Probe-sample Interaction Force Research Articles

Control of the Probe-Sample Interaction Force at the Piconewton Scale by a Magnetic Microprobe in Aqueous Solutions

This article describes the design and implementation of a control system that controls the probe-sample interaction force at the piconewton scale by manipulating a magnetic microprobe in aqueous solutions under a microscope. The control system has two functions, namely accurate force generation by magnetic flux control and precise control of the probe-sample interaction force. Magnetic flux control uses an improved six-input-six-output digital control law along with a disturbance estimator to achieve two control objectives. First, the magnetic flux at each pole tip of a previously developed hexapole electromagnetic actuator can be individually and precisely controlled at frequencies over 4 kHz, and the experimental results precisely followed the theoretical predictions based on the design. Second, together with the optimized flux allocation, the flux control system generates precise magnetic forces, making the six-input hexapole actuator behave like a decoupled three-axis force generator with a bandwidth of more than 4 kHz. Interaction force control compares the measured deformation of the sample with the expected deformation, updated in real time by a deformation predictor, to generate the magnetic force to control the interaction between the probe and the sample. Digital control laws, the estimator, and the predictor are all implemented using a high-speed Field Programmable Gate Array (FPGA) system. Experiments confirm that through mathematical modeling, digital control technology, high-speed electronics, and real-time computation, accurate three-dimensional force generation at the piconewton scale with high bandwidth and precise interaction force control with zero-mean random error, attributed to random thermal force and measurement noise, are achieved.

AFM SMILER: A Scale Model Interactive Learning Extended Reality Toolkit for Atomic Force Microscopy Created With Digital Twin Technology

The Atomic Force Microscope (AFM) is a precision mechatronic system for nanoscale imaging of surfaces. Due to limited instrument access and lack of visualization techniques, understanding its principles can be challenging. Digital twin technology allows the creation of virtual representations of physical systems, which can be particularly useful to address challenges in AFM education. To realistically simulate nanoscale physics, we first developed new efficient algorithms for four virtual scale models, including cantilever mechanics, probe transducers, controller tuning, and contact mechanics. Second, three simulated experiment interactive learning modules are developed for instrument operation, including virtual imaging, system overview, and imaging modalities. In the end, three hardware systems are integrated for an extended reality experience, including a macroscopic AFM scale model, a haptic device for probe-sample interaction force feedback, and an upgraded low-cost educational AFM for nanoscale imaging and instrumentation. This completes the eight total modules for the AFM SMILER: A scale model interactive learning extended reality toolkit. Preliminary studies show the toolkit being helpful for AFM education. In addition to mechatronics and nanotechnology education, techniques developed in this work can be generally applied to computationally efficient realistic digital twin creation.

Оптимизация измерений вектора силы взаимодействия в атомно-силовой микроскопии

The issue of optimization of measurements of three spatial components of the probe-sample interaction force and the corresponding displacement vector of the "ideal cantilever" is considered. To determine these components in an atomic force microscope with an optical beam deflection scheme, it is necessary to register the bending angles at least at two points on the rectangular cantilever and the torsion angle at any of them. It has been proven analytically that one optimal point is the intersection of the probe axis with the console plane. A method to calculate the position of another optimal point has been developed. An experiment was carried out to map the force and displacement vector, and satisfactory agreement with the theory was obtained.

Rapid Probe Engagement and Withdrawal With Force Minimization in Atomic Force Microscopy: A Learning-Based Online-Searching Approach

In this article, the problem of rapid probe engagement and withdrawal in atomic force microscopy (AFM) is addressed. Probe engagement and withdrawal is needed in almost all AFM operations, ranging from imaging to nanomanipulation. However, due to the highly nonlinear force-distance relation, large probe-sample interaction force can be induced during the probe engagement and withdrawal process, resulting in sample deformation and damage and measurement errors. Rapid probe engagement and withdrawal is needed to achieve high-speed AFM operations, particularly, to capture and interrogate dynamic evolutions of the sample. We propose an online-searching-based optimization approach to minimize both the engagement (and withdrawal) time and the interaction force. The force-displacement profile of the probe is partitioned and then optimized sequentially, by immersing optimal trajectory design and iterative learning control into the Fibonacci search process. The proposed approach is illustrated through experimental implementations on two different types of polymer species, a polydimethylsiloxane sample, and a dental silicone sample, respectively.

Adaptive-scanning, near-minimum-deformation atomic force microscope imaging of soft sample in liquid: Live mammalian cell example

In this paper, an adaptive-scanning mode (ASM) of atomic force microscope (AFM) with near-minimum sample deformation is proposed for imaging live biological samples in liquid. Conventional contact mode (CM) imaging of live cells is rather slow (scan rate < 0.2 Hz), and as the imaging speed increases, significant deformation of the soft and highly corrugated cell membrane is induced. Such a low speed CM imaging of live biological samples is not only time consuming, but also incapable of capturing dynamic biological evolutions occurring in seconds to minutes. The proposed ASM approach aims to address these issues through two synergetic efforts integrated together. First, an adaptive-scanning technique is proposed to optimally adjust the lateral scanning speed to accommodate the sample topography variation and the probe-sample interaction force, so that the scanning-caused sample deformation is maintained below the threshold value while the overall imaging time is minimized. Secondly, a data-driven iterative feedforward control is integrated to the vertical feedback loop along with a gradient-based optimization of the deflection set-point to substantially improve the tracking of the sample topography while maintaining the vertical sample deformation around the minimal. The ASM technique is experimentally validated through imaging live human prostate cancer cells on AFM. The experimental results demonstrate that compared to the conventional CM imaging, the imaging speed is increased over eight times without loss of tracking the topography details of the live cell membrane, and the probe-sample interaction force is substantially reduced.

