Tapping Mode Atomic Force Microscopy Research Articles

High-Resolution Atomic Force Microscopy Investigation of Alginate Hydrogel Materials in Aqueous Media.

Alginate hydrogels are frequently used in 3D bioprinting and tissue repair and regeneration. Establishing the structure-property-performance correlation of these materials would benefit significantly from high-resolution structural characterization in aqueous environments from the molecular level to continuum. This study overcomes technical challenges and enables high-resolution atomic force microscopy (AFM) imaging of hydrated alginate hydrogels in aqueous media. By combining a new sample preparation protocol with extremely gentle tapping mode AFM imaging, we characterized the morphology and regional mechanical properties of the hydrated alginate. Upon cross-linking, basic units of these hydrogel materials consist of egg-box dimers, which assemble into long fibrils. These fibrils congregate and pile up, forming a sponge-like structure, whose pore size and distribution depend on the cross-linking conditions. At the exterior, surface tension impacts the piling of fibrils, leading to stripe-like features. These structural features contribute to local, regional, and macroscopic mechanics. The outcome provides new insights into its structural characteristics from nanometers to tens of micrometers, i.e., at the dimensions pertaining to biomaterial and hydrogel-cell interactions. Collectively, the results advance our knowledge of the structure and mechanics from the nanometer to continuum, facilitating advanced applications in hydrogel biomaterials.

Enhancing tapping mode atomic force microscopy using AutoResonance control and a hybrid dynamic model

We present a novel method for analyzing tapping mode atomic force microscopy (AFM), enhancing topography reconstruction’s accuracy and speed. Our approach integrates an automatic resonance-tracking control scheme with a hybrid dynamic model that considers interactions between the AFM tip and the sample, and the oscillating sensor dynamics. A key aspect of our method is a simplified model that captures intricate dynamic behavior as a function of the distance from the topography. This model enables the immediate approximation of the distance from the topography, eliminating the necessity to lock onto the nominal distance as done with classical AFMs. To validate the accuracy, we conducted numerical simulations of Van der Waals and capillary interaction forces, as well as experimental measurements of a coin’s topography. The results affirm the effectiveness of our approach in enhancing AFM imaging and characterization capabilities.

Surface Migration of Fatty Acid to Improve Sliding Properties of Hypromellose-Based Coatings

Hypromellose (HM) is a cellulose-derived polymer of pharmaceutical grade that forms easily from thin films and coatings. As few studies concern HM-formulated systems, this study focuses on the formulation of HM films by incorporating a fatty acid additive, making it possible to control surface properties such as wetting and slip behavior for pharmaceutical or medical applications. The results show that the addition of a very small amount (from 0.1 to 1% w/w) of fatty acid additive reduces HM film affinity for water and water vapor transmission rate, while film appearance and gloss are rather preserved. Surface properties were probed using wettability measurements, Tapping Mode AFM, ATR-FTIR spectrometry, and friction measurements. Tapping Mode AFM images show that the surface roughness reduces by up to 65%. Wettability results show that the surface energy decreases from 43 to 31 mJ.m−2, whereas surface FTIR spectrometry measurements demonstrate that fatty acid molecules migrate on the surface of the formulated films, the driving force being the microphase separation between the polar HM macromolecules and the hydrophobic additive, leading to the formation of a weak boundary layer with poor cohesion. As a consequence, the surface coefficient of friction significantly reduces from 0.38 to 0.08, and fatty acid molecules thus act as a lubricant, improving the sliding properties of HM-based coatings.

