Fabrication of various tip-size AFM probes for evaluating single-molecular retraction force between actin and anti-actin

The authors fabricated a probe tip with various sizes and examined the size dependency of the probe tip on the distribution of retraction forces between actin and anti-actin. Probe tips of various sizes were fabricated by two-photon polymerization methods on a micro cantilever of an atomic force microscope (AFM). The authors succeeded in fabricating a spherical tip having a smooth surface and the tip size varied between φ 0.8 and 5.5 μm. Anti-actin was immobilized on the fabricated probe tips and force curves were measured against an actin-immobilized mica substrate by AFM to analyze the retraction forces. The histograms of retraction forces showed that the single-molecular retraction force between actin and anti-actin was ca. 350–400 pN. It was observed that the average retraction forces for each tip size correlated with the square of the tip radius.     

On the stiction of MEMS materials

Stiction is a serious problem in microelectromechanical systems (MEMS) due to their large surface area-to-volume ratio. Stiction is closely related to surface forces, which greatly depend on the materials used, surface topography and surface treatment process. In this paper, we investigate surface energies and stiction of commonly used MEMS materials by contact angle measurements and atomic force microscopy (AFM). Dispersive and polar components of surface energies are calculated by Owens–Wendt–Rabel–Kaelble method. Silicon and silicon-related materials have higher polar surface energies than SU-8 and poly-methylmethacrylate (PMMA), thereby have larger surface energies and enhanced tendency for stiction. The nano-scale adhesion forces between Si₃N₄ tip and surfaces obtained by AFM further verified that silicon wafer with native oxide has 3–4 times higher adhesion force than SU-8 and PMMA. It has been shown that the materials with higher surface energy have higher sticton/adhesion forces. The topography of surface influences the contact angle and stiction, and is also discussed in the paper.     

Mapping of the receptor-associated protein (RAP) binding proteins on living fibroblast cells using an atomic force microscope

The distribution of the receptor-associated protein (RAP) binding protein and the adhesion forces between RAP and its binding protein on living fibroblast cells were examined using an atomic force microscope (AFM). The distribution of RAP binding protein was obtained on 256 (16x16) locations in 2x2 micro m sections over the surface of living cells. The adhesion forces between RAP and the binding protein were measured with an AFM tip functionalized with RAP. In the presence of RAP in the scanning solution, the number of force curves with large adhesion force decreased. These results indicate that the adhesive forces observed here represent specific binding between RAP and the binding protein. This method will be a useful application of AFM to examine receptors on cell surfaces in high resolution.     

Nanotribological properties of silicon nano-pillars coated by a Z-DOL lubricating film

This paper reports a novel approach for improving the nanotribological properties of silicon (Si) surfaces by topographically and chemically modifying the surfaces. In the first step, Si (100) wafers were topographically modified into nano-pillars by using the photolithography and reactive ion etching (RIE) techniques. Various patterns, including nano-pillars of varying diameters and pitches (distance between pillars), were fabricated. Then, the patterns were coated with a Z-DOL (perfluoropolyether (PFPE)) lubricating film using a dipcoating technique, and this process was followed by thermal treatment. These modified surfaces were tested for their nanotribological properties, namely adhesion and friction forces, using an atomic force microscope (AFM). The results showed that the topographical modification and Z-DOL coating each independently reduced the adhesion and friction forces on the Si surfaces. However, the combination of the two surface treatments was most effective in reducing these forces. This is attributed to the combined effects of the reduction in the real area of contact due to patterning and the low surface energy of the Z-DOL lubricant. Further, it was found that adhesion and friction forces of the surfaces with combined modification varied significantly depending on the diameter of the pillars and the pitch. It is proposed that such a combination of surface modifications promises to be an effective method to improve the nanotribological performance of miniaturized devices, such as MEMS, in which Si is a typical material.     

Fundamental Investigation of the Wear Progression of Silicon Atomic Force Microscope Probes

Atomic force microscopy (AFM) is a key instrument in nanotechnology; however, AFM probe wear is a critical concern with AFM-based technologies. In this work, the wear progression of silicon AFM probes with different radii was thoroughly explored under various normal forces and sliding speeds. The results showed that the initial wear coefficient increased as the normal force increased. However, after a certain sliding distance, the wear coefficient was stable due to the flattening of the probe even with increasing normal force. It was also observed that the wear coefficient decreased with increasing probe radius and the wear of the probe increased as the sliding speed increased. From the overall results, it was concluded that the contact pressure plays a significant role in wear progression and may be responsible for a lower wear coefficient even with increasing adhesion forces due to wear. The wear rate was found to have an exponential dependence on contact stress, as proposed in recent literatures.     

