一维矩形光栅AFM图像盲探针表面重建模拟研究

探针结构参数的合理选取将直接决定扫描图像及其盲探针修正图像的失真程度。基于此,以一维矩形模拟光栅为典型案例,对该模拟光栅的原子力显微镜(AFM)扫描成像过程与盲探针修正过程进行了仿真,阐明了探针结构参数对扫描成像过程与盲探针修正过程的影响规律。通过建立线宽变化度与半高宽相结合的图像重建误差评价指标,确定了针对该模拟光栅的AFM探针建议结构参数,并取得了良好的光栅图像重建效果。研究表明,应用线宽变化度结合半高宽来综合评价光栅的AFM测量和图像重建过程,有利于提升实际光栅AFM图像盲探针重建的准确度。     

Combination of AFM with an objective-type total internal reflection fluorescence microscope (TIRFM) for nanomanipulation of single cells

A new instrument was constructed by combining an objective-type total internal reflection fluorescence microscope with an atomic force microscope (AFM). Our purpose of constructing such an instrument is to detect and confirm the result of cellular level manipulations made with the AFM part through the detection system of the highly sensitive fluorescence microscope part. In this combination, manipulations are now possible from the nanometer to the micrometer scales and the fluorescence detection system is sensitive enough even for localizing single molecules. In this paper, we applied the system as a precise intracellular injector (nanoplanter). Fluorescent beads were first chemically immobilized onto a ZnO whisker that was glued to an AFM tip and were injected into a living BALB/3T3 cell together with the whisker. It was demonstrated that the system could clearly show the result of injection, that is, the presence of a small number of fluorescent beads in the cell.     

Contact mechanics and tip shape in AFM-based nanomechanical measurements

Stiffness-load curves obtained in quantitative atomic force acoustic microscopy (AFAM) measurements depend on both the elastic properties of the sample and the geometry of the atomic force microscope (AFM) tip. The geometry of silicon AFM tips changes when used in contact mode, affecting measurement accuracy. To study the influence of tip geometry, we subjected ten AFM tips to the same series of AFAM measurements. Changes in tip shape were observed in the scanning electron microscope (SEM) between individual AFAM tests. Because all of the AFAM measurements were performed on the same sample, variations in AFAM stiffness-load curves were attributed to differences in tip geometry. Contact-mechanics models that assumed simple tip geometries were used to analyze the AFAM data, but the calculated values for tip dimensions did not agree with those provided by SEM images. Therefore, we used a power-law approach that allows for a nonspherical tip geometry. We found that after several AFAM measurements, the geometry of the tips at the very end is intermediate between those of a flat punch and a hemisphere. These results indicate that the nanoscale tip-sample contact cannot easily be described in terms of simple, ideal geometries.     

Unstable amplitude and noisy image induced by tip contamination in dynamic force mode atomic force microscopy

Liquid 1-decanethiol was confined on an atomic force microscope (AFM) tip apex and the effect was investigated by measuring amplitude-distance curves in dynamic force mode. Within the working distance in the dynamic force mode AFM, the thiol showed strong interactions bridging between a gold-coated probe tip and a gold-coated Si substrate, resulting in unstable amplitude and noisy AFM images. We show that under such a situation, the amplitude change is dominated by the extra forces induced by the active material loaded on the tip apex, overwhelming the amplitude change caused by the geometry of the sample surface, thus resulting in noise in the image the tip collects. We also show that such a contaminant may be removed from the apex by pushing the tip into a material soft enough to avoid damage to the tip.     

Molecular dynamics simulation of the contact process in AFM surface observations

In the present work, several molecular dynamics simulations have been performed to clarify dynamically the contact mechanism between the specimen surface and probe tip in surface observations by an atomic force microscope (SFM) or friction force microscope (FFM). In the simulation, a three‐dimensional model is proposed where the specimen and the probe are assumed to consist of monocrystalline copper and rigid diamond or a carbon atom, respectively. The effect of the cantilever stiffness of the AFM/FFM is also taken into consideration. The surface observation process is simulated on a well‐defined Cu{100} surface. From the simulation results it has been verified that the surface images and the two‐dimensional atomic‐scale stick‐slip phenomenon, just as is the case for real AFM/FFM surface observations, can be detected from the spring force acting on the cantilever. From the evaluation of the behaviour of specimen surface atoms, the importance of the specimen stiffness in deciding the cantilever properties can also be understood. The influence of the probe tip shape on the force images is also evaluated. From the results it can be verified that the behaviour of the specimen surface atoms as well as the solid surface images in AFM/FFM surface observations can be understood using the molecular dynamics simulation of the model presented.     

