Chris Phoenix is providing live blog coverage for us on all the presentations from an important conference on Productive Nanosystems: Launching the Technology Roadmap...
Next talk: "Single-Atom Manipulation and the Chemistry of Mechanosynthesis" by Damian G. Allis, Research Fellow, ICPRFP; Senior Scientist, Nanorex; and Theorist in Residence, Syracuse University
This should be a very interesting talk, because it's about the kind of reaction that'll be used to build diamondoid structures. He starts by talking about what nanotech used to be--atomic precision mechanical chemistry--before the nanoscale researchers started taking it into the realm of imprecise constructions.
Mechanosynthesis is: Positional control of reactants, control of orientation, asymmetric reactants (e.g. putting a small molecule at a chosen location on a surface), control of environmental conditions. Goal is programmable control of assembly processes, making complex covalent structures that may be inaccessible to ordinary chemistry. [Without mechanical input, it's hard to select between chemically similar reaction sites. Also, mechanical force can create conditions that would be really extreme in ordinary chemistry, such as very high pressure.]
Chemists do their work by changing the electronic properties of atoms within a molecule: internal control, which lets them select reaction sites [with difficulty]. Mechanosynthesis selects locations mechanically and directly.
Supramolecular synthesis: molecular building blocks. Instead of targeting between adjacent atoms, it may be easier to build slightly lumpier (but still precise) constructions out of medium-small molecules, and then only have to select between molecules.
He's showing a 222-carbon graphite sheet--this has actually been synthesized--and talking about how hard it would be to target a particular atom by chemistry, and how much easier by mechanical selection.
Now he's showing complex diamondoid machine parts, and talking about how we hope to figure out synthetic pathways to build them. [Chemists would not be able to build such things.] Is there any evidence we can build such things? Yes... scanning probe microscopes have done chemistry. It's primitive, but so was the first transistor.
Tool tip designs: deposit carbon dimers to build diamond. Or even single atoms. (The ultimate level of control of matter.)
There are various levels of precision when simulating atoms. You want at least Hartree-Fock, if not DFT (density functional theory). That takes a lot more computer time. Today's tool tip talk represents the DFT level.
Designing tool tips... If you stick an atom onto an adamantane, then pull it off, you get a dangling bond. But there are other molecules (AL7 and iceene) that rearrange bonds so nothing dangles. (So it'll take less energy to transfer the atom, which is good (at least up to a point)).
The hardest point of designing a tool tip is defect structure analysis. You have to figure out every way it can rearrange that you don't want. Find the transition states, so you can analyze how likely it is to happen. If it's not going to fall apart, then you have to look at the tooltip-workspace transfer energies. Finally, you do molecular dynamics simulations, to find the mechanical properties of the operation.
... Sorry, but he's talking very fast about things I can't quite follow. I'm not sure whether "hydrogen abstraction" is a good thing or a bad thing at the moment, and what energy states are being analyzed. But overall, he's talking about how to analyze whether the structure will fall apart in certain ways. Even if the defect state is lower energy, the transition state may be high-energy enough that it's hard to get there, so the tool tip will be stable [actually, metastable].
... Something about depositing atoms onto a workpiece at edges and corners, not just into the middle of a surface... Something about transferring atoms between tooltips... He's been running out of time and has been talking even faster for the last five minutes.
Question, something about how the defect structures are found. There's no formal way to generate them; use either chemical intuition, or shake them up (in simulation) and see how they fall out.
Drexler says: his intuition is that there won't be practical applications for these kinds of reactions, in vacuum, for several (tech) generations in the future. But the same kind of analysis can be applied to peptide (protein) bonds in water. Damian agrees. [The Nanofactory Collaboration would probably disagree - they want to develop nanofactory-level technology by direct early use of this kind of mechanosynthesis.]
Chris Phoenix
Tags: nanotechnology nanotech nano science technology ethics weblog blog
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