21st CENTURY

RESEARCH REPORT
Microscopes Beyond the Limit
by Charles B. Stevens
Currently we are celebrating the
third century of what, according to Sir
Isaac Newton, should have been a dark
age. Three centuries ago Newton calculated
that telescopes had reached
their ultimate size and none could be
constructed with greater resolution,
because of the limits imposed by aberration,
which he had theoretically
determined.
In a similar mode, contemporary
naysayers have found that microscopes
are also theoretically limited.
But just as scientists over the centuries
have found the means to push back
the apparent limits for telescopes, scientists
today have begun to break
through the supposed limits more recently
proposed for microscopes.
In recent years major advances have
been made in visible light microscopes
and in the development of microscopes
that utilize other forms of
radiation and even particle beams.
Among the latest developments is
the scanning tunneling microscope,
called the STM. The STM works like a
blind man's cane, feeling out the surface
of a microscopic sample on which
the bumps and corners are made by
atoms and molecules.
Scanning Tunneling Microscope
A tiny metal tip is passed along the
surface of the sample. The tip is kept
at a constant distance of a few angstroms
(1 angstrom is a hundred-millionth
of a centimeter). This constant
distance is maintained by keeping a
constant voltage between the tip and
the sample and observing the quantum
tunneling current. This current
depends on the distance between the
tip and the surface. Therefore, keeping
the current constant ensures that
the distance is always the same. The
result is that movements required to
keep the distance constant reveal the
atomic-scale hills and valleys of the
surface of the microscopic sample
(Figures 1-2).
Recent applications of the STM have
included the imaging of biological
molecules submerged in water. Eventually
it is hoped to be able to actively
image these molecules in water while
they are reacting. This could reveal the
fine details of reactions and give crucial
insights into how the reaction of
two compounds is catalyzed by a third.
Another potential application currently
being pursued is to find the actual
sequences or codes for DNA.
The STM is not limited to recording
just the physical shape of a surface.
Other microscopic properties can be
detected. H. Kumar Wickramasinghe
of IBM has developed a technique for
recording the magnetic and thermal
properties of surfaces.
Figure 1
THE SCANNING TUNNELING MICROSCOPE
The scanning tunneling microscope (STM) works like a blind man's cane, feeling out a surface on which the bumps
are made by individual atoms or molecules. A tiny metal tip carrying a voltage scans the surface and electrons
"tunnel" to it in spite of strong repulsion—a phenomenon understood only in terms of quantum mechanics.
Shown at left is an STM image of atoms of BiuTe„Sw a semi-metal. The atoms are 3 angstroms apart. The units on
the image are nanometers, and the scale shows depth in nanometers in terms of light and dark.
At right is the same image in a surface perspective, after computer processing. These images and that in Figure 2
were made on a Digital Instruments (Calif.) NanoScope II scanning tunneling microscope.
22 November-December 1988 21st CENTURY RESEARCH REPORT