Liz Girvan (Microscopy Otago) talks about the problem of artefacts in the scanning electron microscope (SEM). Artefacts look like part of the microscope sample but are actually a side-effect of sample preparation or the conditions in the microscope. It’s easy to be fooled into thinking that artefacts are part of your sample. Liz’s advice: know your sample well so you can spot an artefact when you see one!
It’s quite important to be very open minded about what you’re going to see. You need to make sure what you’re looking at is actually real. So you can get quite a few artefacts, anything from dust that’s fallen on your sample which can look like a piece of sample. You can get some cracking in the vacuum so you actually get cracks forming in your sample which are not there, they’ve been caused by the microscope. Sometimes the coating can cause artefacts. If you coat too thickly, you can start to see the metal particles, and of course, if you’re looking for small particles, they can look just like your sample might look like.
Charging artefact is probably the most common thing we’ll get in the SEM. What that means is the sample isn’t conducting as well as it should be, so we get the electrons building up within the sample forming a little charge. A charging artefact can look really, really different in different samples. So this is a carbon fibre sample that is charging quite badly. All these black and white areas are artefacts – they’re not actually there, but they create quite a nice cool colourful image. So what you can get is smears across your image, you can get lines, you can get black halos, white halos, all kinds of things. Usually the way to tell if something’s an artefact is just from experience, so spending a lot of time looking at a whole lot of different samples, you’ll see these things continually appearing.