| Press Images
This atomic force microscope image by German physicist Franz
Giessibl shows dozens of silicon atoms. Scientists have
debated whether the light and dark crescents - or wing-shaped
features seen on the atoms represent orbitals - the paths
of electrons orbiting the atoms.
To download
high-resolution click here:
Credit: Franz Giessibl |
This blurry atomic force microscope image is a closeup of
one silicon atom. Some scientists argue the whitish vertical
features are orbitals.
To download
high-resolution click here:
Credit: Franz Giessibl
|
A computer simulation by Feng Liu of the University of Utah
produced these topographical map-like images, which represent
the view seen by an atomic force microscope as it moves toward
a single silicon atom. The atom appears as a single "peak"
or object in the top and middle image, but as the microscope
gets closer, there are two "peaks,' which represents
two "wings" or orbitals representing the paths of
the atom's electrons.
To
download high-resolution click here:
Credit: Feng Liu, University of Utah |
Atomic Force Microscopy – Today and Tomorrow
An atomic force microscope has a vibrating tip that narrows to
one or two atoms, usually silicon or tungsten. The tip almost
touches the surface it is scanning. The microscope measures the
chemical bonding force between the tip and the atoms in the scanned
surface. The force is stronger if the tip is above an atom and
weaker if it is above a space between atoms. An image of the surface
is created as the tip scans the surface to make many such measurements.
“The Nobel Prize-winning invention of the scanning tunneling
microscope (STM) and the atomic force microscope (AFM) have allowed
us to directly see individual atoms on a solid surface,”
Liu said. “However, atoms generally appear as a single protrusion
or blur in STM and AFM images because the microscopes can’t
resolve the details of atomic orbitals.”
Because a solid’s material properties depend on the structure
of its surface, atomic force microscopes are used to determine
what materials are suitable as semiconductors, to make electronic
devices and to manipulate atoms into desired structures. Liu says
practical uses of the microscopes will expand when they routinely
make images of atomic orbitals:
-- Some industrial chemical reactions are speeded or “catalyzed”
when they occur on the surface of a piece of metal. If atomic
force microscopes can make images of atomic orbitals on the metal
surface, engineers will be able to “see much more detail
of the atoms and the bonds between them,” helping industry
design better metals and catalysts for a wide variety of manufacturing
processes.
-- An ability to “see” not only atoms but also the
orbital paths of their electrons will aid the field of nanotechnology,
which Liu defines as building materials or even machines “atom
by atom.”
-- Atomic force microscopes now can reveal defects in materials
by showing where atoms are missing. If the microscopes can routinely
“see” orbitals, that would reveal how the remaining
atoms are bonding together around the defect.
-- Conventional atomic force microscopy sees atoms only as spheres,
so it cannot distinguish elements. Atoms of different elements
have different orbital configurations, so the ability to detect
orbitals might let an atomic force microscope not only distinguish
different chemical elements, but also infer details of chemical
bonds between atoms.
The New Study
Liu said his study involved using a supercomputer to simulate
the use of an atomic force microscope to scan a surface of silicon
atoms – the stuff of which computer semiconductor chips
are made – using calculations that follow the rules of quantum
mechanics, the theory that governs the motions of electrons.
“To run these calculations took a half a year on a parallel
supercomputer,” he says.
He and his colleagues used 64 “nodes” or individual
personal computers out of more than 300 PCs that comprise ICE
Box, a “cluster” supercomputer made by assembling
numerous off-the-shelf PCs. The individual PCs run together in
“parallel” to perform complex computations. Some of
the work also was conducted on a Cray supercomputer at a National
Science Foundation supercomputer center in Pittsburgh.
One criticism of the 2000 study by Giessibl – a physicist
at Germany’s University of Augsburg – was that the
apparent image of orbitals on silicon atoms might be due to an
imprecise mathematical description of the interaction between
the atom on the tip of the atomic force microscope and the silicon
atoms being scanned by the microscope.
Liu says he overcame that criticism because his calculations provided
a very accurate simulation of how the microscope tip interacts
with silicon atoms on the scanned surface.
Some scientists argued the crescent-shaped shadows on Giessibl’s
silicon atoms were not orbitals but were artifacts, or non-natural
images somehow introduced by the process of using the microscope.
Liu says his study cannot directly address that criticism. But
he adds that his study does prove the principle that atomic orbitals
can be imaged, whether anyone has already seen them or not.
Reaction to the Utah Findings
Giessibl says that since his 2000 study, his laboratory has used
atomic force microscopy many times to detect subatomic features
on silicon and other materials.
“Experimental and theoretical work on subatomic resolution
opens a new domain of probing matter on atomic and subatomic length
scales – a sure boon to nanoscience, to which Prof. Liu’s
work represents a highly important contribution,” Giessibl
says.
University of Utah researchers who did not work on Liu’s
study say they do not know whether Giessibl actually detected
atomic orbitals, but they say Liu’s study shows an atomic
force microscope theoretically could do so.
Physicist Julio Facelli, director of the Center for High Performance
Computing, says he has “no doubt” Liu is correct in
concluding it is possible to make images of orbitals, but that
does not necessarily prove Giessibl’s image really shows
them.
“In principle, it should be possible to see structure on
the scale of orbitals,” says Clayton Williams, a professor
of physics and an expert on atomic-scale imaging. “There
is not a fundamental constraint saying it’s not possible.”
University of Utah physics Chair Z. Valy Vardeny notes: “People
were skeptical historically about being able to see atoms. But
it was proven to be right.”
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