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'whirlpool' that pulls surface atoms into crystal
E. Kloeppel, Physical Sciences Editor
CHAMPAIGN, Ill. —
Developing novel ways to control the motion of atoms on surfaces is
essential for the future of nanotechnology. Now, researchers at the
University of Illinois at Urbana-Champaign have found a phenomenon of
dislocation-driven nucleation and growth that creates holes that spiral
into a surface and pull atoms into crystalline solids.
The newly discovered mechanism – identified as a series of spiral
steps around dislocations terminating at the surface of titanium nitride,
a technologically important material used in microelectronics and hard
coatings – could potentially be put to use in controlling surface
morphology and in preparing nanoscale structures on surfaces.
“The spiral step dynamics strongly suggests that the cores of
surface-terminated dislocations behave like ‘whirlpools’
sucking surface atoms into the crystal structure,” said Suneel
Kodambaka, a postdoctoral research associate and lead author of a paper
that announced the team’s findings in the May 6 issue of the journal
Dislocations are imperfections in a crystal structure where there is
a missing or an extra half plane of atoms in the lattice. Dislocations
can strongly influence nanostructural and interfacial stability, mechanical
properties and chemical reactions.
“We found that the presence of a dislocation could reverse the
behavior and evolution of the nearby surface substructure,” said
Ivan Petrov, a research professor and director of the Center
for Microanalysis of Materials at the Frederick
Seitz Materials Research Laboratory on the U. of I. campus.
To study the dynamics of dislocation motion and morphological evolution
in single crystals at high temperature (1,300 to 1,400 degrees Celsius),
the researchers used low-energy electron microscopy – a technique
that can visualize the surface at the atomic level.
“We saw steps form at the dislocation site and expand into spiral
structures,” Kodambaka said. “This type of spiral growth
had been seen previously under applied stress, and when depositing or
evaporating material; but never during annealing, when the crystal is
neither gaining nor losing material.”
Resembling steps on a spiral staircase, each step was one layer of atoms
thick and rotated around the dislocation core. The spiral slowly spun
while growing inward, like a bathtub drain sucking water.
“The dislocation provides a path for atoms to move from the surface
to inside the crystal,” Petrov said. “The spiral structure
is a manifestation of the moving material. It is a vortex that consumes
surface atoms and drives the nearby surface kinetics.”
The researchers’ results “provide fundamental insights into
mechanisms that control both the stability of nanostructures and the
formation of nanoscale patterns on surfaces,” Kodambaka said.
“We think this spiral growth process is quite general and will
be observed in many other materials.”
In addition to Kodambaka and Petrov, the research team included materials
science and engineering professor Joseph Greene, electron microscopist
Waclaw Swiech and postdoctoral research associates Sanjay Khare and
Kenji Ohmori. The U.S. Department of Energy funded the work.