Atomic Force Microscope Tip Research Articles

In situ scrutinize the adsorption of sulfamethoxazole in water using AFM force spectroscopy: Molecular adhesion force determination and fractionation

The interaction force of a typical antibiotic molecule during adsorption has never been experimentally determined and fractionated, which hindered the evolution of removal strategies. In this study, sulfamethoxazole (SMX) as a typical antibiotic was stably immobilized onto an atomic force microscopy (AFM) tip without affecting original properties. The SMX modified AFM tip visualized the potential adsorption sites on a graphene oxide (GO) nanosheet for the first time by mapping the SMX adhesion force distribution. Moreover, the interaction force of a single SMX molecule to GO was determined at 38.6 pN which was subsequently fractionated into the hydrophobic (17.9 pN) and π-π (160.0 pN) attractions as well as the electrostatic repulsion (− 139.3 pN) at pH: 5.7. As compared with highly-ordered pyrolytic graphite (HOPG), the introduced oxygen containing groups on GO not only reduced the hydrophobic interaction but also generated an opposite electrostatic repulsion force to SMX. This study experimentally and theoretically revealed the adhesion mechanisms of SMX and potentially other sulfonamide antibiotics in molecular level, which may contribute to the study of antibiotic environmental transportation and the development of next-generation antibiotic remediation protocols.

Nanoscale wafer patterning using SPM induced local anodic oxidation in InP substrates

Atomic force microscopy (AFM) assisted local anodic oxidation (LAO) offers advantages over other semiconductor fabrication techniques as it is a low contamination method. We demonstrate the fabrication of deep and highly reproducible nanohole arrays on InP using LAO. Nanohole and nano-oxide mound radius and depth are controlled independently by altering AFM tip bias and humidity, with a maximum nanohole depth of 15.6 ± 1.2 nm being achieved. Additionally, the effect of tip write speed on oxide line formation is compared for n-type, p-type and semi-insulating substrates, which shows that n-type InP oxidizes at a slower rate that semi-insulated or p-type InP. Finally, we calculate the activation energy for LAO of semi-insulating InP to be 0.4 eV, suggesting the oxidation mechanism is similar to that which occurs during plasma oxidation.

Material transfer mechanism for fabrication of superlubricity interface by reciprocating rubbing on graphite under high contact stress

Superlubricity is attractive in many scientific and technological fields due to energy-saving and wear reduction. Two incommensurate contacting surfaces (superlubricity interface) slide against each other exhibiting ultralow dynamic friction coefficient below 10−3. Fabrication of the superlubricity interface is the essence to obtain superlubricity. Here we developed a plasma-assisted mechanical exfoliation and tribo-transfer method to fabricate graphene nanoflake wrapped atomic force microscopy (AFM) tips. Superlubricity between graphene-wrapped tips with different diameters and graphite was achieved with a friction coefficient as low as 10−4. Systematic investigations were conducted on material transfer mechanism mainly including delamination, deformation and lateral displacement, and fracture and transfer of graphene nanoflake from thick graphene. Negative friction-load phenomenon and abnormal stick-slip motion on graphene were observed, demonstrating top layers of graphene were deformed and delaminated during rubbing process. Weakened strengthening effect on monolayer graphene and bubble on thick graphene after reciprocating rubbing may prove interlayer sliding and lateral displacement. The general nature of the results may apply to other 2D materials. These results not only provide a technical route to acquire superlubricity interface but also intensify the understanding of superlubricity which is the basis of its application in moving mechanical systems.

High-Velocity Shear and Soft Friction at the Nanometer Scale

We study high-speed friction on soft polymer materials by measuring the amplitude dependence of cyclic lateral forces on the atomic force microscope (AFM) tip as it slides on the surface with fixed contact force. The resulting dynamic force quadrature curves separate the elastic and viscous contributions to the lateral force, revealing a transition from stick-slip to free-sliding motion as the velocity increases. We explain force quadratures and describe how they are measured, and we show results for a variety of soft materials. The results differ substantially from the measurements on hard materials, showing hysteresis in the force quadrature curves that we attribute to the finite relaxation time of viscoelastic surface deformation.

