How to make yourself a scanning tunnelling microscope.
Created | Updated Jan 28, 2002
Everyone thinks that scanning tunnelling microscopes are ridiculously complex pieces of equipment. But nothing could be further from the truth. Here is a simple step-by-step theoretical* guide to building your own.
Site Location
While they are not complex, they are sensitive*. So positioning your electron microscope next to the M25 is possibly a bad move*. Even a corridor next to the apparatus will give you large amounts of noise - or even talking.
So, before you begin construction, consider your position carefully.
It'll also help if you plonk the whole thing in a bell jar to keep vibrations out.
The electron 'needle'
The STM*works by having a fine needle as an electrode across a tiny gap to the surface. So you need a very fine needle, right? Wrong. Even at top research universities, the needles are remade every day... not because of accuracy problems, but because it is ridiculously simple.
Building your needle
- Get a piece of electrical wire
Take a single thread of it, so it's about 1mm wide.
Get some pliers
Cut end of wire at 45o
That's it! You've made a needle for your SEM!
Power supply
To get electrons to jump a gap, you usually need a large voltage. In fact, classically*, you shouldn't get any jumping. So most people think you need a couple of teravolts* to get an SEP to work. Nope. In practice about 6 volts are used. Whack this up to a massive 9V and
you can use a standard smoke-alarm battery. Connect one end of this to the needle (the uncut end) and don't connect the other end to anything. But do earth* whatever you are trying to scan.
Measuring something.
To make your SEM work, you actually have to measure something. You get the tip to hover above the surface at a constant current [This means constant distance]. I don't know how you do this yet*, but I'll get it soon. Then you use the movement through a piezo-electric crystal*, to get a different current, which you amplify. Send this to the PC and you are on your way to seeing atoms.
Other important stuff.
Oh, by the way, the whole thing needs to be performed in air, not a vacuum... it makes 'tunnelling'* more likely. And your object needs to be electrically conducting. So you can coat it with silver to get it to work on other materials.
Using it
Once everything is hooked up, program your computer to scan along the sample. Record the information on the PC and marvel at the roughness of polished steel.
The pros
I've been told that DFM have a nice, slightly technical page on it at www.dfm.dtu.dk/spm-cal/descript_SPM-NET.html.
Thanks
Thanks go to Gert for his advice.
