On the road to plasmonics with silver polyhedral nanocrystals: Researchers find simpler approach to making plasmonic materials

 The question of how many polyhedral nanocrystals of silver can be packed into millimeter-sized supercrystals may not be burning on many lips but the answer holds importance for one of today's hottest new high-tech fields -- plasmonics! Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) may 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.

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.

Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division, 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, who also holds appointments with the University of California Berkeley's Chemistry Department and Department of Materials Science and Engineering, is the corresponding author.

"We have shown through experiment and computer simulation that a range of highly uniform, nanoscale silver polyhedral crystals can self-assemble into structures that have been calculated to be the densest packings of these shapes," Yang says. "In addition, in the case of octahedra, 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 the Nature Materials paper Yang and his co-authors describe a polyol synthesis technique that was 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 were then placed in solution where they assembled themselves into dense supercrystals some 25 square millimeters in size through gravitational sedimentation. While the assembly process could be carried out in bulk solution, having the assembly take place in the reservoirs of microarray channels provided Yang and his collaborators with precise control of the superlattice dimensions.

"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," Yang says. "More concentrated solutions or higher angles of tilt caused 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 supercrystals corresponded precisely to their densest lattice packings. Although sedimentation-driven assembly is not new, Yang says this is the first time the technique has been used to make large-scale assemblies of highly uniform polyhedral particles.

"The key factor in our experiments is particle shape, a feature we have found easier to control," Yang says. "When compared with crystal structures of spherical particles, our dense packings 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 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.

"Our self-assembly process for these silver polyhedral nanocrystals may give us access to a wide range of interesting, scalable nanostructured materials with dimensions that are comparable to those of bulk materials," Yang says.

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.

This research was supported in part by the Defense Advanced Research Projects Agency and DOE's Office of Science.

Story Source:

The above story is reprinted from materials provided by DOE/Lawrence Berkeley National Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Joel Henzie, Michael Grünwald, Asaph Widmer-Cooper, Phillip L. Geissler, Peidong Yang. Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices. Nature Materials, 2011; DOI: 10.1038/nmat3178

GRIN plasmonics: A practical path to superfast computing, ultrapowerful optical microscopy and invisibility carpet-cloaking devices

They said it could be done and now they've done it. What's more, they did it with a GRIN. A team of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley, have carried out the first experimental demonstration of GRIN -- for gradient index -- plasmonics, a hybrid technology that opens the door to a wide range of exotic optics, including superfast computers based on light rather than electronic signals, ultra-powerful optical microscopes able to resolve DNA molecules with visible light, and "invisibility" carpet-cloaking devices.

Working with composites featuring a dielectric (non-conducting) material on a metal substrate, and "grey-scale" electron beam lithography, a standard method in the computer chip industry for patterning 3-D surface topographies, the researchers have fabricated highly efficient plasmonic versions of Luneburg and Eaton lenses. A Luneburg lens focuses light from all directions equally well, and an Eaton lens bends light 90 degrees from all incoming directions.

"This past year, we used computer simulations to demonstrate that with only moderate modifications of an isotropic dielectric material in a dielectric-metal composite, it would be possible to achieve practical transformation optics results," says Xiang Zhang, who led this research. "Our GRIN plasmonics technique provides a practical way for routing light at very small scales and producing efficient functional plasmonic devices."

Zhang, a principal investigator with Berkeley Lab's Materials Sciences Division and director of UC Berkeley's Nano-scale Science and Engineering Center (SINAM), is the corresponding author of a paper in the journal Nature Nanotechnology, describing this work titled, "Plasmonic Luneburg and Eaton Lenses." Co-authoring the paper were Thomas Zentgraf, Yongmin Liu, Maiken Mikkelsen and Jason Valentine.

GRIN plasmonics combines methodologies from transformation optics and plasmonics, two rising new fields of science that could revolutionize what we are able to do with light. In transformation optics, the physical space through which light travels is warped to control the light's trajectory, similar to the way in which outer space is warped by a massive object under Einstein's relativity theory. In plasmonics, light is confined in dimensions smaller than the wavelength of photons in free space, making it possible to match the different length-scales associated with photonics and electronics in a single nanoscale device.

"Applying transformation optics to plasmonics allows for precise control of strongly confined light waves in the context of two-dimensional optics," Zhang says. "Our technique is analogous to the well-known GRIN optics technique, whereas previous plasmonic techniques were realized by discrete structuring of the metal surface in a metal-dielectric composite."

Like all plasmonic technologies, GRIN plasmonics starts with an electronic surface wave that rolls through the conduction electrons on a metal. Just as the energy in a wave of light is carried in a quantized particle-like unit called a photon, so, too, is plasmonic energy carried in a quasi-particle called a plasmon. Plasmons will interact with photons at the interface of a metal and dielectric to form yet another quasi-particle, a surface plasmon polariton (SPP).

The Luneburg and Eaton lenses fabricated by Zhang and his co-authors interacted with SPPs rather than photons. To make these lenses, the researchers worked with a thin dielectric film (a thermplastic called PMMA) on top of a gold surface. When applying grey-scale electron beam lithography, the researchers exposed the dielectric film to an electron beam that was varied in dosage (charge per unit area) as it moved across the film's surface. This resulted in highly controlled differences in film thickness across the length of the dielectric that altered the local propagation of SPPs. In turn, the "mode index," which determines how fast the SPPs will propagate, is altered so that the direction of the SPPs can be influenced.

"By adiabatically tailoring the topology of the dielectric layer adjacent to the metal surface, we're able to continuously modify the mode index of SPPs," says Zentgraf. "As a result, we can manipulate the flow of SPPs with a greater degree of freedom in the context of two-dimensional optics."

Says Liu, "The practicality of working only with the purely dielectric material to transform SPPs is a big selling point for GRIN plasmonics. Controlling the physical properties of metals on the nanometer length-scale, which is the penetration depth of electromagnetic waves associated with SPPs extending below the metal surfaces, is beyond the reach of existing nanofabrication techniques."

Adds Zentgraf, "Our approach has the potential to achieve low-loss functional plasmonic elements with a standard fabrication technology that is fully compatible with active plasmonics."

In the Nature Nanotechnology paper, the researchers say that inefficiencies in plasmonic devices due to SPPs lost through scattering could be reduced even further by incorporating various SPP gain materials, such as fluorescent dye molecules, directly into the dielectric. This, they say, would lead to an increased propagation distance that is highly desired for optical and plasmonic devices. It should also enable the realization of two-dimensional plasmonic elements beyond the Luneburg and Eaton lenses.

Says Mikkelsen, "GRIN plasmonics can be immediately applied to the design and production of various plasmonic elements, such as waveguides and beam splitters, to improve the performance of integrated plasmonics. Currently we are working on more complex, transformational plasmonic devices, such as plasmonic collimators, single plasmonic elements with multiple functions, and plasmonic lenses with enhanced performance."

This research was supported by the U.S. Army Research Office and the National Science Foundation's Nano-scale Science and Engineering Center.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Thomas Zentgraf, Yongmin Liu, Maiken H. Mikkelsen, Jason Valentine, Xiang Zhang. Plasmonic Luneburg and Eaton lenses. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2010.282