Download Introduction to Nanoscience

Introduction to Nanoscience
Nanoscience is the study of phenomena on a nanometer scale. Atoms are a few tenths of a
nanometer in diameter and molecules are typically a few nanometers in size. The smallest
structures humans have made have dimensions of a few nanometers and the smallest structures
we will ever make will have the dimensions of a few nanometers. This is because as soon as a
few atoms are placed next to each other, the resulting structure is a few nanometers in size.
One of the first people to point out potential benefits of making devices at the nanoscale was
Richard Feynman. In a speech he gave in 1959 entitled, There's plenty of room at the bottom,
Feynman talked about "the problem of manipulating and controlling things on a small scale."
He estimated that it should be possible to reduce the printing on a page by a factor of 25000
and if that were done, all the books ever printed could be copied in a space of about 35
ordinary pages. Feynman also speculated about moving individual atoms around. Here's what
he had to say about rearranging atoms.
Rearranging the atoms
But I am not afraid to consider the final question as to whether,
ultimately---in the great future---we can arrange the atoms the way we
want; the very atoms, all the way down! What would happen if we
could arrange the atoms one by one the way we want them (within
reason, of course; you can't put them so that they are chemically
unstable, for example).
Richard Feynman
There's Plenty of
Room at the
Bottom.
Up to now, we have been content to dig in the ground to find minerals.
We heat them and we do things on a large scale with them, and we
hope to get a pure substance with just so much impurity, and so on. But
we must always accept some atomic arrangement that nature gives us.
We haven't got anything, say, with a "checkerboard" arrangement, with
the impurity atoms exactly arranged 1,000 angstroms apart, or in some
other particular pattern.
What could we do with layered structures with just the right layers?
What would the properties of materials be if we could really arrange
the atoms the way we want them? They would be very interesting to
investigate theoretically. I can't see exactly what would happen, but I
can hardly doubt that when we have some control of the arrangement
of things on a small scale we will get an enormously greater range of
possible properties that substances can have, and of different things that
we can do.
In 1960 Feynman thought that in the great future we would be able to arrange atoms the way
we want. This is now possible for some kinds of atoms on some kinds of surfaces. Below are
two images of atomic scale structures made in Don Eigler's lab at IBM. A scanning tunneling
microscope (STM) was used to push atoms and molecules around on a metal surface to make
these structures.
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Fig. 1.1. On the left, iron atoms where arranged on a copper surface to
make the kanji character for "atom". On the right is Carbon Monoxide
Man consisting of carbon monoxide molecules on a platinum surface.
Not every possible arrangement of atoms can be made this way but
technology is moving in the direction that Feynman predicted.
Fig. 1.2. Researchers at IBM made a logic circuit based on the motion
of carbon dioxide molecules on a copper surface. This device is a three
input sorter. The left image was made by a scanning tunneling
microscope and on the right is the design of the circuit showing the
placement of all of the atoms. Reference: IBM molecule cascade
website.
K. Eric Drexler is another person who has considered constructing devices atom by atom. In
two influential books, Engines of Creation, and Unbounding the Future: the Nanotechnology
Revolution he described devices called assemblers that could be used to assemble atoms and
molecules into larger structures. He thought that the first generation of assemblers would
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resemble enzymes. Enzymes are catalysts that are often made from proteins. Drexler believes
that in the future, it will be possible to make universal assemblers. These are assemblers that
can be programmed to construct any stable atomic structure. Here is an excerpt from Drexler's
book.
Universal assemblers
These second-generation nanomachines - built of more than just
proteins - will do all that proteins can do, and more. In particular, some
will serve as improved devices for assembling molecular structures.
Able to tolerate acid or vacuum, freezing or baking, depending on
design, enzyme-like second-generation machines will be able to use as
"tools" almost any of the reactive molecules used by chemists - but they
will wield them with the precision of programmed machines. They will
be able to bond atoms together in virtually any stable pattern, adding a
few at a time to the surface of a workpiece until a complex structure is
complete. Think of such nanomachines as assemblers.
K. Eric Drexler
Engines of
Creation.
Because assemblers will let us place atoms in almost any reasonable
arrangement (as discussed in the Notes), they will let us build almost
anything that the laws of nature allow to exist. In particular, they will
let us build almost anything we can design - including more assemblers.