Flexoelectric control of physical properties by atomic force microscopy

The past decade has witnessed the tremendous scientific and technological potential of nanoscale flexoelectricity in solids. The flexoelectric effect describes the universal generation of electric polarization in response to strain gradients and could be inversely enhanced at reduced nanoscale dimensions. Based on this unique scaling effect, nanoscale flexoelectricity has shown exciting physical phenomena, promising novel electronic, electromechanical, and photovoltaic applications. One of the most powerful ways to harness nanoscale flexoelectricity is to press the surface of a material through an atomic force microscope (AFM) tip to generate large strain gradients. This so-called AFM tip pressing allows us to locally break the inversion symmetry in any materials and study all the fascinating physical phenomena associated with inversion asymmetry. Although this technique has recently facilitated many important studies on nanoscale flexoelectricity, its effective use still requires a more solid foundation. In this review, we provide a comprehensive guideline to exploring nanoscale flexoelectricity via AFM tip pressing. We also discuss recent progress and the future research direction of AFM tip pressing-driven nanoscale flexoelectricity.

Reactive-Oxygen-Species-Mediated Surface Oxidation of Single-Molecule DNA Origami by an Atomic Force Microscope Tip-Mounted C60 Photocatalyst.

A tripod molecule incorporating a C60 photocatalyst into a rigid scaffold with disulfide legs was designed and synthesized for the stable and robust attachment of C60 onto an Au-coated atomic force microscope (AFM) tip. The "tripod-C60" was immobilized onto the tip by forming S-Au bonds in the desired orientation and a dispersed manner, rendering it suitable for the oxidation and scission of single molecules on a countersurface, thereby functioning as "molecular shears". A DNA origami with a well-defined structure was chosen as the substrate for the tip-induced oxidation. The gold-coated, C60-functionalized AFM tip was used for both AFM imaging and oxidation of DNA origami upon visible-light irradiation. The localized and temporally controlled oxidative damage of DNA origami was successfully performed at the single-molecule level via singlet-oxygen (1O2) generation from the immobilized C60 on the AFM tip. This oxidative damage to DNA origami can be carried out under ambient conditions in a fluid cell at room temperature, rendering it well-suited for the manipulation of a variety of species on surfaces via a spatially and temporally controlled oxidation reaction triggered by 1O2 locally generated from the immobilized C60 on the AFM tip.

Analysis of the charging kinetics in silver nanoparticles-silica nanocomposite dielectrics at different temperatures

Dielectric nanocomposite materials are now involved in a large panel of electrical engineering applications ranging from micro-/nano-electronics to power devices. The performances of all these systems are critically dependent on the evolution of the electrical properties of the dielectric parts, especially under temperature increase. In this study we investigate the impact of a single plane of silver nanoparticles (AgNPs), embedded near the surface of a thin silica (SiO2) layer, on the electric field distribution, the charge injection and the charge dynamic processes for different AgNPs-based nanocomposites and various temperatures in the range 25°C–110°C. The electrical charges are injected locally by using an Atomic Force Microscopy (AFM) tip and the related surface potential profile is probed by Kelvin Probe Force Microscopy (KPFM). To get deeper in the understanding of the physical phenomena, the electric field distribution in the AgNPs-based nanocomposites is computed by using a Finite Element Modeling (FEM). The results show a strong electrostatic coupling between the AFM tip and the AgNPs, as well as between the AgNPs when the AgNPs-plane is embedded in the vicinity of the SiO2-layer surface. At low temperature (25°C) the presence of an AgNPs-plane close to the surface, i.e., at a distance of 7 nm, limits the amount of injected charges. Besides, the AgNPs retain the injected charges and prevent from charge lateral spreading after injection. When the temperature is relatively high (110°C) the amount of injected charges is increased in the nanocomposites compared to low temperatures. Moreover, the speed of lateral charge spreading is increased for the AgNPs-based nanocomposites. All these findings imply that the lateral charge transport in the nanocomposite structures is favored by the closely situated AgNPs because of the strong electrostatic coupling between them, additionally activated by the temperature increase.

