Contact Mode Atomic Force Microscopy Research Articles

Nonlinear vibrations control of a contact-mode AFM model via a time-delayed positive position feedback

In this work, the nonlinear vibrations of a contact-mode atomic force microscopy (AFM) model subjected to multi excitations are controlled via a time-delayed positive position feedback (PPF) controller. A perturbation method is implemented to obtain the approximate nonlinear dynamical behavior of the AFM system. Different response curves, before and after control, are included utilizing frequency response equations. At multiple resonance cases (primary, sub-harmonic, and internal), the gain effects on the vibration amplitude and stability behavior of the controlled system are studied. The acquired results revealed that PPF controller is efficient in reducing the studied system oscillations. The efficiency of the controller Ea was about 193, means that the controller reduced the vibration by about 99.5%. For increasing feedback signal gain α2, the stability region in τ1-τ2 plane was shrinking. Finally, verification curves are plotted to depict the degree of closeness between analytical predictions and numerical simulation.

Characterizing liver sinusoidal endothelial cell fenestrae on soft substrates upon AFM imaging and deep learning

BackgroundLiver sinusoidal endothelial cells (LSECs) display unique fenestrated morphology. Alterations in the size and number of fenestrae play a crucial role in the progression of various liver diseases. While their features have been visualized using atomic force microscopy (AFM), the in situ imaging methods and off-line analyses are further required for fenestra quantification. MethodsPrimary mouse LSECs were cultured on a collagen-I-coated culture dish, or a polydimethylsiloxane (PDMS) or polyacrylamide (PA) hydrogel substrate. An AFM contact mode was applied to visualize fenestrae on individual fixed LSECs. Collected images were analyzed using an in-house developed image recognition program based on fully convolutional networks (FCN). ResultsKey scanning parameters were first optimized for visualizing the fenestrae on LSECs on culture dish, which was also applicable for the LSECs cultured on various hydrogels. The intermediate-magnification morphology images of LSECs were used for developing the FCN-based, fenestra recognition program. This program enabled us to recognize the vast majority of fenestrae from AFM images after twice trainings at a typical accuracy of 81.6% on soft substrate and also quantify the statistics of porosity, number of fenestrae and distribution of fenestra diameter. ConclusionsCombining AFM imaging with FCN training is able to quantify the morphological distributions of LSEC fenestrae on various substrates. SignificanceAFM images acquired and analyzed here provided the global information of surface ultramicroscopic structures over an entire cell, which is fundamental in understanding their regulatory mechanisms and pathophysiological relevance in fenestra-like evolution of individual cells on stiffness-varied substrates.

Investigation of morphology and texture properties of WSi2 coatings on W substrate based on contact-mode AFM and EBSD

The WSi2 coatings on W substrate were successfully prepared by hot dip silicon-plating (HDS) method. The surface roughness, surface and cross section morphology, and texture evolution were characterized by field-emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), atomic force microscope (AFM), and electron back scattering diffraction (EBSD). The FE-SEM results show that the WSi2 coatings have a good adhesion with the substrate, and no microcracks and pores are observed at the surface. AFM images show that the WSi2 coatings have a similar and homogeneous surface morphology with a granular structure and nanoscale roughness, and most of surface vertical heights are less than 600 nm. The low roughness is observed when the deposition temperature is 1470 °C to 1530 °C, the Ra and Rq are 82.1–115.6 nm and 109.1–140.3 nm, respectively. EBSD analysis indicates that the WSi2 coatings have typical {100} and {110} textures with the coating thickness of 18 to 42 μm and average grain size of 6.78 to 10.92 μm.

