Atomic Force Microscopy Probe Tip Research Articles

Microfabricated double-tilt apparatus for transmission electron microscope imaging of atomic force microscope probe.

Atomic force microscopy (AFM) uses a scanning stylus to directly measure the surface characteristics of a sample. Since AFM relies on nanoscale interaction between the probe and the sample, the resolution of AFM-based measurement is critically dependent on the geometry of the scanning probe tip. This geometry, therefore, can limit the development of related applications. However, AFM itself cannot be effectively used to characterize AFM probe geometry, leading researchers to rely on indirect estimates based on force measurement results. Previous reports have described sample jigs that enable the observation of AFM probe tips using Transmission Electron Microscopy (TEM). However, such setups are too tall to allow sample tilting within more modern high-resolution TEM systems, which can only tilt samples less than a few millimeters in thickness. This makes it impossible to observe atomic-scale crystallographic lattice fringes by aligning the imaging angle perfectly or to view a flat probe tip profile exactly from the side. We have developed an apparatus that can hold an AFM tip for TEM observation while remaining thin enough for tilting, thereby enabling atomic-scale tip characterization. Using this technique, we demonstrated consistent observation of AFM tip crystal structures using tilting in TEM and found that the radii of curvature of nominally identical probes taken from a single box varied widely from 1.4nm for the sharpest to 50nm for the most blunt.

Atomic Force Microscopy Cantilever-Based Nanoindentation: Mechanical Property Measurements at the Nanoscale in Air and Fluid.

An atomic force microscope (AFM) fundamentally measures the interaction between a nanoscale AFM probe tip and the sample surface. If the force applied by the probe tip and its contact area with the sample can be quantified, it is possible to determine the nanoscale mechanical properties (e.g., elastic or Young's modulus) of the surface being probed. A detailed procedure for performing quantitative AFM cantilever-based nanoindentation experiments is provided here, with representative examples of how the technique can be applied to determine the elastic moduli of a wide variety of sample types, ranging from kPa to GPa. These include live mesenchymal stem cells (MSCs) and nuclei in physiological buffer, resin-embedded dehydrated loblolly pine cross-sections, and Bakken shales of varying composition. Additionally, AFM cantilever-based nanoindentation is used to probe the rupture strength (i.e., breakthrough force) of phospholipid bilayers. Important practical considerations such as method choice and development, probe selection and calibration, region of interest identification, sample heterogeneity, feature size and aspect ratio, tip wear, surface roughness, and data analysis and measurement statistics are discussed to aid proper implementation of the technique. Finally, co-localization of AFM-derived nanomechanical maps with electron microscopy techniques that provide additional information regarding elemental composition is demonstrated.

Detection of Strong Light-Matter Interaction in a Single Nanocavity with a Thermal Transducer.

The concept of strong light-matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits the study of the light-matter interaction in single subwavelength-sized nanocavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin (∼150 nm) polymer layer with negligible absorption in the mid-infrared range (5 μm < λ < 12 μm) inside a metal-insulator-metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at λ = 8.3 μm, and the nanocavity is overall 270 nm thick. Acting as a nonperturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using atomic force microscopy (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nanoresonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of a strong light-matter interaction.

The Story behind the First Automatic Atomic Force Microscope

Abstract:Over the past three decades, atomic force microscopy (AFM) has undergone a series of design changes to attain a flat XY scan and stable non-contact mode in ambient atmosphere. AFM has evolved into an ideal method for non-destructive scanning of samples with longer probe tip life, high accuracy, repeatability, and automation. Together with self-optimizing algorithms for scan parameters in the non-contact mode, AFM can become as widely adopted as other microscopies such as optical or scanning electron microscopy (SEM). Even full automation of AFM probe tip exchange is now possible with probe type recognition software and laser beam alignment on the cantilever and position-sensitive photo diode (PSPD). By separating the optics stage from the Z stage, an AFM system can be made with low mechanical noise and improved optical vision.

Advanced mechanism of multiphysics fields tip enhancement induced with varied laser power to fabricate pattern-transformable subdiffraction limit nanostructures.

