INTRODUCTION
The
nanoscale is unique because nothing solid can be made any smaller. It is
also unique because many of the mechanisms of the biological and physical
world operate on length scales from 0.1 to 100 nm. At these dimensions
materials exhibit different physical properties; thus scientists expect
that many novel effects at the nanoscale will be discovered and used for
breakthrough technologies.
A
number of important breakthroughs have already occurred in nanotechnology.
These developments are found in products used throughout the world. Some
examples are catalytic converters in automobiles that help remove air
pollutants, devices in computers that read from and write to the hard
disk, certain sunscreens and cosmetics that transparently block harmful
radiation from the Sun, and special coatings for sports clothes and gear
that help improve the gear and possibly enhance the athlete’s
performance. Still, many scientists, engineers, and technologists believe
they have only scratched the surface of nanotechnology’s potential.
Nanotechnology
is in its infancy, and no one can predict with accuracy what will result
from the full flowering of the field over the next several decades. Many
scientists believe it can be said with confidence, however, that
nanotechnology will have a major impact on medicine and health care;
energy production and conservation; environmental cleanup and protection;
electronics, computers, and sensors; and world security and defense.
II
WHAT IS NANOTECHNOLOGY?
Nanometer
Crystal A germanium crystal only 1 nanometer (nm) wide is seen in this
computer-generated image.AP/Wide World Photos/Paul Sakuma
To
grasp the size of the nanoscale, consider the diameter of an atom, the
basic building block of matter. The hydrogen atom, one of the smallest
naturally occurring atoms, is only 0.1 nm in diameter. In fact, nearly all
atoms are roughly 0.1 nm in size, too small to be seen by human eyes.
Atoms bond together to form molecules, the smallest part of a chemical
compound. Molecules that consist of about 30 atoms are only about 1 nm in
diameter. Molecules, in turn, compose cells, the basic units of life.
Human cells range from 5,000 to 200,000 nm in size, which means that they
are larger than the nanoscale. However, the proteins that carry out the
internal operations of the cell are just 3 to 20 nm in size and so have
nanoscale dimensions. Viruses that attack human cells are about 10 to 200
nm, and the molecules in drugs used to fight viruses are less than 5 nm in
size.
The
possibility of building new materials and devices that operate at the same
scale as the basic functions of nature explains why so much attention is
being devoted to the world below 100 nm. But 100 nm is not some arbitrary
dividing line. This is the length at which special properties have been
observed in materials—properties that are profoundly different at the
nanoscale.
Human
beings have actually known about these special properties for some time,
although they did not understand why they occurred. Glassworkers in the
Middle Ages, for example, knew that by breaking down gold into extremely
small particles and sprinkling these fine particles into glass the gold
would change in color from yellow to blue or green or red, depending on
the size of the particle. They used these particles to help create the
beautiful stained glass windows found in cathedrals throughout Europe,
such as the cathedral of Notre Dame in
Nanotechnologists
are intrigued by the possibility of creating humanmade devices at the
molecular, or nanoscale, level. That is why the field is sometimes called
molecular nanotechnology. Some nanotechnologists are also aiming for these
devices to self-replicate—that is, to simultaneously carry out their
function and increase their number, just as living organisms do. To some
early proponents of the field, this aspect of nanotechnology is the most
important. If tiny functional units could be assembled at the molecular
level and made to self-replicate under controlled conditions, tremendous
efficiencies could be realized. However, many scientists doubt the
possibility of self-replicating nanostructures.
III
APPROACHES to
NANOTECHNOLOGY
Scientists
are currently experimenting with two approaches to making structures or
devices at the scale of 1 to 100 nm. These methods are called the top-down
approach and the bottom-up approach.
A
Top-down Approach
In
the top-down process, technologists start with a bulk material and carve
out a smaller structure from it. This is the process commonly used today
to create computer chips, the tiny memory and logic units, also known as
integrated circuits that operate computers. To produce a computer chip,
thin films of materials, known as a mask, are deposited on a silicon
wafer, and the unneeded portions are etched away. Almost all of today’s
commercial computer chips are larger than 100 nm. However, the technology
to create ever smaller and faster computer chips has already gone below
100 nm. Smaller and faster chips will enable computers to become even
smaller and to perform many more functions more quickly.
The
top-down approach, which is sometimes called microfabrication or
nanofabrication, uses advanced lithographic techniques to create
structures the size of or smaller than current commercial computer chips.
These advanced lithographic techniques include optical lithography and
electron-beam (e-beam) lithography. Optical lithography currently can be
used to produce structures as small as 100 nm, and efforts are being made
to create even smaller features using this technique. E-beam lithography
can create structures as small as 20 nm. However, e-beam lithography is
not suitable for large-scale production because it is too expensive.
