74
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
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111