Computing with Molecules
Source: New
Scientist
Radical departures from present computing design will probably be
needed to exploit molecular computing systems fully.
How fast and powerful can computers become? Will it be possible
someday to create artificial "brains" that have intellectual capabilities
comparable--or even superior--to those of human beings? The answers to these
questions depend to a very great extent on a single factor: how small and dense
we can make computer circuits.
Few if any researchers believe that our present
technology--semiconductor-based solid-state microelectronics--will lead to
circuitry dense and complex enough to give rise to true cognitive abilities. And
until recently, none of the technologies proposed as successors to solid-state
microelectronics had shown enough promise to rise above the pack. Within the
past year, however, scientists have achieved revolutionary advances that may
very well radically change the future of computing. And although the road from
here to intelligent machines is still rather long and might turn out to have
unbridgeable gaps, the fact that there is a potential path at all is something
of a triumph.
The recent advances were in molecular-scale electronics, a field
emerging around the premise that it is possible to build individual molecules
that can perform functions identical or analogous to those of the transistors,
diodes, conductors and other key components of today's microcircuits. After a
period of high hopes but few tangible results, several developments over the
past few years have raised expectations that this technology may one day provide
the building blocks for future generations of ultrasmall, ultradense electronic
computer logic. In a remarkable series of demonstrations, chemists, physicists
and engineers have shown that individual molecules can conduct and switch
electric current and store information.
Last July, in an achievement widely reported in the popular press,
researchers from Hewlett-Packard and the University of California at Los Angeles
announced that they had built an electronic switch consisting of a layer of
several million molecules of an organic substance called rotaxane. By linking a
number of switches, the researchers produced a rudimentary version of an AND
gate, a device that performs a basic logic operation. With well over a million
molecules apiece, the switches are far larger than would be desirable. And they
could be switched only one time before becoming inoperable. Nevertheless, their
assembly into a logic gate was of fundamental significance.
Within months of that announcement, our groups at Yale and Rice
universities published results on a different class of molecules that acted as a
reversible switch. And one month later we described a molecule we had created
that could change its electrical conductivity by storing electrons on demand,
acting as a memory device.
To produce our switch, we inserted regions into the molecules that
trapped electrons, but only when the molecules were subjected to certain
voltages. Thus, the degree to which the molecules resisted a flow of electrons
depended on the voltage applied to them. In fact, by varying the voltage, we
could repeatedly change the molecules at will from a conducting to a
nonconducting state--which is the basic requirement for an electrical switch.
The tiny device actually consisted of a layer of about 1,000 molecules of
nitroamine benzenethiol sandwiched between metal contacts.
After creating the switch, we realized that if we could redesign
the molecule so that it could retain electrons rather than trapping them
briefly, we would have something that could work as a memory element. We went to
work on the trapping region of the molecule, modifying it so that its
conductivity could be changed repeatedly. The resulting "electron sucker" could
retain electrons for nearly 10 minutes--compared with a few milliseconds for
conventional silicon-based dynamic random-access memory.
Although the advances were encouraging, the challenges remaining
are enormous. Creating individual devices is an essential first step. But before
we can build complete, useful circuits we must find a way to secure many
millions, if not billions, of molecular devices of various types against some
kind of immobile surface and to link them in any manner and into whatever
patterns our circuit diagrams dictate. The technology is still too young to say
for sure whether this monumental challenge will ever be surmounted.
The End of the Road Map
Given the magnitude of the challenges ahead, why did researchers
and even the mainstream media pay so much attention to the recent advances? The
answer has to do with industrial society's dependence on microelectronics--and
the limits of the form of the technology we have today.
That form--solid-state and silicon-based--follows one of the most
famous axioms in technology: Moore's Law. It relates that the number of
transistors that can be fabricated on a silicon integrated circuit--and
therefore the computing speed of such a circuit--is doubling every 18 to 24
months. After following this remarkable curve for four decades, solid-state
microelectronics has advanced to the point at which engineers can now put on a
sliver of silicon of just a few square centimeters some 100 million transistors,
with key features measuring 0.18 micron.
These transistors are still far larger than molecular-scale
devices. To put the size differential in perspective, if the conventional
transistor were scaled up so that it occupied the printed page you are reading,
a molecular device would be the period at the end of this sentence. Even in a
dozen years, when industry projections suggest that silicon transistors will
have shrunk to about 120 nanometers in length, they will still be more than
60,000 times larger in area than molecular electronic devices.
