Characterization of materials' nanomechanical properties by force modulation and phase imaging atomic force microscopy with soft cantilevers

Soft cantilevers, although having good force sensitivity, have found limited use for investigating materials' nanomechanical properties by conventional force modulation (FM) and intermittent contact (IC) atomic force microscopy. This is due to the low forces and small indentations that these cantilevers are able to exert on the surface, and to the high amplitudes required to overcome adhesion to the surface. In this paper, it is shown that imaging of local elastic properties of surface and subsurface layers can be carried out by employing electrostatic forcing of the cantilever. In addition, by mechanically exciting the higher vibration modes in contact with the surface and monitoring the phase of vibrations, the contrast due to local surface elasticity is obtained.     

Water distribution at solid/liquid interfaces visualized by frequency modulation atomic force microscopy

Interfacial phenomena at solid/water interfaces play an important role in a wide range of industrial technologies and biological processes. However, it has been a great challenge to directly probe the molecular-scale behavior of water at solid/water interfaces. Recently, there have been tremendous advancements in frequency modulation atomic force microscopy (FM-AFM), enabling its operation in liquids with atomic resolution. The high spatial and force resolutions of FM-AFM have enabled the visualization of one-dimensional (1D) profiles of the hydration force, two-dimensional (2D) images of hydration layers and three-dimensional (3D) images of the water distribution at solid/water interfaces. Here I present an overview of the recent advances in FM-AFM instrumentation and its applications to the study of solid/water interfaces.     

Water distribution at solid/liquid interfaces visualized by frequency modulation atomic force microscopy

Abstract

Interfacial phenomena at solid/water interfaces play an important role in a wide range of industrial technologies and biological processes. However, it has been a great challenge to directly probe the molecular-scale behavior of water at solid/water interfaces. Recently, there have been tremendous advancements in frequency modulation atomic force microscopy (FM-AFM), enabling its operation in liquids with atomic resolution. The high spatial and force resolutions of FM-AFM have enabled the visualization of one-dimensional (1D) profiles of the hydration force, two-dimensional (2D) images of hydration layers and three-dimensional (3D) images of the water distribution at solid/water interfaces. Here I present an overview of the recent advances in FM-AFM instrumentation and its applications to the study of solid/water interfaces.     

Effects of frequencies of AC modulation voltage on piezoelectric-induced images using atomic force microscopy

Lead zirconate titanate (PZT) thin film is prepared by sol-gel method on Pt/Ti electrode/SiO₂/Si wafer. Local poling is performed on the PZT film using an atomic force microscope (AFM). The topography and piezoelectric-induced (PEI) images on the polarized PZT film are recorded using AFM at piezo-responsive mode, operated with an AC voltage at varying frequencies. The best PEI image was obtained at the frequency around 300 kHz. It is explained that the change of piezoelectric vibrations and input noise signals with the frequency of AC modulation voltage affects the intensity of PEI images.     

Techniques for imaging human metaphase chromosomes in liquid conditions by atomic force microscopy

The purpose of this study was to obtain three-dimensional images of wet chromosomes by atomic force microscopy (AFM) in liquid conditions. Human metaphase chromosomes-obtained either by chromosome spreads or by an isolation technique-were observed in a dynamic mode by AFM in a buffer solution. Under suitable operating conditions with a soft triangular cantilever (with the spring constant of 0.08-0.4?N?m(-1)), clear images of fixed chromosomes in the chromosome spread were obtained by AFM. For imaging isolated chromosomes with the height of more than 400?nm, a cantilever with a high aspect ratio probing tip was required. The combination of a Q-control system and the sampling intelligent scan (SIS) system in dynamic force mode AFM was useful for obtaining high-quality images of the isolated chromosomes, in which globular or cord-like structures about 50?nm thick were clearly observed on the surface of each chromatid.     

