“Millipede” – an AFM data storage system at the frontier of nanotribology

The Millipede data storage concept is based on the parallel operation of a large number of micromechanical levers that function as AFM sensors. The technique holds promise to evolve into a novel ultrahigh-density, terabit-capacity, and high-data-rate storage technology. Thermomechanical writing and reading in very thin polymer (PMMA) films is used to store and sense 30–40 nm sized bits of similar pitch size, resulting in 400–500 Gbit/in² storage densities. High data rates are achieved by operating very large arrays (32×32) of AFM sensors in parallel. Batch-fabrication of 32×32 AFM cantilever array chips has been achieved, and array reading and writing have been demonstrated. An important consideration for the Millipede storage project is the polymer dynamics on the size scale of one bit. Scaling of rheological parameters measured for macroscopic polymer samples is likely to be incorrect due to the finite length of the underlying molecular polymer chain, a size that is comparable to the bit itself. In order to shed light on these issues we performed lifetime studies of regular arrays of nanometer size patterns using light-scattering techniques.     

Atomically resolved imaging by low-temperature frequency-modulation atomic force microscopy using a quartz length-extension resonator

Low-temperature ultrahigh vacuum frequency-modulation atomic force microscopy (AFM) was performed using a 1 MHz length-extension type of quartz resonator as a force sensor. Taking advantage of the high stiffness of the resonator, the AFM was operated with an oscillation amplitude smaller than 100 pm, which is favorable for high spatial resolution, without snapping an AFM tip onto a sample surface. Atomically resolved imaging of the adatom structure on the Si(111)-(7x7) surface was successfully obtained.     

基于原子力显微镜的光盘表面微结构的检测

利用原子力显微镜在纳米尺度上观察了光盘(CD)和数字通用光盘(DVD)的表面微观结构,对沟槽间距、摆动幅度等参数进行了测量,进而就这些参数对存储性能的影响进行了分析。实验结果显示:CD-R和DVD-R光盘上以连续的沟槽取代坑,并组成螺旋状的轨道,DVD-R光盘的信息存储密度更大。光盘轨道间距的变化范围是摆动幅度的4倍。写入后,沟槽的深度明显变深。实验结果同时表明,利用AFM能直接测量光盘上信息位的形貌参数,进而找出影响光盘质量的直接因素。     

Atomic force microscopy imaging using a tip-on-chip: opening the door to integrated near field nanotools

We describe in detail how atomic force microscopy (AFM) images can be routinely achieved with macroscopic silicon-based chips integrating mesoscopic tips, paving the way for the development of new near field devices combining AFM imaging with any kind of functionality integrated on a chip. The chips have been glued at the end of the free prong of 100 kHz quartz tuning forks mounted in Qplus configuration. Numerical simulations by modal analysis have been carried out to clarify the nature of the vibration modes observed in the experimental spectra. It is shown that two low frequency modes can be used to drive the system and scan the surface with a great stability in amplitude modulation as well as in frequency modulation AFM under ultrahigh vacuum. The AFM capabilities are demonstrated through a series of examples including phase and dissipation contrast imaging, force spectroscopy measurements, and investigations of soft samples in weak interaction with the substrate. The lateral resolution with the tips grown by focused ion beam deposition already matches the one achieved in standard amplitude modulation mode AFM experiments.     

AFM lithography of aluminum for fabrication of nanomechanical systems

Nanolithography by local anodic oxidation of surfaces using atomic force microscopy (AFM) has proven to be more reproducible when using dynamic, non-contact mode. Hereby, the tip/sample interaction forces are reduced dramatically compared to contact mode, and thus tip wear is greatly reduced. Anodic oxidation of Al can be used for fabricating nanomechanical systems, by using the Al oxide as a highly selective dry etching mask. In our experiments, areas as large as 2 micro m x 3 micro m have been oxidized repeatedly without any sign of tip-wear. Furthermore, line widths down to 10nm have been routinely obtained, by optimization of AFM parameters, such as tip/sample distance, voltage and scan speed. Finally, AFM oxidation experiments have been performed on CMOS processed chips, demonstrating the first steps of fabricating fully functional nanomechanical devices.     