Moreover, no one expects conventional silicon-based
microelectronics to continue following Moore's Law forever. At some point,
chip-fabrication specialists will find it economically infeasible to continue
scaling down microelectronics. As they pack more transistors onto a chip,
phenomena such as stray signals on the chip, the need to dissipate the heat from
so many closely packed devices, and the difficulty of creating the devices in
the first place will halt or severely slow progress.
Indeed, various nagging (though not yet fundamental) problems in
the fabrication of efficient smaller silicon transistors and their
interconnections are becoming increasingly bothersome. Many experts expect these
challenges to intensify dramatically as the transistors approach the 0.1-micron
level. Because of these and other difficulties, the exponential increase in
transistor densities and processing rates of integrated circuits is being
sustained only by a similar exponential rise in the financial outlays necessary
to build the facilities that produce these chips. Eventually the drive to
downscale will run headlong into these extreme facility costs, and the market
will reach equilibrium. Many experts project that this will happen around or
before 2015, when a fabrication facility is projected to cost nearly $200
billion. When that happens, the long period of breathtaking advances in the
processing power of computer chips will have run its course. Further increases
in the power of the chips will be prohibitively costly.
Unfortunately, this impasse will almost certainly occur long
before computer chips have reached the power to fulfill some of the most
sought-after goals in computer science, such as the creation of extremely
sophisticated electronic "brains" that will enable robots to perform on a par
with humans in intellectual and cognitive tasks.
Billions and Billions
The extraordinarily small size of molecular devices brings
advantages beyond the simple ability to pack more of them into a small area. To
grasp these important benefits requires an understanding of how the devices
work--which in turn demands some knowledge of how electrons behave when confined
to regions as small as atoms and molecules.
Free electrons can take on energy levels from a continuous range
of possibilities. But in atoms or molecules, electrons have energy levels that
are quantized: they can only be any one of a number of discrete values, like
rungs on a ladder. This series of discrete energy values is a consequence of
quantum theory and is true for any system in which the electrons are confined to
an infinitesimal space. In molecules, electrons arrange themselves as bonds
among atoms that resemble dispersed "clouds," called orbitals. The shape of the
orbital is determined by the type and geometry of the constituent atoms. Each
orbital is a single, discrete energy level for the electrons.
Even the smallest conventional microtransistors in an integrated
circuit are still far too large to quantize the electrons within them. In these
devices the movement of electrons is governed by physical characteristics--known
as band structures--of their constituent silicon atoms. What that means is that
the electrons are moving in the material within a band of allowable energy
levels that is quite large relative to the energy levels permitted in a single
atom or molecule. This large range of allowable energy levels permits electrons
to gain enough energy to leak from one device to the next. And when these
conventional devices approach the scale of a few hundred nanometers, it becomes
extremely difficult to prevent the minute electric currents that represent
information from leaking from one device to an adjacent one. In effect, the
transistors leak the electrons that represent information, making it difficult
for them to stay in the "off" state.
Building from the Bottom Up
Besides enabling molecular devices to contain their electrons more
securely, quantum mechanical phenomena can also be exploited in specially
designed molecules to perform other functions. For example, to construct a
"wire" we need an elongated molecule through which electrons can flow easily
from one end to the other. Electrons in any quantized structure such as a
molecule tend to move from higher-to lower-energy levels, so in order to channel
electrons we need a molecule that has an empty, low-energy orbital that is
dispersed throughout the molecule from one end to the other. A typical empty,
low-energy electron orbital is known as a pi orbital. And the configuration in
which electron clouds overlap from one molecular component to the next is called
conjugated, so our molecular wire is known as a "pi-conjugated system."
An active device such as a transistor, however, has to do more
than merely allow electrons to flow--it has to somehow control that flow. Thus,
the task of the molecular device engineer is to exploit the quantum world's
discrete energy levels--specifically, by designing molecules whose orbital
characteristics achieve the desired kind of electronic control. For example,
with the right overlap of orbitals in the molecule, electrons flow. But when the
overlap is disturbed--because the molecule has been twisted or its geometry has
been otherwise affected--the flow is blocked. In other words, the key to control
on the molecular scale is manipulating the number of electrons that are allowed
to flow at low orbital energy by perturbing the orbital overlap through the
molecule.
