It's a Small, Small, Small, Small
World
Source: MIT Technology Review
Ralph C. Merkle is a research scientist at Xerox Palo
Alto Research Center (PARC), where he is pursuing research in
computational nanotechnology. He chaired the Fourth Foresight
Conference on Molecular Nanotechnology, and will chair the next such
conference, to be held in November. Before concentrating on
nanotechnology, Merkle specialized in cryptography; he is the
co-inventor of public key encryption.
With the tools of the nanotechnology trade becoming
better defined, the ability to create new materials and devices by
placing every atom and molecule in the right place
is moving closer to reality.
The properties of materials depend on how their atoms
are arranged. Rearrange the atoms in coal and you get diamonds.
Rearrange the atoms in soil, water, and air, and you have grass. And
since humans first made stone tools and flint knives, we have been
manipulating atoms in great thundering statistical herds by casting,
milling, grinding, and chipping materials. We rearrange the atoms in
sand, for example, add a
pinch of impurities, and we produce computer chips. We have gotten
better and better at it, and can make more things at lower cost and
with greater precision than ever before.
Even in our most precise work, we move atoms around in
massive heaps and untidy piles--millions or billions of them at a
time. Theoretical analyses make it clear, however, that we should be
able to rearrange atoms and molecules one by one--with every atom in
just the right place--much as we might arrange Lego blocks to create a
model building or simple machine. This technology, often called
nanotechnology or molecular manufacturing, will allow us to make most
products lighter, stronger, smarter, cheaper, cleaner, and more
precise.
The consequences would be great. We could, for starters,
continue the revolution in computer hardware right down to
molecular-sized switches and wires. The ability to build things
molecule by molecule would also let us make a new class of structural
materials that would be more than 50 times stronger than steel of the
same weight: a Cadillac might weigh 100 pounds; a full-size sofa could
be picked up with one hand.
The ability to build molecule by molecule could also give us surgical
instruments of such precision and deftness that they could operate on
the cells and even molecules from which we are made.
The ability to make such products probably lies a few
decades away. But theoretical and computational models provide
assurances that the molecular manufacturing systems needed for the
task are possible--that they do not violate existing physical law.
These models also give us a feel for what a molecular manufacturing
system might look like. This is an important foundation: after all,
the basic idea of an electrical
relay was known in the 1820s, and the concept of a mechanical computer
that operated off a stored set of instructions--a program--was
understood a few years later. But computers using relays were not
built till much later because no good theoretical comprehension of
"computation" existed. Today, scientists are devising numerous tools
and techniques that will be needed to transform nanotechnology from
computer models into reality. While most remain in the realm of
theory, there appears to be no fundamental barrier to their
development.
A Nano Tool Chest
Imagine putting some wires, transistors, and other
electronic components into a bag, shaking it, and pulling out a
radio--fully assembled and ready to work. Although this sounds
fanciful, such remarkable "self-assembly" is, in essence, what
chemists do whenever they synthesize materials. Mixing solutions in a
beaker, a chemist lets the intrinsic attractions and repulsions of
certain molecules and atoms take over. An art and science has evolved
to arrange conditions so that atoms spontaneously assemble into
particular molecular structures.
Similarly, we are surrounded and inspired by products
that are marvelously complex and yet very inexpensive. Potatoes, for
example, consist of tens of thousands of genes and proteins and
intricate molecular machinery; yet we think nothing of eating this
miracle of biology, mashed with a little butter. Potatoes, along with
many other agricultural products, cost less than a dollar a pound. The
key reason: if provided with a little soil, water, air, and sunlight,
a potato can make more potatoes. Likewise, if we could make a
general-purpose programmable manufacturing device that was able to
make copies of itself--what nanotechnology researchers call an
assembler--then the manufacturing costs for both the device and
anything it made could be kept low.
A basic principle in self-assembly is selective
"stickiness." If two molecular parts have complementary shapes and
charge patterns--that is, one has a hollow where the other has a bump,
or one has a positive charge where the other has a negative
charge--then they will tend to
stick together in a particular way to form a bigger part. This bigger
part can combine in the same way with other parts so that a complex
whole emerges from molecular pieces.
