AFM Cantilever Research Articles

Combined Confocal‐Atomic‐Force Microscope Setup for Quantum Sensing Applications with Sub‐diffractional Spatial Resolution

Quantum sensors find applications ranging from material science to biophysics. Nitrogen‐vacancy (NV) center in diamond has been successfully implemented for measuring various types of signals. Optical NV center readout, routinely used in confocal microscopes, allows achieving high spatial resolution down to the diffraction limit. This work describes in detail the combined confocal‐atomic‐force microscope (confocal‐AFM), which takes advantage of sharp AFM cantilevers, and shows its possible applications for nanoscale resolution. It demonstrates the sub‐diffractional localization of NV centers with platinum‐coated cantilevers and the ability to separately address optically unresolved sensors using cantilevers with ferromagnetic coating. The presented setup exhibits a lateral resolution of 13 nm, providing a tool for nanoscale quantum sensing.

Flexural resonant frequencies of an AFM cantilever in viscoelastic surface contact mode using modified nonlocal elasticity theory

Flexural resonant frequencies of an AFM cantilever in viscoelastic surface contact mode using modified nonlocal elasticity theory

Design and Simulation of the Microcantilever Biosensor for MITF Antigen and D5 Monoclonal Antibody Interaction Finite Element Analysis, and Experimental.

Biosensors and MEMS have witnessed rapid development and enormous interest over the past decades. Constant advancement in diagnostic, medical, and chemical applications has been demonstrated in several platforms and tools. In this study, the analytical and FEA of the microcantilever used in biomolecular analyses were compared with the experimental analysis results. In this study, MITF antigen, which is a melanoma biomarker, and anti-MITF antibody (D5) were selected as biomolecules. A MEMS-type microcantilever biosensor was designed by functionalizing the AFM cantilever by utilizing the specific interaction dynamics and intermolecular binding ability between both molecules. Surface functionalization of cantilever micro biosensors was performed by using FEA. The stress that will occur as a result of the interactions between the MITF-D5 has been determined from the deviation in the resonant frequency of the cantilever. It has been found that the simulation results are supported by analytical calculations and experimental results. The fact that the results of the simulation study overlap with the experimental and mathematical results allows us to get much cheaper and faster answers compared to expensive and time-consuming experimental approaches.

Contactless manufacturing of TERS-active AFM tips by bipolar electrodeposition.

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique for nanoscale chemical imaging. However, its worldwide expansion is still limited by the challenging fabrication of cheap, robust and efficient TERS tips as optical nanosources to amplify the Raman signal. An original method based on bipolar electrodeposition is described here to prepare gold-coated AFM cantilevers used as TERS tips. This wireless method is simple to implement, cost-effective, and allows for the parallel fabrication of several TERS tips with good reproducibility of the metal thickness and a relatively long lifetime. The TERS activity was confirmed by imaging graphene oxide flakes with high spatial resolution (below 10 nm). A promising yield of 64% was achieved for the fabrication of active TERS tips. Therefore, this method could pave the way for the development of new chemical routes for the preparation of TERS tips and other plasmonic nanostructures.

Manufacture and calibration of high stiffness AFM cantilevers

Manufacture and calibration of high stiffness AFM cantilevers

(Invited) Advanced Scanning Probe Microscopy for Studying Biofilm Formation and Single Cell Entities at Electrified Interfaces

