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.



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.

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Published: 11 August 2009

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1. Tip Effects

1.4 Double/multiple tips

if the tip is broken, or contaminated, you can often get a "double tip" effect, where both the tip and the contamination are scanning the surface.
See the example below.



In the example above, each DNA molecule has a "twin", caused by the double tip.

How to avoid it
You can check that this is occurring with a tip characterisation sample. If it happens, 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.

 

 

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Published: 11 August 2009

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1. Tip Artifacts

1.3 Blunt Tip

This artifact is common after scanning for a long time with the same tip. Tips may last very long, or sometimes go blunt quickly. Some samples (known as "tip-eaters") have a tendency to lead to blunt tips very quickly.
In order to find out if your tip is blunt, scan a well-known sample like this one. See the list of tip-characterisation samples.

In the example above, the image on the right is made with a blunt tip, the left with a sharp (new) one.

How to avoid it
Use your tips carefully! If it gets blunt, replace it! In general, scanning with a lower applied force (see set-point) helps to reduce tip wear.

Some low-wear tips which can scan for longer before becoming blunt are also available.

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Published: 11 August 2009

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2. Scanner Artifacts

 

2.1 Piezo Creep

Piezo creep occurs because when you apply a set voltage to the piezo(scanner), and then try to maintain it, in order to move to a certain location, the piezo tends to continue to move in the same direction for a certain period of time. Essentially, this means that when you start a new scan, you will get some stretching or compression of features at the beginnning of the image, and then the image will start to appear more normal over time.
The effect will be particularly pronounced if you move to a new place within your scan range.
See the example below.



In the example above, the image is distorted at the top due to scanner creep.

How to avoid it
The simplest thing to do is just wait until it disappears, and start scanning again. Even better, set the AFM to scan one line continuously. Select the first line of the area you wish to scan, and wait until the image stabilises (i.e. that line is always the same), then start scanning the image properly.
AFMs with linearised scanner do not suffer from this effect. many modern AFMs have such scan linearising features. More details about his can be found in the book "Atomic Force Microscopy".
Note that distorted features all over your image, not just at the start, are due to something else, maybe sample drift.

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Published: 11 August 2009

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