Atomic Force Microscopy Probe Research Articles

AFM probe for measuring ∼10−5 ultra-low friction coefficient: Design and application

Superlubricity provides a novel approach to addressing friction and wear issues in mechanical systems. However, little is known regarding improving the atomic force microscope (AFM) friction coefficient measurement resolution. Accordingly, this study established the theoretical formula for the AFM friction coefficient measurement and deduced the measurement resolution. Then, the formula was applied to the AFM probe with a rectangular cross-section cantilever. The measurement resolution is associated with the dimensional properties of the AFM probe, the mechanical properties of the cantilever material, the properties of the position-sensitive detector (PSD), and probably the anti-vibration performance of the AFM. It is feasible to make the cantilever as short as possible and the tip as high as possible to improve the measurement resolution. An AFM probe for measuring an ultra-low friction coefficient was designed and fabricated. The cantilever’s length, width, and thickness are 50, 35, and 0.6 µm, respectively. The tip height is 23 µm. The measurement resolution can reach 7.1×10−6 under the maximum normal force. Moreover, the AFM probe was applied to measure the superlubricity between graphene layers. The friction coefficient is 0.00139 under 853.08 nN. This work provides a promising method for measuring a ∼10−5 friction coefficient of superlubricity.

The Formation of Au Nanoparticles in SiO2–TiO2 Films by Local Electrochemical Reduction Using an Atomic Force Microscope Probe

The Formation of Au Nanoparticles in SiO2–TiO2 Films by Local Electrochemical Reduction Using an Atomic Force Microscope Probe

Critical analysis of adhesion work measurements from AFM-based techniques for soft contact

HypothesisThe work of adhesion is a thermodynamic quantity that is frequently measured by atomic force microscopy (AFM). Determination of the work of adhesion requires quasi-equilibrium measurements, where we address the question of to what extent atomic force microscopy qualifies for quasi-equilibrium measurements. ExperimentTo measure the work of adhesion, we combined soft colloidal probe AFM (SCP AFM) with reflection interference contrast microscopy (RICM). This allowed us to extract the work of adhesion either from the pull-off force or from the contact radius. With these methods, we investigated the adhesion behavior of poly(N-isopropylacrylamide) (PNIPAM) polymer brushes in the swollen and solvent-induced collapsed state by systematically analyzing contact radii and adhesive forces. FindingsIn the swollen state, the adhesion to the PNIPAM brush was fivefold larger and exhibited significant time dependencies when measured with SCP AFM. A strong rate dependence of the pull-off force method was indicative of a non-equilibrium process. In order to reliably determine the equilibrium work of adhesion, the contact radius method was found to be the better because it is not rate dependent. Our work highlights the important benefits of using optical measurements to determine the contact radius when deriving the works of adhesion between colloidal probes and polymer brush surfaces.

Mechanical Properties of 3-Hydroxybutyric Acid-Induced Vesicles

The vesicle mechanical behaviors were studied upon its exposure to 3-hydroxybutyric acid using an atomic force microscope (AFM). Dipalmitoylphosphatidylcholine (DPPC) and 3-hydroxybutyric acid were used to manufacture the vesicles at their desired ratio. The deflection of an AFM probe with respect to its displacement was measured after characterizing the vesicle adsorption. The movement was analyzed with the Hertzian model to understand the physical behavior of the vesicles. However, in the deflection just prior to the first penetration, the model was a good fit, and the vesicle mechanical moduli were calculated. The moduli became lower with the higher ratio of 3-hydroxybutyric acid to DPPC, but the moduli were saturated at 0.5 of the ratio. These results appear to be the basis for the function of the metabolism associated with 3-hydroxybutyric acid, i.e., anesthetization and glycemic control, on the physical properties of cell membranes.

Beating thermal noise in a dynamic signal measurement by a nanofabricated cavity optomechanical sensor.