Advanced scanning probe microscopy techniques such as atomic force – scanning electrochemical microscopy (AFM-SECM) und scanning electrochemical microscopy (SECM) are highly attractive hybrid techniques for studying biomedically relevant samples down to the single entity level. Our group introduced a new type of AFM-SECM probe having a conductive colloid instead of a sharp AFM tip. As this spherical microelectrode, which is located at the end of an electrically insulated AFM cantilever can be easily modified by electrodeposition with e.g., conductive polymers and electrocatalytic layers, single cell force spectroscopy under potential control allows studying adhesion properties [1,2]. Cell adhesion is a crucial parameter not only for developing new materials serving as substrates for neural scaffolds, electrodes, and biomedical devices but also in biofilm formation. Biofilms are well-organized aggregates of bacteria able to attach to and proliferate at almost every type of solid surface resulting in increased resistance to conventional antimicrobial and antibiofouling agents [3].The first attachment of bacterials is a process highly influenced by the nature of the surface, such as hydrophobicity, chemical structure, surface charge, and presence of antimicrobials [4].In this contribution, we present the potential of various scanning probe microscopy techniques, as suitable tools to locally investigate the early stages of biofilm formation and the effects of various antibiofouling and antimicrobial systems against Escherichia coli and Pseudomonas fluorescens. For example, the properties of polydopamine, a bio-compatible polymer, are studied in respect to early stages of bacterial adhesion in dependence of surface charge density and applied potential [5]. Colloidal conductive AFM probes can also be modified electrocatalytic layers, suitable for detecting signaling molecules and at the same time allow electrochemical force spectroscopy at soft samples like biomedically relevant single entities.

On the Nonlinear Stochastic Dynamics of an Atomic Force Microscope Cantilever

AbstractAtomic force microscopy (AFM) serves characterization and actuation in nanoscale applications. We study the stochastic dynamics of an AFM cantilever under tip-sample interactions represented by the Lennard–Jones and Morse potential energy functions. In both cases, we also study the contrasting dynamic effects of additive (external) and multiplicative (internal) noise. Moreover, for multiplicative noise, we study the two sub-cases arising from the Itô and Stratonovich interpretations of stochastic integrals. In each case, we also investigate the stochastic stability of the system by tracing the time evolution of the maximal Lyapunov exponent. Additionally, we obtain stationary probability densities for the unforced dynamics using stochastic averaging.

An area-variant type MEMS capacitive sensor based on a novel bionic swallow structure for high sensitive nano-indentation measurement

In this paper, a high sensitive area-variant type MEMS capacitive sensor was proposed for nano-indentation measurement. The proposed bionic swallow structure processed a high mechanical sensitivity without influences from the load coupling effect. Six comb arrays were optimized and integrated into the proposed sensor with a novel comb sensing configuration for eliminating electrostatic interferences resulting from multiple combs and maintaining compact chip size. Based on the proposed micro-machined process, comb arrays have been fabricated with great consistency, which exhibited a static differential output of 0.0554 pF. The fabricated sensor has been characterized with rigid glass cube and AFM cantilevers, and the measured force sensitivity can reach 98.54 aF/nN for a larger measurement range of 142 µN with a linearity of 0.9996. The fabricated sensor also exhibited a high resolution of 0.9819 nN, indicating its great feasibility in nano-indentation measurement at the sub-nano-Newton level.

The molecular mechanism of load adaptation by branched actin networks.

Branched actin networks are self-assembling molecular motors that move biological membranes and drive many important cellular processes, including phagocytosis, endocytosis, and pseudopod protrusion. When confronted with opposing forces, the growth rate of these networks slows and their density increases, but the stoichiometry of key components does not change. The molecular mechanisms governing this force response are not well understood, so we used single-molecule imaging and AFM cantilever deflection to measure how applied forces affect each step in branched actin network assembly. Although load forces are observed to increase the density of growing filaments, we find that they actually decrease the rate of filament nucleation due to inhibitory interactions between actin filament ends and nucleation promoting factors. The force-induced increase in network density turns out to result from an exponential drop in the rate constant that governs filament capping. The force dependence of filament capping matches that of filament elongation and can be explained by expanding Brownian Ratchet theory to cover both processes. We tested a key prediction of this expanded theory by measuring the force-dependent activity of engineered capping protein variants and found that increasing the size of the capping protein increases its sensitivity to applied forces. In summary, we find that Brownian Ratchets underlie not only the ability of growing actin filaments to generate force but also the ability of branched actin networks to adapt their architecture to changing loads.

