Atomic Force Microscope Tip Research Articles

Selective Patterning of p‐Type Dopants on MoS2 via Atomic Force Microscopy‐Assisted Surface Modification

The selective patterning of gold chloride (AuCl3) dopants on few‐layer molybdenum disulfide (MoS2) flakes is investigated with the objective of manipulating the electrical properties of 2D material‐based devices. MoS2, renowned for its exceptional properties, is doped with AuCl3 via spin coating. The removal of the dopants is achieved using contact‐mode atomic force microscopy (AFM), with control over the extent of dopant obtained by varying the AFM tip's contact force. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy confirm the patterning of the dopants. The application of this patterning method to MoS2‐based field‐effect transistors results in rectified output characteristics, in contrast to the ohmic behavior observed in pristine and fully coated devices. This method offers a valuable approach for customizing MoS2 surfaces, advancing 2D material‐based electronic devices and next‐generation semiconductor designs with tailored properties.

Finite element modelling of atomic force microscopy imaging on deformable surfaces.

Atomic force microscopy (AFM) provides a three-dimensional topographic representation of a sample surface, at nanometre resolution. Computational simulations can aid the interpretation of such representations, but have mostly been limited to cases where both the AFM probe and the sample are hard and not compressible. In many applications, however, the sample is soft and therefore deformed due to the force exerted by the AFM tip. Here we use finite element modelling (FEM) to study how the measured AFM topography relates to the surface structures of soft and compressible materials. Consistent with previous analytical studies, the measured elastic modulus in AFM is generally found to deviate from the elastic modulus of the sample material. By the analysis of simple surface geometries, the FEM modelling shows how measured mechanical and topographic features in AFM images depend on a combination of tip-sample geometry and indentation of the tip into the sample. Importantly for the interpretation of AFM data, nanoparticles may appear larger or smaller by a factor of two depending on tip size and indentation force; and a higher spatial resolution in AFM images does not necessarily coincide with a more accurate representation of the sample surface. These observations on simple surface geometries also extend to molecular-resolution AFM, as illustrated by comparing FEM results with experimental data acquired on DNA. Taken together, the FEM results provide a framework that aids the interpretation of surface topography and local mechanics as measured by AFM.

How Much Force is Needed to Kill a Single Bacterium?

The interaction between bacteria and nanomaterials, particularly from a physical or mechanical perspective, has emerged as a topic of significant interest in both science and medicine. Mechanobactericidal nanomaterials, which exert antimicrobial effects through purely physical mechanisms, hold promise as alternative strategies to combat bacterial resistance to traditional antibiotics. High-aspect-ratio nanoparticles and surface topographies are being engineered to enhance their mechanobactericidal properties. However, progress in this field is hindered by an incomplete understanding of how these materials induce mechanical cell death in bacteria. This review examines the role of atomic force microscopy (AFM) nanoindentation in quantifying forces required to rupture the bacterial cell wall. The reported values range from nN to a few tens of nN, depending on the type of bacterium and the experimental conditions used. The potential effect of AFM tip properties, loading speed, bacterial immobilization strategy, or environmental conditions on the measured rupture values are discussed. This perspective also highlights the complexities of modeling bacterial cell rupture and the importance of pressure as a parameter for standardizing results across experiments. Furthermore, the implications of these quantitative insights to understand the mechanisms of action of mechanobactericidal nanomaterials are discussed.

The Effects of Short-Range Intermolecular Repulsive Forces on Hamaker Constant Estimation Using Atomic Force Microscopy.

The atomic force microscope (AFM), as it is able to characterize surface topography as well determine the AFM cantilever tip-surface force, proves effective at estimating the value of the Hamaker constant, A, for a given solid material. Two main AFM-based methods have been proposed for estimating values of A. In the approach-to-contact (AtC) method, Hamaker constants are inferred from the deflections at which the AFM tip first jumps into contact with the substrate. In the pull-off (PO) method, the deflections that arise when the AFM tip is finally pulled away from the substrate are used to estimate values of A. In prior applications of these two methods, the short-range intermolecular repulsive forces that are known to arise between the AFM tip and the substrate were, however, ignored. Upon invoking a physically relevant description of these short-range and steeply repulsive forces, we investigate the effects of repulsive interactions on the values of A estimated from the existing AtC and PO methods when applied to neutrally charged systems under low humidity or vacuum conditions. For experimentally relevant surfaces, we find that repulsive forces have a modest effect on the AtC method, although they still need to be accounted for in order to generate improved estimates of the Hamaker constant. On the other hand, repulsive forces have a significant effect on the Hamaker constants inferred from the PO method, and must be properly accounted for when using this approach. Our analysis also includes an explicit incorporation of surface roughness into the PO method, which is not typically done in most prior applications of the PO method.

