AFM Cantilever Research Articles

Modeling and Simulation of Non-Contact Base Excited AFM

AFM has some unique properties such as higher spatial resolution, mapping even a single molecule, simple sample preparation, scanning in different types of medium, and can obtain a 3D scan of the sample surface. Therefore, with the help of AFM, one can obtain a unique understanding of the structure and functional behavior of materials. In this article, to construct the mathematical model of the base excited AFM cantilever mass spring system (lumped parameter model) is used and the solution obtained by the method of Multiple scales. Here, in this work, we consider the AFM operates in the non-contact mode. To study the effect of the non linearity, amplitude of excitation, and damping coefficient, frequency response equation obtained.

Investigation of the influence of the geometric parameters of AFM cantilevers on the resonant frequency of their oscillations

This paper presents the results of theoretical and experimental studies aimed at studying the influence of the basic geometric parameters of micromechanical cantilevers on the resonant frequency of their oscillations. The dependences of the oscillation frequency on the length of the cantilever beam, obtained based on mathematical modeling are theoretically investigated by the finite element method. In experimental studies that used AFM cantilever beams, the beams were shortened stepwise by the method of local ion beam etching by focused ion beam (FIB) Ga+. After each step, the change in the resonant frequency of oscillations was monitored. It was found that the application of the FIB method allows you to accurately change the geometric parameters of the probe beams, which, in turn, allows you to change the resonant frequency of cantilever oscillations with high accuracy. During the analysis of the obtained dependences of the resonant frequency of oscillations on the geometric parameters of the beams, it was found that a decrease in the length of the cantilever from 110 to 80 μm leads to an approximately twofold increase in the resonant frequency of oscillations of the cantilever from 320 to 620 kHz. The obtained graphs of the theoretical and experimental dependencies are compared, which showed a good correlation of the results.

Comparison of Different Excitation Schemes in Bimodal Atomic Force Microscopy in Air and Liquid Environments

Bimodal amplitude modulation atomic force microscopy (AM-AFM) is widely used in nanoscale topography and compositional contrast imaging for various materials. In this work, we use computational simulation to compare the dynamic behaviors of AFM cantilevers in three commonly used excitation schemes in bimodal AM-AFM, i.e., the cantilever base excitation, the cantilever end excitation, and the uniform excitation along the length of the cantilever, in both air and liquid environments. The amplitude and phase spectroscopy curves and the frequency responses acquired from the three excitation schemes are compared and discussed. The results would be useful in guiding the selection of excitation methods and the optimization of imaging conditions.

Traceable Lateral Force Calibration (TLFC) for Atomic Force Microscopy

Efforts to reliably measure AFM lateral forces have been impeded by the difficulties in obtaining appropriate calibration standards, applying those force standards to the apex of the tip, and quantifying calibration uncertainty. Here we propose a new method, Traceable Lateral Force Calibration (TLFC), which combines the reliability of direct methods with the convenience of indirect/semi-direct methods. Like other direct methods, ours comprise three essential steps: (1) fabrication of a spring (the Traceable Reference Lever or TRL); (2) calibration of the TRL spring constant; (3) conversion of measurable TRL deflections into absolute lateral force measurements based on its pre-calibrated spring constant (TLFC method). The TRL device, a simple two-axis cantilever, is easy to design, fabricate, and directly pre-calibrate with a standard laboratory microbalance. Following pre-calibration, the TRL device becomes a convenient absolute standard for AFM lateral force measurements. This paper describes the complete method and demonstrates its primary merits, which include (1) traceability to measurement standards; (2) ease of use by outside user groups; (3) absolute measurement errors 1 N/m normal stiffness); (4) robustness over a wide range of common loads, instruments, probes, and environments. While the method and proof-of-concept devices described in this paper were designed primarily for moderate to high load cantilevers (> 1 N/m), we discuss how a next generation of compliant TRL devices can be used with the TLFC method to reliably calibrate arbitrary AFM cantilevers (< < 1 N/m) and forces.

Hypothetical Efficiency of Electrical to Mechanical Energy Transfer during Individual Stochastic Molecular Switching Events.