Already the cost of building fabrication facilities for producing computer
chips using optical lithography approaches several billion dollars.
Ultimately,
the top-down approach to producing nanostructures is not only likely to be
too costly but also technically impossible. Assembling computer chips or
other materials at the nanoscale is unworkable for a fundamental reason.
To reduce a material in a specifically designed way, the tool that is used
to do the work must have a dimension or precision that is finer than the
piece to be reduced. Thus, a machine tool must have a cutting edge finer
than the finest detail to be cut. Likewise the lithographic mask used to
etch away the locations on a silicon wafer must have a precision in its
construction finer than the material to be removed. At the nanoscale,
where the material to be removed could be a single molecule or atom, it is
impossible to meet this condition.
B
Bottom-up Approach
As
a result, scientists have become interested in another vastly different
approach to creating structures at the nanoscale, known as the bottom-up
approach. The bottom-up approach involves the manipulation of atoms and
molecules to form nanostructures. The bottom-up approach avoids the
problem of having to create an ever-finer method of reducing material to
the nanoscale size. Instead, nanostructures would be assembled atom by
atom and molecule by molecule, from the atomic level up, just as occurs in
nature. However, assembly at this scale has its own challenges.
In
school, children learn about some of these challenges when they study the
random Brownian motion seen in particles suspended in liquids such as
water. The particles themselves are not moving. Rather, the water
molecules that surround the particles are constantly in motion, and this
motion causes the molecules to strike the particles at random. Atoms also
exhibit such random motion due to their kinetic energy. Temperature and
the strength of the bonds holding the atoms in place determine the degree
to which atoms move. Even in solids at room temperature—the chair you
may be sitting on, for example—atoms move about in a process called
diffusion. This ability of atoms to move about increases as a substance
changes from solid to liquid to gas. If scientists and engineers are to
successfully assemble at the atomic scale, they must have the means to
overcome this type of behavior.
A
clear example of such a challenge occurred in 1990 when scientists from
the International Business Machines Corporation (IBM) used a scanning
probe microscope tip to assemble individual xenon atoms so that they
formed the letters IBM on a nickel surface. To prevent the atoms from
moving away from their assigned locations, the nickel surface was cooled
to temperatures close to absolute zero, the lowest temperature
theoretically possible and characterized by the complete absence of heat.
(Absolute zero is approximately -273.16°C [-459.69°F].) At this low
temperature, the atoms possessed very little kinetic energy and were
essentially frozen.
Achieving
this temperature, however, is impractical and uneconomical for the
operation of commercial devices. Nevertheless, the ability of scientists
to manipulate atoms was one of the first indications that the bottom-up
approach might work. It also signaled the emergence of nanotechnology as
an experimental science.
IV
THE EMERGENCE OF NANOTECHNOLOGY
The
concept of nanotechnology originated with American physicist Richard P.
Feynman. In a talk to the American Physical Society in December 1959,
entitled “There’s Plenty of Room at the Bottom: An Invitation to Enter
a New Field of Physics,” Feynman provided examples of the benefits to be
obtained by producing ultrasmall structures. Feynman calculated that the
entire content of Encyclopædia Britannica could be reduced to fit on the
head of a pin, and he estimated that all of printed human knowledge could
be reduced to fit on 35 normal-sized pages.
Although
he did not coin the term nanotechnology, the visionary Feynman predicted
key aspects of today’s nanotechnology, such as the importance of
advanced microscopes and the development of new fabrication methods. He
also emphasized the importance of combining the knowledge, tools, and
methodologies used by physicists, chemists, and biologists. He pointed to
the natural world as an example of how much information and function can
be packed into a tiny volume. A single cell, for example, can move,
perform biochemical processes, and contains within its DNA molecule the
complete knowledge of the design and function of the complex organism of
which it is part.
Feynman
believed the creation of nanoscale devices was possible within the
boundaries set by the laws of physics. He specifically cited the
possibility of atom-by-atom assembly—that is, building a structure (a
molecule or a device) from individual atoms precisely joined by chemical
forces. This possibility led to the concept of a “universal
assembler,” a robotic device at nanoscale dimensions that could
automatically assemble atoms to create molecules of the desired chemical
compounds. Such a device, for example, could assemble carbon atoms to form
low-cost, large diamonds, a potentially important industrial material, now
used only in limited quantities due to the high cost of mining and
synthesis. Such synthetic diamonds could have many industrial and consumer
applications because they are lightweight and yet extremely hard, and are
electrically insulating but excellent conductors of heat. The idea of a
nanoscale robotic assembler continues to be promoted by some researchers,
although there is considerable debate whether such a device is indeed
possible within the known laws of chemistry, physics, and thermodynamics.
