Monday, 29 July 2013

Research helps make advance in "programmable matter" using nanocrystals

Research helps make advance in
"programmable matter" using
These transmission electron
microscope images show the two
different patterns the nanocrystals
could be made to pack in.
When University of Pennsylvania
nanoscientists created beautiful, tiled
patterns with flat nanocrystals, they
were left with a mystery: why did
some sets of crystals arrange
themselves in an alternating,
herringbone style, even though it
wasn't the simplest pattern? To find
out, they turned to experts in
computer simulation at the University
of Michigan and the Massachusetts
Institute of Technology.
The result gives nanotechnology
researchers a new tool for controlling
how objects one-millionth the size of
a grain of sand arrange themselves
into useful materials, it gives a
means to discover the rules for
"programming" them into desired
The study was led by Christopher
Murray, a professor with
appointments in the Department of
Chemistry in the School of Arts and
Sciences and the Department of
Materials Science and Engineering in
the School of Engineering and
Applied Sciences. Also on the Penn
team were Cherie Kagan, a
chemistry, MSE and electrical and
systems engineering professor, and
postdoctoral researchers Xingchen Ye,
Jun Chen and Guozhong Xing.
They collaborated with Sharon
Glotzer, a professor of chemical
engineering at Michigan, and Ju Li, a
professor of nuclear science and
engineering at MIT.
Their research was featured on the
cover of the journal Nature
Chemistry .
"The excitement in this is not in the
herringbone pattern," Murray said,
"It's about the coupling of
experiment and modeling and how
that approach lets us take on a very
hard problem."
Previous work in Murray's group has
been focused on creating
nanocrystals and arranging them into
larger crystal superstructures .
Ultimately, researchers want to
modify patches on nanoparticles in
different ways to coax them into
more complex patterns. The goal is
developing "programming matter,"
that is, a method for designing novel
materials based on the properties
needed for a particular job.
"By engineering interactions at the
nanoscale," Glotzer said, "we can
begin to assemble target structures
of great complexity and functionality
on the macroscale."
Glotzer introduced the concept of
nanoparticle "patchiness" in 2004.
Her group uses computer simulations
to understand and design the
Recently, Murray's team made
patterns with flat nanocrystals made
of heavy metals, known to chemists
as lanthanides, and fluorine atoms.
Lanthanides have valuable properties
for solar energy and medical imaging,
such as the ability to convert
between high- and low-energy light.
They started by breaking down
chemicals containing atoms of a
lanthanide metal and fluorine in a
solution, and the lanthanide and
fluorine naturally began to form
crystals. Also in the mix were chains
of carbon and hydrogen that stuck to
the sides of the crystals, stopping
their growth at sizes around 100
nanometers, or 100 millionths of a
millimeter, at the largest dimensions.
By using lanthanides with different
atomic radii, they could control the
top and bottom faces of the
hexagonal crystals to be anywhere
from much longer than the other
four sides to non-existent, resulting
in a diamond shape.
To form tiled patterns, the team
purified the nanocrystals and mixed
them with a solvent. They spread this
mixture in a thin layer over a thick
fluid, which supported the crystals
while allowing them to move. As the
solvent evaporated, the crystals had
less space available, and they began
to pack together.
The diamond shapes and the very
long hexagons lined up as expected,
the diamonds forming an argyle-style
grid and the hexagons matching up
their longest edges like a
foreshortened honeycomb. The
hexagons whose sides were all nearly
the same length should have formed
a similar squashed honeycomb
pattern, but, instead, they lined up in
an alternating herringbone style.
"Whenever we see something that
isn't taking the simplest pattern
possible, we have to ask why,"
Murray said.
They posed the question to Glotzer's
"They've been world leaders in
understanding how these shapes
could work on nanometer scales, and
there aren't many groups that can
make the crystals we make," Murray
said. "It seemed natural to bring
these strengths together."
Glotzer and her group built a
computer model that could recreate
the self-assembly of the same range
of shapes that Murray had produced.
The simulations showed that if the
equilateral hexagons interacted with
one another only through their
shapes, most of the crystals formed
the foreshortened honeycomb
pattern, not the herringbone.
"That's when we said, 'Okay, there
must be something else going on. It's
not just a packing problem,'" Glotzer
said. Her team, which included
graduate student Andres Millan and
research scientist Michael Engel, then
began playing with interactions
between the edges of the particles.
They found that that if the edges that
formed the points were stickier than
the other two sides, the hexagons
would naturally arrange in the
herringbone pattern.
The teams suspected that the source
of the stickiness was those carbon
and hydrogen chains. Perhaps they
attached to the point edges more
easily, the team members thought.
Since experiment doesn't yet offer a
way to measure the number of
hydrocarbon chains on the sides of
such tiny particles, Murray asked
MIT's Ju Li to calculate how the
chains would attach to the edges at a
quantum mechanical level.
Li's group confirmed that, because of
the way that the different facets cut
across the lattice of the metal and
fluorine atoms, more hydrocarbon
chains could stick to the four edges
that led to points than the remaining
two sides. As a result, the particles
become patchy.
"Our study shows a way forward
making very subtle changes in
building block architecture and
getting a very profound change in the
larger self-assembled pattern,"
Glotzer said. "The goal is to have
knobs that you can change just a
little and get a big change in
structure, and this is one of the first
papers that shows a way forward for
how to do that."
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