Atoms and Substrate Images
Date: March 2009
Back in 1989, scientists at the IBM Research Center in San
Jose, California conducted an interesting activity using atoms. The
scientists manipulated 35 atoms of the gas xenon on a nickel
substrate, to write the letters, IBM.
Why are none of the nickel substrate atoms visible? I have seen
other images, since then, with much the same issue: Where are the
atoms of the substrate? It always looks smooth! Please help me
explain this to my students.
Imaging at atomic scale is quite different than how people see things at our
scale. Nanoscale imaging, unlike our eyes, doesn't use light, lenses and
detectors, and that means the results may seem counter-intuitive compared to
how human eyes work. However, with a little understanding of how nano
imaging works, the images produced will make more intuitive sense.
In the famous IBM image, a type of instrument called an STM (scanning
tunneling microscope) was used. The principle of operation of an STM is that
a very small pointy tip is moved very close to a surface, and when it gets
close enough, a small electrical change can be detected. When you move the
tip across a surface and measure the height at which the change occurs, you
can map out a series of heights associated with different coordinates. Think
of it as an atomic topographic map. You can then use software to create an
image of the heights and coordinates that "looks like" a photograph.
In this case, the instrument was set up to image the xenon atoms, but not to
reach all the way down to the nickel surface between the nickel atoms. The
purpose was to emphasize the xenon atoms, not the nickel substrate. That's
why they got the image they did. However, they could have imaged the nickel
had they so desired. If you Google for STM images, you can see lots of
examples of flat planes of atoms being imaged by STM.
Hope this helps,
The way the "atoms" are imaged is through a process called Atomic Force Microscopy
(AFM). As you know, all atoms exhibit a repulsive force (Van der Waal) as though
the atom were acting like solid object. AFM uses a technique whereby an atom thin
stylus (like in those old style phonographs for vinyl records) that is dragged
through the surface of a substrate and the "bumps" produced by individual atoms
through their Van der Waal's radius, causes the stylus to deflect. The deflection
is measured as a force and converted to an image. The stronger the deflection, the
stronger the force, the higher the mound depicted in the computer generated image.
There are two reasons the substrate does not appear to have any bumps (and therefore
the atoms in the substrate do not get imaged), is (1) a very uniform patch of the
substrate is used so that the bumps are regularly spaced and predictable (usually
densely packed silicon crystals), and (2) since the image is essentially computer
generated from data, the substrate data -which is very uniform and predictable- can
be deleted so that the added information (such as the IBM letters) becomes clearly
visible in relief.
Greg (Roberto Gregorius)
Once you understand what the Scanning Tunneling Microscope (STM) is
doing, the answer becomes more clear. The STM uses a probe that has
been sharpened to an incredibly sharp point... a point only one or two
atoms at its tip. This probe is "hovered" very accurately over the
xenon atoms. In fact, it passes much less than one atom's diameter
over the xenon atoms (which, by the way, are much larger than nickel
atoms). Using the quantum tunneling effect, the STM (operating via its
probe) electrically senses the presence of the Xenon atoms as the
probe passes over, and hence a 3D map can be made of their presence
(which is in essence what you are seeing). Note that you are not
seeing a photo, but only a map of the electric charge.
Whereas the probe passes (and can sense) the xenon atoms at a distance
much smaller than an atom's diameter, the probe is too far away from
the nickel substrate atoms below, to be able to sense the individual
nickel atoms themselves. In a way, it is as if the nickel background
is blurred out of focus and all the STM can "see" is a blurred
background plane that underlies the xenon atoms.
This is not unlike what happens with a closeup shot using a normal
camera. Suppose you were trying to take a shot of an object that was
positioned some distance in front of very fine black and white
checkerboard pattern. If you focused on the object, it would be very
clear, but the background may be so badly out of focus, it might
appear as a solid, featureless grey background.
Click here to return to the Material Science Archives
Update: June 2012