The consequences of this will be profound, because our crude tools
have let us explore only a small part of the range of possibilities that
natural law permits. Assemblers will open a world of new technologies.
Advances in the technologies of medicine, space, computation, and
production - and warfare - all depend on our ability to arrange atoms.
With assemblers, we will be able to remake our world or destroy it. So
at this point it seems wise to step back and look at the prospect as
clearly as we can, so we can be sure that assemblers and
nanotechnology are not a mere futurological mirage.
Building devices atom by atom like this is called molecular nanotechnology. Not everyone
agrees that it will be possible to build a universal assembler. Some people consider Drexler's
book more science fiction than science. It has stimulated a lively debate about what is possible
with nanotechnology. Drexler wrote that aging could be stopped or reversed using
nanotechnology. Some people have had their heads frozen after they die so that they can be
reanimated by nanotechnology in the future. This has given nanotechnology the reputation of
sometimes being a bit wacky. However, it is interesting to speculate on what could be possible.
Another nanotechnology visionary is who has stimulated much debate is Ralph Merkle. Here's
an excerpt from one of his articles.
It's a small, small, small, small world
Ralph Merkle
It's a small,
small, small,
small world.
Manufactured products are made from atoms. The properties of those
products depend on how those atoms are arranged. If we rearrange the
atoms in coal, we get diamonds. If we rearrange the atoms in sand (and
add a pinch of impurities) we get computer chips. If we rearrange the
atoms in dirt, water and air we get grass.
Since we first made stone tools and flint knives we have been arranging
atoms in great thundering statistical heards by casting, milling, grinding,
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chipping and the like. We've gotten better at it: we can make more things
at lower cost and greater precision than ever before. But at the molecular
scale we're still making great ungainly heaps and untidy piles of atoms.
That's changing. In special cases we can already arrange atoms and
molecules exactly as we want. Theoretical analyses make it clear we can
do a lot more. Eventually, we should be able to arrange and rearrange
atoms and molecules much as we might arrange LEGO blocks. In not too
many decades we should have a manufacturing technology able to:
1. Build products with almost every atom in the right place.
2. Do so inexpensively.
3. Make most arrangements of atoms consistent with physical law.
...
What would it mean if we could inexpensively make things with every
atom in the right place? For starters, we could continue the revolution in
computer hardware right down to molecular gates and wires -- something
that today's lithographic methods (used to make computer chips) could
never hope to do. We could inexpensively make very strong and very
light materials: shatterproof diamond in precisely the shapes we want, by
the ton, and over fifty times lighter than steel of the same strength. We
could make a Cadillac that weighed fifty kilograms, or a full-sized sofa
you could pick up with one hand. We could make surgical instruments of
such precision and deftness that they could operate on the cells and even
molecules from which we are made -- something well beyond today's
medical technology. The list goes on -- almost any manufactured product
could be improved, often by orders of magnitude.
We can't yet build most of the nanomachines that are being discussed but they can be
simulated on a computer. Figure 1.3 shows a computer simulation of a nanoscale bearing that
was designed by Ralph Merkle. Using simulations, it is possible to test a nanomachine and see
if it would work the way it was designed to work. More information on the bearing simulation
can be found on the Zyvex website.
Fig. 1.3. A simulation of a nanoscale bearing.
Nanoscience in physics, chemistry, biology, and medicine
Physics is the mother of the natural sciences. In principle, physics can be used to explain
everything that goes on at the nanoscale. There is active physics research going on in
nanomechanics, quantum computation, quantum teleportation, and artificial atoms. While
physics can explain everything, sometimes it is more convenient to think of nanostructures in
terms of chemistry where the molecular interactions are described in terms of bonds and
electron affinities.
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Chemistry is the study of molecules and their reactions with each other. Since molecules
typically have dimensions of a few nanometers, almost all of nanoscience can be reduced to
chemistry. Chemistry research in nanotechnology concerns C60 molecules, carbon nanotubes,
self-assembly, structures built using DNA, and supermolecular chemistry. Sometimes the
chemical description of a nanostructure is insuffient to describe its function. For instance, a
virus can be described best in terms of biology.
Biology is sometimes described as nanotechnology that works. Biological systems contain
small and efficient motors. They self reproduce. They construct tough and strong material.
Biology is an important source for inspiration in nanotechnology. Copying engineering
principles from biology and applying it to create new materials and technologies is called
biomimetrics.
