Enhanced feedback performance in off-resonance AFM modes through pulse train sampling

Dynamic atomic force microscopy (AFM) modes that operate at frequencies far away from the resonance frequency of the cantilever (off-resonance tapping (ORT) modes) can provide high-resolution imaging of a wide range of sample types, including biological samples, soft polymers, and hard materials. These modes offer precise and stable control of vertical force, as well as reduced lateral force. Simultaneously, they enable mechanical property mapping of the sample. However, ORT modes have an intrinsic drawback: a low scan speed due to the limited ORT rate, generally in the low kHz range. Here, we analyze how the conventional ORT control method limits the topography tracking quality and hence the imaging speed. The closed-loop controller in conventional ORT restricts the sampling rate to the ORT rate and introduces a large closed-loop delay. We present an alternative ORT control method in which the closed-loop controller samples and tracks the vertical force changes during a defined time window of the tip-sample interaction. Through this, we use multiple samples in the proximity of the maximum force for the feedback loop, rather than only one sample at the maximum force instant. This method leads to improved topography tracking at a given ORT rate and therefore enables higher scan rates while refining the mechanical property mapping. Keywords: atomic force microscopy (AFM); off-resonance tapping (ORT); pulsed-force mode; feedback contro

PLL-based high-speed demodulation of FM signals for real-time AFM applications

In this paper we present a new architecture for PLL-based high-speed demodulation of frequency-modulated AFM signals. In our approach, we use single-sideband frequency up-conversion to translate the AFM signal from the position sensitive detector to a fixed intermediate frequency of 10MHz. In this way, we fully benefit from the excellent noise performance of PLL-based FM demodulators still avoiding the intrinsic bandwidth limitation of such systems. Furthermore, the system becomes independent of the cantilever's resonance frequency. To investigate if the additional noise introduced by the single-sideband upconverter degrades the system noise figure we present a model of the AM-to-FM noise conversion in the PLL phase detector. Using this model, we can predict an upper corner frequency for the demodulation bandwidth above which the converted noise from the single-sideband upconverter becomes the dominant noise source and therefore begins to deteriorate the overall system performance. The approach is validated by measured data obtained with a PCB-based prototype implementing the proposed demodulator architecture. © 2013 IEEE

Piezoresistive AFM cantilevers surpassing standard optical beam deflection in low noise topography imaging

Optical beam deflection (OBD) is the most prevalent method for measuring cantilever deflections in atomic force microscopy (AFM), mainly due to its excellent noise performance. In contrast, piezoresistive strain-sensing techniques provide benefits over OBD in readout size and the ability to image in light-sensitive or opaque environments, but traditionally have worse noise performance. Miniaturisation of cantilevers, however, brings much greater benefit to the noise performance of piezoresistive sensing than to OBD. In this paper, we show both theoretically and experimentally that by using small-sized piezoresistive cantilevers, the AFM imaging noise equal or lower than the OBD readout noise is feasible, at standard scanning speeds and power dissipation. We demonstrate that with both readouts we achieve a system noise of ≈0.3 Å at 20 kHz measurement bandwidth. Finally, we show that small-sized piezoresistive cantilevers are well suited for piezoresistive nanoscale imaging of biological and solid state samples in air

Data-Driven Controller Design for Atomic-Force Microscopy

A novel method to design data-driven, fixed-structure controllers with H2 and H∞ performance objectives is presented. The control design problem is transformed into a convex optimization problem with linear matrix inequality constraints, which can be solved efficiently with standard solvers. The method is used to design a data-driven controller for an atomic-force microscope. The closed-loop performance of the calculated controller is validated on a real setup

Microfluidic bacterial traps for simultaneous fluorescence and atomic force microscopy

The atomic force microscope has become an established research tool for imaging microorganisms with unprecedented resolution. However, its use in microbiology has been limited by the difficulty of proper bacterial immobilization. Here, we have developed a microfluidic device that solves the issue of bacterial immobilization for atomic force microscopy under physiological conditions. Our device is able to rapidly immobilize bacteria in well-defined positions and subsequently release the cells for quick sample exchange. The developed device also allows simultaneous fluorescence analysis to assess the bacterial viability during atomic force microscope imaging. We demonstrated the potential of our approach for the immobilization of rod-shaped Escherichia coli and Bacillus subtilis. Using our device, we observed buffer-dependent morphological changes of the bacterial envelope mediated by the antimicrobial peptide CM15. Our approach to bacterial immobilization makes sample preparation much simpler and more reliable, thereby accelerating atomic force microscopy studies at the single-cell level

Reducing uncertainties in energy dissipation measurements in atomic force spectroscopy of molecular networks and cell-adhesion studies

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) is a valuable tool in biophysics to investigate the ligand-receptor interactions, cell adhesion and cell mechanics. However, the force spectroscopy data analysis needs to be done carefully to extract the required quantitative parameters correctly. Especially the large number of molecules, commonly involved in complex networks formation; leads to very complicated force spectroscopy curves. One therefore, generally characterizes the total dissipated energy over a whole pulling cycle, as it is difficult to decompose the complex force curves into individual single molecule events. However, calculating the energy dissipation directly from the transformed force spectroscopy curves can lead to a significant overestimation of the dissipated energy during a pulling experiment. The over-estimation of dissipated energy arises from the finite stiffness of the cantilever used for AFM based SMFS. Although this error can be significant, it is generally not compensated for. This can lead to significant misinterpretation of the energy dissipation (up to the order of 30%). In this paper, we show how in complex SMFS the excess dissipated energy caused by the stiffness of the cantilever can be identified and corrected using a high throughput algorithm. This algorithm is then applied to experimental results from molecular networks and cell-adhesion measurements to quantify the improvement in the estimation of the total energy dissipation

Single-Cycle-PLL Detection for Real-Time FM-AFM Applications

In this paper we present a novel architecture for phase-locked loop (PLL) based high-speed demodulation of fre- quency-modulated (FM) atomic force microscopy (AFM) signals. In our approach, we use single-sideband (SSB) frequency upcon- version to translate the AFM signal from the position sensitive detector to a fixed intermediate frequency (IF) of 10 MHz. In this way, we fully benefit from the excellent noise performance of PLL-based FM demodulators still avoiding the intrinsic band- width limitation of such systems. In addition, the upconversion to a fixed IF renders the PLL demodulator independent of the cantilever’s resonance frequency, allowing the system to work with a large range of cantilever frequencies. To investigate if the additional noise introduced by the SSB upconverter degrades the system noise figure we present a model of the AM-to-FM noise conversion in PLLs incorporating a phase-frequency detector. Using this model, we can predict an upper corner frequency for the demodulation bandwidth above which the converted noise from the single-sideband upconverter becomes the dominant noise source and therefore begins to deteriorate the overall system performance. The approach is validated by both electrical and AFM measurements obtained with a PCB-based prototype imple- menting the proposed demodulator architecture