atomic force microscopy (AFM) and related methods such as Kelvin-probe force microscopy (KFM). Here we show, on the examples of functionalized nanoparticles and collagen fibrils, that water desorption after sample preparation affects their electrostatic potential determined by KFM in a predictable manner. We explain this effect with a simple, analytical model based on the capacitance of the partially dielectric-filled tip-sample system. We also propose practical measures to avoid false interpretation of electrical AFM-based experiments. As the phenomenon is very generic it may have significant implications in the application of AFM to nanoparticles and other nanostructures including biological ones.

Active damping by Q-control for fast force-distance curve measurements in atomic force microscopy.

This paper investigates the benefit of active damping by an analog Q-control circuit for measuring fast force-distance curves in atomic force microscopy. By active damping of the cantilever oscillation after snap-off, the down-ring time-constant is reduced significantly from 385 µs to 23 µs. Experimental results demonstrate that the number of force-distance curves per second can be increased by a factor of more than 30.

Automated spherical aberration correction in scanning confocal microscopy.

Mismatch between the refractive indexes of immersion media and glass coverslips introduces spherical aberrations in microscopes especially for high numerical aperture objectives. This contribution demonstrates an automated adjustment of the coverslip correction collar in scanning confocal microscopy to compensate for spherical aberrations due to coverslip thickness mismatch. With a motorized coverslip correction collar, the adjustment procedure consists of xz image scans, image processing, correction quality evaluation, the mismatch estimation, and eventually the optimal adjustment of the correction collar. For fast correction with less photodamage, coarse-fine Gaussian fitting algorithms are proposed and evaluated with various specimen for their estimation accuracy. The benefits of the proposed automated correction are demonstrated for various coverslips with biological specimens, showing the optimized resolution of the confocal microscope.

High precision wavelength estimation method for integrated optics.

A novel and simple approach to optical wavelength measurement is presented in this paper. The working principle is demonstrated using a tunable waveguide micro ring resonator and single photodiode. The initial calibration is done with a set of known wavelengths and resonator tunings. The combined spectral sensitivity function of the resonator and photodiode at each tuning voltage was modeled by a neural network. For determining the unknown wavelengths, the resonator was tuned with a set of heating voltages and the corresponding photodiode signals were collected. The unknown wavelength was estimated, based on the collected photodiode signals, the calibrated neural networks, and an optimization algorithm. The wavelength estimate method provides a high spectral precision of about 8 pm (5 · 10(-6) at 1550 nm) in the wavelength range between 1549 nm to 1553 nm. A higher precision of 5 pm (3 · 10(-6)) is achieved in the range between 1550.3 nm to 1550.8 nm, which is a factor of five improved compared to a simple lookup of data. The importance of our approach is that it strongly simplifies the optical system and enables optical integration. The approach is also of general importance, because it may be applicable to all wavelength monitoring devices which show an adjustable wavelength response.

Local strain and damage mapping in single trabeculae during three-point bending tests.

The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals is of limited value. However, there is growing evidence that information on trabecular microarchitecture can improve the assessment of fracture risk. One current strategy is to exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular bone structures obtained from non-invasive imaging modalities for the prediction of apparent mechanical properties. However, there is a lack of FE damage models, based on solid experimental facts, which are needed to validate such approaches and to provide criteria marking elastic-plastic deformation transitions as well as microdamage initiation and accumulation. In this communication, we present a strategy that could elegantly lead to future damage models for FEA: direct measurements of local strains involved in microdamage initiation and plastic deformation in single trabeculae. We use digital image correlation to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values. Our results show that the whitening zones, i.e. damage formation, in the presented loading case of a three-point bending test correlate best with areas of elevated tensile strains oriented parallel to the long axis of the samples. The average local strains along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagating over a large range of tensile strains.

Model-based aberration correction in a closed-loop wavefront-sensor-less adaptive optics system.

In many scientific and medical applications, such as laser systems and microscopes, wavefront-sensor-less (WFSless) adaptive optics (AO) systems are used to improve the laser beam quality or the image resolution by correcting the wavefront aberration in the optical path. The lack of direct wavefront measurement in WFSless AO systems imposes a challenge to achieve efficient aberration correction. This paper presents an aberration correction approach for WFSlss AO systems based on the model of the WFSless AO system and a small number of intensity measurements, where the model is identified from the input-output data of the WFSless AO system by black-box identification. This approach is validated in an experimental setup with 20 static aberrations having Kolmogorov spatial distributions. By correcting N=9 Zernike modes (N is the number of aberration modes), an intensity improvement from 49% of the maximum value to 89% has been achieved in average based on N+5=14 intensity measurements. With the worst initial intensity, an improvement from 17% of the maximum value to 86% has been achieved based on N+4=13 intensity measurements.

