Atomic topographical image

Figure Bl.19.7. A series of time-lapse STM topographic images at room temperature showing a 40 mn x 40 mn area of Au(l 11). The time per frame is 8 mm, and each took about 5 min to scan. The steps shown are one atomic unit in height. The second frame shows craters left after tip-sample contact, which are two and three atoms deep. During a 2 h period the small craters have filled completely with diflhismg atoms, while the large craters continue to fill. (Taken from [29], figure 1.)...

In this section, we discuss the effect of tip states. Those tip states will create inverted images, where the sites of surface atoms are minima rather than maxima in the topographic images (Barth et al. 1990). 

The normal tip-sample distance in STM experiments can be obtained accurately from this experiment. In Fig. 8.8, the equilibrium distance, where the net force is zero, is taken as the origin of z. As shown for the case of aluminum, because the attractive force has a longer range than the repulsive force, the absolute equilibrium distance between the apex atom and the counterpart on the sample surface is slightly less than the sum of the atomic radii of both atoms, which is about 2 A. The normal topographic images on Si(lll) are usually taken at / =1 nA, corresponding to a distance of 3 A from the equilibrium point, or 5 A from nucleus to nucleus. 

Schematically, the steps in this process are shown in Fig. 14.4. At the beginning, the local radius of the tip end is small. Field emission can be easily established. A high current though the tip end then causes local melting. The local curvature at the end of the tip suddenly decreases. The field emission current is then reduced dramatically. The tip end recrystallizes to have a relatively large radius. Feenstra et al. (1987a) observed that the tips prepared in this way always provide reproducible tunneling spectra, although atomic-resolution topographic images are generally not observed.

For a solid surface with two-dimensional periodicity, such as a defect-free crystalline surface, all the measurable quantities have the same two-dimensional periodicity, for example, the surface charge distribution, the force between a crystalline surface and an inert-gas atom (Steele, 1974 Goodman and Wachman, 1976 Sakai, Cardino, and Hamann, 1986), tunneling current distribution, and STM topographic images (Chen, 1991). These quantities can be expanded into two-dimensional Fourier series. Usually, only the few lowest Fourier components are enough for describing the physical phenomenon, which requires a set of Fourier coefficients. If the surface exhibits an additional symmetry, then the number of independent Fourier coefficients can be further reduced. 

For few micrometer or sub-micron sized electrodes, an afm (atomic force microscope) is the only tool for contacting. Hereby a conductively coated cantilever of the afm is used to contact the top electrode and also to scan the sample surface to retrieve a topographic image to find the top electrode position. But when we look at afm measurements of sub-micron capacitors, we sometimes face hysteresis loops with strangely increased coercive voltages of e.g. 10 Volts, at a film thickness of e.g. 170 nm (Figure 17.7). 

Topographic image of a reconstructed silicon surface showing outermost atomic positions, photograph of Dr. James Batteas. National Institute of Standards and Technology. Reproduced by permission p. 

The SECM, which does not provide atomic level spatial resolution, cannot compete with STM or AFM as a tool for topographic imaging. However, SECM is well suited for high resolution mapping of surface reactivity. This can be done in either feedback or collection mode. The former can provide a spatial distribution of the rate of a redox reaction responsible for mediator regeneration at the substrate. By proper choice of solution components to control the tip... 

One of the reasons as to why we should use electron microscopy rather than optical microscopy to obtain topographical images is shown in Fig. 12.17 the resolution obtainable can reach almost atomic dimensions. The... 

Can undertake topographical imaging of surfaces with atomic-scale lateral resolution, down to 1A... 

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