Thermal fluctuations often impose both fundamental and practical measurement limits on high-performance sensors, motivating the development of techniques that bypass the limitations imposed by thermal noise outside cryogenic environments. Here, we theoretically propose and experimentally demonstrate a measurement method that reduces the effective transducer temperature and improves the measurement precision of a dynamic impulse response signal. Thermal noise-limited, integrated cavity optomechanical atomic force microscopy probes are used in a photothermal-induced resonance measurement to demonstrate an effective temperature reduction by a factor of ≈25, i.e., from room temperature down as low as ≈12 K, without cryogens. The method improves the experimental measurement precision and throughput by >2×, approaching the theoretical limit of ≈3.5× improvement for our experimental conditions. The general applicability of this method to dynamic measurements leveraging thermal noise-limited harmonic transducers will have a broad impact across a variety of measurement platforms and scientific fields.

Chitosan-Based Architectures as an Effective Approach for the Removal of Some Toxic Species from Aqueous Media.

Modified uncrosslinked and crosslinked chitosan derivatives were investigated as green sorbents for the removal of copper (Cu2+) and lead (Pb2+) cations from simulated solutions. In this regard, N, O carboxymethyl chitosan (N, O CMC), chitosan beads (Cs-g-GA), chitosan crosslinked with glutaraldehyde/methylene bisacrylamide (Cs/GA/MBA), and chitosan crosslinked with GA/epichlorohydrin (Cs/GA/ECH) were prepared and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy analyses. Atomic force microscopy investigation was carried out to compare the surface topography of the prepared samples before and after the metal uptake. The kinetics of the removal process were investigated by pseudo-first-order and -second-order models. Moreover, the adsorption isotherms were carefully studied by applying Langmuir and Freundlich models. The data reveal that upon adsorption of copper(II) metal ions, all chitosan-modified products followed the Langmuir isotherm except for Cs/GA/ECH which followed the Freundlich isotherms, and the highest adsorption capacity (q e) was obtained for Cs/GA/MBA due to the formation of stable chelate structures between the metal cation and the functional groups present on the modified chitosan product. The order of metal uptake at the optimum pH value is as follows: Cs/GA/MBA (Cu: 95.7 mg/g, Pb: 99.15 mg/g), Cs/GA/ECH (Cu: 80.4 mg/g, Pb: 93.14 mg/g), Cs-g-GA (Cu: 77 mg/g, Pb: 88.4 mg/g), and N, O CMCh (Cu: 30.2 mg/g, Pb: 44.8 mg/g). The AFM data confirmed the metal uptake process by comparing the roughness and height measurements of the free sorbents and the metal-loaded sorbents.

Creating Skyrmions in Thin CoPt Films with an Atomic Force Microscope Probe

Creating Skyrmions in Thin CoPt Films with an Atomic Force Microscope Probe

Formation of skyrmions in thin CoPt films with an atomic force microscope probe

Methods of magnetic force microscopy have been developed that make it possible to visualize the evolution of the domain structure when scanning a sample with a magnetic probe. These methods were used to study the processes of formation of skyrmions in thin CoPt films, a characteristic feature of which is the presence of the Dzyaloshinskii–Moriya interaction. A change in the position, shape, and size of skyrmions under the action of a spatially inhomogeneous magnetic field of the probe has been experimentally demonstrated.

A New Method of AFM‐Based Nanolithography Using Frequency Enhanced Electrochemical Pressure Solution Etching

AbstractA new method of direct‐write nanolithography that is able to rapidly etch silica surfaces under a scanning atomic force microscopy (AFM) probe in tapping mode (TM) is reported. In this lithography technique, silica surfaces are etched using a recently described electro‐chemo‐mechanical phenomenon of frequency enhanced electrochemical pressure solution (FEEPS). In FEEPS, the tapping of the AFM tip generates oscillations of the Stern potential at the silica‐water interface that can accelerate the silica dissolution kinetics by more than 5–6 orders of magnitude when surface resonance states are achieved; i.e., when the oscillation frequency is in phase with the dynamics of interfacial chemical reaction steps. By scanning silica surfaces in TM, silica is selectively dissolved below the tapping tip as it is scanned. The FEEPS accelerated silica dissolution rates can generate etched features with depths of more than 60 nm in a single AFM tip pass. The rate of etching can be controlled easily by varying the scanning rate or by modulating the tapping frequency. This fine control over the silica etching process and because material is removed (dissolved) rather than displaced as with nanoscratching, the FEEPS process lends itself to gray‐scale nanolithography which is demonstrated.

