Note: A scanning electron microscope sample holder for bidirectional characterization of atomic force microscope probe tips

A novel sample holder that enables atomic force microscopy (AFM) tips to be mounted inside a scanning electron microscopy (SEM) for the purpose of characterizing the AFM tips is described. The holder provides quick and easy handling of tips by using a spring clip to hold them in place. The holder can accommodate two tips simultaneously in two perpendicular orientations, allowing both top and side view imaging of the tips by the SEM.     

Combined low-temperature scanning tunneling/atomic force microscope for atomic resolution imaging and site-specific force spectroscopy

We present the design and first results of a low-temperature, ultrahigh vacuum scanning probe microscope enabling atomic resolution imaging in both scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) modes. A tuning-fork-based sensor provides flexibility in selecting probe tip materials, which can be either metallic or nonmetallic. When choosing a conducting tip and sample, simultaneous STM/NC-AFM data acquisition is possible. Noticeable characteristics that distinguish this setup from similar systems providing simultaneous STM/NC-AFM capabilities are its combination of relative compactness (on-top bath cryostat needs no pit), in situ exchange of tip and sample at low temperatures, short turnaround times, modest helium consumption, and unrestricted access from dedicated flanges. The latter permits not only the optical surveillance of the tip during approach but also the direct deposition of molecules or atoms on either tip or sample while they remain cold. Atomic corrugations as low as 1 pm could successfully be resolved. In addition, lateral drifts rates of below 15 pm/h allow long-term data acquisition series and the recording of site-specific spectroscopy maps. Results obtained on Cu(111) and graphite illustrate the microscope's performance.     

Optimizing AC-mode atomic force microscope imaging

We have operated an atomic force microscope in ambient air with several oscillating cantilever modes to establish the optimal scanning parameters to maximize image resolution and to minimize probe and sample damage. This was done by scanning a surface in air and correlating scan parameters such as oscillation amplitude and damping with image resolution. We also examined the geometry of the probe with a scanning electron microscope, before and after scanning, in order to determine whether the scanning technique had an effect on the geometry of the probe tip. If the probe is oscillated such that it contacts the surface on each oscillation, substantial damage or “wear” to the probe occurs and significant degradation of image quality was observed. In ambient air, the optimal conditions are achieved when the probe penetrates the contamination layer and reverses direction before touching the surface. Under these “near-contact” conditions no probe damage is observed and high-image resolution can be maintained indefinitely.     

An integrated approach to piezoactuator positioning in high-speed atomic force microscope imaging

In this paper, an integrated approach to achieve high-speed atomic force microscope (AFM) imaging of large-size samples is proposed, which combines the enhanced inversion-based iterative control technique to drive the piezotube actuator control for lateral x-y axis positioning with the use of a dual-stage piezoactuator for vertical z-axis positioning. High-speed, large-size AFM imaging is challenging because in high-speed lateral scanning of the AFM imaging at large size, large positioning error of the AFM probe relative to the sample can be generated due to the adverse effects--the nonlinear hysteresis and the vibrational dynamics of the piezotube actuator. In addition, vertical precision positioning of the AFM probe is even more challenging (than the lateral scanning) because the desired trajectory (i.e., the sample topography profile) is unknown in general, and the probe positioning is also effected by and sensitive to the probe-sample interaction. The main contribution of this article is the development of an integrated approach that combines advanced control algorithm with an advanced hardware platform. The proposed approach is demonstrated in experiments by imaging a large-size (50 microm) calibration sample at high-speed (50 Hz scan rate).     

Note: Seesaw actuation of atomic force microscope probes for improved imaging bandwidth and displacement range

The authors describe a method of actuation for atomic force microscope (AFM) probes to improve imaging speed and displacement range simultaneously. Unlike conventional piezoelectric tube actuation, the proposed method involves a lever and fulcrum "seesaw" like actuation mechanism that uses a small, fast piezoelectric transducer. The lever arm of the seesaw mechanism increases the apparent displacement range by an adjustable gain factor, overcoming the standard tradeoff between imaging speed and displacement range. Experimental characterization of a cantilever holder implementing the method is provided together with comparative line scans obtained with contact mode imaging. An imaging bandwidth of 30 kHz in air with the current setup was demonstrated.     

