Reconstructing the future
Last week, I talked about the mind blowing impossibility of making sub-atomic devices. It’s a lot like trying to make a fruit salad using only half an apple- it doesn’t even make sense to think about it.
But I want to talk about what we can do, and what the constraints are. What are the rules of the game? Well, just over 20 years ago, Don Eigler managed to position individual atoms to spell out the letters IBM. It’s pretty amazing that we can mechanically manipulate the smallest building blocks of matter, but it doesn’t mean we’ve mastered the nano-scale.
We can’t just build whatever we want atom-by-atom. Atomic manipulation is a painstaking process, but even if you have unlimited time and patience, there are still limits to what you can do. What happens is that atoms tend to move about of their own accord, they can drift across a surface or stick together in ways you don’t want.
For a nano-technologist, this sort of thing could be a massive problem. But there’s another. Say you get every atom exactly where you want- you find a way to do that, getting around the problems above.
If you have a flat atomic surface; the flattest surface possible with no lumps or bumps even at an atomic level, atoms can spontaneously rearrange themselves. Why? Well normally they like to stick together in certain regularly repeating patterns, sticking to a preferred number of neighbouring atoms. At a solid surface, there’s a break in continuity. Coming up through the material, the regular pattern suddenly stops, so the surface atoms are no longer bound to the ideal number of atoms.
Effectively, in the solid, each atom is held in place by all the ones around it. At the surface, where the regular pattern is broken, the atoms can rearrange into what you could think of as a more comfortable arrangement.
Silicon does this. It forms a complex pattern in the top few layers of atoms in a way that baffled surface scientists for years. You can think of it as a problem for those wanting to build mechanical devices at the nanometre scale, but I prefer look at this as something absolutely fascinating that nature just does.
This is the silicon 111 7×7 reconstruction. 111 refers to the alignment of the plane you cut through the silicon crystal to create a surface, while the 7×7 tells you the size of the unit cell- blocks of atoms making up the new repeating pattern. This reconstruction happens in vacuum, when the surface is perfectly flat and clean. The video was produced by Yan Liang at the Dept. of Chemical Engineering and Materials at the University of Minnesota. The text’s a bit small, so you might want to play in full screen mode.Atomic manipulation, Explaining nano comment below, or link to this permanent URL from your own site.