Tapping Mode Atomic Force Microscopy Research Articles

Investigation on the Impact of Excitation Amplitude on AFM-TM Microcantilever Beam System's Dynamic Characteristics and Implementation of an Equivalent Circuit.

Alterations in the dynamical properties of an atomic force microscope microcantilever beam system in tapping mode can appreciably impact its measurement precision. Understanding the influence mechanism of dynamic parameter changes on the system's motion characteristics is vital to improve the accuracy of the atomic force microscope in tapping mode (AFM-TM). In this study, we categorize the mathematical model of the AFM-TM microcantilever beam system into systems 1 and 2 based on actual working conditions. Then, we analyze the alterations in the dynamic properties of both systems due to external excitation variations using bifurcation diagrams, phase trajectories, Lyapunov indices, and attraction domains. The numerical simulation results show that when the dimensionless external excitation g < 0.183, the motion state of system 2 is period 1. When g < 0.9, the motion state of system 1 is period 1 motion. Finally, we develop the equivalent circuit model of the AFM-TM microcantilever beam and perform related software simulations, along with practical circuit experiments. Our experimental results indicate that the constructed equivalent circuit can effectively analyze the dynamic characteristics of the AFM-TM microcantilever beam system in the presence of complex external environmental factors. It is observed that the practical circuit simulation attenuates high-frequency signals, resulting in a 31.4% reduction in excitation amplitude compared to numerical simulation results. This provides an essential theoretical foundation for selecting external excitation parameters for AFM-TM cantilever beams and offers a novel method for analyzing the dynamics of micro- and nanomechanical systems, as well as other nonlinear systems.

Effect of excitation frequencies on phase contrast in tapping mode atomic force microscope

Effect of excitation frequencies on phase contrast in tapping mode atomic force microscope

Tailored Microcantilever Optimization for Multifrequency Force Microscopy.

Microcantilevers are at the heart of atomic force microscopy (AFM) and play a significant role in AFM-based techniques. Recent advancements in multifrequency AFM require the simultaneous excitation and detection of multiple eigenfrequencies of microcantilevers to assess more data channels to quantify the material properties. However, to achieve higher spatiotemporal resolution there is a need to optimize the structure of microcantilevers. In this study, the architecture of the cantilever with gold nanoparticles using a dip-coating method is modified, aiming to tune the higher eigenmodes of the microcantilever as integer multiples of its fundamental frequency. Through the theoretical methodology and simulative model, that integer harmonics improve the coupling in multifrequency AFM measurements is demonstrated, leading to enhanced image quality and resolution. Furthermore, via the combined theoretical-experimental approach, the interplay between induced mass and stiffness change of the modified cantilever depending on the attached particle location, size, mass, and geometry is found. To validate the results of this predictive model, tapping-mode AFM is utilized and bimodal Amplitude Modulation AFM techniques to examine and quantify the impact of tuning higher-order eigenmodes on the imaging quality of a polystyrene-polymethylmethacrylate (PS-PMMA) block co-polymer assembly deposited on a glass slide and Highly Ordered Pyrolytic Graphite (HOPG).

Synthesis of Metal Nanoparticles Encapsulated with Skewered Porphyrins Assembled by Siloxane Coupling

A protocol for encapsulation of metal nanoparticles with organic shells of porphyrin molecules via silane coupling is described. A strategy with silicon tetrachloride was used to produce a skewered arrangement of porphyrins that are linked through a central silicon atom by siloxane, Si-O-Si bridges. The planar macrocycles align cofacially to surround the periphery of metal nanoparticles (e.g. gold, iron oxide).  Skewered ‘shish kebob’ assemblies of porphyrins form an encapsulating shell by attachment to metal cores with silicon-oxygen-metal bridges. Free-base porphyrins were skewered through siloxane coupling using SiCl4, with the silicon atom inserted to the center of the macrocycles. The Si atom binds to the four nitrogens at the center of the macrocycles, and also links to adjacent macrocycles through siloxane bridges. Iron and gold nanoparticles were used as core materials, while the organic shells were prepared with tetraphenyl porphyrin or octaethyl porphyrin. The thickness of the shells can be tuned by synthetic parameters such as concentration and immersion intervals. Structural changes were tracked using UV/Vis spectroscopy to evaluate spectral shifts. Nanoparticle samples were examined with tapping-mode atomic force microscopy to directly view changes in the size and shapes of nanoparticles before and after encapsulation with porphyrins. Phase images enabled sensitive mapping of the nanoparticle composition, revealing a soft organic shell surrounding the hard metal core. The synthetic approach with skewering porphyrins to metal nanoparticles should be generic for preparing metal core-shell nanoparticles encapsulated with shells of macrocyclic porphyrinoid molecules.