Exploring oscillations with a nonlinear restoring force

When exploring the oscillation of an object for small displacements from the equilibrium position, the magnitude of the applied force is approximately considered to be proportional to the object’s displacement. However, for bigger displacements, studied in this paper, the magnitude of the applied force is best approximated to an equation of the form F = S|x| n , n > 0. In this paper accurate and approximate equations regarding the period of the oscillation and also approximate functions regarding the object’s position with respect to time are being derived. The interesting result is the simplicity of the equation that relates the period to the exponent n and the oscillation’s amplitude at the domain 0 < n ⩽ 2. It is also shown that the force data in many well-known cases can be fitted to the aforementioned equation; typical examples are the polynomial force law, the impact of a sphere on a surface, the free oscillations of an atomic force microscopy tip at the end of a cantilever, the simple pendulum’s free oscillation, etc. Thus, the methodology and the findings presented in this paper can be applied to a variety of different situations under the restriction that the force data can be approximately described by an equation of the form F = S|x| n , n > 0.

Ectoine Effect on Mechanical Properties of Vesicles in Aqueous Solution.

The mechanical properties of the vesicles incorporated with ectoine were studied using atomic force microscope (AFM). The vesicles were prepared with dipalmitoylphosphatidylcholine (DPPC) by changing only the ratio of the ectoine to DPPC. After the vesicles were adsorbed on the mica substrate and their morphology were characterized, the plot of an AFM tip displacement versus the tip deflection was acquired by monitoring the behavior of the tip into the vesicle. The breakthrough of the tip into the vesicle was observed to occur twice. Each breakthrough represented a penetration of the tip into the top and bottom portions of the vesicle, respectively. The force data between the pre-contact and the first breakthrough were comparable with the Hertzian model to estimate Young's modulus and the bending modulus of the vesicles. Both moduli decreased proportionally with the increase in the ratio of ectoine to lipid up to 0.5. However, above 0.5, the moduli were slightly changed with the increase. These results of the mechanical properties appear to be due to the osmotic and volumetric effect on the headgroup packing disruption.

Investigation of the morphological and fractal behavior at nanoscale of patterning lines by scratching in an atomic force microscope.

In this work, the topographical effect of the scratching trajectory and the feed direction on the formation of lithographed lines on the (001) InP surface was investigated using an atomic force microscope (AFM) tip-based nanomachining approach. Nanoscratching tests were carried out using the sharp face of a diamond AFM tip in contact mode. From the topographic maps obtained by AFM, several morphological and fractal parameters were obtained and analyzed. Surface morphology presented a surface smoothing for surfaces with scratches produced in [011] and [001] directions. The height parameters confirmed this behavior because scratches in [001] direction exhibited lower roughness. Moreover, this scratch direction promoted the height distribution most symmetrical and platykurtic. The other morphological parameters revealed that this direction provided a more irregular surface (smaller Smc and Sxp ), peak distribution, denser and pointed, smaller portion of material in the core, less deep furrows, higher spatial frequency components, and high isotropy. Fractal parameters revealed that FRE90 has the highest spatial complexity, it is dominated by higher spatial frequencies, and has the lowest surface percolation. Furthermore, all samples exhibited high topographic uniformity.

Nanomechanical Induction of Autophagy-Related Fluorescence in Single Cells with Atomic Force Microscopy.

Mechanistic understanding of how living systems sense, transduce, and respond to mechanical cues has important implications in development, physiology, and therapy. Here, the authors use an integrated atomic force microscope (AFM) and brightfield/epifluorescent microscope platform to precisely simulate living single cells or groups of cells under physiological conditions, in real time, concomitantly measuring the single‐cell autophagic response and its transmission to neighboring cells. Dual‐color fluorescence monitoring of the cellular autophagic response reveals the dynamics of autophagosome formation, degradation, and induction in neighboring contacting and noncontacting cells. Autophagosome formation is dependent on both the applied force and contact area of the AFM tip. More importantly, the enhancement of the autophagic responses in neighboring cells via a gap junction‐dependent mechanism is observed. This AFM‐based nanoacupuncture platform can serve as a tool for elucidating the primary mechanism underlying mechanical stimulation of living systems and other biomechanical therapeutics.

