1. Tip Effects


1.2 Dirty or Contaminated Tips

If the tip is dirty or contaminated (often with parts of the sample sticking to it), you can get strange repeating shapes in the image. Sometimes it is obvious that this has occurred, but sometimes it is not. In the example below there are two images of the same sample, one taken with a clean tip, and one with a badly contaminated tip.
Using a sample you already know can help you diagnose this problem. There are a number of tip-characterisation samples available, a list of them can be found in the AFM references and standards page.

AFM images shwoing dirty tip

In the example above, the image on the right was scanned using a very badly contaminated tip, the one on the left with a
clean tip, but the sample is the same!

This shows the drastic effects than can occur when the tip is dirty.

How to avoid it
Change the tip. Once a tip is broken, you must change it. If it is contamination, you might be able to clean it. But most often, even in this case, you will have to change to a new tip.

Note that the image above is of a sample of BOPP film (Biaxially Oriented PolyPropylene). The use of this sample in tip characterization, and even to clean the tip has been discussed by Nie et al in:
H.-Y. Nie, M. J. Walzak, and N. S. Mcintyre, "Use of biaxially oriented polypropylene film for evaluating and cleaning contaminated atomic force microscopy probe tips:
an application to blind reconstruction," Review of Scientific Instruments, vol. 73, pp. 3831-3836, 2002.

However, you could use any well-known sample to characterise the state of your tip. The advantage of BOPP is that it is also possible to clean the tip by performing indentation into the sample. In principle you could use
other samples for this. It's worth remmbering, however that you are very unlikely to be able to return to the pristine state of a new tip, by any cleaning method.