The nanofabrication platform was carried out using an atomic force microscope (AFM) system and a continuous wave (cw) laser to investigate the influence of laser power on the underlying mechanism of nanostructures fabricated by multiphysics fields tip enhancement (MFTE) induced by a cw laser irradiating the AFM probe tip. The nanostructure fabrication of nanopits and grooves and nanodots and lines on a polymethyl methacrylate thin film was conducted in an ambient environment by changing the incident laser power. The dependence of the MFTE on laser power was numerically analyzed, too. The lateral dimensions of nanopits and grooves and nanodots and lines characterized in situ were 154 nm, 96 nm, 188 nm, and 25 nm, respectively, breaking the optical diffraction limit. It turned out that the nanostructures converted from craters (pits and grooves) to protrusions (dots and lines) when altered with the laser power. Different laser powers can trigger the MFTE to change, thus, inducing varied coupling energy, which is the essential mechanism for nanostructure conversion. We also established a model to analyze the nanostructures transition and to predict the dimensions of nanostructures. The simulation results demonstrate that the MFTE has an essential effect on the formation of nanostructures, which are in good agreement with the experimental results.

Nanogenerator with direction sensitive piezoelectric response based on InGaN nanowires over ridge textured Si (1 1 0) substrate with inter-facet topography modulation

Direction sensitive piezoelectric response is demonstrated from a nanogenerator based on an InGaN nanowires array on a ridge textured Si (1 1 0) substrate with inter-facet topography modulation. Under stationary molecular beam epitaxy, distinct variations of the inclination orientation of the InGaN nanowires are created due to the different beam flux incidence angle between the facets, which act as the primary factor for the asymmetric piezoelectric output. The piezoelectric properties were first studied using an electrical atomic force microscope (AFM) in contact scan mode at zero bias. The InGaN NWs on opposite ridge facets show different piezoelectric output distribution under the same scanning geometry. The difference of the tangential force component due to the varied NWs tilting angle, applied on the NWs by the AFM probe tip, explains the non-uniform piezoelectric signal distribution. A capacitor type piezoelectric nanogenerator (PNG) device was fabricated, which exhibits pronounced direction sensitive piezoelectric response under rotated external pressure. This work gives a universal strategy on synthesis of composite semiconductor materials with anisotropic piezoelectric response in terms of a self-powered force direction sensitive angle sensor piezoelectric nanogenerator device.

Case Report: Bayesian Statistical Inference of Experimental Parameters via Biomolecular Simulations: Atomic Force Microscopy.

The atomic force microscopy (AFM) is a powerful tool for imaging structures of molecules bound on surfaces. To gain high-resolution structural information, one often superimposes structure models on the measured images. Motivated by high flexibility of biomolecules, we previously developed a flexible-fitting molecular dynamics (MD) method that allows protein structural changes upon superimposing. Since the AFM image largely depends on the AFM probe tip geometry, the fitting process requires accurate estimation of the parameters related to the tip geometry. Here, we performed a Bayesian statistical inference to estimate a tip radius of the AFM probe from a given AFM image via flexible-fitting molecular dynamics (MD) simulations. We first sampled conformations of the nucleosome that fit well the reference AFM image by the flexible-fitting with various tip radii. We then estimated an optimal tip parameter by maximizing the conditional probability density of the AFM image produced from the fitted structure.

Lymphocyte cytoskeleton reorganization caused by a short-term action of an extremely low frequency electromagnetic field (50 Hz) on the blood in vitro

Objective: to detect changes in the parameters of the structure and mechanical properties of the surface layer of lymphocytes in the blood of a rat after a single action (30 min and 2 h) of an extremely low frequency electromagnetic field (ELF EMF) on the rat`s whole blood in vitro.Material and methods. We exposed the whole blood of a Wistar rat (4.5 months) to the effect of ELF EMF (50 Hz, B = 0.79 mT), generated by an experimental setup based on a Helmholtz coil. Lymphocytes were isolated from the blood using the Histopaque-1077 density gradient sedimentation method, then were placed on the surface of glass slides and fixed with glutaraldehyde, and finally were dried. We performed atomic force microscopy (AFM) using the atomic force microscope Bioscope Resolve in the PeakForce QMN mode in air.Results. On the basis of the results of the analysis of the AFM images, we classified the lymphocytes into several morphological types depending on the size of the lamellopodium. We analyzed the mechanical properties of the surface areas sized 250 nm × 250 nm for lymphocytes with the lamellopodium sized 400–1100 nm. The analysis found an rise in the elastic modulus of the lymphocyte surface after a short-term action (30 min) of ELF EMF. When the time of the ELF EMF exposure was increased up to 2 hours, the elastic (elastic modulus) and adhesive properties (the adhesion force between the AFM probe tip and cell surface) of the lymphocytes were significantly weakened.Conclusion. For the first time using the AFM method we have detected nanoscale changes in the structure and mechanical properties of the surface of the rat`s lymphocytes after a short-term action of ELF EMF (50 Hz, 0.79 mT, 30 min), which are indicative of the activation of the functions of lymphocytes as an immune system component in response to the short-term exposure of ELF EMF.

