Atomic Force Microscope Tip Research Articles

Conductive filament nanostructures and their magnetization reversal in NiO resistive switching memory cells

We demonstrate that conductive filament (CF) nanostructures in NiO thin film resistive switching memory (RRAM) cells present unique magnetic properties. The CF nanostructures of nanodots and nanorods formed using a conductive atomic force microscopy tip were fabricated through the forming process of CFs for the NiO RRAM cells. In the current–voltage curve, unipolar resistive switching behavior was observed. The ferromagnetic domain and magnetization switching of CF nanostructures were characterized using magnetic force microscopy. In principle, nanoscale-sized magnetic nanostructures possess superparamagnetism due to their thermal fluctuations. In this study, however, CF nanorods with a length of 30 nm exhibited ferromagnetism because the exchange bias effect occurred at the interface between the antiferromagnetic NiO and the CF nanorod. Therefore, the ferromagnetism of the CF nanostructures is the origin of the magnetization switching and can be favorable in magnetic storage device applications by overcoming the superparamagnetic limits with the exchange bias effect.

Electron beam lithography on non-planar, suspended, 3D AFM cantilever for nanoscale thermal probing

Electron beam lithography (EBL) on non-planar, suspended, curved or bent surfaces is still one of the most frequently stated problems for fabricating novel and innovative nano-devices and sensors for future technologies. Although spin coating is the most widespread technique for electron resist (e-resist) deposition on 2D or flat surfaces, it is inadequate for suspended and 3D architectures because of its lack of uniformity. In this work, we use a thermally evaporated electron sensitive resist the QSR-5 and study its sensitivity and contrast behaviour using EBL. We show the feasibility of utilizing the resist for patterning objects on non-planar, suspended structures via EBL and dry etching processes. We demonstrate the integration of metal or any kind of thin films at the apex of an atomic force microscopy (AFM) tip. This is showing the great potential of this technology in various fields, such as magnetism, electronic, photonics, phononics and other fields related to near field microscopy using AFM probe like for instance scanning thermal microscopy.

Graphical Direct Writing of Macroscale Domain Structures with Nanoscale Spatial Resolution in Nonpolar-Cut Lithium Niobate on Insulators

We report on a graphical domain engineering technique with the capability to fabricate macroscale domain structures with nanoscale spatial resolution in nonpolar-cut lithium-niobate thin film on insulators through the biased probe tip of scanning atomic force microscopy. It is found that the domain writing process is asymmetric with respect to the spontaneous polarization ${\mathrm{P}}_{s}$ even though the tip-induced poling field is mirror symmetric. Various domain structures, with a dimension larger than millimeters while consisting of nanoscale domain elements and with arbitrary domain-wall inclination angle with respect to ${\mathrm{P}}_{s}$ , are designed graphically and then written directly into nonpolar-cut lithium-niobate crystals. As a proof-of-principle demonstration, periodically poled $x$-cut lithium-niobate thin film on insulators with a period of 600 nm, a depth of 460 nm, and a length of approximately $1$ mm is fabricated. This technique could be useful for device applications in integrated optics and optoelectronics and domain-wall nanoelectronics based on lithium niobate on insulators.

Nanomechanical tribological characterisation of nanostructured titanium alloy surfaces using AFM: A friction vs velocity study

Medical-grade titanium alloys used for orthopaedic implants are at risk from infections and complications such as wear and tear. We have recently shown that hydrothermally etched (HTE) nanostructures (NS) formed on the Ti6AlV4 alloy surfaces impart enhanced anti-bacterial activity which results in inhibited formation of bacterial biofilm. Although these titanium alloy nanostructures may resist bacterial colonisation, their frictional properties are yet to be understood. Orthopaedic devices are encapsulated by bone and muscle tissue. Contact friction between orthopaedic implant surfaces and these host tissues may trigger inflammation, osteolysis and wear. To address these challenges, we performed simulation of the contact behaviour between a smooth control Ti6Al4V alloy and HTE surfaces against a hardwearing SiO2 sphere using Atomic Force Microscopy (AFM) in Lateral Force Microscopy mode. The friction study was evaluated in both air and liquid environments at high (5 Hz) and low (0.5 Hz) scan velocities. Lower scan velocities demonstrated opposing friction force changes between the two mediums, with friction stabilising at higher velocities. The friction measured on the NS alloy surfaces was reduced by ~20% in air and ~80% in phosphate buffered saline, in comparison to the smooth control surface, displaying a non-linear behaviour of the force applied by the AFM tip. Changes in friction values and cantilever scan velocities on different substrates are discussed with respect to the Stribeck curve. Reduced friction on nanostructured surfaces may improve wear resistance and aid osseointegration.