Electric-Field Control of the Local Thermal Conductivity in Charge Transfer Oxides.

Phonons, the collective excitations responsible for heat transport in crystalline insulating solids, lack electric charge or magnetic moment, which complicates their active control via external fields. This presents a significant challenge in designing thermal equivalents of basic electronic circuit elements, such as transistors or diodes. Achieving these goals requires precise and reversible modification of thermal conductivity in materials. In this work, the continuous tuning of local thermal conductivity in charge-transfer SrFeO3-x and La0.6Sr0.4CoO3-x oxides using a voltage-biased Atomic Force Microscopy (AFM)tip at room temperature is demonstrated. This method allows the creation of micron-sized domains with well-defined thermal conductivity, achieving reductions of up to 50%, measured by spatially resolved Frequency Domain Thermoreflectance (FDTR). By optimizing the oxide's chemical composition, the thermal states remain stable under normal atmospheric conditions but can be reverted to their original values through thermal annealing in air. A comparison between Mott-Hubbard and charge-transfer oxides reveals the critical role of redox-active lattice oxygen in ensuring full reversibility of the process. This approach marks a significant step toward fabricating oxide-based tunable microthermal resistances and other elements for thermal circuits.

Enhanced mechanical hardness of mixed-phase BiFeO3 films through quenching

Due to epitaxial strain, BiFeO3 (BFO) thin films exhibit a morphotropic phase boundary with coexisting rhombohedral-like (R-like) and tetragonal-like (T-like) phases. The T-like phase, distinguished by its large c/a ratio and giant polarization, has garnered extensive interest. In this work, by quenching an epitaxial mixed-phase BFO thin film grown on a LaAlO3 substrate, a pronounced transition from the R-like to the T-like phase is observed. This transition is concomitant with improved phase structure stability under the force field induced by an atomic force microscope tip. PeakForce Quantitative NanoMechanics mapping reveals that the T-like phase exhibits a higher Young's modulus than the R-like phase, signifying an overall enhancement in the mechanical hardness of the BFO film. This work introduces a simple but powerful approach to manipulating the fraction of the T-like phase in the mixed-phase BFO films, presenting prospects for enhancing their performance and expanding their application range in advanced techniques.

Anomalous Nernst Effect-Based Near-Field Imaging of Magnetic Nanostructures.

The anomalous Nernst effect (ANE) gives rise to an electrical response transverse to magnetization and an applied temperature gradient in a magnetic metal. A nanoscale temperature gradient can be generated by the use of a laser beam applied to the apex of an atomic force microscope tip, thereby allowing for spatially resolved ANE measurements beyond the optical diffraction limit. Such a method has been previously used to map in-plane magnetized magnetic textures. However, the spatial distribution of the out-of-plane temperature gradient, which is needed to fully interpret such ANE-based imaging, was not studied. We therefore use a well-known magnetic texture, a magnetic vortex core, to demonstrate the reliability of the ANE method for imaging of magnetic domains with nanoscale resolution. Moreover, since the ANE signal is directly proportional to the temperature gradient, we can also consider the inverse problem and deduce information about the nanoscale temperature distribution. Our results together with finite element modeling indicate that besides the out-of-plane temperature gradients there are even larger in-plane temperature gradients. Thus, we extend the ANE imaging to study the out-of-plane magnetization in a racetrack nanowire by detecting the ANE signal generated by in-plane temperature gradients. In all cases, a spatial resolution of ≈70 nm is obtained. These results are significant for the rapidly growing field of thermoelectric imaging of antiferromagnetic spintronic device structures.

Submillisecond Electric Field Sensing with an Individual Rare-Earth Doped Ferroelectric Nanocrystal.