Effects of air damping on quality factors of different probes in tapping mode atomic force microscopy

Abstract The AFM probe in tapping mode is a continuous process of energy dissipation, from moving away from to intermittent contact with the sample surfaces. At present, studies regarding the energy dissipation mechanism of this continuous process have only been reported sporadically, and there are no systematic explanations or experimental verifications of the energy dissipation mechanism in each stage of the continuous process. The quality factors can be used to characterize the energy dissipation in TM-AFM systems. In this study, the vibration model of the microcantilever beam was established, coupling the vibration and damping effects of the microcantilever beam. The quality factor of the vibrating microcantilever beam under damping was derived, and the air viscous damping when the probe is away from the sample and the air squeeze film damping when the probe is close to the sample were calculated. In addition, the mechanism of the damping effects of different shapes of probes at different tip–sample distances was analyzed. The accuracy of the theoretical simplified model was verified using both experimental and simulation methods. A clearer understanding of the kinetic characteristics and damping mechanism of the TM-AFM was achieved by examining the air damping dissipation mechanism of AFM probes in the tapping mode, which was very important for improving both the quality factor and the imaging quality of the TM-AFM system. This study’s research findings also provided theoretical references and experimental methods for the future study of the energy dissipation mechanism of micro-nano-electromechanical systems.

Enhancing open-circuit voltage and suppression of energy loss in ternary organic photovoltaics utilizing carbazole/bicarbazole-based guest donors

The third component in binary organic photovoltaics (OPV) plays a crucial role in finely adjusting light absorption, reducing energy loss, and optimizing the blend morphology. We introduce four A-π-D-π-A type small molecules as guest-donor materials, employing easily accessible starting materials. These molecules incorporate carbazole or bicarbazole as the core structure, along with pyran-4-ylidene-malononitrile or pyran-4-ylidene-indenedione derivatives as electron-accepting end groups. We explore the miscibility of guest donors with Y6 and analyze the energy loss in corresponding OPVs. Tapping mode atomic force microscopy and grazing incidence wide-angle X-ray (GIWAX) scattering analysis confirm that the twisted configuration of the guest donor CY-3 disperses effectively into the Y6 domain, promoting the formation of an alloy-like structure and maximizing the energy gap (Egap) of OPVs. CY-3 improves the molecular packing of Y6 in ternary blend and optimizes the blend morphology, leading to the minimum non-radiative energy recombination (ΔEnon-rad) and energy loss (Eloss) in the corresponding ternary OPV which exhibits an optimized Voc of (0.891 ± 0.006 V). The optimized blend morphology also, suppresses bimolecular and trap-assisted recombination, and improves charge collection. Ternary devices employing PM6/PY-IT, PM6/Y18, PM6/L8-BO, and PM6/BTP-eC9 demonstrate significant improvements in their Voc. Particularly remarkable is the attainment of the highest PCE of 17.18 % in the OPV based on PM6/BTP-eC9 with the incorporation of CY-3.

An atomic force microscope-like dual-stage force controlled fast tool servo for in-process inspection of micro-structured surfaces

This study presents the development of a dual-stage force sensor integrated fast tool servo (FS-FTS) for in-process inspection of micro-structures. With the cutting tool serving as a probe, the measurement principle of the FS-FTS is similar to that of an atomic force microscope (AFM). The force modulation method, inspired by the tapping mode AFM, is employed to detect the weak contact between the cutting tool and the workpiece. The primary stage of the dual-stage FTS is responsible for tracking the surface profile of the micro-structures while the secondary stage offers the high-frequency oscillation to modulate the force signal. An experimental method is proposed to determine the optimal oscillation frequency. The transfer function of the FS-FTS, from the input voltage of the primary stage to the output force of the force detection module, is identified, and a notch filter integrated feedback controller is designed for the force control. With the designed dual-stage FS-FTS and force controller, the tip of the cutting tool could follow the unknown workpiece contour with a specified constant contact force, and the surface profile is obtained through the measured displacement of the primary stage. The feasibility and superiority of the developed system are validated through the in-process inspection of micro-lens with a depth of 19μm. The results demonstrate that the developed FS-FTS realizes the sub-micron profile measurement with a scanning speed of 150μm/min, and the depth of the resulting surface scratches is less than 20 nm.

Mixed-mode oscillations of an atomic force microscope in tapping mode.

