Mode Atomic Force Microscopy Research Articles

Visuo-haptic virtual exploration of single cell morphology and mechanics based on AFM mapping in fast mode

Today the combination of several microscopy techniques over a sample can be performed at the same time on the same region. To fully capitalize on their complementarities and reveal new properties or behaviors as well as to present them in a new and attractive way, enhancing the experimental data exploration is a key issue. Especially, science education at micro and nanoscale is often limited by the complexity of making complicated knowledge accessible and easily remembered by the larger public. We propose an “out of the box” way to introduce complex scientific subjects like cell biology and polymer micro-rheology using haptic display and virtual reality. To reach this ambitious goal, firstly we combined the advantages of the Atomic Force Microscopy (AFM) and Fluorescence Microscopy (FM) in order to get complementary real experimental data on fixed and living isolated animal cells adhering on a protein micro-pattern or on a collagen-coated soft hydrogel. Secondly, thanks to the recent and fast AFM modes (PeakForce and Quantitative Imaging) dedicated to nanomechanical investigations, we achieved high resolution mapping images of the cell morphology, architecture and local mechanical properties. Then this set of data was implemented in a free simulation engine connected to a low-cost haptic device to create a virtual and interactive cell or polymer environment. In this environment, the user can explore the stiffness across three soft samples and can relate it to specific sample components. This dedicated visuo-haptic environment provides a novel sensory approach of the cell biology and/or micro(bio)mechanics through an active exploration of a set of scientific experimental data. This work paves the way to an affordable interactive and multisensory VR platform where various AFM images recorded in Peak Force or Quantitative Imaging, could be load in order to be actively explored; this approach fits into the emerging field of touching data.

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.

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.

Measurement of the Poisson’s ratio and Young’s modulus of an isotropic material with T-shape contact resonance atomic force microscopy

The Poisson’s ratio and the Young’s modulus play an important role in the characterization of nanomaterial mechanical properties. They are the vital parameters of understanding nanoscale material behavior. Here we report a method of quantitatively determining the values of the Poisson’s ratio and the Young’s modulus with a T-shape contact resonance atomic force microscopy. Unlike the cantilever of a traditional atomic force microscopy, the flexural and torsional modes of the T-shape cantilever are simultaneously excited and coupled in the contact mode. Through the analysis, the bifurcation of the coupled contact resonance frequencies is found in higher modes with the increasing contact stiffness. More importantly, the frequency bifurcation point can be used to decouple the Poisson’s ratio and the Young’s modulus, which leads to the determination of their separate values. In contrast to the previous methods, in which the Poisson’s ratio and the Young’s modulus are intrinsically coupled and there is no effective way of decoupling, the method presented in this study offers a new way of decoupling and determining these two parameters. This efficient and accurate method can be of significant help to the characterization of various nanomaterials.

Subnanometer-scale imaging of nanobio-interfaces by frequency modulation atomic force microscopy.

Recently, there have been significant advancements in dynamic-mode atomic force microscopy (AFM) for biological applications. With frequency modulation AFM (FM-AFM), subnanometer-scale surface structures of biomolecules such as secondary structures of proteins, phosphate groups of DNAs, and lipid-ion complexes have been directly visualized. In addition, three-dimensional AFM (3D-AFM) has been developed by combining a high-resolution AFM technique with a 3D tip scanning method. This method enabled visualization of 3D distributions of water (i.e. hydration structures) with subnanometer-scale resolution on various biological molecules such as lipids, proteins, and DNAs. Furthermore, 3D-AFM also allows visualization of subnanometer-scale 3D distributions of flexible surface structures such as thermally fluctuating lipid headgroups. Such a direct local information at nano-bio interfaces can play a critical role in determining the atomic- or molecular-scale model to explain interfacial structures and functions. Here, we present an overview of these recent advancements in the dynamic-mode AFM techniques and their biological applications.

