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Researchers at the Aono Atomcraft Project in Japan, using an STM, are now able to extract a single silicon atom from the surface of a silicon crystal and rebond it to the surface at a different location. Atoms translocated in this manner can be re-removed without disarranging the underlying atomic layers. Atoms brought from afar can be used to repair holes in the silicon surface, or they can be used to build structures on top of the surface. [J. Vac. Sci. Technol. B 12(4): 2429-2433, Jul/Aug94] Folks, this looks an awful lot like real nanotechnology.

Pretty exciting stuff!

But if you look closely at the citation, you'll see that it happened a decade ago. So what happened since? Why didn't this lead straight to "real nanotechnology"?

The tool they were using, the STM (scanning tunneling microscope), has a limitation: it can only detect objects that it can send electrons through. And pure silicon crystal is an insulator. Add a few impurities, and it conducts; so they presumably were working on the surface of a "doped" crystal, and were able to make 2D patterns. But I guess they couldn't figure out how to make small conductive 3D silicon structures. Or, maybe, the project's funding ran out before they were able to: it was only funded through 1994.

Any lessons for today? Yes, though they're tentative. One lesson is that a single technique, no matter how impressive, probably won't be enough to get us to the ultimate goal: digitally-controlled nano-building-nano.

Another lesson is that a decade ago, people were already able to do covalent single-atom chemistry with atomic precision. They even had the process automated; their web site shows patterns consisting of many (dozens? hundreds?) displaced atoms. Take a simple system--the surface of a silicon crystal--and you can make intricate patterns with atomic precision by doing simple things to it repeatedly. I found that blurb in a google search, and reading it out of context, I first assumed that it must have been done within the last year. But no, they did it with decade-old computers, tools, and theory.

Now let's go back to that tool limitation. An STM probably can't build 3D silicon shapes. But there's a related tool called an AFM (atomic force microscope) that measures force rather than conductivity. It can "see" silicon just fine. And last year, a silicon atom was removed from a surface and then replaced using an AFM.

Can nanomachine-building nanomachines be made out of silicon? It seems pretty likely. Micron-scale AFMs have already been built using MEMS processes. So does this represent a path to molecular manufacturing, and if so, how close are they? We can't know, because no one in the U.S. has bothered to develop the theory of "stiff nanomachines" in enough detail to compare the possibilities with the experiments.

Chris

Comments

[Michael Vassar]

Silicon doesn't have the variety of forms of carbon, but I would be shocked if there wasn't some possible hybrid technique that would use silicon to manipulate carbon. Prehaps something like forming molds from Silicon and then using plasma deposition of diamond to fill the mold, then cutting out the diamond part, possibly with an x-ray laser or with a diamond knife, or possibly by reacting the non-desired parts with some chemical. Once diamond parts were assembled, they might be brought together mechanically possibly using simple manipulator tools that had been built from silicon, or by self-assembly.
Any idea why the Aono Atomcraft project didn't get funding to continue?

[Michael Vassar]

BTW, their lack of funding is a rebuttle to the current "if only Eric Drexel actually did experiments he would be credible" rhetoric.

[John Michelsen]

I studied these papers in great detail years ago. Conductivity is not a problem. But silicon is too mushy to make nanomachines out of. Diamond is the way to go.

[Chris Phoenix, CRN]

Michael: It's worth pointing out that all we know is that they didn't get _non-secret_ funding. Anyone know if Aono has "stopped publishing"?

John: Do you mean STMs can build 3D silicon shapes after all? And would they still be too mushy at cryogenic temperatures?

Chris

[John Michelsen]

With great effort, you could probably do precise 2D patterning in silicon. For 3D you need a material that won't just melt at the slightest touch like silicon and most other materials will. Temperature does nothing to make bonds stronger.

[Brett Bellmore]

No, but cryogenic temperatures CAN deny a surface the activation energy necessary to reconstruct, provided that simply depositing an atom doesn't provide the energy.

Another candidate material would be cubic boron nitride, which is strongly covalently bonded, and apparently resists reconstruction as much as, perhaps more than, diamond. The downside, of course, is that the chemistry is at least twice as complex, since it's a compound, not a single element.

[Chris Phoenix, CRN]

John: I know that temperature doesn't make bonds stronger. But can you explain more about the failure modes and conditions?

On boron nitride: part of what makes mechanosynthetic chemistry a pain is avoiding reconstructions. I'd guess (without study) that using two different atoms would reduce the opportunities for reconstruction, and so might make the chemistry easier or more reliable. And so might be worth doing even if you needed more tool tips.

Chris

[Brett Bellmore]

By the way, boron nitride really IS a good analog of carbon; Not only does it form a hard covalent solid, but there are also a whole class of boron nitride fullerenes.

[Michael Vassar]

Boron MNT might slow takeoff due to a lesser abundance of Boron vs Carbon. OTOH, it might be used to immediately build nanocomputers to run high power simulations followed by carbon MNT.