(Off Topic) Your child's child's child Atomic TinkerToy Set from Radio Shack.
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Some Assembly Required, by Sasha Nemecek
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Scientists can grab an individual atom and place it exactly where they want.
Welcome to the new and exciting world of atomic engineering
Everything around us
--from concrete blocks to computer chips--
is made of atoms.
They are nature's Tinkertoy set,
but it can take a Herculean effort for humans to rearrange individual,
all but weightless, atoms.
Consider how minuscule they are:
Some two trillion would fit in this letter o.
But researchers have now developed tools that enable them to see,
grasp and move these tiny particles.
Image: Hongjie Dai, Stanford University
RING OF IRON: By using a scanning tunneling microscope to pick up
individual atoms, scientists at the IBM Almaden Research Center
positioned 48 iron atoms in a circle on top of a copper surface.
The ripples inside the ring are the result of the wavelike behavior
of electrons in the system. The technology dates back to the early 1980s,
when two European physicists, Gerd Binnig and Heinrich Rohrer, working
at the IBM Research Laboratories in Zurich, built the first instrument
that could display images of atoms: the scanning tunneling microscope.
Despite its name, though, the STM is not a true microscope. Rather than
capturing direct images with the help of lenses, optics and light, an
STM relies instead on translating electric current (from the surfaces of
conductors--metals, semiconductors or superconductors) into images of atoms.
The most important feature of any STM is its ultrasharp probe--typically
a thin wire designed so that a single atom hangs from the tip. Atoms
consist of a positively charged nucleus at their center surrounded by
negatively charged electrons, in what scientists call an electron cloud.
In the case of atoms positioned at the surface of any material, these
electron clouds protrude just slightly above the plane, like rows of
tiny foothills. Once the STM probe comes close enough to one of the
surface atoms--around a nanometer (one billionth of a meter) away--the
electron cloud of the atom on the end of the probe and that of the
surface atom begin to overlap, causing an electronic interaction.
When a low voltage is applied to the STM tip, a so-called quantum tunneling
current flows between the two electron clouds. This current turns out to be
highly dependent on the distance between the tip and the surface.
Image: Hongjie Dai, Stanford University
MIX-AND-MATCH molecule: Atomic engineers eventually hope to create
molecules from scratch, adding atoms exactly as needed to perform
specific functions. This molecule, with 18 cesium and 18 iodine atoms,
was built--one atom at a time--with a scanning tunneling microscope (or
STM). A helpful way to think of the STM probe is like a finger reading
Braille. Researchers using an STM typically program the computer
controlling the probe to keep the current between the tip and the
surface atoms at a constant level. So as the feedback probe scans back
and forth across a sample, it also shifts up and down, following the
contours of the electron clouds. For instance, as an electron cloud
emerges from the plane of the surface and the tip comes closer to the
atom, the tunneling current at the probe would ordinarily increase.
As soon as the computer registers this difference, however, it tells
the tip to pull back from the surface and in this way maintains a stable
current reading.
Alternatively, as the electron cloud falls below the surface plane and
the tip separates from the atom, the probe would normally detect a lower
tunneling current. Once again, though, the probe responds to this
change, coming closer to the surface to preserve a constant current
level. Over time the probe generates a topographical survey of the
surface, essentially "feeling" the size and location of atoms.
The results of STM scans can be stunning. Scientists use computer
programs to translate the probe's motion into images of the surprisingly
rugged terrain of seemingly smooth surfaces, often adding color to
emphasize the peaks and valleys of the atomic geography. Indeed, early
work with the STM centered on generating images of the atoms at the
surface of metals, semiconductors and superconductors, revealing
unexpected and often informative patterns and imperfections.
Image: Hongjie Dai, Stanford University
SHORT LIST: A carbon nanotube--essentially a "buckyball" stretched into
a hollow tube of carbon atoms some 10 nanometers wide--has been
transformed into a writing implement. Using an atomic force microscope
with a nanotube tip, researchers at Stanford University removed hydrogen
atoms from the top of a silicon base. The exposed silicon oxidized,
leaving behind a visible tracing. More recently researchers have
discovered they can also use the STM to move individual atoms. Instead
of just hovering right above the atoms, the STM tip can actually reach
down and pick up a single atom. This trick is possible because the
interaction between the atom on the probe's tip and the surface atom
becomes stronger as the tip moves closer to the surface. Eventually this
interaction leads to a temporary chemical bond between the two atoms,
which is stronger than those between the surface atom and its neighbors.
Once this bond forms, the tip essentially holds on to the surface atom,
permitting scientists to move the probe and guest to the desired location.
Today the technology behind the STM has been adapted for use in a
variety of similar imaging devices. The atomic force microscope, or AFM,
for instance, enables scientists to study biological systems, from DNA
to molecular activity within a cell. Instead of relying on changes in
the quantum tunneling current between the tip and surface atoms, the AFM
exploits fluctuations in other types of atomic and molecular scale
forces--mechanical or electrostatic forces, for instance--again feeling
the surface geography. AFM has become a significant tool for biologists
and chemists.
The holy grail for these atomic engineers is to build a molecule
atom by atom, with the goal of one day constructing a new type of material.
Physicist Donald M. Eigler, who works at the IBM Almaden Research Center
in San Jose, has produced in his laboratory a molecule consisting of 18
cesium and 18 iodine atoms [see STM image above]--the largest molecule
ever to be assembled in atomic installments. And although there is no
immediate use for such a compound, there is plenty of interest in the
technology. The dream is to build new materials that might serve, say,
as ultrahigh-density data storage for future computers or as a novel
medical device. All of this with a few atomic Tinkertoys.
About the Author
Sasha Nemecek is co-editor of this issue of Scientific American Presents. |
