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The writing is less than
half an Angstrom (0.00005 microns) "tall" off the surface. The
characters are less than 500 Angstrom (0.05 microns) high. They
were created when Weiss and his team moved hydrogen atoms
underneath a palladium surface using a custom-built, ultrastable,
low-temperature, scanning tunneling microscope.
Manipulation of
subsurface hydorgen atoms in palladium by scanning tunneling
microscopy to form the subsurface hydride.
Schematic of the effect
of populating subsurface sites with hydrogen atoms from the bulk.
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Observations of the effects of the resulting
subsurface hydrides - hydrogen atoms with a partial negative charge -
confirmed the existence of the stable sites, which had been predicted
but previously had neither been deliberately assembled nor directly
observed. The research was led by Paul S. Weiss, Distinguished
Professor of Chemistry and Physics at Penn State.
After moving absorbed hydrogen atoms to just below the crystal surface,
the researchers were able to observe how the presence of the hydride
in specific sites within a metal crystal affects the chemical,
physical, and electronic properties of the metal. Understanding these
effects could advance efforts to improve chemical reactions involving
metal catalysts. In addition, the subsurface hydride may provide a
model material for application in hydrogen storage and fuel cells. The
ability to prepare the subsurface hydride provides an important
research tool for these applications.
Weiss points out that hydrogen atoms just below the
surface of the metal have been thought to be important in a number of
chemical reactions. "Indirect experimental data have shown that
chemically reactive hydrogen atoms were located at such sites, but
there was no way to test them," says Weiss. "This material will allow
us to test the predictions and to apply data from direct observation."
The researchers carried out the experiments in a
low-temperature scanning tunneling microscope (STM) under ultrahigh
vacuum by exposing the crystal to a hydrogen atmosphere. They removed
excess hydrogen from the surface by cycles of exposure to heat and
oxygen. After the surface had been cleaned, the researchers were able
to use electrons from the STM tip to move hydrogen atoms that had been
absorbed into the bulk metal up into the stable subsurface sites. As
the hydride formed underneath the surface of the material, Weiss and
his team observed that the surface of the crystal distorted, the
positive charge of palladium atoms above them increased, and
interactions occurred with hydrogen atoms on the surface of the
palladium crystal. "One of the most interesting aspects of the
research was the ability to move atoms beneath the surface," Weiss
says. "The observation of the effects of the populated sites, such as
surface distortion, confirmed the existence of the stable sites and
the theoretical predictions of the physical and electrical properties
of the hydrides."
Years ago, Weiss was the first on an IBM team to manipulate xenon
atoms on a metal surface. His coworkers later moved atoms to spell out
their corporate logo. By extending the ability to manipulate atoms
beyond the surface of a material, this research is expected to advance
the understanding and control of important chemical reactions in a
variety of commercial applications. In addition, this ability has
potential as a model system of a technologically important material.
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