Nanotechnology
began being promoted as a key component of future technology in the late
1970s. The term nanotechnology was first used in 1974 by Japanese
scientist Norio Taniguchi in a paper titled “On the Basic Concept of
Nanotechnology.” However, the term was also used by American engineer K.
Eric Drexler in the book Engines of Creation (1986), which had a greater
impact and helped accelerate the growth of the field. By this time, major
breakthroughs had been achieved in industry, such as the formation of
nanoparticle catalysts made of nonreactive metals and used in catalytic
converters found in automobiles. These catalysts chemically reduced
noxious nitrogen oxides to benign nitrogen and simultaneously oxidized
poisonous carbon monoxide to form carbon dioxide.
A
The Tools of Nanotechnology
The
scientific community began serious work in nanoscience when tools became
available in the late 1970s and early 1980s—first to probe and later to
manipulate and control materials and systems at the nanoscale. These tools
include the transmission electron microscope (TEM), the atomic force
microscope (AFM), and the scanning tunneling microscope (STM). See also
Microscope.
A1
Transmission Electron Microscope (TEM)
The
TEM uses a high-energy electron beam to probe material with a sample
thickness of less than 100 nm. The electron beam is directed onto the
object to be magnified. Some of the electrons are absorbed by or bounce
off the object, while others pass through the object and form a magnified
image of the material. A photographic plate, fluorescent screen, or
digital camera placed behind the material records the magnified image.
TEMs can magnify an object up to 30 million times. By contrast a
conventional optical microscope can magnify objects up to 1,000 times.
TEMs are suitable for imaging objects with dimensions of less than 100 nm,
and they yield information on the size of the nanostructure, its
composition, and its crystal structures.
The
TEM is a popular and powerful instrument within the nanoscience community.
Most of the images published in scientific journals on nanocrystals found
in semiconductors were recorded with this instrument. TEMs can easily
visualize individual atoms within semiconductor nanocrystals.
A2
Atomic Force Microscope (AFM)
An
AFM uses a tiny silicon tip, usually less than 100 nm in diameter, as a
probe to create an image of a sample material. As the silicon probe moves
along the surface of the sample, the electrons of the atoms in the sample
repel the electrons in the probe. The AFM adjusts the height of the probe
to keep the force on the sample constant. A sensing mechanism records the
up-and-down movements of the probe and feeds the data into a computer,
which creates a three-dimensional image of the surface of the sample.
Thus, the exact surface topography can be recorded with precise height
information, and individual atoms in the surface can be imaged. The
lateral resolution of this technique, however, is sometimes poor.
A3
Scanning Tunneling Microscope (STM)
An
STM uses a tiny probe, the tip of which can be as small as a single atom,
to scan an object. An STM takes advantage of a wavelike property of
electrons called tunneling. Tunneling allows electrons emitted from the
probe of the microscope to penetrate, or tunnel into, the surface of the
object being examined. The rate at which the electrons tunnel from the
probe to the surface is related to the distance between the probe and the
surface. These moving electrons generate a tiny electric current that the
STM measures. The STM constantly adjusts the height of the probe to keep
the current constant. By tracking how the height of the probe changes as
the probe moves over the surface, scientists can get a detailed map of the
surface. The map can be so detailed that individual atoms on the surface
are visible.
B
Manipulating Atoms
In
addition to imaging, AFM and STM are also useful for manipulating
nanostructures. In this regard, the tips resemble “arms” that can be
used to manipulate individual atoms. For example, not only did scientists
at IBM move and align individual atoms on a flat surface so that the atoms
spelled IBM, but also they used an STM to position 48 iron atoms into a
circular structure, where interesting phenomenon could be visually
inspected. This manipulation was only possible at extremely low
temperatures.
Although
the AFM and STM are capable of moving atoms and individual nanostructures,
the process is very slow and time-consuming. Scientists hope to develop
this technique further by using massive arrays of scanning tips instead of
just using one. Such arrays could help speed up the manipulation of atoms,
although it would also require extensive micro- and nanofabrication.
