Once the tip has properly engaged the surface and conditions have been optimized, imaging can then begin. Getting features which remain in each frame are considered part of the sample surface. Some of those features might be a result of a poor tip, damged during scanning or contaminated by the sample.
Why is identifying a tip artifact so important? First, one doesn't want to attribute a feature as being unique to the sample when in reality it is part of the tip. Second, the quality of the tip affects the quality of the image. As an example, the occurance of a double tip can limit the vertical resolution - it can be similar to having a blunted tip.
What one looks for as a sign of tip artifacts is the repetition of topographic features. If some patterns show up in the same shape and orientation then they are probably due to a bad tip. Debris on the surface is a good indicator of tip conditions since these are often spherical in shape - images of these should therefore also be nearly spherical.
Changing the scan direction electronically will have no effect on the affected image - to verify that these features are due to the tip one must physically rotate the sample relative to the tip and see if the artifacts have also rotated. Even this procedure will not always work since the sample topography might not be the same after rotation, such as with fibrous material.
The following images are examples of artifacts I've encountered. Spotting the problems becomes more difficult as one goes down the list. Click on the small picture to get a full view of the images mentioned, then the BACK key on your browser to restore this page.
|We start off with an example of a good AFM image of 300 nm polystyrene spheres.....|
|followed by similar spheres imaged with a supposedly sharp tip.|
|This image should only contain images of large polystyrene spheres.|
|Classic example of tip artifact, showing DNA and debris.|
|Larger image in the same style as the previous one. This artifact from an STM image of a gold surface became more evident once the original was flattened and the colour changed. Note the scale - this is a 1 x 1 micron scan!|
|As with some art, I can't make this one out. It is supposed to be an image of one of the thin film standards.|
|So, if the tip is bad, does that mean that if can't be used? The answer is - it depends. Here is an image with a double tip, but the tip and sample were still good enough to obtain atomic resolution of the flatter regions.|
|Here is a cute example of some serious artifacts. The obtained image looks like the three bears, made more evident when a freaky colour table is used.|
|Here is a complex image of a polymer structure. The white portions indicate the polymer and the black are the substrate. Take a good look at the blue regions, which are at a height between the other two.|
|Here is an artifact which might not be too easy to spot - both left and right handed filaments were found, but there was something odd.|
Last revised on 01-07-1999 by Peter Markiewicz