Methods

SXM (STM/AFM/spectroscopy) methods are powerful tools for the investigation of surfaces in the nano- and micrometer range. The topography, mechanical properties (stiffness [1], lateral forces [2]) and electronic properties (conductivity, band structure, barrier height) can be investigated. Titanium substrates are used for corrosion protection as well as for medical implants. SXM investigations on these surfaces have already been performed [3-9], but the substrates were often not well defined or modified and the analytical techniques were only partially completed. Table 1 shows the possible methods applied to titanium surfaces so far. Most of these investigations have been performed either with one or two methods only, or without variation of the sample preparation. Some aspects of SXM (especially STM) investigations cannot be found in the pictures, but only in the associated text: several authors report that stable STM conditions could not be achieved under certain circumstances. However, an interpretation was not yet given. A combined STM/AFM-investigation on the same sample position has not yet been published for titanium surfaces. A combination of STM/AFM with other methods (light microscope, infrared light, SEM) has been demonstrated partially, but not systematically. These investigations have shown that the resulting SXM data depend highly on

- the tip material used;

- the voltage applied in STM;

- the analyzing conditions (vacuum, dry nitrogen, wet air);

- the analyzing techniques (initial conditions, prepulsed or linear spectra, scan rate);

- the preparation / modification of the surface;

- the orientation of the investigated grains;

- the thickness of the oxide.

An understanding of these surfaces can only be achieved if all SXM methods are applied together to a well-defined substrate, dependending on all the parameters mentioned above. In addition to these difficulties, some more problems should be mentioned. The absolute z-position of the tip is usually unknown. On metallic surfaces one can safely assume that the tip is not in contact with the surface, but on semiconducting or even insulating oxide surfaces, the tip can be in contact with the surface or it may even penetrate into the oxide. This will result in bad STM conditions. Another problem is the xy position in the micrometer range. If an experiment is conducted on one xy position, then (after retracting the tip) it is very difficult to find the same position again, which is important for microstructured surfaces.

Table 1. Survey of SPM*-3' investigations on titanium surfaces.

Titanium

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