In situ Growth and Characterization of Lubricious Carbon-Based Films Using Colloidal Probe Microscopy

Silicon oxide-doped hydrogenated amorphous carbon (a-C:H:Si:O) is an important form of diamond-like carbon (DLC) for tribological applications, primarily because of its enhanced thermal stability and reduced dependence of friction on environmental humidity. As with other DLCs, its mechanisms of lubrication are still an active area of research, though it is now known that surface passivation and tribofilm growth are important factors. In this study, tribofilm formation for a-C:H:Si:O is examined at the microscale by using steel colloid atomic force microscopy probes as the sliding counterface. This approach provides some inherent advantages over macroscale tribology experiments, namely that the tribofilm thickness and stiffness can be tracked in situ and correlated directly with the friction response. The results of these experiments show that the tribofilm grows rapidly on the steel colloid following a period of counterface wear and high friction. The friction drops more than 80% upon nucleation of the tribofilm, which is attributed to a decrease of more than 80% in adhesion combined with a decrease in the estimated interfacial shear strength of at least 65%. Approximately 80% of the friction decrease occurs before the tribofilm reaches a thickness of 2 nm, suggesting that only the near-surface properties of the tribofilm provide the needed functionality for its effective lubrication mechanisms.

Bottom-Up Generated Height Gauges for Silicon-Based Nanometrology.

Steady progress in integrated circuit design has forced basic metrology to adopt silicon lattice parameter as a secondary realization of the SI meter that lacks convenient physical gauges for precise surface measurements at a nanoscale. To employ this fundamental shift in nanoscience and nanotechnology, we propose a set of self-organized silicon surface morphologies as a gauge for height measurements within the whole nanoscale (0.3-100 nm) range. Using 2 nm sharp atomic force microscopy (AFM) probes, we have measured the roughness of wide (up to 230 μm in diameter) singular terraces and the height of monatomic steps on the step-bunched and amphitheater-like Si(111) surfaces. For both types of self-organized surface morphology, the root-mean-square terrace roughness exceeds 70 pm but has a little effect on step height measurements having 10 pm accuracy for AFM technique in air. We implement a step-free 230-μm-wide singular terrace as a reference mirror in an optical interferometer to reduce the systematic error of height measurements from >5 nm to about 0.12 nm, which allows visualizing 136-pm-high monatomic steps on the Si(001) surface. Then, using a "pit-patterned" extremely wide terrace with dense but counted monatomic steps in a pit wall, we have optically measured mean Si(111) interplanar spacing (313.8 ± 0.4 pm) that agrees well with the most precise metrological data (313.56 pm). This opens up avenues for the creation of silicon-based height gauges using bottom-up approaches and advances optical interferometry among techniques for metrology-grade nanoscale height measurements.

