Lipid membrane: inelastic deformation of surface structure by an atomic force microscope

The stability of the 1,2-Dioleoyl-sn-Glycero-3-[phospho-rac-1-Glycerol-Na] liposome in the liquid crystalline state have been investigated using an atomic force microscope (AFM). We have observed the inelastic deformation of the sample surface. The AFM tip causes persistent deformation of the surface of the lipid membrane, in which some of the lipid molecules are eventually pushed or dragged by the AFM tip. The experiment shows how the surface structure of the lipid membrane can be created by the interaction between the AFM tip and lipid membrane. When the operating force exceeds 10^-8 N, it leads to large deformations of the surface. A square region of about 1×1μm² is created by the scanning probe on the surface. When the operating force is between 10^-11N and 10^-8N, it can image the topography of the surface of the lipid membrane. The stability of the sample is related to the concentration of the medium in which the sample is prepared.     

原子力显微镜探针耦合变形下的微观扫描力研究

原子力显微镜(AFM)的微探针系统是典型的微机械构件，它在接触扫描过程处于耦合变形状态.采用数值模拟方法探究恒力模式下探针耦合变形对微观扫描力信号、微观形貌信号的影响.研究表明，AFM的恒力模式扫描中，法向扫描力并不是恒定大小，与轴向扫描力存在耦合作用，在粗糙峰峰值增加阶段，二力均增加；在粗糙峰峰值减小阶段，二力均减小；该耦合作用随形貌坡度、针尖长度等增加而加强.微观形貌的测试信号和横向扫描侧向力信号受探针耦合变形影响较小，但侧向力与形貌斜率密切相关，且其极值点与形貌极值点存在位置偏差，这些结果均与原子力 关键词： 原子力显微镜 探针悬臂梁 耦合变形 扫描力     

qPlus型原子力显微镜技术

扫描探针显微镜主要包括扫描隧道显微镜和原子力显微镜,其利用尖锐的针尖逐点扫描样品,可在原子和分子尺度上获取表面的形貌和丰富的物性,改变了人们对物质的研究范式和基础认知。近年来,qPlus型高品质因子力传感器的出现将扫描探针显微镜的分辨率和灵敏度推向了一个新的水平,为化学结构、电荷态、电子态、自旋态等多自由度的精密探测和操控提供了前所未有的机会。文章首先简要介绍原子力显微镜的发展历史和基本工作原理,然后重点描述qPlus型原子力显微镜技术的优势及其在单原子、单分子和低维材料体系中的应用,最后展望该技术的未来发展趋势和潜在应用。     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

The influence of a probe on topographical images in atomic force microscopy

Examples of the errors that arose under the studies of metric characteristics of micro- and nanoobjects with atomic force microscopy (AFM) are presented. Degradation of the tip of the probe both under successive scanning of the same part of a sample surface and in the course of a single scan is revealed. AFM images acquired with the probe having twin apexes located at different levels are demonstrated.     

Organized molecular assemblies for scanning probe microscope lithography

Nanostructures down to a few ten nanometers in size have been fabricated with Langmuir–Blodgett (LB) films and self-assembled monolayers (SAMs) using scanning probe microscope lithography. The SAMs have been prepared with organosilane and bipolar amphiphiles, alkanethiol molecules as ultrathin resists on Si and Au substrates. The LB films on silicon substrates using both the polymer of thiophene derivatives and the mixture of palmitic acid and hexadecylamine were prepared and fabricated. The effect of functional groups of molecules on the atomic force microscope (AFM) anodization has been studied in the optimized process conditions. Applied voltage between the AFM tip and sample, the scanning speed and the relative humidity in the laboratory are also important factors for nanometer-scale lithography of the ultrathin films. The STM lithography with an octadecanethiol SAM on Au film in the air was carried out at the pulse mode of tip bias with respect to the suitability of STM lithography. The high structural orderness and perfect thickness of ultrathin organic molecular assemblies are the major advantages as required for nanoscale lithography.     

Tip cleaning and sharpening processes for noncontact atomic force microscope in ultrahigh vacuum

Tip cleaning and sharpening processes for noncontact atomic force microscope (AFM) operated in ultrahigh vacuum (UHV) were carried out and evaluated by a scanning Auger microscope (SAM) with a field emission electron gun and a noncontact AFM in UHV combined with a scanning tunneling microscope and a field emission microscope. The cantilever used in this study was piezoresistive, which can be heated by passing a current through the resistive legs of the cantilever. As a pretreatment, the tip was irradiated with ultraviolet light in oxygen to remove carbon contaminants. It was heated at about 750°C to form a clean oxide layer in oxygen of 5×10⁻⁵ Torr in an SAM chamber. The desorption of the layer can make a remained tip apex sharper by heating under electron beam irradiation. A thermally oxidized layer was also eliminated by HF etching to sharpen the tip apex. The procedures are useful to obtain a well-defined Si tip suitable for a noncontact AFM.     