Adaptable Optical Microwaveguides From Mechanically Flexible Crystalline Materials.

Flexible organic crystals (elastic and plastic) are important materials for optical waveguides, tunable optoelectronic devices, and photonic integrated circuits. Here, we present highly elastic organic crystals of a Schiff base, 1-((E)-(2,5-dichlorophenylimino)methyl)naphthalen-2-ol (1), and an azine molecule, 2,4-dibromo-6-((E)-((E)-(2,6-dichlorobenzylidene)hydrazono)methyl)phenol (2). These microcrystals are highly flexible under external mechanical force, both in the macroscopic and the microscopic regimes. The mechanical flexibility of these crystals arises as a result of weak and dispersive C-H⋅⋅⋅Cl, Cl⋅⋅⋅Cl, Br⋅⋅⋅Br, and π⋅⋅⋅π stacking interactions. Singly and doubly-bent geometries were achieved from their straight shape by a micromechanical approach using the AFM cantilever tip. Crystals of molecules 1 and 2 display a bright-green and red fluorescence (FL), respectively, and selective reabsorption of a part of their FL band. Crystals 1 and 2 exhibit optical-path-dependent low loss emissions at the termini of crystal in their straight and even in extremely bent geometries. Interestingly, the excitation position-dependent optical modes appear in both linear and bent waveguides of crystals 1 and 2, confirming their light-trapping ability.

Effect of air film squeeze force and surface force on the vibration characteristics of AFM cantilever

Effect of air film squeeze force and surface force on the vibration characteristics of AFM cantilever

Supramolecular Polymerization Powered by Escherichia coli : Fabricating a Near-Infrared Photothermal Antibacterial Agent in Situ

Supramolecular Polymerization Powered by <i>Escherichia coli</i> : Fabricating a Near-Infrared Photothermal Antibacterial Agent in Situ

(Invited) In Situ TEM Nano Electrochemistry

Since the inception of the 1st open cell nanobattery study, significant progress has been made in the field of in situ nano electrochemistry. Many new techniques are emerging, which include combining transmission electron microscopy ― scanning probe microscopy (TEM-SPM) with environmental TEM (ETEM) to enable metal ― gas battery studies; combining TEM-SPM with microelectromechanical system (MEMS) heating devices to enable high temperature battery studies; using large diameter carbon nanotuble as an electrochemical reaction cell to enable in situ liquid cell studies. Our latest results on Li/Na-O2/CO2 batteries, Li/Na-S/Se batteries as well as quantitative measurements of the mechanical properties of Li and Na dendrites will be highlighted [1]. Fig. 1 shows an example of measuring the mechanical property of lithium whiskers. Progress on the in situ studies of solid state batteries will also be presented. Liqiang Zhang, Yongfu Tang, Jianyu Huang et al., Science 330, (2010) 1515; Nature Nano 15, (2020) 94; ACS Nano 14, (2020) 13232; Nano Lett. 18, (2018) 3723; Eng. Env. Sci. 4 (2011) 3844; 14(2021) 602; Adv. Mater. 1900608, 2019; ACS Energy Lett. 5, (2020) 2546; Materials Today 42, (2021) 137. Fig. 1 In-situ ETEM-AFM characterization of stress generation during Li dendrite growth. (a) Schematic of the ETEM-AFM setup used for imaging and measurement of Li dendrite growth. An arc-discharged CNT was attached to a conducting AFM, and this assembly was used as cathode; the scratched Li metal on the top of a sharp tungsten needle was used as anode, and the naturally formed Li2O on the Li surface as a solid electrolyte. (b) A TEM image showing an AFM cantilever was approaching the counter electrode of Li metal. (c) A TEM image showing a CNT was attached to a flattened AFM tip. (d) Time-lapse TEM images of the Li dendrite growth. A nano-sized Li ball nucleated from the CNT, Li2O and gas triple point (1863 s). As the Li ball grew to about 1.26 μm in size, the dendrite emerged underneath the ball (2028 s) which pushed the AFM cantilever up. When the dendrite reached 4.08 mm in length, it collapsed (2177 s) due to the axial compression by the AFM tip. The blue dotted line indicates a fixed reference position, and the red arrow indicates the upward displacement of the AFM tip. Figure 1

