Tapping Mode Research Articles

Liquid-Phase Peak Force Infrared Microscopy for Chemical Nanoimaging and Spectroscopy.

Peak force infrared (PFIR) microscopy is an emerging atomic force microscopy that bypasses Abbe's diffraction limit in achieving chemical nanoimaging and spectroscopy. The PFIR microscopy mechanically detects the infrared photothermal responses in the dynamic tip-sample contact of peak force tapping mode and has been applied for a variety of samples, ranging from soft matters, photovoltaic heterojunctions, to polaritonic materials under the air conditions. In this article, we develop and demonstrate the PFIR microscopy in the liquid phase for soft matters and biological samples. With the capability of controlling fluid compositions on demand, the liquid-phase peak force infrared (LiPFIR) microscopy enables in situ tracking of the polymer surface reorganization in fluids and detecting the product of click chemical reaction in the aqueous phase. Both broadband spectroscopy and infrared imaging with ∼10 nm spatial resolution are benchmarked in the fluid phase, together with complementary mechanical information. We also demonstrate the LiPFIR microscopy on revealing the chemical composition of a budding site of yeast cell wall particles in water as an application on biological structures. The label-free, nondestructive chemical nanoimaging and spectroscopic capabilities of the LiPFIR microscopy will facilitate the investigations of soft matters and their transformations at the solid/liquid interface.

A dynamical approach to topography estimation in atomic force microscopy based on smooth orthogonal decomposition

Atomic force microscope (AFM) is one of the most versatile and powerful devices capable of producing high-resolution images of nanomaterial. Many researchers are widely investigating to improve the scanning speed and image quality of AFM by proposing different techniques. Here, we aim to present a novel approach based on the smooth orthogonal decomposition for the estimation of the surface topography in AFM. The technique proposed in this research not only eliminates the need for a closed-loop controller but also acquires the surface three-dimensional shape (topography) very quickly and accurately. The proposed technique relies on the fact that in the tapping mode of atomic force microscopy, the tip displacements are very fast compared to the topography changes, and the surface topography as a slowly varying parameter can be estimated using the smooth orthogonal decomposition algorithm. To this aim, the state space is reconstructed based on Takens’ theorem and used only the tip displacement measurement data. According to Takens’ theorem, using the delay time and embedding dimension parameters, we are able to create a system in which dynamical behaviors are similar to the original system. The results demonstrate that the proposed estimation approach is robust to noise and does not require large data or computational resources to be implemented. Also, the performance of the proposed method is appropriate for any type of force interactions between the tip and sample.

Tapping modes in the Atomic Force Microscope model with Lennard-Jones force and slow-fast base motion

Abstract Based on the idea of slow-fast dynamics, two tapping modes are investigated in the Atomic Force Microscope (AFM) model with Lennard-Jones force and slow-fast base motion. According to the approximation derived from Taylor expansion in the vicinity of static equilibrium of the linearized system, we give a prediction about harmonic responses of the fast sub-system (FSS). Then a truncated version of harmonic balancing method (HBM) is combined with the Floquet theory to give the distribution and stability of the harmonic responses, which indicates the contact and non-contact modes dominated by cycle manifolds. By directly integrating the FSS, the numerical bifurcation diagrams and the frequency spectrums corresponding to the periodic solutions are obtained to verify the results given by the half-analytic process. Moreover, distribution of basins of attraction shows that, when the tri-stability is destroyed by fold bifurcation of cycle, due to the highly fragmented fractal structures in basins, transition between different mode manifolds may present an weak irregular way. On the other hand, when the slow excitation is introduced, the regular and irregular “cycle-cycle” type mixed mode oscillations (MMOs) triggered by “fold of cycle/fold of cycle” hysteresis can be observed, which results in two tapping mode, .i.e., the slow tapping mode and the “compound of contact/snap tapping” mode. Besides that, our results show that, for the AFM under slow and near-resonant excitation, the beneficial effects of constant contact mode and snap tapping mode can be utilized.

