Fact: Mechanical systems can do precisely positioned, covalent chemistry in
vacuum.
Several different covalent reactions have been demonstrated. Iron has been
bonded to carbon monoxide. A single silicon atom has been removed from a silicon
crystal surface and then put back in the same place. Richard Terra has written
a good overview
of work circa 1999. Such work demonstrates that precise positional chemistry
can be achieved for a variety of reactions.
Theory: Nanoscale mechanical systems can do the
same.
Most scanning probe microscopes use large piezoelectric ceramic actuators.
MEMS microscopes have been built using electrostatic actuators. Nanoscale
mechanical elements should also be able to function as scanning probes. Stiffness
in the face of thermal noise is a significant engineering issue, but calculations indicate
that diamond-like materials should be suitable even at room temperature. A
variety of actuation methods should be feasible, including stepping drives
controlled by a small number of relatively large actuators. Nanoscale machinery
should be able to construct probes with six or more degrees of freedom.
Theory: A small set of reactions can construct
3D covalent solids, a few atoms at a time, from simple feedstock of small
molecules.
The basic theory is developed in Nanosystems (Drexler,
1992). Several simulations of carbon depositions have been done such as (Merkle
and Freitas, 2003). For covalent surfaces that are not prone to reconstruction
at suitable temperatures (which should include at least some diamond surfaces),
the same deposition reaction should work at numerous points on the surface.
A small, fixed number of reactions should be sufficient to build a variety
of parts with a large number of atoms.
Theory: Such 3D covalent solids can implement
nanoscale mechanical systems.
Just as a rapid prototyping system can deposit dots or beads of material in
layers to build 3D shapes, a mechanochemical system depositing a few atoms
at a time should be able to build up a covalent surface to create 3D shapes.
Different reactions may be required for edges, corners, curves, valleys, and
so on. It is not yet known what atomic-scale features will be easily accessible
to mechanochemistry, but it seems unlikely that building pixellated shapes
will turn out to be impossible. Useful questions are: How small can the pixels
be? What other features (e.g. single-bond bearings, springs, hinges) can be
achieved with a compact and reliable chemistry? How compactly can general-purpose
NEMS be manufactured with this technology?
Friction and efficiency in stiffly-built NEMS are questions of particular
interest. Smooth, stiff surfaces should be able to slide past each other with
low friction at reasonable speeds, because there will not be many mechanisms
to transfer energy or force between the surfaces. If the atoms are spaced
differently on each surface, energy barriers to motion should also be low,
and may be made low enough that thermal noise can cause the machinery to "float" between
states as biomolecules do. See "A
Proof About Molecular Bearings" for discussion of such surfaces. Nested
carbon nanotubes have been observed to experience extremely low friction,
e.g. (Zettl,
2000). Given the range of conditions under which they can be grown,
it is likely that buckytubes could be fabricated by carbon mechanochemistry.
Fact: Ordinary covalent chemistry is digital: the bond is either there, or not.
In a covalent bond, electrons are shared between two (occasionally more) atoms.
This requires a close association, and in general, there's no such thing as
half a covalent bond. Many covalent bonds are quite strong: thermal noise at
room temperature would need billions of years to break them. Although there
are some covalent molecules with strained bonds, many molecules—including
useful three-dimensional shapes—do not put significant strain on any bond.
Bond stability and thermochemical damage are discussed in detail in Nanosystems Section
6.4, and diamond surface reconstruction in Section 8.6.3.
Theory: Mechanical chemistry can be extremely reliable,
with extremely high yields.
Although many conditions can be found in which mechanically guided chemistry
will produce unreliable results, other conditions should produce reliable
reactions. Mechanical chemistry proposals involve stiff covalent surfaces
and high vacuum. Many critics of the concept are incorrectly extending knowledge
about chemistry in solvents or with floppy molecules (i.e. biochemistry) to
a very different domain. In the absence of anything like a complete study
of useful (e.g. diamond-forming) reactions, we must depend on theory.
There are three issues to consider: How fast will the reaction happen (energy
barrier)? How often will it go to completion (equilibrium constant)? Will
other reactions happen instead (side reactions)? These issues are considered
in detail in Chapter 8 of Nanosystems;
the summary below is extremely incomplete.
If a sharp reactive "tip" is pushed hard enough into a receptive surface,
energy barriers can be reduced to zero. Barriers less than 33 zJ (4.7 kcal/mol)
allow physically constrained exoergic reactions to equilibrate in 0.1 microsecond
at room temperature.