Active Damping of a Piezoelectric Tube Scanner using Self-Sensing Piezo Actuation.

In most Atomic Force Microscopes (AFM), a piezoelectric tube scanner is used to position the sample underneath the measurement probe. Oscillations stemming from the weakly damped resonances of the tube scanner are a major source of image distortion, putting a limitation on the achievable imaging speed. This paper demonstrates active damping of these oscillations in multiple scanning axes without the need for additional position sensors. By connecting the tube scanner in a capacitive bridge circuit the scanner oscillations can be measured in both scanning axes, using the same piezo material as an actuator and sensor simultaneously. In order to compensate for circuit imbalance caused by hysteresis in the piezo element, an adaptive balancing circuit is used. The obtained measurement signal is used for feedback control, reducing the resonance peaks in both scanning axes by 18 dB and the cross-coupling at those frequencies by 30 dB. Experimental results demonstrate a significant reduction in scanner oscillations when applying the typical triangular scanning signals, as well as a strong reduction in coupling induced oscillations. Recorded AFM images show a considerable reduction in image distortion due to the proposed control method, enabling artifact free AFM imaging at a speed of 122 lines per second with a standard piezoelectric tube scanner.

Extracting hysteresis from nonlinear measurement of wavefront-sensorless adaptive optics system.

In many scientific and medical applications wavefront-sensorless adaptive optics (AO) systems are used to correct the wavefront aberration by optimizing a certain target parameter, which is nonlinear with respect to the control signal to the deformable mirror (DM). Hysteresis is the most common nonlinearity of DMs, which can be corrected if the information about the hysteresis behavior is present. We report a general approach to extract hysteresis from the nonlinear behavior of the adaptive optical system, with the illustration of a Foucault knife test, where the voltage-intensity relationship consists of both hysteresis and some memoryless nonlinearity. The hysteresis extracted here can be used for modeling and linearization of the AO system.

In situ observation of fluoride-ion-induced hydroxyapatite-collagen detachment on bone fracture surfaces by atomic force microscopy.

The topography of freshly fractured bovine and human bone surfaces was determined by the use of atomic force microscopy (AFM). Fracture surfaces from both kinds of samples exhibited complex landscapes formed by hydroxyapatite mineral platelets with lateral dimensions ranging from ∼90 nm × 60 nm to ∼20 nm × 20 nm. Novel AFM techniques were used to study these fracture surfaces during various chemical treatments. Significant topographical changes were observed following exposure to aqueous solutions of ethylenediaminetetraacetic acid (EDTA) or highly concentrated sodium fluoride (NaF). Both treatments resulted in the apparent loss of the hydroxyapatite mineral platelets on a timescale of a few seconds. Collagen fibrils situated beneath the overlying mineral platelets were clearly exposed and could be resolved with high spatial resolution in the acquired AFM images. Time-dependent mass loss experiments revealed that the applied agents (NaF or EDTA) had very different resulting effects. Despite the fact that the two treatments exhibited nearly identical results following examination by AFM, bulk bone samples treated with EDTA exhibited a ∼70% mass loss after 72 h, whereas for the NaF-treated samples, the mass loss was only of the order of ∼10%. These results support those obtained from previous mechanical testing experiments, suggesting that enhanced formation of superficial fluoroapatite dramatically weakens the protein-hydroxyapatite interfaces. Additionally, we discovered that treatment with aqueous solutions of NaF resulted in the effective extraction of noncollagenous proteins from bone powder.

Fast contact-mode atomic force microscopy on biological specimen by model-based control.

The dynamic behavior of the piezoelectric tube scanner limits the imaging rate in atomic force microscopy (AFM). In order to compensate for the lateral dynamics of the scanning piezo a model based open-loop controller is implemented into a commercial AFM system. Additionally, our new control strategy employing a model-based two-degrees-of-freedom controller improves the performance in the vertical direction, which is important for high-speed topographical imaging. The combination of both controllers in lateral and vertical direction compensates the three-dimensional dynamics of the AFM system and reduces artifacts that are induced by the systems dynamic behavior at high scan rates. We demonstrate this improvement by comparing the performance of the model-based controlled AFM to the uncompensated and standard PI-controlled system when imaging pUC 18 plasmid DNA in air as well as in a liquid environment.