Nanoscale mechanical and thermal properties of a multilayer polyethylene film with varying densities

Various atomic force microscopy methods for the imaging and measurement of the modulus of polyethylene (PE) layers in a five-layer multilayer film are described. The film includes linear low-density polyethylene (LLDPE) products with densities ranging from 0.940 to 0.912 gm/cm3. Conventional TappingMode AFM phase imaging can reveal the layers but is sensitive to measurement conditions. PeakForce Tapping AFM using quantitative nanomechanics (QNM) mode can resolve all five layers based on at least three different signals – stiffness mapping, penetration, and energy dissipation. In particular, the difference is statistically significant for the modulus and penetration data. This is remarkable considering that the method is sensitive to density differences of only 0.004 density units. Quasistatic AFM indentation was also done using the same probe and peak load. Here we see the effect of time dependence on the mechanical properties, where the slower interaction times yield estimated moduli much closer to the long-time moduli measured in bulk tensile experiments, compared to the value obtained at much higher frequency using PeakForce Tapping AFM. Nano-thermal analysis of the same five-layer film using heated tip technology can also discriminate the layers statistically, based on softening temperature, which shows excellent correlation with bulk properties.

Periodic nanostructures on single-crystal copper for SERS substrate fabricated by using AFM dynamic lithography

The fabrication of periodic nanostructures on noble metals as surface enhanced Raman scattering (SERS) substrate is still challenging through facile and cost-effective process. Dynamic lithography under AFM tapping mode has been applied to the fabrication of nanogrooves on single-crystal copper to reduce the feature size. However, there was a lack of fundamental understanding of the effect of the fabrication of periodic nanostructures due to dynamic lithography and then SERS enhancement. To this end, this study aims to investigate the variation of the energy dissipation and the corresponding fabricated nanogrooves at different laser spot positions. It was found that improper laser position was contributed to the increased energy dissipation and irregular nanogrooves were thus obtained with much larger machined depth. On the contrary, at the optimal laser position, the minimal sensitivity was associated with the maximum voltage value on the photodiode. In this case, regular nanogrooves with controlled feature size can be produced for the fabrication of periodic nanostructures. And checkerboard nanodots with a period of 200 nm were fabricated by the combination of arrays of nanogrooves, which indicated advantages of high stability for AFM dynamic lithography. In addition, rhodamine 6G (R6G) was selected as the detecting target to measure SERS spectra of fabricated substrates. Results indicated the Raman enhancement factors can be promoted due to uniform shape of nanogroove and high resolution in horizon dimension.

Large-range high-speed dynamic-mode atomic force microscope imaging: adaptive tapping towards minimal force

In this paper, a software-hardware integrated approach is proposed for high-speed, large-range tapping mode imaging of atomic force microscope (AFM). High speed AFM imaging is needed in various applications, particularly in interrogating dynamic processes at nanoscale such as polymer crystallization process. Achieving high speed in tapping-mode AFM imaging is challenging as the probe-sample interaction during the imaging process is highly nonlinear, making the tapping motion highly sensitive to the probe sample spacing, and thereby, difficult to maintain at high speed. Increasing the speed via hardware bandwidth enlargement, however, leads to a substantially reduction of the imaging area. Contrarily, the imaging speed can be increased without loss of the scan size through control (algorithm)-based approach. For example, the recently-developed adaptive multiloop mode (AMLM) technique has demonstrated its efficacy in increasing the tapping-mode imaging speed without loss of scan size. Further improvement, however, has been limited by the hardware bandwidth and the online signal processing speed and computation complexity involved. Thus, in this paper, the AMLM technique is further enhanced to optimize the probe tapping regulation, and integrated with a field programmable gate array platform to further increase the imaging speed without loss of quality and scan range. Experimental implementation of the proposed approach demonstrates that high-quality imaging can be achieved at a high-speed scanning rate of 100 Hz and higher, and over a large imaging area of over 20 μm.