Nano-Mechanical Properties of Modified Poly(Methyl Methacrylate) Films Studied by Atomic Force Microscopy

Atomic force microscopy (AFM) has been used to study the effect of various photoinitiators doped into poly(methyl methacrylate) (PMMA) on the mechanical properties of PMMA films at the nanometer scale. Pure and modified PMMA films (containing four different photoinitiators) were exposed to a mercury vapor lamp in air atmosphere. Force–distance curves for hardness, Young modulus, and adhesion forces were obtained using different AFM modes (tapping or contact-mode) and different tips (diamond or silicon nitride). The results revealed that the added photoinitiators slightly changed the nanomechanical properties of PMMA as a result of alterations in the photochemical reactions and physical processes occurring in the studied systems. tert-Butyl peroxybenzoate had the most efficient effect on the measured parameters in UV-irradiated PMMA, whereas benzoyl peroxide was less active. The mechanism of the observed processes is discussed.     

Molecular lubricants for silicon‐based microelectromechanical systems (MEMS): a novel assembly strategy

Microelectromechanical systems (MEMS) are poised to bring the next technology revolution. At present, many of these are fabricated from silicon using lithographic techniques developed in the microelectronics industry. Due to the large surface area to volume ratio on the micrometer scale, surface forces, such as adhesion and friction, are often detrimental to the fabrication and operation of MEMS devices. Thus, one of the key issues in MEMS is surface engineering to reduce adhesion and friction. Here, we present a general strategy for the efficient assembly of organic layers directly onto the silicon surface in both vacuum environment and solution phases via the robust Si–N or Si–O linkage. This is achieved by the reaction between an amine or alcohol functional group and a chlorinated Si surface. The resulting surface assemblies are thermally stable. Characterization by interfacial force microscope (IFM) reveals that these assemblies have very low surface energy and are mechanically stable. This revised version was published online in September 2006 with corrections to the Cover Date.     

Polysaccharide properties probed with atomic force microscopy

In recent years, polysaccharides have been extensively studied using atomic force microscopy (AFM). Owing to its high lateral and vertical resolutions and ability to measure interaction forces in liquids at pico‐ or nano‐Newton level, the AFM is an excellent tool for characterizing biopolymers. The first imaging studies showed the morphology of polysaccharides, but gradually more quantitative image analysis techniques were developed as the AFM grew easier to use in aqueous liquids and in non‐contact modes. Recently, AFM has been used to stretch polysaccharides and characterize their physicochemical properties by application of appropriate polymer stretching models, using a technique called single‐molecule force spectroscopy. From application of such models as the wormlike chain, freely jointed chain, extensible‐freely jointed chain, etc., properties such as the contour length, persistence length and segment elasticity or spring constant can be calculated for polysaccharides. The adhesion between polysaccharides and surfaces has been quantified with AFM, and this application is particularly useful for studying polysaccharides on microbial and other types of cells, because their adhesion is controlled by biopolymer characteristics. This review presents a synthesis of the theory and techniques currently in use to probe the physicochemical properties of polysaccharides with AFM.     

MEMS-based fast scanning probe microscopes

Scanning probe microscopy is a frequently used nanometer-scale surface investigation technique. Unfortunately, its applicability is limited by the relatively low image acquisition speed, typically seconds to minutes per image. Higher imaging speeds are desirable for rapid inspection of samples and for the study of a range of dynamic surface processes, such as catalysis and crystal growth. We have designed a new high-speed scanning probe microscope (SPM) based on micro-electro mechanical systems (MEMS). MEMS are small, typically micrometer size devices that can be designed to perform the scanning motion required in an SPM system. These devices can be optimized to have high resonance frequencies (up to the MHz range) and have very low mass (10⁻¹¹ kg). Therefore, MEMS can perform fast scanning motion without exciting resonances in the mechanical loop of the SPM, and hence scan the surface without causing the image distortion from which conventional piezo scanners suffer. We have designed a MEMS z-scanner which we have integrated in commercial AFM (atomic force microscope) and STM (scanning tunneling microscope) setups. We show the first successful AFM experiments.     

Characterization of Microscale Wear in a Polysilicon-Based MEMS Device Using AFM and PEEM–NEXAFS Spectromicroscopy

Mechanisms of microscale wear in silicon-based microelectromechanical systems (MEMS) are elucidated by studying a polysilicon nanotractor, a device specifically designed to conduct friction and wear tests under controlled conditions. Photoelectron emission microscopy (PEEM) was combined with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) to quantitatively probe chemical changes and structural modification, respectively, in the wear track of the nanotractor. The ability of PEEM–NEXAFS to spatially map chemical variations in the near-surface region of samples at high lateral spatial resolution is unparalleled and therefore ideally suited for this study. The results show that it is possible to detect microscopic chemical changes using PEEM–NEXAFS, specifically, oxidation at the sliding interface of a MEMS device. We observe that wear induces oxidation of the polysilicon at the immediate contact interface, and the spectra are consistent with those from amorphous SiO₂. The oxidation is correlated with gouging and debris build-up in the wear track, as measured by AFM and scanning electron microscopy (SEM).     