Self-assembly is not by itself sufficient, however, to
make the wide range of products that nanotechnology promises. If the
parts are indiscriminately sticky, for example, then stirring them
together would yield messy blobs instead of precise molecular
machines. We can solve this problem by holding the molecular parts in
the proper position and orientation so that when they touch they will
join together the way we want them to. At the macroscopic scale, the
idea that we can hold parts in our hands and assemble them by properly
positioning them with respect to each other goes back to prehistory:
we celebrate ourselves as the tool-using species. But the idea of
holding and positioning molecules is new and almost shocking.
Nanoscale equivalents of "arms" and "hands" must be developed.
Current proposals for molecular-scale positional devices
resemble normal-sized robotic devices, but they are about one
ten-millionth as big. A molecular robotic arm could sweep
systematically back and forth, adding and withdrawing atoms from a
surface to build any structure that the computer instructed it to.
Such an arm, composed of a few million atoms, might be 100 nanometers
long and 30 nanometers around. Although it would have roughly 100
moving parts, it would use no lubricants--at this scale, a lubricant
molecule is more like a
piece of grit. Such ultraminiature tools should be able to position
their tips to within a small fraction of an atomic diameter. Trillions
of such devices would occupy little more than a few cubic millimeters
(a speck slightly larger than a pinhead).
Molecular arms would be buffeted by something we don't
worry about at the macroscopic scale: thermal noise. Atoms and
molecules are in a constant state of wiggle and jiggle; the higher the
temperature, the more vigorous the motion. To maintain its position,
therefore, a nanoscale arm must be extremely stiff.
The stiffest material around is diamond. The strength
and lightness of a material depends on the number and strength of the
bonds that hold its atoms together, and on the lightness of the atoms.
The element that best fits both criteria is carbon, which is
lightweight and forms stronger bonds than any other atom. The
carbon-carbon bond is especially strong; each carbon atom can bond to
four neighboring atoms. In diamond, then, a dense network of strong
bonds creates a strong, light, and stiff material. Indeed, just as we
named the Stone Age, the Bronze Age, and the Steel Age after the
materials that humans
could make, we might call the new technological epoch we are entering
the Diamond Age.
How can a diamond device of this scale be produced? One
answer comes from looking at how we grow diamond today. In a process
somewhat reminiscent of spray painting, we build up layer after layer
of diamond by holding a surface in a cloud of reactive hydrogen atoms
and hydrocarbon molecules. When these molecules bump into the surface
they change it, either by adding, removing, or rearranging atoms. By
carefully controlling the pressure, temperature, and the exact
composition of the gas in this process, called chemical vapor
deposition (CVD), we can create conditions that favor the growth of
diamond on the surface.
But randomly bombarding a surface with reactive
molecules does not offer fine control over the growth process; it is
akin to trying to build a wristwatch using a sand blaster. We want the
chemical reactions to occur at precisely the places on the surface
that we specify. A second problem is how to make the diamond surface
reactive at the particular spots where we want to add another atom or
molecule. A diamond surface is normally covered with a layer of
hydrogen atoms. Without this layer, the raw diamond surface would be
highly reactive because it would be studded with the carbon atoms'
unused (or "dangling") bonds. While hydrogenation prevents unwanted
reactions, it also renders the entire surface inert, making it
difficult to add carbon (or anything else) to it.
To overcome this problem, we could use a set of
molecular-scale tools that would, in a series of steps, prepare the
surface and create structures on the layer of diamond, atom by atom
and molecule by molecule. The first step in the process would be to
remove a hydrogen atom from a specific spot on the diamond surface,
leaving behind a reactive dangling bond. This can be done with a
"hydrogen abstraction tool"--a molecular structure that has a high
chemical affinity for hydrogen at one end but is elsewhere inert. The
tool's unreactive region serves as a kind of handle. The tool would be
held by a molecular positional device, such as the molecular robotic
arm discussed earlier, and moved directly over particular hydrogen
atoms on the surface we wish to abstract.