Usefulness of electronic stimulation in restless legs syndrome: a pilot study

Purpose This study aims to investigate the effect of electronic stimulation (ES) as a non-pharmacological treatment in restless legs syndrome (RLS). Methods This is a randomized, single-blind study. A total of 46 patients were included, consisting of an active group and a sham group with 22 and 24 members, respectively. The stimulation was administered to bilateral lower legs using the tapping mode (3 Hz) on a handheld ES device, and symptom changes were measured in both groups. The effects of the stimuli were analyzed with repeated measures ANOVA. Results The symptom severity was significantly reduced in the active group, and showed significant interaction effects in the time * group (F = 4.441, p = 0.031). Although both the active and sham groups reported improved symptoms upon receiving longer periods of treatment, the effect of the ES was greater in the active group. Conclusions ES treatment resulted in symptom improvement when using ideal levels of stimulation intensity. ES can be considered as a non-pharmacological treatment option for RLS.

Optical Micro/Nanofiber-Enabled Compact Tactile Sensor for Hardness Discrimination.

Optical micro/nanofibers (MNFs) can be applied for ultrasensitive tactile sensing with fast response and compact size, which are attractive for restoring tactile information in minimally invasive robotic surgery and tissue palpation. Herein, we present a compact tactile sensor (CTS) with a diameter of 1.5 mm enabled by an optical MNF. The CTS provides continuous readouts for high-fidelity transduction of touch and pressure stimuli into interpretable optical signals, which permit instantaneous sensing of contact and pressure with pressure-sensing sensitivity as high as 0.108 mN-1 and a resolution of 0.031 mN. Working in pressing mode, the CTS can discriminate the difference in the hardness of two poly(dimethylsiloxane) (PDMS) slats (with shore A of 36 and 40) directly, a hardness resolving ability even beyond the human hands. Benefitting from the fast response feature, the CTS can also be operated in either scanning or tapping mode, making it feasible for hardness identification by analyzing the shape of the response curve. As a proof of concept, the hardness discrimination of a pork liver and an adductor muscle was experimentally demonstrated. Such MNF-enabled compact tactile sensors may pave the way for hardness sensing in tissue palpation, surgical robotics, and object identification.

Atomic force microscopy studies of enamel, inner enamel, dentin, and cementum in canine teeth

The main goal of the present work is to explore the three dimensional (3-D) atomic force microscopy (AFM) images of human teeth and investigating their micromorphology. For this purpose, 10 fresh and permanent canine teeth were selected from a group of 40-year-old men who were candidate for the experimental processes. Afterward, they were all applied for studying the morphology of their hard tissues. The tapping mode of AFM was used to characterize the surface micromorphology on the square areas of 1μm × 1μm (512 × 512 pts). AFM results and surface stereometric analysis indicate the relationships between the micromorphology of the surface and the structural properties of these tissues across the length scales. As can be seen, the surface of cementum has the most irregular topography (D=2.87 ± 0.01) while the most regular topography (D=2.43 ± 0.01) is found in dentin. Furthermore, the more and less regularity of the surface have been found in inner enamel (Sq=26.26 nm) and dentin (Sq=41.28 nm), respectively. Stereometric and fractal analyses give valuable information about human canine teeth via 3-D micromorphology.

Periodic Solutions of a Tapping Mode Cantilever in an Atomic Force Microscope with Harmonic Excitation

Periodic Solutions of a Tapping Mode Cantilever in an Atomic Force Microscope with Harmonic Excitation

Discrimination of adhesion and viscoelasticity from nanoscale maps of polymer surfaces using bimodal atomic force microscopy.

The simultaneous excitation and measurement of two eigenmodes in bimodal atomic force microscopy (AFM) during sub-micron scale surface imaging augments the number of observables at each pixel of the image compared to the normal tapping mode. However, a comprehensive connection between the bimodal AFM observables and the surface adhesive and viscoelastic properties of polymer samples remains elusive. To address this gap, we first propose an algorithm that systematically accommodates surface forces and linearly viscoelastic three-dimensional deformation computed via Attard's model into the bimodal AFM framework. The proposed algorithm simultaneously satisfies the amplitude reduction formulas for both resonant eigenmodes and enables the rigorous prediction and interpretation of bimodal AFM observables with a first-principles approach. We used the proposed algorithm to predict the dependence of bimodal AFM observables on local adhesion and standard linear solid (SLS) constitutive parameters as well as operating conditions. Secondly, we present an inverse method to quantitatively predict the local adhesion and SLS viscoelastic parameters from bimodal AFM data acquired on a heterogeneous sample. We demonstrate the method experimentally using bimodal AFM on polystyrene-low density polyethylene (PS-LDPE) polymer blend. This inverse method enables the quantitative discrimination of adhesion and viscoelastic properties from bimodal AFM maps of such samples and opens the door for advanced computational interaction models to be used to quantify local nanomechanical properties of adhesive, viscoelastic materials using bimodal AFM.