Atomic force microscope tip cleaning by using gratings as micro‐washboards

The sharpness of atomic force microscope (AFM) tips is essential for acquiring high quality AFM images. However, AFM tips would easily get contaminated during scanning and storage at ambient condition, which influences image resolution and causes image distortion. Replacing the probe frequently is a solution, but uneconomical. To solve this problem, several tip cleaning methods have been proposed but there is space for further improvement. Therefore, this article developed a method of tip cleaning by using a one‐dimensional grating (600 lines/mm) as a micro‐washboard to “wash” contaminated tips. We demonstrate that the contaminants can be scrubbed away by rapidly scanning such micro‐washboard against the tip in the aids of Z‐dithering (10–20 Hz) exerted on the washboard. This method is highly efficient and proved to be superior to traditional ones. Experiments show that AFM images acquired with “washed” tips have higher resolution and less distortion compared with images acquired using contaminated tips, even comparable to those scanned by new ones. Microsc. Res. Tech. 76:1131–1134, 2013. © 2013 Wiley Periodicals, Inc.     

Using defined structures on very thin foils for characterizing AFM tips

Knowledge of tip geometry is necessary for reproducible atomic force microscope (AFM) measurements. This is particularly important for measurements in contact mode, in which a certain wear of the tip will always occur. For small or flat structures or for structures of larger dimensions, knowledge of the tip radius and the entire tip geometry is important. Additionally, the tilt of the tip in relation to the sample is of importance. Normally, very complicated lithographically manufactured structures for tip characterization are used. In contrast, the structures shown in this work are very simple. For measuring the tip geometry very thin foils patterned by focused ion beam (FIB) were used. In this work we demonstrate the possibility of determining the AFM tip geometry and the tilt based on several different large structures. A proven algorithm was developed for the reconstruction of the tips. The shape of FIB-structured foils was determined by electron microscopy prior to AFM measurements. This new method for determining tip shape is also presented as it compares to other current methods. In this case a discussion on the stability and advantages of the new method is presented.     

Atomic force microscopy and cells: Indentation profiles around the AFM tip,cell shape changes,and other examples of experimental factors affecting modeling

We use atomic force microscopy in conjunction with a fluorescence microscope capable of optical sectioning to acquire images of white blood cells while force is applied with the AFM tip. The indentation profile within the cell is compared to the profile of the AFM tip: examples are shown for indentations at the center of the cell which are reasonable matches to the tip profile, and an additional example is shown for an indentation that is on the tilted side of a highly rounded cell and that differs from the tip shape. We also demonstrate that the AFM tip can interact with internal cell structures, we show that the contact area between the cell and the substrate can increase under applied pressure, that the main body of the cell can fuse with the extended lamellipodium, and that the cell can be displaced laterally by the AFM tip. The features illustrated here are relevant to the interpretation of indentation experiments that measure cell elasticity properties, as is discussed briefly. Microsc. Res. Tech. 78:626–632, 2015. © 2015 Wiley Periodicals, Inc.     

The substructure of immunoglobulin G resolved to 25 kDa using amplitude modulation AFM in air

Amplitude modulation (or tapping-mode) atomic force microscopy (AM AFM or TM AFM) in air can reveal sub-molecular details of isolated multi-subunit proteins, such as immunoglobulin G (IgG) antibodies, on atomically flat support surfaces such as mica [A. San Paulo, R. Garcia, Biophys. J. 78(3) (2000) 1599]. This is achieved by controlling the microscope imaging parameters (e.g. cantilever drive frequency and set-point amplitude) to keep the AFM tip predominantly in the attractive force regime. Under these conditions, the 50 kDa F(c) and F(ab) subunits can be resolved when the molecule has the appropriate orientation on the surface. The presence of a water layer on hydrophilic mica is an important factor affecting imaging contrast, a consequence of capillary neck formation between tip and surface [L. Zitzler, S. Herminghaus, F. Mugele, Phys. Rev. B 66(15) (2002) 155436]. Desiccation of samples to remove surface bound water layers can yield reproducible imaging of the IgG substructure [N.H. Thomson, J. Microsc. (Oxford) 217(3) (2004) 193]. This approach has also given higher resolution than previously achieved, down to about 25 kDa, and these data are detailed here. These subdomains are formed as two immunoglobulin folds from the light and heavy peptide chains of the IgG crossover. This result has been validated by comparing the AFM images with X-ray crystallography data from the protein data bank. These data show that the AFM can obtain 25 kDa resolution on isolated protein molecules with commercially available silicon tips, but, as expected for a local probe technique, resolution is highly dependent on the macromolecular orientation on the support surface.     

