Atomic force microscope material

Burnham N A and Colton R J 1989 Measuring the nanomechanical properties and surface forces of materials using atomic force microscope J. Vac. Sc/. Technol. A 7 2906... 

Friedbacher G, Hansma P K, Ramil E and Stucky G D 1991 Imaging powders with the atomic force microscope from biominerals to commercial materials Sc/e/ ce 253 1261... 

The development of a host of scanning probe devices such as the atomic force microscope (AFM) [13-17] and the surface forces apparatus (SFA) [18-22], on the other hand, enables experimentalists to study almost routinely the behavior of soft condensed matter confined by such substrates to spaces of molecular dimensions. However, under conditions of severe confinement a direct study of the relation between material properties and the microscopic structure of confined phases still remains an experimental challenge. 

There are of course many other similarities and differences, and some of them are listed in Table 5.1 without further explanations. In general, STM is very versatile and flexible. Especially with the development of the atomic force microscope (AFM), materials of poor electrical conductivity can also be imaged. There is the potential of many important applications. A critically important factor in STM and AFM is the characterization of the probing tip, which can of course be done with the FIM. FIM, with its ability to field evaporate surface atoms and surface layers one by one, and the capability of single atom chemical analysis with the atom-probe FIM (APFIM), also finds many applications, especially in chemical analysis of materials on a sub-nanometer scale. It should be possible to develop an STM-FIM-APFIM system where the sample to be scanned in STM is itself an FIM tip so that the sample can either be thermally treated or be field evaporated to reach into the bulk or to reach to an interface inside the sample. After the emitter surface is scanned for its atomic structure, it can be mass analyzed in the atom-probe for one atomic layer,... 

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

The resolution of the atomic force microscope depends on the radius of curvature of the tip and its chemical condition. Solid crystal surfaces can often be imaged with atomic resolution. At this point, however, we need to specify what Atomic resolution is. Periodicities of atomic spacing are, in fact, reproduced. To resolve atomic defects is much more difficult and usually it is not achieved with the atomic force microscope. When it comes to steps and defects the scanning tunneling microscope has a higher resolution. On soft, deformable samples, e.g. on many biological materials, the resolution is reduced due to mechanical deformation. Practically, a real resolution of a few nm is achieved. 

Since 1981, researchers have developed a number of modifications of the STM for uses in which the original instrument is not suitable. For example, the atomic force microscope (AFM) was invented in 1986 by Binnig and Christoph Gerber at IBM-ZRL and Calvin Quate at Stanford University. The AFM can be used on nonconductive surfaces, such as organic materials, on which the STM cannot he used. Today the STM, AFM, and related devices are collectively known as scanning probe microscopes (SPMs). 

Fig. 4. Monolayers of poly(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyl methacry-late)-block-poly-(2-perfluorooctylethyl acrylate) (left) and its hybrid with per-fluorooctadecanoic acid at [polymer] [Ci7F35COOH] =1 4 (right), (a) Atomic force microscopic (AFM) images of LB films (b) depth vs. scattering length density (SLD) profiles of Langmuir films (c) schematic illustration of molecular arrangements on water subphase. Reprinted with permission from Ref. [86], 2004, The Materials Research Society of Japan Ref. [87], 2006, American Scientific Publishers.

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. 

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