Making Plasmonics with Silver Polyhedral Nanocrystals

Nanoscale fabrication (such as focused ion beam lithography techniques) allow one to make new materials with increasing sophistication and freedom of design, but controlling light at the nanoscale remains a challenge. Traditionally light can only be controlled on length scales down to a little below the wavelength of light, a few hundred nanometers, hence the usual resolution limit of optical microscopes and telescopes.
Plasmonics is the phenomenon by which a beam of light is confined in ultra-cramped spaces allowing it to be manipulated as desired. Plamonics is thought to embody the strongest points of both optical and electronic data transfer, allowing the fast transmission of information over very small wires. Plasmonics, sometimes called "light on a wire," would allow the transmission of data at optical frequencies along the surface of a tiny metal wire, despite the fact that the data travels in the form of electron density distributions rather than photons.
Plasmonics will be useful in controlling the optical properties of molecules, and in using optics to monitor molecules. Understanding the new physics involved will enable progress in biology, chemistry and materials science, and will enable new devices and new materials to be made. Plasmonics holds great promise for super fast 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 at U.S. Department of Energy Lawrence Berkeley Lab have developed a simple approach to fabricate plasmonic materials by inducing polyhedral-shaped silver nanocrystals to self-assemble into three-dimensional super crystals of the highest possible density.
Silver nanocrystals
Researchers made silver nanocrystals of a variety of polyhedral shapes self-assembled into millimeter-sized superstructures through a simple sedimentation technique based on gravity. Nanoscale silver polyhedral crystals can self-assemble into structures with densest packing of these shapes.
Polyol synthesis technique can be used to generate silver nanocrystals in various shapes, including cubes, truncated cubes, cuboctahedra, truncated octahedra and octahedra over a range of sizes from 100 to 300 nanometers. These uniform polyhedral nanocrystals when placed in solution get assembled themselves into dense super crystals through gravitational sedimentation.
Precise control of the super lattice dimensions can be obtained in bulk solution when the assembly takes place in the reservoirs of micro array channels.
Thus a dilute solution of nanoparticles is loaded into a reservoir and then tilted to make the particles to gradually sediment and assemble at the bottom of the reservoir and more concentrated solutions or higher angles of tilt causes the assemblies to form more quickly.
The assemblies generated by this sedimentation procedure exhibited both translational and rotational order over exceptional length scales. In the cases of cubes, truncated octahedra and octahedra, the structures of the dense super crystals corresponded precisely to their densest lattice packing. Although sedimentation-driven assembly is not new, for the first time the technique has been used to make large-scale assemblies of highly uniform polyhedral particles.
When compared with crystal structures of spherical particles, dense packing of polyhedra 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.
The silver nanocrystals used by researchers are excellent plasmonic materials for surface-enhanced applications, such as sensing, nanophotonics and photo catalysis. Packing the nanocrystals into three-dimensional super crystals allows them to be used as metamaterials with the unique optical properties that make plasmonic technology so intriguing.


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