Analyses of vibration responses on nanoscale processing in a liquid using tapping-mode atomic force microscopy

An analytical solution of the vibration responses of biological specimens using atomic force microscopy (AFM), which often requires operation in a liquid, is developed. In this study, the modal superposition method is employed to analyze the vibration responses of AFM cantilevers in tapping mode (TM) operated in a liquid and in air. The hydrodynamic force exerted by the fluid on AFM cantilevers is approximated by additional mass and hydrodynamic damping. The tip–sample interaction forces were transformed into axial, distributed transversal, and bending loading, and then applied to the end region of the AFM through the tip holder. The effects of transverse stress and bending stress were adopted to solve the dynamic model. With this model, a number of simulations were carried out to investigate the relationship between the transient responses of the cantilever in a liquid and the parameters considered in nanoscale processing. The simulations show that the vibration of AFM cantilevers in a liquid has dramatically different dynamic characteristics from these of that in air. The liquid reduces the magnitude of the transversal response and reduces the cantilever resonances. Moreover, the magnitudes of response become larger with increasing intermolecular distances and smaller with decreasing tip length. The cantilever vibration amplitudes significantly depend on the damping constant and the mass proportionality constant.     

Metrology of electromagnetic static actuation of MEMS microbridge using atomic force microscopy

The objective of this paper is to describe application of atomic force microscopy (AFM) for characterization and calibration of static deflection of electromagnetically and/or thermally actuated micro-electromechanical (MEMS) bridge. The investigated MEMS structure is formed by a silicon nitride bridge and a thin film metal path enabling electromagnetic and/or thermal deflection actuation. We present how static microbridge deflection can be measured using contact mode AFM technology with resolution of 0.05 nm in the range of up to tens of nm. We also analyze, for very small structure deflections and under defined and controlled load force varied in the range up to ca. 32 nN, properties of thermal and electromagnetical microbridge deflection actuation schemes.     

Study of Schottky contact between Au and NiO nanowire by conductive atomic force microscopy (C-AFM): The case of surface states

In this work, NiO nanowires have been synthesized by a hydrothermal reaction of NiCl₂ with Na₂C₂O₄ in the presence of ethylene glycol at 180 °C for 12 h, then calcinated at 400 °C for 2 h. The NiO nanowires were analyzed by means of scanning electron microscope (SEM), atomic force microscope (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The resulting current–voltage (I–V) characteristics of the NiO nanowires exhibited a clear rectifying behavior. This rectify behavior was attributed to the formation of a Schottky contact between Au coated atomic force microscopy (AFM) tip and NiO nanowires (nano-M/SC) which was dominated by the surface states in NiO itself. Photo-assisted conductive AFM (PC-AFM) was used to demonstrate how the I–V characteristics are influenced by the surface states. Our I–V results also showed that the nano-M/SCs had a good photoelectric switching effect at reverse bias.     

Study on the interactions between anti-cancer drug oxaliplatin and DNA by atomic force microscopy

Oxaliplatin is one of the most important anticancer drugs at present. However, the mechanism of action of oxaliplatin is still controversial. In this study, the interactions between oxaliplatin and a plasmid DNA have been studied so as to reveal the structural basis of its activity. The structural characteristic of pUC19 DNA (2 ng/μL) incubated with 100 μmol/L and 1000 μmol/L of oxaliplatin for the different time on a freshly cleaved highly ordered pyrolytic graphite (HOPG) surface was investigated by atomic force microscopy (AFM). High resolution AFM observation indicated that oxaliplatin can induce pUC19 DNA molecules change from the extended conformation to the entangled structures with many nodes, and finally to the compact particles. The present AFM results provide structural evidence about the interactions between oxaliplatin and circular duplex DNA containing multiple targets.     

Micro-force measurement of drag on a small flat plate in the presence of a corona discharge

The design of a micro-force sensor suitable for the measurement of corona drag and other low velocity drag studies in a small laboratory wind tunnel facility is described. Example drag data are given for dc corona discharge generated by sharp parallel electrodes mounted on a microscope glass slide with discharge parallel to the air flow. The arrangement simulates two-dimensional flow over a flat plate useful for theoretical analysis. Measurements of free stream wind velocities in the range 0–210 cm/s with attendant drag down to 10⁻⁷ N can be detected in this facility depending on the calibration. The force sensor utilizes two strain gages mounted on a 0.127 mm stainless steel “feeler gage” in a cantilever arrangement. A bridge circuit provides sensitivities in the range 40–250 N/mV using a gravitational calibration technique. Anomalous effects from suspension wires and the interaction of electrostatic forces with the surroundings are discussed.     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