Measurement of thin film magnetostriction using field-dependent atomic force microscopy

Measurement of thin film magnetostriction is a challenging task, as magnetostrictive material deformations in parts per million, in conjunction with materials at small dimensions, require high precision, often with dedicated set-ups, for reproducible results. We have developed a novel approach employing a commercial atomic force microscope (AFM) with attached electromagnets. Magnetostriction measurements are demonstrated on 50–500 nm thick Fe81Al19 films sputter deposited directly on high aspect ratio commercial AFM micro-cantilevers. A magnetostrictive deflection of the cantilever bimorph translates into a deflection force acting in a contact mode measurement, which is interpreted and recorded as a change in height. For determination of the magnetostriction coefficient λs, we have developed a modified version of the equation for the magnetostrictive deflection of a cantilever bimorph by Guerrero and Wetherhold, taking into account long-range attractive forces acting during contact mode AFM measurements in air. The sub-atomic precision of the AFM, combined with the widespread availability of all components and the simple set-up, makes the measurement of magnetostriction on films of just a few tens of nanometers thickness easily accessible.

Nanoscale triboelectrification gated transistor

Tribotronics has attracted great attention owing to the demonstrated triboelectrification-controlled electronics and established direct modulation mechanism by external mechanical stimuli. Here, a nanoscale triboelectrification-gated transistor has been studied with contact-mode atomic force microscopy and scanning Kevin probe microscopy. The detailed working principle was analyzed at first, in which the nanoscale triboelectrification can tune the carrier transport in the transistor. Then with the manipulated nanoscale triboelectrification, the effects of contact force, scan speed, contact cycles, contact region and charge diffusion on the transistor were investigated, respectively. Moreover, the manipulated nanoscale triboelectrification serving as a rewritable floating gate has demonstrated different modulation effects by an applied tip voltage. This work has realized the nanoscale triboelectric modulation on electronics, which could provide a deep understanding for the theoretical mechanism of tribotronics and may have great applications in nanoscale transistor, micro/nano-electronic circuit and nano-electromechanical system.

A Proportional Derivative (PD) Controller for Suppression the Vibrations of a Contact-Mode AFM Model

The nonlinear dynamics control of a contact-mode atomic force microscopy (AFM) system with multi forces (harmonic and parametric excitation force) utilizing the time delay proportional derivative (PD) controller is investigated. The perturbation method is utilized to calculate the first-order approximate solutions for the AFM system. The stability of the AFM system is investigated at the worst resonance case by Lyapunov's first method. We also show the bifurcation diagrams of response curves using frequency response equations before and after control are performed. Furthermore, we focus on the effect of the time-delayed control and returns signals gain on the vibration amplitude and the stability analysis of the controlled system in primary, sub-harmonic resonance for different parameter variations. In addition, the numerical results are procured using MATLAB program. Eventually, validation curves are presented to estimate the nearness degree between the analytical predictions also numerical simulation. The obtained results exhibited that, the time-delayed PD control efficiency to put down the nonlinear oscillations of the system.

The Study of Nanoindentation of Atomically Flat GaAs Surface using the Tip of Atomic-Force Microscope

It was shown that the nanoindentation treatment of the atomically flat GaAs surface with the tip of atomic-force microscope in contact mode allows to produce small size pits with the depth in the range from a few tenths of nm up to a 1.5 nm. The experimental data can be qualitatively described on the base of kinetic concept of fracture of solid state developed by Zhurkov, which suppose the generation of defects and subsequent destruction of the GaAs surface. The molecular dynamics modelling confirmed thermally activated destruction of a few top atomic layers under indentation. The presented technology could be used to form the shape of solid state surfaces with subnanometer resolution in depth without wet etching processes.

