Nanoparticles Coaxed into Self-Assembly

A simple and inexpensive technique directs the self-assembly of nanoparticles into device-ready materials and has applications in fields including computer memory storage, energy storage and harvesting, remote sensing, catalysis, light management and plasmonics.

A team of scientists with the Berkeley National Laboratories (Berkeley Lab) and the University of California, Berkeley, developed a technique to direct the self-assembly of nanoparticles using solutions of block copolymer supramolecules. Earlier this year, the researchers, led by Ting Xu, were able to induce nanorod semiconductors to self-assemble into three-dimensional macroscopic structures.

Berkeley Lab researchers have developed a relatively simple and inexpensive technique for directing the self-assembly of nanoparticles into device-ready thin films with microdomains of lamellar (left) or cylindrical morphologies. (Image: Courtesy of Ting Xu group)

"Block copolymer supramolecules self-assemble and form a wide range of morphologies that feature microdomains typically a few to tens of nanometers in size," Xu said. "As their size is comparable to that of nanoparticles, the microdomains of block copolymer supramolecules provide an ideal structural framework for the co-self-assembly of nanoparticles."

The team's current research uses the block copolymer solution technique to produce multiple layers of 100- to 200-nm films from highly ordered arrays of gold nanoparticles. The microdomains of these films were organized into two morphologies, cylindrical and lamellar. The former formed 1-D chains, one particle wide, that were packed into hexagonal lattices parallel to the surface. The lamellar morphology formed 2-D sheets stacked into multiple layers that were also hexagonal and parallel to the surface.

The interatomic spacing in these structures was 8 to 10 nm. This opens up possibilities for applications in plasmonics, wherein a beam of light is confined to very small spaces. A major challenge for the development of plasmonic technology has been fabricating metamaterials using noble metals, but the ability of the gold nanoparticles to self-assemble into complex structures will allow for the production of novel nanoparticle-based devices 1000 times smaller than today's microtechnologies.

From left, Peter Bai, Joseph Kao and Ting Xu incorporated gold nanoparticles into solutions of block copolymer supramolecules to form multiple layers of self-assembled thin films. (Image: Photo by Roy Kaltschmidt, Berkeley Lab)

"Our gold thin films display strong plasmonic coupling along the interparticle spacing in the 1-D chains and 2-D sheets, respectively," Xu says. "We should therefore be able to use these films to investigate unique plasmonic properties for next-generation electronic and photonic devices. Our supramolecular technique might also be used to fabricate plasmonic metamaterials."

The research was published in the journal Nano Letters as "Nanoparticle Assemblies in Thin Films of Supramolecular Nanocomposites." The paper was co-authored by Joseph Kao, Peter Bai, Vivian Chuang, Zhang Jiang and Peter Ercius.

This research was supported by the US Department of Energy Office of Science.

For more information, visit: www.lbl.gov