Physical, rheological and morphological characterization of modified asphalt binder with differing crumb rubber contents

Waste crumb rubber (CR) which is obtained from scrap tires can be successfully used as an asphalt binder modifier for the purpose of attaining the economic benefits with a desirable performance. The wet process was used in this study to investigate the rheological, morphological, and physical changes that would occurred to asphalt binder as a result of the interaction with different rubber contents, that is, 5%, 7%, 10%, 12% and 25%. The penetration, viscosity, softening point, ductility, Superpave performance grades, multiple stress creep recovery (MSCR) of the prepared modified binders were measured under different ageing conditions and temperature ranges. Furthermore, atomic force microscopy (AFM) scanning technique was also conducted in tapping mode to characterize the microstructure of these binders. The obtained results showed that as the rubber content increased, the viscosity and softening point values increased with a reduction in the penetration and ductility values. Rubber content of 10% was ranked as the higher percentage of rubber in terms of bumping the performance grade by at least two grades at high testing temperatures. The grading performance tests based on rutting index (G*/sin δ°) and non-recoverable creep compliance index (Jnr) showed a comparable result for ranking the stiffer binder with a greater resistance to rutting. However, most of crumb rubber modified binders failed to meet the stress sensitivity criteria of MSCR test. AFM images can be considered as a new effective and suitable tool that should be adopted for further microstructural analysis.

Stability and collapse of amphiphilic copolymer aggregates in contact with hydrophilic mica surfaces

Amphiphilic copolymers with a main chain containing polyethylene glycol (PEG) and side chains of different molecular weight and hydrophobicity form spherical aggregates (micelles) in aqueous solution. The suitability of these for incorporation and delivery of small hydrophobic molecules depends strongly on their stability in solution and on their structural changes in contact with surfaces. Atomic force microscopy (AFM) in tapping mode in liquid was used to characterize adsorbed aggregates of a series of copolymers on a model hydrophilic surface (mica). Adsorbed aggregates did not retain the core-shell character they had in solution, but opened up completely to form a layer only 0.5−1 nm thick which corresponds to one or two cross-sectional diameters of individual PEG chains. A good correlation was found between the number of polymer chains in individual adsorbed aggregates as determined with AFM and that known from light scattering studies on the corresponding structures in solution, indicating that each aggregate on the surface originated from one aggregate in solution. Adsorption could be reduced with a more stable hydrophobic core and some copolymers did not adsorb on mica within the time studied.

Total Internal Reflection Peak Force Infrared Microscopy.

Total internal reflection (TIR) infrared spectroscopy is a convenient measurement tool for collecting spectra for chemical identification. However, TIR infrared microscopy lacks high spatial resolution due to the optical diffraction limit and difficulty to preserve a high-quality wave front for focus. In this article, we present the peak force infrared microscopy in the TIR geometry to achieve a 10 nm spatial resolution. Instead of optical detection, photothermal responses of the sample are collected in the peak force tapping mode of atomic force microscopy. We demonstrate the technique on two representative samples: structured polymers for soft matters and a hexagonal boron nitride flake for two-dimensional materials. As an extension of the apparatus, we also demonstrate nanoinfrared imaging with the TIR excitation for photoinduced force microscopy. The combination of TIR geometry with nanoinfrared microscopies simplifies the optical alignment, providing alternative instrument-designing principles for atomic force microscopy-based infrared microscopy.

