A comparison of scanning electron microscopes (SEM) and transmission electron microscopes (TEM), with Dr Richard Tilley of Victoria University of Wellington using a TEM.
DR RICHARD TILLEY
The microscope that we use is actually quite a special microscope, so it’s an electron microscope. When you have a normal optical microscope, it’s called optical because you are using light, but with an electron microscope, instead of having a ray of light, or a beam of light come down your microscope, you actually have a beam of electrons. And because we have a beam of electrons, we can actually use our electron microscope to see things on a very, very small scale – far smaller than if you use an optical microscope. And we can actually see atoms with our electron microscope – that is really what we are trying to do. There are several different types of electron microscope – some of them are designed for more biological purposes, and some of them are designed for more materials purposes. We have two microscopes at Victoria University as part of the MacDiarmid Institute. So one of the microscopes is a scanning electron microscope, and a scanning electron microscope has a beam of electrons that comes along and scans along the surface of the material, and by detecting the electrons coming off the surface, we can build up an image of what the surface looks like. The resolution of a scanning electron microscope is about five nanometres. The transmission electron microscope is quite different – this is the one where we can actually see atoms. So with the transmission electron microscope, you have your beam of electrons come down, and they actually pass through or transmit through your material, and then you collect your image, or see your image of the electrons that have passed through.
So the difference between the two microscopes is, with the scanning electron microscope, we can only see down to five nanometres, but we can look at any surface. With the transmission electron microscope, we can see down to the atom scale, but we have to have materials which are very, very thin, so the electrons can pass through. So actually you are looking at two quite different samples in the two microscopes.
Stem images of nanoparticles with kind permission of Dr Richard Haverkamp and Springer Science and Business Media (published in Journal of Nanoparticle Research (2007), 9:697-700, Pick your carats: nanoparticles of gold-silver-copper alloy produced in vivo by R.G Haverkamp, A.T Marshall and D. van Agterveld.)