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nanotimes News in Brief

11-06/07 :: June/July 2011

Luminescent Nanocrystal Molecule // Berkeley Lab Scientists Find Unique Luminescence in Tetrapod Nanocrystals

© Text: UC Berkeley / LBNL T

he research team of Paul Alivisatos at the Uni- versity of California (UC, USA) Berkeley and

Lawrence Berkeley National Laboratory (Berkeley Lab) has created artificial molecules of semiconduc- tor nanocrystals and watched them break a funda- mental principle of photoluminescence known as “Kasha’s rule.” Named for chemist Michael Kasha, who proposed it in 1950, Kasha’s rule holds that when light is shined on a molecule, the molecule will only emit light (fluorescence or phosphorescence) from its lowest energy excited state. This is why pho- toluminescent molecules emit light at a lower ener- gy than the excitation light. While there have been examples of organic molecules, such as azulene, that break Kasha’s rule, these examples are rare. Highly luminescent molecular systems crafted from quan- tum dots that break Kasha’s rule have not been reported – until now.

”We have demonstrated a semiconductor nanocry- stal molecule, in the form of a tetrapod consisting of a cadmium-selenide quantum dot core and four cadmium sulfide arms, that breaks Kasha’s rule by emitting light from multiple excited states,” says Paul Alivisatos, director of Berkeley Lab and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley. “Becau- se this nanocrystal system has much higher quantum

yield and is relatively more photostable than organic molecules, it holds promi- sing potential for optical sensing and light emission- based applications, such as LEDs and imaging labels.”

“For the study of nanocrystal molecules, it is im- portant to be able to grow complex nanocrystals in which simple nanocrystal building blocks are connected together in well-defined ways,” Charina Choi, lead author of the Nano Letters paper, ex- plains. “Although there are many versions of elec- tronically coupled nanocrystal molecules, semicon- ductor tetrapods feature a beautiful symmetry that is analogous to the methane molecule, one of the fundamental units of organic chemistry.”

In this study, Choi, Alivisatos and their co-authors designed a cadmium-selenide (CdSe)and cadmium- sulfide (CdS) core/shell tetrapod whose quasi-type-I band alignment results in high luminescence quan- tum yields of 30- to 60-percent. The highest oc- cupied molecular orbital (HOMO) of this tetrapod involves an electron “hole” within the CdSe core, while the lowest unoccupied molecular orbital (LUMO) is centered within the core but is also likely