Fig. 1.4. A chloroplast, where photosynthesis takes place in plants, is
filled with nanoscale molecular machinery. Reference: Nanotechnology:
Shaping the world atom by atom - by the U.S. National Science and
Technology Council.
An example of biomimetrics is trying to produce materials with the strength and lightness of
seashells. Shells are strong, light materials composed of aragonite (a crystalline form of
calcium carbonate which is basically chalk) and a rubbery biopolymer. Another example is a
thin glass fiber grown by an ocean sponge. These glass fibers capable of transmitting light
better than industrial fiber optic cables used for telecommunication. The "Venus flower basket
sponge" grows the flexible fibers at cold temperatures using natural materials. If this could be
reproduced in the laboratory it might avoid the problems created by current fiber optic
manufacturing methods that require high temperatures and produce relatively brittle cable.
Other biomimetric discoveries are enzymes taken from bacteria that break down fat in cold
water have been used to improve laundry detergent and proteins from jellyfish are used during
surgery to illuminate cancerous tissue. Trying to make a computer that mimics how a brain
works is another example of biomimetics.
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Fig. 1.5 (Top-left) fracture surface and (bottom-left) polished section of mollusk shell showing
the micro-laminated construction responsible for its hardness, strength, and toughness. Taken
from http://www.mrti.utep.edu/Full%20Pages/selfass.htm. (right) A scanning electron
micrograph of the cell wall of a diatom. Diatoms are unicellular algae and major component of
plankton. The circular cell membrane is about 5 μm in diameter and is made of silica (a form
of glass). The algae make features in silica on the nanometer scale using just seawater and
sunlight. Diatom Home Page http://www.indiana.edu/~diatom/diatom.html.
Some of the applications of nanotechnology are in the field of medicine. In science fiction,
miniature submarines sometimes travel through the body effecting repairs. While researcher
are actively working on drug delivery systems that encapsulate drugs and release them at the
point they are needed, vessels that can travel through the body would have to be far less
complex than those described in the science fiction books.
Nanocamera
A nanodevice that often appears in science fiction is a nanocamera. This is used to view the
inside of the body or in other confined spaces where an ordinary camera would not fit.
Unfortunately, it is not possible to make such a camera using conventional far field optics.
Light sources and light detectors can be made very small; a single molecule is large enough to
serve as a simple light source or light detector. However, the amplitude of a light wave cannot
change over a distance much shorter than the wavelength of the light. This means that if
detectors are placed together spaced more closely than a wavelength they will all measure the
same light intensity and no image will be formed. The wavelength of visible light is about one
half micron so the detectors that make up a camera should be spaced at least this far apart. For
an image consisting of 1000 by 1000 pixels, the camera would have to be about one half a
millimeter on a side. This is small but clearly visible to the naked eye. It is too large to pass
through the smallest blood vessels in the body.
Using a camera of any size it is not possible to view something with the dimensions of a few
nanometers using visible light. Since the amplitude of the light cannot change on a scale
shorter than the wavelength, any nanoscale object viewed with visible light will be no more
than a shadowy blur. One way to get around this problem is to use shorter wavelengths. X-rays
have short enough wavelengths but are difficult to focus. An electron microscope can have a
resolution of a few nanometers. However, there are no nanoversions of imaging systems that
use these short wavelengths and x-ray photon or energetic electrons have enough energy to
damage a nanoscale structure. Building a nanoscale camera that uses short wavelengths does
not seem feasible.
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Fig. 1.5. An artist's impression of a nano-louse immunizing a single red blood cell. The details
shown in this drawing are too small to be seen with visible light.
Recognition at the nanometer scale is done by touch. Two molecules that react with each other
will bump together in random orientations until by chance they find the correct configuration
and react. The molecules have to be in intimate contact for this to happen. Scanning probe
microscopes also touch nanoscale objects in order to image them. This is done by bringing a
sharp tip closer than 1 nm to the object that should be imaged and measuring the interactions
between the nano-object and the tip. The tip is then moved around until an image is formed.
Some of scanning probe imaging techniques are described in more detail in the section on topdown nanotechnology. No nanoscale versions of scanning probe microscopes are presently
available or are likely to be made available in the foreseeable future.