There are now many examples of single molecule rotors, motors, and switches in the literature that, when driven by photons, electrons, or chemical reactions, exhibit well-defined motions. As a step toward using these single molecule devices to perform useful functions, one must understand how they interact with their environment and quantify their ability to perform work on it. Using a single molecule rotary switch, we examine the transfer of electrical energy, delivered via electron tunneling, to mechanical motion and measure the forces the switch experiences with a noncontact q-plus atomic force microscope. Action spectra reveal that the molecular switch has two stable states and can be excited resonantly between them at a bias of 100 mV via a one-electron inelastic tunneling process which corresponds to an energy input of 16 zJ. While the electrically induced switching events are stochastic and no net work is done on the cantilever, by measuring the forces between the molecular switch and the AFM cantilever, we can derive the maximum hypothetical work the switch could perform during a single switching event, which is ∼55 meV, equal to 8.9 zJ, which translates to a hypothetical efficiency of ∼55% per individual inelastic tunneling electron-induced switching event. When considering the total electrical energy input, this drops to 1 × 10-7% due to elastic tunneling events that dominate the tunneling current. However, this approach constitutes a general method for quantifying and comparing the energy input and output of molecular-mechanical devices.

Frictional Properties of the Wax Coverings in Nepenthes alata Slippery Zone: Results from AFM Scanning

Slippery zone of Nepenthes alata presents anisotropic friction behaviors depending on its evolved structures. Studies have demonstrated the contribution of lunate cells, but the role of wax coverings has not been specially investigated. In this paper, we showed findings obtained from AFM scanning and SEM observation on characterizing the frictional properties of wax coverings. Friction data generated by the AFM cantilever on wax coverings along downward and upward directions were different. The difference was insignificant (2.56%) when applying the load force of 300 nN, but increased (4.83-8.40%) when applying the load force of 500-1500 nN. The scanned wax coverings showed no detectable damage when applying the load force of 300 nN, whereas observable damage was presented when the load force exceeded 500 nN. When the load force increased to 1500 nN, the wax coverings were destroyed completely. The results suggest the wax coverings depend on their surface morphology and structural characteristic to generate different frictional properties in different directions, but the difference was inconspicuous to declare the surface anisotropy. This study is helpful for further understanding the anisotropic properties of slippery zone, and motivates the slippery zone to be a bionic prototype for designing anisotropic surfaces.

In-situ TEM Coupled with AFM Cantilever for Direct Observation of Li Dendrite Nucleation and Growth Under Stress

In-situ TEM Coupled with AFM Cantilever for Direct Observation of Li Dendrite Nucleation and Growth Under Stress

A MEMS nanoindenter with an integrated AFM cantilever gripper for nanomechanical characterization of compliant materials

This work presents the development of a MEMS nanoindenter that uses exchangeable AFM probes for quasi-static nanomechanical characterization of compliant and ultra-compliant materials. While the electrostatic micro-force transducer of the MEMS nanoindenter provides a maximum indentation depth up to 9.5 µm with a maximum output force of 600 µN, experimental investigations reveal that it can achieve a depth and force resolution better than 4 pm Hz−1/2 and 0.3 nN Hz−1/2, in air for f≥ 1 Hz. A passive AFM probe gripper is integrated into the MEMS nanoindenter, allowing the nanoindenter to utilize various AFM probes as an indenter for material testing. A proof-of-principle experimental setup has been built to investigate the performance of the MEMS nanoindenter prototype. In proof-of-principle experiments, the prototype with a clamped diamond AFM probe successfully identified an atomic step (∼0.31 nm) within a Si < 111 > ultraflat sample using the scanning probe microscopy mode. The nanomechanical measurement capability of the MEMS nanoindenter prototype has been verified by means of measurements of reference polymer samples using a silicon AFM probe and by means of measurements of the elastic properties of a PDMS sample using a spherical diamond-coated AFM probe. Owing to its compact and low-cost but high-resolution capacitive readout system, this MEMS nanoindenter head can be further applied for in-situ quantitative nanomechanical measurements in AFMs and SEMs.

Optical excitation of atomic force microscopy cantilever for accurate spectroscopic measurements

Reliable operation of frequency modulation mode atomic force microscopy (FM-AFM) depends on a clean resonance of an AFM cantilever. It is recognized that the spurious mechanical resonances which originate from various mechanical components in the microscope body are excited by a piezoelectric elemen that is intended for exciting the AFM cantilever oscillation and these spurious resonance modes cause the serious undesirable signal artifacts in both frequency shift and dissipation signals. We present an experimental setup to excite only the oscillation of the AFM cantilever in a fiber-optic interferometer system using optical excitation force. While the optical excitation force is provided by a separate laser light source with a different wavelength (excitation laser : λ=1310 nm), the excitation laser light is still guided through the same single-mode optical fiber that guides the laser light (detection laser : λ=1550 nm) used for the interferometric detection of the cantilever deflection. We present the details of the instrumentation and its performance. This setup allows us to eliminate the problems associated with the spurious mechanical resonances such as the apparent dissipation signal and the inaccuracy in the resonance frequency measurement.

