Basic Principles of AFM

FIGURE 13.12 Basic AFM components. Scanner can be attached either to sample or to cantilever and tip (dashed). 

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Atomic Force Microscopy Atomic force microscopy is a direct descendant of STM and was first described in 1986 [254], The basic principle behind AFM is straightforward. An atomically sharp tip extending down from the end of a cantilever is scanned over the sample surface using a piezoelectric scanner. Built-in feedback mechanisms enable the tip to be maintained above the sample surface either at constant force (which allows height information to be obtained) or at constant height (to enable force information to be obtained). The detection system is usually optical whereby the upper surface of the cantilever is reflective, upon which a laser is focused which then reflects off into a dual-element photodiode, according to the motion of the cantilever as the tip is scanned across the sample surface. The tip is usually constructed from silicon or silicon nitride, and more recently carbon nanotubes have been used as very effective and highly sensitive tips. 

Biosensors Using Atomic Force Microscopes, Fig. 1 (a) Basic components and working principle of AFM. A sharp tip fixed at the end of a fiexible cantilever is raster scanned over the surface of a sample. As the tip interacts with the surface, the cantilever deflects, and its deflections are monitored by a laser and a photodiode and then used to reconstruct the topography of the sample, (b) A schematic diagram of AFM as a biosensor in detecting... 

Within the numerous proximal probe techniques developed in the years following the breakthrough inventions of the STM (1) and AFM (5), scanning force microscopy (8) represents a family of scanning probe techniques that rely in their contrast mechanism on various forces between probe tip and sample (9-12) (see Atomic Force Microscopy). In order to provide a basis for an understanding and appreciation of the SFM work on polymers (13-18), as presented in this review, the basic principles of SFM, as well as selected imaging modes, are briefly discussed. 

In this chapter, the instrumentation and biological applications of FM-AFM in liquid are described. In section 18.2, the basic principle of FM-AFM and the special requirements for the operation in liquid are described. In section 18.3, applications of FM-AFM to the molecular-resolution imaging of biological systems are reviewed. In section 18.4, the present state of liquid-environment FM-AFM and the future prospects are summarized. 

From the general principles described in the previous section the following basic components of an AFM can be identified ... 

A new alternative to solve this problem is atomic force microscopy (AFM) which is an emerging surface characterization tool in a wide variety of materials science fields. The method is relatively easy and offers a subnanometer or atomic resolution with little sample preparation required. The basic principle involved is to utilize a cantilever with a spring constant weaker than the equivalent spring between atoms. This way the sharp tip of the cantilever, which is microfabricated from silicon, silicon oxide or silicon nitride using photolithography, mechanically scans over a sample surface to image its topography. Typical lateral dimensions of the cantilever are on the order of 100 pm and the thickness on the order of 1 pm. Cantilever deflections on the order of 0.01 nm can be measured in modem atomic force microscopes. 

To assess the scratch behavior of polsmiers, scratch tests are performed. A compendium of the use of various instruments and methods applied is given in Reference (20). Although different setups have been used for scratch studies, the basic principle relies on the use of a stylus to scratch a surface (2,3,21 3). The stylus can be made of materials ranging from diamond and sapphire, to tool steel, and silicon nitrite for the case of AFM tips. It also comes with a variety of geometries including conical tips with different included angles, spherical tips of different radii, and Berkovich tip. In addition, the observation length scale and controlled experimental parameters are instrument specific. 

Thus by contact angle measurements using three different liquids (L), of which two must be polar, with known y Y and y values, the ys", Ys and ys of any solid (S) can, in principle, be determined. The value of yl must be known or determined independently [108]. The apolar component of the surface tension of solids (yj" ) can be determined by contact angle measurements using strictly apolar liquids for which yL = y These surface tension components can be related to experimentally determined pull-off forces between chemically modified AFM tips and an oxyfluorinated isotactic polypropylene surface in CFM approaches [110]. It was observed that the pull-off force measured with carboxylic acid tips in ethanol depended hnearly on the basic term of the surface tension (y,") on the modified polymer surface. 

Successful syntheses of a wide variety of zeolite and zeotype materials have been developed over the last 50 years. Much is known about the way in which these structures are formed, particularly from recent detailed studies using advanced techniques of microscopy (HRTEM and AFM). However, a number of issues surrounding the mechanism of synthesis remain incompletely resolved. This is partly due to experimental difficulty but is also a reflection of the fact that not all systems arc the same. Nevertheless, in the foregoing account an attempt has been made to review our existing knowledge in terms of basic similarities between one reaction regime and another. In this way, it is hoped to establish some overall principles which, with appropriate modification, may be found to be generally applicable, or at least to provide a framework for further analysis. 

The AFM is a force measuring instrument [2]. It operates on broadly similar principles to the surface force apparatus [3], except that instead of probing the interaction forces between two macroscopic surfaces, the forces measured are those between a very sharp tip and a surface. The tip is attached to a cantilever - the spring - which, as the sample is scanned under the tip (or the tip scanned with respect to the sample), or moved in a direction normal to the tip, deflects in accordance with the force experienced between the tip and the surface. This basic concept is depicted in Fig. 1. Importantly, the AFM can be used to image any surface irrespective of sample conductivity - this is in contrast to STM where the substrate must be (semi)conducting. AFM probes are typically microfabricated from Si3N4 or Si [4]. 

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