Unique Luminescence Found in Nanocrystals

A fundamental principle of photoluminescence known as “Kasha’s rule” was broken by scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory when they created artificial molecules of semiconductor nanocrystals.

Berkeley Lab researchers have developed unique semiconductor tetrapods that, under illumination, break Kasha’s rule for photoluminescence by emitting two colors of light. (Image: Alivisatos’ research group)

Named for chemist Michael Kasha, who proposed it in 1950, Kasha’s rule holds that, when light is shone on a molecule, the molecule will emit light (fluorescence or phosphorescence) only from its lowest energy excited state. This is why photoluminescent molecules emit light at a lower energy than the excitation light. Although 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 quantum dots that break Kasha’s rule have not been reported – until now.

“We have demonstrated a semiconductor nanocrystal 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,” said Paul Alivisatos, director of the Berkeley Lab and the Larry and Diane Bock Professor of Nanotechnology at the University of California, Berkeley. “Because this nanocrystal system has much higher quantum yield and is relatively more photostable than organic molecules, it holds promising potential for optical sensing and light-emission-based applications, such as LEDs and imaging labels.”

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. (Image: Alivisatos’ research group)

Alivisatos, an internationally recognized authority on nanochemistry, is one of two corresponding authors, along with Sanjeevi Sivasankar of the DoE’s Ames Laboratory and Iowa State University, on a paper describing this work in the journal Nano Letters. Co-authors were Charina Choi, Prashant Jain and Andrew Olson, all members of Alivisatos’ research group, plus Hui Li, a member of Sivasankar’s research group.

“For the study of nanocrystal molecules, it is important to be able to grow complex nanocrystals in which simple nanocrystal building blocks are connected together in well-defined ways,” Choi said. “Although there are many versions of electronically coupled nanocrystal molecules, semiconductor tetrapods feature a beautiful symmetry that is analogous to the methane molecule, one of the fundamental units of organic chemistry.”

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

Charina Choi is a member of Paul Alivisatos' group at Berkeley Lab. (Image: Roy Kaltschmidt, Berkeley Lab)

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 tetrapods emit two colors was a surprise,” Choi said. “If we can learn to control the frequency and intensity of the emitted colors, then these tetrapods may be useful for multicolor emission technologies.”

“In the field of optical sensing with light emitters, it is impractical to rely simply on changes in emission intensity, as emission intensity can fluctuate significantly due to background signal,” Jain said. “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 nanoscale sensors for measuring forces. Previous work by Alivisatos and Choi showed that the emission wavelengths of these tetrapods will shift in response to local stress on their four arms.

“When a stress bends the arms of a tetrapod, it perturbs the electronic coupling within the tetrapod’s heterostructure, which in turn changes the color of the emitted light, and also likely alters the ratio of emission intensity from the two excited states,” Choi said. “We are currently trying to use this dependence to measure biological forces; for example, the stresses exerted by a beating heart cell.”

By adjusting the length of a CdSe/CdS core/shell tetrapod’s arms, it is possible to tune band alignment and electronic coupling within the heterostructure. The result would be tunable emissions from multiple excited states, an important advantage for nano-optic applications.

“We’ve demonstrated that the oscillator strength of LUMO+1 to HOMO light emissions can be tuned by changing the arm length of the tetrapod,” Choi said. “We predict that the lifetime and energy of the emissions can also be controlled through appropriate structural modifications, including arm thickness, number of arms, chemical composition and particle strain.”

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