Highly Responsive Al/PTB7/Si/Al Vertical Structure-Based White Light Photodetector Using FTM Method

This article reports a p-type Poly [[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno [3,4-b] thiophenediyl ]] (PTB7) thin film based Al/PTB7/Si/Al vertical structure for white light detection application. The thin film of PTB7 polymer was deposited via a low-cost and facile floating-film transfer method (FTM). The morphology of the PTB7 film is analyzed by tapping mode Atomic Force Microscopy (AFM) which gives an average surface roughness as ~0.703 nm. The maximum responsivity and detectivity of the fabricated device were obtained as ~146.13 mA/W and ~4.137 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Jones at ~0.228 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> white light intensity under a reverse bias of -2 V. The rise/fall time of the fabricated sensing device was measured as ~0.016/~0.112 s respectively.

Determination of electric and thermoelectric properties of molecular junctions by AFM in peak force tapping mode

Molecular thin films, such as self-assembled monolayers (SAMs), offer the possibility of translating the optimised thermophysical and electrical properties of high-Seebeck-coefficient single molecules to scalable device architectures. However, for many scanning probe-based approaches attempting to characterise such SAMs, there remains a significant challenge in recovering single-molecule equivalent values from large-area films due to the intrinsic uncertainty of the probe-sample contact area coupled with film damage caused by contact forces. Here we report a new reproducible non-destructive method for probing the electrical and thermoelectric (TE) properties of small assemblies (10–103) of thiol-terminated molecules arranged within a SAM on a gold surface, and demonstrate the successful and reproducible measurements of the equivalent single-molecule electrical conductivity and Seebeck values. We have used a modified thermal-electric force microscopy approach, which integrates the conductive-probe atomic force microscope, a sample positioned on a temperature-controlled heater, and a probe-sample peak-force feedback that interactively limits the normal force across the molecular junctions. The experimental results are interpreted by density functional theory calculations allowing quantification the electrical quantum transport properties of both single molecules and small clusters of molecules. Significantly, this approach effectively eliminates lateral forces between probe and sample, minimising disruption to the SAM while enabling simultaneous mapping of the SAMs nanomechanical properties, as well as electrical and/or TE response, thereby allowing correlation of the film properties.

Flotation and adsorption characteristics of albite and quartz with oleic acid-based collector

The separation of feldspars from quartz is a challenging processing method as they have identical physical and physicochemical surface properties. To put forward the possible ways to separate them from each other is essential. Thus, from this point of view, this study aimed to determine the flotation behavior of both minerals with an oleic acid-based collector and its adsorption characteristics with both minerals. According to the results, the collector-treated albite displayed stronger characteristic adsorption FTIR bands of the CH2 group at 2957 cm−1, 2926 cm−1 and 2854 cm−1 than quartz. But, in contrast to the similar adsorption FTIR bands determined, albite and quartz presented selective flotation recovery results in at moderately alkaline environment. While 90% flotation recovery of albite could be obtained at certain pH conditions, less than 10% of quartz recovery was obtained regardless of collector concentration and pH. When the tapping mode AFM in the micro topographical aspect was correlated with the experimental data, the collector-treated albite, compared to the original albite sample, displayed collector adsorption patches with different intensities. However, quartz and collector-treated quartz samples displayed similar micro topographies which indicated no/low collector-quartz interaction.

