h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Electron Microscopes - A637067
Author: Hell - U171578

This entry is a part of an entry complex on microscopy. It can perfectly stand alone.

I am aware of the University, nevertheless I prefer to submit this to peer review. The links will have to be added after the (potential) inclusion to the guide.

HELL

Ok, this is good stuff, but it concentrates mainly on the SEM. The TEM was the one invented by Ruska, and according to this 'ere textbook, the first dedicated SEM was described by Zworykin et al in 1942. So if anything, the SEM is the more modern type.

I think maybe the thing to do here is to loose the last paragraph, and change the title to 'The Scanning Electron Microscope' or similar. If you want to include TEM as well I think this will require a fair amount of rewriting.

Jamie

Ummm... Ruska invented the common electron microscope in 1931. According to the Nobel foundation. It was a scanning electron microscope.

The invention of the TEM is historically obscure (many scientists claim to have invented it). First so-called transmission electron microscopes existed in the 1950s. I had the impression that at that time people didn't gain much different information from a TEM compared with a SEM. Our insitute has recently bought a TEM. People here made such a fuss about it (I heard someone say: There are only three TEMs in Germany), that I gathered that it must be a very modern kind of apparatus. (I'm gonna ask them folks)

About skipping/rewriting TEM: Hmm... On the other hand the method is not really different from SEM except the electrons are a little 'faster' and pierce through the material, everything else is more or less the same. So... There's not much to say about TEMs really. Why not leave it in here?

HELL

Ok, I've double-checked this. Yes, Ruska did invent the 'common' electron microscope, but this was a TEM. Have a look at http://www.unl.edu/CMRAcfem/em.htm , for example (I just did a quick Google for electron microscopy history). The 'proper' SEM was not invented till about 1942, as I said earlier (although the concept was apparently first mooted around 1935). Things are complicated a bit because the first STEM was constructed in 1938, according to the book I happen to have on my desk (Scanning Electron Microscopy and Microanalysis by Goldstein et al, published by Plenum Press. Which reminds me, I must get it back to the library...).

I am pretty sure that there are more than three TEMs in Germany Might I ask what institute you are at? It may be that there are only three of a particular type of microscope in Germany.

For example, some people in my group (http://www.ssp.gla.ac.uk/) have collaborated with the University of Regensburg in the past, and I know they have at least one TEM. In fact, I had a look around their website and eventually (took a while, I don't speak German. Or read it, either) found http://www.physik.uni-regensburg.de/forschung/zweck/geraet.htm , which shows the two TEM systems they have (very nice systems they are too).

It may be that there are only 3 300 kV microscopes in Germany, or more likely only 3 400 kV microscopes, as these are pretty rare.

I just wonder if we are working to different definitions here, and this may be causing misunderstandings. Basically, the types of electron microscopes are:

TEM. Basically an analogue of the conventional transmission light microscope. A electron source and series of lenses forms an plane electron wave, that illuminates a sample. A series of lenses after the sample magnifies the image and projects the image onto a phosphor screen, giving you an optical image you can look at. You can also project the image onto film, TV cameras, CCD cameras etc to record the image. Most TEMs work with electrons accelerated by 100 - 300 kV, although some specialist systems can go up to 3 MV. For a 200 kV microscope the sample needs to be less than 100 nm thick (roughly) for it to be electron transparent.

SEM. Again, you have an electron source and pre-specimen lenses. Hovever, in this case the electron beam is foccused down to a small spot (generally as small as possible). This is then scanned across the sample by magnetic coils. (In fact, your standard cathode ray tube based TV does pretty much the same thing, on a larger scale). Where the beam hits the sample, you get more electrons emitted from that spot. These secondary electrons are picked up by some kind of detector, which gives out an amplified signal. This signal governs the brightness of the beam in a TV tube. As the beam in the TV tube is scanned at the same rate (and in the same pattern) as the electron beam is scanned over the sample, you get a picture of the sample surface. As most SEMs tend to work at 30 kV or lower, the electrons do not penetrate very far into the sample.

STEM. Take a TEM, and add scan coils just above the sample. Use the pre-specimen optics to focus the beam down onto a small spot as with the SEM. Scan the beam over the sample. Now you have a SEM, but you also can look at the electrons that pass through the sample as well as the ones that bounce back off it. Generally, the main disadvantage of a STEM is that the sample has to be small (generally a disc about 3 mm in diameter), and thin enough to get the electrons through. In a SEM you are only looking at the electrons reflected from a sample, and so the sample chambers are a lot bigger, and can often take samples several cm in size.

Hmmm, I didn't mean to go on that long, sorry. Anyway, does this lot make sense? I know what I'm talking about (my Ph.D. involved a lot of electron microscopy) but I am not very good at explaining things on paper (my long-suffering supervisor will tell you that)

I'll try to comment on the SPM article in the next few days.

Jamie

Hmmm... 3400kV. That might explain why everybody went so apes about this. I was just wondering, if TEMs are then the 'common' electron microscope why then is everybody in this house so happy that they can finally look INTO stuff with the TEM. They could have bought a cheaper one and get therir research going a lot earlier. (OK with lower resolution, but that would at least keep them off the streets... Oh, FYI the TEM has been BOUGHT but it's not INSTALLED yet - this can take another 2 years... Luckily, I am doing ordinary optical microscopy for a living. That might also explain why I got the explanations for the EMs so messed up.

Maybe if I just leave the 'general' electron microscopy bits in there and leave the rest for someone else to write a full-detailed entry on TEMs and STEMs and SEMs?

What do you think?

HELL

Jamie, would you mind co-authoring this entry?

Have another read it's far more general now, and doesn't mix up SEMs and TEMs.

What do you think?

HELL

Looking better now, however there are a few things that could maybe be improved. Would you mind if I have a go at re-drafting a few bits? What I would do is to take a copy of the text and put it in a separate entry, and then make the changes. Then you could look at my version and incorporate the changes (assuming you agree with them of course) into this version. Would this be ok with you?

Sure. Great idea. The entry can only become better.

Do you think I should remove this from PR for a while? If we're fast we could leave it in, on the other hand this is not a very clean procedure. On yet another hand nobody else noticed, yet... ... Don't tell anyone.

HELL

PS: I think it could be better if you put the co-ordinates for your version of the entry on my space, so not to confuse this thread.

Nah, leave it in. One of the copies has to remain in, so it might as well be this one. I've grabbed a copy of the text, and I'll start on it now.

The new version is in. The skeleton of the entry is the same, passages in the SEM and the TEM sections have been immensely improved (by Jamie).

Now, let's just wait for reviewers.

HELL

You'll be pleased to know that I recommended this entry and it has now gone into the Editorial Process for future inclusion in the Edited Guide. When it does get into the Edited Guide, the editors will email to let you know, but please bear in mind it can take a while for entries to go through the process. Please see this for more details.

http://www.bbc.co.uk/dna/h2g2/alabaster/SubEditors-ProcessSubEditors-Process.


Oh - you know the drill anyway Congratulations.

you might be surprised to hear that an adventurous father/son combination built a working electron microscope in their garage a few years ago for only $200. not bad eh?

One of my lab-pals is planning to build a single molecule detecting fluorescence microscope for under $2000 next year.

H