Understanding and Controlling Spatial Resolution, Sensitivity, and Surface Selectivity in Resonant-Mode Photothermal-Induced Resonance Spectroscopy

Photothermal-induced resonance (PTIR) is increasingly used in the measurement of infrared absorption spectra of submicrometer objects. The technique measures IR absorption spectra by relying on the photothermal effect induced by a rapid pulse of light and the excitation of the resonance spectrum of an AFM cantilever in contact with the sample. In this work, we assess the spatial resolution and depth response of PTIR in resonant mode while systematically varying the pulsing parameters of the excitation laser. We show that resolution is always much better than predicted by existing theoretical models. Higher frequency, longer pulse length, and shorter interval between pulses improve resolution, eventually providing values that are comparable to or even better than tip size. Pulsing parameters also affect the intensity of the signal and the surface selectivity in PTIR images, with higher frequencies providing increased surface selectivity. The observations confirm a difference in signal generation between resonant PTIR and other photothermal techniques that we ascribe to nonlinearity in the PTIR signal. In analogy with optical imaging, we show that PTIR takes advantage of such nonlinearity to perform photothermal measurements that are super-resolved when compared to the resolution allowed by the thermal wavelength. Finally, we show that by controlling the pulsing parameters of the laser we can devise high resolution surface sensitive measurements that do not rely on the use of optical enhancement effects.

Modeling atomic force microscopy and shell mechanical properties estimation of coated microbubbles.

We present an extensive comparison with experimental data of our theoretical/numerical model for the static response of coated microbubbles (MBs) subject to compression from an atomic force microscope (afm). The mechanics of the MB's coating is described in the context of elastic thin shell theory. The encapsulated fluid is treated as compressible/incompressible pertaining to a gas/liquid, while the thinning of the liquid film between the MB and the afm cantilever is modeled via introduction of an interaction potential and the resulting disjoining pressure. As the external force increases, the experimental force-deformation (f-d) curves of MBs covered with polymer have an initial linear response (Reissner regime), followed by a non-linear curved downwards response (Pogorelov regime) where buckling takes place. On the other hand, the f-d curve for MBs covered with lipid monolayers initially follows the Reissner regime, but buckling is bypassed to a curved upwards regime where internal gas pressure dominates. The elastic properties, namely Young's modulus and shell thickness, for MB's covered with polymer can be estimated by combining the buckling point and the slope of the Reissner regime or the slopes of Reissner and Pogorelov regimes. Comparison of the present model with afm f-d curves for polymer shows satisfactory agreement. The area dilatation and bending moduli are shown to be the appropriate independent elastic parameters of MBs covered with phospholipid monolayers and are estimated by combination of the transition from Reissner to pressure dominated regime. Simulations and experiments in this case are in excellent agreement.

Calculation of Resonant Frequencies of Silicon AFM Cantilevers

A theoretical study of the quality and the validity range of different numerical and analytical methods of calculating of the frequency shift in measurement using microcromechanical sensor is presented. This work considered a calculating method for natural frequency in comparison with experimentally measured oscillation frequencies of microcromechanical sensor immersed in air and/or viscous medium. The numerical methods for solving the equation of resonance oscillation of the console fixed on one side are considered for calculating the natural oscillation frequencies of standard AFM cantilevers.

Vibration analysis of AFM cantilever considering the surface force (Vibration model assuming the linear spring element with surface force deviation)

Vibration analysis of AFM cantilever considering the surface force (Vibration model assuming the linear spring element with surface force deviation)

Additive Manufacturing of Sub-Micron to Sub-mm Metal Structures with Hollow AFM Cantilevers.