Quantifying geometric tip-sample effects in AFM measurements using certainty graphs

Abstract One of the challenges faced in accurately measuring nanoscale surface roughness using Atomic Force Microscopy (AFM) techniques are the geometric interactions of the surface with the AFM probe-tip. What is the significance of these effects, and to what extent do they reduce the effectiveness of an AFM as a nanospatial measurement tool of surfaces with roughness? The aim of this work is to develop a framework by which an AFM user would be able to automatically [1] quantify the magnitude these effects from geometric tip-sample interactions have on measurement accuracy. In turn, the AFM user would then be able to make an informed decision on whether additional methods need to be applied to correct these measurements. To accomplish this, a numerical model is developed that can generate sets of ‘true’ stochastic surfaces of various statistical roughness parameters as well as various probe-tip shapes. The model then simulates, using such probe-tips, the scanning procedure of an AFM on such sets of surfaces to produce corresponding measurement images. The difference, due to geometric tip-sample interactions, between the ‘true’ surfaces and the measured images is quantified, over various ranges of roughness parameters for different probe-tips, with a novel certainty metric to create certainty graphs. These graphs show for a given tip, the significance of the tip effects due to surface roughness and thus the extent of measurement inaccuracy.

Role of phase transformation in possible wear mechanisms in silicon microelectromechanical-system devices

Wear on microelectromechanical-system (MEMS)/nanoelectromechanical-system (NEMS) devices is an inevitable and critical issue. It reduces the lifespan of the device and increases the failure rate. In this study, an atomic force microscope (AFM) was used to investigate wear mechanisms and to devise precautions against wear. AFM made it possible to demonstrate the operational behaviors of MEMS/NEMS silicon-based devices. It was found that the dynamic tip–sample interaction process induces phase transformation of the silicon tip apart from the conventional wear mechanisms. This weakens the mechanical strength of the tip and promotes wear. Evidence of phase transformation in probe tips during scanning was found, and the possibility of the phase-transformation path was also investigated. In addition, the distribution of stress in AFM probe tips with applied force was simulated. Worn probe tips were analyzed based on the Kikuchi patterns, and then high pressure was applied on monocrystalline silicon to determine the possible phase-transformation path. The results provided useful insights into wear mechanisms in silicon-based MEMS devices.

Visualizing molecular charging

Surface Chemistry High-resolution atomic force microscopy (AFM) has been used to control and image the charge state of organic molecules adsorbed on multilayer sodium chloride films. Fatayer et al. biased an AFM probe tip with a voltage to charge and discharge molecules such as azobenzene and porphine from cations to anions. Subsequent imaging with carbon monoxide–functionalized tips revealed changes in the conformation, bond order, and aromaticity of the organic molecules resulting from charge-state changes. Science , this issue p. [142][1] [1]: /lookup/doi/10.1126/science.aax5895

On the surface hydrophilization of a blended polysulfone membrane: atomic force microscopy measurement and molecular dynamics simulation

This study elucidates a mechanism to improve fouling resistance for a polysulfone (PSf) membrane by blending polyvinylpyrrolidone (PVP). PSf is generally hydrophobic in nature and tends to be fouled by natural organic matter (NOM) due to a hydrophobic bonding during membrane filtration. Thus, to reduce hydrophobic bonding, a hydrophilic treatment is conducted using non-solvent induced phase separation, in which PVP is added to the PSf membrane. To investigate the chemical properties of the membrane surface, a surface elemental analysis using x-ray photoelectron spectroscopy and contact angle measurement of water droplets is conducted. It is found that, with an increase in PVP additive, macroscopic hydrophilicity increases. Next, to measure the surface adsorption of hydrophobic bonding, atomic force microscopy (AFM) is used. In this study, the AFM probe tip is chemically modified with hydrophobic matter that mimics hydrophobic NOM. With this AFM tip, contact force measurements are conducted in order to measure the absorption characteristics between the membrane surface and the NOM. Furthermore, a cross-flow filtration test is carried out to measure adsorption amounts of tannic acid (as a model hydrophobic substance) into the PSf membrane. Finally, these experimental results were phenomenologically investigated using a molecular dynamics simulation to clarify the hydrophilicity mechanism and improvement of anti-fouling performance of the membrane.