Morphology of synthetic DOPA-eumelanin deposited on glass and mica substrates: An atomic force microscopy investigation

Bright field microscopy and atomic force microscopy techniques are used to investigate morphological properties of synthetic eumelanin, obtained by oxidation of l-DOPA solution, deposited on glass and mica substrates. Deposits of eumelanin are characterized by aggregates with different shape and size. On a micrometric scale, filamentous as well as granular structures are present on glass and mica substrates, with a larger density on the former than on the latter. On a nanometric scale, filamentous aggregates, several microns long and about 100 nm wide and high, and granular aggregates, ∼50 nm high and 100 nm wide, are found on both substrates, whereas point-like deposits less than 10 nm high and less than 50 nm wide are found on mica substrate. Dynamic light scattering measurements and atomic force microscopy images support the evidence that eumelanin presents only nanometric point-like aggregates in aqueous solution, whereas such nanoaggregates organize themselves according to granular and filamentous structures when deposition occurs, as a consequence of interactions with the substrate surface.     

Controlled fabrication of gallium nitride nano- and micro-wires by dielectrophoretic force and torque

This paper demonstrates the manipulation of neutral dielectric wires with high aspect ratio by a pulsed electric field. Dielectrophoretic (DEP) force and torque were employed to align the randomly positioned GaN nano- and micro-wires. A simulation of the DEP force alignment process confirmed the experimentally observed dependence on alignment yield to frequency and bias of the electric field. Current–voltage measurements of the GaN micro-wires, aligned by DEP force and torque to pre-patterned metal contacts, confirms that the direct manipulation of micro-sized wire with an electric field oscillated at a frequency of 10 kHz–5 MHz.     

Electric current measurement using fiber-optic curvature sensor

A novel fiber-optic curvature sensor, which can measure curvature directly, has been developed in recent years. The electric current measurements system based on fiber-optic curvature sensor and electromagnetic principle is developed. A fiber-optic curvature sensor is bonded to a thin-walled cantilever and two circular magnet targets with the same parameters are configured at the tip of the cantilever symmetrically. In this case, the throughput of the sensor will be changed due to the bending deformation of cantilever, which is proportional to the electromagnetic force caused by measured electric current. Direct and alternate characteristics of the proposed measurement system are studied experimentally. The results show that the measurement errors are within the range of ±5.5 mA and the corresponding accuracy is within 1% at the current measurement range from −300 mA to 300 mA, which indicate the feasibility of the proposed measurement system.     

原子力显微镜在轻敲模式下的动力学模型

基于Hamaker假设、Lennard-Jones势能定律及经典弹性理论建立了一种新型的球体与平面黏着接触的弹性模型，该模型显示黏着力在原子力显微镜(AFM)针尖趋近和撤离样品表面，即加载和卸载的两个过程中存在黏着滞后现象，表明了AFM在轻敲工作模式中存在能量耗散.同时，根据所建的黏着接触弹性模型，建立了AFM在轻敲工作模式下的动力学模型，研究了AFM在轻敲工作模式下的振动幅度、相位差及耗散功率随针尖与样品表面间距的变化规律，仿真结果与现有的实验结果相一致.     

Theory of superfluidity and drag force in the one-dimensional Bose gas

The one-dimensional Bose gas is an unusual superfluid. In contrast to higher spatial dimensions, the existence of non-classical rotational inertia is not directly linked to the dissipationless motion of infinitesimal impurities. Recently, experimental tests with ultracold atoms have begun and quantitative predictions for the drag force experienced by moving obstacles have become available. This topical review discusses the drag force obtained from linear response theory in relation to Landau’s criterion of superfluidity. Based upon improved analytical and numerical understanding of the dynamical structure factor, results for different obstacle potentials are obtained, including single impurities, optical lattices and random potentials generated from speckle patterns. The dynamical breakdown of superfluidity in random potentials is discussed in relation to Anderson localization and the predicted superfluid–insulator transition in these systems.     

Simultaneous measurement of the temperature and force using a steel cantilever soldered with a partially nickel coated in-fibre Bragg grating

A method for the simultaneous, independent measurement of temperature and force using a single in-fiber Bragg grating (FBG) sensors was proposed and demonstrated. The FBGs were partially metallized using a partial nickel coating method. A Bragg reflex peak was successfully divided into two reflex peaks during the partial nickel plating process. The metallized part of the FBG was soldered on a steel cantilever, and the non-metallized part hanged in air. Using a structure that is composed of the steel cantilever soldered with a dual-wavelength FBG, the temperature and pressure can be simultaneously measured and discriminated. The metallized part of the FBG is highly temperature-sensitive (about 26.1 pm/°C), and for the non-metallized part the original Bragg wavelength of the FBG remains unchanged. The force sensitivity of the metallized part which soldered on the cantilever remained at 82 pm/N from 30 to 90 °C.     