Review Article: Active scanning probes: A versatile toolkit for fast imaging and emerging nanofabrication

With the recent advances in the field of nanotechnology, measurement and manipulation requirements at the nanoscale have become more stringent than ever before. In atomic force microscopy, high-speed performance alone is not sufficient without considerations of other aspects of the measurement task, such as the feature aspect ratio, required range, or acceptable probe-sample interaction forces. In this paper, the authors discuss these requirements and the research directions that provide the highest potential in meeting them. The authors elaborate on the efforts toward the downsizing of self-sensed and self-actuated probes as well as on upscaling by active cantilever arrays. The authors present the fabrication process of active probes along with the tip customizations carried out targeting specific application fields. As promising application in scope of nanofabrication, field emission scanning probe lithography is introduced. The authors further discuss their control and design approach. Here, microactuators, e.g., multilayer microcantilevers, and macroactuators, e.g., flexure scanners, are combined in order to simultaneously meet both the range and speed requirements of a new generation of scanning probe microscopes.

Design and characterisation of cantilevers for multi‐frequency atomic force microscopy

The experimental characterisation of a set of microcantilevers targeted at use in multi-frequency atomic force microscope is presented. The aim of this work is to design a cantilever that naturally amplifies its harmonic oscillations which are introduced by nonlinear probe-sample interaction forces. This is performed by placing the modal frequencies of the cantilever at integer multiples of the first modal frequency. The developed routine demonstrates the placement of the frequency of the second to fifth mode. The characterisation shows a trend that lower-order modes are more accurately placed than higher-order modes. With two fabricated designs, the error in the second mode is at most 2.26% while the greatest error in the fifth mode is at 10.5%.

Quantum state atomic force microscopy

New classical modalities of atomic force microscopy continue to emerge to achieve higher spatial, spectral, and temporal resolution for nanometrology of materials. Here, we introduce the concept of a quantum mechanical modality that capitalizes on squeezed states of probe displacement. We show that such squeezing is enabled nanomechanically when the probe enters the van der Waals regime of interaction with a sample. The effect is studied in the non-contact mode, where we consider the parameter domains characterizing the attractive regime of the probe-sample interaction force.

Piezoelectric bimorph-based shear force microscopy for the construction of noble metal plasmonic structures in air

In this article, we present an alternative method to directly manipulate the noble metal structures in air. The method is carried out based on a home-made shear force microscope, which mainly consists of a fiber probe-bimorph beam shear force sensor, a scanning probe microscope controller, and a nano-positioning stage. The magnitude of the probe-sample interaction forces as a function of the set-point ratio is estimated, which shows that the magnitude of the forces is inversely proportional to the set-point ratio. The microscopic imaging and manipulation can be realized at the higher and lower set-point values, respectively. Typical results of imaging at the set-point ratio of larger than 90% and manipulation at that of lower than 60% are demonstrated. The results suggest that this method would provide a promising way to study the relation between plasmonic structures and optical properties on the micro/nano scale.

High-speed atomic force microscope imaging: adaptive multiloop mode.

In this paper, an imaging mode (called the adaptive multiloop mode) of atomic force microscope (AFM) is proposed to substantially increase the speed of tapping mode (TM) imaging while preserving the advantages of TM imaging over contact mode (CM) imaging. Due to its superior image quality and less sample disturbances over CM imaging, particularly for soft materials such as polymers, TM imaging is currently the most widely used imaging technique. The speed of TM imaging, however, is substantially (over an order of magnitude) lower than that of CM imaging, becoming the major bottleneck of this technique. Increasing the speed of TM imaging is challenging as a stable probe tapping on the sample surface must be maintained to preserve the image quality, whereas the probe tapping is rather sensitive to the sample topography variation. As a result, the increase of imaging speed can quickly lead to loss of the probe-sample contact and/or annihilation of the probe tapping, resulting in image distortion and/or sample deformation. The proposed adaptive multiloop mode (AMLM) imaging overcomes these limitations of TM imaging through the following three efforts integrated together: First, it is proposed to account for the variation of the TM deflection when quantifying the sample topography; second, an inner-outer feedback control loop to regulate the TM deflection is added on top of the tapping-feedback control loop to improve the sample topography tracking; and, third, an online iterative feedforward controller is augmented to the whole control system to further enhance the topography tracking, where the next-line sample topography is predicted and utilized to reduce the tracking error. The added feedback regulation of the TM deflection ensures the probe-sample interaction force remains near the minimum for maintaining a stable probe-sample interaction. The proposed AMLM imaging is tested and demonstrated by imaging a poly(tert-butyl acrylate) sample in experiments. The experimental results demonstrate that the image quality achieved by using the proposed AMLM imaging at a scan rate of 25 Hz and over a large-size imaging (50 μm × 25 μm) is at the same level of that obtained using TM imaging at 1 Hz, while the probe-sample interaction force is noticeably reduced from that achieved using TM imaging at 2.5 Hz.

