Peak Force Tapping Mode Research Articles

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.

Atomic force microscopy (AFM) and its applications to bone-related research

Bone consists of organic (mostly collagen) and inorganic (mostly bioapatite mineral) components that are organized into hierarchical structures from nano-to macro-scales that provide load-bearing functions. The structures and properties of bone are affected by bone remodeling activities, which are affected by mechanotransduction, a process through which mechanical signals are converted to biochemical signals in cellular signaling. Atomic Force Microscopy (AFM) technique can be used to characterize the surface morphology and mechanical properties of the specimens and can achieve atomic resolution in the resulting images. Therefore, the AFM technique has been applied in bone research and has provided new understandings of the structures and properties of hierarchical structures in bone across multiple length scales. This review begins by introducing the tip-surface interactions and the operation modes of AFM, including the recently developed sub-resonance modes, including PeakForce Tapping mode. Then the contact adhesion theories used in analyzing AFM data are reviewed, followed by a systematic review of the applications of the AFM technique to bone and bone-related tissues and cells, including surface morphology imaging, contact indentation testing, and other mechanical tests. The applications of sub-resonance tapping mode to bone and other biological molecules, cells, and tissues are also reviewed.

Study on mechanical properties of the interfacial transition zone in carbon nanofiber-reinforced cement mortar based on the PeakForce tapping mode of atomic force microscope

In this study, the PeakForce quantitative Nanoscale Mechanical Characterization (PF-QNM) mode was used to explore the micro-zone mechanical properties of the interfacial transition zone (ITZ) in carbon nanofiber (CNF)-reinforced cement mortar. It was found that adding CNFs increased the Young’s modulus of neat cement paste and cement mortar specimens by 15.5% and 36.7%, respectively. The morphology of the interface in a mortar specimen under the PF-QNM mode showed the close embedding of fine aggregates in set cement, which may facilitate force transmission and energy dissipation. The filling and bridging effects of CNFs may increase the Young’s modulus of set cement. The flexural strengths and Young’s modulus of specimens were measured by macro-mechanical tests; the results of which were found to be consistent with the results of micro-zone mechanical property tests. The convenience and accuracy of the test results of the PF-QNM mode were found to be promising.

(Digital Presentation) Activating the Ion Transmission at the Cathode-Electrolyte Interface in All-Solid-State Batteries

