Influencing structure in the heart of nanoland Light used
to control size of nanopores
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A graphic representation of light influencing the pore size of a self assembled
nanostructure. The area struck by light is being shrunk.
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Pianos and cars may be tuned with wrenches and screwdrivers,
but the old clunky tools just won’t do to adjust the sizes of trillions of nanoscopic
pores in fine filters, sensors, or diffraction gratings to make them work better.
Now, an Albuquerque-based research group reports precise size adjustments of nanopores
through use of a simple ultraviolet beam of light.
The precision is so great that it could achieve the long-sought goal of membrane-based
separation of oxygen from nitrogen, a difference in size of 0.2 Angstroms [0.02
nanometers], says Jeff Brinker, senior scientist at the Department of Energy’s Sandia
National Laboratories, professor at the University of New Mexico, and lead researcher on
the multi-institution project.
A joint patent application has been filed on the process by Sandia and UNM.
“The Holy Grail of membrane science is air separation,” says Brinker.
“People would love a filter that could separate oxygen from nitrogen. Industrialists
have invested hundreds of millions of dollars to do that. Now, in a systematic way, we
might go from pores of, say, 3.4 to 3.6 angstroms in diameter, tuning the membrane to
optimize oxygen-nitrogen separation.”
The work is an extension of a four-paper series published by Brinker’s group in
the journal Nature, detailing the group’s inquiries into the properties of
nanostructures that self-assemble to produce repeating patterns of pores of exactly the
same size.
In this case, the honeycomb-like structure, which might be visualized as a group of
soda straws lying together in a bundle, have pores that shrink in unison when illuminated
by a beam of light.
“Using light to change pore size is a kind of nanostructural engineering,”
Brinker says. “In addition to creating an overall pattern as achieved by conventional
lithography, this kind of lithography also can help us define the internal structure of
the films on the nanoscale. Even though the overall shapes we create are at the high end
of the nanostructure regime, the light influences pore size and connectivity in the heart
of nanoland, varying pore sizes continuously over a range within that illuminated
pattern.”
Creating
tunable zeolites
In effect, he says, the process creates a kind of tunable zeolite, thereby enhancing the
capability of membranes to separate molecules by size. This is done merely by exposing the
membrane — in this case, self-assembled thin-film silica that is photosensitive
— to the proper amount of light. “Tuning a ten-angstrom hole to 8 or 9 angstroms
should make a huge difference in membrane performance,” says Brinker.
Key to the pore size changes are photoacid molecules that self-assemble and uniformly
incorporate into a periodic nanostructure. A light shone on these molecules breaks them
apart to form an acid that causes silica to solidify locally. The amount of
solidification, which necessarily shrinks pore sizes to create the denser material, is
proportional to the amount of light shone on the membrane.
Zeolites are crystalline structures with tiny but unalterable pore sizes widely used by
industry to separate materials. As filters they are precise, but their pore sizes are
unchangeable.
In the paper’s introduction, the authors write, “The ability to optically
define and continuously control both structure and function on the macro- and mesoscales
is of interest for sensor arrays, nanoreactors, photonic and fluidic devices, and
low-dielectric-constant films.”
A further feature involves shining light through a lithographic mask that varies its
intensity, producing so-called “gray-scale” patterning, which theoretically
allows for a broad continuous spatial variation of the materials’ structure and
properties.
The same process also can be used to produce optical diffraction gratings —
devices that can redirect and filter light — made entirely of laser-damage-resistant
silica.
Dhaval Doshi (a UNM graduate student), Kelly Simmons-Potter, and B.G. Potter, both at
Sandia, used an evaporation-induced, self-assembly process to prepare and characterize
photosensitive films. The work incorporated molecular photoacid generators
compartmentalized within a silica-surfactant mesophase.
“While modifying pore sizes by small amounts so far seems to be completely
controllable, we haven’t yet demonstrated control in going from huge to teeny
pores,” says Brinker. “We’re not sure what the dynamic range of the process
is.”
Other researchers on this project include Alan Hurd from Sandia and researchers from
the Vienna University of Technology and Applied Materials Corp. in Santa Clara, Calif.
The work is funded by DOE’s Office of Basic Energy Sciences, the Defense Advanced
Research Projects Agency (DARPA), and Sandia’s Laboratory-Directed Research and
Development (LDRD) program.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a
Lockheed Martin Company, for the United States Department of Energy under contract
DE-AC04-94AL85000. With main facilities in Albuquerque, N.M., and Livermore, Calif.,
Sandia has major research and development responsibilities in national security, energy
and environmental technologies, and economic competitiveness.
This news release is available at http://www.sandia.gov/media/NewsRel/NR2000/light.htm
For more information
Research contact : Jeff
Brinker
Tel: (505) 272-7627
E-Mail : cjbrink@sandia.gov
Media contact: Neal Singer
Tel: (505) 845-7078
E-Mail : nsinger@sandia.gov
Sandia's home page :
http://www.sandia.gov
Brinker's research group : http://www.unm.edu/~solgel/
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