Air-bearing sliders and plane-plane-concave tips for atomic force microscope cantilevers

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.

This paper describes a process to perform nanoscale welding of multi-walled carbon nanotubes (MWCNTs) using a conductive atomic force microscopy (AFM) tip. An AFM tip can be used not only to apply a mechanical force to push, drag or scratch a sample, but also to exert an electrostatic force between the conductive tip and a sample substrate by applying a bias voltage. This electrostatic field will be extremely high, due to the phenomenon of local field enhancement. Such a high electric field will provide enough energy for the metal atoms to evaporate from the conductive tip. In this paper, a model is developed to simulate the electrostatic field by adjusting the distance and the bias voltage between the conductive tip and substrate. The simulation results show that the radius of the conductive tip, the bias voltage and the gap distance will all affect the electric field intensity. In our experiments, MWCNTs are first assembled between a pair of electrodes using a dielectrophoresis (DEP) technique. Then, an electrostatic field is applied to weld MWCNTs onto the surface of microelectrodes. The experimental results show that MWCNTs can effectively be welded and the electronics performance can also be greatly improved.

The cantilevers of atomic force microscope (AFM) are mounted under a certain tilt angle, which is commonly assumed to have negligible effect on friction measurements in AFM. We present a theoretical study of the effect of the tilt angle on AFM based friction measurements. A method for correcting the friction coefficient between sample surfaces and AFM tips is also presented to minimize the effects of the tilt. The frictional forces between a silicon tip and a silicon surface at tilt angles ranging from 5 degrees to 25 degrees were measured. The results show that the measured friction coefficient increases with the tilt angle effectively, whereas the variation range of the corrected friction coefficient is within 10%.

With the advent of scanning probe microscopes, probe-based datarecording technologies are being developed for ultrahigh areal densityrecording, where the probe tip is expected to be scanned at velocities up to100 mm s−1. In one technique, a conductive atomic force microscope (AFM) tip is scanned over a phasechange chalcogenide medium and phase change is accomplished by applying either ahigh or low magnitude of current which heats the interface. Another technique isferroelectric data storage, where a conducting AFM tip is scanned over a leadzirconate titanate (PZT) film, a ferroelectric material. Ferroelectric domains can bepolarized by applying short voltage pulses between the AFM tip and the bottomelectrode layer that exceed the coercive field of the PZT film, resulting in nonvolatilechanges in the electronic properties. Tip wear is a serious concern in both datastorage methods. The understanding and improvement of tip wear, particularly atthe high velocities needed and at high interface temperatures for high data raterecording, is critical to the commercialization of these data storage technologies.This paper presents a review of nanotribological and nanomechanical studieson the materials used in phase change and ferroelectric probe-based recording.Although this work is aimed at probe-based data recording, it is also relevantto the development of robust AFM probes and to the study of nanocontacts ingeneral.

A facile approach for functionalizing atomic force microscopy (AFM) tips with nanoparticles (NPs) will provide exciting opportunities in the field of tip-enhanced vibrational spectroscopy and in probing the interactions between NPs and biological systems. In this study, through successive exposure to polydopamine and AgNO3 solutions, the apex of AFM tips was functionalized with silver nanoparticles (AgNPs). The AgNP-modified AFM tips were used to measure the interaction forces between AgNPs and supported lipid bilayers (SLBs) formed on mica, as well as to probe the penetration of SLBs by AgNPs, with an emphasis on the effect of human serum albumin (HSA) proteins. AgNPs experienced predominantly repulsive forces when approaching SLBs. The presence of HSA resulted in an enhancement in the repulsive interactions between AgNPs and SLBs, likely through steric repulsion. Finally, the forces required for AgNPs to penetrate SLBs were higher in the presence of HSA probably due to the increase in the effective size of the nanoscale protuberances on the AFM tip stemming from the formation of protein coronas around the AgNPs.

Under ambient humidity, water condenses as a nanometre meniscus between an atomic force microscope (AFM) tip and a surface, giving rise to a strong capillary force on the tip. To examine the molecular features of the meniscus, we performed an all-atom molecular dynamics simulation. By varying the tip–surface distance, we have simulated the formation, thinning and snap-off of the water meniscus. The meniscus is several nanometres wide and substantially fluctuates in its periphery when its neck is narrow. The density profile of the meniscus shows that its periphery is not sharp but has a fuzzy boundary whose thickness ranges from 0.4 to 0.9 nm. We obtained the neck radius of the meniscus and the radius of curvature of its periphery. Due to the sharp asperity of the AFM tip, these two structural parameters are comparable in size, in contrast to the case of a macroscopic tip, where the neck radius is much greater. We found that the meniscus periphery is often far from a circle in shape. With the structural parameters of the meniscus, we calculated the capillary force by using the Laplace–Kelvin equation. Our calculation reproduces the typical behaviour of the force–distance curve in the AFM experiment.

