一种新颖的点衍射干涉轻敲模式原子力显微镜

论述了一种新颖的原子力显微镜,它利用硅微探针的特殊结构和相关光学系统所引起的点衍射干涉现象[1]来扫描成像,因为硅微探针被用作反射型点衍射板,故光路完全共路,再结合锁相检测技术,使得该仪器抗干扰力极强且结构精巧紧凑,可适用于测试软硬不同材料样品,对软质高分子膜材料检测得到了实际的链状结构。     

Dynamic analysis of tapping mode atomic force microscope (AFM) for critical dimension measurement

Transient dynamics of tapping mode atomic force microscope (AFM) for critical dimension measurement are analyzed. A simplified nonlinear model of AFM is presented to describe the forced vibration of the micro cantilever-tip system with consideration of both contact and non-contact transient behavior for critical dimension measurement. The governing motion equations of the AFM cantilever system are derived from the developed model. Based on the established dynamic model, motion state of the AFM cantilever system is calculated utilizing the method of averaging with the form of slow flow equations. Further analytical solutions are obtained to reveal the effects of critical parameters on the system dynamic performance. In addition, features of dynamic response of tapping mode AFM in critical dimension measurement are studied, where the effects of equivalent contact stiffness, quality factor and resonance frequency of cantilever on the system dynamic behavior are investigated. Contact behavior between the tip and sample is also analyzed and the frequency drift in contact phase is further explored. Influence of the interaction between the tip and sample on the subsequent non-contact phase is studied with regard to different parameters. The dependence of the minimum amplitude of tip displacement and maximum phase difference on the equivalent contact stiffness, quality factor and resonance frequency are investigated. This study brings further insights into the dynamic characteristics of tapping mode AFM for critical dimension measurement, and thus provides guidelines for the high fidelity tapping mode AFM scanning.     

Simulation-aided design and fabrication of nanoprobes for scanning probe microscopy

We proposed and demonstrated a flexible and effective method to design and fabricate scanning probes for atomic force microscopy applications. Computer simulations were adopted to evaluate design specifications and desired performance of atomic force microscope (AFM) probes; the fabrication processes were guided by feedback from simulation results. Through design-simulation-fabrication iterations, tipless cantilevers and tapping mode probes were successfully made with errors as low as 2% in designed resonant frequencies. For tapping mode probes, the probe tip apex achieved a 10 nm radius of curvature without additional sharpening steps; tilt-compensated probes were also fabricated for better scanning performance. This method provides AFM users improved probe quality and practical guidelines for customized probes, which can support the development of novel scanning probe microscopy (SPM) applications.     

Localized etching of silicon in water using a catalytically active platinum-coated atomic force microscopy probe

This paper presents a novel atomic force microscopy (AFM)-based nanofabrication technique for Si in water that is based on highly localized catalytic etching with a Pt-coated AFM probe. It has been shown that nanoscale grooves can be fabricated on the Si surface at room temperature via Pt-assisted catalytic chemical etching in water without the addition of any chemicals. Furthermore, dissolved oxygen (O₂) in water has been found to be a key element for driving the chemical reaction of Si with water in the Si removal process. Experimental results have also suggested that an oscillating cantilever of the Pt-coated AFM probe for the stirring of water is essential in order to overcome the oxygen mass-transfer limitations and enhance the Si removal rate. The elementary chemical reactions taking place during the etching of Si has been estimated on the basis of electrochemical theory. It is proposed that in the first step, dissolved oxygen is reduced and forms hydroxide ions (OH⁻) with water molecules (H₂O) on the surface of the Pt-coated tip. In the second step, Si atoms are oxidized on reaction with OH⁻ ions and water soluble silicates are formed. The catalytic reaction taking place on the surface of a Pt-coated tip can be enhanced by the application of an anodic potential to an additional Pt wire electrode, resulting in a dramatic fifty-fold increase in the Si removal rate.     