AFM Research in Catalysis and Medicine

Background: In this study, we show that the AFM method not only allows monitoring the morphological changes in biological structures fixed on the surface due to H-bonds, but also makes it possible to study the self-organization of metal complexes by simulating the active center of enzymes due to intermolecular H-bonds into stable nanostructures; the sizes of which are much smaller than the studied biological objects. The possible role of intermolecular hydrogen bonds in the formation of stable supramolecular metal complexes, which are effective catalysts for the oxidation of alkyl arenes to hydroperoxides by molecular oxygen and mimic the selective active sites of enzymes, was first studied by AFM. Methods and Results: The formation of supramolecular structures due to intermolecular hydrogen bonds and, possibly, other non-covalent interactions, based on homogenous catalysts and models of active centers enzymes, heteroligand nickel and iron complexes, was proven by AFM-technique. AFM studies of supramolecular structures were carried out using NSG30 cantilever with a radius of curvature of 2 nm, in the tapping mode. To form nanostructures on the surface of a hydrophobic, chemically modified silicon surface as a substrate, the sample was prepared using a spin-coating process from solutions of the nickel and iron complexes. The composition and the structure of the complex Ni2(acac)(OAc)3·NMP·2H2O were determined in earlier works using various methods: mass spectrometry, UV- and IR-spectroscopy, elemental analysis, and polarography. Self-assembly of supramolecular structures is due to intermolecular interactions with a certain coordination of these interactions, which may be a consequence of the properties of the components themselves, the participation of hydrogen bonds and other non-covalent interactions, as well as the balance of the interaction of these components with the surface. Using AFM, approaches have been developed for fixing on the surface and quantifying parameters of cells. Conclusion: This study summarizes the authors' achievements in using the atomic force microscopy (AFM) method to study the role of intermolecular hydrogen bonds (and other non-covalent interactions) and supramolecular structures in the mechanisms of catalysis. The data obtained from AFM based on nickel and iron complexes, which are effective catalysts and models of active sites of enzymes, indicate a high probability of the formation of supramolecular structures in real conditions of catalytic oxidation, and can bring us closer to understanding enzymes activity. With a sensitive AFM method, it is possible to observe the self-organization of model systems into stable nanostructures due to H-bonds and possibly other non-covalent interactions, which can be considered as a step towards modeling the active sites of enzymes. Methodical approaches of atomic force microscopy for the study of morphological changes of cells have been developed.

In-Situ Preparation of GaN Sacrificial Layers on Sapphire Substrate in MOVPE Reactor for Self-Separation of the Overgrown GaN Crystal

GaN layers on sapphire substrates were prepared by using metal organic vapor phase epitaxy (MOVPE) combined with an in-situ H2 etching process for the purpose of later self-separation of thick GaN crystals produced by hydride vapor phase epitaxy (HVPE) on such substrates. The etching process results in deep pits and long voids that formed on the surface and along the lower interface between GaN and sapphire, respectively. The pits, which were investigated by SEM analysis, can be modified in their aspect ratio and density by controlling the etching parameters. Using a proper set of in-situ etching parameters, a seed layer with internal voids can be prepared, which is suitable for HVPE overgrowth and the self-separation process. The quality of the in-situ-etched seed GaN layer and overgrown GaN crystal were characterized by X-ray diffraction (XRD) and defect selective etching (DSE). With the aid of atomic force microscopy (AFM) in tapping mode, the interface morphology of the separated GaN crystal was analyzed. The crystal quality of the separated HVPE-GaN crystal is comparable to the crystal grown on untreated GaN MOVPE-seed, which did not separate from the sapphire substrate. The introduced technique to promote the crystal separation during the HVPE process has no obvious drawback on the quality of the grown GaN crystals. Using this technique, the self-separation occurs more gently due to a weakened interface between GaN/sapphire. The conventional separation from an untreated seed by pure thermomechanical action results in higher mechanical forces on the crystal and consequently much higher risk of crystal breakage.

(Invited) Investigation of Corrosion Inhibitor Adsorption on Mild Steel By Electrochemical Atomic Force Microscopy