The closest thing to a nanoscale camera that we have is probably the drugs that are used in
medicine. If the structure of a undesirable object such as a virus or a cancer cell is known, then
a drug with a complementary structure can be made. The virus and the drug should fit together
like a lock and key. When the drug comes in contact with the virus it binds to it. This either
disables the virus or marks it in such a way that the body can destroy it by other means. This
kind of camera can only recognize a certain shape and it is difficult to extent this concept to a
general purpose camera that can see any shape.
Estimate the size of an atom
The object of this last section is to estimate the the size of an atom. The simplest atom,
hydrogen, consists of a positively charged proton and a negatively charged electron. The force
of attraction between the proton and the electron is given by Coulomb's law (Understanding
Physics, Eq. 15.8),
F=
q1q2
r^.
4πεor2
The potential energy of the electron can be calculated by integrating the force as a function of
the position (see Understanding Physics, page 87). The potential energy is,
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U=-
e²
.
4πεor
The potential energy is negative. The electron lowers its energy as it approaches the proton. It
would seem that the electron would want to get as close to the proton as possible. However,
there is another energy that prevents the electron from getting too close to the proton. This is
the quantum confinement energy.
The quantum confinement energy can be estimated using the Heisenberg uncertainty relation,
ΔxΔpx ≥ h/(4π). For a hydrogen atom, the uncertainty in the position of the electron is about
the radius of the atom Δx = r. This means that the then Δpx ≥ h/(4πr). The definition of the
uncertainty in momentum is,
Δpx = √ <px²> - <px>²
Here the angular brackets < > signify that the average value of the quantity in the brakets
should be used. Since the electron is confined to a region near the proton, the average
momentum of the electron is zero, <px> = 0. This means that the average kinetic energy of the
electron in the x-direction is,
<px²>
h²
≥
2m
32π²mr²
There is an analogous increase in the kinetic energy in the y and z directions so the total
increase in kinetic energy is three times the increase calculated above.
Econfinement =
<px²> <py²> <pz²>
3h²
+
+
≥
2m
2m
2m
32π²mr²
The electron decreases its potential energy by getting close to the proton but increases its
confinement energy by concentrating itself too much around the proton. The radius of an atom
can be estimated by determining the radius that minimizes the total energy of the electron. The
radius that minimizes U + Econfinement is,
r=
3h²εo
. = 4.0 × 10-11 [m].
4πme²
This is 3/4 of the Bohr radius, the true radius of an electron in hydrogen that can be calculated
using a full quantum mechanical analysis. The important result of this calculation is that the
kinetic energy of a particle increases as the particle is confined to a smaller region. It is the
balance between this confinement energy and the electrostatic energy that is largely
responsible for determining the size of atoms. This discussion also shows that quantum effects
have to be considered when discussing phenomena at the nanometer scale.
Reading for week 1

There's plenty of room at the bottom - by Richard Feynman. This is essential reading for
anyone interested in nanotechnology.
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References




V.C. Sundar, A.D. Yablon, J. L. Grazul, M. Ilan and J. Aizenberg, "Fibre-optical features
of a glass sponge," Nature vol. 424 p. 899 (2003).
nanoTechnologie, Hoofdstuk 1 - VWO Masterclass gegeven door J. M. van Ruitenbeek
en J. W. M. Frenken.
Browse through Drexler's book, Engines of Creation.
Nanotechnology: Shaping the world atom by atom - by the U.S. National Science and
Technology Council.
Fantastic Voyage by Isaac Asimov - Four men and a woman
are reduced to a microscopic fraction of their original size, sent
in a miniaturized atomic sub through a dying man's carotid
artery to destroy a blood clot in his brain. If they fail, the entire
world will be doomed.
Problems
1. Estimate how long it would take for a single universal assembler to make a structure that
weighs one gram. Estimate the maximum size of a structure that a universal assembler could
create in one hour.
2. To image an object with the dimensions of a few nanometers with electromagnetic radiation,
the wavelength of the radiation would have to be smaller than the obect being imaged. What
kind of radiation would be necessary? What would the energy of a single photon be? Would
these photons have enough energy to destroy the object being observed?
3. In Star Trek, a device call a replicator was used to create near-perfect reproductions of small
objects by using a molecular pattern stored in computer memory. Most food service on
starships is provided by food replicators located throughout the ship. Could this work? Could
you use it to produce fuel? The science of Star Trek.
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