|
Even at the nanoscale, hybrids show promise—as evidenced by new
efforts to pair inorganic nanoparticles with conductive polymers to convert
sunlight into electricity or build better biosensors. To make the most of these
molecular matchups, however, scientists need to understand the small-scale
details of charge transfer—and how to control it.
Scientists working at the Center for Functional Nanomaterials
(CFN) at the U.S. Department of Energy’s Brookhaven National Laboratory
together with their colleagues at Syracuse University recently demonstrated
this kind of precision control in a hybrid composed of light-absorbing quantum dots
and a conjugated polymer—two types of semiconducting materials that have been
widely studied for photovoltaic and other optoelectronic applications and
biosensors. They’ve published a paper describing the details of the hybrid
nanocomposite self-assembly enabling control of the charge transfer rate in ACS Nano.
“Photovoltaic solar cells that incorporate organic and inorganic
semiconducting materials could be inexpensive to manufacture,” said Brookhaven
physical chemist Mircea Cotlet, who led the research team. “But we need to
design them in such a way that they are easy to assemble and efficient at
absorbing light and separating charges to produce electric current.”
On the other hand, scientists are also interested in using such
hybrid composites to produce light emitting diodes and biosensors, where the
production of such separated charges reduces the amount of light generated via
the quantum dots’ photoluminescence, and is therefore detrimental. “We need to
understand the details of how these materials operate under various conditions
so we can optimize the properties for the desired application,” Cotlet said.
For the current study, the scientists took advantage of
electrostatic attraction to get negatively charged quantum dots and a
positively charged polymer to self-assemble to form hybrids, and then tested
the strength of one kind of charge transfer under a variety of conditions. By
illuminating the quantum dots with a particular wavelength of light, they could
measure how positive charges known as “holes” were extracted in a controlled
way by the nearby polymer at the expense of the photoluminescence emitted by
the quantum dot.
The key variable they were interested in studying was how the
hole transfer rate was affected by the thickness of an optically inert shell
surrounding the core of each quantum dot. To test this, the scientists at Syracuse chemically
synthesized a series quantum dots with varying shell thickness, from very thin
(1 nm) to thick (4 nm), and capped them with the conjugated polymers.
“We were really excited to design and synthesize the quantum
dots needed for this study,” said Syracuse
University chemist Mathew
Maye, whose team regularly uses the CFN facility for its research. “There are
still many unknowns in nanoscale energy transfer, and thanks to this study, we
now are closer to optimizing all the fine details.”
The results clearly show that photoluminescence and charge
transfer are competing processes: As the shell thickness increased, the rate of
charge transfer decreased and photoluminescence increased. Conversely, thicker
shells resulted in more intense photoluminescence and a lower hole transfer
rate.
This implies that the shell acts as a “tunneling barrier”
slowing down the flow of holes from the dot to the polymer, Cotlet said.
“If you want to design such hybrid materials for photovoltaic
solar cell applications, you may want to use dots with a very thin shell or no
shell at all to produce as many charges as you can,” he said. “But because such
hole transfer quenches, or kills, the photoluminescence of the dot, a thicker
shell may be a better option for applications like light emitting diodes or
biosensors.”
The scientists found the same inverse relationship between shell
thickness and charge transfer rate no matter whether the hybrid materials were
in solution or thin films, but hole transfer rates were better overall in the
thin film condition than in solution. “This is good news since thin films are
the most likely configuration for photovoltaic devices,” Cotlet said.
Additional experiments looking at single hybrid particles also
found fluctuations in the rate of charge transfer that were dependent on the
dot’s shell thickness.
“These kinds of fluctuations can affect the amount of current
that comes out of a device, so we’d want to find ways to stabilize this in a
device.” Cotlet said. “Only by looking at single particles, as we’ve done here
for the first time, can you track this kind of fluctuating behavior,” he said.
Overall, the findings show that the nanoparticle synthesis and
self-assembly methods used to engineer quantum dots with various shell thicknesses
can give scientists the control they need over optoelectronic properties. “Whether we want to maximize hole transfer rate for photovoltaic applications,
or alternatively, photoluminescence for biosensors, we now have a way to do
that,” Cotlet said.
Source: Brookhaven National Laboratory