Improving tapping mode atomic force microscopy with piezoelectric cantilevers

This article summarizes improvements to the speed, simplicity and versatility of tapping mode atomic force microscopy (AFM). Improvements are enabled by a piezoelectric microcantilever with a sharp silicon tip and a thin, low-stress zinc oxide (ZnO) film to both actuate and sense deflection. First, we demonstrate self-sensing tapping mode without laser detection. Similar previous work has been limited by unoptimized probe tips, cantilever thicknesses, and stress in the piezoelectric films. Tests indicate self-sensing amplitude resolution is as good or better than optical detection, with double the sensitivity, using the same type of cantilever. Second, we demonstrate self-oscillating tapping mode AFM. The cantilever's integrated piezoelectric film serves as the frequency-determining component of an oscillator circuit. The circuit oscillates the cantilever near its resonant frequency by applying positive feedback to the film. We present images and force-distance curves using both self-sensing and self-oscillating techniques. Finally, high-speed tapping mode imaging in liquid, where electric components of the cantilever require insulation, is demonstrated. Three cantilever coating schemes are tested. The insulated microactuator is used to simultaneously vibrate and actuate the cantilever over topographical features. Preliminary images in water and saline are presented, including one taken at 75.5 μm/s—a threefold improvement in bandwidth versus conventional piezotube actuators.     

In-situ transmission electron microscopy studies of polymer–carbon nanotube composite deformation

This paper demonstrates the viability of in-situ transmission electron microscopy for studying the deformation mechanisms of polymer nano-composites. In-situ straining studies are performed on carbon multiwalled nanotube (MWNT)–polystyrene composite films. The experiments show that load transfer across the nanotube–polystyrene interface is operative well into the plastic deformation regime of the composite film. The MWNTs are observed to bridge cracks propagating through the polystyrene, providing closure stresses across the crack wake. Although some MWNTs fracture by either a sword-in-sheath mechanism or transverse shear fracture, most of the MWNTs eventually debond at the MWNT–polymer interface and subsequently pull out of the matrix.     

Analysis of dynamic cantilever behavior in tapping mode atomic force microscopy

Tapping mode atomic force microscopy (AFM) provides phase images in addition to height and amplitude images. Although the behavior of tapping mode AFM has been investigated using mathematical modeling, comprehensive understanding of the behavior of tapping mode AFM still poses a significant challenge to the AFM community, involving issues such as the correct interpretation of the phase images. In this paper, the cantilever's dynamic behavior in tapping mode AFM is studied through a three dimensional finite element method. The cantilever's dynamic displacement responses are firstly obtained via simulation under different tip‐sample separations, and for different tip‐sample interaction forces, such as elastic force, adhesion force, viscosity force, and the van der Waals force, which correspond to the cantilever's action upon various different representative computer‐generated test samples. Simulated results show that the dynamic cantilever displacement response can be divided into three zones: a free vibration zone, a transition zone, and a contact vibration zone. Phase trajectory, phase shift, transition time, pseudo stable amplitude, and frequency changes are then analyzed from the dynamic displacement responses that are obtained. Finally, experiments are carried out on a real AFM system to support the findings of the simulations. Microsc. Res. Tech. 78:935–946, 2015. © 2015 Wiley Periodicals, Inc.     

Optimizing the driving scheme of a self-actuated atomic force microscope probe for high-speed applications

We investigate the optimum driving scheme of a dynamic atomic force microscope with a self-actuated probe for high-speed applications by performing numerical simulations. We compare the recently developed methods such as Q-control, dynamic PID control, and modified Q-control methods to the standard tapping mode by considering scan speed and peak transient forces. In addition, the effects of driving frequency and set-point amplitude on the maximum achievable scan speed for the same probe-sample system are discussed. We find that the scan speed can be increased significantly at the expense of increased peak transient forces.     

Analysis of simulated scanning of atomic-scale silicon surface by atomic force microscopy

This study constructs a contact-mode atomic force microscopy (AFM) simulation measurement model with constant force mode to simulate and analyze the outline scanning measurement by AFM. The simulation method is that when the probe passes the surface of sample, the action force of the atom of sample received by the atom of the probe can be calculated by using Morse potential. Through calculation, the equivalent force on the cantilever of probe can be acquired. By using the deflection angle equation for the cantilever of probe developed and inferred by this study, the deflection angle of receiving action force can be calculated. On the measurement point, as the deflection angle reaches a fixed deflection angle, the scan height of this simulation model can be acquired. By scanning in the right order, the scan curve of the simulation model can be obtained. By using this simulation measurement model, this study simulates and analyzes the scanning of atomic-scale surface outline. Meanwhile, focusing on the tip radii of different probes, the concept of sensitivity analysis is employed to investigate the effects of the tip radius of probe on the atomic-scale surface outline. As a result, it is found from the simulation on the atomic-scale surface that within the simulation scope of this study, when the tip radius of probe is greater than 12 nm, the effects of single atom on the scan curve of AFM can be better decreased or eliminated.     