Will Polycrystalline Platinum Tip Sliding on a Gold(111) Surface Produce Regular Stick–Slip Friction?

Friction measurements by an atomic force microscope (AFM) frequently showed regular stick-slip friction signals with atomic-scale resolutions. Typically, for an AFM metal tip sliding on a metal crystal surface, the microstructure of the tip made from the thermally evaporated metal coating on a silicon cantilever was polycrystalline. Our detailed molecular dynamics(MD) simulations of a polycrystalline Pt tip (R = 10 nm in radius) sliding on an Au(111) surface revealed how the geometry of the polycrystalline tip took effect on the friction behavior at the contact interface. We found that the apex of the Pt tip with multiple grains near the edge of contact could induce severe plastic deformations of the gold substrate, leading to irregular stick-slip frictions upon sliding. Simulation results showed that in order to achieve a clear stick-slip friction signal with single atomic slips, the apex of the Pt tip must adopt a single crystalline protrusion without any neighboring grains involved in the metal contact. We showed that such a single crystalline protrusion, which presumably could be achieved during initial run-in or wear-out of high-energy Pt atoms in the neighboring grains, was passivated by a large number of gold atoms due to metal adhesion in the contact periphery. Using such a crystalline protrusion tip, we demonstrated that the stick-slip friction produced was very "tolerant" to the adhesion of a large number of gold atoms on the tip apex. We further showed that AFM tip mass used in MD simulations also played an important role in determining the transition between friction regimes, which could be well explained by the Prandtl-Tomlinson thermal activation model.

Precisely controlling the twist angle of epitaxial MoS2/graphene heterostructure by AFM tip manipulation

Two-dimensional (2D) moiré materials have attracted a lot of attention and opened a new research frontier of twistronics due to their novel physical properties. Although great progress has been achieved, the inability to precisely and reproducibly manipulate the twist angle hinders the further development of twistronics. Here, we demonstrated an atomic force microscope (AFM) tip manipulation method to control the interlayer twist angle of epitaxial MoS2/graphene heterostructure with an ultra-high accuracy better than 0.1°. Furthermore, conductive AFM and spectroscopic characterizations were conducted to show the effects of the twist angle on moiré pattern wavelength, phonons and excitons. Our work provides a technique to precisely control the twist angle of 2D moiré materials, enabling the possibility to establish the phase diagrams of moiré physics with twist angle.

Enhancing sensitivity in atomic force microscopy for planar tip-on-chip probes

We present a new approach to tuning-fork-based atomic force microscopy for utilizing advanced “tip-on-chip” probes with high sensitivity and broad compatibility. Usually, such chip-like probes with a size reaching 2 × 2 mm2 drastically perturb the oscillation of the tuning fork, resulting in poor performance in its intrinsic force sensing. Therefore, restoring initial oscillatory characteristics is necessary for regaining high sensitivity. To this end, we developed a new approach consisting of three basic steps: tuning-fork rebalancing, revamping holder-sensor fixation, and electrode reconfiguration. Mass rebalancing allows the tuning fork to recover the frequency and regain high Q-factor values up to 104 in air and up to 4 × 104 in ultra-high vacuum conditions. The floating-like holder-fixation using soft wires significantly reduces energy dissipation from the mounting elements. Combined with the soft wires, reconfigured electrodes provide electrical access to the chip-like probe without intervening in the force-sensing signal. Finally, our easy-to-implement approach allows converting the atomic force microscopy tip from a passive tool to a dedicated microdevice with extended functionality.

Size and Semiconducting Effects on the Piezoelectric Performances of ZnO Nanowires Grown onto Gravure-Printed Seed Layers on Flexible Substrates

Zinc oxide (ZnO) nanogenerators have attracted increasing interest in the scientific community for use in energy harvesting and mechanical sensing applications. Understanding the interplay between piezoelectricity and semiconductor physics is fundamental to enhancing these devices’ performances, although direct characterization at the nanoscale is challenging. With this work, we present a new strategy to improve piezoresponse force microscopy (PFM) measurements and analysis. This strategy was applied to study the piezoelectric performances of ZnO nanowires grown on seed layers deposited by gravure printing onto flexible substrates. We demonstrate the influence of nanowire diameter and atomic force microscope (AFM) tip position on the piezoresponse amplitude. We also explain our results with simulations showing the importance of considering semiconducting properties in the analysis.

