Page 67 - Plastics News July 2019

teChNoLogy

at the question of the position of the doping components, curve. To improve efficiency beyond the current-voltage
whether the materials clump or not and how much they trade-off, one must move the entire trade-off curve,
clump, or cluster, as we call it. It turns out that clustering she says. This unexpected finding should provide a
is a critical variable." The team turned to chemist Michael new path for designing more efficient polymers for
Barnes, a co-author on their recent paper, who used thermo-electric devices, the researchers say. DV notes
Kelvin Probe Force Microscopy to probe the dopants at the that until now, chemists and materials scientists have been
nano level and show that clustering is indeed present in trying to organize polymers to be more like the inorganics,
polymers doped at room temperature, but not at higher "nicely aligned and very regular, which is difficult to do,"
temperatures. With that confirmation, the researchers he adds. "It turns out that this may not be the way to
turned to modeling an expanded trade-off curve, says go; you can take another road or another approach. We
Upadhyaya. From their theoretical modeling, she and hope this paper provides a basis to move polymer-based
Aksamija found that clustering alters the shape of that thermo-electrics forward."

A new way of making complex structures in thin films

elf-assembling materials called block copolymers, which spontaneously arrange themselves into periodic structures.
Sare known to form a variety of predictable, regular Researchers had found that if there was a repeating pattern
patterns, can now be made into much more complex of lines or pillars created on a substrate, and then a thin
patterns that might someday be useful for making optical film of the block copolymer was formed on that surface,
or plasmonic devices (in which electromagnetic waves the patterns from the substrate would be duplicated in
interact with electrons), according to a new study. The new the self-assembled material. But this method could only
findings appear in the journal Nature Communications, in produce simple patterns such as grids of dots or lines. In
a paper by postdoc Yi Ding, professors of materials science the new method, there are two different, mismatched
and engineering Alfredo Alexander-Katz and Caroline Ross, patterns. One is from a set of posts or lines etched on a
substrate material, and the other is an inherent pattern that
is created by the self-assembling copolymer. For example,
there may be a rectangular pattern on the substrate and
a hexagonal grid that the copolymer forms by itself. One
would expect the resulting block copolymer arrangement
to be poorly ordered, but that's not what the team found.
Instead, "it was forming something much more unexpected
and complicated," Ross says.There turned out to be a subtle
but complex kind of order -- interlocking areas that formed
slightly different but regular patterns, of a type similar
to quasicrystals, which don't quite repeat the way normal
crystals do. In this case, the patterns do repeat, but over
longer distances than in ordinary crystals. "We're taking
advantage of molecular processes to create these patterns
on the surface" with the block copolymer material, Ross
and three others. "This is a discovery that was in some sense says. This potentially opens the door to new ways of making
fortuitous," says Alexander-Katz. "Everyone thought this was devices with tailored characteristics for optical systems or
not possible," he says, describing the team's discovery of for "plasmonic devices" in which electromagnetic radiation
a phenomenon that allows the polymers to self-assemble resonates with electrons in precisely tuned ways, the
in patterns that deviate from regular symmetrical arrays. researchers say. Such devices require very exact positioning
Self-assembling block copolymers are materials whose and symmetry of patterns with nanoscale dimensions,
chain-like molecules, which are initially disordered, will something this new method can achieve.Katherine Mizrahi

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