Improving Adhesion Between Nanoparticles and Surface of Mica Substrate by Aminosilane Modification

In the manipulation of nanoparticles for precise placement, the relatively low adhesion of the nanoparticles to the substrate surface has emerged as a problem. Owing to the fact that nanoparticles manipulated using atomic force microscopy (AFM) often cannot be accurately placed at their predetermined destinations or may even go astray, becoming “lost,” the success rate of manipulation attempts is low. We investigated the possibility of enhancing the adhesion between magnetic nanoparticles and a substrate surface by modifying a mica substrate with a solution of 3-aminopropyltriethoxysilane (APTES). The morphology of the mica surface before and after modification was analyzed, and the adhesive force was calculated by using AFM in contact mode. The effect of different APTES-solution concentrations on the adhesive force was analyzed as well. The results demonstrate that the adhesion of the nanoparticles to the modified substrate was substantially stronger than their adhesion to an unmodified surface, a finding that can be used to improve the success rate of nanoparticle manipulation.

Scan Rate Adaptation for AFM Imaging Based on Performance Metric Optimization

Constant-force contact-mode atomic force microscopy (AFM) relies on a feedback control system to regulate the tip-sample interaction during imaging. Due to limitations in actuators and control, the bandwidth of the regulation system is typically small. Therefore, the scan rate is usually limited in order to guarantee a desirable image quality for a constant-rate scan. By adapting the scan rate online, further performance improvement is possible, and the conditions to this improvement have been explored qualitatively in a previous study for a wide class of possible scan patterns. In this article, a quantitative assessment of the previously proposed adaptive scan scheme is investigated through experiments that explore the impact of various degrees of freedom in the algorithm. Further modifications to the existing scheme are proposed and shown to improve the closed-loop performance. The flexibility of the proposed approach is further demonstrated by applying the algorithm to tapping-mode AFM.

Quantum Hall Effect Measurement of Spin-Orbit Coupling Strengths in Ultraclean Bilayer Graphene/WSe2 Heterostructures.

We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe2 heterostructure devices. Contact mode atomic force microscopy (AFM) cleaning yields ultraclean interfaces and high-mobility devices. In a perpendicular magnetic field, we measure the quantum Hall effect to determine the Landau level structure in the presence of out-of-plane Ising and in-plane Rashba SOC. A distinct Landau level crossing pattern emerges when tuning the charge density and displacement field independently with dual gates, originating from a layer-selective SOC proximity effect. Analyzing the Landau level crossings and measured inter-Landau level energy gaps yields the proximity-induced SOC energy scale. The Ising SOC is ∼2.2 meV, 100 times higher than the intrinsic SOC in graphene, whereas its sign is consistent with theories predicting a dependence of SOC on interlayer twist angle. The Rashba SOC is ∼15 meV. Finally, we infer the magnetic field dependence of the inter-Landau level Coulomb interactions. These ultraclean bilayer graphene/WSe2 heterostructures provide a high mobility system with the potential to realize novel topological electronic states and manipulate spins in nanostructures.

Heat accumulation effects in laser processing of diamond-like nanocomposite films with bursts of femtosecond pulses

In this paper, we have investigated the burst mode (BM) ablation and surface structuring of diamondlike nanocomposite (DLN) a-C:H:Si:O films with femtosecond laser pulses (wavelength λ = 515 nm, pulse duration τ = 320 fs, and pulse repetition rate f = 100 kHz) under different scanning conditions (single spots and linear structures). The pulse separation in the bursts is 25 ns (intraburst frequency f = 40 MHz), and the pulse number is varied from 1 to 8. The ablation depth and specific ablation rates (μm3/μJ) are found to be higher for the burst mode compared to single-pulse irradiation, increasing with the pulse number in the burst. The obtained experimental data of the higher ablation efficiency are shown to correlate with computer simulations of the BM ablation. In correlation with the ablation findings, Raman spectra of single spots and microgrooves have evidenced a growing graphitization of the amorphous film structure with the pulse number in the bursts (at an equal energy deposited into the films). Contact-mode atomic force microscopy (AFM) is applied to reveal an influence of the BM processing on the surface properties (nanoscale relief, friction) of laser-structured films. Based on the ablation and Raman data analysis, AFM examination of ablated/redeposited layers, and computer simulations of the burst mode ablation, the heat accumulation is identified as the main factor responsible for the enhanced ablation efficiency during the BM processing of DLN films. In addition, results of the high precision surface microstructuring of DLN films in the burst mode are presented.