Influence of form deviations on the radius and roundness measurement of micro spheres

Abstract Micro and nano coordinate measuring machines (CMMs) require small and well characterized micro spheres as probing elements. However, established strategies and instruments have mostly been designed for and applied to the characterization of larger spheres in the range of millimetres or above. That is why we have recently focused our attention towards a novel strategy which is based on a set of atomic force microscope (AFM) surface scans in conjunction with a stitching algorithm. Initial experimental results are promising, but point to several influences which require further attention. We have, therefore, begun to model the measurement strategy and applied it on simulated spheres, in order to investigate and reduce some of these influences. The model is currently limited to effects which are related to the radius and form of the sphere. Other influences, like the AFM tip, are being ignored. In this paper, we introduce the essential parts of this model and apply it on spheres of different mean radii (60 µm, 100 µm and 150 µm) and of different qualities (Grade 3 and Grade 5). The investigations illustrate that the measurement object can have a significant influence on the measurement result and needs to be considered.

Energy dissipation during collision for anti-relaxation coatings in alkali-metal vapor cells

Abstract Anti-relaxation surface coatings can reduce collisions between alkali-metal atoms and cell walls, thereby prolonging the spin-polarization lifetime of alkali-metal atoms. This phenomenon can considerably improve the performance of quantum sensors. Researchers have focused on the surface topography and composition of anti-relaxation coatings in alkali-metal vapor cells and on methods that provide an increase in the relaxation time of alkali metals. We proposed a novel method to characterise the coating performance of alkali-metal cells by directly measuring the energy dissipation during collisions between the anti-relaxation coating surface and atoms in the cell. We found that the interactions between alkali-metal atoms and the anti-relaxation coating resemble elastic collisions, which can be indirectly characterised by the energy dissipation during interactions between an atomic force microscopy tip and atoms. In addition, we confirmed that the energy dissipation between collisions is positively correlated with the relaxation characteristics of alkali metals at various temperatures. This discovery is of great significance as it can be utilised to characterise the anti-relaxation coating performance of alkali-metal cells by directly analysing the energy dissipation of anti-relaxation coatings instead of measuring the relaxation time.

Characterization of electronic charged states of high density self-aligned Si-based quantum dots evaluated with AFM/Kelvin probe technique

We have characterized charged states of a self-aligned Si quantum dot (QD) structure with an areal density as high as ∼1013 cm−2 by surface potential measurements by using atomic force microscopy (AFM)/Kelvin probe force microscopy (KFM). By line scanning with the electrically-biased AFM tip with respect to the substrate in a contact mode, the corresponding area was negatively charged caused by electron injection from the tip into QDs. Cross-sectional profiles of the surface potential were clearly changed, which depends on the tip bias voltages. Also, a stepwise shape in a relationship between the change of the surface potential (ΔV) and the tip bias voltages was observed, reflecting electron charging into the discrete energy levels due to the quantum confinement effect. By assuming a simple equivalent circuit model for KFM measurements of the self-aligned Si-QDs, calculated results for ΔV are in good agreement with the measured results. The agreements precisely clarified the charged states in Si-QDs.

Nanoscale thermal imaging of VO2 via Poole–Frenkel conduction

We present a method for nanoscale thermal imaging of insulating thin films using atomic force microscopy (AFM), and we demonstrate its utility on VO2. We sweep the applied voltage V to a conducting AFM tip in contact mode and measure the local current I through the film. By fitting the IV curves to a Poole–Frenkel conduction model at low V, we calculate the local temperature with spatial resolution better than 50 nm using only fundamental constants and known film properties. Our thermometry technique enables local temperature measurement of any insulating film dominated by the Poole–Frenkel conduction mechanism and can be extended to insulators that display other conduction mechanisms.