C
Synthesizing Carbon Molecules and Other Developments
Smallest
Ultraviolet Laser Hundreds of nanowires—tiny forms of carbon molecules
only five to ten atoms wide—make up the world’s smallest ultraviolet
laser, which was created by researchers in the early 21st century.Peidong
Yang,
Several
other developments in the 1980s and 1990s stimulated interest in the
potential of nanotechnology. In 1985 chemists at
Then
in 1991 Japanese physicist Sumio Iijima published a widely noticed report
that appeared to build on the buckyball discovery. While studying
fullerenes, Iijima reported finding a tubular version known as a carbon
nanotube, a thin, extraordinarily stiff form of carbon that has been
described as “the strongest material that will ever be made.” In 1993
two researchers working independently—Iijima in
The
increasing focus of the scientific community on the nanoscale led the
In
addition to the support of federal governments, state governments also
became active in support of nanotechnology. Examples in the
By
2003 significant commercial products had already been developed based on
nanotechnologies. The devices on computers known as read-write heads,
which read data from a computer hard disk and also write data to the disk,
were built from multilayer nanometer-thick film. These films increased the
sensitivity of the read-write heads so that many more bits of data can be
packed on the surface of the hard disks. Consequently, the memory capacity
found in modern personal computers dramatically increased, and relatively
inexpensive 60-gigabyte hard disks became available in competitively
priced computers.
Another
nanotechnology product line was nanoparticle formulations of zinc or
titanium oxides that absorb harmful ultraviolet radiation from the Sun but
are invisible to the eye. This technology has enabled cosmetic companies
to offer skin protection in their products without compromising
appearance. The usually white skin creams become transparent upon
application because the nanoparticles are too small to scatter light.
Nanocoating technology on clothes has yielded the most stain-resistant
clothes ever produced. Olympic-level swimmers have been aided in setting
many new world records by using swimsuits with clothing fibers bonded to
hydrophobic (not compatible with water) molecules. These nanocoated
swimsuits create less friction with water so swimmers can swim faster.
In
the early 21st century corporations began to identify nanoscience and
nanotechnology as a field of development unto itself with many common
concepts and approaches that could impact broadly across multiple product
lines. It became common for major high-tech corporations to have a
specific manager or leading scientist assigned to the development of
corporate nanotechnology strategy, research, and development. In addition
to the larger corporations, the field also began to yield many small
start-up companies. As of 2003 most of these companies were involved in
nanomaterials production, simple nanodevice fabrication, and the
production of tools used to research and manufacture at the nanoscale. In
the investment community, an increasing number of venture capitalist
enterprises began to follow nanotechnology closely, and the first funds
devoted solely to investment in nanotechnology companies were created.
V
CHALLENGES CONFRONTING NANOTECHNOLOGY
The
Tiniest Wires An image from a scanning tunneling microscope (STM) reveals
metallic wires only eight to ten atoms wide. Researchers at
Hewlett-Packard Company in
A
major challenge facing nanotechnology is how to make a desired
nanostructure and then integrate it into a fully functional system visible
to the human eye. This requires creating an interface between structures
built at the nanometer scale and structures built at the micrometer scale.
A common strategy is to use the so-called “top-down meets bottom-up”
approach. This approach involves making a nanostructure with tools that
operate at the nanoscale, organizing the nanostructures with certain
assembly techniques, and then interfacing with the world at the micrometer
scale by using a top-down nanofabrication process.
However,
technical barriers exist on the road toward this holy grail of
nanotechnology. For example, the bottom-up approach generally yields
nanocrystals of 1 nm, a dimension that is too small for current
nanofabrication techniques to interact with. As a result, interfacing a
nanocrystal with the outside world is a highly complex and expensive
process. A novel procedure must be developed to overcome this barrier
before many of the synthetic nanostructures can become part of mainstream
industrial applications.
Also,
as the size of the nanostructure gets increasingly thinner, the surface
area of the material increases dramatically in relation to the total
volume of the structure. This benefits applications that require a big
surface area, but for other applications this is less desirable. For
example, it is undesirable to have a relatively large surface area when
carbon nanotubes are used as an electrical device, such as a transistor.
This large surface area tends to increase the possibility that other
unwanted layers of molecules will adhere to the surface, harming the
electrical performance of the nanotube devices. Scientists are tackling
this issue to improve the reliability of many nanostructure-based
electronic devices.
Another
important issue relates to the fact that the properties of nanocrystals
are extremely sensitive to their size, composition, and surface
properties. Any tiny change can result in dramatically different physical
properties. Preventing such changes requires high precision in the
development of nanostructure synthesis and fabrication. Only after this is
achieved can the reproducibility of nanostructure-based devices be
improved to a satisfactory level. For example, although carbon nanotubes
can be fashioned into high-performance transistors, there is a significant
technical hurdle regarding their composition and structure. Carbon
nanotubes come in two “flavors”; one is metallic and the other is
semiconducting. The semiconducting flavor makes good transistors. However,
when these carbon nanotubes are produced, mixtures of metallic and
semiconducting tubes are entangled together and so do not make good
transistors. There are two possible solutions for this problem. One is to
develop a precise synthetic methodology that generates only semiconductor
nanotubes. The other is to develop ways to separate the two types of
nanotubes. Both strategies are being researched in labs worldwide.