All-solid-state batteries (ASSBs) have garnered increasing attention due to the enhanced safety, featuring nonflammable solid electrolytes as well as the potential to achieve high energy density. 1 The advancement of the ASSBs is expected to provide, arguably, the most straightforward path towards practical, high-energy, and rechargeable batteries based on metallic anodes.1 However, the sluggish ion transmission at the cathode-electrolyte (solid/solid) interface would result in the high resistant at the contact and limit the practical implementation of these all solid-state materials in real world batteries.2 Several methods were suggested to enhance the kinetic condition of the ion migration between the cathode and the solid electrolyte (SE).3 A composite strategy that mixes active materials and SEs for the cathode is a general way to decrease the ion transmission barrier at the cathode-electrolyte interface.3 The active material concentration in the cathode is reduced as much as the SE portion increases by which the energy density of the ASSB is restricted. In addition, the mixing approach generally accompanies lattice mismatches between the cathode active materials and the SE, thus providing only limited improvements, which is imputed by random contacts between the cathode active materials and the SE during the mixing process. Implementing high-pressure for the electrode and electrolyte of ASSB in the assembling process has been verified is a but effective way to boost the ion transmission ability between the cathode active materials and the SE by decreasing the grain boundary impedance. Whereas the short-circuit of the battery would be induced by the mechanical deformation of the electrolyte under high pressure. 4 Herein, we demonstrate a novel way to address the ion transmission problem at the cathode-electrolyte interface in ASSBs. Starting from the cathode configuration, the finite element method (FEM) was employed to evaluate the current concentration and the distribution of the space charge layer at the cathode-electrolyte interface. Hierarchical three-dimensional (HTD) structures are found to have a higher Li+ transfer number (tLi+), fewer free anions, and the weaker space-charge layer at the cathode-electrolyte interface in the resulting FEM simulation. To take advantage of the HTD structure, stereolithography is adopted as a manufacturing technique and single-crystalline Ni-rich (SCN) materials are selected as the active materials. Next, the manufactured HTD cathode is sintered at 600 °C in an N2 atmosphere for the carbonization of the resin, which induces sufficient electronic conductivity for the cathode. Then, the gel-like Li1.4Al0.4Ti1.6(PO4)3 (LATP) precursor is synthesized and filled into the voids of the HTD structure cathode sufficiently. And the filled HTD structure cathodes are sintered at 900 °C to achieve the crystallization of the LATP gel. Scanning transmission electron microscopy (STEM) is used to unveil the morphology of the cathode-electrolyte interface between the sintered HTD cathode and the in-situ generated electrolyte (LATP). A transient phase has been found generated at the interface and matched with both lattices of the SCN and the SE, accelerating the transmission of the Li-ions, which is further verified by density functional theory calculations. In addition, Electron Energy Loss Spectroscopy demonstrates the preserved interface between HTD cathode and SEs. Atomic force microscopy is employed to measure the potential image of the cross-sectional interface by the peak force tapping mode. The average potential of modified samples is lower than the sample that mix SCN and SEs simply in the 2D planar structure, which confirms a weakened space charge layer by the enhanced contact capability as well as the ion transmission ability. To see if the demonstrated method is universally applicable, LiNi0.8Co0.1Mn0.1O2 (NCM811) is selected as the cathode active material and manufactured in the same way as the SCN. The HTD cathode based on NCM811 exhibits higher electrochemical performance compared with the reference sample based on the 2D planar mixing-type cathode. We believe such a demonstrated universal strategy provides a new guideline to engineer the cathode/electrolyte interface by revolutionizing electrode structures that can be applicable to all-solid-state batteries.Figure 1. Schematic of comparing of traditional 2D planar cathode and HTD cathode in ASSB Tikekar, M. D., et al., Nature Energy (2016) 1 (9), 16114 Banerjee, A., et al., Chem Rev (2020) 120 (14), 6878 Chen, R., et al., Chem Rev (2020) 120 (14), 6820 Cheng, X., et al., Advanced Energy Materials (2018) 8 (7) Figure 1

Internal topology and water transport in tetrafunctional epoxy resins

AbstractWater sorption into epoxy resins limits the performance lifetime of many composites, adhesives, and coatings. One hypothesis for the well‐known hygroscopicity of epoxies is that the well‐known internal nodular topology of epoxy networks enhances permeability. This theory has however remained untested to date, since previous attempts to manipulate the internal topology have been accompanied by changes in the hydrogen bonding capability of resins, which dominates water uptake. Here, the nanostructure of highly cross‐linked network polymers based on tetra glycidyl diaminodiphenyl methane is varied in the absence of significant polarity effects by careful optimization of the cure schedule. The internal morphology of resins cured at 100, 150, and 200°C are characterized using Peakforce tapping mode atomic force microscopy, whilst the cure progression at each temperature is analyzed using in‐situ transmission mode infrared spectroscopy. Distinct nodular morphologies are found to form at high cure temperatures (150 and 200°C), where spectroscopic analysis demonstrates that excess epoxy consumption occurs prior to the gel point, as a result of intra‐cluster cross‐linking and reduced reaction selectivity. Surprisingly, the establishment of an internal nanostructure is, however, found to have negligible effect on the extent and kinetics of moisture sorption.

Nanomechanical Atomic Force Microscopy to Probe Cellular Microplastics Uptake and Distribution

The concerns regarding microplastics and nanoplastics pollution stimulate studies on the uptake and biodistribution of these emerging pollutants in vitro. Atomic force microscopy in nanomechanical PeakForce Tapping mode was used here to visualise the uptake and distribution of polystyrene spherical microplastics in human skin fibroblast. Particles down to 500 nm were imaged in whole fixed cells, the nanomechanical characterization allowed for differentiation between internalized and surface attached plastics. This study opens new avenues in microplastics toxicity research.

Super-resolution mid-infrared spectro-microscopy of biological applications through tapping mode and peak force tapping mode atomic force microscope.