In the phenomenon of mixed-mode oscillations, transitions between large-amplitude and small-amplitude oscillations may lead to anomalous jitter in the probe of a tapping mode atomic force microscope (TM-AFM) during the scanning process, thereby affecting the accuracy and clarity of the topographical images of the tested sample's surface. This work delves deeply into various mixed-mode oscillations and the corresponding formation mechanisms in TM-AFM under low-frequency resonant excitation. Through a detailed analysis of bifurcation sets of the fast subsystem, we found that the system's mixed-mode oscillations encompass the typical two coexisting branches and the novel three coexisting branches of equilibrium point attractors. In the stable case, a certain transition pattern in phase trajectory can be observed involving two jumps and four jumps, switching between quiescent and spiking states. In the bi-stable case, the trajectory undergoes distinct transitions decided by whether to pass through or crossover the middle branch of attractors when bifurcation occurs. By applying basin of attraction and fast-slow analysis methods, we unfold the dynamic mechanism of mixed-mode oscillations with distinct switching patterns. Our research contributes to a better understanding of complex oscillations of TM-AFM and provides valuable insights for improving image quality and measurement precision while mitigating detrimental oscillations.

Thickness measurement of thin films using atomic force microscopy based scratching

Thin-film thickness measurements using atomic force microscopy (AFM) comprise two steps: 1. AFM scratching in order to produce an exposed film edge, and 2. subsequent AFM measurement of the corresponding step height across the exposed edge. Although the technique is known, many open questions have limited its wider applications. In order to clarify the open questions, here we first demonstrate how to determine the normal force applied during the scratching in contact mode needed to completely remove films from substrates. In order to determine film thickness from processed AFM images, we discuss two procedures based on the histogram method and polynomial step-function fitting. Mechanisms of the scratching process are elucidated by the analysis of lateral forces and their enhancement during the film peeling. Phase maps of scratched domains recorded in amplitude modulation AFM (tapping) mode display a clear contrast compared to pristine films. Therefore, we suggest their utilization as simple indicators of spatial domains with completely removed films. As an example, here the measurements were done on polymer films fabricated by layer-by-layer deposition of oppositely charged polyelectrolytes composed of poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate), while the applicability of the presented method on other materials is discussed in detail.

Study of Phase Imaging Contrast Optimization in Tapping Mode Atomic Force Microscopy

In tapping mode atomic force microscopy (TM‐AFM), the phase image can reveal more about the physicochemical properties of the sample surface than the topography image, making it widely utilized in fields such as materials science and life sciences. However, obtaining high phase contrast in phase images during experiments is challenging, thus studying the phase imaging theory of TM‐AFM and optimizing phase contrast methods hold significant importance. This paper derives a theoretical expression for phase contrast based on phase theory, discovering that phase contrast is directly related to the frequency ratio and background dissipation quality factor. Through theoretical, experimental, and simulation approaches, optimization methods for phase contrast were proposed from the perspectives of frequency ratio and background dissipation quality factor as follows: (1) By fitting experimental results with the theoretical expression for phase contrast, the optimal frequency ratio for scanning can be determined, and imaging with this optimal ratio can enhance phase contrast and (2) reducing the width ratio between the free and fixed ends of the probe can decrease background dissipation, increase the proportion of core dissipation in the total energy dissipation of the probe, and thereby improve phase imaging contrast. These findings are significant in guiding TM‐AFM phase scanning experiments and optimizing phase imaging contrast.

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

There are several imaging modes in AFM, and the tapping mode is the most commonly used scanning mode. Tapping mode can acquire the height information and phase information of the sample surface, among which the phase information has more value, which can reflect the physical properties of the sample surface. In order to understand the phase imaging mechanism of AFM, this paper uses the vibration theory to derive the theoretical expression of phase, and finds that the excitation frequency will directly affect the phase contrast. Based on this, this paper finds, through theoretical and experimental analysis, that there exists an optimal excitation frequency that maximizes the phase contrast during the scanning process. These results are important for interpreting the phase image of AFM and thus optimizing the phase imaging in experiments.

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.