Color-Filtered STM
One of the most astounding inventions of the late 20th century, the
scanning tunneling microscope, or STM, yields atomic-scale landscapes
of electrically conducting surfaces such as metals. Now, researchers
at the Colorado School of Mines have demonstrated a powerful new technique
for filtering the images. Just as color filters make it easier to discern
desired features in a photograph, "color-filtered STM" makes
it easier to see desired atoms and chemical bonds on a surface. In the
technique, electrons of different energies are analogous to different
colors. Only electrons in desired energy ranges are allowed to jump
or "tunnel" to the STM tip, to build up images of the atoms
or chemical bonds of interest. In the image above, the left-hand side
shows the atomic structure of the silicon (111) surface. Two different
types of silicon atoms (marked in red and blue) exist at that surface
(they differ by their position and the chemical-bonding environment
on the surface). The atoms show up as bright spots in the two grayscale
images on the right.
The next image is the kind that would be obtained with a
conventional STM that has a metal tip. What shows up in this image are
the atoms (shown in blue) that have the highest energy electrons associated
with them.
The final image is a color- or energy-filtered image, in
which the researchers have suppressed the blue atoms, and can now observe
others that have electronic states at lower energy (shown in red). This
silicon surface is actually a special case, in which the 'red' atoms
actually lie sort of beneath the blue ones. Via energy filtering, researchers
can thus "see through" the blue atoms and selectively image
the red ones!
(Thanks to Peter Sutter of the Colorado School of Mines for the images
and the caption.)