Micro- and Nanowear of Self-Mated Steel Generated and Studied With an AFM at the Single Asperity Level

Single asperity nanowear phenomena are fundamental for understanding basic tribological mechanisms. Yet, they are studied mostly through theoretical and simulation works. Few experiments were conducted in the past decades, usually with materials which are commonly used in micro- and nanotechnology, but not for macroscopic components with relevance in tribology. In the present work, we show for the first time tribotests performed with self-mated 100Cr6 steel, a very widespread material at the macroscale, taking advantage of an AFM, employed as a tribometer for the tribotests as well as for the inspection of wear of both tribopartners. Emphasis is put on the morphology of the scars, on wear particles, and on wear of the “colloidal” particles glued on the AFM cantilever. Measurements demonstrate the possibility of characterizing single asperity events leading to very small wear (scars with isolated, down to 1-nm-deep scratches). We highlight several phenomena, for example, transfer of wear particles and their negative contribution to wear volume, which are elementary key constituents of tribological processes. Such phenomena, probably occurring also at the macroscale, can be detected, identified, and characterized with high spatial and time resolution only at the nanoscale, thus giving insight into conditions and causes of their emergence.

Numerical modeling of strain hardening effect on an AFM cantilever undergoing stress in tapping mode

AbstractTapping mode is one of the scanning modes possible when using an AFM. In tapping mode, the cantilever is driven to oscillate up and down at or near its resonance frequency. Nano‐measurement researchers and metrologists have been concerned with errors in feature dimensions arising from bad cantilever as a result of strain hardening. Strain hardening is when a metal is strained beyond the yield point. This work involves modeling an AFM cantilever with a point load at the tip undergoing transverse vibration. The numerical work involves mapping out the stress field and identifying areas susceptible to plastic flow to advise equipment makers on how to design against strain hardening. The numerical analysis was carried out using COMSOL Multiphysics 5.0. Different dimensions of the cantilever (length = 100‐500 µm and width = 20‐50 µm) and Eigen frequencies were varied to investigate the optimal dimensions for designing an AFM cantilever against strain hardening. The numerical simulation shows that the AFM cantilever experiences increased deformation and strain hardening between a range of length 400‐500 µm and 200‐300 µm with the highest at 100‐200 µm and width of 20‐40 µm. It is also observed that the effect of strain hardening on the cantilever is minimal at lengths 300‐400 µm and widths 40‐45 µm when undergoing tapping mode with the highest Eigen frequencies.

Validity of point-mass model in off-resonance dynamic atomic force microscopy

The quantitative measurement of viscoelasticity of nano-scale entities is an important goal of nanotechnology research and there is considerable progress with advent of dynamic atomic force microscopy. The hydrodynamics of cantilever, the force sensor in AFM measurements, plays a pivotal role in quantitative estimates of nano-scale viscoelasticity. The point-mass (PM) model, wherein the AFM cantilever is approximated as a point-mass with mass-less spring is widely used in dynamic AFM analysis and its validity, particularly in liquid environments, is debated. It is suggested that the cantilever must be treated as a continuous rectangular beam to obtain accurate estimates of nano-scale viscoelasticity of materials it is probing. Here, we derived equations, which relate stiffness and damping coefficient of the material under investigation to measured parameters, by approximating cantilever as a point-mass and also considering the full geometric details. These equations are derived for both tip-excited as well as base-excited cantilevers. We have performed off-resonance dynamic atomic force spectroscopy on a single protein molecule to investigate the validity of widely used PM model. We performed measurements with AFMs equipped with different cantilever excitation methods as well as detection schemes to measure cantilever response. The data was analyzed using both, continuous beam model and the PM model. We found that both models yield same results when the experiments are performed in truly off-resonance regime with small amplitudes and the cantilever stiffness is much higher than the interaction stiffness. Our findings suggest that a simple PM approximation based model is adequate to describe the dynamics, provided care is taken while performing experiments so that the approximations used in these models are valid.