We describe our force-controlled 3D printing method for layer-by-layer additive micromanufacturing (µAM) of metal microstructures. Hollow atomic force microscopy cantilevers are utilized to locally dispense metal ions in a standard 3-electrode electrochemical cell, enabling a confined electroplating reaction. The deflection feedback signal enables the live monitoring of the voxel growth and the consequent automation of the printing protocol in a layer-by-layer fashion for the fabrication of arbitrary-shaped geometries. In a second step, we investigated the effect of the free parameters (aperture diameter, applied pressure, and applied plating potential) on the voxel size, which enabled us to tune the voxel dimensions on-the-fly, as well as to produce objects spanning at least two orders of magnitude in each direction. As a concrete example, we printed two different replicas of Michelangelo’s David. Copper was used as metal, but the process can in principle be extended to all metals that are macroscopically electroplated in a standard way.

Functionalization of Atomic Force Microscope Cantilevers with Single-T Cells or Single-Particle for Immunological Single-Cell Force Spectroscopy.

Atomic force microscopy based single cell force spectroscopy (AFM-SCFS) is a powerful tool for studying biophysical properties of living cells. This technique allows for probing interaction strengths and dynamics on a live cell membrane, including those between cells, receptor and ligands, and alongside many other variations. It also works as a mechanism to deliver a physical or biochemical stimulus on single cells in a spatiotemporally controlled manner, thus allowing specific cell activation and subsequent cellular events to be monitored in real-time when combined with live-cell fluorescence imaging. The key step in those AFM-SCFS measurements is AFM-cantilever functionalization, or in other words, attaching a subject of interest to the cantilever. Here, we present methods to modify AFM cantilevers with a single T cell and a single polystyrene bead respectively for immunological studies. The former involves a biocompatible glue that couples single T cells to the tip of a flat cantilever in a solution, while the latter relies on an epoxy glue for single bead adhesion in the air environment. Two immunological applications associated with each cantilever modification are provided as well. The methods described here can be easily adapted to different cell types and solid particles.

The interplay between drift and electrical measurement in conduction atomic force microscopy.

In Conduction Atomic Force Microscopy (CAFM), it is sometimes required to monitor electrical data at a single location over an extended period of time. However, thermal drift of the microscope will cause the tip to move with respect to the sample and thus limit the collection of data. We investigate a method to prolong the time a tip dwells at a location by choosing the AFM cantilever to have small spring constants in the lateral directions. The basis of the approach is that the tip can only move (or slip) once the lateral forces caused by drift overcome the friction force pinning the tip to the surface. We demonstrate the effect experimentally using platinum wire tips and diamond coated tips on SiO2 and HfO2 dielectric films. Simultaneous measurement of the current flow and lateral force signals show that the onset of tip slipping correlates with the beginning of a decrease in the measured current flow, and the onset of slip is prolonged for blunt tips or cantilevers having soft lateral spring constants. The approach not only provides a way to improve the CAFM method for time dependent measurements but also assists in interpreting CAFM data in the presence of drift.

Investigating the motion modes of smooth/rough micro/nanoparticles with circular crowned roller geometry and computing the maximum force

The exact positioning of micro/nanoparticles is essential in the construction of micro/nanoscale structures. Of course, the shape of a particle and the external force application location on it influence the type of behavior exhibited by that particle during its displacement on a surface. In former works, biological and non-biological particles have been modeled as cylindrical or spherical particles, while the shape of many such particles is a combination of these two geometries. In this paper, the driving and displacement of particles with circular crowned roller geometry on a substrate by means of an AFM cantilever tip have been modeled three-dimensionally. For a more realistic modeling of actual particle conditions, the Cooper model has been developed for this geometry and the effects of surface roughness have been incorporated into the model. By using the presented model, the forces applied on particle during the manipulation process, the deformations induced in the AFM cantilever, the rotation center and the maximum force applied on particle can be computed and the motion modes of particle (sliding, rolling, rotating) at the onset of displacement can be predicted. The findings indicate that the critical forces associated with rough particles are smaller than those for smooth particles. Furthermore, the effects of particle dimensions and roughness parameters such as the radius and height of asperities and also the effect of AFM tip location on a particle on its motion modes have been studied and the suitable conditions for creating different motion modes have been discussed.

Size-dependent on vibration and flexural sensitivity of atomic force microscope

In this paper, the free vibration behaviors and flexural sensitivity of atomic force microscope cantilevers with small-scale effects are investigated. To study the small-scale effects on natural frequencies and flexural sensitivity, the consistent couple stress theory is applied. In this theory, the couple stress is assumed skew-symmetric. Unlike the classical beam theory, the new model contains a material-length-scale parameter and can capture the size effect. For this purpose, the Euler–Bernoulli beam theory is used to develop the AFM cantilever. The tip interacts with the sample that is modeled by a spring with constant of. The equation of motion is obtained through a variational formulation based on Hamilton’s principle. In addition, the analytical expressions for the natural frequency and sensitivity are also derived. At the end, some numerical results are selected to study the effects of material-length-scale parameter and dimensionless thickness on the natural frequency and flexural sensitivity.