The role of the probe tip material in distinguishing p- and n-type domains in bulk heterojunction solar cells by atomic force microscopy based methods

Visualization of functionally different domains in bulk heterojunction (BHJ) solar cells is of paramount importance to understand the routes of optimization of their structure for best performance. In this work, a concept of detecting n-type and p-type semiconductor domains in BHJ structures by methods based on atomic force microscopy (AFM) is proposed. It assigns an active role to the semiconducting coating of the AFM probe tip which is able to form different junctions, i.e., p-n anisotype or p+-p, n+-n isotype, with the surfaces inspected. Here, we illustrate this concept on the example of BHJ structures composed of the n-type inorganic microcrystalline semiconductor CdS and mechanochemically prepared p-type kesterite nanopowder and two types of AFM probe tip coatings, i.e., p-type boron-doped diamond and n-type nitrogen-doped diamond coating, respectively. Conductive AFM (CAFM) measurements demonstrated unequivocally the different diode behavior when contacting n- or p-type semiconductor domains in the BHJ structures. Simulation of the energy level alignment at the probe-sample interfaces allowed us to explain the formation of anisotype or isotype junctions depending on the sample domain and probe used. Kelvin probe force microscopy measurements were consistent with the CAFM results and indicated the different contact potentials from the diverse types of domains in the BHJ structure.

Tip-Enhanced Infrared Difference-Nanospectroscopy of the Proton Pump Activity of Bacteriorhodopsin in Single Purple Membrane Patches.

Photosensitive proteins embedded in the cell membrane (about 5 nm thickness) act as photoactivated proton pumps, ion gates, enzymes, or more generally, as initiators of stimuli for the cell activity. They are composed of a protein backbone and a covalently bound cofactor (e.g. the retinal chromophore in bacteriorhodopsin (BR), channelrhodopsin, and other opsins). The light-induced conformational changes of both the cofactor and the protein are at the basis of the physiological functions of photosensitive proteins. Despite the dramatic development of microscopy techniques, investigating conformational changes of proteins at the membrane monolayer level is still a big challenge. Techniques based on atomic force microscopy (AFM) can detect electric currents through protein monolayers and even molecular binding forces in single-protein molecules but not the conformational changes. For the latter, Fourier-transform infrared spectroscopy (FTIR) using difference-spectroscopy mode is typically employed, but it is performed on macroscopic liquid suspensions or thick films containing large amounts of purified photosensitive proteins. In this work, we develop AFM-assisted, tip-enhanced infrared difference-nanospectroscopy to investigate light-induced conformational changes of the bacteriorhodopsin mutant D96N in single submicrometric native purple membrane patches. We obtain a significant improvement compared with the signal-to-noise ratio of standard IR nanospectroscopy techniques by exploiting the field enhancement in the plasmonic nanogap that forms between a gold-coated AFM probe tip and an ultraflat gold surface, as further supported by electromagnetic and thermal simulations. IR difference-spectra in the 1450–1800 cm–1 range are recorded from individual patches as thin as 10 nm, with a diameter of less than 500 nm, well beyond the diffraction limit for FTIR microspectroscopy. We find clear spectroscopic evidence of a branching of the photocycle for BR molecules in direct contact with the gold surfaces, with equal amounts of proteins either following the standard proton-pump photocycle or being trapped in an intermediate state not directly contributing to light-induced proton transport. Our results are particularly relevant for BR-based optoelectronic and energy-harvesting devices, where BR molecular monolayers are put in contact with metal surfaces, and, more generally, for AFM-based IR spectroscopy studies of conformational changes of proteins embedded in intrinsically heterogeneous native cell membranes.

New insights into the contact angle and formation process of nanobubbles based on line tension and pinning

In this paper, the effect of a probe tip on the contact angle measurements and the formation process of nanobubbles based on line tension and pinning theory was investigated. In this investigation, the images of nanobubbles and micropancakes on mica(001) and molybdenite(0001) surfaces were obtained during an alcohol-water exchange by means of atomic force microscopy (AFM) in tapping mode (TMAFM). Based on the image characteristics and the theory of line tension and pinning, a new point of view is presented: the contact angle of a nanobubble is not always a large obtuse angle but is an uncertain value; its true value is affected by the curvature radius of the AFM probe tip, and the value is close to the macroscopic contact angle. Meanwhile, the formation of nanobubbles is affected by the surface tension, line tension and pinning.