Physicochemical and biological characteristics of chitosan/κ-carrageenan thin layer-by-layer films for surface modification of nitinol

Thin films based on the natural polysaccharides κ-carrageenan (Carr) and chitosan (Chit) were formed by layer-by-layer deposition technique. Surface topography and mechanical characteristics (Young's modulus, adhesion strength) of the polymer films with different number of layers were determined using various modes of atomic force microscopy (AFM). Polymer films were used to deposit on the surface of nitinol (NiTi) plates. The creation of polysaccharide coatings on nitinol led to a change in surface properties, such as hydrophilicity and root mean square roughness. in vitro cytotoxicity assay for nitinol plates with and without polymer coating by the MG-63 osteoblast-like cell line was conducted and was shown that all the studied samples are not toxic. A decrease in cytotoxicity for samples with a polymer film consisting of 4 layers of chitosan and carrageenan was shown as compared to an uncoated nitinol plates.

A horizontal-type scanning near-field optical microscope with torsional mode operation toward high-resolution and non-destructive imaging of soft materials.

We design and build a horizontal-type aperture based scanning near-field optical microscope (a-SNOM) with superior mechanical stability toward high-resolution and non-destructive topographic and optical imaging. We adopt the torsional mode in AFM (atomic force microscopy) operation to achieve a better force sensitivity and a higher topographic resolution when using pyramidal a-SNOM tips. The performance and stability of the AFM are evaluated through single-walled carbon nanotube and poly(3-hexyl-thiophene) nanowire samples. An optical resolution of 93 nm is deduced from the a-SNOM imaging of a metallic grating. Finally, a-SNOM fluorescence imaging of soft lipid domains is successfully achieved without sample damage by our horizontal-type a-SNOM instrument with torsional mode AFM operation.

Neural network module for tuning an atomic force microscope in the study of photonic crystal films

The article describes the use of neural network algorithms for setting the modes of investigation of photonic crystal films by tapping mode of atomic force microscopy. The photonic crystal thin films and monolayers obtained from colloidal suspensions by methods of sedimentation, centrifugation, vertical deposition, electrophoresis, and Langmuir-Blodgett are studied in this work. Study of films growth kinetics revealed that many factors were critical for reproducible deposition of the structure. To obtain high-quality images of samples, it is very important to get rid of artifacts and correctly configure the scanning parameters. To solve these problems it was proposed to use neural network algorithms. They allow to choose individual settings for each sample. We got acceptable results using a multilayer perceptron with one hidden layer.

Quantitative mapping of high modulus materials at the nanoscale: comparative study between atomic force microscopy and nanoindentation.

Local mechanical properties of submicron features are of particular interest due to their influence on macroscopic material performance and behaviour. This study is focused on local nanomechanical measurements, based on the latest Atomic Force Microscopy (AFM) mode, where the peak force set point is finely controlled at each pixel. After probe calibration, we evaluate the impact of spring constant of two AFM hand-crafted natural full diamond tips with steel cantilevers, used for mapping. Based on the fast capture of the cantilever deflection at each pixel and real time force curve analysis in the elastic region, AFM local measured contact moduli mappings of the silica beads (>50GPa) incorporated in an epoxy resin matrix, are compared with those determined using classical instrumented nanoindentation tests. Our analyses show that with the two AFM probes, without local residual deformation, the high moduli of the silica beads measured with this advanced AFM mode are within the standard deviation of the values determined by classical nanoindentation. LAY DESCRIPTION: The knowledge of material properties at the nanometer scale is a key parameter for well understanding and determining the behavior of material at macroscopic scale. In this paper, we compare two methods (an advanced mode and a classical one) based on the analysis of probes in interaction with the surface of studied material. We focus on a latest developed mode for determining local mechanical properties with a very high spatial resolution. For the advanced mode, we also consider two different hand-crafted probes. Our analyses show that with the high spatial resolution advanced mode, local mechanical properties are well determined. We also highlight the impact of the properties of the used probes for this advanced mode. In a final step, the power of the presented investigation lies in the fact that it does not modify the topography of the surface.