Already the standard methods of chemical synthesis allow
researchers to design and produce molecules with specific atoms, geometries and
orbital arrangements. Moreover, enormous quantities of these molecules are
created at the same time, all of them absolutely identical and flawless. Such
uniformity is extremely difficult and expensive to achieve in other
batch-fabrication processes, such as the lithography-based process used to
produce the millions of transistors on an integrated circuit.
The methods used to produce molecular devices are the same as
those of the pharmaceutical industry. Chemists start with a compound and then
gradually transform it by adding prescribed reagents whose molecules are known
to bond to others at specific sites. The procedure may take many steps, but
gradually the pieces come together to form a new potential molecular device with
a desired orbital structure. After the molecules are made, we use analytical
technologies such as infrared spectroscopy, nuclear magnetic resonance and mass
spectrometry to determine or confirm the structure of the molecules. The various
technologies contribute different pieces of information about the molecule,
including its molecular weight and the connection point or angle of a certain
fragment. By combining the information, we determine the structure after each
step as the new molecule is synthesized.
One of our simplest active devices was a molecule based on a
string of three benzene rings, in which the orbitals overlapped (were
conjugated) throughout. We made the connections between the benzene rings
structurally weak, so that slight twists or kinks weakened or strengthened the
conjugation of the orbitals. All we needed was a way to control this twisting
and we would have a molecular device in which we could control current flow--a
switch, in other words.
To the center benzene ring in the molecule, we added NO2 and NH2
groups, projecting outward from the string on opposite sides of the center ring.
This asymmetrical configuration left the molecule with a strongly perturbed
electron cloud. That asymmetric, perturbed cloud in turn made the molecule very
susceptible to distortion by an electric field: applying an electric field to
the molecule twisted it. We now had an active device: every time we applied a
voltage to the molecule, an electric field was set up that twisted the molecule
and blocked current flow. With the voltage removed, the molecule sprang back to
its original shape, and the current flowed again. In follow-up experiments, we
found that for our infinitesimal device the abruptness of the switching from one
state to the other was superior to that of any comparable solid-state device.
Of course, a lot of advanced technology and years of research were
necessary before we could even test one of these devices. The basic challenge is
reaching into an unfathomably Lilliputian domain in order to contact and
interact with a single molecule and bring information about the behavior of that
molecule into our macroscopic world.
The task was all but impossible before the invention, in the
1980s, of the scanning tunneling microscope (STM) at IBM's research laboratories
in Zurich. The STM gives scientists a window on the atomic world, letting them
visualize and manipulate single atoms or molecules. With an atomically sharp tip
of metal held precisely over a surface, the topography of the surface is sensed
by the minute current of tunneling electrons that flows between the surface and
the tip. Rastering the tip back and forth creates a picture of the hills and
valleys on the surface.
Although scanning tunneling microscopy is crucial for testing and
constructing individual devices, any useful molecular circuit will consist of
vast numbers of devices, orderly arranged and securely affixed to a solid
structure to keep them from interacting randomly with one another. Progress
toward solving this huge challenge has emerged from studies of self-assembly, a
phenomenon in which atoms, molecules or groups of molecules arrange themselves
spontaneously into regular patterns and even relatively complex systems without
intervention from outside.
Molecular Glue
Once the assembly process has been set in motion, it proceeds on
its own to some desired end [see "Self-Assembling Materials," by George M.
Whitesides; Scientific American, September 1995]. In our research we use
self-assembly to attach extremely large numbers of molecules to a surface,
typically a metal one [see illustration on self-assembly]. When attached, the
molecules, which are often elongated in shape, protrude up from the surface,
like a vast forest with identical trees spaced out in a perfect array.
The Basics
The inexorable drive to produce smaller devices may leave
technologists no choice but to migrate to a new form of electronics in which
specially designed individual molecules replace the transistors of today's
circuits. That forced migration could come about within the next decade, some
researchers believe.
The bare requirements for a general-purpose computer are a
switching device (like a transistor), memory and a way of connecting arbitrarily
large numbers of the devices and memory elements. So far scientists have managed
to produce single-molecule switches and memory elements. The switch, however,
had only two terminals. Realistically, to construct complex logic circuits
requires a device with more than two terminals, in which, for example, current
flow between two is controlled by a third (that is the way transistors work).
Even more imposing, scientists lack a method of connecting huge
numbers of the devices. Although no potential solutions to this problem are
apparent yet, researchers suspect that radically new architectures and
conventions will be needed to exploit molecular devices fully.