Analysis of the Wear-Assisted Corrosion of a Super-Duplex Stainless-Steel Using an Interface Film Growth Model

The wear-assisted corrosion response of a super-duplex stainless-steel alloy from a single asperity contact was measured using the contact mode of an atomic force microscope (AFM). The AFM tip, as illustrated in the figure below, applied increasing loads to an isolated surface region that was polarized to two different static potentials in a 0.6 M NaCl solution. The AFM tip was swept across the isolated, exposed, surface region of the alloy while the alloy response was measured by a potentiostat. The wear-assisted corrosion response was modeled using an interfacial film growth model (IFM) that was used to estimate kinetics parameters for the response of the alloy as a function of the applied loads and polarization potentials.The average response values from the IFM were then approximated using an Arrhenius expression that incorporated the peak applied stress and activation volume for the reaction as modifiers to the energy barrier for oxide film formation. This Arrhenius model suggested that the stress fields from the single asperity contact induced damage in the protective oxide and also disrupted other oxide-formation processes at the oxide-electrolyte interface. The induced damage and the disruption of the oxide-formation processes, as a function of the applied stress, slowed repassivation and allowed increasingly high corrosion currents as the stress fields increased.AcknowledgmentsThe U.S. Naval Research Laboratory sponsored this work under work unit 1E19 as part of its Basic Research Program. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government.Figure CaptionSchematic diagram of experimental setup of the AFM tip loading the isolated region on the super-duplex stainless-steel alloy surface while the potentiostat is used to both polarize the alloy and measure its response to the load. Figure 1

Recognition Imaging on Native Chromatin Extracted from Cancerous and Non-Cancerous Cell Lines

Atomic force microscopy (AFM) recognition imaging is a label-free technique which can identify specific molecules from compositionally complex samples while mapping the topography of the samples simultaneously in a liquid environment. It required a sensitive imaging mode. Typically, the so-called MAC mode is used, where the AFM tip is driven directly by a magnetic field which will reduce the movement of liquid surrounding the tip and increase signal to noise ratio. The AFM tips for MAC mode need to have a ferromagnetic coating like nickel in order to be driven by a magnetic field. Commercial MAC mode probes are typically silicon nitride or quartz. We developed a protocol to coat sharp commercial silicon probes with nickel, making them reliable MAC mode tips. We applied our new probes to samples prepared from the supernatant fraction of a nuclear extract of EPC2 (non-cancerous) and CPD (cancerous) esophageal cell lines. Salt fractionation is an efficient method for the extraction of intact chromatin fragments from cell nuclei, but the supernatant fraction was not studied in previous work. With anti-H3 antibody modified MAC mode tips, we identified histones H3 in samples of both cell lines, indicating that this fraction contains chromatin. We proved the specificity by blocking with a peptide mimicking the H3 binding site for our antibody. Further, we demonstrated the dependence of the recognition signals on the oscillation amplitude of the probe, indicating the robustness of our recognition signal. We also analyzed the nuclear extracts for the presence of SMC2, a structural maintenance protein for chromosomes. We applied AFM recognition imaging with anti-SMC2 antibody modified probes to the prepared chromatin samples and identified SMC2 proteins in samples of both cell lines. Western blots confirmed the presence of SMC2 proteins in the samples. Overall, AFM recognition imaging provides a robust way to analyze the structure and molecular components of nuclear extracts from human cells, providing insights into the link between chromatin structure and cell phenotypes.