VI
FUTURE IMPACT OF NANOTECHNOLOGY
Nanotechnology
is expected to have a variety of economic, social, environmental, and
national security impacts. In 2000 the National Science Foundation began
working with the National Nanotechnology Initiative (NNI) to address
nanotechnology’s possible impacts and to propose ways of minimizing any
undesirable consequences.
For
example, nanotechnology breakthroughs may result in the loss of some jobs.
Just as the development of the automobile destroyed the markets for the
many products associated with horse-based transportation and led to the
loss of many jobs, transformative products based on nanotechnology will
inevitably lead to a similar result in some contemporary industries.
Examples of at-risk occupations are jobs manufacturing conventional
televisions. Nanotechnology-based field-emission or liquid-crystal display
(LCD), flat-panel TVs will likely make those jobs obsolete. These new
types of televisions also promise to radically improve picture quality. In
field-emission TVs, for example, each pixel (picture element) is composed
of a sharp tip that emits electrons at very high currents across a small
potential gap into a phosphor for red, green, or blue. The pixels are
brighter, and unlike LCDs that lose clarity in sunlight, field-emission
TVs retain clarity in bright sunlight. Field-emission TVs use much less
energy than conventional TVs. They can be made very thin—less than a
millimeter—although actual commercial devices will probably have a bit
more heft for structural stability and ruggedness. Samsung claims it will
be releasing the first commercial model, based on carbon nanotube
emitters, by early 2004.
Other
potential job losses could be those of supermarket cashiers if
nanotechnology-based, flexible, thin-film computers housed in plastic
product wrappings enable all-at-once checkout. Supermarket customers could
simply wheel their carts through a detection gateway, similar in shape to
the magnetic security systems found at the exits of stores today. As with
any transformative technology, however, nanotechnology can also be
expected to create many new jobs.
The
societal impacts from nanotechnology-based advances in human health care
may also be large. A ten-year increase in human life expectancy in the
Nanomaterials
could also have adverse environmental impacts. Proper regulation should be
in place to minimize any harmful effects. Because nanomaterials are
invisible to the human eye, extra caution must be taken to avoid releasing
these particles into the environment. Some preliminary studies point to
possible carcinogenic (cancer-causing) properties of carbon nanotubes.
Although these studies need to be confirmed, many scientists consider it
prudent now to take measures to prevent any potential hazard that these
nanostructures may pose. However, the vast majority of nanotechnology-based
products will contain nanomaterials bound together with other materials or
components, rather than free-floating nano-sized objects, and will
therefore not pose such a risk.
At
the same time, nanotechnology breakthroughs are expected to have many
environmental benefits such as reducing the emission of air pollutants and
cleaning up oil spills. The large surface areas of nanomaterials give them
a significant capacity to absorb various chemicals. Already, researchers
at Pacific Northwestern National Laboratory in
Finally,
nanotechnology can be expected to have national security uses that could
both improve military forces and allow for better monitoring of peace and
inspection agreements. Efforts to prevent the proliferation of nuclear
weapons or to detect the existence of biological and chemical weapons, for
example, could be improved with nanotech devices.
VII
NANOTECHNOLOGY RESEARCH
Major
centers of nanoscience and nanotechnology research are found at
universities and national laboratories throughout the world. Many
specialize in particular aspects of the field. Centers in nanoelectronics
and photonics (the study of the properties of light) are found at the
Albany Institute of Nanotechnology in Albany, New York; Cornell University
in Ithaca, New York; the University of California at Los Angeles (UCLA);
and Columbia University in New York City. In addition, Cornell hosts the
Universities
with departments specializing in nanopatterning and assembly include
Other
major research efforts are taking place at national laboratories, such as
the Center for Integrated Nanotechnologies at Sandia National Laboratories
in Albuquerque and at Los Alamos National Laboratory, both in New Mexico;
the Center for Nanophase Materials Sciences at Oak Ridge National
Laboratory in Tennessee; the Center for Functional Nanomaterials at
Brookhaven National Laboratory in Upton, New York; the Center for
Nanoscale Materials at Argonne National Laboratory outside Chicago,
Illinois; and the Molecular Foundry at the Lawrence Berkeley National
Laboratory in Berkeley, California.
Internationally,
the Max-Planck Institutes in