Study of charged object sensing properties via an organic nanobelt

A flexible wearable electronic skin (e-skin) that is able to sense non-touch objects is strongly required for the improvement and advancement of next-generation high-performance sensing systems. Here, a facile two-terminal organic device is employed for studying the sensing properties of charged objects including proximity and contact stimuli. By adjusting the amount of charges on the stimuli, the sensing response of the organic device was confirmed to rely on the charge quantity of stimuli rather than the applied pressure. In addition, the carrier concentration in the organic semiconductor is effectively accumulated or depleted, depending on the carrier type in the semiconductor and the polarity of voltage at the atomic force microscope (AFM) tip. These results show a potential application in ultrahigh-precision automatic operation, for example, humanoid robots to sensitively detect and safely grab a charged-object without damage. • The flexible organic sensor applies the two-terminal device configuration. • The sensing characteristics of charged objects are studied by organic nanobelt sensor. • The organic semiconductor sensor can sense both proximity and contact stimuli. • The amount of charges on the stimuli is used to confirm the sensing mechanism. • The flexible sensor can identify the majority carrier type in semiconductor materials.

A qPlus-based scanning probe microscope compatible with optical measurements.

We design and develop a scanning probe microscope (SPM) system based on the qPlus sensor for atomic-scale optical experiments. The microscope operates under ultrahigh vacuum and low temperature (6.2K). In order to obtain high efficiency of light excitation and collection, two front lenses with high numerical apertures (N.A. = 0.38) driven by compact nano-positioners are directly integrated on the scanner head without degrading its mechanical and thermal stability. The electric noise floor of the background current is 5 fA/Hz1/2, and the maximum vibrational noise of the tip height is below 200 fm/Hz1/2. The drift of the tip-sample spacing is smaller than 0.1 pm/min. Such a rigid scanner head yields small background noise (oscillation amplitude of ∼2pm without excitation) and high quality factor (Q factor up to 140 000) for the qPlus sensor. Atomic-resolution imaging and inelastic electron tunneling spectroscopy are obtained under the scanning tunneling microscope mode on the Au(111) surface. The hydrogen-bonding structure of two-dimensional (2D) ice on the Au(111) surface is clearly resolved under the atomic force microscope (AFM) mode with a CO-terminated tip. Finally, the electroluminescence spectrum from a plasmonic AFM tip is demonstrated, which paves the way for future photon-assisted SPM experiments.

Intrinsic memory characteristic of polycrystalline ZnTe film originating from Mott variable range hopping conduction

The resistive switching characteristics of Au/ZnTe/ITO structure with polycrystalline ZnTe film as resistive switching layer is investigated. Macroscopically, 100 bipolar switching cycles under the direct current (dc) voltages were carried out and the conduction states can retain for several hours. Microscopically, reading and writing operations can be achieved on ZnTe film with Au top electrode replaced by conductive Atomic Force Microscopy (c-AFM) tip. The I–V characteristic in low resistance state (LRS) is linear in the whole range of voltage. The I–V characteristic in high resistance state (HRS) is linear in the low voltage while it obeys Schottky emission in the high voltage, and Schottky barrier height is symmetric in the positive and negative voltage. During linear I–V characteristic voltage range, the electrons transport between adjacent point defects via Mott variable range hopping. The higher hopping distance and higher activation energy in HRS contribute to the higher resistance value in HRS compared with LRS. Impedance spectroscopy in HRS and LRS both behave as a semicircle, which accords with the semiconductor-like characteristic of conductive point defects. Photoluminescence (PL) spectroscopy indicates the decisive role of deep level defects in conduction. This study confirms the intrinsic resistive switching characteristic of ZnTe film and provides a new choice for intrinsic non-oxides material in nonvolatile memory application.

Structure Fabrication on Silicon at Atomic and Close-To-Atomic Scale Using Atomic Force Microscopy: Implications for Nanopatterning and Nanodevice Fabrication.

In this paper, the atomic-scale structure fabrication on Si (100) substrate using atomic force microscopy (AFM) with the aid of electrochemical and mechanical processes in a humid environment and under ambient conditions is studied. The local oxidation patterns are formed using platinum-coated tips with the aid of bias applied to the tip-substrate junction, and direct removal has been achieved using single crystal diamond tips, enabling the structure fabrication at the atomic and close-to-atomic scale. The depth and height of the etched trenches reached about 1 nm, which provides an approach for the fabrication of atomic-scale electrodes for molecular device development. Furthermore, material removal close to about three silicon atoms (~3.2 Å) has been achieved. This is important in molecular device fabrication. A detailed comparison among the nanopatterns and the material removal over bare and hydrofluoric acid (HF) treated silicon substrates is provided. This comparison is useful for the application of fabricating atomic-scale electrodes needed for the molecular electronic components. A deep understanding of atomic-scale material removal can be pushed to fabricate a single atomic protrusion by removing the neighbouring atoms so that the molecule can be attached to a single atom, thereby the AFM tip and Si substrate could act as the electrodes and the molecule between them as the channel, providing basic transistor actions in a molecular transistor design. In this paper, platinum-coated and single-crystal diamond tips are used to explain the oxide formations and direct material removal, respectively.