Mild steel has wide application in oil and gas industry due to its excellent mechanical properties and low costs. However, it is susceptible to corrosion attack in a typical water-containing service environment. Organic corrosion inhibitors can adsorb on steel surfaces to form protective layers even when added in a small amount. As conceptualized in the literature, the decrease in corrosion rates due to addition of inhibitor should be directly related to their surface coverage. However, studies have so far not provided any direct evidence of such a link, primarily due to limitations of conventional electrochemical analysis that have been used to investigate them. In the present work, a specially designed holder for electrochemical atomic force microscopy (EC-AFM) has been used to study the influence of a tetra-decyl-dimethyl-benzyl-ammonium inhibitor model compound on mitigation of corrosion. AFM results shows that when the bulk solution inhibitor concentration was at 0.5 CMC, partial coverage of the surface by the inhibitor film and patchy corrosion (which could lead to localized attack) of the UNS G1018 steel surface were observed by contact mode AFM imaging. When inhibitor bulk concentration was above the surface saturation value (1 CMC), full surface protection was indicated by the almost unchanged AFM topography images from 10 minutes to 4 hours exposure to the inhibited solution. The LPR results obtained with EC-AFM showed that the corrosion rates of this steel decreased as the bulk solution concentration of inhibitor in separate tests was increased from 0, 0.5 CMC to 1 CMC, while further increase of concentrations (from 1 CMC to 2 CMC) didn’t lead to any apparent reduction of general corrosion rate. Although the LPR corrosion rates at 1 CMC and 2 CMC were extremely low (0.05 - 0.1 mm/year), the values didn’t reduce to 0 under these full coverage conditions. Additional AFM tapping mode imaging results on mica indicated the formation of a porous film (a layer with ‘holes’) which is used to explain the types of adsorption layers possible and their probable influence on corrosion.

Squeeze Film Damping Effect on Different Microcantilever Probes in Tapping Mode Atomic Force Microscope.

During the operation of tapping mode atomic force microscope (TM-AFM), the gap between the cantilever and sample surface is very small (several nanometers to micrometers). Owing to the small gap distance and high vibration frequency, squeeze film force should be considered in TM-AFM. To explore the mechanism of squeeze film damping in TM-AFM, three theoretical microcantilever simplified models are discussed innovatively herein: tip probe, ball probe, and tipless probe. Experiments and simulations are performed to validate the theoretical models. It is of great significance to improve the image quality of atomic force microscope.

Lead-Free Bi3.15Nd0.85Ti3O12 Nanoplates Filler-Elastomeric Polymer Composite Films for Flexible Piezoelectric Energy Harvesting.

Nowadays, wearable and flexible nanogenerators are of great importance for portable personal electronics. A flexible piezoelectric energy harvester (f-PEH) based on Bi3.15Nd0.85Ti3O12 single crystalline nanoplates (BNdT NPs) and polydimethylsiloxane (PDMS) elastomeric polymer was fabricated, and high piezoelectric energy harvesting performance was achieved. The piezoelectric output performance is highly dependent on the mass ratio of the BNdT NPs in the PDMS matrix. The as-prepared f-PEH with 12.5 wt% BNdT NPs presents the highest output voltage of 10 V, a peak-peak short-circuit current of 1 μA, and a power of 1.92 μW under tapping mode of 6.5 N at 2.7 Hz, which can light up four commercial light emitting diodes without the energy storage process. The f-PEHs can be used to harvest daily life energy and generate a voltage of 2–6 V in harvesting the mechanical energy of mouse clicking or foot stepping. These results demonstrate the potential application of the lead-free BNdT NPs based f-PEHs in powering wearable electronics

Microstructural, chemical and rheological evaluation on oxidative aging effect of SBS polymer modified asphalt

With employment of Atomic Force Microscope (AFM), more knowledge on asphalt surface and mechanical property at micro-scale could be obtained. Yet, aging effect on styrene–butadiene-styrene polymer modified asphalt (SBSMA) has not been well investigated with AFM. In this study, two SBSMAs were prepared by two SBS polymers and one base binder, and then all samples were subjected to Thin Film Oven Test (TFOT), 20-hour Pressure Aging Vessel (PAV), and 40-hour PAV aging. Topographical and phase images of samples at each aging condition were obtained using tapping mode of AFM, and two indices of area ratio of bee structures and surface roughness were obtained to present microstructural evolution as aging severity. Meanwhile, micro-mechanical property was characterized in terms of Young’s modulus and Elastic Index as well as Adhesion, obtained from AFM scanning at contact mode. Additionally, chemical and macro-rheological change were evaluated by ATR-FTIR and DSR tests, including temperature sweep, multiple stress creep recovery (MSCR) and linear amplitude sweep (LAS) tests. Results highlight that the introduction of SBS can make a difference in terms of micro-structure and macro-property, as compared with the base binder. The bee structures and surface roughness of SBSMAs and base binder showed a different behavior trend during aging process, while aging hardening could be observed in both micro- and macro- scale. Also, the adhesion evolution as aging can also be distinguished, with and without SBS polymer. ATR-FTIR results show that the long-term aging (PAV aging) contributes the most of SBS polymer degradation and asphalt binder oxidation during the whole aging process. Moreover, the certain microstructural, micromechanical and rheological properties are well correlated with the chemical change. This study aims to provide insights to better understand aging behavior of SBSMAs from multi-scale aspect.