Small biomolecules at the subcellular level are building blocks for the manifestation of complex biological activities. However, non-intrusive in situ investigation of biological systems has been long daunted by the low spatial resolution and poor sensitivity of conventional light microscopies. Traditional infrared (IR) spectro-microscopy can enable label-free visualization of chemical bonds without extrinsic labeling but is still bound by Abbe's diffraction limit. This review article introduces a way to bypass the optical diffraction limit and improve the sensitivity for mid-IR methods - using tip-enhanced light nearfield in atomic force microscopy (AFM) operated in tapping and peak force tapping modes. Working principles of well-established scattering-type scanning near-field optical microscopy (s-SNOM) and two relatively new techniques, namely, photo-induced force microscopy (PiFM) and peak force infrared (PFIR) microscopy, will be briefly presented. With∼10-20nm spatial resolution and monolayer sensitivity, their recent applications in revealing nanoscale chemical heterogeneities in a wide range of biological systems, including biomolecules, cells, tissues, and biomaterials, will be reviewed and discussed. We also envision several future improvements of AFM-based tapping and peak force tapping mode nano-IR methods that permit them to better serve as a versatile platform for uncovering biological mechanisms at the fundamental level.

Study on Mechanical Properties of the Interfacial Transition Zone in Carbon Nanofiber-Reinforced Cement Mortar Based on the Peakforce Tapping Mode of Atomic Force Microscope

Study on Mechanical Properties of the Interfacial Transition Zone in Carbon Nanofiber-Reinforced Cement Mortar Based on the Peakforce Tapping Mode of Atomic Force Microscope

Open-loop amplitude-modulation Kelvin probe force microscopy operated in single-pass PeakForce tapping mode.

The open-loop (OL) variant of Kelvin probe force microscopy (KPFM) provides access to the voltage response of the electrostatic interaction between a conductive atomic force microscopy (AFM) probe and the investigated sample. The measured response can be analyzed a posteriori, modeled, and interpreted to include various contributions from the probe geometry and imaged features of the sample. In contrast to this, the currently implemented closed-loop (CL) variants of KPFM, either amplitude-modulation (AM) or frequency-modulation (FM), solely report on their final product in terms of the tip–sample contact potential difference. In ambient atmosphere, both CL AM-KPFM and CL FM-KPFM work at their best during the lift part of a two-pass scanning mode to avoid the direct contact with the surface of the sample. In this work, a new OL AM-KPFM mode was implemented in the single-pass scan of the PeakForce Tapping (PFT) mode. The topographical and electrical components were combined in a single pass by applying the electrical modulation only in between the PFT tip–sample contacts, when the AFM probe separates from the sample. In this way, any contact and tunneling discharges are avoided and, yet, the location of the measured electrical tip–sample interaction is directly affixed to the topography rendered by the mechanical PFT modulation at each tap. Furthermore, because the detailed response of the cantilever to the bias stimulation was recorded, it was possible to analyze and separate an average contribution of the cantilever to the determined local contact potential difference between the AFM probe and the imaged sample. The removal of this unwanted contribution greatly improved the accuracy of the AM-KPFM measurements to the level of the FM-KPFM counterpart.

Robustness of surfactant-laden latex films

Despite the growing popularity of latex inks in the inkjet printing industry, little is known about the influence of additives present in the ink mixtures on the robustness of the latex films. Here, we investigate the influence of a non-ionic surfactant, Dynol 607, on the abrasion robustness of a water-based latex film. For fixation temperature below the minimal film formation temperature (MFFT), we observe an improvement of robustness of the film with increasing surfactant concentration due to the improved film coalescence. However, for temperatures exceeding the MFFT, Dynol clearly has a negative impact on the abrasion robustness. Using AFM in the PeakForce tapping mode we have demonstrated that the surfactant forms ultra-thin surfactant layers at the top of the latex film. Force spectroscopy performed with the colloidal probes showed that these layers can increase the adhesion between the film and abrasive material leading to decreased abrasion robustness. However, other effects such as: macroscopic corrugation of the film or the surfactant layer between latex film and the substrate that may have influence on the mechanical performance of the film, were also identified. Our research showcases that mechanical robustness of latex films is affected by surfactant in various ways, which are poorly understood, even though surfactants are commonly used in latex ink formulations.

