Use of silver nanocrystals in Plasmonic Technology

Peidong Yang, chemist, Materials Sciences Division, Berkeley Laboratory

CALIFORNIA, USA:Plasmonics is the phenomenon by which a beam of light is confined in ultra-cramped spaces allowing it to be manipulated into doing things a beam of light in open space cannot. This phenomenon holds great promise for superfast computers, microscopes that can see nanoscale objects with visible light, and even the creation of invisibility carpets. A major challenge for developing plasmonic technology, however, is the difficulty of fabricating metamaterials with nano-sized interfaces between noble metals and dielectrics. ! Researchers with the U.S. Department of Energy (DOE)’s and Lawrence Berkeley National Laboratory (Berkeley Lab)  have opened the door to a simpler approach for the fabrication of plasmonic materials by inducing polyhedral-shaped silver nanocrystals to self-assemble into three-dimensional supercrystals of the highest possible density.

Peidong Yang, chemist, Materials Sciences Division, Berkeley Lab led a study in which silver nanocrystals of a variety of polyhedral shapes self-assembled into exotic millimeter-sized superstructures through a simple sedimentation technique based on gravity. This first ever demonstration of forming such large-scale silver supercrystals through sedimentation is described in a paper in the journal Nature Materials titled “Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices.” Yang.

“We have shown a range of highly uniform, nanoscale silver polyhedral crystals that can self-assemble into structures that have been calculated to be the densest packings of these shapes,” said Yang. “In addition, in the case of octahedral, we showed that controlling polymer concentration allows us to tune between a well-known lattice packing structure and a novel packing structure that featured complex helical motifs.”

“In a typical experiment, a dilute solution of nanoparticles was loaded into a reservoir that was then tilted, causing the particles to gradually sediment and assemble at the bottom of the reservoir, More concentrated solutions or higher angles of tilt caused the assemblies to form more quickly.” said Yang

The assemblies generated by this sedimentation procedure exhibited both translational and rotational order over exceptional length scales. In the cases of cubes, truncated octahedral, the structures of the dense supercrystals corresponded precisely to their densest lattice packings.

“The key factor in our experiments is particle shape, a feature we have found easier to control, When compared with crystal structures of spherical particles, our dense packings of polyhedral are characterized by higher packing fractions, larger interfaces between particles, and different geometries of voids and gaps, which will determine the electrical and optical properties of these materials.”said Yang.

The silver nanocrystals used by Yang and his colleagues are excellent plasmonic materials for surface-enhanced applications, such as sensing, nanophotonics and photocatalysis.   Packing the nanocrystals into three-dimensional supercrystals allows them to be used as metamaterials with the unique optical properties that make plasmonic technology so intriguing.

Co-authoring the Nature Materials paper with Yang were Joel Henzie, Michael Grünwald, Asaph Widmer-Cooper and Phillip  Geissler, who also holds joint appointments with Berkeley Lab and UC Berkeley.