Observation of angiotensin II-induced changes in fixed and live mesangial cells by atomic force microscopy

Glomerular mesangial cells (MCs) are centrally located in the glomerulus. MCs control not only glomerular filtration, but also the response to local injury, including cell proliferation and basement membrane remodeling. Angiotensin II (Ang II) plays an important role in kidney function regulation, and participates in the progression of renal damage, as well as mesangial injury. However, studies on Ang II effects on MCs have used indirect methods, such as gene and protein expression after MC injury. In this study, we visually observed structural and mechanical changes to MC after Ang II treatment using atomic force microscopy (AFM). We obtained AFM topography and deflection images of live MCs, as well as fixed MCs in liquid, before and after Ang II treatment. Real-time imaging showed the dynamic movement of live MCs induced by Ang II. Changes in MC elastic property after Ang II treatment were measured using force–distance curves. AFM images of fixed and live MCs showed that cells contracted after Ang II exposure, with the nucleus height increasing within 20 min of Ang II stimulation. Force–distance analysis showed that Ang II caused MCs to stiffen (p < 0.0001). In conclusion, we demonstrated that AFM is an effective tool for real-time monitoring of live cell responses to drugs and stimuli.     

Microfluidic acoustic trapping force and stiffness measurement using viscous drag effect

It has recently been demonstrated that it was possible to individually trap 70 μm droplets flowing within a 500 μm wide microfluidic channel by a 24 MHz single element piezo-composite focused transducer. In order to further develop this non-invasive approach as a microfluidic particle manipulation tool of high precision, the trapping force needs to be calibrated to a known force, i.e., viscous drag force arising from the fluid flow in the channel. However, few calibration studies based on fluid viscosity have been carried out with focused acoustic beams for moving objects in microfluidic environments.In this paper, the acoustic trapping force (F_trapping) and the trap stiffness (or compliance k) are experimentally determined for a streaming droplet in a microfluidic channel. F_trapping is calibrated to viscous drag force produced from syringe pumps. Chebyshev-windowed chirp coded excitation sequences sweeping the frequency range from 18 MHz to 30 MHz is utilized to drive the transducer, enabling the beam transmission through the channel/fluid interface for interrogating the droplets inside the channel. The minimum force (F_min,trapping) required for initially immobilizing drifting droplets is determined as a function of pulse repetition frequency (PRF), duty factor (DTF), and input voltage amplitude (V_in) to the transducer. At PRF = 0.1 kHz and DTF = 30%, F_min,trapping is increased from 2.2 nN for V_in = 22 V_pp to 3.8 nN for V_in = 54 V_pp. With a fixed V_in = 54 V_pp and DTF = 30%, F_min,trapping can be varied from 3.8 nN at PRF = 0.1 kHz to 6.7 nN at PRF = 0.5 kHz. These findings indicate that both higher driving voltage and more frequent beam transmission yield stronger traps for holding droplets in motion.The stiffness k can be estimated through linear regression by measuring the trapping force (F_trapping) corresponding to the displacement (x) of a droplet from the trap center. By plotting F_trapping – x curves for certain values of V_in (22/38/54 V_pp) at DTF = 10% and PRF = 0.1 kHz, k is measured to be 0.09, 0.14, and 0.20 nN/μm, respectively. With variable PRF from 0.1 to 0.5 kHz at V_in = 54 V_pp, k is increased from 0.20 to 0.42 nN/μm. It is shown that a higher PRF leads to a more compliant trap formation (or a stronger F_trapping) for a given displacement x. Hence the results suggest that this acoustic trapping method has the potential as a noninvasive manipulation tool for individual moving targets in microfluidics by adjusting the transducer’s excitation parameters.     

Atomic force microscopy enabled roughness analysis of nanostructured poly (diaminonaphthalene) doped poly (vinyl alcohol) conducting polymer thin films

The Atomic Force Microscopy (AFM) helps in evaluating parameters like amplitude or height parameters, functional or statistical parameters and spatial parameters which describe the surface topography or the roughness. In this paper, we have evaluated the roughness parameters for the native poly (vinyl alcohol) (PVA), monomer diaminonaphthalene (DAN) doped PVA, and poly (diaminonaphthalene) (PDAN) doped PVA films prepared in different solvents. In addition, distribution of heights, skewness and Kurtosis moments which describe surface asymmetry and flatness properties of a film were also determined. At the same time line profiles, 3D and 2D images of the surface structures at different scanning areas i.e. 5 × 5 μm² and 10 × 10 μm² were also investigated. From the roughness analysis and the surface skewness and coefficient of Kurtosis parameters, it was concluded that for PVA film the surface contains more peaks than valleys and the PDAN doped PVA film has more valleys than peaks. It was also found that the PDAN doped PVA film with acetonitrile solvent was used for substrate in electronics applications because the film gives less fractal morphology. Thus, the AFM analysis with different parameters suggested that the PDAN doped PVA films are smooth at the sub-nanometer scale.     