Molecular Dynamics Simulations of Ripple Nanostructure Formation During Nanomachining via Atomic Force Microscopy

Molecular dynamics (MD) constant force simulation mode is first used to study the formation of the ripple nanostructures on the single-crystal copper surface by combining the topography generated by machined grooves with the accumulated pile-up material on the side of these grooves via a triangular pyramid atomic force microscopy (AFM) tip. Groove morphologies, lateral forces, the evolution of subsurface defects, and atomic flow laws generated by different machining parameters are discussed. Specifically, the normal load, the feed value between grooves, and the scratching direction significantly affect groove formation. The simulated morphologies of the ripple nanostructures indicate that groove consistency and symmetry increase with increasing feed, and an optimized feed for various normal loads can be determined. The interaction between adjacent grooves produces a variation in the lateral forces with the groove number and feed. This is mainly related to the evolution of subsurface defects, the atomic flow law, and the volume of material pile-ups. The distributions of subsurface defects affect the atomic flow directions, and the atomic flow laws determine groove formation. The simulations provide important guidance for ripple nanostructure fabrication on single-crystal copper surface via AFM tips.

Quantitative near-field characterization of surface plasmon polaritons on monocrystalline gold platelets

Near-field microscopy allows for visualization of both the amplitude and phase of surface plasmon polaritons (SPPs). However, their quantitative characterization in a reflection configuration is challenging due to complex wave patterns arising from the interference between several excitation channels. Here, we present near-field measurements of SPPs on large monocrystalline gold platelets in the visible. We study systematically the influence of the incident angle of the exciting light on the SPPs launched by an atomic force microscope tip. We find that the amplitude and phase signals of these SPPs are best disentangled from other signals at grazing incident angle relative to the edge of the gold platelet. Furthermore, we introduce a simple model to extract the wavelength and in particular the propagation length of the tip-launched plasmons. Our experimental results are in excellent agreement with our theoretical model. The presented method allows the quantitative analysis of polaritons occurring in different materials at visible wavelengths.

Imaging Ferroelectrics: Reinterpreting Charge Gradient Microscopy as Potential Gradient Microscopy

AbstractCharge gradient microscopy (CGM) is a scanning probe imaging mode, particularly well‐suited for the characterization of ferroelectrics. The implementation of the technique is straightforward; it involves monitoring currents that spontaneously develop between a passive conducting atomic force microscopy tip and Earth, as the tip is scanned across the specimen surface. However, details on the fundamental origin of contrast and what images mean, in terms of associated ferroelectric microstructures, are not yet fully understood. Here, by comparing information from CGM and Kelvin probe force microscopy, obtained from the same sets of ferroelectric domains (in both lithium niobate and barium titanate), it is shown that CGM reasonably reflects the spatial derivative of the measured surface potential. This is conceptually different from measuring local gradients in the surface bound‐charge density or in any associated screening charges: after all, clear CGM signals are seen, even when polarization is entirely in‐plane (where the bound charge density is uniformly zero, but gradients in surface potential are still fully expected). It is therefore suggested that CGM in ferroelectrics may be more accurately called potential gradient microscopy.

Influence of Thin Film Deposition on AFM Cantilever Tips in Adhesion and Young's Modulus of MEMS Surfaces.

Adhesion is a critical factor in microelectromechanical systems (MEMSs) and is influenced by many parameters. In important fields, such as microassembly, an improved understanding of adhesion can result in higher precision. This study examines the influence of deposition of gold and titanium onto the atomic force microscope (AFM) tips in adhesion forces and Young’s modulus, between a few MEMS substrates (silicon, gold, and silver) and the AFM tips. It was found that, except for gold substrate, an AFM tip coated with gold has the highest adhesion force of 42.67 nN for silicon substrates, whereas the titanium-coated AFM tip decreases the force for all the samples. This study suggests that such changes must be taken into account while studying the adhesion force. The final results indicate that utilizing gold substrate with titanium AFM tip led to the lowest adhesion force, which could be useful in adhesion force measurement during microassembly.