Gina Miller, <firstname.lastname@example.org>, writes regarding nano-assembly:
> That's the molecular building blocks approach. Many people are considering
> this as a first step towards mature molecular manufacturing and for some
> purposes molecular building blocks may be all that we need. But if you want
> to build very strong diamondoid materials you need to be able to put every
> atom in it's place. It's a first step, it may be sufficient for some
> purposes, but it won't give you diamondoid.
Drexler's work does seem to be based on using small molecular units for
assembly. Nanosystems in section 8.6.4 describes several methods for
diamond construction, depending on what crystal face you are working on.
The first, for adding to what is called the 111 surface, consists of
(1) removing a hydrogen, (2) adding a unit consisting of two carbons
and a hydrogen, (3) adding a hydrogen to the carbons just added, and (4)
removing two hydrogens farther along. This keeps the face of the carbon
terminated with hydrogen atoms to preserve the necessary crystalline
structure. They also a one at a time carbon atom addition to the 110
crystal surface, and a pairwise carbon atom addition to the 100 face.
In 8.6.5 he discusses building with even larger units, Cumulene strands,
which are one-dimensional strings of pure carbon, or hexagonal strands
like tightly rolled tubes of graphite, can be laid down parallel to
produce diamond structures.
I think the bottom line is that the particular tools and units that are
used will depend on the mechanical and chemical constraints of what is
to be built. It may turn out that you add some atoms in a unit, and
then remove some of what you just added. In the 111 diamond synthesis,
it is necessary to keep putting on and taking off hydrogen atoms to
prevent incorrect bonds from forming as the carbon loops back on itself.
The best synthesis approach for a given structure will depend on messy
details of chemistry.
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