STM of finegrain titanium

In order to probe the electronic conductivity, STM experiments were performed on these surfaces. A result is shown in Fig. 2. It is not possible to get a clear picture of the surface, the grain and the grain boundaries. The feedback system regulates the piezos to maintain a setpoint of InA (resulting in scratch-lines all over the picture, where the hard W tip is moved into the softer TiCh this is indicated by the unreal z range, which is 10 times higher than in the AFM pictures). It does not matter whether the bias is -5 V or +5 V, or even larger. Smaller values than 2 V (positive and negative) results in totally instable conditions (the piezos are extended to their maximum towards the surface, indicating that it is impossible to get a current of 1 nA or -1 nA, respectively). If the bias is -3 V (-1 nA setpoint), then pictures can be recorded. However, no clear structures can be seen and the pictures are irreproducible. Some regions in these pictures seem to be reproducible, and obviously in these areas the feedback system can maintain the current setpoint. This indicates that the electronic conductivity of anodically grown Ti02 films is not sufficient for STM experiments if they are performed in dry nitrogen. However, some grains have a larger number of defect states (over which the current arises), resulting in poor, but possible, STM conditions. If the experiments were performed directly after the oxide film

Fig.2. Example of an irreproducible STM picture of fine-grain titanium covered with 20nm Ti02, xy range: 75 pm, -3 V bias, -1 nA setpoint, zmax = 643 nm, scan rate 100 pm/s, W tip, normal air. Horizontal white or black lines between homogeneous areas indicate feedback irregularities.

was prepared electrochemically (without drying the surface in nitrogen), then it was possible to get pictures in some areas (not all), with the same structures as seen in the AFM picture (Fig. 1(d)). It was not possible to make large scans on these surfaces without tip crashes.

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