Water-in-Water Emulsions, Ultralow Interfacial Tension, and Biolubrication

Water-in-Water Emulsions, Ultralow Interfacial Tension, and Biolubrication

Scanning thermal microscopy for accurate nanoscale device thermography

We investigate the accuracy and reliability of temperature mapping using scanning thermal microscopy (SThM) in contact and PeakForce tapping mode on the example of a GaN-on-SiC high electron mobility transistor (HEMT). HEMT steady-state and transient surface temperatures are extracted from SThM measurements to study the method’s accuracy and transient thermal response of the SThM probe; the results are verified by 3D finite element thermal simulation calibrated by Raman thermography. A reliable pixel-by-pixel calibration method to convert the measured electromotive force into surface temperature was developed. Discrete point measurements show good agreement (±3 °C) with the simulation in both contact and PeakForce tapping modes proving the feasibility of the SThM for accurate device thermography at the nanoscale. However, the measured temperature in calibrated 2D temperature maps deviates by as much as ~15–44% from the simulation, suggesting the SThM probe did not reach the temperature steady state due to limitations in pixel dwell time during the recording of the 2D map.

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

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

Total Internal Reflection Peak Force Infrared Microscopy.

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

Nanoscale Chemical Features of the Natural Fibrous Material Wood.

Peak force infrared (PFIR) microscopy is a recently developed approach to acquire multiple chemical and physical material properties simultaneously and with nanometer resolution: topographical features, infrared (IR)-sensitive maps, adhesion, stiffness, and locally resolved IR spectra. This multifunctional mapping is enabled by the ability of an atomic force microscope tip in the peak force tapping mode to detect photothermal expansion of the sample. We report the use of the PFIR to characterize the chemical modification of bio-based native and intact wooden matrices, which has evolved into an increasingly active research field. The distribution of functional groups of wood cellulose aggregates, either in native or carboxylated states, was detected with a remarkable spatial resolution of 16 nm. Furthermore, mechanical and chemical maps of the distinct cell wall layers were obtained on polymerized wooden matrices to localize the exact position of the modified regions. These findings shall support the development and understanding of functionalized wood materials.

Applications of atomic force microscopy in immunology.

Cellular mechanics, a major regulating factor of cellular architecture and biological functions, responds to intrinsic stresses and extrinsic forces exerted by other cells and the extracellular matrix in the microenvironment. Cellular mechanics also acts as a fundamental mediator in complicated immune responses, such as cell migration, immune cell activation, and pathogen clearance. The principle of atomic force microscopy (AFM) and its three running modes are introduced for the mechanical characterization of living cells. The peak force tapping mode provides the most delicate and desirable virtues to collect high-resolution images of morphology and force curves. For a concrete description of AFM capabilities, three AFM applications are discussed. These applications include the dynamic progress of a neutrophil-extracellular-trap release by neutrophils, the immunological functions of macrophages, and the membrane pore formation mediated by perforin, streptolysin O, gasdermin D, or membrane attack complex.

Water Molecule-Triggered Anisotropic Deformation of Carbon Nitride Nanoribbons Enabling Contactless Respiratory Inspection

The exploitation of the interaction between nanostructured matter and small molecules, such as H2O at interfaces via dynamic hydrogen bonding, is essentially the key for smart, responsive nanodevic...

Surface potential response from GaP nanowires synthesized with mixed crystal phases

In this work, we investigate variations of surface potentials along a single gallium phosphide (GaP) nanowire (NW) synthesized with a mixed crystal phase along the growth direction. GaP NWs synthesized with both wurtzite (WZ) and zincblende (ZB) phases were studied. The measurements were performed on a standard Atomic Force Microscopy (AFM) set-up equipped with Kelvin Probe Force Microscopy (KPFM) module in PeakForce Tapping Mode. KPFM Measurements from two structures were analyzed. Variations of surface poten-tials were observed in a single GaP NW with WZ/ZB segments. An average difference in surface potential was 55±11 mV. This is explained by different crystal structures along the NW. The work expands the understanding of crystal structure-dependent electrical transport properties of GaP NWs.