A metrological scanning force microscope

In this paper, the design, construction, and characterization of a metrological scanning force microscope (SFM) for the purposes of dimensional measurement of surface features is discussed. Using this instrument, precision measurements of engineering surfaces can be performed in air with subnanometer resolution. In this design, scanning of the specimen in the x and y planes and surface profiling in z-axis are each monitored directly by capacitance sensors. The present SFM is capable of a resolutions of approximately 0.1 nm over 15 μm range in z-axis and about 1 nm over 50 pm scanning range in x− and y-axes with a repeatability of less than 1 nm. The linearity error was measured to be within the noise level. Specimens ranging from soft polymeric films to polished zerodur are used to illustrate its metrological capability.     

新型电化学原子力显微镜的研制

电化学原子力显微镜将电化学分析技术与原子力显微镜结合起来,能对生物传感器,新型电池和电腐蚀进行原位电化学扫描探针显微测量分析。为了实现电化学与扫描探针功能的系统集成,在控制电路设计中采用现场可编程门阵列,提高了系统的可靠性。电化学控制箱与原子力显微镜的头部紧密集成,保证微弱信号不受干扰,并具有多种电化学工作模式。系统具有稳定性好,重复性高,抗干扰能力强等优点。     

A high-pressure atomic force microscope for imaging in supercritical carbon dioxide

A high-pressure atomic force microscope (AFM) that enables in situ, atomic scale measurements of topography of solid surfaces in contact with supercritical CO(2) (scCO(2)) fluids has been developed. This apparatus overcomes the pressure limitations of the hydrothermal AFM and is designed to handle pressures up to 100 atm at temperatures up to ～350 K. A standard optically-based cantilever deflection detection system was chosen. When imaging in compressible supercritical fluids such as scCO(2), precise control of pressure and temperature in the fluid cell is the primary technical challenge. Noise levels and imaging resolution depend on minimization of fluid density fluctuations that change the fluid refractive index and hence the laser path. We demonstrate with our apparatus in situ atomic scale imaging of a calcite (CaCO(3)) mineral surface in scCO(2); both single, monatomic steps and dynamic processes occurring on the (1014) surface are presented. This new AFM provides unprecedented in situ access to interfacial phenomena at solid-fluid interfaces under pressure.     

Metrological atomic force microscope using a large range scanning dual stage

We developed a metrological atomic force microscope (MAFM) using a large range scanning dual stage and evaluated the performance in the measurement of lateral dimension. AFMs are widely used in nanotechnology for very high spatial resolution, but the limitation in measurement range should be overcome to expand its application in nanometrology. Therefore, we constructed new MAFM having a large measurement of 200 mm × 200 mm by using a dual stage and an AFM head module. The dual stage is composed of a coarse and a fine stage to obtain large scanning range and high resolution simultaneously. Precision surfaces and PTFE sliding pads guide the motion of coarse stage, drove by a fine pitch screw and DC motors. Flexure hinges and PZT actuators are utilized for the fine stage. Multi-axis interferometers measure the five degrees of freedom motion of the dual stage for the position control and the compensation of parasitic angular motions. The vertical displacement of AFM tip is measured by a built-in capacitive sensor in the AFM head module within the range of 38 μm. The performance of the dual stage was evaluated and the expanded uncertainty (k = 2) in the measurements of 1-D displacement L was estimated as $ U(L) = \sqrt {(2.8nm)^2 + (3.0 \times 10^{ - 7} \times L)^2 } $ U(L) = \sqrt {(2.8nm)^2 + (3.0 \times 10^{ - 7} \times L)^2 } . The relative uncertainty in pitch measurement was less than 0.02 % and the improvement of accuracy was verified by comparing with other MAFM, which are mostly due to the expansion of scan range and the compensation of angular motion. To enhance the performance, we will reduce the vibration and examine the motion of stage in the vertical direction during a long range scan.     