Influence of squeeze film damping on quality factor in tapping mode atomic force microscope

The micro-cantilever is one of the crucial components of atomic force microscopy in tapping mode (TM-AFM). Therefore, a thorough dynamic analysis of cantilevers is very important to further enhancing the performances of TM-AFM. In general, the quality factor can be used to characterize the energy dissipation of the micro-cantilever. In order to examine the mechanism of squeeze film damping on the vibration energy dissipation of micro- cantilevers, three types of micro-cantilevers with different tips, i.e. (1) tipless; (2) spherical; and (3) conical tips, were innovatively designed in this paper. The theoretical models of the three types of probes are established, and the simulation results of pressure and quality factor are given. The quality factor varying with the tip-sample distance is obtained by tune experiment method. Through the comparison of theory and experiment, the shortcomings of the theoretical model are analyzed and the contribution of the conical tip to the squeeze film damping is explored. This study is of great significance to understand the cantilever dynamics in the air at the micro-nano scale.

AFM imaging of extracellular ice nucleators.

Ice nucleators are substances that can initiate nucleation of pure water at and above -10 °C.Some plant pathogens possess a gene which encodes for a protein that acts as an ice nucleator, activity of which is enhanced when it is combined with the sugar and lipid components from the cell membrane. This matter retains its ice nucleation activity even after it is detached from the cell wall, and is termed extracellular ice nucleator (ECIN). In this paper, surface morphology of ECINs was investigated using atomic force microscopy (AFM) in tapping mode. ECINs were immobilized onto polyelectrolyte multilayers using the layer by layer deposition method. Effect of layer build-up, method of ECIN production, and polyelectrolytes used for multilayer fabrication were investigated. Globular and rod-like structures were observed on the AFM images of the nano-thin ECIN layers. Activity of ECINs, tested in food solutions in earlier studies, was retained when applied as a nano-thin layer onto a silicon wafer surface. Protein aggregate sizes decreased when higher centrifugation speeds were applied, and ice nucleation activity also decreased. Nucleation occurred faster and at higher temperatures when substrates were immersed in solutions of higher ECIN concentration, whereas number of bilayers formed did not have a significant effect. Higher concentration ECIN dipping solutions also led to the formation of thicker and denser ECIN layers as observed via AFM imaging. PRACTICAL APPLICATION: This study demonstrates the properties of nano-thin ECIN layers, which can crystallize pure water into ice at higher temperatures and in shorter time. Accelerating ice nucleation can potentially be utilized to freeze liquids in shorter time hence using less energy, or improve frozen foods' stability against the risk of cold chain breakage.

Multiparametric analytical quantification of materials at nanoscale in tapping force microscopy

Atomic force microscopy (AFM) is a powerful technique for accurate, reliable and non-destructive imaging and characterization of materials at the nanoscale. Among the numerous AFM methods, amplitude modulation or tapping mode AFM (AM-AFM) is an established method for imaging and characterization for most commercial AFM systems. Despite its high spatial resolution and sensitivity, quantitative characterization by AM-AFM lag behind other advanced AFM methods as far as quantification of materials properties is concerned. In this paper a fully analytical multiparametric approach for AM-AFM is proposed which simultaneously quantifies the Hamaker constant and viscoelastic properties of materials. The main advantage of the proposed method lies in the inclusion of adhesion to calculate viscoelasticity, which makes it superior to the current equations used in the AFM community. The accuracy of the proposed method is validated by several simulations and experiments and comparison with nanoindentation results, which strongly support its candidacy as a method of choice for material properties quantification by dynamic AFM.