Influences of planarization modification and morphological filtering by AFM probe-tip on the evaluation accuracy of fractal dimension

The scaling characteristic of surface roughness could reveal the fractal property of a surface, which is common for the thin films fabricated via physical vapor deposition. In our previous study, a roughness scaling extraction (RSE) method was proposed to accurately calculate the fractal dimension (D), which is an effective parameter to represent the irregularity and fragmental property of a fractal surface. RSE method was based on a single morphological image of the concerned surface, and was much more accurate than the traditional methods with a low mean relative error of 0.64%. In this study, RSE method was firstly optimized by using the planarization modification on the morphological images of artificial surfaces with ideal dimensions (Di) ranging from 2.1 to 2.9, which were generated through Weierstrass-Mandelbrot (W-M) function. The calculation accuracy of RSE method could be enhanced by using the second order planarization modification, with a lowest mean relative error of only 0.42% between Di and the calculated dimensions (Dc). Secondly, artificial surfaces with high resolution and Di from 2.2 to 2.8 were utilized to investigate the influence of atomic force microscopy (AFM) probe-tip geometry on the calculation accuracy. A typical geometry with a probe-tip radius (r) of 10 nm was employed to carry out the simulation of morphological filtering for the surface images, and the filtered images with various scales (L) from 0.5 μm to 7.5 μm were generated. RSE method was found to be robust only when the ratio of L/r was above 400, while a significant deviation occurred at lower ratio values, which would be instructive for the fractal analysis on the AFM images of various surface morphologies.

Diamond tool cutting edge measurement in consideration of the dilation induced by AFM probe tip

Measurement of diamond cutting edge is of great importance in ultra-precision machining. Atomic force microscope (AFM) that takes advantage of its three-dimensional (3D) imaging in nanoscale becomes an indispensable means to acquire the 3D surface morphology of the diamond tool cutting edge. However, the dilation induced by AFM probe tip in diamond tool cutting edge measurement is a key error source in cutting edge evaluation, especially for the very sharp edge. In this paper, analysis and elimination method of this dilation effect are presented based on the principle of AFM imaging in terms of mathematical morphology theory. Our work effectively improves the evaluation accuracy of diamond tool cutting edge, which is beneficial to the diamond tool manufacture and selection in ultra-precision machining.

Molecular dynamics simulation of cell membrane penetration by atomic force microscopy tip

In recent years, a delivery system based on atomic force microscopy (AFM) has been developed to transport nucleic acids, proteins and drugs to single adherent cell by controlling the indentation process. However, the transportation efficiency is limited by the low penetration rate of the common commercial AFM probe. The tip of commercial AFM probe is blunt and it is hard for blunt tip to insert into the soft cell membrane. In this study, dissipative particle dynamics (DPD) simulations were applied to investigate the penetration process of the AFM probe into the cell membrane subjected to different strain states. It was observed that the AFM tip moved down a shorter distance to penetrate the stretched lipid membrane compared with unstretched membrane. Moreover, the threshold value of penetrating force decreased as cell membrane extended. The short indentation time and small force can reduce the probability of cell membrane collapse, therefore it is easier for the AFM tip to penetrate the cell. We also performed the AFM indentation experiments via AFM to investigate the relationship between penetrating force and indentation speed. This work provides a potential way to improve the efficiency of cell transfection by using the AFM delivery system.

Atomic Force Microscopy Force Mapping Analysis of an Adsorbed Surfactant above and below the Critical Micelle Concentration.

Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.

Angle-Dependent Atomic Force Microscopy Single-Chain Pulling of Adsorbed Macromolecules from Planar Surfaces Unveils the Signature of an Adsorption-Desorption Transition.

The adsorption–desorption behavior of polymer chains is at the heart of macromolecular surface science and technology. With the current developments in atomic force microscopy (AFM), it has now become possible to address the desorption problem from the perspective of a single macromolecule. Here, we report on desorption of single polymer chains on planar surfaces by AFM-based single molecule force spectroscopy (SMFS) as a function of the pulling angle with respect to the surface-normal direction. SMFS experiments were performed in water with various substrates using different polymers covalently attached to the AFM probe tip. End-grafting at the AFM tip was achieved by surface-initiated polymerization using initiator functionalized tips. We found that the desorption force increases with a decreasing pulling angle, i.e., an enhanced adhesion of the polymer chain was observed. The magnitude of the desorption force shows a weak angular dependence at pulling angles close to the surface normal. A significant increase of the force is observed at shallower pulling from a certain pulling angle. This behavior carries the signature of an adsorption–desorption transition. The angular dependence of the normalized desorption force exhibits a universal behavior. We compared and interpreted our results using theoretical predictions for single-chain adsorption–desorption transitions.