Ferrule-top atomic force microscope

Ferrule-top cantilevers are a new generation of all-optical miniaturized devices for utilization in liquids, harsh environments, and small volumes [G. Gruca et al., Meas. Sci. Technol. 21, 094033 (2010)]. They are obtained by carving the end of a ferruled fiber in the form of a mechanical beam. Light coupled from the opposite side of the fiber allows detection of cantilever deflections. In this paper, we demonstrate that ferrule-top cantilevers can be used to develop ultra compact AFMs for contact mode imaging in air and in liquids with sensitivity comparable to that of commercial AFMs. The probes do not require any alignment procedure and are easy to handle, favoring applications also outside research laboratories.     

Note: vibration reduction control of an atomic force microscope using an additional cantilever

Since an atomic force microscope is used to measure sub-nanometer level precision, it is sensitive to external vibration. If the vibration can be measured by using an additional sensor, we can obtain the vibration-free signal by subtracting the vibration signal from the signal containing the vibration. To achieve a highly effective vibration rejection ratio, it is important to decide where to locate the additional sensor. This is because the vibration measured at the sensing position should have the same phase as that of the vibration in the signal. Vibration reduction control using this electrical sensing method is verified through time domain analysis and topology images of a standard grid sample.     

Calibration of the scanning (atomic) force microscope with gold particles

Scanning force microscopy (SFM) holds great promise for biological research. Two major problems that have confronted imaging with the scanning force microscope have been the distortion of the image and overestimation in measurements of lateral size due to the varying geometry and characteristics of the scanning tip. In this study, spherical colloidal gold particles (10, 20 and 40 nm in diameter) were used to determine (1) tip parameters (size, shape and semivertical angle); (2) the distortion of the image caused by the tip; and (3) the overestimation or broadening of lateral dimensions. These gold particles deviate little in size, are rigid and have a size similar to biological macromolecules. Images of the colloidal gold particles by SFM were compared with those obtained by electron microscopy (EM). The height of the gold particles as measured by SFM and EM was comparable and was little affected by the tip geometry. The measurements of the lateral dimensions of colloidal gold, however, showed substantial differences between SFM and EM in that SFM resulted in an overestimate of the lateral dimensions. Moreover, the distortion of images and broadening of lateral dimensions were specific to the SFM tip used. The calibration of the SFM tip with mica provided little clue as to the type of distortion and the amount of lateral broadening observed when the larger gold particles were scanned. The SFM image also depended on the orientation of the tip with respect to the specimen. Our results suggest that quantitative SFM imaging requires calibration to identify and account for both the distortions and the magnitude of lateral broadening caused by the cantilever tip. Calibration with gold particles is fast and nondestructive to the tip. The raw imaging data of the specimen can be corrected for the tip effect and true structural information can be derived. In summary, we present a simple and practical method for the calibration of the SFM tip using gold particles with a size in the range of biomacromolecules that allows: (1) selection of a cantilever tip that produces an image with minimal distortion; (2) quantitative determination of tip parameters; (3) reconstruction of the shape of the tip at different heights from the tip apex; (4) appreciation of the type of distortion that may be introduced by a specific tip and quantification of the overestimation of the lateral dimensions; and (5) calculation of the true structure of the specimen from the image data. The significance is that such calibration will permit quantitative and accurate imaging with SFM.     

Print your atomic force microscope

Progress in scanning probe microscopy profited from a flourishing multitude of new instrument designs, which lead to novel imaging modes and as a consequence to innovative microscopes. Often these designs were hampered by the restrictions, which conventional milling techniques impose. Modern rapid prototyping techniques, where layer by layer is added to the growing piece either by light driven polymerization or by three-dimensional printing techniques, overcome this constraint, allowing highly concave or even embedded and entangled structures. We have employed such a technique to manufacture an atomic force microscopy (AFM) head, and we compared its performance with a copy milled from aluminum. We tested both AFM heads for single molecule force spectroscopy applications and found little to no difference in the signal-to-noise ratio as well as in the thermal drift. The lower E modulus seems to be compensated by higher damping making this material well suited for low noise and low drift applications. Printing an AFM thus offers unparalleled freedom in the design and the rapid production of application-tailored custom instruments.     