Contrast and resolution enhancement of a near-field optical microscope by using a modulation technique

A new design of a tunneling near-field optical microscope (TNOM) combined with an atomic force microscope (AFM) is presented. This design can be used to generate three different images of the sample's surface: a non-contact (tapping mode) AFM image, a conventional TNOM and an image of a modulation signal of the conventional TNOM, which we call AC-TNOM. The images are obtained simultaneously, using a single light source. It is shown that the AC-TNOM has better resolution ( approximately 200A) and contrast compared to conventional TNOM ( approximately 400A).     

Study of the tip mass and interaction force effects on the frequency response and mode shapes of the AFM cantilever

The vibrational characteristics of an atomic force microscope (AFM) cantilever beam play a key role in dynamic mode of the atomic force microscope. As the oscillating AFM cantilever tip approaches the sample, the tip–sample interaction force influences the cantilever dynamics. In this paper, we present a detailed theoretical analysis of the frequency response and mode shape behavior of a cantilever beam in the dynamic mode subject to changes in the tip mass and the interaction regime between the AFM cantilever system and the sample. We consider a distributed parameter model for AFM and use Euler–Bernoulli method to derive an expression for AFM characteristics equation contains tip mass and interaction force terms. We study the frequency response of AFM cantilever under variations of interaction force between AFM tip and sample. Also, we investigate the effect of tip mass on the frequency response and also the quality factor and spring constant of each eigenmodes of AFM micro-cantilever. In addition, the mode shape analysis of AFM cantilever under variations of tip mass and interaction force is investigated. This will incorporate the presentation of explicit analytical expressions and numerical analysis. The results show that by considering the tip mass, the resonance frequencies of the cantilever are decreased. Also, the tip mass has a significant effect on the mode shape of the higher eigenmodes of the AFM cantilever. Moreover, tip mass affects the quality factor and spring constant of each modes.     

Feasibility of multi-walled carbon nanotube probes in AFM anodization lithography

Multi-walled carbon nanotube (CNT) tips were used in atomic force microscope (AFM) anodization lithography to investigate their advantages over conventional tips. The CNT tip required a larger threshold voltage than the mother silicon tip due to the Schottky barrier at the CNT-Si interface. Current-to-voltage curves distinguished the junction property between CNTs and mother tips. The CNT-platinum tip, which is more conductive than the CNT-silicon tip, showed promising results for AFM anodization lithography. Finally, the nanostructures with high aspect ratio were fabricated using a pulsed bias voltage technique as well as the CNT tip.     

Use of AFM for imaging and measurement of the mechanical properties of light-convertible organelles in plants

We obtained topographic images of etioplasts and chloroplasts and measured their elasticity in a physiological buffer using an atomic force microscope (AFM) and found a possible correlation between the morphological and mechanical properties during the light conversion of etioplasts to chloroplasts. Alcian blue 8GX dye was found to be effective for immobilizing the plant organelles stably on a glass surface for AFM experiments. We employed the tapping-mode AFM with a cantilever soft enough to measure the elasticity of the organelles in a liquid solution. The best images of soft, spherical organelles were obtained using the tapping-mode AFM with oscillation at the thermal vibration frequency of the cantilever of around 3 kHz. Whereas etioplasts were found to be smooth-surfaced and stiff against compression by the AFM tip, before light conversion to chloroplasts, they became rough-surfaced and mechanically soft after exposure to light. The elasticity of etioplasts was 20 times higher than that of chloroplasts, probably reflecting changes in their inner structures.     

Polystyrene spheres on mica substrates: AFM calibration, tip parameters and scan artefacts

Atomic force microscopy (AFM), in various versions, has had major impact as a surface structural and spectroscopic tool since its invention in 1986. At its present state of development, however, the interpretation of AFM images is limited by the current state of methodologies for calibration over the wide dynamic range of magnification. Also, the parameters of individual tips, as well as the generic characteristics of different kinds of tips, affect both the quality of the images and their interpretation. Finally, the very nature of the tip-to-surface interaction will generate artefacts, in addition to those associated with tip shape, which need to be fully understood by the practitioners of force microscopy. This project seeks to address and shed light on some of these issues. Polystyrene beads deposited on mica substrates form hexagonal close-packed layers. The unit cell parameters are suitable for calibration of the AFM in the lateral plane, while the perpendicular spacing of the layers is appropriate for calibration along the vertical axis. Using different size fractions, it is straightforward to determine the extents of linearity, orthogonality, thermal and instrumental drifts over distances from 100 nm to tens of micrometres. The present results show that the methodologies for contact mode operation can be adapted to noncontact modes. It is known that an AFM image arises from a convolution of surface topography and tip shape, and is mediated by the interaction. In principle it is possible to carry out a deconvolution, if we have complete knowledge about two of the three elements (i.e. tip, surface and interaction). In practice we rarely have the requisite information. Prominent artefacts will occur when the characteristic parameters of the tip are comparable to those of the surface topography, and/or if there is a variable strength, or extent of localization, of the interaction. The present results demonstrate artefacts due to effects of geometry as well as interaction.     