Non-Smooth Dynamics of Tapping Mode Atomic Force Microscopy

Abstract This study examines the nonlinear dynamics in tapping-mode atomic force microscopy (AFM) with tip-surface interactions that include van der Waals and Derjaguin-Müller-Toporov contact forces. We investigate the periodic solutions of the hybrid system by performing numerical pseudo-arclength continuation. Through the use of bifurcation locus maps in the set of parameters of the discontinuous model, the overall dynamical response scenario is assessed. We demonstrate the influence of various dissipation mechanisms that are related with the AFM touching or lacking contact with the sample. Local and global analyses are used to investigate the stability of the stable solution in the repulsive regime. The impacting nonsmooth dynamics framed within a higher-mode Galerkin discretization is able to capture windows of irregular and complex motion.

Molecular dynamics models of tapping mode atomic force microscopy

Macro-mechanical simulation software cannot easily simulate the atomic resolution of the tapping mode atomic force microscope (TM-AFM), so the accuracy of the corresponding mechanical model is questioned. In this paper, a TM-AFM simulation model is established using classical molecular dynamics (MD). The model simulated the tapping of gold (Au) and aluminum (Al) by probes with various amplitudes. The simulation yielded the z-direction force curves, trajectory curves and indentation curves of the probe. The amplitude change and the phase shift of the probe at various amplitudes were calculated from the direct measurement results. A contact jump and detachment jump become evident and are significant to energy and force results. The recovery ability of Al after indenting is smaller than that of Au. The energy calculations can be fitted to a high goodness of fit, reaching 0.99 and better; hence, the amplitude and phase shift variations of the probe can be used to fit the stored and dissipated energies, the sample energies when the sample is tapped. In this way, the TM-AFM is able to calculate the mechanical properties of the sample, and thus characterize the sample.

Blunting and wear of AFM tips during dynamic lithography

Mechanical scanning probe lithography using AFM tips has been applied to the fabrication of nanostructures on material surfaces. Particularly, the dynamic lithography under the AFM tapping mode is promising in fabricating the surface nanostructures at the width of around 20 nm. However, tip blunting and gradual atom-by-atom wear occurred during the process although the tip tapping motion has largely reduced the tip-surface interaction force and time. This study aims to investigate the blunting rate and mechanism of such AFM tips. It was found that the maximum contact stress was in the range of 0.7 to 1.0 GPa when a silicon AFM tip (initial tip radius under 20 nm) was tapped on a PMMA thin-film surface. And the contact stress was mainly attributed to the tip radius, which was the key factor to limit machinability and machining efficiency of the dynamic lithography. Furthermore, the mechanism of the tip blunting was through its plastic deformation and tip wear. A stress analysis indicated that it was the large stress gradient that contributed to the high density of wrinkles in the tips, and that ninety percent of the blunting of a tip were caused by the plastic deformation, leading to a significant increase of the tip radius.

Probing the morphological and mechanical changes in a C. elegans model of Huntington's disease.

Huntington's Disease (HD) is a fatal neurodegenerative disease caused by a mutation in the huntingtin gene, in which CAG nucleotides are excessively repeated. In turn, translated huntingtin protein (htt) contains an expanded polyglutamine (polyQ) domain, which triggers toxic protein aggregation, the hallmark feature of this disease. Common symptoms of HD, such as chorea, muscle atrophy, and behavioral and neurological decline, continue to worsen as the disease progresses. This study aims to investigate the relationship between polyQ domain length and age-dependent morphological and mechanical changes in a Caenorhabditis elegans model of HD. C. elegans are an established model species to investigate aging, as morphological and mechanical changes—such as epidermal roughening and whole body decrease in Young's modulus, respectively—occur as a function of age. Moreover, worm mobility exhibits a deficit as a result of such changes. We expect to observe age-dependent morphological changes much earlier in the pathogenic C. elegans (Q128), as opposed to age-matched, non-pathogenic C. elegans (Q15). Morphological analysis was performed using tapping mode atomic force microscopy (AFM), and images were processed using custom MATLAB scripts to determine RMS roughness. Further studies will be conducted to investigate the role of age-dependent changes in mechanical properties, such as Young's modulus, as they relate to polyQ domain length. In-situ contact mode AFM will be used to achieve this. We expect to observe age-dependent mechanical changes much earlier in the Q128 worms, in comparison to the Q15 worms. Pending results, additional studies may be conducted to determine if these changes serve as a peripheral biomarker of HD, as well as the role of such a biomarker in diagnostic, prognostic, and therapeutic strategies.