Review—Lithium Carbon Composite Material for Practical Lithium Metal Batteries

Comprehensive SummaryLithium (Li) metal is considered ideal for high‐energy‐density batteries due to its extremely high specific capacity and low electrochemical potential. However, uncontrolled Li dendrite growth and interfacial instability during repeated Li plating/stripping have limited the practical applicability of Li metal batteries (LMBs). Over the past decades, substantial efforts have been devoted to solving the challenges associated with Li metal anodes. Our research team has developed several Li‐carbon (Li‐C) microsphere composites in recent years to suppress the formation of Li dendrites and achieve a decent cycle life. In this account, we summarize our advances in the design and application of Li‐C composites, which include the developments in the structure and chemical composition of high‐specific‐capacity Li‐C composites, strategies for surface passivation of the micro‐spherical Li‐C composites, and applications of the Li‐C composite in next‐generation high‐energy‐density Li‐ion, Li‐air, and solid‐state LMBs. Finally, we discuss future perspectives for developing high‐performance Li metal anodes and endeavors to realize the practical applications of LMBs.What is the most favorite and original chemistry developed in your research group?Accurate control of surface chemistry of battery materials in general, and Li metal in particular, with self‐assembled monolayers (SAM).How do you get into this specific field? Could you please share some experiences with our readers?Li metal is an ideal anode material for rechargeable batteries except that it is extremely reactive towards the environment and that the conversion reaction tends to deposit Li metal into dendrites. My group developed a Li‐C composite by infiltrating molten Li into carbon nanotube microspheres. This composite largely alleviated the dendrite growth problem of Li anode, but the reactivity of Li metal caused the Li‐C microsphere impossible to process outside Ar gloveboxes. A passivation method is needed.I recalled the molecular self‐assembly technique I learned and used in my Ph.D. dissertation, where I used the Au‐thiol SAM chemistry to control the surface chemical interactions between atomic force microscopy (AFM) probes and samples. The question is whether we can find the right chemistry between Li metal and self‐assembling molecules. This was how we started the journey of SAM on Li metal, and later further developed the idea to other battery materials such as high‐Ni NCM cathode, etc. How do you supervise your students?Students first need to build their own knowledge base, a set of scientific concepts and theoretical framework they are comfortable with and a set of experimental skills they are confident in. Then we define research directions together. Once we agree on a direction and the student starts to explore the direction with his/her knowledge and skills, here it comes the most enjoyable part of supervising students: I get to ask questions. It is through addressing questions, students learn to practice critical thinking, polish logic and reasoning, and eventually they observe and ask questions on their own. They become independent.What are your hobbies? What's your favorite book(s)?Jogging and reading. I like many books. To recommend just a couple: Oracle bones by Peter Hessler, and the comic books Calvin and Hobbes.Who influences you mostly in your life?Historical figures: Su Shi and Wang yangming. Scientists: Professors Charles Lieber and Louis Brus.If you have anything else to tell our readers, please feel free to do so.Genuinely interested in other people's interest seems to be the key of making friends; I also recommend it as a chemist's approach to make friends with the earth.

In Situ Atomic Force Microscopy and X‐ray Computed Tomography Characterization of All‐Solid‐State Lithium Batteries: Both Local and Overall

All‐solid‐state lithium batteries (ASSLBs) are promising due to their high‐energy output and low‐risk profile, but their development has only just begun. Atomic force microscopy (AFM) and related techniques have had an impact on ASSLBs research by elucidating the interfacial, morphological, mechanical, electrical, and electrochemical properties of a wide range of electrodes and electrolytes. However, because a cross‐section cut is necessary to define the solid–solid interface, true in situ analysis is not practical. The first part of this review will assess recent advancements in the study of ASSLBs utilizing AFM and other scanning probe microscopy techniques. The interior solid–solid interfaces can be illuminated in situ using X‐ray computed tomography (X‐CT) and other nondestructive characterization techniques, whereas, in contrast, to deepen the subject, it is further examined how X‐CT vary from the use of other instruments for solid‐state battery characterization, compare the information that various methods may give, and assess how well they can accurately reflect real batteries. This review may serve as a reference and point researchers in the direction of future study on the solid–solid interface of ASSLBs.

New Mechanical Models to Study the Impact of Contact, Wear, and Adhesion of Lunar Dust on Space Materials

Lunar dust significantly impacts spacecraft and spacesuits during lunar exploration. The mechanical properties of lunar dust, including adhesion, contact, and wear, are investigated using new mechanical models proposed herein. First, the main contribution for mechanics of this paper was implementing the conical Johnson, Kendall, and Roberts (JKR) model using the energy method for investigating the normal and tangential mechanical characteristics of lunar dust. Additionally, the mechanical properties of lunar dust, such as adhesion, contact, and wear are proposed considering the adhesion effect using the mechanical model. The validity and accuracy of the conical JKR model were verified by comparing the results with experimental data and existing spherical and cylindrical JKR models. Furthermore, a new wear mechanical model for typical lunar dust is proposed considering the adhesion effect. The wear width and coefficients based on the wear model fit well with the existing experiment; therefore, the proposed model can be used to calculate the wear coefficient. Additionally, it can be used to estimate the contact force using an atomic force microscope (AFM) probe and analyze the adhesion between rough particles. The mechanical properties of lunar dust based on our models can be valuable for the characteristic research of in situ resource development and manned lunar landings.