Microscopic Analysis of PEMFC Catalyst Layers

Proton exchange membrane fuel cell (PEMFC) electrodes have a complex structure of carbon supported platinum (Pt) nanoparticles intermixed with proton conducting ionomer. This structures creates a porous network facilitating transport of electrons, protons, and reactants. These catalyst layers have been shown to exist with a non-homogenous distribution of ionomer, with aggregates and agglomerates of carbon and ionomer. To better understand the catalyst layer structure, we have applied atomic force microscopy (AFM) to investigate fuel cell catalyst layer properties at high resolution. Various ionomer-carbon (I/C) ratios were analyzed in order to identify correlations between carbon agglomerate size, pore distribution, and structure. Surface and cross-sectional images of topography, adhesion, and roughness were collected. Scanning electron microscopy (SEM) was performed to allow for additional depictions of the samples. Several other techniques are planned to further explore structures. Electrochemical capabilities of the AFM will be utilized to inspect the effects of current on the aforementioned properties and provide better control in measurements. Additionally, samples will be placed in a water-filled holder to study effects of swelling through in situ AFM. Transmission electron microscopy (TEM) will also be performed to complement these measurements. A Bruker AFM was used in PeakForce tapping mode with a ScanAsyst Air tip to obtain the images in Figure 1. Figure 1a shows the height map of a sample with an I/C ratio of 1. Figure 1b is the adhesion map of the same sample, showing clear spatial adhesion contrast. In this map, lighter colors correspond to the more adhesive ionomer. Darker areas signify hard materials like carbon. Figure 1c depicts the adhesion map overlaid on the height map, indicating how the adhesion and topography relate. The arrows in Figure 1c illustrate where the ionomer thickness can be measured between carbon aggregates. Figure 2a summarizes the average ionomer thickness calculated for a range of I/C ratios from 0.25 to 1.25. There is a positive correlation between I/C ratio and average ionomer thickness, suggesting that increased availability of ionomer leads to thicker surface coatings and more separation between the carbon aggregates. Figure 2b shows the relationship between I/C ratio and aggregate diameter. These values were measured by processing the images in ImageJ and reducing aggregates to approximated spheres. Although no clear relationship is apparent now, further investigation with differing I/C ratios may reveal a trend. Acknowledgements : This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). This work was also performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Figure 1

Cationic-Surfactant-Coated Mica Surfaces below the Critical Micellar Concentration: 1. Patchy Structures As Revealed by Peak Force Tapping AFM Mode.

The morphology and structure of the self-assembled surfactant aggregates at the solid-liquid interface remain controversial. For the well-studied system of cationic cetyltrimethylammonium bromide (C16TAB) adsorbed onto the opposite negatively charged, atomically smooth mica surface, a variety of surface aggregates have been previously reported: AFM imaging pointing to cylinders and surface micelles as opposed to mono/bilayer-like structures revealed by neutron and X-ray reflectometry, NMR, spectroscopic techniques, and numerical simulations. To reconcile with the latter results, we revisit the morphometry of the C16TAB-coated mica surfaces using the recent peak force tapping (PFT-AFM) mode that allows fragile structures to be imaged with the lowest possible applied force. The evolution of the structural organization at the mica-water interface is investigated above the Krafft boundary over a wide concentration range (from 1/1000 to 2 cmc) after long equilibration times to ensure thermodynamic equilibrium. A complex but fairly complete picture has emerged: At very low concentrations, the C16TAB surfactants adsorb as isolated molecules before forming small clusters. Above 1/140 cmc, monolayer-like stripes are formed. As the concentration is increased, a connected network of these patches progressively covers the mica substrate. Above 1/80 cmc, bilayer-like patches build on top of the underlying monolayer, and ultimately a complete bilayer (at about half the cmc) covers the entire mica substrate. Thanks to the less invasive PFT-AFM imaging mode, our observations not only agree with the theoretical predictions and numerical simulations but also reconcile, at last, the direct observations by means of the AFM imaging technique with the results obtained with other techniques.