Electric force microscope

Electric Force Microscope Observations of Electric Surface Potentials... 

Figure 4. The electric field potential of the same area of the carbon-black-polymer composite surface shown in Figure 3, as imaged in the electric force microscope mode of the scanning probe microscope. (Reproduced with permission fi-om ref. 4. Copyright 1995 American Physical Society.)...

There are many other experiments in which surface atoms have been purposely moved, removed or chemically modified with a scanning probe tip. For example, atoms on a surface have been induced to move via interaction with the large electric field associated with an STM tip [78]. A scaiming force microscope has been used to create three-dimensional nanostructures by pushing adsorbed particles with the tip [79]. In addition, the electrons that are tunnelling from an STM tip to the sample can be used as sources of electrons for stimulated desorption [80]. The tuimelling electrons have also been used to promote dissociation of adsorbed O2 molecules on metal or semiconductor surfaces [81, 82]. 

FIG. 1 Schematic representation of the operation of the scanning polarization force microscope (SPFM). An electrically biased AFM tip is attracted toward the surface of any dielectric material. The polarization force depends on the local dielectric properties of the substrate. SPFM images are typically acquired with the tip scanning at a height of 100-300 A. (From Ref. 32.)... 

Another alternative prototype of memory array, consisting of data stored as electrostatic charge or molecular dipole in a two-dimensional network of streptavidin cross-linked by biotinylated porphyrin derivative, was also suggested. Information reading was expected to be carried out using the electric force mode of the atomic force microscope [70]. 

The force microscope, in general, has several modes of operation. In the repulsive-force or contact mode, the force is of the order of 1-10 eV/A, or 10 -10 newton, and individual atoms can be imaged. In the attractive-force or noncontact mode, the van der Waals force, the exchange force, the electrostatic force, or magnetic force is detected. The latter does not provide atomic resolution, but important information about the surface is obtained. Those modes comprise different fields in force microscopy, such as electric force microscopy and magnetic force microscopy (Sarid, 1991). Owing to the limited space, we will concentrate on atomic force microscopy, which is STM s next of kin. 

The field ion microscope is perhaps the simplest of all atomic resolution microscopes as far as mechanical and electrical designs are concerned. The atomic resolution microscopes, at the present time, include also different types of electron microscopes,1 the scanning tunneling microscope (STM)2 and the atomic force microscope (AFM)2 Before we discuss the general design features of the field ion microscope it is perhaps worthwhile to describe the first field ion microscope,3 and a very simple FIM4 which can be constructed in almost any laboratory. The first field ion microscope, shown in Fig. 3.1, is essentially a field emission microscope5 except that it is now equipped with a palladium tube with... 

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,... 

Another device that yields results of the same kind as STM is atomic force microscopy (AFM) (Binning, 1986). This avoids dependence on an electron stream (which cannot be obtained from insulators)58 and relies on the actual interatomic forces between a microtip and nearby surface atoms. The forces experienced at a given point by the tip are sensed by a cantilever spring. The movements of this are slight, but they can be measured by means of interf erometry and in this way the movement of the tip can be quantified. The sensitivity of the atomic force microscope is less than that of STM, but its action is independent of the electrical conductivity of the surface and it is therefore to be preferred over STM, particularly for studies in bioelectrochemistiy. 

Finally, a very simple molecular electronic component is benzene-1,4-dithiol, which is readily used as a linker between gold electrodes in the same way as the cobalt terpyridine complex shown in Figure 11.40. Benzene dithiol has been used to demonstrate the thermoelectric effect in molecular electronic systems as a linker between a gold surface and the gold tip of a modified atomic force microscope. Thermoelectricity (termed the Seebeck effect) is the generation of an electrical potential... 

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