Fabricated cantilever for AFM measurements and manipulations: Pre-stress analysis of stress fibers

The atomic force microscope (AFM) is a highly successful instrument for imaging of nanometer-sized samples and measurement of pico- to nano-Newton forces acting between atoms and molecules, especially in liquid. Generally, commercial AFM cantilevers, which have a sharp tip, are used for AFM experiments. In this review, we introduce micro-fabricated AFM cantilevers and show several applications for cell biology. In manipulation of samples on a cellular scale with a force of tens to hundreds of nano-Newtons, attempts have been made to secure the formation of covalent/non-covalent linkages between the AFM probe and the sample surface. However, present chemistry-based modification protocols of cantilevers do not produce strong enough bonds. To measure the tensile strength and other mechanical properties of actin-based thin filaments in both living and semi-intact fibroblast cells, we fabricated a probe with a hooking function by focused ion beam technology and used it to capture, pull and eventually break a chosen thin filament, which was made visible through fusion with fluorescent proteins. Furthermore, we fabricated a microscoop cantilever specifically designed for pulling a microbead attached to a cell. The microscoop cantilevers can realize high-throughput measurements of cell stiffness.     

Scanning probe microscope with interchangeable AFM-FFM and STM heads

Summary A scanning probe microscope operating in air with interchangeable atomic force-friction force (AFM-FFM) and electronic-tunnelling (STM) heads is presented. Our AFM operates in the so-called contact mode and utilizes the optical-lever detection method which allows simultaneous measurement of the topography as well as the lateral force. The set-up also contains an optical microscope to control both the sample and the probe laser spot on the cantilever. The experimental method to change from AFM to STM operation is based on the use of the probe laser beam and the optical microscope. The maximum scanning area is (24×24) μm² and it is well embraced in the optical-microscope visual field. The microscope attains atomic resolution in air in both AFM and STM configuration. Its performance is demonstrated on the surface of different samples. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Instrumentation of STM and AFM combined with transmission electron microscope

A scanning tunneling microscope (STM) combined with a transmission electron microscope (TEM) is a powerful tool for direct investigation of structures, electronic properties, and interactions at the atomic scale. Here, we report on two different designs of such TEM-STM as well as an extension with an atomic force microscope (TEM-AFM). In the first TEM-STM design, a stepper motor, combined with a one-dimensional inertial slider, was used to perform the coarse approach. The advantage of this design was the strong pulling force that enabled notched metallic wires to be broken inside the TEM, which lead to clean sample surfaces. A second design, with a three-dimensional inertial slider, allowed lateral motion inside the TEM, which simplified the adjustment of tip location on the sample. By replacing the STM tip with a standard AFM-cantilever chip, a new combination was demonstrated: TEM-AFM. Here the force was simply measured by direct TEM imaging of the motion of the AFM tip. Some experimental results are included to illustrate the capabilities of TEM-STM and TEM-AFM.     

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.     

Evaluation of the contact resonance frequencies in atomic force microscopy as a method for surface characterisation (invited)

The combination of ultrasound with atomic force microscopy (AFM) opens the high lateral resolution of scanning probe techniques in the nanometer range to ultrasonics. One possible method is to observe the resonance frequencies of the AFM sensors under different tip-sample interaction conditions. AFM sensors can be regarded as small flexible beams. Their lowest flexural and torsional resonance frequencies are usually found to be in a range between several kHz and several MHz depending on their exact geometrical shape. When the sensor tip is in a repulsive elastic contact with a sample surface, the local indentation modulus can be determined by the contact resonance technique. Contact resonances in the ultrasonic frequency range can also be used to improve the image contrast in other dynamic techniques as, for example, in the so-called piezo-mode. Here, an alternating electric field is applied between a conducting cantilever and a piezoelectric sample. Via the inverse piezoelectric effect, the sample surface is set into vibration. This excitation is localised around the contact area formed by the sensor tip and the sample surface. We show applications of the contact resonance technique to piezoelectric ceramics.     

Atomic-force microscope observation and molecular-scale flattening of the single-crystal surface of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST)

The surface of an organic electro-optic crystal tends to be covered with a degenerate rough layer, which may cause light scattering or unfavorable transmission of light. We demonstrate a novel method of removing this layer and flattening the (001) surface of a 4-dimethylamino- N -methyl-4-stilbazolium tosylate (DAST) crystal on a molecular scale by applying suitable force on the tip of an atomic-force microscope (AFM). When the loading force on the AFM tip is kept near 10 nN, the DAST molecules can be removed layer by layer. This method produced a large, flat terrace of 250,000 nm(2) , and the molecular-scale flatness of this area was confirmed by AFM observation.     

