Number 284, September 3, 1996 by Phillip F. Schewe and Ben Stein
MOVING NATIVE ATOMS ON A SURFACE IS NOW POSSIBLE with scanning tunneling
microscopes (STMs), providing a tool for addressing one of the central
questions in surface science: when foreign atoms and molecules are adsorbed
onto surfaces, do they sit directly on top of the substrate atoms or do
they settle into the crevices between them? Gerhard Meyer (gmeyer@axpried.physik.fu-berlin.de)
and his colleagues at the Free University in Berlin use STMs to manipulate
the native copper atoms on a surface. When the STM tip is moved close to
the surface, it can exert a force sufficient to overcome the energy barrier
that ordinarily prevents the copper atoms from moving along the surface.
By moving this copper atom about in the vicinity of an adsorbed CO molecule,
a sort of surveyor's grid can be established which allows the position
of the visitor to be located with new accuracy. With this technique, Meyer
was able to infer that the CO molecule lies directly on top of a surface
atom, instead of between two neighboring atoms. Furthermore, with this
grid in place, the carbon monoxide then became a "marker," providing
information on how other species, such as nearby C2H4 molecules and lead
atoms, registered with the surface. This technique can potentially be used
to study adsorption on other metallic surfaces, even very complex ones
which cannot be studied with conventional crystallography methods. (Gerhard
Meyer et al., Phys. Rev. Lett., 2 September 1996; a color figure illustrating
this story will in the next day be posted on the Web at this address: /png/)
PHASE IMAGING WITH HIGH-ENERGY X-RAYS . Conventional x-ray imaging methods
detect how many x rays are absorbed as they pass through a sample. Contrast
in the resulting image arises from variations in absorption. But as objects
get thicker it is necessary to use high-energy x rays (which are more penetrating)
and larger amounts of x rays (to build up a good contrast). But this can
cause considerable radiation damage to the sample. An alternative approach,
requiring much smaller amounts of x-rays, is to image objects by measuring
changes in phase. (If one considers the x ray as a wave with crests and
valleys, a phase shift is the amount by which the crests and valleys are
shifted when the x ray penetrates the object.) The phase shift can provide
information about both the composition of the material and its thickness.
This technique normally requires that an x-ray beam be split in two, one
part passing through the sample and the other serving as a reference beam.
Then the two waves are brought together again to form an interference pattern.
Unfortunately, these interferometric methods are difficult to carry out
because the beam of x rays must be coherent, and currently lasers are only
available at lower (softer) x-ray energies. However, Keith Nugent of the
University of Melbourne in Australia (k.nugent@physics.unimelb.edu.au)
and his colleagues have demonstrated a phase imaging technique that does
not require the use of interferometry. Instead they convert measurements
of how the x rays are re-directed through a sample into information about
the phase shifts at different points in the sample. This "phase map,"
in turn, can be transformed into a direct physical image. So far the researchers
have used this method to image a carbon grid with lines 330 microns apart.
Ultimately, the researchers expect potential resolutions of about 1 micron
with current detector technology and are exploring the goal of making phase-based
CAT scans of the interiors of objects.(K.A. Nugent et al., Physical Review
Letters, 30 September 1996.)
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