Compact variable-temperature scanning force microscope

A compact design for a cryogenic variable-temperature scanning force microscope using a fiber-optic interferometer to measure cantilever deflection is presented. The tip-sample coarse approach and the lateral tip positioning are performed by piezoelectric positioners in situ. The microscope has been operated at temperatures between 6 and 300 K. It is designed to fit into an 8 T superconducting magnet with the field applied in the out-of-plane direction. The results of scanning in various modes are demonstrated, showing contrast based on magnetic field gradients or surface potentials.     

Electrostatic interactions observed when imaging proteins with the atomic force microscope

The atomic force microscope (AFM) is now an established and valuable tool for the study of biological macromolecules in aqueous environments. In this paper we form a patterned boundary via the microcontact printing of individually isolated proteins, covalently attached to a solid support. We use this boundary to investigate electrostatic interactions that can occur between an AFM tip and a protein surface during imaging in solution. The observed height variations of the protein film are found to be a combination of not only structural considerations and thickness of the protein film, but also the repulsive contribution from electrostatic interactions between the AFM tip and the sample. These variations in measured heights of the protein surface can be described by Derjaguin, Landau, Verway, Overbeek (DLVO) theory. Our experimental results show that height measurements can be manipulated either negatively or positively by adjusting the pH and concentration of the electrolyte buffer that is utilised.     

卧式原子力显微镜的研制

提出原子力显微镜 (AFM)的新设计 ,讨论卧式 AFM的工作原理及其性能特点 ,简要介绍 AFM的控制电路系统及其图像扫描和图像处理软件系统 ,给出 AFM扫描获得的部分样品的图像结果。     

Lateral force microscope calibration using a modified atomic force microscope cantilever

A proof-of-concept study is presented for a prototype atomic force microscope (AFM) cantilever and associated calibration procedure that provide a path for quantitative friction measurement using a lateral force microscope (LFM). The calibration procedure is based on the method proposed by Feiler et al. [Rev. Sci. Instrum. 71, 2746 (2000)] but allows for calibration and friction measurements to be carried out in situ and with greater precision. The modified AFM cantilever is equipped with lateral lever arms that facilitate the application of normal and lateral forces, comparable to those acting in a typical LFM friction experiment. The technique allows the user to select acceptable precision via a potentially unlimited number of calibration measurements across the full working range of the LFM photodetector. A microfabricated version of the cantilever would be compatible with typical commercial AFM instrumentation and allow for common AFM techniques such as topography imaging and other surface force measurements to be performed.     

A novel cleaning method of gold-coated atomic force microscope tips for their chemical modification

For chemical modification of gold-coated AFM tips with thiol or sulfide compounds, a new two-step precleaning procedure was studied. The two-step cleaning procedure involves (i) oxidation of organic contaminants on the AFM tips with ozone treatment and (ii) reduction of the oxidized gold surface by immersing the oxidized tip into pure hot ethanol at ca. 65 degrees C. The chemically modified tips prepared from gold-coated AFM tips precleaned by the two-step procedure gave almost the same tip characteristics as those chemically modified immediately after gold vapor deposition in a factory. The present two-step cleaning procedure can be used widely for chemical modification of commercially available gold-coated AFM tips with thiol or disulfide compounds for chemical force microscopy.     

Note: A scanning thermal probe microscope that operates in liquids

We have developed a scanning thermal probe microscope that operates in liquid environments. The thermal sensor is a fluorescent particle glued at the end of a sharp tungsten tip. Since light emission is a strongly thermally sensitive effect, the measurement of the particle fluorescence variations allows the determination of the temperature. No electrical wiring of the probe is needed. As a demonstrative example, we have measured the temperature map of a Joule-heated microheater immersed in a water∕glycerol solution. Both topographical and thermal images are obtained with a good sensitivity.