This creates a chicken-and-egg problem: we need a
molecular robotic arm to build another molecular robotic arm. To solve
this problem, we must at some point build a molecular robotic arm with
something other than a molecular robotic arm. We could, for example,
use a macroscopic positional device--such as an improved version of an
existing atomic-force microscope--to make our first molecular robotic
arm. Alternatively, we could self-assemble a simplified molecular
positional device. These first crude positional devices could then be
used to make better ones.
One suitable molecule for a hydrogen abstraction tool is
the acetylene radical--two carbon atoms triple bonded together. One
carbon would be the handle, and would link to a nanoscale positioning
tool. The other carbon has a dangling bond where a hydrogen atom would
be in ordinary acetylene. The environment around the tool would be
inert (typical proposals involve the use of either vacuum or a noble
gas, such as neon).
Once this tool has created a reactive spot by
selectively removing hydrogen atoms from the diamond surface, it
becomes possible to deposit carbon atoms at the desired sites. In this
way a diamond structure is built, molecule by molecule, according to
plan. One proposal for this function is the dimer deposition tool. A
dimer is a molecule consisting of two of the same atoms or molecules
stuck together. In this case, the dimer would be C2--two carbon atoms
connected by a triple bond. In the deposition tool, each carbon in the
dimer would be connected to a larger molecule by single bonds with
oxygen atoms.
The hydrogen abstraction tool and dimer deposition tool
would work together. First, the abstraction tool would remove two
adjacent hydrogen atoms from the diamond surface. The two dangling
bonds would react with the ends of the carbon dimer. This reaction
would break the carbon-oxygen bonds and then transfer the carbon dimer
from the tool to the surface. Because the energy released during the
reaction is much larger than thermal noise, the dimer will "snap" onto
the surface
and stay there.
A third proposed tool for making nanostructures is the
carbene insertion tool. Carbenes--highly reactive carbon atoms with
two dangling bonds--will react with (and add a carbon atom to) many
molecular structures. Carbenes will readily insert into double or
triple bonds, like the bond in the carbon-carbon dimer described
above. A positionally controlled carbene could be attached almost
anywhere on a growing molecular workpiece, leading to the construction
of virtually any desired shape.
A fourth proposal is for a hydrogen deposition tool.
Where the hydrogen abstraction tool is intended to make an inert
structure reactive by creating a dangling bond, the hydrogen
deposition tool would do the opposite: make a reactive structure inert
by terminating dangling bonds. Such a tool would let us stabilize
reactive surfaces and prevent the surface atoms from rearranging in
unexpected and undesired ways. The key requirement for such a tool is
that it include a weakly attached hydrogen atom. While many molecules
fit that description, the bond between hydrogen and tin is especially
weak; thus, a tin-based hydrogen deposition tool should be effective.
These four molecular tools should enable us to make a
wide range of stiff structures--but only those that are composed of
hydrogen and carbon. This is a much less ambitious goal than
attempting to use all 100 or so elements in the periodic table. But in
exchange for confining ourselves to this more limited class of
structures, we make it much easier to analyze those that can be
fabricated and the synthetic reactions needed to make them. In any
case, this narrower proposal can be more readily and more thoroughly
investigated than full nanotechnology. And diamond and its
shatterproof variants fall
within this category, as do the "fullerenes"--sheets of carbon atoms
rolled into spheres, tubes, and other shapes. These materials can
compose all the parts needed for basic mechanical devices such as
struts, bearings, gears, and robotic arms.
Ultimately we'd like to add other elements--to create
diamond electronic devices, for example, or add some nitrogen to the
internal surface of a bearing in order to relieve strain (the
carbon-nitrogen bond is shorter than the carbon-carbon bond). Such
structures, composed primarily of carbon and hydrogen in combination
with nitrogen, oxygen, fluorine, silicon, phosphorous, sulfur, or
chlorine, constitute what we call the class of "diamondoid" materials.
