Nanotech Scenario Series
Notes on the Theory of Molecular Manufacturing
Compiled by Chris Phoenix, Director of Research
Center for Responsible Nanotechnology
This page provides further in-depth information to
EPA panel presentation.
Fact: Mechanical systems can do precisely positioned, covalent chemistry in
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
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
(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,
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
Nanosystems Section 6.4, and diamond surface reconstruction in
Theory: Mechanical chemistry can be extremely reliable, with extremely high
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
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
- 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
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
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
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
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
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
Theory: The range of hardware could include a system capable of copying its
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
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
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.