Three-Dimensional (3D) Conductive Network of CNT-Modified Short Jute Fiber-Reinforced Natural Rubber: Hierarchical CNT-Enabled Thermoelectric and Electrically Conductive Composite Interfaces.

Jute fibers (JFs) coated with multiwall carbon nanotubes (MWCNTs) have been introduced in a natural rubber (NR) matrix creating a three-dimensional (3D) electrically conductive percolated network. The JF-CNT endowed electrical conductivity and thermoelectric properties to the final composites. CNT networks fully covered the fiber surfaces as shown by the corresponding scanning electron microscopy (SEM) analysis. NR/JF-CNT composites, at 10, 20 and 30 phr (parts per hundred gram of rubber) have been manufactured using a two-roll mixing process. The highest value of electrical conductivity (σ) was 81 S/m for the 30 phr composite. Thermoelectric measurements revealed slight differences in the Seebeck coefficient (S), while the highest power factor (PF) was 1.80 × 10−2 μW/m K−2 for the 30 phr loading. The micromechanical properties and electrical response of the composite’s conductive interface have been studied in peak force tapping quantitative nanomechanical (PFT QNM) and conductive atomic force microscopy (c-AFM) mode. The JF-CNT create an electrically percolated network at all fiber loadings endowing electrical and thermoelectric properties to the NR matrix, considered thus as promising thermoelectric stretchable materials.

Active fault tolerant control for high-precision positioning of a non-contact mode uncertain atomic force microscopy

A non-contact mode atomic force microscope with chaotic dynamics may exposed to unknown faults, disturbances or uncertain parameters that are not always be compensated using classical control methods. Therefore, a fault tolerant controller must be designed for accurate tracking of the tip-position of the end-effector. In this paper, first, an unscented Kalman filter is designed for joint estimation of the states and parameters for an atomic force microscopy under process noise. The velocity of the end-effector, sample height and unknown fault are simultaneously estimated by measuring the tip position of randomly excited microscopy. Second, unscented Kalman filtering based model predictive controller is proposed for the accurate tracking of the tip-position. To prevent the disadvantage of the model-based controller design, an uncertainty or unknown fault function of the system is estimated by unscented Kalman filter such that the unmodeled dynamics of the system are compensated while the control signal is produced. Note that the controller voltage being applied to the microscopy is produced based on the estimated states and parameters of the atomic force microscopy. The numerical applications present that satisfactory tracking performance for tip position is obtained by the proposed fault tolerant controller such that extended Kalman filtering-based tracking results are also compared and discussed.

Rapid characterization of nano-scale structures in large-scale ultra-precision surfaces

Characterizations of nano-scale structures using conventional stylus profilometer and optical microscopy are respectively restricted by tip radius and diffraction limit. Atomic force microscope (AFM) can measure smaller structures but it is difficult and time-consuming to find them in large-scale ultra-precision surfaces. In this paper, a new profilometer for rapid positioning and measurement of nano-scale structures in large-scale ultra-precision surfaces is proposed. A technique based on white light interference is integrated into an AFM to detect probe deflection. The adapted profilometer has two measurement modes: a white light interferometry mode used to find an area of interest, an AFM mode for high-resolution measurement. Using this profilometer, the time of positioning an area of interest can be significantly reduced and the measurement efficiency is thus improved. Furthermore, a tracking measurement method is proposed to improve the measurement range of the AFM mode, which can enhance the measuring ability of the profilometer for complex large-scale surfaces. Experiments demonstrate that the profilometer can quickly positioning nano-scale structures in large-scale surfaces and achieve three-dimensional measurement with nanometer resolution.