Tribological interaction between multi-walled carbon nanotubes and silica surface using lateral force microscopy

This work presents the tribological interaction between multi-walled carbon nanotubes (MWCNTs) and silica surface using lateral manipulation in the atomic force microscope (AFM). The MWCNT is mechanically manipulated by a pyramidal silicon probe of an AFM using the same scan mechanism as in the imaging mode. With a controlled normal force of the AFM probe, it was found that lateral force applied to the MWCNT could overcome the tribological adhesion between MWCNT and silica surface, causing individual MWCNT to rotate on the silica. According to the results, the shear stresses due to tribological interacting with the MWCNTs and the silica are 59.6 MPa and 64.8 MPa for the MWCNT 1 (100 nm diameter) and the MWCNT 2 (60 nm diameter), respectively. Experimental results show that the shear stress increases with the increasing rotation angle for each manipulation, from which we determine the linear fitting function. In addition, we determine the relationship between push point and pivot point to realize the rotation behavior. The implications of tribological interaction between the MWCNTs and silica surface are discussed in detail.     

Prototype cantilevers for quantitative lateral force microscopy

Prototype cantilevers are presented that enable quantitative surface force measurements using contact-mode atomic force microscopy (AFM). The "hammerhead" cantilevers facilitate precise optical lever system calibrations for cantilever flexure and torsion, enabling quantifiable adhesion measurements and friction measurements by lateral force microscopy (LFM). Critically, a single hammerhead cantilever of known flexural stiffness and probe length dimension can be used to perform both a system calibration as well as surface force measurements in situ, which greatly increases force measurement precision and accuracy. During LFM calibration mode, a hammerhead cantilever allows an optical lever "torque sensitivity" to be generated for the quantification of LFM friction forces. Precise calibrations were performed on two different AFM instruments, in which torque sensitivity values were specified with sub-percent relative uncertainty. To examine the potential for accurate lateral force measurements using the prototype cantilevers, finite element analysis predicted measurement errors of a few percent or less, which could be reduced via refinement of calibration methodology or cantilever design. The cantilevers are compatible with commercial AFM instrumentation and can be used for other AFM techniques such as contact imaging and dynamic mode measurements.     

Adhesion and Friction Studies of Nano-textured Surfaces Produced by Self-Assembling Au Nanoparticles on Silicon Wafers

This article presents the results of nanoscale friction and adhesion of nanoparticle-textured surfaces (NPTS) using atomic force microscope (AFM). The effects of coverage ratio, texture height, and packing density on the adhesion and friction of the NPTS were investigated. The nano-textured surfaces were produced by self-assembling Au nanoparticles (NPs) with diameters of 20 nm and 50 nm on the silicon (100) surfaces, respectively. Surface morphology of the NPTS was characterized by field emission scanning electron microscopy and AFM. The results show that the NPTS significantly reduced the adhesive force compared to the smooth surface. The adhesion of NPTS is mainly dependent on the coverage ratio of NPs rather than the texture height and higher coverage ratio resulted in smaller adhesive force. The reduced adhesion of textured surfaces was attributed to the reduced real area of contact. The friction of NPTS is mainly dependent on the spacing between asperities. The lowered frictional force was obtained when the spacing between asperities is less than the size of AFM tip, because of the effectively reduced real area of contact between the AFM tip and the NPTS surface.     

The effect of water on friction of MEMS

Water plays a significant role in the performance of micro electro mechanical systems (MEMS). A special apparatus was employed to investigate the adhesive friction attributed to water at low coverages, i.e., in the nanometer range, where friction and adhesion are a function of the water layer thickness. In addition, the history of the sample surface also plays a significant role. The friction forces associated with hydrophobic samples are negligibly affected by humidity changes, whereas those of hydrophilic samples show a strong dependence. Sample coverage and the friction force are also influenced by the sample temperature. High forces were measured for high humidities at low sample temperatures, for hydrophilic silicon. In contrast, hydrophobic samples show an increase of the friction force with increasing temperature. Experiments performed under high vacuum demonstrated that decreasing the water layer thickness by desorption decreases the friction force with several sub‐minima and sub‐maxima. The friction signal is accompanied by sudden fluctuations. For submonolayer coverage the friction force starts to increase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adhesion and friction studies of a nano-textured surface produced by spin coating of colloidal silica nanoparticle solution

This paper presents the results of adhesion and friction studies on a nano-textured surface. The nano-textures were produced by spin coating colloidal silica nanoparticle solution on a flat silicon substrate. Surface morphology was characterized by environmental scanning electron microscopy (ESEM) and scanning probe microscopy (SPM). Adhesion and friction studies were conducted using a TriboIndenter employing diamond tips with 5 μm and 100 μm nominal radii of curvature. The results show that the adhesion forces and coefficients of friction of the nano-textured surface measured by the 100 μm tip were reduced up to 98 and 88%, respectively, compared to those of a baseline silicon oxide film surface.     