Researchers have studied a variety of self-assembly systems. Our
work often requires us to attach molecular devices to a metal (usually gold)
surface. So we frequently work with a molecular fragment that we attach to one
or both ends of our device and that has a high affinity for gold atoms. The
specific fragment we commonly use, called a "sticky" end group for obvious
reasons, is based on an atom of sulfur and is known in chemical terminology as
thiol.
To initiate the self-assembly, we need only dip a gold surface
into a beaker. In solution in this container are our molecular devices, each
with thiol end groups on both ends. Spontaneously and in unimaginably large
numbers, the devices attach themselves to the gold surface.
Handy though it is, self-assembly alone will not suffice to
produce useful molecular-computing systems, at least not initially. For some
time, we will have to combine self-assembly with fabrication methods, such as
photolithography, borrowed from conventional semiconductor manufacturing. In
photolithography, light or some other form of electromagnetic radiation is
projected through a stencil-like mask to create patterns of metal and
semiconductor on the surface of a semiconducting wafer. In our research we use
photolithography to generate layers of metal interconnections and also holes in
deposited insulating material. In the holes, we create the electrical contacts
and selected spots where molecules are constrained to self-assemble. Thus, the
final system consists of regions of self-assembled molecules attached by a
mazelike network of metal interconnections.
The first successful demonstration of self-assembly in molecular
electronics occurred just four years ago, in 1996, when Paul S. Weiss's group at
Pennsylvania State University tested self-assembled molecules. One of us (Tour),
then at the University of South Carolina, synthesized the devices. Weiss and his
colleagues found that by mixing a small amount of a solution of molecules that
were designed to have conducting properties with another containing a known
inert insulating molecule, they could get a self-assembled layer in which
conductive molecules were very sparsely interspersed among nonconductive ones.
By positioning the tip of an STM directly over one of the isolated conducting
molecules, they could qualitatively measure the conductivity. As expected, it
was significantly greater than that of the surrounding molecules. Similar
results were also obtained by a group at Purdue University, which tagged the top
of the conductive molecules with minute gold particles.
At the same time at Yale, one of us (Reed) performed the first
quantitative electrical measurements of a single molecule, which was also
fabricated by self-assembly. Specifically, Reed and his group measured how much
current could flow across a single molecule. The heart of the experimental setup
was an STM modified to enable it to position two tips opposite each other with
sufficient precision and mechanical stability to contain a single molecule in
between [see illustration]. A very simple molecule was used to convey mobile
electrons: a single benzene ring with sticky thiol end groups on both ends to
contact the metal leads of the STM tips. It turned out that the resistance of
the molecule was in the range of tens of millions of ohms.
The Yale researchers also found that the molecule could sustain a
current of about 0.2 microampere at five volts--which meant that the molecule
could channel through itself roughly a million million (1012) electrons per
second. The number is impressive--all the more so in light of the fact that the
electrons can pass through the molecule only in single file (one at a time). The
magnitude of the current was far larger than would be expected from simple
calculations of the power dissipated in a molecule, leading to the conclusion
that the electrons traveled through the molecule without generating heat by
interacting or colliding.
These initial observations of conduction in molecules were
followed quickly by demonstrations of basic devices. The simplest electronic
device is a diode, which can be thought of as a one-way valve for electrons. In
1997, only a year after the first measurements of conduction in molecules, two
separate research groups built diodes. At the University of Alabama, Robert M.
Metzger's group synthesized a molecule that had an internal energetic lineup of
orbitals, which varied depending on the polarity of the voltage applied to it.
The lineup of orbitals was analogous to the rungs on a ladder. With the voltage
applied in one direction, the lineup corresponded to a ladder propped against a
house. In this orientation, it takes considerable effort to climb the ladder.
With the opposite voltage polarity, the orbital lineup was analogous to the
rungs of a ladder lying flat on the ground, where it can be traversed with
little effort.
In the other group at Yale, Chong-Wu Zhou took a slightly
different tack. With this molecular diode, the differences in the lineup of the
energy levels occurred externally to the molecule, where it contacted the metal.
This scheme also worked well and helped to set the stage for the design of more
useful and interesting molecular devices and circuits.