原子力显微镜微悬臂梁杨氏模量动态测试方法

原子力显微镜微悬臂梁是微纳米领域重要的微力传感器,而微悬臂梁的杨氏模量又是决定其力学性能的重要参数.由于微悬臂梁的尺寸处于微米级,有些特征尺寸甚至达到纳米级,常规的测试结构材料特性的检测方法已经难以满足需求,急需研究新的测试方法和装置对微悬臂梁的机械特性进行研究和分析.本文提出了一种基于微悬臂梁振动固有频率测试的杨氏模量测试方法.使用本方法时,首先建立待测微悬臂梁在空气中的振动模型,并使用数值仿真的方法计算结构尺寸相同但杨氏模量不同的各种微悬臂梁在空气中的振动固有频率,然后实际测量微悬臂梁的振动固有频率,和实验结果最接近的仿真结果所对应的杨氏模量参数就是待测微悬臂梁的杨氏模量.本文最后对Mikromaseh公司生产的NSC型探针的杨氏模量进行了测试,实验结果证实了本文提出的方法的正确性.     

Multimodal atomic force microscopy: Biological imaging using atomic force microscopy combined with light fluorescence and confocal microscopies and electrophysiologic recording

Because the atomic force microscope (AFM) allows molecular resolution imaging of hydrated specimens, it provides a unique window to the microscopic biological world. A high signal-to-noise ratio in AFM images sets them apart from the images obtained from other techniques: One does not need extensive image analyses often required by other techniques to obtain high-resolution information. AFM can provide molecular details on crystalline as well as amorphous materials. However, it is often limited in providing identity of the imaged structures, especially in a complex system such as a cellular membrane. AFM's application for biological imaging will rely on an unambiguous identification of imaged structures. For mixed macromolecules, it may be essential to make critical comparisons of the same structural features imaged with AFM and other techniques such as light fluorescence and confocal microscopies, electron microscopy and X-ray diffraction, and biochemical, immunologic, and pharmacologic techniques and electrophysiologic recordings. Significantly, the simple design of AFM allows it to be integrated with other techniques for simultaneous multimodal imaging. Recent combined multimodal imaging include light fluorescence, confocal, and near-field optical imaging as well as electrophysiologic recordings. Preliminary studies from such multimodal imaging include 1) an independent identification of macromolecules in a complex specimen using appropriately labeled markers such as fluorescent-dye labeled antibodies or dark-field microscopy; 2) imaging real-time reorganization of surface features using laser confocal and AFM; 3) a direct correlation of structural features and ion transfer via pores in a membrane; and 4) macromolecular complexes such as receptor-ligand and antigen-antibody. These features of a multimodal imaging system will provide new and significant avenues for a direct real-time structure-function correlation studies of biological macromolecules. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 293–300, 1997     

Accuracy of the spring constant of atomic force microscopy cantilevers by finite element method

Atomic force microscopy (AFM) probe with different functions can be used to measure the bonding force between atoms or molecules. In order to have accurate results, AFM cantilevers must be calibrated precisely before use. The AFM cantilever's spring constant is usually provided by the manufacturer, and it is calculated from simple equations or some other calibration methods. The spring constant may have some uncertainty, which may cause large errors in force measurement. In this paper, finite element analysis was used to obtain the deformation behavior of the AFM cantilever and to calculate its spring constant. The influence of prestress, ignored by other methods, is discussed in this paper. The variations of Young's modulus, Poisson's ratio, cantilever geometries, tilt angle, and the influence of image tip mass were evaluated to find their effects on the cantilever's characteristics. The results were compared with those obtained from other methods.     

Contact force identification using the subharmonic resonance of a contact-mode atomic force microscopy

We propose a step-by-step experimental procedure for characterization of the nonlinear contact stiffness on surfaces using contact-mode atomic force microscopy. Our approach directly estimates the first-, second-, and third-order coefficients of the contact stiffness. It neither uses nor requires the underlying assumptions of the Hertzian contact theory. We use a primary resonance excitation of the probe to estimate the linear coefficient of the contact stiffness. We use the method of multiple scales to obtain closed-form expressions approximating the response of the probe to a subharmonic resonance excitation of order one-half. We utilize these expressions and higher-order spectral measurements to independently estimate the quadratic and cubic coefficients of the contact stiffness.     