Film thickness and roughness of ZDDP antiwear films

The two experimental techniques, spacer layer interferometry imaging (SLIM) and atomic force microscopy (AFM), have been used to measure the thickness and roughness of zinc dialkyl dithiophosphate (ZDDP) reaction films formed in a rolling-sliding minitraction machine (MTM) tribometer. The AFM method has been complemented by a novel ZDDP film removal method based on ethylenediaminetetraacetic acid (EDTA) solution. It has been found that the two approaches measure very similar ZDDP film thickness values, lending credence to both methods. However the AFM approach measures much rougher ZDDP reaction films than MTM-SLIM and it is believed that SLIM underestimates the film roughness. Based on this, the use of MTM-SLIM is recommended for monitoring the evolution of antiwear film thickness during rubbing, while AFM should be employed for studying the morphology of antiwear films.     

Atomic force microscopic study of the structure of high‐density polyethylene deformed in liquid medium by crazing mechanism

A procedure has been developed for the direct atomic force microscopic (AFM) examination of the native structure of high‐density polyethylene (HDPE) deformed in an adsorption‐active liquid medium (AALM) by the crazing mechanism. The AFM investigation has been carried out in the presence of a liquid medium under conditions preventing deformed films from shrinkage. Deformation of HDPE in AALM has been shown to proceed through the delocalized crazing mechanism and result in the development of a fibrillar‐porous structure. The structural parameters of the crazed polymer have been determined. The obtained AFM images demonstrate a nanosized nonuniformity of the deformation and enable one to observe the structural rearrangements that take place in the deformed polymer after removal of the liquid medium and stress relaxation. A structural similarity has been revealed between HDPE deformed in the AALM and hard elastic polymers.     

Optimization of adhesion mode atomic force microscopy resolves individual molecules in topography and adhesion.

The force sensor of an atomic force microscope (AFM) is sensitive enough to measure single molecular binding strengths by means of a force-distance curve. In order to combine high-force sensitivity with the spatial resolution of an AFM in topography mode, adhesion mode has been developed. Since this mode generates a force-distance curve for every pixel of an image, the measurement speed in liquid is limited by the viscous drag of the cantilever. We have equipped our adhesion mode AFM with a cantilever that has a low viscous drag in order to reach pixel frequencies of 65 Hz. Optimized filtering techniques combined with an auto-zero circuitry that reduces the drift in the deflection signal, limited high- and low-frequency fluctuations in the height signal to 0.3 nm. This reduction of the height noise, in combination with a thermally stabilized AFM, allowed the visualization of individual molecules on mica with an image quality comparable to tapping mode. The lateral resolution in both the topography and the simultaneously recorded adhesion image are only limited by the size of the tip. Hardware and software position feedback systems allows individual molecules to be followed in time during more than 30 min with scan sizes down to 60 x 60 nm2.     

Low temperature ultrahigh vacuum noncontact atomic force microscope in the pendulum geometry

A noncontact atomic force microscope (nc-AFM) operating in magnetic fields up to ±7 T and liquid helium temperatures is presented in this article. In many common AFM experiments the cantilever is mounted parallel to the sample surface, while in our system the cantilever is assembled perpendicular to it; the so called pendulum mode of AFM operation. In this mode measurements employing very soft and, therefore, ultrasensitive cantilevers can be performed. The ultrahigh vacuum conditions allow to prepare and transfer cantilevers and samples in a requested manner avoiding surface contamination. We demonstrate the possibility of nc-AFM and Kelvin force probe microscopy imaging in the pendulum mode. Ultrasensitive experiments on small spin ensembles are presented as well.     