The Diamond Age
Natural diamond is expensive, we can't make it in the
shapes we want, and it shatters. Nanotechnology will let us
inexpensively make shatterproof diamond (with a structure that might
resemble diamond fibers) in exactly the shapes we want. This would let
us make a Boeing 747 that would weigh one fiftieth of today's versions
without any sacrifice in strength. The benefit to space travel would
also be dramatic. The strength-to-weight ratio and the cost of
components are critical to the performance and economy of space ships:
nanotechnology could improve both of these parameters by about two
orders of magnitude.
Nanotechnology could also radically alter the economics
of energy production. The sun could provide orders of magnitude more
power than people now use--and do so more cleanly and less expensively
than fossil fuels and nuclear reactors--if only we could make low-cost
solar cells and batteries. We already know how to make efficient solar
cells: nanotechnology could cut their costs, finally making solar
power economical. In this application we need not make new or
technically superior devices; just by making inexpensively what we
already know how to make expensively we would move solar power into
the mainstream.
The manufacture of computer chips could undergo a
profound change. There seem to be fundamental limits in how much
further we can improve lithography, the process by which chips are now
made. In lithography (literally, "stone writing"), we draw fine lines
on a silicon wafer using methods borrowed from photography. A
light-sensitive film--called a "resist"--is spread over the silicon
wafer. The resist is exposed to a complex pattern of light and dark,
like a negative in a camera, and developed. By repeating this process,
an intricate set
of interlocking patterns can be made that defines the complex logic
elements of a computer chip.
But arranging atoms by throwing photons (or other
particles) at a surface from a distance doesn't seem like the best
approach, especially if we want to use three dimensions instead of
just two; imagine building a car by throwing tools at it from more
than a mile away. Thus if improvements to computer hardware are to
continue at the current pace, in a decade or so we'll have to move
beyond lithography to some new manufacturing technology. Designs for
computer logic elements composed of fewer than 1,000 atoms have
already been suggested--but each atom in such a small device has to be
in exactly the right place. And spraying chemicals around simply can't
arrange atoms with the needed precision.
Fortunately, diamond is an excellent electronic
material. It outperforms silicon in several key respects. For one
thing, electrons move faster in diamond than in silicon. Diamond can
also work better than silicon at high temperatures. This is important
because as chips get faster and faster, their performance is limited
by the need to dissipate the heat that builds up in the circuitry.
Diamond has this advantage for two reasons. First,
diamond has greater thermal conductivity than silicon, which lets heat
move out of a diamond transistor more quickly. Second, diamond has a
larger "bandgap" than silicon--5.5 electron volts, as opposed to 1.1
electron volts in silicon. The bandgap is the minimum amount of energy
required to boost an electron from its relatively immobile state into
the semiconductor's conduction band, where the electron moves freely
under the influence of a voltage. As the temperature increases, more
electrons gain the energy needed to jump into the conduction band.
When too many electrons do this, the device changes from a
semiconductor into a conductor; the transistor shorts out and stops
working. Diamond's higher bandgap means it shorts out at a higher
temperature.
With nanotechnology, we should be able to build mass
storage devices that can store more than 100 billion billion bytes in
a volume the size of a sugar cube, and massively parallel computers of
the same size that can deliver a billion billion instructions per
second--a billion times more than today's desktop computers.
The availability of nanoscale devices could radically
redefine surgery, too. There is today a fundamental mismatch between
what's needed to treat injuries and the capabilities of our tools. The
cellular and molecular machinery in our tissue is small and precise,
yet today's scalpels are, as seen by a cell, crude scythes that rip
through tissue, leaving dead and maimed cells in their wake. The only
reason that modern surgery works is the remarkable ability of cells to
regroup, bury their dead, and heal over the wound.
Surgical tools that are molecular in both size and
precision should let us directly heal, at the molecular and cellular
level, the injuries that cause disease. A molecular robotic arm less
than 100 nanometers long, for example, would easily fit into the
circulatory system (a single red blood cell is about 8,000 nanometers
in diameter) and would even be able to squeeze inside individual
cells.
