Re: Tip reactions... does Smalley have a point?
From: Jeremy (lord_psi_at_hotmail.com)
Date: 10/31/04
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Date: 31 Oct 2004 05:14:36 GMT
Chris Phoenix <cphoenixNOSPAM@crnano.org> wrote in message
news:<cm0k9r02h2u@enews1.newsguy.com>...
> This is a very thoughtful discussion. It's refreshing to see thoughtful
> inquiry after years of careless skepticism. Thanks!
Hi Chris.
> Handwaves below...
>
> Jeremy wrote:
>
> > John Larkin <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in message
> > news:<cls8um02avd@enews3.newsguy.com>...
> >
> >>On 28 Oct 2004 18:50:25 GMT, lord_psi@hotmail.com (Jeremy) wrote:
> >>>In the process of constructing the fine motion controller, there are
> >>>going to be times when a site is dehydrogenated immediately adjacent
> >>>to a surface bound oxygen atom, or some other reactive moiety.
>
> Hm. Even if you're restricted to adding atoms one at a time to a
> mostly-flat layer, then when trying to build a 1-nm cube (~100 atoms)
> you have still have about 10^42 choices as to the sequence of atom
> deposition. Of course most of these choices won't work. But if one
> does work, and if you can find it, then you're fine. Is it possible
> that the oxygens could generally be deposited last in their layer?
Certainly, but does that avoid the problem? If any of those mobile
atoms are to be adjacent to each other, then you have potential
"hopping" problems. In some cases, the atoms/molecules can only hop
along specific crystal axes, which you might be able to use to
restrict the behavior to something more controllable, but I have no
idea exactly how it would work on various diamond surfaces,
particularly at elevated non-liquid helium temperatures.
> Other options: maybe a small set of tooltips can build a wide variety of
> oxygen-containing molecules that can be deposited in one piece, so you
> never have to have a reactive oxygen floating around.
A not unreasonable solution, and one which keeps the synthetic
chemists gainfully employed. Of course, that limits MNT progress to
the progress of some very complicated synthetic chemistry.
> >>>With sufficient mechanical energy, we
> >>>shouldn't necessarily have to mimic enzymes ability to catalyze the
> >>>reaction by binding to the transition state, but we still need
> >>>stabilizing moieties surround the dedicated reaction tip without
> >>>affecting the reaction. This is not a simple tip.
>
> I agree, stabilizing lots of nearby reactive atoms using a non-bonded
> tip seems quite difficult. Here are a couple of tongue-in-cheek ideas
> from a non-chemist.
>
> 1) Before you insert the reactive molecule for deposition, plunk down a
> ring of some weakly-binding atom like lead. Anything loose and reactive
> on the surface will form weak bonds to the passivation ring. Now inject
> the moiety to be deposited through the center of the ring. Now pull
> away the ring, and if you haven't stirred things up too much, the
> surface will be unchanged.
This resembles my extremely handwavy solution. Instead of surrounding
the reactive site with something on the surface, surround the tip with
a ring of highly electronegative (or charged, or whatever) atoms
several angstroms beyond reaction distance, set slightly "above" the
reactive tip. As you bring the tip toward the reaction site, that
ring could apply enough force to shift the oxygen back away from the
reaction site. This avoids a large number of reaction steps, and
could potentially work on any flat surface. The corners and edges
might be more of a problem. One of the problems is, this is a much
more complicated tip, and I don't know if we could synthesize it.
> 2) Pin down reactive floppy atoms temporarily, by oxygen-bonding them to
> something out of the way. Peroxide bonds appear strong enough to
> survive at room temperature, but weak enough that it shouldn't be too
> much trouble to remove the pinning atom when you want to bond something
> to the pinned atom.
Okay, again, that's a lot of work for each surface reaction, but
should certainly be doable. That actually raises a very good
question. I've generally been considering these oxygen atoms as they
behave in isolation, but that isn't really the situation after the
first molecule goes down. The atoms on the surface will interact with
each other, which might increase the kinetic barrier to hole
migration. I need to go back to the STM literature on that one.
> >>>This may already have been addressed, and if so, can I get some
> >>>references? If not... are we arguing past Smalley and his ilk? Yes,
> >>>diamond should work. But can we make an assembler out of only
> >>>diamond?
>
> Actually, it's expected that the answer is yes. At least according to
> Freitas and Merkle. Freitas even put up a chemical formula for his
> assembler architecture: something like C400,000 H200,000 Si30 Sn30 (I'm
> making up the numbers).
I assume this is based on diamondoid mechanics and the weakened link
chemistry at the tips? I don't know if I buy it, but that would
simplify the design rules a lot. Probably at the expense of
capability, like the ability to make or incorporate complex
nanoelectronic systems. An early generation design?
> >>I work with tomographic 3D atom probes, which *disassemble* things one
> >>atom at a time. The samples have to be cooled to numbers like 20-40K,
> >>because every time you remove one atom, the neighbors all want to move
> >>around, and that messes up the structure you're trying to analyze.
>
> But this is great news! If we can prevent rearrangement by cooling to
> 20-40K (which isn't all *that* cold) then it looks like we're home free.
You want a liquid helium cooled desktop nanofactory? That would be
fun to design. There might be a cheaper way to do it (compressor on a
liquid nitrogen system?). If necessary, then yes, it deals with the
mobility issue, and answers my version of Smalley's argument in an
expensive way. All the technical work I've seen emphasizes very
specifically that these things should work at room temperature.
That's based on the stiffness of the diamond parts. It seems like a
point worth noting if that assertion is incorrect.
> > I've seen lovely
> > models of carbon mechanosynthesis, apparently indicating that at room
> > temperature such shifts aren't a problem when a carbon dimer is placed
> > on a reconstructed surface,
>
> Um, do you mean a depassivated surface? The Freitas models I've seen
> use the 110 surface, unreconstructed.
Sure, depassivated. But unreconstructed, with dangling radicals
across the entire surface? Can you post/send me the specific
references? I'm not sure I've seen that model.
-Jeremy
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