This research regards to a two-dimensional lateral pushing nanomanipulation using Atomic Force Microscope (AFM). Yet a reliable control of the AFM tip position during the AFM-based manipulation process is a chief issue since the tip can jump over the target nanoparticle and then the process can fail. However, a detailed Modeling and understanding of the interaction forces on the AFM tip is important for prosperous manipulation control and a nanometer resolution tip positioning. In the proposed model, Lund-Grenoble (LuGre) dynamic friction model is used as friction force on the contact surface between the nanoparticle and the substrate. This model leads to a stick-slip behavior of the nanoparticle that is so similar to the experimental behavior in nanoscale. Derjaguin interaction force is applied between the AFM tip and the nanoparticle which considers both attractive and repulsive interactions. The AFM is modeled by lumped-parameters model. A controller is designed based on the proposed dynamic model in order for positioning the AFM tip during a desired nanomanipulation task. Optimal sliding mode approach is used to design the controller. In this approach sliding surface which is used in the sliding mode approach is selected optimally based on the linear quadratic (LQ) method.

It has been confirmed experimentally that the adhesion force measured between an atomic force microscope (AFM) tip and the self-assembled monolayers (SAMs) has a direct correlation with the chain length of SAMs, and that the adhesion force decreases with the increase of the chain length. In this paper, a theoretical model is put forward to calculate the adhesion force between the AFM tip and the SAMs by integrating the Lennard–Jones potential. The theoretical results are in good agreement with the existing experimental results.

We use scaling theory to model the interactions between a layer of end-grafted polymers in a poor solvent and an atomic force microscope (AFM) tip. We focus on two distinct types of interactions as the tip is brought in contact with the polymeric substrate. In the first case, the polymers preferentially wet the surface of the tip, whereas in the second case, chains form chemical bonds with sites on the tip surface. For both cases, we analyze the morphological changes and determine the force profiles when the tip is embedded in the polymer layer and subsequently drawn away from this interface. The profiles for both cases exhibit a plateau region, where the force does not depend on the degree to which the polymer layer is stretched or deformed. This plateau region is more pronounced in the case where the chains are chemically bound to the tip. The results are in agreement with recent AFM studies on a collapsed polymer layer and help rationalize the experimental findings. The results also provide useful insi...

<p>An unconventional fabrication technique of nanoelectrode was developed using atomic force microscopy (AFM) and hydrogel. Until now, the precise control of electroactive area down to a few nm<sup>2</sup> has always been an obstacle, which limits the wide application of nanoelectrodes. Here, the nanometer-sized contact between the boron-doped diamond (BDD) as conductive AFM tip and the agarose hydrogel as solid electrolyte was well governed by the feedback amplitude of oscillation in the non-contact mode of AFM. Consequently, this low-cost and feasible approach gives rise to new possibilities for the fabrication of nanoelectrodes. The electroactive area controlled by the set point of AFM was investigated by cyclic voltammetry (CV) of the ferrocenmethanol (FcMeOH) combined with quasi-solid agarose hydrogel as an electrolyte. Single copper (Cu) nanoparticle was deposited at the apex of the AFM tip using this platform whose electrocatalytic activity for nitrate reduction was then investigated by CV and Field Emission-Scanning Electron Microscopy (FE-SEM), respectively.</p>

The analysis of the dynamic behavior of the micro- cantilevers employed in atomic force microscopy (AFM) is often limited to linear or weakly nonlinear behavior without damping. Finite element simulations are used here to study the cantilever dynamics outside of these restrictions. The nonlinear contact mechanics between the AFM tip and the material surface are modeled using the JKR model with different damping. This model is most appropriate for AFM cantilevers that are most compliant than the specimen. The focus is on the contact case in all analyses to simplify the problem. Thus, the AFM cantilever tip is assumed to remain in contact with the specimen surface at all times during the motion. Applications for both weakly and strongly nonlinear behavior are examined. The properties of the vibration, the influence of different initial loads and different damping models on the behavior, like nonlinear shifts of the resonance frequencies, the eccentricity and asymmetry of the amplitude, of the nonlinear vibration are calculated by FEM. The numerical analysis shows that the eccentricity and the asymmetry of the amplitude are more sensitive to the change of damping and the contact stiffness than the resonance frequencies. The response of the cantilever and the evaluation of elastic properties of the sample can be studied appropriately using this model.