Structural identification of graphene films and nanoislands on 6H-SiC(0001) by direct height measurement

By combining non-contact atomic force microscopy (nc-AFM) and Kelvin probe microscopy (KPFM) in ultra high vacuum environment (UHV), we directly measure the height and work function of graphene monolayer on the Si-face of 6H-SiC(0001) with a precision that allows us to differentiate three different types of graphene structures : zero layer graphene (ZLG), Quasi free-standing monolayer graphene (QFMLG) and bilayer graphene (BLG). The height and work function of ZLG are 2.62 ± 0.22 Å and 4.42 ± 0.05 eV respectively, when they are 4.09 ± 0.11 Å and 4.63 ± 0.05 eV for QFMLG. The work function is 4.83 ± 0.05 eV for the BLG. Unlike any other available technique, the local nc-AFM/KPFM dual probe makes it possible to directly identify the nature of nanometer-sized graphene islands that constitute the early nuclei of graphene monolayer grown on 6H-SiC(0001) by chemical vapor deposition.

Characterization of adhesion strength between carbon nanotubes and cementitious materials

Multi-walled carbon nanotube (MWCNT) and carbon nanofiber (CNF) additions increase the elastic modulus, flexural strength, and toughness of Portland cement concrete. However, the interaction mechanism between cement constituents and these nanomaterials is not fully understood. A modified MWCNT-coated atomic force microscopy (AFM) probe is developed by coating a silica particle with oxidized MWCNT through layer-by-layer assembly and adhering it to a tipless AFM cantilever. The probe allows measurement of adhesion between MWCNT and the substrate with a force control procedure. SEM-EDS is acquired in the same region as AFM measurements through a benchmarking scheme to correlate chemistry with the measured adhesion. Statistical deconvolution shows C–S–H regions have lower adhesion to MWCNT than intermixed regions (C–S–H/Clinker). Furthermore, in C–S–H regions, the normalized adhesion strength increases with calcium concentration. This result is due to the higher interaction between the oxygen functional groups in the MWCNT surface and the calcium in the substrate.

Single bubble probe atomic force microscope and impinging-jet technique unravel the interfacial interactions controlled by long chain fatty acid in anaerobic digestion

Anaerobic digestion of lipid-rich wastewater generally suffers from foaming induced by long chain fatty acid (LCFA). However, a systematic understanding of LCFA inhibition, especially the physical inhibition on interfacial interaction still remains unclear. Here, we combined bubble probe atomic force microscope and impinging-jet technique to unravel the interfacial interactions controlled by long chain fatty acids in anaerobic digestion. We showed that LCFA had a significant inhibition on methane production in anaerobic reactors for the inhibition of the conversion of VFAs to methane. By measuring the LCFA influence on methanogenic archaea Methanosarcina acetivorans C2A, the results demonstrated that methanogenesis was limited for substrates utilization but not metabolic pathways. The impinging-jet technique results indicated that LCFA enhanced bubble separation from anaerobic granules and reduced the bubble-bubble coalescence probability. In addition, the bubble probe atomic force microscope (AFM) revealed that LCFA enhanced the adhesion force between bubbles by enhancing electrical double layer (EDL) repulsion and decreasing hydrophobic interactions. Overall, these results complement framework of LCFA inhibition in anerobic digestion and provide a nanomechanical insight into the fundamental interfacial interactions related to bubbles in anaerobic reactors.

Interplay of Fluid Mechanics and Matrix Stiffness in Tuning the Mechanical Behaviors of Single Cells Probed by Atomic Force Microscopy.