Understanding the dynamics of electrical signals within neuronal assemblies is crucial to unraveling complex brain functions. Despite recent advances in employing optically active nanostructures in transmembrane potential sensing, there remains room for improvement in terms of response time and sensitivity. Here, we report the development of such a nanosensor capable of detecting electric fields with a submillisecond response time at the single-particle level. We achieve this by using ferroelectric nanocrystals doped with rare-earth ions that produce upconversion (UC). When such a nanocrystal experiences a variation of surrounding electric potential, its surface charge density changes, inducing electric polarization modifications that vary, via a converse piezoelectric effect, the crystal field around the ions. The latter variation is finally converted into UC spectral changes, enabling optical detection of the electric potential. To develop such a sensor, we synthesized erbium and ytterbium-doped barium titanate crystals of ≈160 nm in size. We observed distinct changes in the UC spectrum when individual nanocrystals were subjected to an external field via a conductive atomic force microscope tip, with a response time of 100 μs. Furthermore, our sensor exhibits a remarkable sensitivity of 4.8 kV/cm/, enabling time-resolved detection of a fast-changing electric field of amplitude comparable to that generated during a neuron action potential.

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.

Accurate Modelling of AFM Force-Indentation Curves with Blunted Indenters at Small Indentation Depths.

When testing biological samples with atomic force microscopy (AFM) nanoindentation using pyramidal indenters, Sneddon's equation is commonly used for data processing, approximating the indenter as a perfect cone. While more accurate models treat the AFM tip as a blunted cone or pyramid, these are complex and lack a direct relationship between applied force and indentation depth, complicating data analysis. This paper proposes a new equation derived from simple mathematical processes and physics-based criteria. It is accurate for small indentation depths and serves as a viable alternative to complex classical approaches. The proposed equation has been validated for ℎ < 3R (where h is the indentation depth and R is the tip radius) and confirmed through simulations with blunted conical and pyramidal indenters, as well as experiments on prostate cancer cells. It is a reliable method for experiments where the tip radius cannot be ignored, such as in shallow indentations on thin samples to avoid substrate effects.

Novel hybrid interference and atomic force microscopy

Abstract A novel hybrid microscope which combines an interference microscopic (IM) and an atomic force microscopic (AFM) measurement mode is introduced. It is realised by adding an AFM probe to an IM, where the AFM probe can be mechanically switched in or out of the beam path of the IM. When the AFM cantilever is out of the beam path, the system works in the IM measurement mode for noncontact and fast optical measurements. When the AFM cantilever is in the beam path, it works in the AFM measurement mode, where the AFM tip interacts with sample surfaces for measurements. The deformation of the AFM cantilever induced by the tip-sample interaction force is detected from interference fringes, which are formed by the interference of the light beam reflected from the backside of the AFM cantilever and a reference beam in the IM. This novel design has a high-level synergy of AFM and IM technologies and provides several promising application potentials. For instance, it can bring high-resolution AFM measurements whenever and wherever needed in IM measurements, thus, to complement the lateral resolution limits of IM; The AFM measurement mode can provide measurement results with higher topography fidelity, thus, to offer in-situ reference areal surface metrology for IM measurements. In the paper, design concept, realisation of a prototype instrument, and experimental results illustrating the performance of the prototype instrument are detailed. &amp;#xD;

On-demand nanoengineering of in-plane ferroelectric topologies.

Hierarchical assemblies of ferroelectric nanodomains, so-called super-domains, can exhibit exotic morphologies that lead to distinct behaviours. Controlling these super-domains reliably is critical for realizing states with desired functional properties. Here we reveal the super-switching mechanism by using a biased atomic force microscopy tip, that is, the switching of the in-plane super-domains, of a model ferroelectric Pb0.6Sr0.4TiO3. We demonstrate that the writing process is dominated by a super-domain nucleation and stabilization process. A complex scanning-probe trajectory enables on-demand formation of intricate centre-divergent, centre-convergent and flux-closure polar structures. Correlative piezoresponse force microscopy and optical spectroscopy confirm the topological nature and tunability of the emergent structures. The precise and versatile nanolithography in a ferroic material and the stability of the generated structures, also validated by phase-field modelling, suggests potential for reliable multi-state nanodevice architectures and, thereby, an alternative route for the creation of tunable topological structures for applications in neuromorphic circuits.

Physicochemical Changes of Apoferritin Protein during Biodegradation of Magnetic Metal Oxide Nanoparticles.

