Tiny but Mighty

By altering the size and thickness of metallic nanoshells, engineers can manipulate their interaction with light.

Joining nanotechnology with optics, a new class of materials called metal nanoshells can absorb selected colors of visible or infrared (IR) light, offering the potential for new applications in thin films and coatings, chemical and biological sensors, and optical switching. In conventional materials, bulk-material effects dominate. In the nano regime, surface effects dominate. We use this effect in metal nanoshells.

Metal nanoshells are nanoparticles consisting of a dielectric core (typically silica) and a metal shell (typically silver or gold) whose optical resonant properties are determined by the relative size of the core and shell layers. By varying the core diameter and shell thickness, the nanoparticle resonance can be placed anywhere across most of the visible and IR regions of the spectrum. Scaling the absolute size of a metal nanoshell up or down allows one to manipulate the relative amount of scattering versus absorption occurring at the resonant extinction wavelength of the nanoparticles.

This powerful resonant effect is based on the electromagnetic properties of layered nanostructures, and in fact has been known for more than 50 years. Recently in our laboratory at Rice University (Houston, TX), we have developed a method for reproducible and reliable fabrication of these types of nanostructures.

Making a nanoshell

The growth method for metal nanoshells is essentially planar fabrication on a nanoparticle surface and incorporates a variety of wet chemical procedures from colloid chemistry, chemical self-assembly, and electroless plating. Silica nanoparticle cores ranging in diameter from approximately 50 nm to 1 µm are either synthesized or obtained from commercial sources.

The nanoparticle cores by themselves are inert. To become functional, the surfaces must be chemically modified using a siloxane species with an amine or thiol group. This coating process terminates the nanoparticle surface with a chemical species capable of binding to a small metal colloid; a metal colloid of 1 to 2 nm in diameter is then bound to the nanoparticle surface.

We introduce the seed particle into an electroless plating solution where metal can be reduced onto its surface. This procedure produces a uniform metal shell on the dielectric core, with thickness ranging from nominally 5 to 20 nm. The method permits nanometer-by-nanometer precision of metal deposition, which allows us to tune the optical properties of the completed nanoshell with a great deal of control.

Metal nanoshells fabricated using this approach possess resonant properties that agree quantitatively with Mie scattering theory. We use the theory to design resonant nanoparticles by calculating the appropriate shell and core sizes based on the resonant wavelength desired. The current fabrication method produces nanoshells whose resonant wavelengths comfortably span the near IR spectral region, which has a wide variety of technological applications.

Freedom and flexibility

Unlike photonic crystals, which rely on long-range 2-D or 3-D periodicity to achieve tunable resonant properties, the optical properties of nanoshells are controlled by the structure of the individual nanoparticles. This offers enormous freedom and flexibility in developing optically active coatings, materials, or components. Metal nanoshells can be incorporated into a wide variety of media, such as polymers, gels, or liquids, imparting their optical properties to the resultant composite material. The shells can act as an intelligently designed optical absorber, for example, which may be of use in the development of filters, coatings, and other components in the optics industry. They also can produce enormous enhancements of Raman scattering of nearby molecules, an effect that can be optimized to specific pump-laser wavelengths and that could be of great utility in the development of ultrasensitive biological or chemical sensors.

Because of their metal coating, illuminating nanoshells gives rise to a very large photothermal response. This effect, when combined with temperature-sensitive polymers, has been used to develop new optomechanical nanoshell-polymer composite materials. When illuminated, these materials collapse to approximately 30% of their original volume. This effect is optically driven, so the photothermal collapse can be induced remotely. The addition of metal nanoshells has also been shown to dramatically increase the active lifetime of luminescent conducting polymer materials by effectively inhibiting their photo-oxidation, quite possibly circumventing the need for encapsulation of the final material or device.

(By Naomi Halas, Rice University, Oemagazine, December, 2001)

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