Mircea Cotlet (standing) and Zhihua Xu. Image: Brookhaven Lab |
In a step toward engineering ever-smaller electronic devices,
scientists at the U.S. Department of Energy’s (DOE) Brookhaven National
Laboratory have assembled nanoscale pairings of particles that show
promise as miniaturized power sources. Composed of light-absorbing,
colloidal quantum dots linked to carbon-based fullerene nanoparticles,
these tiny two-particle systems can convert light to electricity in a
precisely controlled way.
“This
is the first demonstration of a hybrid inorganic/organic, dimeric
(two-particle) material that acts as an electron donor-bridge-acceptor
system for converting light to electrical current,” said Brookhaven
physical chemist Mircea Cotlet, lead author of a paper describing the
dimers and their assembly method in Angewandte Chemie.
By
varying the length of the linker molecules and the size of the quantum
dots, the scientists can control the rate and the magnitude of
fluctuations in light-induced electron transfer at the level of the
individual dimer. “This control makes these dimers promising
power-generating units for molecular electronics or more efficient
photovoltaic solar cells,” said Cotlet, who conducted this research with
materials scientist Zhihua Xu at Brookhaven’s Center for Functional Nanomaterials (CFN).
Scientists
seeking to develop molecular electronics have been very interested in
organic donor-bridge-acceptor systems because they have a wide range of
charge transport mechanisms and because their charge-transfer properties
can be controlled by varying their chemistry. Recently, quantum dots
have been combined with electron-accepting materials such as dyes,
fullerenes, and titanium oxide to produce dye-sensitized and hybrid
solar cells in the hope that the light-absorbing and size-dependent
emission properties of quantum dots would boost the efficiency of such
devices. But so far, the power conversion rates of these systems have
remained quite low.
“Efforts
to understand the processes involved so as to engineer improved systems
have generally looked at averaged behavior in blended or layer-by-layer
structures rather than the response of individual, well-controlled
hybrid donor-acceptor architectures,” said Xu.
The
precision fabrication method developed by the Brookhaven scientists
allows them to carefully control particle size and interparticle
distance so they can explore conditions for light-induced electron
transfer between individual quantum dots and electron-accepting
fullerenes at the single molecule level.
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The
entire assembly process takes place on a surface and in a stepwise
fashion to limit the interactions of the components (particles), which
could otherwise combine in a number of ways if assembled by
solution-based methods. This surface-based assembly also achieves
controlled, one-to-one nanoparticle pairing.
To
identify the optimal architectural arrangement for the particles, the
scientists strategically varied the size of the quantum dots — which
absorb and emit light at different frequencies according to their size —
and the length of the bridge molecules connecting the nanoparticles.
For each arrangement, they measured the electron transfer rate using
single molecule spectroscopy.
“This
method removes ensemble averaging and reveals a system’s heterogeneity —
for example fluctuating electron transfer rates — which is something
that conventional spectroscopic methods cannot always do,” Cotlet said.
The
scientists found that reducing quantum dot size and the length of the
linker molecules led to enhancements in the electron transfer rate and
suppression of electron transfer fluctuations.
“This
suppression of electron transfer fluctuation in dimers with smaller
quantum dot size leads to a stable charge generation rate, which can
have a positive impact on the application of these dimers in molecular
electronics, including potentially in miniature and large-area
photovoltaics,” Cotlet said.
“Studying
the charge separation and recombination processes in these simplified
and well-controlled dimer structures helps us to understand the more
complicated photon-to-electron conversion processes in large-area solar
cells, and eventually improve their photovoltaic efficiency,” Xu added.
A
U.S. patent application is pending on the method and the materials
resulting from using the technique, and the technology is available for
licensing. This work was funded by the DOE Office of Science.