The biodegradation of therapeutic magnetic-oxide nanoparticles (MONPs) in the human body raises concerns about their lifespan, functionality, and health risks. Interactions between apoferritin proteins and MONPs in the spleen, liver, and inflammatory macrophages significantly accelerate nanoparticle degradation, releasing metal ions taken up by apoferritin. This can alter the protein's biological structure and properties, potentially causing health hazards. This study examines changes in apoferritin's shape, electrical surface potential (ESP), and protein-core composition after incubation with cobalt-ferrite (CoFe2O4) oxide nanoparticles. Using atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM), we observed changes in the topography and ESP distribution in apoferritin nanofilms over time. After 48 h, the characteristic apoferritin hole (∼1.35 nm) vanished, and the protein's height increased from ∼3.5 to ∼7.5 nm due to hole filling. This resulted in a significant ESP increase on the filled-apoferritin surface, attributed to the formation of a heterogeneous chemical composition and crystal structure (γ-Fe2O3, Fe3O4, CoO, CoOOH, FeOOH, and Co3O4). These changes enhance electrostatic interactions and surface charge between the protein and the AFM tip. This approach aids in predicting and improving the MONP lifespan while reducing their toxicity and preventing apoferritin deformation and dysfunction.

Kinetics of monolayer MoS2-encapsulated nanobubbles on hexagonal boron nitride substrates

Understanding the kinetics of nanobubbles encapsulated by ultrathin two-dimensional (2D) layered van der Waals crystal membranes on atomically flat substrates is important to the applications of 2D materials and the pursuit of 2D nanobubble technologies. Here, we investigate the controlled motion of monolayer molybdenum disulfide (MoS2)-encapsulated nanobubbles on flat hexagonal boron nitride substrates using atomic force microscopy (AFM). Our study reveals a distinct transition from standstill bubble deformations to stable, stepwise bubble translations on flat substrates. The membrane tension-dominated 2D nanobubble behaves like an elastic soft body in its collision interaction with the AFM tip. This delicate motion-control technique enables neighboring 2D nanobubbles to move closer and eventually coalesce into larger nanobubbles. These findings pave the way for high-precision manipulation of nanobubbles and facilitate the exploration of their emerging applications.

Magnetic Monopole-Like Behavior in Superparamagnetic Nanoparticle Coated With Chiral Molecules.

Superparamagnetic iron oxide nanoparticles (SPIONs) have attracted wide attention due to their promising applications in biomedicine, chemical catalysis, and magnetic memory devices. In this work, the force is measured between a single SPION coated with chiral molecules and a ferromagnetic substrate by atomic force microscopy (AFM), with the substrate magnetized either toward or away from the approaching AFM tip. The force between the coated SPION and the magnetic substrate depends on the handedness of the molecules adsorbed on the SPION and on the direction of the magnetization of the substrate. By inserting nm-scale spacing layers between the coated SPION and the magnetic substrate it is shown that the SPION has a short-range magnetic monopole-like magnetic field. A theoretical framework for the nature of this field is provided.

Application of the nuclear tracks methodology for the validation of atomic force microscope probe tips

Atomic force microscopy (AFM) is a well-established technique for studying materials at nanoscale dimensions. The resolution of this instrument strongly relies on the sharpness of its tip, which can become compromised through wear and breakage, often due to contamination at the AFM tip apex. To evaluate the condition of both new and used tips, scanning electron microscopy (SEM) inspection is commonly employed. However, SEM services may not always be readily available or in high demand. In this study, we present an AFM tip calibration device that utilizes a pattern of etched tracks on CR-39 material. To construct this device, you will require a radioactive alpha particle source, typically Americium-241, as well as a controlled temperature bath set at 60 degrees Celsius, which contains a 6.25 M KOH solution. This endeavor serves as another intriguing and practical application of the nuclear tracks methodology.

Growth phase-dependent changes in conformational and physicochemical properties of bacterial surface biopolymers lead to changes in bacterial adhesion strength at nanoscale and macroscale

This study presents a detailed investigation into conformational, physicochemical and adhesive characteristics of biopolymers on the surfaces of Escherichia coli and potential probiotic Bacillus subtilis harvested from middle-exponential phase (mid-EP), late-exponential phase (late-EP) and early-stationary phase (early-SP) of growth. The lengths to which bacterial surface biopolymers extend (biopolymer brush lengths), densities of grafted bacterial surface biopolymers indicating the amounts of molecules covering the bacterial surfaces (biopolymer grafting densities), adhesion forces of bacterial surface biopolymers to the model inert surfaces of silicon nitride (Si3N4), and the pull-off distances of biopolymers from Si3N4 were measured in water by atomic force microscopy (AFM). The Weibull analysis of AFM adhesion data showed that as the culture aged, the adhesive bonds between Si3N4 AFM tips and surface molecules of E. coli harvested from the culture were broken with a higher applied force. However, the highest applied force to break the bonds was required for B. subtilis in late-EP, followed by those required for cells in early-SP and mid-EP, respectively. The results of a steric model fitting to AFM approach force-distance (FD) curves and analysis of the pull-off distances in the AFM retraction FD curves showed higher biopolymer grafting density for E. coli in early-SP and longer biopolymer brush layer for B. subtilis in late-EP, which were associated with stronger adhesion to Si3N4 in water. The results of thermodynamic adhesion energy calculations based on the Wu model showed that polar interaction energy dominated the bacterial adhesion at the macroscale, the strength of which varied as a function of the growth phase for both E. coli and B. subtilis. The growth phase-dependent variation in polar components of thermodynamic adhesion energies between the bacteria and Si3N4 in water was consistent with the growth phase-dependent variation in the bond strengths between the bacteria and Si3N4 in water as revealed by the Weibull analysis of AFM adhesion data. Therefore, information obtained by Weibull analysis of nanoscale AFM bacterial adhesion data can be used to predict macroscale bacterial adhesion to the model inert Si3N4 surface in water.