Measuring Diffusion Coefficients of the Ceria Phase in a Dual-Phase Oxygen Permeation Membrane Using a Combined Polarization - Kelvin Probe Force Microscopy Method

Dual-phase membranes with FeCo2O4 or its iron-rich pendant Fe2CoO4 and Gd-doped ceria as an oxygen ion conductor have already been successfully applied as oxygen permeation membranes with high permeability in the temperature range above 800 °C [1-3]. Apart from Gd-doped ceria, Sm-doped ceria also can be an interesting alternative in this kind of composite due to its high ionic conductivity [4].As especially ceria or ceria-based composites have gained more and more interest for low-temperature applications, e.g. as catalyst materials, current research efforts are aiming to improve the composition and microstructure of such dual-phase membranes for application in a membrane reactor at considerably lower temperatures (below 600 °C) to perform partial oxidation reactions. For temperatures between 600-1200 °C the defect chemistry of these materials is well established, but experimental data for low temperature charge transport are limited and not fully understood, yet.Kelvin Probe Force Microscopy (KPFM) is an Atomic Force Microscopy (AFM)-based measurement method which can measure the local surface potential (or from the physics view: the Volta potential) of the sample [5, 6]. The surface potential is a measure for local changes of the defect chemistry, as it is directly related to the local Fermi niveau [7]. The implications of local oxidation and reduction at low temperatures for charge carrier mobility in pure ceria, and especially in dual-phase materials, are not very well understood so far, but KPFM can be used to measure the surface potential difference directly at the polarized contact area as well as to map the time-dependent relaxation process of the introduced gradient, also giving information about the local surface potential distribution, lateral extent of the gradient, variations of the shape of the gradient etc. [8, 9].In the present study, single ceria grains in a dual-phase Ce0.8Sm0.2O2-δ + FeCo2O4 oxygen permeation membrane were polarized at room temperature in ambient air using a Pt-coated AFM tip and a large silver paste back contact. Subsequently, the introduced defect concentration gradient (which is visible as a gradient of the surface potential) and the relaxation process over time was monitored by KPFM.By comparing the measurement results to previously acquired data of single phase ceria samples with different doping concentrations and crystallinity, we were able to show that by this technique, chemical diffusion coefficients of a single phase in a composite material are accessible. Acknowledgements: The work was funded by the German Research Foundation - project #387282673. Literature: Ramasamy, M.; Baumann, S.; Palisaitis, J.; Schulze‐Küppers, F.; Balaguer, M.; Kim, D.; Meulenberg, W. A.; Mayer, J.; Bhave, R.; Guillon, O. J Amer Ceram Soc 2016, 99 (1), 349-355.Ramasamy, M.; Persoon, E. S.; Baumann, S.; Schroeder, M.; Schulze-Küppers, F.; Görtz, D.; Bhave, R.; Bram, M.; Meulenberg, W. A. Journal of membrane science 2017, 544, 278-286.Lin, Y.; Fang, S.; Su, D.; Brinkman, K. S.; Chen, F. Nature Comm 2015, 6 (1), 1-9.Mori, T.; Drennan, J.; Lee, J.-H.; Li, J.-G.; Ikegami, T. Solid State Ionics 2002, 154-155, 461-466.Örnek, C.; Leygraf, C.; Pan, J. Corr Eng Sci Tech 2019, 54 (3), 185-198.Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Surf Sci Rep 2011, 66 (1), 1-27.Zhang, Z.; Yates, J. T. Chem Rev 2012, 112 (10), 5520-5551.Neuhaus, K.; Schulze-Küppers, F.; Baumann, S.; Ulbrich, G.; Lerch, M.; Wiemhöfer, H.-D. Solid State Ionics 2016, 288, 325-330.Lee, W.; Lee, M.; Kim, Y.-B.; Prinz, F. B. Nanotechnology 2009, 20 (44), 445706.

