Imaging with STM

15) Thermal evaporation or organic molecular 16) In general applications, the logarithm of the 

By applying appropriate voltage ramps to the x- and y-piezo-drives (Fig. 10.7), the tip is scanning along the surface line by line, controlled by the feedback and thus follows the electronic contour of the studied object. 

Perpendicular to the scan direction, the scanner steps from one line to the next line. The scan speed is typically defined as the number of lines acquired per unit of time and is usually given in Hz. 

The step size is given by the size of the scan area and the pixel resolution of the image . Tire acquired signal - either the voltage needed to move the tip up and down in z direction (constant current mode) or the tunneling current itself (con- 

19) In real-time applications, optimizing the feedback parameters is particularly crucial. With respect to this a lot can be learned from the papers of the SPM community. 

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Molecular resolution images from both STM and AFM on compact monolayers show a wealth of lattice structures, which can conveniently be analyzed in terms of lattice parameters using their two-dimensional Fourier transforms. Using an NSOM the molecular orientation in an LB monolayer was presented in polarized fluoresence images" ). With STM it is not only possible to image the arrangement of... 

Adlayers of metal free and preformed potassium complexes of dibenzo-lS-crown-6-ether molecules (organized on a Au(lll) surface under potential control) were in-situ imaged with STM [143]. The potassium ion appeared as ball shaped protrusion in the inclusion complex. 

Substituted thioureas, isothiocyanates, thiophenes and imidazole-2-thiones also form stable self-assembled layers on the gold surface. Tetramethylurea layers can be imaged with STM. Appfication of FT-IR spectroscopy to these layers shows that the... 

Appropriate sample handling and preparation procedures need to be developed for each material type to be exanuned [9]. Organic molecules can be imaged with STM when placed on a conducting substrate. It is important to minimize resolution loss due... 

A novel modification of the STM supplements images with those due to the thermopower signal across the tip-sample temperature gradient [49]. Images of guanine on graphite illustrate the potential of this technique. 

Fig. XVin-3. AFM image of DNA strands on mica. Lower figure image obtained in the contact mode under water. The contrast shown covers height variations in the range of 0-2 nm. Upper figure observed profile along the line A-A of the lower figure. (From S. N. Magnov and M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, New Yoric, 1996.)...

More recently, studies employing STM have been able to address surface self-diffiision across a terrace [16, 17. 18 and 19], It is possible to image the same area on a surface as a fiinction of time, and watch the movement of individual atoms. These studies are limited only by the speed of the instrument. Note that the performance of STM instruments is constantly improving, and has now surpassed the 1 ps time resolution mark [20]. Not only has self-diflfiision of surface atoms been studied, but the diflfiision of vacancy defects on surfaces has also been observed with STM [18]. 

Yackoboski K, Yeo Y H, McGonigal G C and Thomson D J 1992 Molecular position at the liquid/solid interface measured by voltage-dependent imaging with the STM Ultramicroscopy 42-44 963... 

Like STM, SFM employs a piezoelectric transducer to scan the tip across the sample (Figure 2b), and a feedback loop operates on the scanner to maintain a constant separation between the tip and the sample. As with STM, the image is generated by monitoring the position of the scanner in three dimensions. 

Profilometry of softer materials, such as polymers, is also possible with SFM, and with STM if the sample is conducting. Low forces on the SFM tip allow imaging of materials whose surfaces are degraded by traditional stylus profilometry. However, when the surface is soft enough that it deforms under pressure from the SFM tip, resolution will be degraded and topography may not be representative of the true... 

The main difficulty with STM and SFM techniques is the problem of tip imaging. Neither technique is recommended for obtaining accurate measurements of edge profiles of vertical or undercut surfaces. In addition, SFM tips cannot accurately image the lateral dimensions of features with sides steeper than 55° at present. Obtaining SFM tips with more suitable aspect ratios is an area of active research. 

Figure 8.3 STM image with chemical contrast of the Pt25Rh75(100) surface. Arrows point at brighter Rh atoms at the step edge. (Reprinted with permission from Hebenstreit et al. [1999]. Copyright 1999. Elsevier.)...

Figure 9.11 Thiophene adsorbed at 500 K on an H-atom pretreated MoS2 cluster (50 x 54 A2). Beam-like features at the metallic edge [scan line (i)] and the shifted intensity of the outermost edge protrusions relative to the clean edge (triangles refer to the clean edge). These shifts in intensity [line scan (ii)] are associated with changes in the local electronic structure after adsorption of thiophene observed with STM. All the images were taken at room temperature subsequent to thiophene adsorption at 500 K. (Reproduced from Ref. 34).

As we have seen in the previous chapter, the apparent topography and corrugation of thin oxide films as imaged by STM may vary drastically as a function of the sample bias. This will of course play an important role in the determination of cluster sizes with STM, which will be discussed in the following section. The determination of the size of the metallic nanoparticles on oxide films is a crucial issue in the investigation of model catalysts since the reactivity of the particles may be closely related to their size. Therefore, the investigation of reactions on model catalysts calls for a precise determination of the particle size. If the sizes of the metal particles on an oxidic support are measured by STM, two different effects, which distort the size measurement, have to be taken into account. 

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