11-06/07 :: June/July 2011

nanotimes News in Brief

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Berkeley Lab researchers have developed unqiue semiconductor tetrapods that under illumination break Kasha’s rule for photoluminescence by emitting two colors of light. Smal image left: Artificial molecules consisting of a cadmium-selenide quantum dot core and four cadmium sulfide arms, with the fourth arm sticking out of the plane and appearing as a black dot in the center of each tetrapod. © University of California / Berkeley Lab

to be present in the four arms as well. The next lowest unoccupied molecular orbital (LUMO+1) is located primarily within the four CdS arms. Through single particle photoluminescence spectroscopy carried out at Ames, it was determined that when a CdSe/CdS core/shell tetrapod is excited, not only is a photon emitted at the HOMO-LUMO energy gap as expected, but there is also a second photon emitted at a higher energy that corresponds to a transition to the HOMO from the LUMO+1.

“The discovery that these CdSe/CdS core/shell tetra- pods emit two colors was a surprise,” Choi says. “If we can learn to control the frequency and intensity of the emitted colors then these tetrapods may be useful for multi-color emission technologies.” For example, says co-author Prashant Jain, “In the field of optical sensing with light emitters, it is impracti- cal to rely simply on changes in emission intensity as emission intensity can fluctuate significantly due

to background signal. However, if a molecule emits light from multiple excited states, then one can design a ratiometric sensor, which would provide more accurate readouts than intensity magnitude, and would be more robust against fluctuations and background signals.”

Another promising possibility for CdSe/CdS core/shell tetrapods is their potential application as nanosca- le sensors for measuring forces. Previous work by Alivisatos and Choi showed that the emission wave- lengths of these tetrapods will shift in response to local stress on their four arms.

Charina L. Choi, Hui Li, Andrew C. K. Olson, Prashant K. Jain, Sanjeevi Sivasankar, A. Paul Alivisatos: Spatially Indi- rect Emission in a Luminescent Nanocrystal Molecule, In: NANO Letters, Vol. 11(2011), Issue 6, June 8, 2011, Pages 2358-2362, DOI:10.1021/nl2007032: http://dx.doi.org/10.1021/nl2007032