In-situ Vapor-Phase Lubrication of MEMS

In-situ vapor-phase lubrication of sidewall MicroElectroMechanical System (MEMS) devices is investigated with 1-pentanol vapor. The 1-pentanol vapor successfully maintains lubricating properties between silicon contacts of MEMS devices. This is attributed to the ability of alcohol to adsorb on the silicon surface and sustain a lubricating layer, which prevents wear of the MEMS surfaces and minimizes friction. In the presence of these vapors, MEMS devices with sliding contacts operated without failure for up to a factor of 1.7 × 10⁴ longer than in dry N₂ gas alone, representing a dramatic improvement in operating life. Adhesion and friction were also investigated as a function of alcohol vapor pressure. The adhesive force between microfabricated MEMS sidewall surfaces increases from ∼30 to ∼60 nN as the alcohol vapor pressure is increased from 0 to 20% of saturation, and then only slightly increases to ∼75 nN at 95% of saturation vapor pressure. This increase in force is well within the capabilities of even the lowest force on-chip actuators, such as electrostatic comb drives which can typically generate a few μN of force. The static friction force was found to be independent of alcohol vapor pressure within the uncertainties in the measurement.     

Effect of ambient humidity on the strength of the adhesion force of single yeast cell inside environmental-SEM

A novel method for measuring an adhesion force of single yeast cell is proposed based on a nanorobotic manipulation system inside an environmental scanning electron microscope (ESEM). The effect of ambient humidity on a single yeast cell adhesion force was studied. Ambient humidity was controlled by adjusting the chamber pressure and temperature inside the ESEM. It has been demonstrated that a thicker water film was formed at a higher humidity condition. The adhesion force between an atomic force microscopy (AFM) cantilever and a tungsten probe which later on known as a substrate was evaluated at various humidity conditions. A micro-puller was fabricated from an AFM cantilever by use of focused ion beam (FIB) etching. The adhesion force of a single yeast cell (W303) to the substrate was measured using the micro-puller at the three humidity conditions: 100%, 70%, and 40%. The results showed that the adhesion force between the single yeast cell and the substrate is much smaller at higher humidity condition. The yeast cells were still alive after being observed and manipulated inside ESEM based on the result obtained from the re-culturing of the single yeast cell. The results from this work would help us to understand the ESEM system better and its potential benefit to the single cell analysis research.     

猪笼草蜡质滑移区表面反粘附特性的研究

利用环境扫描电子显微镜(ESEM)和原子力显微镜(AFM)表征红瓶猪笼草蜡质滑移区表面微观形貌,并提取粗糙度的相关参数。利用AFM分别在低载荷和高载荷下对蜡质区表面同一区域进行扫描,在不同条件下的扫描形貌一致,且扫描后的探针针尖上未发现附着污染物。利用胶体探针技术在无针尖的探针悬臂上粘附15 μm SiO2小球,模拟单根刚毛与猪笼草蜡质区表面的接触,并测试蜡质区表面的粘附力和摩擦力,并与不同粗糙度的抛光纸表面做对照。考虑到表面物理化学性质对其粘附特性的重要影响,利用接触角测量仪测量蜡质区表面和同粗糙度范围抛光纸表面对水和二碘甲烷的表观接触角并利用二滴法计算其表面能。研究结果表明：蜡质滑移区表面单个蜡质晶体具有力学稳定性,不会因脱落而污染昆虫的粘附器官,污染学说不成立;表面微粗糙度能有效地减小界面间的接触面积,降低了蜡质滑移区表面的粘附力和摩擦力;蜡质滑移区超疏水特性和低表面能是降低表面粘附力和摩擦力的另一个重要因素,两者共同作用形成了猪笼草蜡质滑移区的反粘附特性。     

An active MEMS probe for fine position and force measurements

This paper deals with the development and calibration of a single degree-of-freedom probe that is capable of regulating an input position and measuring force or applying a constant input force and measuring deflection. Such a probe is useful in making sensitive measurements on thin films, nano- and microstructures, and fluids. The probe is actuated by an electrostatic comb drive with an integrated capacitive sensor. COTS electronics and a capacitance-to-voltage IC are used to develop a closed-loop controller for the system, capable of regulating position over a range of about 40 μm to within a 5 nm resolution and controlling forces up to 300 μN with a resolution of 25 nN. The design and fabrication of the probe are discussed. The calibration of the device is performed using multiple methods to cross check each other. The use of the probe is demonstrated in the measurement of surface tension and probing the response of a soft polymer to small forces.     

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm²) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.