Two-dimensional dopant profiling by electrostatic force microscopy using carbon nanotube modified cantilevers

A two-dimensional (2D) dopant profiling technique is demonstrated in this work. We apply a unique cantilever probe in electrostatic force microscopy (EFM) modified by the attachment of a multiwalled carbon nanotube (MWNT). Furthermore, the tip apex of the MWNT was trimmed to the sharpness of a single-walled carbon nanotube (SWNT). This ultra-sharp MWNT tip helps us to resolve dopant features to within 10?nm in air, which approaches the resolution achieved by ultra-high vacuum scanning tunnelling microscopy (UHV STM). In this study, the CNT-probed EFM is used to profile 2D buried dopant distribution under a nano-scale device structure and shows the feasibility of device characterization for sub-45?nm complementary metal-oxide-semiconductor (CMOS) field-effect transistors.     

Nanoscale capacitance imaging with attofarad resolution using ac current sensing atomic force microscopy

Nanoscale capacitance imaging with attofarad resolution (～1?aF) of a nano-structured oxide thin film, using ac current sensing atomic force microscopy, is reported. Capacitance images are shown to follow the topographic profile of the oxide closely, with nanometre vertical resolution. A comparison between experimental data and theoretical models shows that the capacitance variations observed in the measurements can be mainly associated with the capacitance probed by the tip apex and not with positional changes of stray capacitance contributions. Capacitance versus distance measurements further support this conclusion. The application of this technique to the characterization of samples with non-voltage-dependent capacitance, such as very thin dielectric films, self-assembled monolayers and biological membranes, can provide new insight into the dielectric properties at the nanoscale.     

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.     

Thermomagnetic fluctuations and hysteresis loops of magnetic cantilevers for magnetic resonance force microscopy

We have used frequency-shift cantilever magnetometry to study individual nickel magnets patterned at the end of ultra-sensitive silicon cantilevers for use in magnetic resonance force microscopy (MRFM). We present a procedure for inferring a magnet's full hysteresis curve from the response of cantilever resonance frequency versus magnetic field. Hysteresis loops and small-angle fluctuations were determined at 4.2 K with an applied magnetic field up to 6 T for magnets covering a range of dimensions and aspect ratios. Compared to magnetic materials with higher anisotropy, we find that nickel is preferable for MRFM experiments on nuclear spins at high magnetic fields.     

Energy dissipation distributions and dissipative atomic processes in amplitude modulation atomic force microscopy

Instantaneous and average energy dissipation distributions in the nanoscale due to short and long range interactions are described. We employ both a purely continuous and a semi-discrete approach to analyze the consequences of this distribution in terms of rate of heat generation, thermal flux, adhesion hysteresis, viscoelasticity and atomic dissipative processes. The effects of peak values are also discussed in terms of the validity of the use of average values of power and energy dissipation. Analytic expressions for the instantaneous power are also derived. We further provide a general expression to calculate the effective area of interaction for fundamental dissipative processes and relate it to the energy distribution profile in the interaction area. Finally, a semi-discrete approach to model and interpret atomic dissipative processes is proposed and shown to lead to realistic values for the atomic bond dissipation and viscoelastic atomic processes.     

Nanofabrication with atomic force microscopy

Atomic force microscopy (AFM) was developed in 1986. It is an important and versatile surface technique, and is used in many research fields. In this review, we have summarized the methods and applications of AFM, with emphasis on nanofabrication. AFM is capable of visualizing surface properties at high spatial resolution and determining biomolecular interaction as well as fabricating nanostructures. Recently, AFM-based nanotechnologies such as nanomanipulation, force lithography, nanografting, nanooxidation and dip-pen nanolithography were developed rapidly. AFM tip (typical radius ranged from several nanometers to tens of nanometers) is used to modify the sample surface, either physically or chemically, at nanometer scale. Nanopatterns composed of semiconductors, metal, biomolecules, polymers, etc., were constructed with various AFM-based nanotechnologies, thus making AFM a promising technique for nanofabrication. AFM-based nanotechnologies have potential applications in nanoelectronics, bioanalysis, biosensors, actuators and high-density data storage devices.     