Spectroscopy with scanning near-field optical microscopy using photon tunnelling mode

We have demonstrated Raman spectroscopy using scanning near-field optical microscopy (SNOM). Photon tunnelling mode was employed, in which the sample is illuminated using an attenuated total reflection (ATR) configuration and the evanescent wave perturbed by the sample is picked up by a sharpened optical fibre probe. By this experimental arrangement Raman scattering from the optical fibre probe was greatly reduced, therefore we were able to excite the sample using more intense laser light compared to the illumination mode SNOM. Raman spectra of copper phthalocyanine (CuPc) were obtained in the off-resonance condition and without using surface-enhanced Raman scattering (SERS).     

Fabrication of carbon nanotube AFM probes using the Langmuir–Blodgett technique

Carbon nanotube (CNT)-tipped atomic force microscopy (AFM) probes have shown a significant potential for obtaining high-resolution imaging of nanostructure and biological materials. In this paper, we report a simple method to fabricate single-walled carbon nanotube (SWNT) nanoprobes for AFM using the Langmuir–Blodgett (LB) technique. Thiophenyl-modified SWNTs (SWNT-SHs) through amidation of SWNTs in chloroform allowed to be spread and form a stable Langmuir monolayer at the water/air interface. A simple two-step transfer process was used: (1) dipping conventional AFM probes into the Langmuir monolayer and (2) lifting the probes from the water surface. This results in the attachment of SWNTs onto the tips of AFM nanoprobes. We found that the SWNTs assembled on the nanoprobes were well-oriented and robust enough to maintain their shape and direction even after successive scans. AFM measurements of a nano-porous alumina substrate and deoxyribonucleic acid using SWNT-modified nanoprobes revealed that the curvature diameter of the nanoprobes was less than 3 nm and a fine resolution was obtained than that from conventional AFM probes. We also demonstrate that the LB method is a scalable process capable of simultaneously fabricating a large number of SWNT-modified nanoprobes.     

The effect of amorphous carbon layer on the field emission characteristics of carbon nanotube film

Carbon nanotube (CNT) has excellent field emission characteristics and could play as a good cold cathode in the application of vacuum electronic devices. However, the practical application faces a big obstacle regarding current fluctuation and deterioration of the CNT cathode. In this research, the formation of amorphous carbon (ac) layer between the CNT film and the substrate, and the effect of the existence of this layer on field emission stability of the CNT film are studied. The formation of the ac layer could be controlled by adjustment of growth temperature and hydrocarbon flow rate. The field emission character and current stability of the CNT film without ac layer are better than those of the CNT film with ac layer. The results attribute to the ac layer a thermal disequilibrium state under high current level. Moreover, adhesion capacity of the CNT film without ac layer is also better than that with the ac layer. It is concluded that the ac layer between the CNT film and substrate is a key factor in the stability of field emission characteristics and should be eliminated before applications.     

Electrocatalytic determination of captopril using a modified carbon nanotube paste electrode: Application to determination of captopril in pharmaceutical and biological samples

A carbon paste electrode modified with carbon nanotube and benzoylferrocene (BF) was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of captopril (CAP), was described. The electrode was employed to study the electrocatalytic oxidation of CAP, using cyclic voltammetry (CV), chronoamperometry (CHA) and square wave voltammetry (SWV) as diagnostic techniques. It has been found that the oxidation of CAP at the surface of modified electrode occurs at a potential of about 85 mV less positive than that of an unmodified CPE. SWV exhibits a linear dynamic range from 1.0 × 10⁻⁷ to 3.5 × 10⁻⁴ M and a detection limit of 3.0 × 10⁻⁸ M for CAP. Finally the modified electrode was used for determination of CAP in CAP tablet and urine sample.     