Connecting from the Top Down
As they began constructing such devices, the Yale group adapted a
structure first made by Kristin Ralls and Robert A. Buhrman of Cornell
University. The structure contained an extremely minute hole, called a nanopore,
in which an "active region" was created by self-assembling a relatively small
number of molecular devices in a single layer, or monolayer. In a hole just 30
nanometers wide, approximately 1,000 of the molecular devices were allowed to
self-assemble. Evaporating a metal contact onto the top of the self-assembled
monolayer ("SAM") completed the device.
After using this configuration to produce and test molecular
diodes, the Yale group quickly moved on to more complex devices, namely,
switches. A controllable switch of some kind is a minimum requirement for a
general-purpose computer. Even more desirable is a switch that can amplify a
current, besides merely turning it on and off. Such amplification is necessary
to connect vast numbers of the switches, as is required to build complex logic
circuits. The silicon transistor fulfills both those requirements, which is why
it is one of the great success stories of the 20th century.
The molecular equivalent of a transistor that can both switch and
amplify current is yet to be discovered. But researchers have taken the first
steps along the path by constructing switches, such as the twisting switch
described earlier. In fact, Jia Chen, a graduate student in Reed's Yale group,
observed impressive switching characteristics, such as an on/off ratio greater
than 1,000, as measured by the current flow in the two different states. For
comparison, the analogous device in the solid-state world, called a resonant
tunneling diode, has an on/off ratio of around 100.
Similar behavior was observed in the U.C.L.A./HP experiments. In
their demonstration, they showed that the conductivity of a molecular layer of
rotaxanes, molecules that resemble a core with a surrounding barbell, could be
predictably interrupted when a high voltage was applied to a junction containing
the molecules. At this voltage, the molecules reacted and changed configuration,
altering the lineup of orbitals and interrupting the flow of current through the
molecule. Combining a series of these junctions, they built a device that
performed a simple logic function.
Perhaps most encouragingly, molecular devices have already proved
themselves as memory elements. Besides active, transistorlike devices, memory is
the other main requirement for a useful, general-purpose computer. Recall our
twisting switch. We altered the internal electrically active unit (the lopsided
center benzene ring with opposing NO2 and NH2 groups) by keeping just the
"electron-sucking" nitro group, NO2. The change made the molecular orbitals
susceptible to becoming modified--either spread out or localized depending on
the charge state of the internal group. Absence or presence of charge in the
internal node would modify the conduction of electrons through the molecule. By
storing charge on the nitro group, we blocked the conduction, which represents a
binary "0." Conversely, with no charge stored on the group, the conduction was
high, representing a binary "1." Significantly, the molecular memory cell
retained (or "remembered," if you will) the stored bit for nearly 10 minutes--an
astounding amount of time in comparison with an ordinary silicon dynamic
random-access memory (DRAM) element, which can hang on to a bit for only a few
milliseconds (silicon DRAMs must be frequently refreshed by an external circuit
to retain their data). The construction of the memory element, which involved a
relatively straightforward modification to the twisting switch, also
demonstrated the ease and flexibility in which molecular-scale devices can be
redesigned.
Given the enormous potential advantages of molecular devices, why
don't we scrap silicon research and proceed wholeheartedly to molecular-based
systems? Because despite the recent auspicious advances, a number of significant
obstacles, some fundamental, still stand in the way of fabulously complex and
powerful circuits.
Needed: The Next Transistor
Foremost among them is the challenge of making a molecular device
that operates analogously to a transistor. A transistor has three terminals, one
of which controls the current flow between the other two. Effective though it
was, our twisting switch had only two terminals, with the current flow
controlled by an electrical field. In a field-effect transistor, the type in an
integrated circuit, the current is also controlled by an electrical field. But
the field is set up when a voltage is applied to the third terminal.
A three-terminal molecular device will make possible the chemical
synthesis of tremendously efficient and complex circuits. Even before then,
combinations of molecular systems with conventional electronics will probably be
used in places where the advantages of self-assembly are natural. But
interfacing between the molecular and microelectronic worlds will present its
own challenges. Computer chips today have two levels of size scale. From the
macroscopic level of the chip we can see and hold in our hand, there is a factor
of 1,000 in size reduction to get to the gross wiring level, encompassing the
largest connections on the chip, which are smaller than a human hair. Then
another factor-of-1,000 reduction is necessary to get to the level of the
smallest connections and components of the transistors. If molecular devices are
to be added to a chip, they will represent yet another factor-of-1,000 reduction
in scale down from the smallest microelectronic device components.