Measuring phase shifts and energy dissipation with amplitude modulation atomic force microscopy

By recording the phase angle difference between the excitation force and the tip response in amplitude modulation AFM it is possible to image compositional variations in heterogeneous samples. In this contribution we address some of the experimental issues relevant to perform phase contrast imaging measurements. Specifically, we study the dependence of the phase shift on the tip-surface separation, interaction regime, cantilever parameters, free amplitude and tip-surface dissipative processes. We show that phase shift measurements can be converted into energy dissipation values. Energy dissipation curves show a maximum (～10?eV/cycle) with the amplitude ratio. Furthermore, energy dissipation maps provide a robust method to image material properties because they do not depend directly on the tip-surface interaction regime. Compositional contrast images are illustrated by imaging conjugated molecular islands deposited on silicon surfaces.     

Real time atomic force microscopy imaging during nanogap formation by electromigration

We present real time atomic force microscopy imaging during nanogap fabrication by feedback controlled electromigration of a gold nanowire. The correlated measurements of electrical resistance and atomic force microscopy reveal that the major structural changes appear at the early stage of the process. Moreover, despite important morphological changes, the resistance of the nanowire shows a weak increase of just a few ohms. The detailed analysis of the atomic force microscopy images clearly shows that the electromigration process is strongly influenced by the initial microstructure of the nanowire.     

Mapping van der Waals forces with frequency modulation dynamic force microscopy

Nanometre-size gold clusters supported on MoS(2)(0001) are investigated by means of ultrahigh-vacuum frequency modulation dynamic force microscopy. Topography and frequency shift images are simultaneously obtained using the average tunnelling current to regulate the tip-substrate distance. Two families of clusters are observed, giving different frequency shift images. While the topographic and frequency shift profiles have similar shapes on small clusters (size [Formula: see text]?nm), they are quite different near the top of large clusters (size [Formula: see text]?nm): the topographic profile is rounded, but the frequency shift profile exhibits rather steep edges and a depression near the centre of the island. It is demonstrated that these differences result from the finite range of van der Waals forces. On small islands, the frequency shift is dominated by the interaction of the tip with the substrate. On large islands, it is dominated by the interaction with the island. The particular observed shape results from the geometry of the island. These interpretations are comforted by analytical and numerical calculations. In particular, the characteristic shape of the frequency shift profiles on large islands can be reproduced by introducing realistic parameters and considering only the contribution of van?der?Waals forces.     

Electrostatic nanolithography in polymers using atomic force microscopy

The past decade has witnessed an explosion of techniques used to pattern polymers on the nano (1-100 nm) and submicrometre (100-1,000 nm) scale, driven by the extensive versatility of polymers for diverse applications, such as molecular electronics, data storage, optoelectronics, displays, sacrificial templates and all forms of sensors. Conceptually, most of the patterning techniques, including microcontact printing (soft lithography), photolithography, electron-beam lithography, block-copolymer templating and dip-pen lithography, are based on the spatially selective removal or formation/deposition of polymer. Here, we demonstrate an alternative and novel lithography technique--electrostatic nanolithography using atomic force microscopy--that generates features by mass transport of polymer within an initially uniform, planar film without chemical crosslinking, substantial polymer degradation or ablation. The combination of localized softening of attolitres (10(2)-10(5) nm3) of polymer by Joule heating, extremely non-uniform electric field gradients to polarize and manipulate the soften polymer, and single-step process methodology using conventional atomic force microscopy (AFM) equipment, establishes a new paradigm for polymer nanolithography, allowing rapid (of the order of milliseconds) creation of raised (or depressed) features without external heating of a polymer film or AFM tip-film contact.