A relocated technique of atomic force microscopy (AFM) samples and its application in molecular biology

A kind of simple atomic force microscopy (AFM) relocated technique, which takes advantage of homemade sample locator system, is used for investigating repeatedly imaging of some specific species on the whole substrate (over 1 x 1 cm2) with resolution about 400 nm. As applications of this sample locator system, single extended DNA molecules and plasmid DNA network are shown in different AFM operational modes: tapping mode and contact mode with different tips after the substrates have been moved.     

Nanometer-scale heat-conductivity measurements on biological samples

With semiconductor structures reaching the nanometer scale, heat conductivity measurements on the mesoscopic range of some tens of nanometers become an increasingly important aspect for the further improvement in digital processing and storage. Also the attempt to use atomic-force microscopy (AFM) technology for high-density data storage by writing information bits as nanometer-sized indentations into a polymer substrate with a heated cantilever tip asks for a careful investigation of the nano-scale heat-conductivity properties of polymers. Furthermore, in many AFM imaging applications, heat conductivity can provide additional information about the material the imaged structures consist of. In this respect, heat conductivity can also become very interesting in studies of usually quite heterogeneous biological samples, if the resolution can attain the nanometer range. In standard scanning thermal microscopy application, the tip forms a thermocouple, which precludes high-resolution imaging, as thermocouples cannot be made sufficiently small. In this paper, which focuses on biological applications, we demonstrate that by using an ultra sharp AFM cantilever with a Joule heating element above the tip structure different molecular components can be distinguished thanks to their different heat-conductivity properties. In this case, the resolution is determined by the actual tip size, and it can reach 10nm.     

Simple high vacuum evaporation system with low-temperature substrate

An ultrahigh vacuum system has been constructed which permits deposition of thin films onto 4.2-K substrates. The evaporation mechanism is contained within a small cylindrical vessel which is totally immersed in liquid (4)He. This device has been used to study superconductivity in PbBi and Ga, PdH and PdD, and most recently, transport properties of ultrathin films.     

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.     

Morphology and amine accessibility of (3-aminopropyl) triethoxysilane films on glass surfaces

3-Aminopropyl) triethoxysilane (APTES) is commonly used to functionalize glass substrates because it can form an amine-reactive film that is tightly attached to the surface. In this study, we investigated the morphology and chemical reactivity of APTES films prepared on glass substrates using common deposition techniques. Films were prepared using concentrated vapor-phase deposition, dilute vapor-phase deposition, anhydrous organic-phase deposition and aqueous-phase deposition. All films were annealed, or cured, at 150 degrees C. The morphology of the films was quantified by fluorescence and by atomic force microscopy (AFM). The optical equivalent of the AFM images was computed and then used to directly compare optical and AFM images. Reactive amine density was determined by a picric acid assay and by a method that employed N-succinimidyl 3-[2-pyridyldithio]-propionamido (SPDP) cross-linked rhodamine. Fluorescence and AFM images showed that silane films prepared from dilute vapor-phase and aqueous-phase deposition were more uniform and had fewer domains than those deposited by the other methods. The ratio of picric acid-accessible amino groups to SPDP cross-linked rhodamine-accessible groups varied with the preparation method, suggesting reactant size-dependent difference in amine accessibility. We found a larger number of accessible amino groups on films prepared by vapor-phase deposition than on those prepared from solution deposition. The dilute vapor-phase deposition technique produced relatively few domains, and it should be a good choice for bioconjugation applications. There were appreciable differences in the films produced by each method. We suggest that these differences originate from differences in film rearrangement during annealing.     

The analysis of morphological distortion during AFM study of cells

Atomic force microscopy (AFM) has been widely applied in cellular morphology study. However, morphological information including volume and roughness obtained by AFM are usually affected by different kinds of factors, which include the microscopic system itself, imaging mode, or external factors such as AFM probe or tip condition. In this study, based on red blood cell model, the dependence of cellular morphology, volume, and roughness on several parameters of the imaging was evaluated and, furthermore, a general rule and resolution for trustful analysis had been suggested. In addition, the potential effects that resulted from sample itself had also been analyzed based on adhesive force analysis. The results indicated that the scanning range and the imaging mode affect cellular volume and roughness, and the distorted images should be ascribed to blunt tip, contaminated tip, and the shape of tip. The analysis of morphological distortion during AFM investigation of cells provides a reference for researchers using AFM.     