Optimization of signal intensity in intermittent contact scanning nonlinear dielectric microscopy

Scanning nonlinear dielectric microscopy (SNDM) is a useful method for the nano-scale carrier distribution imaging of semiconductor materials and devices. This method is normally combined with contact mode atomic force microscopy (AFM) but the combination of SNDM with intermittent contact mode AFM is preferable to avoid the damages of tips and samples. However, signal-to-noise (S/N) ratio may significantly reduce due to a shorter contact time in the intermittent contact mode. Here we discuss the S/N ratio of the intermittent contact mode and show that the existence of the optimal condition on contact time and measurement bandwidth to maximize S/N ratio. We also experimentally demonstrate that signal intensity is actually improved by controlling contact time in the carrier distribution imaging on a Si test sample and atomically-thin layered semiconductors.

Caenorhabditis elegans revisited by atomic force microscopy – Ultra-structural changes of the cuticle, but not in the intestine after treatment with Combretum mucronatum extract

Assessing the internal morphology of Caenorhabditis elegans by a topographical technique like atomic force microscopy (AFM) is a challenging process. As a prerequisite for a successful image acquisition, direct contact between the structure of interest and the AFM probe needs to be established. To gain this insight into the morphology of cuticle and intestine in C. elegans before and after treatment with a tannin-enriched hydro-ethanolic extract from Combretum mucronatum, we developed an approach based on polyethylene glycol embedding, ultra-sectioning, de-embedding and hexamethyldisilazane-dehydration prior to measuring in ambient conditions by intermittent contact mode AFM. The used experimental protocol allowed a facile and fast insight into the ultrastructure of treated versus untreated C. elegans individuals, directly leading to the identification of treatment-associated morphological alterations in the cuticle but not the intestine of C. elegans. Additionally, the presented ultra-microtomy based protocol could allow future insight into virtually any tissue or organism by AFM.

The evolution of surface cleanness and electronic properties of graphene field-effect transistors during mechanical cleaning with atomic force microscopy

The evolution of surface cleanliness and the electronic properties-Dirac voltage(VDirac), hysteresis and mobility (μ) of a graphene field-effect transistor (GFET)—were monitored by measuring lateral force microscopy and drain current (ID) as a function of gate voltage (VG), after mechanically cleaning the surface, scan-by-scan, with contact-mode atomic force microscopy. Both the surface cleanliness and the electronic properties evolved, showing a sudden improvement and then saturation for a mobility of around 2200 cm2 V−1 s−1. We found that the mobility suppression of the as-fabricated GFET deviated from a randomly distributed impurities model, which predicted a greater mobility than obtained from the measured VDirac. Therefore, the substrate impurities are excluded from the origins of the extraordinary suppression of the mobility, and the possible origin will be discussed.

Nickel Nanofilms Electrolessly Deposited on Organosilane Nanorings and Characterized by Contact Mode AFM Combined with Magnetic Sample Modulation

Surface structures of magnetic nanorings were made using electroless deposition of Ni onto patterned templates of an amine-functionalized organosilane. Samples were prepared by chemical approaches based on colloidal lithography employing a surface mask of size-sorted, monodisperse silica spheres. Surface changes were evaluated after key points of the reactions using imaging modes of atomic force microscopy (AFM). Nanopatterns of 3-aminopropyltriethoxysilane (APTES) were prepared on Si(111) by applying a heated vapor to a surface mask of silica spheres. After rinsing, the particle mask was removed to reveal ring-shaped nanopatterns presenting amine groups at the interface. Organosilane nanopatterns were then immersed in a solution of Pd catalyst followed by treatment in a Ni plating bath. Changes in surface morphology after each reaction step were characterized ex situ using tapping-mode AFM to follow the time course of nanofabrication. Images of the Ni nanorings acquired with AFM were compared with SEM micrographs to further elucidate the morphology of the metal coatings. The magnetic character of the nanostructures was investigated with magnetic sample modulation (MSM-AFM), which is a hybrid of contact mode AFM combined with magnetic actuation of samples. Surface maps of the vibration of diamagnetic Pd and magnetic Ni nanorings were obtained with MSM-AFM, providing insight on processes of electroless plating. Fine details of the surface corrugation and grain structure of the Ni coated areas of the sample detected with SEM were sensitively resolved with MSM-AFM that were not apparent in AFM topography frames. Chemistry-based steps with electroless deposition (ELD) of metal and colloidal lithography provide a practical route for reproducible nanofabrication of highly regular geometries with high-throughput.

