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SEEING ATOMS
from:
http://www.physicscentral.com
About Seeing Atoms
What does it mean to see an atom?
Suppose you tried to use the world’s strongest
optical microscope to see an atom. What would happen? You would
probably reflect light from the atoms into your microscope. Light
has wave properties, so imagine waves of light shining on an atom.
The wavelength of visible light is about ten thousand times the
length of a typical atom.
To help think about this, let’s switch to thinking
about water waves rolling in on a beach. If you stand in the water,
the waves roll past you, unaffected. Hardly any wave energy is
reflected. That’s because the size of your body is so much less than
the wavelength of the water waves. The waves move by as if you were
not there, so reflection of water waves will not reveal the presence
of a person in the water.
The wavelength of visible light is about 10-6
m (the same as 103 nm), as shown in the drawing . The
size of a typical atom is about 10-10 m, which is 10,000
times smaller than the wavelength of light. Since an atom is so much
smaller than the wavelength of visible light, it’s much to small to
change the way light is reflected, so observing an atom with an
optical microscope will not work.
How about radiation like light but with a shorter
wavelength? X-ray wavelengths are about the same size as atoms, but
reflecting x-rays from matter forms a complex pattern of spots that
depends on the arrangement of the atoms. Analysis of these patterns
reveals a lot of important information about crystals, but the x-ray
images do not show individual atoms.
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Research
The best way to image atoms is with a device
called a scanning tunneling microscope. It is based on tunneling, a
quantum-mechanical effect roughly analogous to water leaking right
through the sides of a glass. If a small needle comes within about
10-9 m of a metal surface, an electric current, due to
the tunneling, starts to flow. The size of this current depends upon
the separation of the needle and the atom and decreases as the
separation increases. (See diagram.)
Schematic view of the scanning tunneling microscope (image courtesy
of
IAP/TU Wein STM Gallery)
An automatic control system based on this current
produces a detailed map of the surface. If the current starts to go
down, the needle is moved towards the atoms to bring the current
back up, and vice versa. Monitoring the movement of the needle makes
an image of the surface, right down to the size of individual atoms.
The image shows what is called the “Quantum Corral.” A ring of 48
iron atoms corrals electrons, shown in blue, inside the ring and
prevents them from escaping. The ripples inside the ring display the
wave properties of electrons.
How do you move an atom? It turns out that the atom
will stick to the tip of the needle if the current is just right.
When the atom is in the desired position, the current is reversed
and the atom remains in place on the surface. The image shows the
quantum corral being assembled.
This technique makes possible a whole new kind of
chemistry investigation, where chemical reactions are studied in
exquisite detail by bringing together atoms to make molecules one at
a time. After the reaction, the molecule remains on the surface,
where it is easy to observe with the scanning tunneling microscope.
A corral made by placing 48 iron atoms in a circle, one at a time,
onto a surface of gold (Reprinted with permission from
IBM)
Several stages in the assembly of the quantum corral (Reprinted with
permission from
IBM)
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