High-speed adaptive contact-mode atomic force microscopy imaging with near-minimum-force.

In this paper, an adaptive contact-mode imaging approach is proposed to replace the traditional contact-mode imaging by addressing the major concerns in both the speed and the force exerted to the sample. The speed of the traditional contact-mode imaging is largely limited by the need to maintain precision tracking of the sample topography over the entire imaged sample surface, while large image distortion and excessive probe-sample interaction force occur during high-speed imaging. In this work, first, the image distortion caused by the topography tracking error is accounted for in the topography quantification. Second, the quantified sample topography is utilized in a gradient-based optimization method to adjust the cantilever deflection set-point for each scanline closely around the minimal level needed for maintaining stable probe-sample contact, and a data-driven iterative feedforward control that utilizes a prediction of the next-line topography is integrated to the topography feeedback loop to enhance the sample topography tracking. The proposed approach is demonstrated and evaluated through imaging a calibration sample of square pitches at both high speeds (e.g., scan rate of 75 Hz and 130 Hz) and large sizes (e.g., scan size of 30 μm and 80 μm). The experimental results show that compared to the traditional constant-force contact-mode imaging, the imaging speed can be increased by over 30 folds (with the scanning speed at 13 mm/s), and the probe-sample interaction force can be reduced by more than 15% while maintaining the same image quality.

Dynamic Force Sensing Using an Optically Trapped Probing System

This paper presents the design of an adaptive observer that is implemented to enable real-time dynamic force sensing and parameter estimation in an optically trapped probing system. According to the principle of separation of estimation and control, the design of this observer is independent of that of the feedback controller when operating within the linear range of the optical trap. Dynamic force sensing, probe steering/clamping, and Brownian motion control can, therefore, be developed separately and activated simultaneously. The adaptive observer utilizes the measured motion of the trapped probe and input control effort to recursively estimate the probe-sample interaction force in real time, along with the estimation of the probing system's trapping bandwidth. This capability is very important to achieving accurate dynamic force sensing in a time-varying process, wherein the trapping dynamics is nonstationary due to local variations of the surrounding medium. The adaptive estimator utilizes the Kalman filter algorithm to compute the time-varying gain in real time and minimize the estimation error for force probing. A series of experiments are conducted to validate the design of and assess the performance of the adaptive observer.

Measurements of Electrostatic Potential Across p–n Junctions on Oxidized Si Surfaces by Scanning Multimode Tunneling Spectroscopy

We investigated the variation in contact potential difference (CPD) voltage across p–n junctions on oxygen-passivated Si(110) surfaces by scanning multimode tunneling spectroscopy, which detects probe–sample interaction force simultaneously with tunneling current. The enhancement of sensitivity to electrostatic force was achieved with a small amplitude of probe vibration (0.3 nm) when the tip–sample gap was adjusted to reduce short-range interactions by maintaining the tunneling current at a specified bias voltage. At the optimal tip–sample gap, the CPD voltage, derived from force gradient spectra, agrees with the expected built-in potential across the p–n junction. The CPD voltage showed a standard deviation of ∼30 mV on atomically flat terraces. Larger fluctuations were ascribed to structural and charge variations on the oxidized surfaces.

Preliminary indications from atomic force microscopy of the presence of rapidly-formed nanoscale films on aquifer material surfaces

The objective of this study was to determine if there is a nanoscale surface film on aquifer-like materials exposed to deep groundwaters, as has previously been found on surfaces exposed to surface and soil waters. Such surface films will modify surface properties that are so important in determining the mobility of many groundwater pollutants. Muscovite mica was used because a) it is a good analogue for the main sorbing phases of many clastic aquifers and b) its cleavage planes are atomically flat allowing high resolution imaging. Freshly-cleaved muscovite plates were exposed to groundwater from a sandstone aquifer for 30 min, and surface properties (morphology, coverage, roughness and tip–substrate force interactions) were measured using atomic force microscopy (AFM). A patchy surface film of several nanometres in depth, incorporating larger separate particles, was found on the mica surface. This film was associated with significantly increased roughness values and AFM probe–sample interaction forces compared with pure water and inorganic (synthetic groundwater) solution controls. Although the results reported are preliminary in nature, if confirmed, such films are likely to affect sorption reactions, surface-facilitated redox interactions, non-aqueous phase liquid wetting angles, and colloid–pathogen–rock attachment, and will thus be of importance in understanding natural attenuation and migration of dissolved, non-aqueous and particulate phases in groundwaters.