N.M. - A wish list for nanotechnologists might consist
of a simple, inexpensive means - actually, any means
at all - of self-assembling nanocrystals into robust
orderly arrangements, like soup cans on a shelf or
bricks in a wall, each separated from the next by
an insulating layer of silicon dioxide.
The silica casing could be linked to compatible semiconductor
devices. The trapped nanocrystals might function as
a laser, their frequency dependent on their size,
or as a very fine catalyst with unusually large surface
area, or perhaps a memory device tunable by particle
size and composition.
Or perhaps the technologist might want to stop nanocrystals
from clumping. Agglomeration prevents them from being
used as light-emitting tagging mechanisms to track
cancer cells in the body and from being used in light-emitting
devices needed for solid state lighting.
In this week's journal Science, researchers at the
National Nuclear Security Administration's Sandia
National Laboratories and the University of New Mexico
describe a simple, commercially feasible method for
doing both these things.
"The paper overcomes barriers to using nanocrystals
routinely," said Jeff Brinker, Sandia Fellow
and UNM chemical engineering professor, who with Sandia's
Hongyou Fan led the self-assembling effort. "The
question in nanotechnology isn't 'where's the beef,'
it's 'where's the connectors'? How does one make connections
from the macroscale to the nanoscale? This question
lies at the heart of nanotechnology."
The self-assembly approach developed by the SNL/UNM
teams allows nanocrystal arrays to be integrated into
devices using standard microelectronic processing
techniques, bridging huge gaps in scale.
Said IBM staff researcher Chuck Black at T. J. Watson
Research Center in Yorktown Heights, NY, "One
thing that's nice is that these materials are hard
materials. Often they come with an organic surfactant
layer that makes it difficult to process materials,
like a kind of grease. This material is embedded in
oxide. It sounds like a neat thing and a new approach."
The Sandia/UNM approach scrubs the surfactants with
an ozone compound.
"Also, quantum dots can be important for biolabeling
and biosensing," said Fan, who initiated the
effort to use the nanocrystals for those purposes.
"The beauty of our approach is that it makes
these quantum dots both water-soluble and biocompatible,
two essential qualities if we want to use them for
in vivo imaging. The functional organic groups on
the quantum dots can link with a variety of peptides,
proteins, DNA, antibodies, etc. so that the dots can
bind to and help locate targets like cancer cells,
a critical issue in biomedicine."
Sandia has applied for a patent on this approach,
which should aid attempts at several major universities
to identify individual cancer cells before they increase
(Researchers have found that at the nanoscopic realm,
changing merely the size of a material changes the
frequency it emits when 'pumped' by outside energy;
thus, quantum dots of particular sizes and material
will emit at predictable frequencies, which makes
them useful adjuncts when bound to molecules created
to bind to particular cancer molecules.)
The process uses a simple surfactant (similar to dishwashing
soap) to surround the nanocrystals - in this case,
made of gold - to make them water soluble. Further
processing involving silica causes the gold nanocrystals
to arrange themselves within a silica matrix in a
lattice - a kind of artificial solid with properties
that can be adjusted through control of nanocrystal
composition, diameter, properties of the surfactant,
and/or stabilizing ligands used in formation of the
water soluble nanocrystals.
The robust 3-D solids, which are stable indefinitely,
demonstrate the incorporation of nanocrystalline arrays
into device architectures.
A further use allows physicists to go beyond modeling
to determine how current scales with voltage in nanodevices.
"Before," says Brinker, "there was
no way to make precisely ordered 3-D nanocrystalline
solids, integrate them in devices, and characterize
their behavior. There was no theoretical model. How
does the current decide which way to hop between crystals?"
The new material can be used as an artificial solid
to test out theories. "It should be a dream for
physicists; they don't just have to model anymore,"
A kind of choreographed transmission possibility exists
with the so-called "coulomb blockade," he
said: No current is passed at low voltages because
each crystal is separated by a thin (several nanometer
thick) layer of silica dioxide, creating an insulator
between the stored charges. Each nanocrystal charges
separately. "This could be configured into a
flash memory," said Brinker, "with a huge
number of charges stored in an array of nodes."
Researchers at UNM's Center for High Technology Materials
performed experiments to establish the current/voltage
scaling characteristics of the gold/silica arrays
as a function of temperature. Sandia researcher Tim
Boyle made and provided nanocrystal semiconductor
(cadmium selinide) quantum dots.
Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin company, for
the U.S. Department of Energy's National Nuclear Security
Administration. With main facilities in Albuquerque,
N.M., and Livermore, Calif., Sandia has major R&D
responsibilities in national security, energy and
environmental technologies, and economic competitiveness.
Sandia National Laboratories
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