Experimental Setup for Dynamic Analysis of Micro- and Nano-Mechanical Systems in Vacuum, Gas, and Liquid.

An experimental setup to perform dynamic analysis of a micro- and nano-mechanical system in vacuum, gas, and liquid is presented. The setup mainly consists of a piezoelectric excitation part and the chamber that can be either evacuated for vacuum, or filled with gas or water. The design of the piezoelectric actuator was based on a Langevin transducer. The chamber is made out of materials that can sustain: vacuum, variety of gases and different types of liquids (mild acids, alkalies, common alcohols and oils). All the experiments were performed on commercial cantilevers used for contact and tapping mode Atomic Force Microscopy (AFM) with stiffness 0.2 N/m and 48 N/m, respectively, in vacuum, air and water. The performance of the setup was evaluated by comparing the measured actuator response to a finite element model. The frequency responses of the two AFM cantilevers measured were compared to analytical equations. A vacuum level of 0.6 mbar was obtained. The setup has a bandwidth of 10–550 kHz in vacuum and air, and a bandwidth of 50–550 kHz in liquid. The dynamic responses of the cantilevers show good agreement with theory in all media.

Film Thickness and Friction of ZDDP Tribofilms

Tribofilm formation by several zinc dialkyl- and diaryldithiophosphate (ZDDP) solutions in thin film rolling-sliding conditions has been investigated. A primary, a secondary alkyl and a mixed alkyl ZDDP show similar rates of film formation and generate films typically 150 nm thick. Another secondary ZDDP forms a tribofilm much faster and the film is partially lost after extended rubbing. An aryl ZDDP forms a tribofilm much more slowly. The films all have a pad-like structure, characterised by flat pad regions separated by deep valleys. Three different techniques have been used to analyse the thickness and morphology of the tribofilms: spacer layer imaging (SLIM), scanning white light interferometry (SWLI) of the gold-coated film and contact mode atomic force microscopy (AFM). The SLIM method measures considerably thicker films than the other two techniques, probably because of lack of full conformity of a glass disc loaded against the rough tribofilm. No evidence of a highly viscous layer on top of the solid tribofilm is seen. SWLI and contact mode AFM measure similar film thicknesses. The importance of coating the tribofilm with a reflective layer prior to using SWLI is confirmed. As noted in previous work, the formation of a ZDDP tribofilm is accompanied by a marked shift in the Stribeck friction curve towards higher entrainment speed. For a given ZDDP this shift is found to correlate with the measured tribofilm roughness, proving that it results from the influence of this roughness on fluid entrainment in the inlet.

