Tip treatment

In the STM literature, there are many scattered discussions of the problem of tip treatment. A brief summary of the facts found in the STM literature is as follows  

A tungsten tip, prepared by electrochemical etching, with a perfectly smooth end of very small radius observed by SEM or TEM, would not provide atomic resolution immediately. 

Atomic resolution might happen spontaneously by repeated tunneling and scanning for an unpredictable time duration. 

A crashed tip often recovers to resume atomic resolution, unexpectedly and spontaneously. 

During a single scan, the tip often undergoes unexpected and spontaneous changes that may dramatically alter the look and the resolution from one half of an STM image to another half. 

---

Fig. 1.22. Local tunneling spectrum of Si(lll)-2xl. Using the tip treatment procedure developed by Feenstra, Stroscio, and Fein (1987a), reproducible tunneling spectra can be obtained. The tunneling spectrum shown here (Feenstra, 1991) reproduces exactly all the features found in an early measurement (Stroscio, Feenstra, and Fein, 1986). (Reproduced from Feenstra, 1991, with permission.)...

A systematic method of obtaining local tunneling spectra with STM was developed by Feenstra, Thompson, and Fein (1986). The details of this method will be described in Chapter 14. As expected from the Bardeen formula, the tip DOS plays an equal role as the sample DOS in determining the tunneling spectra. Because the tip is made of transition metals or semiconductors, the tip DOS is usually highly structured. In order to obtain reproducible tunneling spectra of the sample, special tip treatment procedures have to be conducted... 

The directional walk of a single atom on the tip surface is not only an interesting observation, but is also relevant to STM experiments. Many tip treatment methods for STM rely on the application of a high electric field near the tip end, which will induce directional walks of atoms at the tip surface, among other effects. Figure 1.35 is an example of the directional walk of a W atom on the surface of a W tip (Tsong, 1990). 

Fig. 13.7. Tip treatment by annealing in a field. Top At a temperature close to the melting point of tungsten, the shape of the tip is basically determined by diffusion process. A rather round-shaped thermal tip is formed. Bottom left, at lower temperature, the directional effect of the field dominates. A built-up tip is formed. Bottom right, in an oxygen atmosphere, the corrosion process further generates a nanotip. After Binh (1988a).

The tip treatment can be done during actual tunneling. Often, these in situ tip treatments take a few seconds to complete. The effect of the tip treatment process can be verified by actual imaging immediately. If one action is not successful, another action can proceed immediately—it takes a few more seconds. As we have mentioned at the beginning of this chapter, these methods was already used at the birth of the STM by the inventors in their first set of experiments (Binnig and Rohrer, 1982). 

As we showed in the previous section, in order to obtain reproducible tunneling spectra, the STM tip must have reproducible DOS, preferably a flat DOS, that is, with a free-electron-metal behavior. However, the tips freshly made by mechanical or electrochemical methods, especially those providing atomic resolution, often show nonreproducible tunneling spectra. The DOS of such tips is often highly structured. To obtain reproducible STS data, a special and reproducible tip treatment procedure is required. 

Fig. 14,4. Tip treatment for tunneling spectroscopy. (A) By applying a relatively large positive bias on the sample, a sharp tip generates a field-emission current. (B) When the field-emission current is very high, the tip end melts. (C) The tip end recrystallizes to form facets with low surface energy. In the case of tungsten, the W(llO) facets are preferred. Its surface DOS resembles a free electron metal. (After Feenstra et al., 1987a.)...

Tip treatment 281—293, 301 annealing 286 annealing with a field 288 atomic metallic ion emission 289 controlled collision 293 controlled deposition 288 field evaporation 287 for scanning tunneling spectroscopy 301 high-field treatment 291 Tip wavefunctions 76—81 explicit forms 77 Green s functions, and 78 Tip-state characterization 306, 308 ex situ 306 in situ 308... 

---