Nanotribology: tip–sample wear under adhesive contact

In this report, the irreversible variation of mass of the probe tip of an atomic force microscope (AFM) is considered from theoretical and numerical points of view through statistical methods. The tip–sample interaction due to the intermittent-contact operating mode of an AFM is modelled as a double-well potential where the wear mechanism, which reveals itself as mass sticking to the probe tip, is described as a transition between the two potential wells. We evaluate the interaction of a silicon nitride AFM/FFM tip with gold in order to compare the results with those obtained from previous experimental and numerical studies.     

Three-channel false colour AFM images for improved interpretation of complex surfaces: A study of filamentous cyanobacteria

Imaging signals derived from the atomic force microscope (AFM) are typically presented as separate adjacent images with greyscale or pseudo-colour palettes. We propose that information-rich false-colour composites are a useful means of presenting three-channel AFM image data. This method can aid the interpretation of complex surfaces and facilitate the perception of information that is convoluted across data channels. We illustrate this approach with images of filamentous cyanobacteria imaged in air and under aqueous buffer, using both deflection-modulation (contact) mode and amplitude-modulation (tapping) mode. Topography-dependent contrast in the error and tertiary signals aids the interpretation of the topography signal by contributing additional data, resulting in a more detailed image, and by showing variations in the probe-surface interaction. Moreover, topography-independent contrast and topography-dependent contrast in the tertiary data image (phase or friction) can be distinguished more easily as a consequence of the three dimensional colour-space.     

Blind estimation of general tip shape in AFM imaging

The use of flared tip and bi-directional servo control in some recent atomic force microscopes (AFM) has made it possible for these advanced AFMs to image structures of general shapes with undercut surfaces. AFM images are distorted representations of sample surfaces due to the dilation produced by the finite size of the tip. It is necessary to obtain the tip shape in order to correct such tip distortion. This paper presents a noise-tolerant approach that can for the first time estimate a general 3-dimensional (3D) tip shape from its scanned image in such AFMs. It extends an existing blind tip estimation method. With the samples, images, and tips described by dexels, a representation that can describe general 3D shapes, the new approach can estimate general tip shapes, including reentrant features such as undercut lines.     

碳纳米管原子力显微镜针尖的制作研究

研究在光学显微镜下，运用两个独立的三维工作台分别控制针尖和碳纳米管的位置，将碳纳米管吸附在传统的原子力显微镜针尖上。首先将碳纳米管粘附在导电的胶带上，然后用涂胶的针尖与其接触将碳纳米管粘附到针尖上，最后运用电蚀的方法优化碳纳米管针尖的长度，以达到高分辨率的要求。运用制作的碳纳米管针尖对硅表面的深槽进行成像，获得了传统针尖无法得到的信息。     

A novel atomic force microscope operating in liquid for in situ investigation of electrochemical preparation of porous alumina

A novel atomic force microscope (AFM) operating in liquid is described in this article. The specially designed AFM probe involves a tip attached to a cantilever, a tip holder, and a circular Plexiglas window. When the probe dives into the fluid, a circular meniscus is established around the Plexiglas window, preventing the tip from being affected or destroyed by surface tension of the liquid. In this setup, the whole scanning probe and the sample can completely dive into fluid. Meanwhile, the probe tip scans over the sample surface when the instrument works. These advantages enable the instrument to scan comparatively large or heavy samples with a high speed. The highest scan rate is about 30 lines/s or 14 s for a 400 x 400-pixel, 3 x 3 microm image. Using the new AFM, we carry out in-situ investigation of the formation processes of porous alumina during electrochemical anodic oxidation. A lead ring and an aluminum foil serve as cathode and anode, respectively. They are entirely immersed in the bath electrolyte, which is oxalic acid solution. During anodic oxidation, the AFM images of the sample surface are successively acquired without elevating the sample out of the solution. Experiments reveal that electrochemical reactions take place soon after the power supply is switched on, and with the progression of anodization, nanostructures of porous alumina gradually occur on the aluminum substrate, finally yielding ordered arrays of nanopores. As a typical example of applications, the results of this work show that the new AFM is an ideal and powerful tool for in-situ observation and study of materials or samples in aqueous solutions.