Imaging the Nanophase-separated Structure of Block Copolymer Thin Film by Atomic Force Microscopy in Aqueous Solution

We employed two modes of atomic force microscopy to characterize the nanophase-separated structure of a block copolymer thin film from poly(2-methacryloyloxyethyl phosphorylcholine) and poly(3-(met...

Chemical and steric effects in simulating noncontact atomic force microscopy images of organic molecules on a Cu (111) substrate

Noncontact mode of atomic force microscopy (nc-AFM) employing a CO-functionalized tip is a very powerful tool for studying molecular structures. However, interpreting nc-AFM images for nonplanar molecules can sometimes be problematic. To illustrate and resolve the nature of such problematic systems, we employ real-space pseudopotentials constructed within density functional theory to simulate nc-AFM images. We focus on several representative nonplanar organic molecules (pentacene, naphthanthrone, olympicene, and 6-phenylhexa-1,3,5-triynybenzene (PHTB)) on a typical substrate: the Cu (111) surface. This substrate results in significant distortions in the molecular geometries of pentacene and naphthanthrone. Including these distortions in simulated nc-AFM imaging notably improves the agreement between the simulated and measured images. In naphthanthrone, the relatively large interaction between the O atom and the Cu substrate offers a straightforward explanation for the absence of the $\mathrm{C}=\mathrm{O}$ bond in the measured image. Nonplanar features such as tilting or twisting are also apparent in olympicene and PHTB. A ``triangular'' bright feature associated with the $\ensuremath{-}\mathrm{C}{\mathrm{H}}_{2}$ group in olympicene appears in simulated and measured nc-AFM images. This feature is directly related to the tilting angle of the molecule with respect to the Cu substrate. The ``defective'' benzene ring feature and the faint ellipsoidal $\mathrm{C}\ensuremath{\equiv}\mathrm{C}$ feature in PHTB can be ascribed to its twisted nature. The ability to simulate such subatomic images in nc-AFM reflects the accuracy and efficiency of calculating quantum forces in real space.

Relationship between rheological properties and actin filaments of single cells investigated by atomic force microscopy

We investigated how rheological properties of single cells are spatially correlated with actin filaments (F-actin) by atomic force microscopy (AFM). This was achieved by measuring the power-law rheological properties of mouse fibroblast cells adhered on micropatterned substrate by multi-frequency force modulation AFM in the combination of the immunofluorescent observation of F-actin. We observed that G0, a scale factor of the modulus at a certain frequency, of cells was decreased as the F-actin was disrupted with a chemical treatment, showing that the spatial-averaged stiffness is associated with the F-actin formation, as observed in various cell types and microrheological methods. Interestingly, we found that G0 measured in local locations within cells displayed a negative correlation with the corresponding F-actin density. The results indicate that the local cell stiffness is not necessarily correlated with the F-actin density, but rather depending on how network structures of F-actin are formed in local regions.

Instrument-Dependent Factors Affecting the Precision in the Atomic Force Microscopy Stiffness Measurement of Nanoscale Liposomes.

The mechanical strength (stiffness) of liposomes affects their cellular uptake efficiency and drug release in drug delivery processes. We recently developed a tip shape evaluation method for improving the precision of liposome stiffness measurement by quantitative imaging (QI)-mode atomic force microscopy (AFM). The present study applied our method to the widely-used AFM instruments equipped for intermittent contact (IC)-mode force curve measurements, and examined instrument-dependent factors that affect the liposome stiffness measurements. We demonstrated that the evaluation of the tip shape for cantilever selection can be applicable to the IC mode as well as the QI mode. With the cantilever selection, the improved precision of the liposome stiffness was obtained when the stiffness of each liposome was determined from the slope in the force-deformation curve by the IC-mode force curve measurement. Further, the stiffness values were found to be similar to that measured by QI-mode measurements. These results indicate that our developed method can be widely used via IC-mode force curve measurements as well as via QI mode. It was also revealed that spatial drift of the cantilever position was instrument-dependent factors which could affect the precision of liposome stiffness measurements in the case of IC-mode force curve measurement. Therefore, in case of stiffness measurement by IC-mode force curve measurement, it is vital to obtain force-deformation curves immediately after imaging a liposome for the precise stiffness measurement of liposomes. These findings will promote the usage of the AFM stiffness measurement method for the characterization of lipid nanoparticle-based drug delivery systems.