The Interaction and Lift-Off Forces of an Atomic Force Microscope Tip from Single Fibers Extracted from Protective Clothing Fabric.

The widespread use of engineered nanoparticles (ENPs) poses a potential health hazard to humans, especially to those involved in either nanoparticle manufacturing or the usage and assembly of a final product. In this study, we performed systematic force vs distance experiments (F(z)) using an atomic force microscope (AFM) on fibers commonly used in street clothing and protective laboratory clothing to better characterize the relevant interaction forces between engineered nanoparticles (ENPs) and the contacted fabric fibers. The intent of this study is to identify those factors that influence the interaction of ENPs with fabrics with an aim to improve the efficacy of protective clothing against ENP exposure and mitigate potential health risks. A ∼14 nm diameter AFM SiOx tip (with nanoscale radius of curvature) is considered as an effective oxide ENP. Features present (or absent) in a well-executed F(z) AFM experiment provide a fingerprint that distinguishes the relevant forces and interaction mechanisms in play. Measurements of F(z) as a function of relative humidity were also performed to assess the importance of thin surface water layers in binding nanometer-size oxide ENPs to a fabric fiber. The F(z) data indicate the dominant mechanism for adhesion of the oxide tip to the various fabric fibers (cotton, Tyvek (HD polyethylene), polypropylene, and polyester) can be attributed to a van der Waals interaction. The analysis provides no evidence for long-range electrostatic forces or capillary-induced adhesion of the AFM tip to the fibers studied.

Flexoelectricity Modulated Electron Transport of 2D Indium Oxide.

The phenomenon of flexoelectricity, wherein mechanical deformation induces alterations in the electron configuration of metal oxides, has emerged as a promising avenue for regulating electron transport. Leveraging this mechanism, stress sensing can be optimized through precise modulation of electron transport. In this study, the electron transport in 2D ultra-smooth In2O3 crystals is modulated via flexoelectricity. By subjecting cubic In2O3 (c-In2O3) crystals to significant strain gradients using an atomic force microscope (AFM) tip, the crystal symmetry is broken, resulting in the separation of positive and negative charge centers. Upon applying nano-scale stress up to 100nN, the output voltage and power values reach their maximum, e.g. 2.2mV and 0.2pW, respectively. The flexoelectric coefficient and flexocoupling coefficient of c-In2O3 are determined as ≈0.49nCm-1 and 0.4V, respectively. More importantly, the sensitivity of the nano-stress sensor upon c-In2O3 flexoelectric effect reaches 20nN, which is four to six orders smaller than that fabricated with other low dimensional materials based on the piezoresistive, capacitive, and piezoelectric effect. Such a deformation-induced polarization modulates the band structure of c-In2O3, significantly reducing the Schottky barrier height (SBH), thereby regulating its electron transport. This finding highlights the potential of flexoelectricity in enabling high-performance nano-stress sensing through precise control of electron transport.

Tunable topological domain structures in high-density PbTiO3 nanodots array

In this work, we demonstrated that tunable topological domain structures, e.g., center-type domains and skyrmion-like polar bubbles, can be generated at room temperature in high-density epitaxial PbTiO3 nanodots fabricated via the template-assisted tailoring of thin films. These topological domain structures can be manipulated electrically by applying an appropriate bias on the conductive atomic force microscopy tip, allowing for writing, erasing, and rewriting of topological domains into the nanodot. Moreover, ring-shaped conductive channels are observed around the center-type domain states. These findings provide a playground for further exploring their emerging functionalities and application potentials for nanoelectronics.