It is well known that both fluid mechanics and matrix stiffness present within the cellular microenvironments play an essential role in the physiological and pathological processes of cells. However, so far, knowledge of the interplay of fluid mechanics and matrix stiffness in tuning the mechanical behaviors of single cells is still extremely limited. Particularly, atomic force microscopy (AFM) is now an important and standard tool for characterizing the mechanical properties of single living cells. Nevertheless, studies of utilizing AFM to detect cellular mechanics are commonly performed in static medium conditions, which are unable to access the effects of fluidic media on cellular behaviors. Here, by integrating AFM with a fluidic cell medium device and hydrogel technology, the combined effects of fluid mechanics and matrix stiffness on cell mechanics were investigated. A fluidic medium device with tunable fluid mechanics was established to simulate the shear flow effects, and hydrogels were used to fabricate substrates with different stiffnesses for cell growth. Especially, the cantilever of the AFM probe was modified with a microsphere to indent cells for probing cell mechanics. Based on the established experimental platform, the elastic and viscous properties of single living cells grown on substrates with tunable matrix stiffness under fluidic microenvironments were quantitatively measured, and the remarkable alterations in the mechanical properties of cells were unraveled. The subcellular structure changes of cells in fluidic microenvironments were observed by fluorescence microscopy. Further, AFM morphological imaging was used to image living cells grown in fluidic medium conditions, and significant changes in the surface structure and roughness of cells were observed. The study provides a novel way to investigate the synergistic effects of fluid mechanics and matrix stiffness on the behaviors of single cells, which will benefit unveiling the underlying mechanical cues involved the interactions between microenvironments and cells.

Fundamental and higher eigenmodes of qPlus sensors with a long probe for vertical-lateral bimodal atomic force microscopy.

The detection of vertical and lateral forces at the nanoscale by atomic force microscopy (AFM) reveals various mechanical properties on surfaces. The qPlus sensor is a widely used force sensor, which is built from a quartz tuning fork (QTF) and a sharpened metal probe, capable of high-resolution imaging in viscous liquids such as lubricant oils. Although a simultaneous detection technique of vertical and lateral forces by using a qPlus sensor is required in the field of nanotribology, it has still been difficult because the torsional oscillations of QTFs cannot be detected. In this paper, we propose a method to simultaneously detect vertical and lateral force components by using a qPlus sensor with a long probe. The first three eigenmodes of the qPlus sensor with a long probe are theoretically studied by solving a set of equations of motion for the QTF prong and probe. The calculation results were in good agreement with the experimental results. It was found that the tip oscillates laterally in the second and third modes. Finally, we performed friction anisotropy measurements on a polymer film by using a bimodal AFM utilizing the qPlus sensor with a long probe to confirm the lateral force detection.

Tribochemical nanolithography: selective mechanochemical removal of photocleavable nitrophenyl protecting groups with 23 nm resolution at speeds of up to 1 mm s-1.

We describe the mechanochemical regulation of a reaction that would otherwise be considered to be photochemical, via a simple process that yields nm spatial resolution. An atomic force microscope (AFM) probe is used to remove photocleavable nitrophenyl protecting groups from alkylsilane films at loads too small for mechanical wear, thus enabling nanoscale differentiation of chemical reactivity. Feature sizes of 20-50 nm are achieved repeatably and controllably at writing rates up to 1 mm s-1. Line widths vary monotonically with the load up to 2000 nN. To demonstrate the capacity for sophisticated surface functionalisation provided by this strategy, we show that functionalization of nanolines with nitrilo triacetic acid enables site-specific immobilization of histidine-tagged green fluorescent protein. Density functional theory (DFT) calculations reveal that the key energetic barrier in the photo-deprotection reaction of the nitrophenyl protecting group is excitation of a π-π* transition (3.1 eV) via an intramolecular charge-transfer mechanism. Under modest loading, compression of the adsorbate layer causes a decrease in the N-N separation, with the effect that this energy barrier can be reduced to as little as 1.2 eV. Thus, deprotection becomes possible via either absorption of visible photons or phononic excitation transfer, facilitating fast nanolithography with a very small feature size.