Pulsed Force Kelvin Probe Force Microscopy.

Measurement of the contact potential difference (CPD) and work functions of materials are important in analyzing their electronic structures and surface residual charges. Kelvin probe force microscopy (KPFM), an imaging technique of atomic force microscopy, has been widely used for surface potential and work function mapping at the nanoscale. However, the conventional KPFM variants are often limited in their spatial resolution to 30-100 nm under ambient conditions. The continuingly decreasing size and increasing complexity of photoactive materials and semiconductor devices will present future challenges in uncovering their nanometer-scale electrical properties through KPFM. Here, we introduce a KPFM technique based on the pulsed force mode of atomic force microscopy. Our technique, named pulsed force Kelvin Probe Force Microscopy (PF-KPFM), is a single-pass technique that utilizes the intrinsic Fermi level alignment between the AFM tip and the conductive sample without the need for an external oscillating voltage. Induced cantilever oscillations generated by a spontaneous redistribution of electrons between tip and sample are extracted and used to obtain the cantilever oscillation amplitude and to derive the surface potential. The spatial resolution of PF-KPFM is shown to be <10 nm under ambient conditions. The high spatial resolution surface potential mapping enables in situ determination of ohmic and nonohmic contacts between metals and semiconductors, mapping boundaries of ferroelectric domains of BaTiO3, as well as characterization of protein aggregates. High spatial resolution measurements with PF-KPFM will facilitate further studies directed at uncovering electrical properties for emerging photoactive materials, biological samples, and semiconductor devices.

Dynamic analysis of tapping mode atomic force microscope (AFM) for critical dimension measurement

Transient dynamics of tapping mode atomic force microscope (AFM) for critical dimension measurement are analyzed. A simplified nonlinear model of AFM is presented to describe the forced vibration of the micro cantilever-tip system with consideration of both contact and non-contact transient behavior for critical dimension measurement. The governing motion equations of the AFM cantilever system are derived from the developed model. Based on the established dynamic model, motion state of the AFM cantilever system is calculated utilizing the method of averaging with the form of slow flow equations. Further analytical solutions are obtained to reveal the effects of critical parameters on the system dynamic performance. In addition, features of dynamic response of tapping mode AFM in critical dimension measurement are studied, where the effects of equivalent contact stiffness, quality factor and resonance frequency of cantilever on the system dynamic behavior are investigated. Contact behavior between the tip and sample is also analyzed and the frequency drift in contact phase is further explored. Influence of the interaction between the tip and sample on the subsequent non-contact phase is studied with regard to different parameters. The dependence of the minimum amplitude of tip displacement and maximum phase difference on the equivalent contact stiffness, quality factor and resonance frequency are investigated. This study brings further insights into the dynamic characteristics of tapping mode AFM for critical dimension measurement, and thus provides guidelines for the high fidelity tapping mode AFM scanning.

Comparison of fresh and aged lithium iron phosphate cathodes using a tailored electrochemical strain microscopy technique.

Electrochemical strain microscopy (ESM) is a powerful atomic force microscopy (AFM) mode for the investigation of ion dynamics and activities in energy storage materials. Here we compare the changes in commercial LiFePO4 cathodes due to ageing and its influence on the measured ESM signal. Additionally, the ESM signal dynamics are analysed to generate characteristic time constants of the diffusion process, induced by a dc-voltage pulse, which changes the ionic concentration in the material volume under the AFM tip. The ageing of the cathode is found to be governed by a decrease of the electrochemical activity and the loss of available lithium for cycling, which can be stored in the cathode.