Submicron gold structures formed by atomic force microscopy lithography on thin films of poly(methyl methacrylate)

Thin films of poly(methyl methacrylate) (PMMA) (25 nm) deposited on gold coated glass substrates have been locally modified by exposure to biased atomic force microscopy (AFM) tips. Constant exposure currents in the range of 0.5–0.65 nA leads to removal of PMMA both by direct ablation and enhanced solubility in a developer solution. Possible mechanisms causing such modifications have been discussed. Submicron Au structures were formed by combining AFM lithography, involving localized removal of PMMA, with sputter deposition of gold and a lift-off process.     

Tip cleaning and sharpening processes for noncontact atomic force microscope in ultrahigh vacuum

Tip cleaning and sharpening processes for noncontact atomic force microscope (AFM) operated in ultrahigh vacuum (UHV) were carried out and evaluated by a scanning Auger microscope (SAM) with a field emission electron gun and a noncontact AFM in UHV combined with a scanning tunneling microscope and a field emission microscope. The cantilever used in this study was piezoresistive, which can be heated by passing a current through the resistive legs of the cantilever. As a pretreatment, the tip was irradiated with ultraviolet light in oxygen to remove carbon contaminants. It was heated at about 750°C to form a clean oxide layer in oxygen of 5×10⁻⁵ Torr in an SAM chamber. The desorption of the layer can make a remained tip apex sharper by heating under electron beam irradiation. A thermally oxidized layer was also eliminated by HF etching to sharpen the tip apex. The procedures are useful to obtain a well-defined Si tip suitable for a noncontact AFM.     

Mechano-chemical AFM nanolithography of metallic thin films: A statistical analysis

A mechano-chemical atomic force microscope (AFM) nanolithography on a metallic thin film (50 nm in thickness) covered by a spin-coated soft polymeric mask layer (50–60 nm in thickness) has been introduced. The surface stochastic properties of initial grooves mechanically patterned on the mask layer (grooves before chemical wet-etching) and the lithographed patterns on the metallic thin film (the grooves after chemical wet-etching) have been investigated and compared by using the structure factor, power spectral density, and AFM tip deconvolution analyses. The effective shape of cross section of the before and after etching grooves have been determined by using the tip deconvolution surface analysis. The wet-etching process improved the shape of the grooves and also smoothed the surface within them. We have indicated that relaxation of the surface tension of the deposited mask layer after the AFM scribing is independent from surface density of the grooves and also their length scale. Based on the statistical analysis, it was found that increase of the width of the grooves after the wet-etching and also relaxation of surface tension of the mask layer resulted in a down limit in the size of the metallic nanowires made by the combined nanolithography method. An extrapolation of the analyzed statistical data has indicated that, in this method, the minimum obtainable width and length of the metallic nanowires are about 55 nm and 2 μm, respectively.     

Effect of pressure on structure and extinction of near-limit hydrogen counterflow diffusion flames

Results of measurements of critical conditions for extinction and of temperature profiles in counterflow diffusion flames are reported. The fuel was a hydrogen–nitrogen mixture with 14 mole percent hydrogen, and the oxidizer was air. Pressures ranged from 0.1 MPa to 1.5 MPa; measurements were made in a facility especially constructed for carrying out counterflow combustion experiments at high pressures. With increasing pressure, the strain rate at extinction first increases and then decreases, in qualitative agreement with predictions, but there are observable quantitative differences. Temperature profiles, obtained employing an R-type thermocouple at a fixed strain rate of 100/s, agree well with predictions, within experimental uncertainty. The results may help to improve knowledge of underlying chemical-kinetic and transport parameters at elevated pressures.     

Material anisotropy revealed by phase contrast in intermittent contact atomic force microscopy

Phase contrast in intermittent-contact atomic force microscopy (AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This is surprising, because measurements of nanoscale in-plane properties typically require contact mode microscopies. Our measurements are possible because the tip oscillates not just perpendicular but also parallel to the sample surface along the long axis of the cantilever. This lateral tip displacement is virtually universal in AFM, implying that any oscillating-tip AFM technique is sensitive to in-plane material properties.