11-04 :: April/May 2011

nanotimes News in Brief

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The plasmonic phenomenon was discovered in nanostructures at the interfaces between a noble metal, such as gold or silver, and a dielectric, such as air or glass. Directing an electromagnetic field at such an interface generates electronic surface waves that roll through the conduction electrons on a metal, like ripples spreading across the surface of a pond that has been plunked with a stone. Just as the energy in an electromagnetic field is carried in a quantized particle-like unit called a photon, the energy in such an electronic surface wave is carried in a quantized particle-like unit called a plasmon. The key to plasmonic properties is when the oscillation frequency between the plasmons and the incident photons matches, a phenomenon known as localized surface plasmon resonance (LSPR). Conventional scientific wisdom has held that LSPRs require a metal nanostructure, where the conduction electrons are not strongly attached to individual atoms or molecules. Now, this has proved not to be the case. ©LBL

freezes the nanocrystals into a relatively vacancy-free state, which we can then dope in a controlled man- ner using common chemical oxidants.”

By introducing enough free electrical charge carriers via dopants and vacancies, Jain and his colleagues were able to achieve LSPRs in the near-infrared ran- ge of the electromagnetic spectrum. The extension of plasmonics to include semiconductors as well as metals offers a number of significant advantages, as Jain explains.

“Unlike a metal, the concentration of free charge carriers in a semiconductor can be actively controlled by doping, temperature, and/or phase transitions,” he says. “Therefore, the frequency and intensity of LSPRs in dopable quantum dots can be dynamical- ly tuned. The LSPRs of a metal, on the other hand,

once engineered through a choice of nanostructure parameters, such as shape and size, is permanently locked-in.” Jain envisions quantum dots as being integrated into a variety of future film and chip-based photonic devices that can be actively switched or controlled, and also being applied to such optical ap- plications as in vivo imaging. In addition, the strong coupling that is possible between photonic and electronic modes in such doped quantum dots holds exciting potential for applications in solar photovol- taics and artificial photosynthesis.

Joseph M. Luther, Prashant K. Jain, Trevor Ewers & A. Paul Alivisatos: Localized surface plasmon resonances ari- sing from free carriers in doped quantum dots, In: Nature Materials, Volume 10(2011), No. 5, May 2011, Pages 361- 366, DOI:10.1038/nmat3004: http://dx.doi.org/10.1038/nmat3004