Dependence of adhesion force on contact time due to water thin-film flow revealed with an AFM cantilever at low humidity

The contact time dependence of adhesion force is not well understood. Adhesion forces between a tipless silica cantilever and some samples with different hydrophilicity were measured to study the dependence at low relative humidity (16 ± 1%). Dependent adhesion forces were observed on hydrophilic samples. The adhesion force has three distinct trends: sharply increasing at first, then followed by moderately increasing and at last remaining almost unchanged (reaching saturation). The saturation time is varied for different samples: from several seconds to tens of seconds. The more hydrophilic the sample surface is, the longer the saturation time is. The increasing tendency is well explained by a model of water thin-film flow. It takes some time for the liquid absorbed on a surface to flow into a growing water bridge due to a large viscosity. The viscosity of the film on a more hydrophilic sample may be larger because of stronger interaction between the film and the sample surface, eventually resulting in a longer saturation time. The reported results may help deepen the understanding of the mechanism of adhesion and explain other experimental results concerning AFM parameters.

Modal Actuation and Sensing With an Active AFM Cantilever

Modal actuation and sensing are implemented on a microfabricated AFM cantilever with a two-layer piezoelectric stack transducer. The top transducer is shaped as the second derivative of the first mode shape. The bottom transducer is uniformly distributed along the cantilever length. The purpose of this work is to demonstrate that modal actuation and sensing can be used to eliminate other resonances from frequency response function of the active cantilever, except the first mode. First-order mathematical formulations are presented to model transverse vibrations of the cantilevers under this study. Microfabrication steps and characterization of these cantilevers are reported. Their respective actuation and sensing gains are determined for comparison with conventional active cantilevers. Electromechanical coupling coefficients are also calculated to exhibit reciprocity of modal actuation and sensing in eliminating higher modes from the frequency response. The advantages of modal actuation and sensing in avoiding instabilities in feedback controlled AFM active cantilevers are also demonstrated.

Single-molecule mechanics of synthetic aromatic amide helices: Ultrafast and robust non-dissipative winding

Summary Because of proteins’ many degrees of conformational freedom, programming protein folding dynamics, overall elasticity, and motor functions remains an elusive objective. Instead, smaller and simpler objects, such as synthetic foldamers, may be amenable to design. However, little is known about their mechanical performance. Here, we show that reducing molecular size may not compromise mechanical properties. We report that helical aromatic oligoamides as small as 1 nm possess outstanding elasticity and outperform most natural helices. Using single-molecule force spectroscopy, we characterize their folding trajectories and intermediate states. We show that they cooperatively and reversibly unwind at high forces. They extend up to 3.8 times their original length and rewind against considerable forces on a timescale of 10 μs. Pulling and relaxing cycles follow the same trace up to a very high loading rate, indicating that the mechanical energy accumulated during the stretching does not dissipate and is immediately reusable.

Pulsed laser activated impulse response encoder (PLAIRE): sensitive evaluation of surface cellular stiffness on zebrafish embryos.

Mechanical properties of cells and tissues closely link to their architectures and physiological functions. To obtain the mechanical information of submillimeter scale small biological objects, we recently focused on the object vibration responses when excited by a femtosecond laser-induced impulsive force. These responses are monitored by the motion of an AFM cantilever placed on top of a sample. In this paper, we examined the surface cellular stiffness of zebrafish embryos based on excited vibration forms in different cytoskeletal states. The vibration responses were more sensitive to their surface cellular stiffness in comparison to the Young's modulus obtained by a conventional AFM force curve measurement.