Thermal challenges are also staggering, especially if engineers
wind up with no alternatives to using molecular devices in modes and
configurations similar to those used now with transistors in conventional chips.
At present, a state-of-the-art microprocessor with 10 million transistors and a
clock cycle of half a gigahertz (half a billion cycles per second) emits almost
100 watts--greater in radiant heat than a range-top cooking surface in the home.
Such a unit is close to the thermal limitation of semiconductor technology.
Knowing the minimum amount of heat that a single molecular device emits would
help put a limit on the number of devices we could put on a chip or substrate of
some kind.
This fundamental limit of a molecule, operating at room
temperature and at today's speeds, is about 50 picowatts (50 millionths of a
millionth of a watt). That figure suggests an upper limit to the number of
molecular devices we can closely aggregate: it is roughly 100,000 times more
that what we can now do with silicon microtransistors on a chip. Although that
may seem like a vast improvement, it is still far below the density that would
be possible if we did not have to worry about heat.
For these calculations, we followed the convention in silicon
microelectronics that every device is addressable--or, put another way, that any
device can be picked out from among the countless millions through the
interconnections, like a house with a unique street address. This kind of
addressing (which is called random access) would be required, for example, to
retrieve the contents of a particular memory location.
Right now no one knows how to create such an interconnect
structure on the molecular level. Straightforward extensions of the present
techniques we employ to fabricate complex microelectronics are not practical for
molecular-scale electronics, because the lithography needed for creating the
interconnections to single molecules is far beyond the capability of known
technologies. Is the ability to address every device, the common architecture we
use today, necessary or efficient at molecular-scale densities? What will
large-scale circuits of this technology look like? Can we use nanotubes,
single-walled structures of carbon with diameters of one or two nanometers and
lengths of less than a micron, as the next generation of interconnects between
molecular-scale devices?
Decades from now, radical departures from present computing design
will probably be needed to exploit molecular computing systems fully if we are
to extend electronics significantly beyond Moore's Law. We have only very
limited ideas about what these departures might be. The ability to construct
complex molecular devices, with new paradigms and lists of rules about
connecting the various devices, will open up an entirely different way to think
about computer design.
Although such departures are fraught with problems, we have no
alternative but to solve them if electronics is to continue advancing at
something like its current pace well into the next century. And difficult though
the challenges may be, the rewards for those who solve the problems could be
staggering. By pushing Moore's Law past the limits of the tremendously powerful
technology we already have, these researchers will take electronics into vast,
uncharted terrain. If we can get to that region, we will almost certainly find
some wondrous things--maybe even the circuitry that will give rise to our
intellectual successor.
Further Information:
CONDUCTANCE OF A MOLECULAR JUNCTION. M. A. Reed, C. Zhou, C. J.
Muller, T. P. Burgin and J. M. Tour in Science, Vol. 278, pages 252-254; October
10, 1997.
A DEFECT-TOLERANT COMPUTER ARCHITECTURE: OPPORTUNITIES FOR
NANOTECHNOLOGY. J. R. Heath, P. J. Kuekes, G. S. Snider and R. S. Williams in
Science, Vol. 280, pages 1716-1721; June 12, 1998.
MOLECULAR ELECTRONICS: SCIENCE AND TECHNOLOGY. Edited by A. Aviram
and M. Ratner. Annals of the New York Academy of Sciences, Vol. 852; 1998.
The Authors
MARK A. REED and JAMES M. TOUR began collaborating on molecular
electronics research in 1990. Reed is chairman of the department of electrical
engineering and the Harold Hodgkinson Professor of Engineering and Applied
Science at Yale University. His research interests include nanotechnology and
the fundamental limits of electronic conduction. A former research scientist at
Texas Instruments, he recently founded with Tour the Molecular Electronics
Corporation in Chicago with the aim of making molecular electronics commercially
viable. He is author of over 100 publications and holds 17 patents on quantum
effect, heterojunction and molecular devices. Tour is a synthetic organic
chemist who has been designing and synthesizing molecules for molecular
electronics for 10 years. He is with the department of chemistry and the Center
for Nanoscale Science and Technology at Rice University, where he pursues
chemical aspects of molecular electronics. Previously, he was at the University
of South Carolina, where he spent 11 years on the faculty of the department of
chemistry.
by Mark A.
Reed and James M. Tour
http://www.sciam.com/2000/0600issue/0600reed.html