30-05-2012, 04:25 PM
Productive nanosystems: the physics of molecular fabrication
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Early and advanced productive nanosystems
Biological systems provide not only examples,
but also tools to use and models to emulate.
The attention given to advanced systems must
neither obscure howfar we are from implementing
them nor discourage investigation of early steps.
Practical work must begin with what we can make
today. Not surprisingly, many proposals for early
development goals centre on exploiting natural
productive nanosystems to build next-generation
systems. Each generation could make a wider
range of materials and structures, moving further
and further beyond biological models. Recently
developed capabilities in the design of novel
protein and nucleic acid structures—including
simple molecular machines—have opened the
door to a path of progressive improvement. The
discussion here, however, will focus on where this
path can lead as it moves closer to fundamental
physical limits.
Quantum and classical phenomena
We live in a quantum mechanical world: the
properties of matter on all scales are consequences
of quantum mechanics. Nonetheless, many
macroscopic systems can be described with
good accuracy by classical models, which are
often more intuitive and tractable.
Classical continuum scaling laws
Models of macroscopic machines ignore not only
quantum mechanics but also atoms. This level
of analysis is surprisingly useful at the nanometre
scale, though only as a rough guide to expectations
rather than as a basis for detailed investigation. In
particular, continuum models yield scaling laws
that suggest that nanoscale machinery can have
remarkable performance.
Classical atomistic models
Although classical continuum models offer only
a rough guide, atomistic classical models have a
well-established role in analysing nanoscale systems.
These models have their roots in the Born–
Oppenheimer approximation, which treats electronic
degrees of freedom as responding instantaneously
to changes in nuclear positions. The result
is a potential function that determines the dynamics
of nuclear motion, which then is often treated
classically.