With suitable choice of tip atoms, it should be possible to obtain energy
differences between unreacted and reacted states, 145 zJ or 21 kcal/mol,
corresponding to equilibrium constants >1015: this is less
than the difference in bond energy between carbon and silicon. For some
reactions, the physical trajectory of the tip can be adjusted to break
bonds by shear or torque. Missed reactions can also be sensed sterically
and retried.
In vacuum with full positional control, most side reactions can be avoided.
Reconstructions of the tip must be avoided by design, but the design space
is huge. Surface reconstructions must be considered for each material being
built; diamond surface reconstruction is considered in Section 8.6.3. Positional
error is an engineering problem; a stiffness of 20 N/m allows reliable (10-15 error
rate) avoidance of side reactions that would be accessible with only 1.35
angstroms of jitter. Reconstruction of the reaction complex will usually
require the breaking of one or more covalent bonds, which will not be energetically
feasible if the bonds are more or less unstrained. Since applied mechanical
and bond forces will be distributed among multiple bonds away from the reaction
site, only the atoms closest to the reaction site need to be considered
in designing the reaction.
Of course, this does not absolutely prove that a sufficient set of diamond-building
reactions can be found. But given the huge number of options for tool tip
design and the stability of diamond surfaces at room temperature, it is likely
that at least a basic diamond-building capability can be designed.
Theory: An extremely reliable and repeatable manufacturing
system can be based on positional mechanical chemistry.
With the ability to select a sequence of reactions from a predesigned set
and specify a position for each reaction, large and complicated covalent shapes
could be built a few atoms at a time. The expected reliability rates would
allow billion-atom structures (~200-nm cube of diamond) to be built with low
probability of even a single error. Scaling laws and reaction rates suggest
that a billion-atom structure could be built by a single, relatively slow
mechanochemical manipulator in a few hours.
Most products would consist of multiple pieces. Pieces could be fabricated
separately and then assembled by direct manipulation. The shape of a stiff
piece could be calculated within a fraction of an atomic diameter, allowing
gripping without feedback. The soft nature of atomic electron clouds would
reduce the need for precise alignment. Van der Waals forces have traditionally
been viewed as a challenge, but may also be beneficial in reducing the mechanical
complexity required of grippers. (Note that the grippers are applied to large
molecules, not individual atoms.)
Theory: Such a manufacturing system could be completely
automated.
A molecular manufacturing system would use a small set of simple and well-characterized
operations with extremely low error rates for both fabrication and assembly
of precise parts. A manufacturing program, designed and tested in simulation,
could be expected to work reliably and produce a large number of billion-atom
products without error.
Theory: With good engineering, the advantages of
molecular manufacturing can outweigh its limitations.
Molecular manufacturing will have to compete against other technologies and
methods. As currently understood, it appears to have substantial advantages
over 3D printing, lithography, and biomimetic manufacturing. All these technologies
will take substantial time and effort to develop, but molecular manufacturing
can probably beat the alternatives: its basic capabilities, which might be
developed in a decade, appear better than the advanced capabilities or even
the ultimate limits of competing technologies.
3D printing manufactures parts by depositing or fusing small amounts of material
in a raster-scanned pattern. Current 3D printing is limited to one or a few
materials. Currently, these materials are no better than those available with
other manufacturing technologies, and often worse. Printing whole products
in assembled form would be quite difficult. Rates of scanning or deposition
are slow; a printer might take months to manufacture its own mass of product.
No technology has been proposed that can fabricate 1-nm features, much less
with atomic precision.
Lithography is a proven and valuable technology. The feature size is shrinking
steadily. However, 1-nm features are still decades away, materials are very
limited, and products are essentially two-dimensional. Lithography is also
a very expensive technology, suitable for making only small devices.
Biomimetic manufacturing would build products out of biomolecules such as
protein and DNA, and possibly biosystems such as chemically driven motors.
As Richard
Smalley noted recently, "Biology ... can't make a crystal of silicon,
or steel, ... or virtually any of the key materials on which modern technology
is built." It appears that, although biomimetic engineering can make small
precise devices and machines, their material properties will be sharply
limited by the chemistry involved. Design is another problem: biomimetic
engineering depends on the folding and interaction of solvated linear polymers,
and this is a difficult process to predict or to engineer.
As in biomimetics, the feature size of molecular manufacturing products would
naturally be atomic-scale. The ability to build nanoscale mechanical parts
using nanoscale mechanical systems would allow a fabricator to produce its
own weight in a few hours or less. Carbon lattice—diamond and buckytubes—appears
to be an obvious and relatively easy material to fabricate with this technology.
