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Observation of a scientific rule being broken can sometimes
lead to new knowledge and important applications. Such would seem the case when
scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley
National Laboratory (Berkeley Lab) created artificial molecules of
semiconductor nanocrystals and watched them break a fundamental 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 photoluminescent molecules emit light at a lower
energy 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 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,” says Paul Alivisatos, director of Berkeley Lab and
the Larry and Diane Bock Professor of Nanotechnology at the University of
California (UC) 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.”
Alivisatos is one of two corresponding authors, along with
Sanjeevi Sivasankar of DOE’s Ames Laboratory and Iowa State University, on a paper describing this work
in Nano Letters. The paper is titled “Spatially Indirect Emission in a
Luminescent Nanocrystal Molecule.” Co-authoring the paper were Charina Choi,
Prashant Jain, and Andrew Olson, all members of Alivisatos’ research group,
plus Hui Li, a member of Sivasankar’s research group.
Semiconductor tetrapods make exceptionally good subjects for
the study of electronically coupled nanocrystals as Charina Choi, lead author
of the Nano Letters paper, explains.
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: Lawrence Berkeley National Laboratory |
“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 says. “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 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 quantum
yields of 30 to 60%. The highest occupied 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 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 tetrapods 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 impractical 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 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 says. “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 nanooptic 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 says. “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.”