Highly Efficient Ternary Near-Infrared Organic Photodetectors for Biometric Monitoring.

Near-infrared (NIR) small-molecule acceptors that absorb at wavelengths of up to 1000 nm are attractive for applications in organic photodetectors (OPDs) and biometrics. In this study, we incorporated IEICO-4F as the third component for PffBT4T-2OD:PC71BM-based OPDs to provide an efficient NIR response while greatly suppressing the leakage current at reverse bias. By varying the blend ratio and thickness (250-600 nm), we obtained an NIR OPD displaying an ultralow dark-current density (JD = 2.62 nA cm-2), ultrahigh detectivity [D* = 7.2 × 1012 Jones (850 nm)], high sensitivity, and photoresponsivity covering the region from the ultraviolet to the NIR. We used tapping-mode atomic force microscopy, optical microscopy, grazing-incidence wide-angle X-ray scattering, and contact angle measurements to investigate the effect of IEICO-4F on the performance of the ternary OPDs. The low compatibility of PffBT4T-2OD and IEICO-4F, originating from weak intermolecular interactions, allowed us to manipulate the degree of phase separation between the donor and acceptor in the ternary blends, leading to an optimized blend morphology featuring efficient charge separation, transport, and collection. To demonstrate its applicability, we integrated our OPD with two light-emitting diodes and used the system for precisely calculated transmissive pulse oximetry.

Strengthening Polylactic Acid by Salification: Surface Characterization Study.

Polylactic acid (PLA) is one of the market's most commonly used biodegradable polymers, with diverse applications in additive manufacturing, specifically fused deposition modeling (FDM) 3D printing. The use of PLA in complex and sophisticated FDM applications is continually growing. However, the increased range of applications requires a better understanding of the material properties of this polymer. For example, recent studies have shown that PLA has the potential to be used in artificial heart valves. Still, the durability and longevity of this material in such a harsh environment are unknown, as heart valve failures have been attributed to salification. Additionally, there is a gap in the field for in situ material characterization of PLA surfaces during stiffening. The present study aims to benchmark different dynamic atomic force microscopy (AFM) techniques available to study the salification phenomenon of PLA at micro-scales using different PLA thin films with various salt concentrations (i.e., 10%, 15%, and 20% of sodium chloride (NaCl)). The measurements are conducted by tapping mode AFM, bimodal AFM, the force spectroscopy technique, and energy quantity analysis. These measurements showed a stiffening phenomenon occurring as the salt solution is increased, but the change was not equally sensitive to material property differences. Tapping mode AFM provided accurate topographical information, while the associated phase images were not considered reliable. On the other hand, bimodal AFM was shown to be capable of providing the topographical information and material compositional mapping through the higher eigenmode's phase channel. The dissipated power energy quantities indicated that how the polymers become less dissipative as salt concentration increases can be measured. Lastly, it was shown that force spectroscopy is the most sensitive technique in detecting the differences in properties. The comparison of these techniques can provide a helpful guideline for studying the material properties of PLA polymers at micro- and nano-scales that can prove beneficial in various fields.

Friction hysteretic behavior of supported atomically thin nanofilms

Hysteretic friction behavior has been observed on varied 2D nanofilms. However, no unanimous conclusion has yet been drawn on to the exact mechanism or relative contribution of each mechanism to the observed behavior. Here we report on hysteretic friction behavior of supported atomically thin nanofilms studied using atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. Load dependent friction measurements were conducted on unheated and heated samples of graphene, h-BN, and MoS2 supported by silica substrates. Two diverging friction trends are reported: the unheated samples showed higher friction during unloading than during loading, and the heated samples showed a reversed hysteresis. Further, the friction force increased sub-linearly with normal force for heated samples, compared with unheated samples. Tapping mode AFM suggested that the interaction strength of the substrate was increased with heating. Roughened substrates in the MD simulations that mimicked strong/weak interaction forces reproduced the experimental observations and revealed that the evolution of real contact area in different interface interaction situation caused the diverging behaviors. Surface roughness and interaction strength were found to be the key parameters for controlling the out-of-plane deformation of atomically thin nanofilms.