LOCAL ELECTROCHEMICAL DEPOSITION OF METAL ISLANDS MECHANICALLY INDUCED WITH THE TIP OF AN ATOMIC FORCE MICROSCOPE

The investigation of electrochemical processes on the nanometer scale is of great scientific as well as technological interest. Here we study the electrodeposition of copper on a polycrystalline gold surface, and demonstrate that copper deposition can be locally induced by mechanical activation with the tip of an atomic force microscope (AFM). Whereas at higher values of the deposition voltage (>100mV), a solid copper film can grow on the gold surface without tip activation, at lower voltages (approx. 30-60mV), copper deposition only occurs at the position where the surface is activated by the AFM tip due to scanning in mechanical contact with the sample. With this mechano-electrochemical "writing" process, which can be performed at ambient conditions, the controlled local deposition of metallic islands is possible, at applied force loads of the order of 10nN. Both the size-dependence of the locally induced structures on the deposition time and the reversibility of the local deposition process are studied. Depending on the deposition parameters, individual copper islands between 50nm and 200nm in size were deposited at predefined locations on the gold surface. The investigations open perspectives for the controlled mechano-electrochemical writing of more complex nanostructures with the AFM tip.     

Force probing of protein crystals: An atomic force microscopy study

The atomic force microscope (AFM) was used for measuring force-distance curves on horse spleen ferritin crystals in liquid environment. In the region of the approach curve which corresponds to tip-surface contact, discrete jumps were recorded, as predicted by molecular dynamics simulations in the case of low tip-sample interaction. The observed jumps can be related to the removal of individual molecules from the surface by the AFM tip. A simple steric model, which takes into account tip and ferritin molecule size, can explain the displacements observed with excellent agreement. The elemental force jump resulting from the approach curves is a direct measure of the force required to remove a single molecule from the crystal face. We discuss the conditions under which the cantilever potential energy difference along the elemental force step provides the energy of extraction of a single molecule. The estimate of the intermolecular binding energy turns out to be in good agreement with the value calculated independently from the surface free energy of ferritin crystals. Received 10 February 2000 and Received in final form 4 May 2000     

Theoretical Study on the Capillary Force between an Atomic Force Microscope Tip and a Nanoparticle

Considering that capillary force is one of the most important forces between nanoparticles and atomic force microscope （AFM） tips in ambient atmosphere, we develop an analytic approach on the capillary force between an AFM tip and a nanoparticle. The results show that the capillary forces are considerably affected by the geometry of the AFM tip, the humidity of the environment, the vertical distance between the AFM tip and the nanoparticle, as well as the contact angles of the meniscus with an AFM tip and a nanoparticle. It is found that the sharper the AFM tip, the smaller the capillary force. The analyses and results are expected to be helpful for the quantitative imaging and manipulating of nanoparticles by AFMs.     

A dislocation mechanism of friction between a nanoprobe and solid surface

A dislocation mechanism of friction between an atomic-force microscope (AFM) probe and an atomically smooth solid surface is put forward. In this mechanism, the contact region is represented by an edge dislocation. The triboacoustic emission measured with an AFM shows the dislocation nature of friction. The friction force is calculated for a parabolic tip.     

Force spectroscopy in noncontact mode

We have analyzed the possibility of using noncontact scanning force microscopy (NCAFM) to detect variations in surface composition, i.e., to detect a ‘spectroscopic image' of the sample. This ability stems from the fact that the long-range forces, acting between the AFM tip and sample, depend on the composition of the AFM tip and sample. The long-range force can be magnetic, electrostatic, or van der Waals forces. Detection of the first two forces is presently used in scanning force microscopy technique, but van der Waals forces have not been used. We demonstrate that the recovery of spectroscopic image has a unique solution. Furthermore, the spectroscopic resolution can be as good as lateral one.     

Electrical Measurement Techniques in Atomic Force Microscopy

A conductive tip in an atomic force microscope (AFM) has extended the capability from conventional topographic imaging to electrical surface characterization. The conductive tip acts as a voltage electrode to provide stimuli and monitor electrical surface properties. In this review article, we have organized the AFM electrical techniques based on whether the electrical properties are monitored at the cantilever tip or across the sample. Furthermore, the techniques are organized based on probe detection signal. A number of acronyms are used in the literature, and the more commonly used ones are identified. The principle of each technique is described, and representative applications are presented. A better understanding of the spectrum of techniques should serve as the driver to expand the application of electrical techniques to study interdisciplinary phenomena at the nanoscale.     

Manipulations with diamond nanoparticles in SPM: the effect of electric field of the conductive probe tip

The role of the electric field during manipulations with diamond nanoparticles on a silicon substrate by a scanning probe microscope (SPM) tip is studied. It is found that the attractive force appearing in the contact between nanodiamond and an electrically charged tip is sufficient to detach and displace a chosen nanoparticle from initial to goal position under moderate mechanical stresses of the probe to nanoparticle. The problem of the control of the tip motion trajectory during manipulations is solved by visualizing the tip trace of the sample surface. The results obtained will be used for precision positioning of single-photon emitters based on luminescent nanodiamonds in microcavities.