Plasmonic Chains Act Like Polymers

New research that seeks to establish points of reference between plasmonic particles and polymers might lead to smaller computer chips, better antennae and improvements in optical computing.

Interactions between small things have been in the spotlight recently with the discovery of signs of the Higgs boson, along with discussion about how the most elemental particles interact to give the universe its form.

Scientists at Rice University study nanoparticles with the goal of understanding how the more elemental electromagnetic particles within behave. They have now detailed patterns — similar to those of polymers that self-assemble as a result of strong interactions between chemicals — in the way that surface plasmons influence each other in gold nanoparticle chains.

This is important to electronics engineers, who are perpetually looking for ways to shrink the size of computer chips and other devices through ever-smaller components such as waveguides. The ability of nanoparticles to pass waves that can be interpreted as signals may open the door to new methods for optical computing, and may contribute to more finely tuned sensors and antennae.

How far a wavelength of light can be transmitted along a gold nanoparticle chain to where it dies down (the “extinction” point) depends on the configuration of the repeat units — the nanoparticles themselves — according to Rice University researchers. Their study was intended to draw parallels between nanoparticle arrays and polymers that also depend on chemical repeat units for their characteristics. (Image: Liane Slaughter, Rice University)

The Rice team looked for ways plasmons influence each other across tiny gaps — as small as 1 nm — between gold nanoparticles. They engineered chains of 50-nm particles in single and double rows that mimic the repeating molecular patterns of polymers, and observed the superradiant and subradiant signals collectively sustained by the individual nanoparticle assemblies. The composition of the chain in terms of the nanoparticles’ shape, size and positioning determined the frequencies of light with which it can characteristically interact.

“In plasmonics, we use individual nanoparticles as building blocks to make higher-order structures,” said Stephan Link, an assistant professor of chemistry and electrical and computer engineering. “Here, we’re taking concepts known to polymer scientists to analyze the structures of longer chains of nanoparticles that we think resemble polymers.”

By changing the atoms that repeat in a long chain of molecules, it will change the polymer’s properties, said graduate student Liane Slaughter.

“What we changed in our assembly structures was the repeat unit — a single particle row versus a dimer (in the double row) — and we found that this fit the analogy with chemical polymers because that change very clearly alters the interactions along the chain,” Link said.

This basic structure change from a single to a double row led to pronounced differences, as demonstrated by additional subradiant modes and a lower-energy superradiant mode.

The team also discovered two effects that seem to be universal among its plasmonic polymers: The first is that the energy of the superradiant mode decreases with the addition of nanoparticles along the length, up to about 10 particles, and then levels off; the second is that disorder among the repeat units seems to matter only at the small scale.

“With chemically prepared nanoparticles, there’s always a distribution of sizes and perhaps shapes,” Link said. “As you bring them close together, they couple really strongly, and that’s a big advantage. But at the same time, we can never make structures that perfect.”

The researchers had set out to understand the effect of disorder, and what they found was pretty amazing, he said.

“As the system grows in size, the effect of disorder is less and less important on the optical properties. That also has a strong analogy in polymers, in which disorder can be seen as chemical defects.”

“If the plasmonic interactions over the chain tolerate disorder, it gives promise to designing functional structures more economically and maybe with higher throughput,” Slaughter said. “With a whole bunch of small building blocks, even if they’re not all perfectly alike, you can make a great variety of shapes and structures with broad tenability.”

The research appeared in Nano Letters.

For more information, visit: www.rice.edu