(Invited) Nanoscale Probing of Field-Driven Ion Migration in TaOx for Neuromorphic Memristor Applications

In semiconducting metal oxides, application of electric fields often causes migration of the constituent ions (oxygen ions, metal cations, and ionic vacancies) via ionic conduction and diffusion. The field-driven ion migration has been considered a key phenomenon in the recent development of oxide-based resistive random access memory (ReRAM) and memristive neuromorphic devices because their operations are generally made through the migration and resultant changes in the valence/resistance states of the oxide layers. Semiconducting amorphous TaOx is known as a practical material of switching layers in ReRAMs, and moreover, has recently been considered of critical importance in the development of neuromorphic hardware because of its variety of neuromorphic resistive switching functions. Many important functions, including multilevel (or analog) resistive switching, stochastic switching, and spike-rate/spike-timing dependent plasticity, have been demonstrated in junctions with a structure of metal/TaOx/metal, and the origin of the neuromorphic functionality has been ascribed to the high ionic conductivities (both for O2− and Ta5+) and degree of structural freedom of amorphous TaOx.Regarding the mechanisms of the neuromorphic switching, however, limited understanding has been achieved in the previous works. The mechanisms have so far been inferred from the switching behaviors in electrical measurements or numerical modeling because of the experimental difficulty in observing nanoscale ion migration during the neuromorphic resistive switching. To directly design of the neuromorphic functions and establish oxide-based neuromorphic computing, therefore, experimental probing of the ion migration processes is now importantly required for neuromorphic resistive switching of TaOx.In this study, we fabricated ultra-flat thin films of amorphous TaOx (3.0–10 nm thick) with a root-mean-square roughness of <0.2 nm by the pulsed laser deposition method, which will offer suitable conditions for ion migration analysis by probe microscopy. In the conductive tip atomic force microscopy investigations, we observed a stable resistive switching phenomenon at arbitrary positions on the TaOx surface by voltage applications from a probe. The positional independence of the switching characteristics ensures the experimental advantages of the ultra-flat films in the investigations. Additionally, this indicates that "easy spots" for resistive switching, of which the formation has been suggested in other metal oxides, are not present in the thin films of amorphous TaOx, and initial clustering of oxygen vacancies is not importantly involved in the switching. When multiple voltage stresses of >3.0 V were applied from a probe, we observed a multilevel switching phenomenon in the TaOx thin films. In the measurements, we observed that the multilevel switching appears with very small amount of structural changes of TaOx (<1.0 nm in the out-of-plane direction), and relatively large-scale structural deformations (2.0 nm in the out-of-plane direction and 150 nm in the in-plane direction) started to be observed only after breakdown-like behaviors in the resistance with formation of a 10 nm-sized conductive spot. This indicates that the intrinsic ion migration distance involved in the multilevel switching of TaOx is in a range of <1.0 nm in out-of-plane direction, which is smaller than previous predictions. From these results, the spatial extents of neuromorphic resistive switching of TaOx were revealed in this study, which will offer important insights into the process development for TaOx-based neuromorphic memristor.

Controlled growth of a single carbon nanotube on an AFM probe

Carbon nanotubes (CNTs) can be used as atomic force microscopy (AFM) tips for high-resolution scanning due to their small diameter, high aspect ratio and outstanding wear resistance. However, previous approaches for fabricating CNT probes are complex and poorly controlled. In this paper, we introduce a simple method to selectively fabricate a single CNT on an AFM tip by controlling the trigger threshold to adjust the amount of growth solution attached to the tip. The yield rate is over 93%. The resulting CNT probes are suitable in length, without the need for a subsequent cutting process. We used the CNT probe to scan the complex nanostructure with a high aspect ratio, thereby solving the long-lasting problem of mapping complex nanostructures.

AFM study of organic ligand packing on gold for nanoparticle drug delivery applications

