Tip treatment for spectroscopy studies

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. 

Feenstra et al. (1987a) developed an in-situ tip DOS flattening procedure. This procedure uses field emission current to locally heat up the end of the tip, which eventually causes a local melting and recrystallization. Details of this process is as follows  

Start with an electrochemically etched W tip. The cleaning or sharpening is not critical. 

The sample, or the anode, can be any conducting material. Because the process will cause local damage to the anode, it is better to use a scratch sample or an unused area on the sample. 

Increase the bias slowly up to some 100 V until a field emission current is observed. 

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