Effect of Silver Nanoparticles on the Electrical Characteristics of Oxide/Semiconductor Heterojunctions

Electrostatic properties at the nanoscale under relevant environment is of potential use in novel electrical and optical applications that bulk materials and devices can’t provide. For instance, Nanoparticles (NPs) based structures have drawn a lot of attention for possible memory-based applications. Embedding different kind of NPs in thin dielectric oxides has shown to play a key role in memory devices like memristors. NPs based non-volatile memory devices can be a suitable candidate for the next generation memory technology, especially with the industry aggressive scaling requirements. With the aid of these NPs, one can improve the efficiency, switching time and device sizes of heterojunction-based devices. Towards this purpose, the effect of silver nanoparticles (40nm) on the electrical characteristics of oxide/semiconductor heterojunctions are investigated by using the conductive atomic force microscopy (CAFM).The CAFM allows us to bias an individual NP to investigate their electrical characteristics. For this purpose, the uniform and individual dispersion of these nanoparticles is optimized by employing the appropriate sonication, dispersion methods (spin coating and drop casting) and thermal treatment on n-type Si substrate (5 ohm-cm) as well as on alumina coated Si substrate. The size and dispersion of these nanoparticles on both substrates is confirmed by means of AFM topography and SEM micrographs. Under these optimized dispersion parameters, we prepared two sets of structures with stacking [Ag-NPs/Si] and [Al2O3(3nm)/Ag-NPs/Al2O3(1nm)/Si]. For comparison of electrical characteristics, another two sets of control structures were prepared including HF-clean bare Si and [Al2O3/Si] stack. The effect of these individual NPs on the electrical characteristics was confirmed against the control structures. For this purpose, we initially carried out the topography of above mentioned structures in tapping mode of AFM then executed the electrical characteristic (I-V) by soft engaging an individual NP using the CAFM Au coated cantilever tip. The electrical response for both control structures during the application of sweep voltage of +3V ~0 ~ -3V, showed the rectifying behavior for the positive bias. Interestingly, electrical characteristics of stacked samples with NPs appeared to have enhanced current (reversed rectification) for negative bias during the application of same voltage sweep as shown in the attached figure (Inset show AFM image of the NPs embedded between two layer of oxide).This indicates that the NPs play a key role in the current transport. The NPs can form tunneling paths connecting the Si substrate to the AFM tip, and thus a large current can be expected due to the local field enhancement between the NP and the substrate interface. The current observed with the stacking of [Al2O3/Ag-NPs/Al2O3/Si] was more significant as compare to others and can be best candidate of the memory technology. The results are rationalized by a model that include a physics based simulation which is proposed for AFM cantilever tip and nanoparticle geometries to validate the I-V characteristics.This research will contribute to miniaturization of the memory application in NPs based data storage devices. Figure 1

Surface-Induced Silica Scaling during Brackish Water Desalination: The Role of Surface Charge and Specific Chemical Groups.

Silica scaling of membranes used in reverse osmosis desalination processes is a severe problem, especially during the desalination of brackish groundwater due to high silica concentrations. This problem limits the water supply in inland arid and semiarid regions. Here, we investigated the influence of surface-exposed organic functional groups on silica precipitation and scaling. A test solution simulating the mineral content of brackish groundwater desalination brine at 75% recovery was used. The mass and chemical composition of the precipitated silica was monitored using a quartz crystal microbalance, X-ray photoelectron spectroscopy, and infrared spectroscopy, showing that surfaces with positively charged groups induced rapid silica precipitation, and the rate of silica precipitation followed the order -NH2 ∼ -N+(CH3)3 > -NH2/-COOH > -H2PO3 ∼ -OH > -COOH > -CH3. Force vs distance AFM measurements showed that the adhesion energy between a silica colloid glued to AFM cantilever and the studied surfaces increased as the surface charge changed from negative to positive. Thus, for the first time direct measurements of molecular forces and specific chemical groups that govern silica scaling during brackish water desalination is reported here. The influence of the different functional groups and the effect of the surface charge on silica precipitation that were found here can be used to design membranes that resist silica scaling in membrane-based desalination processes.