When the AFM cantilever is mounted with a sharp diamond tip, in addition to controlling both the applied normal load on the tip and other machining parameters by the AFM electrical components, it is capable of conducting micro/nano-machining on the surface of single crystal silicon. Firstly in this research work the diamond tip based single asperity cutting experiments are conducted on silicon surface with different normal loads and cutting speed, in aiming to investigate the AFM based microcutting process and material removal mechanism for silicon-like brittle material on nanoscale. Secondly, the micro/nano-machining experiments are conducted on single crystal silicon surface with different detecting approaches for characterizing the features of the micro/nano-machined region in terms of material removal mechanism and chips forming characteristic under different normal loads. Furthermore, the contact interaction mechanism existed between the AFM diamond tip and the machined silicon surface during the micro/nano-machining process is simulated with finite element method. Finally, the wear mechanism of an atomic force microscope (AFM) diamond tip when conducting micro/nano-machining on a single crystal silicon surface is empirically analyzed. The results indicate that the AFM based v has a precisely dimensional controllability and a good surface quality on nanometer scale.

Biodegradable polymeric nanofibrous scaffold comprises individual nanofibers where their stiffnesses can promote or undermine the various cellular functions as well as structural integrity of the scaffold. As such, there is a need to investigate the nanomechanical properties of these individual nanofibers. However, conducting mechanical tests of individual fibers at the nanometer scale can pose great challenges and difficulties. Here, we present novel techniques to perform nanomechanical testing of individual polymeric nanofibers. For demonstration of the nano tensile tests, polycaprolactone (PCL) nanofibers were produced via electrospinning. These fibers were deposited across two parallel edges of a cardboard frame so that a single nanofiber can be isolated for tensile test using a nano tensile tester. For nanoscale three-point bend test, a Poly (L-lactic acid) (PLLA) nanofiber was suspended across a microsized groove etched on a silicon wafer. An atomic force microscope (AFM) tip was then used to apply a point load on the mid-span of the suspended fiber. Beam bending theory was then used to calculate the elastic modulus of the nanofiber. For nanoindentation test, a PLLA nanofiber was deposited on a mica substrate and an AFM tip used to indent the nanofiber. Modified Hertz theory for normal contact was then used to evaluate the elastic modulus of the nanofiber.

Using friction force microscopy (FFM) under controlled environments, we have systematically investigated the humidity effect on the frictional properties of two important classes of self-assembled monolayers (SAMs), i.e., N-octadecyltrimethoxysilane (OTE, CH3(CH2)17Si(OCH3)3) on SiO2(OTE/SiO2), and N-alkanethiols on Au(111), together with their respective substrates. Experimental results show that both OTE and alkylthiol SAMs can decrease the friction force between a Si3N4 atomic force microscope (AFM) tip and substrates. The nearly humidity-independent friction of the two kinds of SAMs indicates that these SAMs are ideal lubricants in applications of micro-electro-mechanical systems (MEMS) under different environments. The humidity dependence—as the humidity increases, the friction first increases and then decreases—of the two substrates, SiO2 and Au(111), can be explained by the adsorption of water. The decrease in the friction at high humidity is attributed to the low viscosity in the multilayers of water, while the increase in the friction at low humidity can be explained by the high viscosity between the water monolayer and the surfaces (AFM tip and sample), possibly due to the confinement effects. The effect of modification of the AFM tip with alkanethiol molecules on the humidity dependence of Au(111) friction has also been investigated.

Scattering-type scanning near-field microscopy (s-SNOM) at terahertz (THz) frequencies could become a highly valuable tool for studying a variety of phenomena of both fundamental and applied interest, including mobile carrier excitations or phase transitions in 2D materials or exotic conductors. Applications, however, are strongly challenged by the limited signal to noise ratio. One major reason is that standard atomic force microscope (AFM) tips, which have made s-SNOM a highly practical and rapidly emerging tool, provide weak scattering efficiencies at THz frequencies. Here we report a combined experimental and theoretical study of commercial and custom-made AFM tips of different apex diameter and length, in order to understand signal formation in THz s-SNOM and to provide insights for tip optimization. Contrary to common beliefs, we find that AFM tips with large (micrometer-scale) apex diameter can enhance s-SNOM signals by more than one order of magnitude, while still offering a spatial resolution of about 100 nm at a wavelength of 119 micron. On the other hand, exploiting the increase of s-SNOM signals with tip length, we succeeded in sub-15 nm resolved THz imaging employing a tungsten tip with 6 nm apex radius. We explain our findings and provide novel insights into s-SNOM via rigorous numerical modeling of the near-field scattering process. Our findings will be of critical importance for pushing THz nanoscopy to its ultimate limits regarding sensitivity and spatial resolution.