Scientists at the U.S. Department
of Energy’s Argonne National Laboratory have begun to use molecular
“stencils” to pave the way to new materials that could potentially
find their way into future generations of solar cells, catalysts and photonic
crystals.
Researchers at Argonne’s Center for Nanoscale Materials and Energy Systems
Division have developed a technique known as sequential infiltration synthesis
(SIS), which relies on the creation of self-assembled nanoscale chemical domains
into which other materials can be grown. In this technique, a film composed
of large molecules called block copolymers acts as a template for the creation
of a highly-tunable patterned material.
This film of block copolymers shows the material's characteristic tendency to separate into distinct regions.
This new method represents an extension of atomic layer deposition (ALD), a
popular technique for materials synthesis that is routinely used by Argonne
scientists. Instead of just layering two-dimensional films of different nanomaterials
on top of one another, however, SIS allows scientists to construct materials
that have much more complex geometries.
“This new technique allows us to create materials that just weren’t
possible with ALD or block copolymers alone,” said Seth Darling, an Argonne
nanoscientist who helped to develop SIS in collaboration with Argonne chemist
Jeff Elam. “Having the ability to control the geometry of the material
we’re making as well as its chemical composition opens the door to a whole
universe of new materials.”
According to Darling, the success of the technique relies on the unique chemistry
of block copolymers. Every block copolymer is composed of two chemically distinct
subunits; for instance, one subunit might have an affinity for water while the
other might repel water. In such a case, like would seek out like, creating
a heterogeneous matrix of interspersed homogenous regions.
“You can think of a block copolymer as like a pair of molecular Siamese
twins where one likes to talk and one likes to read quietly,” Darling
said. “If you put a bunch of these twins together in a room, the talkative
ones are going to try to be near the talkative ones and the readers are going
to try to be near the readers, but they can’t simply all separate themselves
to either side of the room, and it’s this action that gives us the geometries
we’re looking for.”
Depending on the initial substrate, the block copolymers, and the processing
that materials scientists use, regions can form that have many different shapes,
from spherical to cylindrical to planar. While there are many types of block
copolymers, in general they cannot serve as wide an array of purposes as inorganic
materials. The challenge, according to Darling, is to bring the self-assembly
of block copolymers together with the functionality of inorganic materials.
The physical and chemical properties of a material generated using SIS depend
on how block copolymer chemistry and morphology interact with the chemistry
of ALD techniques. “We can tailor our materials synthesis efforts in a
much more precise way than we ever could before,” Darling said.
Darling and Elam have spent most of their careers at Argonne focused on the
development of new types of materials, including the development of solar cells
that combine organic and inorganic components. They believe that the types of
materials that SIS can generate will drive fundamental solar energy technologies
to greater efficiencies and lower cost.
“Our solar energy future does not have a one-size-fits-all solution,”
Elam said. “We need to investigate the problem from many different angles
with many different materials, and SIS will give researchers like us many new
routes of attack.”