It is often claimed that molecular manufacturing systems would be inefficient
compared with biological methods. There are two answers to this. The first
is that the products of molecular manufacturing would include extremely useful
devices that biological methods simply cannot build. Some inefficiency in
manufacturing and even in operation would be acceptable. The second, and stronger,
answer is that the efficiency of biology depends on its use of physics phenomena
(such as using thermal noise to cross low energy barriers), not on its use
of particular chemical or material properties. Nanoscale machinery can use
these same phenomena and achieve the same efficiencies.
Fact: Incredibly complex software has been built using reliable flexible digital
operations.
Today's commercial software is approaching the human genome in sheer amount
of information. (A full CD-ROM contains 640 megabytes, and the human genome
contains only 1,500 megabytes.) Such massive programs are built by using a concept
called "levels of abstraction." A piece of functionality is specified, designed,
tested thoroughly, and can then be re-used in a variety of contexts by other
designers who do not have to think about the details of its design.
Theory: We could build incredibly complex hardware
with reliable programmable chemical operations.
Design of mechanical systems can be approached by using levels of abstraction
to divide design issues and hide most of the details. A small set of well-characterized
chemical operations would be recombined to build any part. The shapes built
by molecular manufacturing would be specified directly by the sequence of
operations used to fabricate them. Once a suitable set of reactions was known,
design of new shapes would be straightforward. Likewise, once it was known
how to produce shapes reliably, these shapes could be combined into a variety
of parts. Standardized parts could be combined into a large variety of machines.
Standardized machines could be combined into a large variety of systems, and
so on. At each step, the design would be amenable to direct human understanding,
with simple repetition sufficient to produce large numbers of identically
placed atoms (i.e. a crystal) or large numbers of parallel machines (e.g.
a computer from a few repeated logic elements).
Design at each level would be nearly independent of designs at higher and
lower levels. Competence in each level could be developed in parallel, enabling
rapid increase in engineering ability. Design skills and practices could be
transferred from today's engineering disciplines. Isolating adjacent systems
to allow independent treatment is an engineering problem, not a fundamental
limitation; engineering today deals with effects such as heat and vibration,
and these will be as easy if not easier to deal with on the nanoscale.
With the shape of each part and the deviation caused by thermal noise known
precisely, each step of operation (including compound operations) could be
tested in simulation for reliability and repeatability. Products, as well
as the process to manufacture them, could be verified before they were ever
built. Theory: The range of hardware could include a system capable
of copying its structure.
A mechanical system capable of doing programmed positional chemical operations
could be very small. The manipulator is likely to require only a few degrees
of freedom; a Stewart platform or similar stiff 6DOF manipulator should be
adequate to do a wide range of reactions. Diamond is stiff enough to do room-temperature
diamond-building mechanochemistry. With a palette of shapes and parts to choose
from, a six-actuator mechanical system could be built. The "tips"—the
small reactive molecules that do the deposition reactions—may require
some combination of solution chemistry and mechanochemistry, but the rest
of the structure should be buildable entirely with mechanochemistry. A billion
atoms and a few hundred nanometers should be sufficient to encompass a simple
mechanochemical fabrication system.
Fact: Rapid prototyping and automated assembly are already valuable technologies.
Automated assembly is widely used today to save labor costs in factories, in
some cases doing jobs that would be impossible for humans. With reliable part
shapes, placement operations can be extremely reliable. 3D printing is still
developing, but has already found use in rapid prototyping and custom-built
toys; services are available on the Internet.
Theory: Automated production of molecular machine
parts from straightforward design appears possible.
In a mechanochemical system building covalent solids, atoms will stay where
they are placed. This means that the shape of the part can be predicted directly
from the sequence of assembly operations. If the chemical operations are as
reliable as expected, large numbers of billion-atom systems can be produced
without a single error and without complicated error detection or correction
mechanisms. A system that can exactly duplicate its structure in a few hours
and makes errors only rarely does not need to be repaired; it can simply be
thrown away after the first error.
Theory: Systems and products, including macroscopic
products, can be produced from arrays of nanoscale chemical fabricators
and larger assembly robotics.
As discussed in this
paper, molecular manufacturing can be the basis for manufacturing large
products. Given a mechanochemical fabricator (just the manipulator, not
the computer or power source) that fits within 200 nm and can undertake
all the necessary diamond-forming operations, it appears fairly straightforward
to combine large numbers of such fabricators with control and power supplies
into a nanofactory. The billion-atom products of the fabricators can be
fastened together into much larger products. A nanofactory should be able
to fabricate another nanofactory. Design of the nanofactory and products
should be straightforward.