One application would be in cancer therapy. We could
design a small device able to identify and kill cancer cells. The
device, which would incorporate a nanoscale computer and several
binding sites that are shaped to fit specific molecules, would
circulate freely throughout the body, periodically sampling its
environment by determining whether
its binding sites were occupied. The more frequently a site was
occupied, the higher the concentration of the molecule for which that
site was designed. A nanodevice with a dozen different types of
binding sites could in this way monitor the concentrations of a dozen
different types of molecules that occur normally in the body but whose
concentrations relative to one another change when cancer is present.
The computer could determine if the profile of concentrations fit a
preprogrammed profile and would, when a cancerous profile was
encountered, release a poison that selectively kills the cancer cells.
Each device could incorporate a nanoscale pressure
sensor that would allow the cancer killer to receive instructions
through ultrasonic signals in the megahertz range. By "listening" to
several macroscopic acoustic signal sources, the device could
determine its location within the body much as a radio receiver on
earth can use the transmissions from several satellites to determine
its position. Awareness of its own location within the body would help
the device decide whether it was near the cancer. In the absence of
location information, it might sometimes mistakenly release poison in
a cell that seemed to be a cancer cell. If the objective was to kill a
colon cancer, for example, a cancer killer in the big toe would not
release its poison no matter what its cancer sensors told it.
How Can We Get There?
The wondrous capabilities described here are, for the
most part, theoretical. How can they be made real? How can we build a
general-purpose, programmable manufacturing system using highly
reactive, positionally controlled tools that could inexpensively
manufacture most diamondoid structures?
The magnitude of this challenge should not be
underestimated. Present proposals for an assembler able to fabricate
diamondoid structures involve hundreds of millions or billions of
atoms--with no atom out of place. Even a simple robot arm, which might
be composed of only a few million atoms, would have to be accompanied
by other components. The robotic arms would work in a vacuum, for
instance, dictating the need
for a shell around the arm to maintain that vacuum. Other ancillary
gadgets that will be needed include acoustic receivers, computers,
pressure-actuated ratchets, and binding sites. If each operation, such
as hydrogen abstraction or carbene deposition, typically handles one
or a few atoms, then the error rate must be fewer than one in a
billion.
Although such perfection is theoretically attainable,
today's technology is not up to the task. A chemical synthesis process
that chemists view as very good converts 99 percent of the reactants
to the desired product. Yet that 99 percent yield represents an error
rate of one in 100, which is ten million times less perfect than we
desire for a mature nanotechnology. The synthesis of proteins from
amino acids by
ribosomes has an error rate of perhaps one in 10,000. DNA, by relying
on extensive error detection and correction along with built-in
redundancy (the molecule has two complementary strands), achieves an
error rate of roughly one base in a billion when replicating itself.
No existing technology can approach this level of
performance. One technique that can position individual atoms, for
example, is the scanning probe microscope (SPM), in which a sharp tip
is brought down to the surface of a sample so that a signal is
generated that lets us map out the surface being probed, like a blind
person tapping with a cane to sense the path ahead. Some SPMs
literally push on the surface and note how hard the surface pushes
back. Others connect the surface and probe to a voltage source, and
measure the current flow when the
probe gets close to the surface. A host of other probe-surface
interactions can be measured, and are used to make different types of
SPMs.
The SPM can not only map a surface but can change
it--depositing individual atoms and molecules in a desired pattern,
for example. In a well-publicized case, scientists arranged 35 xenon
atoms on a nickel surface to form the letters identifying their
employer: IBM. But this SPM manipulation required cooling to 4 degrees
above absolute zero--not exactly ideal conditions for large-scale
manufacturing. More recently, IBM scientists have precisely arranged
molecules at room temperature on a copper surface. However, SPMs have
error rates high enough that they must use relatively sophisticated
error detection and correction methods. And while these systems can
move around a few atoms or molecules, they can't manufacture large
amounts of precisely structured diamond of the kind that might be used
to build a car or a plane.
Finally, today's SPMs are much too slow. In nature,
ribosomes take tens of milliseconds to add a single amino acid to a
growing protein. But if an assembler is to manufacture a copy of
itself in about a day, and if this takes a few hundred million
operations, then each operation must take place in a fraction of a
millisecond. An SPM, by contrast, takes hours to arrange a few atoms
or molecules. Rather than attempting to solve all these problems in a
single giant leap, we
might approach them more incrementally--developing a series of
intermediate systems. One approach, for example, would be to eliminate
the requirement that the assembler be made from diamondoid structures.