Conductive composite of UHMWPE and CB as a dynamic contact analysis sensor

There has long been a need to experimentally measure the dynamic contact conditions of important engineering tribological systems, especially those with polymeric bearing surfaces that prove difficult to model. In order to experimentally quantify the dynamic contact conditions of geometrically complex polymeric bearing surfaces, a composite sensor material has been developed. In this study, qualitative morphological analysis of virgin ultrahigh molecular weight polyethylene (UHMWPE) and carbon black (CB) powders, as well as UHMWPE and CB powder mixtures of varying percentages was performed using field emission scanning electron microscopy (FESEM). Quantitative structure and friction analysis using atomic force microscopy (AFM) was performed on cryoultrasectioned block surfaces of compression-molded CB/UHMWPE composite. In addition, the mechanical properties of the composites were quantified using tensile testing, and the force dependence of the electrical properties was examined under dynamic compressive loading.     

Note: Electrical and thermal characterization of a ferroelectric thin film with an electro-thermal nanoprobe

The localized temperature-dependent piezoelectric response of ferroelectric barium strontium titanate (BST) thin films is studied using an electro-thermal (ET) nanoprobe. The ET probe provides independent electrical and thermal excitation to a nanometer-scale volume of the specimen and is capable of detecting the phase transition temperature of the BST thin films. The piezoresponse measured by the ET probe follows the temperature dependence of the piezoelectric constant, whereas with bulk heating the response follows the temperature dependence of the spontaneous polarization. The observed differences stem from the localized inhomogeneous electro-thermal field distribution at the specimen.     

A study of positive charge effect on AFM anodization lithography using metal phosphate monolayers

Monolayer films of metal phosphate were prepared by sequential adsorption of phosphoric acid (PA) and metal ions (Zr4+ or Ca2+). The monolayer formation was confirmed and characterized by ellipsometry and atomic force microscopy (AFM). Nanometer-scale patterning on the self-assembled metal phosphate films has been achieved using AFM. The difference in AFM lithography on positively charged surfaces in the case of Zr4+ and Ca2+ ions was investigated and summarized. Since the presence of the positive charges allows electrons to move from a tip to a layer easily, the lithographic process initiates at a lower voltage in the layer of Zr2+ phosphate than in the layer of Ca phosphate or PA. Other lithographic results on metal phosphate layers were compared with those on a monolayer of PA.     

Sample preparation procedures for biological atomic force microscopy

Since the late 1980s, atomic force microscopy (AFM) has been increasingly used in biological sciences and it is now established as a versatile tool to address the structure, properties and functions of biological specimens. AFM is unique in that it provides three-dimensional images of biological structures, including biomolecules, lipid films, 2D protein crystals and cells, under physiological conditions and with unprecedented resolution. A crucial prerequisite for successful, reliable biological AFM is that the samples need to be well attached to a solid substrate using appropriate, nondestructive methods. In this review, we discuss common techniques for immobilizing biological specimens for AFM studies.     

Topographic and phase-contrast imaging in atomic force microscopy

Phase-contrast imaging in the tapping mode atomic force microscopy (AFM) is a powerful method in surface characterization. This method can provide fine details about rough surfaces, which are normally obscured in topographic imaging. To illustrate some of the capabilities of phase-contrast imaging, AFM studies of Pt/Ti/SiO2/Si and Pb(Zr0.52Ti0.48)O3 (PZT) films were carried out. Phase-contrast imaging revealed fine details of their microstructures, including grain boundaries, triple junctions and twinning, which could not be detected by topographic imaging. The studies showed that phase-contrast imaging is capable of providing superior information about surface characteristics when compared to the standard topographic imaging.