Multifrequency nanoscale impedance microscopy (m-NIM): A novel approach towards detection of selective and subtle modifications on the surface of polycrystalline boron-doped diamond electrodes

In this paper, we describe the modification of Nanoscale Impedance Microscopy (NIM), namely, a combination of contact-mode atomic force microscopy with local impedance measurements. The postulated approach is based on the application of multifrequency voltage perturbation instead of standard frequency-by-frequency analysis, which among others offers more time-efficient and accurate determination of the resultant impedance spectra with high spatial resolution. Based on the impedance spectra analysis with an appropriate electric equivalent circuit, it was possible to map surface resistance and contact capacitance.Polycrystalline heavy boron-doped diamond (BDD) electrodes were the research object. Recent studies have shown that the exposure of such electrodes to oxidizing environment may result in the modification of termination type, and thus it is a key factor in describing the electric and electrochemical properties of BDD. We have successfully applied multifrequency NIM, which allowed us to prove that the modification of termination type is selective and occurs with different propensity on the grains having specific crystallographic orientation. Furthermore, our approach enabled the detection of even subtle submicroscopic surface heterogeneities, created as a result of various oxidation treatments and to distinguish them from the surface heterogeneity related to the local distribution of boron at the grain boundaries.

Controllable Tunneling Triboelectrification of Two-Dimensional Chemical Vapor Deposited MoS2

Tunneling triboelectrification of chemical vapor deposited monolayer MoS2 has been characterized at nanoscale with contact-mode atomic force microscopy (AFM) and Kelvin force microscopy (KFM). Although charges can be trapped on insulators like SiO2 by conventional triboelectrification, triboelectric charges tunneling through MoS2 and localized at the underlying substrate exhibit more than two orders of magnitude longer lifetime. Their polarity and density can be modified by triboelectric process with various bias voltages applied to Pt-coated AFM tips, and the saturated density is almost 30 times higher than the reported result of SiO2. Thus, the controllable tunneling triboelectric properties of MoS2 on insulating substrates can provide guidance to build a new class of two-dimensional (2D) MoS2-based nanoelectronic devices.

Zinc Oxide- and Magnesium-Doped Zinc Oxide-Decorated Nanocomposites of Reduced Graphene Oxide as Friction and Wear Modifiers

Zinc oxide (ZnO) and magnesium-doped zinc oxide nanoparticles, Zn0.88Mg0.12O (ZMO), were prepared by autocombustion method. Further, nanocomposites of the as-prepared nanoparticles with microwave-synthesized reduced graphene oxide (rGO) nanosheets, ZnO-rGO and ZMO-rGO, have also been prepared with a view to see the effect of doping of magnesium in zinc oxide on the tribological properties of the nanocomposite. Morphologies of nanoparticles/nanosheets and their nanohybrids have been studied by employing scanning electron microscopy (SEM)/high-resolution (HR) SEM with energy-dispersive X-ray (EDX), transmission electron microscopy (TEM)/HR-TEM, X-ray diffraction, Fourier transform infrared, UV-visible, Raman, and X-ray photoelectron spectroscopy (XPS) techniques. Triboactivity of the additives in paraffin oil has been interpreted considering the parameters mean wear scar diameter, coefficient of friction, load-carrying capacity, and wear rates obtained from ASTM D4172 and ASTM D5183 tests using a four-ball lubricant tester at optimized concentration (0.125% w/v). The performance of base lube and its admixtures has been found to lie in the order ZMO-rGO > ZnO-rGO > ZMO > ZnO > rGO > paraffin oil. Outstanding enhancement in triboactivity of nanocomposites, particularly that of ZMO-rGO indicates that nanoparticles are irrefutably instrumental in reinforcement of rGO, and on the other hand, rGO is associated with abatement of agglomeration of the nanoparticles. Thus, interactions between rGO and nanoparticles are vehemently synergic in nature. It is noteworthy that the best results were obtained with the following optimized concentrations: ZnO/ZMO 0.25%; rGO 0.15% and composites 0.125% w/v. Morphological studies of the wear track lubricated with different additives have been performed using SEM and contact mode atomic force microscopy. Results are in conformity with the order given above. The EDX analysis of ZMO-rGO exhibits the presence of zinc and magnesium on the worn surface, supporting their role in the formation of in situ tribofilm. Their role is further corroborated by XPS studies. Owing to their excellent tribological behavior, these sulfur- and phosphorus-free composites may be recommended as potential wear and friction modifiers.