Make Yourself a Scanning Tunnelling Microscope Content from the guide to life, the universe and everything

Make Yourself a Scanning Tunnelling Microscope

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A scanning tunnelling microscope, or STM, is a device that allows us to see individual atoms. It's generally thought that they must be ridiculously complex pieces of equipment, but nothing could be further from the truth. Here is a simple step-by-step guide to building your own.

Site Location

Before you begin construction, consider your position carefully. While they're not complex, STMs are sensitive so positioning your electron microscope next to a motorway is possibly a bad move. Even a corridor next to the apparatus will give you large amounts of noise, and talking should be kept to a minimum. 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 using a fine needle as an electrode, placed a tiny distance from any surface. It seems obvious that you'll need a very fine needle; but what's obvious is not always true. Even at top research universities, the needles are remade every day; not because of accuracy problems, but because it's a ridiculously simple process.

Building Your Needle

  1. Get a piece of electrical wire.

  2. Take a single thread of it, so it's about 1mm wide.

  3. Get some pliers.

  4. Cut end of wire at a 45° angle.

That's it - you've made a needle for your STM!

Power Supply

To get electrons to jump a gap, you usually need to use a high voltage. In fact, classically1, you shouldn't get any jumping. Most people think you need a couple of tera-volts2 to get an STM to work, whereas in practice about 6 volts are used. Whack this up to a massive 9V3and connect one end of the power supply to the needle (the uncut end) and don't connect the other end to anything. But do earth4 whatever you are trying to scan.

Looking at Something Very Small

To make your STM work, you actually have to give it something to scan and tunnel. Get the tip to hover above the surface you want to scan so that a constant current passes through it; this ensures it's always a constant distance from the sample. This involves using a piezo-electric crystal5 to produce very fine movements in the needle. Send this to a PC and you are on your way to seeing atoms.

Other Essentials

The whole thing needs to be performed in air, not in a vacuum... it makes tunnelling6 more likely. Also, your object needs to be electrically conducting and to do this you can coat an object with silver if it isn't electrically conducting on its own.

Using Your New Toy

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 Professionals

DFM have a nice if somewhat technical page on Scanning Probe Microscopes - the family of microscopes to which the STM belongs.

1Classical science is what physicists knew in the 18th Century and what you learned in school.2A tera-volt is 1012V.3For this you can use a standard smoke-alarm battery4That is, connect it to the electrical earth, sometimes known as 'ground'.5A piezo-electric crystal changes shape when you run voltage across it, and produces a voltage difference when you change its shape.6Tunnelling is when an electron crosses the needle-sample gap.

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