Insights and guidelines to interpret forces and deformations at the nanoscale by using a tapping mode AFM simulator: dForce 2.0.

Amplitude modulation (tapping mode) AFM is the most versatile AFM mode for imaging surfaces at the nanoscale in air and liquid environments. However, it remains challenging to estimate the forces and deformations exerted by the tip. We introduce a new simulator environment to predict the values of the observables in tapping mode AFM experiments. The relevant feature of dForce 2.0 is the incorporation of contact mechanics models aimed to describe the properties of ultrathin samples. These models were essential to determine the forces applied on samples such as proteins, self-assembled monolayers, lipid bilayers, and few-layered materials. The simulator incorporates two types of long-range magnetic forces. The simulator is written in an open-source code (Python) and it can be run from a personal computer.

CuAAC-methacrylate interpenetrating polymer network (IPN) properties modulated by visible-light photoinitiation.

Interpenetrating polymer networks (IPNs) are a class of materials with interwoven polymers that exhibit unique blended or enhanced properties useful to a variety of applications, ranging from restorative protective materials to conductive membranes and hydrophobic adhesives. The IPN formation kinetics can play a critical role in the development of the underlying morphology and in turn the properties of the material. Dual photoinitiation of copper-catalyzed azide-alkyne (CuAAC) and radical mediated methacrylate polymerization chemistries enable the manipulation of IPN microstructure and properties by controlling the kinetics of IPN formation via the intensity of the initiating light. Specifically, azide and alkyne-based polyethylene glycol monomers and tetraethylene glycol dimethacrylate (TEGDMA) were polymerized in a single pot to form IPNs and the properties were evaluated as a function of the photoinitiating light intensity. Morphological differences as a function of intensity were observed in the IPNs as determined by thermomechanical properties and phase-contrast imaging in tapping mode atomic force microscopy (AFM). At moderate intensities (20 mW/cm2) of visible light (470 nm), the TEGDMA polymerization gels first and therefore forms the underlying network scaffold. At low intensities (0.2 mW/cm2), the CuAAC polymerization can gel first. The ability to switch sequence of gelation and IPN trajectory (simultaneous vs. sequential), affords control over phase separation behavior. Thus, light not only allows for spatial and temporal control over the IPN formation but also provides control over their thermomechanical properties, representing a route for facile IPNs design, synthesis, and application.

Morphological Effects of Strain Localization in the Elastic Region of Magnetorheological Elastomers.

Strain localization is a significant issue that poses interesting research challenges in viscoelastic materials because it is difficult to accurately predict the damage evolution behavior. Over time, the damage mechanism in the amorphous structure of viscoelastic materials leads to subsequent localization into a shear band, gradually jeopardizing the materials' elastic sustainability. The primary goal of this study is to further understand the morphological effects and the role of shear bands in viscoelastic materials precipitated by strain localization. The current study aims to consolidate the various failure mechanisms of a sample and its geometry (surface-to-volume ratio) used in torsional testing, as well as to understand their effects on stress relaxation durability performance. A torsional shear load stress relaxation durability test was performed within the elastic region on an isotropic viscoelastic sample made of silicon rubber and a 70% weight fraction of micron-sized carbonyl iron particles. Degradation was caused by a shear band of localized plasticity that developed microscopically due to stress relaxation durability. The failure pattern deteriorated as the surface-to-volume ratio decreased. A field-emission scanning electron microscope (FESEM) and a tapping-mode atomic force microscope (AFM) were used for further observation and investigation of the sample. After at least 7500 cycles of continuous shearing, the elastic sustainability of the viscoelastic materials microstructurally degraded, as indicated by a decline in stress performance over time. Factors influencing the formation of shear bands were observed in postmortem, which was affected by simple micromanipulation of the sample geometry, making it applicable for practical implementation to accommodate any desired performance and micromechanical design applications.