Chemical Bond: Selective

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Chemical Bond: Selective
Chemical Bond: Selective

Selective Lawnmower

Filing around on a molecule overwhelms any precision mechanic - it usually results in more botch than anything else. New tools should help.


Oh, from all the odds and ends in the chemical construction kit such beautiful things could be put together! Theoretically, it would be quite simple – just pluck an atom from a molecule here and there, stick another here and there – and a drug is already tailor-made from a toxin. Or anything else useful, the possibilities would be endless.

In practice, nobody is anywhere near that far: manipulating atoms in a targeted manner simply fails because the fragile individual molecular parts crumble en masse under the thick fingers of the would-be engineers instead of obeying - the manipulation attempts are too clumsy, that too clumsy sensitivity of the instruments. Touching an average-sized, i.e. tiny, molecule in the right place in order to cut off an atom on the other side is a job for a scalpel, not a sledgehammer.

It has long been clear what such a scalpel for atomic modeling of molecules could be: a laser, i.e. a sharply bundled beam of photons. You could simply use it to hold onto the molecule to be modified – or preferably onto the bond between the molecule and an interfering atom – and pull the trigger. It's just a shame that molecules are "pretty uncooperative," as Yale University chemist John Tully comments. Unfortunately, nobody can target the highly mobile target precisely enough: Bonds in molecules hardly resemble the rigid lines between "C" and "H" in a chemistry book, but more like a moving tangle of confusedly darting electrons that are dragged back and forth between two atoms form a diffuse charge cloud of putty.

Bombarded molecules are uncooperative

(John Tully) If the vaguely targeted beam of photons hits the molecule, the radiated energy is usually simply distributed over the entire system of the ordered, vibrating atom-electron mixture, which looks so simple in chemistry books and is called a molecule. The scattered excess energy then usually does what simple heating would have done: not the targeted, but simply the weakest of the bonds in the molecule breaks. Which really could have been easier.

It seems more promising not to aim more precisely, but to use the right laser ammunition. And this is where Philip Cohen of the University of Minnesota and his colleagues have come a long way. For some time now, they have been practicing targeted laser bombardment on a bond that is becoming increasingly important industrially – that between silicon and hydrogen. Si-H is found plentifully in the manufacturing plants of the computer chip and solar panel industries, where silicon surfaces are protected from oxidation by passivation with hydrogen. The protective layer "H" on the surface "Si" then interferes at the latest as soon as another layer of silicon is to be applied - and the hydrogen has to go again. Cohen and Co elegantly show how this works and could also work with completely different bindings.

Your not entirely new approach: bonding is never the same as bonding in a molecule. Rather, two atoms move electrons towards and away from each other at a typical frequency, depending on their electrochemical properties between their connecting cement mass. However, Cohen and colleagues excited this characteristic bonding frequency of Si-H very precisely with a free-electron laser that radiates infrared at a suitable resonance frequency. Molecular bonds vibrating at other frequencies, on the other hand, should hardly be able to absorb the narrow-frequency laser light - which is why the diffuse distribution of the energy over the entire molecule should be omitted.

Indeed: Wherever the laser shone, it specifically only broke the bond between hydrogen and silicon. In a control experiment, the researchers also passivated more than four-fifths of their silicon surface with heavy deuterium ("D") in addition to hydrogen. However, the frequency-precise laser differentiated very precisely between Si-H and Si-D and evaporated hardly any deuterium from the silicon surface: Only less than five percent of the gases released by the laser contained heavy hydrogen.

Even an industrial application of their technology may not be far away, the authors are pleased - and this means that even mass products such as solar cells and computer chips may soon be produced much more efficiently. After all, the molecular manipulators have not only come closer to a precise scalpel for atomic shaving - their lasers whizzing around on silicon even resemble a selective lawn mower that plucks hydrogen stalks from silicon grass and leaves the very similar deuterium stalks in place. A device like this eliminates the need for accurate aiming - you can just have it done.

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