Gold nanoparticles (AuNPs) functionalized with polyethylene glycol (PEG) and peptide ligands are promising drug delivery vectors for use in cancer therapy. However, to control the interaction of the ligands with blood proteins and specific cell receptors, more needs to be known about the arrangement and properties of these grafted ligands. This issue is examined here principally with atomic force microscopy (AFM) for ligands grafted to both gold films and gold nanoparticles. Macro-level sessile drop measurements on the films indicate that, in dilute solutions, the PEG and peptide ligands exhibit similar affinity to the gold surface. However, at the nano-level, AFM experiments show differences in the morphology and cohesion of the adsorbed ligand films. Differences in tip/surface adhesion are also measured, indicative of variations of hydrophilicity for the top surface of these ligand films. Analysis of force-separation curves suggests a low packing density of PEG ligands. AFM investigations of functionalized AuNPs shows that the ligands improve dispersion and modify the tip/surface adhesion behaviour on the nanoparticles. However, AFM images of as-synthesised citric-capped and functionalized AuNPs gave similar nanoparticle diameters. Analysis of the AFM tip tapping on the AuNPs suggests that Tapping-AFM is not suitable for detection of low packing density ligands.

Investigating the effect of nanoscale triboelectrification on nanofriction in insulators

Nanofriction and nanoscale triboelectrification is a universally existing phenomenon and important in many branches of nanotechnology, but how the nanoscale triboelectrification affects the nanofriction during the relative sliding at the contact interface is not fully investigated. Here, nanoscale triboelectrification and nanofriction on the insulating SiO2 surface were investigated with atomic force microscopy (AFM) and scanning Kelvin probe microscopy. By the AFM tip sliding on the SiO2 surface, both triboelectric charges and nanofriction were investigated with the applied load, sliding cycle, and external electric field. Whether the tribocharges on the SiO2 surface were negative or positive, the nanofriction increases with the enhanced tribocharge density by nanoscale triboelectrification, while decreases with the reduced tribocharges by charge diffusion. The change of nanofriction correlates strongly with the variation in tribocharge density due to the tribocharge induced excess electrostatic interactions at the sliding interface. These findings not only add to our understanding of the role of nanoscale triboelectrification on nanofriction, but also help to control energy dissipation of sliding nanofriction, which is also important in many branches of nanotechnology, such as data storage and nanoelectromechanical system.

Quantitative Local Probing of Polarization with Application on HfO2 -Based Thin Films.

Owing to their switchable spontaneous polarization, ferroelectric materials have been applied in various fields, such as information technologies, actuators, and sensors. In the last decade, as the characteristic sizes of both devices and materials have decreased significantly below the nanoscale, the development of appropriate characterization tools became essential. Recently, a technique based on conductive atomic force microscopy (AFM), called AFM-positive-up-negative-down (PUND), is employed for the direct measurement of ferroelectric polarization under the AFM tip. However, the main limitation of AFM-PUND is the low frequency (i.e., on the order of a few hertz) that is used to initiate ferroelectric hysteresis. A significantly higher frequency is required to increase the signal-to-noise ratio and the measurement efficiency. In this study, a novel method based on high-frequency AFM-PUND using continuous waveform and simultaneous signal acquisition of the switching current is presented, in which polarization-voltage hysteresis loops are obtained on a high-polarization BiFeO3 nanocapacitor at frequencies up to 100kHz. The proposed method is comprehensively evaluated by measuring nanoscale polarization values of the emerging ferroelectric Hf0.5 Zr0.5 O2 under the AFM tip.

Exceptional Elasticity of Microscale Constrained MoS2 Domes.

The outstanding mechanical performances of two-dimensional (2D) materials make them appealing for the emerging fields of flextronics and straintronics. However, their manufacturing and integration in 2D crystal-based devices rely on a thorough knowledge of their hardness, elasticity, and interface mechanics. Here, we investigate the elasticity of highly strained monolayer-thick MoS2 membranes, in the shape of micrometer-sized domes, by atomic force microscopy (AFM)-based nanoindentation experiments. A dome’s crushing procedure is performed to induce a local re-adhesion of the dome’s membrane to the bulk substrate under the AFM tip’s load. It is worth noting that no breakage, damage, or variation in size and shape are recorded in 95% of the crushed domes upon unloading. Furthermore, such a procedure paves the way to address quantitatively the extent of the van der Waals interlayer interaction and adhesion of MoS2 by studying pull-in instabilities and hysteresis of the loading–unloading cycles. The fundamental role and advantage of using a superimposed dome’s constraint are also discussed.