Diamondoid is attractive, as we've seen, because of its
strength, stiffness, and electrical properties. But an intermediate
system need only be able to make a more advanced system, and perhaps
products that are impressive in comparison with today's products. It
doesn't have to be diamondoid itself.
This suggests what might be called building blockÜbased
nanotechnology. Rather than building diamond, we'll build some other
material from relatively large molecular units consisting of tens,
hundreds, or even thousands of atoms. Such large building blocks
reduce the number of assembly steps, so fewer unit operations are
required, and they need not be as reliable. Soluble building blocks
that stick only to other building blocks, not to the solvent or low
concentrations of contaminants, eliminate the need for working in a
vacuum.
In selecting such building blocks, we have many choices:
any of the many molecules that chemists have synthesized, or could
reasonably synthesize, with the desired properties. Each molecular
building block should have at least three sites where it can link to
other building blocks. Units with two bonding sites suggest the
polymers ubiquitous in biological systems, such as DNA, RNA, and
proteins. Building blocks that have three bonding sites make the
design of stiff
three-dimensional structures much easier.
Such building blocks could be linked to each other using
any one of a variety of well-understood chemical reactions. A
particularly attractive possibility is the Diels-Alder reaction, in
which a diene (a hydrocarbon with two carbon-carbon double bonds) can
be made to react with a specific molecule.
Answering the Doubters
Despite the plausibility of developing nanotechnology,
there are skeptics. Their criticisms, however, are poorly informed.
For example, chemist David Jones, a Nature columnist, was quoted in
Scientific American that the construction of a molecular assembler was
doomed because individual atoms are "amazingly mobile and reactive.
They will
combine instantly with ambient air, water, each other, the fluid
supporting the assemblers, or the assemblers themselves."
Proposals involving reactive molecular tools, however,
specify that the environment should be inert--either vacuum or a noble
gas; there would be no "ambient air" to react with. And because the
molecular tools are positionally controlled, they will not react with
each other or the assembler itself--for the same reason that a hot
soldering iron does not react with the skin of the person who wields
it.
I am commonly asked how long it will be before we can
make molecular computers, before inexpensive photovoltaic cells bring
cheap, clean solar power, before ultralightweight spacecraft
dramatically lower the cost of space exploration. The scientifically
correct answer is: I don't know. But looking at one technology that
nanotechnology can improve--computing--gives one perspective. From
electromechanical relays to vacuum tubes to transistors to integrated
circuits, we have seen steady declines in the size and cost of logic
elements and steady
increases in their performance for the last 50 years. Extrapolation of
these trends suggests that for the computer hardware revolution to
stay "on schedule" will require the development of molecular
manufacturing by about 2010 or 2020.
Of course, extrapolating past trends is a
philosophically debatable method of technology forecasting. While no
fundamental law of nature prevents us from developing nanotechnology
on this schedule (or even faster), there is equally no law that says
this schedule will not slip. Much worse, though, such trends imply
that there is some ordained schedule--that nanotechnology will
inevitably appear regardless of what we do or don't do. Nothing could
be further from the truth. How long it takes to develop this
technology depends very
much on what we do. If we pursue it systematically, it will happen
sooner. If we ignore it, or simply hope that someone will stumble over
it, it will take much longer. Fortunately, by using theoretical,
computational, and experimental approaches together, we can reach the
goal more quickly and reliably than by using any single approach
alone. Just as Boeing can design, "build," and "fly" airplanes in a
computer before making them in the real world, we can do the same for
molecular manufacturing. We can quickly eliminate most of the false
starts and blind alleys and rapidly focus on the best approaches.
Like the first human landing on the moon, the Manhattan
project, or the development of the modern computer, the advent of
molecular manufacturing will require the coordinated efforts of many
people for many years. How long will it take? A lot depends on when we
start.
by Ralph C. Merkle
http://www.techreview.com