Optimized hand fabricated AFM probes for simultaneous topographical and electrochemical tapping mode imaging

In this work hybrid AFM-electrochemical (SECM) probes to be used in dynamic atomic force microscopy are presented. These nanosensors are hand fabricated from gold microwires using a simple benchtop method. They display proportions close to commercially available silicon and silicon nitride cantilevers giving comparable performance in terms of resolution and imaging stability. The remarkable characteristic of these hybrid nanosensors is that they allow the coupling of 3D imaging ability and versatility of atomic force microscopy with the power of electrochemical methods. Local measurement of electrochemical-activity of a test sample consisting of gold bands functionalized by redox-labeled nanometer-sized polyethylene glycol chains has been achieved with simultaneous imaging of the 3D surface topography at high resolution. These hybrid AFM-SECM tips are capable of sensing local electrochemical currents down to ∼10 fA emphasizing the sensitivity and resolution of this technique.     

Manipulation, dissection, and lithography using modified tapping mode atomic force microscope

A modified tapping mode of the atomic force microscope (AFM) was introduced for manipulation, dissection, and lithography. By sufficiently decreasing the amplitude of AFM tip in the normal tapping mode and adjusting the setpoint, the tip-sample interaction can be efficiently controlled. This modified tapping mode has some characteristics of the AFM contact mode and can be used to manipulate nanoparticles, dissect biomolecules, and make lithographs on various surfaces. This method did not need any additional equipment and it can be applied to any AFM system.     

Phase evolution in cholesterol/DPPC monolayers: atomic force microscopy and near field scanning optical microscopy studies

A combination of atomic force microscopy (AFM) and near field scanning optical microscopy has been used to study domain formation in dipalmitoylphosphatidylcholine (DPPC)/cholesterol monolayers with cholesterol concentrations ranging from 0 to 50%. The results show a clear evolution from a mixture of liquid expanded and liquid condensed phases for cholesterol concentrations < 10% to a mixture of liquid expanded and two cholesterol‐containing phases at intermediate concentrations, and finally to a single homogeneous liquid ordered phase for 33% cholesterol. Mixtures of the various phases are clearly identified by height differences in AFM and in some cases by fluorescence imaging for samples containing 0.5% BODIPY dye, which localizes preferentially in the more fluid phase. Note that fluorescence imaging, at least with the dye used here, is unable to distinguish between the cholesterol‐rich and cholesterol‐poor phases detected at intermediate cholesterol concentrations. The combination of fluorescence and AFM imaging provides a more complete picture of the phase evolution for cholesterol/DPPC monolayers than could be obtained by either technique alone, and presents substantial advantages over conventional fluorescence microscopy in that submicrometre‐sized domains can be readily detected.     

High-speed force load in force measurement in liquid using scanning probe microscope

This article presents an inversion-based iterative feedforward-feedback (II-FF/FB) approach to achieve high-speed force load in force measurement of soft materials in liquid using scanning probe microscope (SPM). SPM force measurement under liquid environment is needed to interrogate a wide range of soft materials, particularly live biological samples. Moreover, when dynamic evolution of the sample occurs during the measurement, and/or measuring the rate-dependent viscoelasticity of the sample, the force measurement also needs to be acquired at high-speed. Precision force load in liquid, however, is challenged by adverse effects including the thermal drift effect, the reduction of the signal to noise ratio, the distributive hydrodynamic force effect, and the hysteresis and vibrational dynamics effects of the piezoelectric actuators (for positioning the probe relative to the sample), particularly during high-speed measurement. Thus, the main contribution of the article is the development of the II-FF/FB approach to tackle these challenges. The proposed method is illustrated through an experimental implementation to the force-curve measurement of a poly (dimethylsiloxane) sample in liquid at high-speed.     

Study on effect of hydrogen treatment on amorphous carbon film using scanning probe microscopy

Amorphous carbon film was treated by hydrogen plasma. The change of surface structure, conductivity, and work function distribution is characterized by scanning probe microscope technique and local electron emission is also analyzed. We find that chemical effect of hydrogen plasma on the a-C film is small, but the etching effect is strong and the surface morphology and conductance are obviously changed after hydrogen treatment. Electron emission enhancement is not due to the decrease of work function or existence of sp² conductive channels, but from the mutual effect between sp² and sp³ phase. We suggest that the enhancement is due to the internal electron injection from the sp²-rich interface layer into the surface sp³-rich grains.