Transmission electron micrographs and (inset) showing the electron diffraction patterns of three quantum dot samples with average size of (a) 2.4 nm (b) 3.6 nm, and (c) 5.8 nm. Image: Alivisatos group |
With its promise of superfast computers and ultra-powerful optical
microscopes among the many possibilities, plasmonics has become one of the
hottest fields in high-technology. However, to date plasmonic properties have
been limited to nanostructures that feature interfaces between noble metals and
dielectrics. Now, researchers with the U.S. Department of Energy (DOE)’s
Lawrence Berkeley National Laboratory (Berkeley Lab) have shown that plasmonic
properties can also be achieved in the semiconductor nanocrystals known as
quantum dots. This discovery should make the field of plasmonics even hotter.
“We have demonstrated well-defined localized surface plasmon resonances
arising from p-type carriers in vacancy-doped semiconductor quantum dots that
should allow for plasmonic sensing and manipulation of solid-state processes in
single nanocrystals,” says Berkeley Lab director Paul Alivisatos, a
nanochemistry authority who led this research. “Our doped semiconductor quantum
dots also open up the possibility of strongly coupling photonic and electronic
properties, with implications for light harvesting, nonlinear optics, and
quantum information processing.”
Alivisatos is the corresponding author of a paper in the journal Nature
Materials titled “Localized surface plasmon resonances arising from free
carriers in doped quantum dots.” Co-authoring the paper were Joseph Luther and
Prashant Jain, along with Trevor Ewers.
The term “plasmonics” describes a phenomenon in which the confinement of
light in dimensions smaller than the wavelength of photons in free space make
it possible to match the different length-scales associated with photonics and
electronics in a single nanoscale device. Scientists believe that through
plasmonics it should be possible to design computer chip interconnects that are
able to move much larger amounts of data much faster than today’s chips. It
should also be possible to create microscope lenses that can resolve nanoscale
objects with visible light, a new generation of highly efficient light-emitting
diodes, and supersensitive chemical and biological detectors. There is even
evidence that plasmonic materials can be used to bend light around an object,
thereby rendering that object invisible.
The plasmonic phenomenon was discovered in nanostructures at the interfaces
between a noble metal, such as gold or silver, and a dielectric, such as air or
glass. Directing an electromagnetic field at such an interface generates
electronic surface waves that roll through the conduction electrons on a metal,
like ripples spreading across the surface of a pond that has been plunked with
a stone. Just as the energy in an electromagnetic field is carried in a
quantized particle-like unit called a photon, the energy in such an electronic
surface wave is carried in a quantized particle-like unit called a plasmon. The
key to plasmonic properties is when the oscillation frequency between the
plasmons and the incident photons matches, a phenomenon known as localized
surface plasmon resonance (LSPR). Conventional scientific wisdom has held that
LSPRs require a metal nanostructure , where the conduction electrons are not
strongly attached to individual atoms or molecules. This has proved not to be
the case as Prashant Jain, a member of the Alivisatos research group and one of
the lead authors of the Nature Materials paper, explains.
“Our study represents a paradigm shift from metal nanoplasmonics as we’ve
shown that, in principle, any nanostructure can exhibit LSPRs so long as the
interface has an appreciable number of free charge carriers, either electrons
or holes,” Jain says. “By demonstrating LSPRs in doped quantum dots, we’ve
extended the range of candidate materials for plasmonics to include
semiconductors, and we’ve also merged the field of plasmonic nanostructures,
which exhibit tunable photonic properties, with the field of quantum dots,
which exhibit tunable electronic properties.”
Jain and his co-authors made their quantum dots from the semiconductor
copper sulfide, a material that is known to support numerous copper-deficient stoichiometries.
Initially, the copper sulfide nanocrystals were synthesized using a common hot
injection method. While this yielded nanocrystals that were intrinsically
self-doped with p-type charge carriers, there was no control over the amount of
charge vacancies or carriers.
“We were able to overcome this limitation by using a room-temperature ion
exchange method to synthesize the copper sulfide nanocrystals,” Jain says. “This freezes the nanocrystals into a relatively vacancy-free state, which we
can then dope in a controlled manner using common chemical oxidants.”
By introducing enough free electrical charge carriers via dopants and
vacancies, Jain and his colleagues were able to achieve LSPRs in the
near-infrared range of the electromagnetic spectrum. The extension of
plasmonics to include semiconductors as well as metals offers a number of
significant advantages, as Jain explains.
“Unlike a metal, the concentration of free charge carriers in a
semiconductor can be actively controlled by doping, temperature, and/or phase
transitions,” he says. “Therefore, the frequency and intensity of LSPRs in
dopable quantum dots can be dynamically tuned. The LSPRs of a metal, on the
other hand, once engineered through a choice of nanostructure parameters, such
as shape and size, is permanently locked-in.”
Jain envisions quantum dots as being integrated into a variety of future
film and chip-based photonic devices that can be actively switched or
controlled, and also being applied to such optical applications as in vivo
imaging. In addition, the strong coupling that is possible between photonic and
electronic modes in such doped quantum dots holds exciting potential for
applications in solar photovoltaics and artificial photosynthesis
“In photovoltaic and artificial photosynthetic systems, light needs to be
absorbed and channeled to generate energetic electrons and holes, which can
then be used to make electricity or fuel,” Jain says. “To be efficient, it is
highly desirable that such systems exhibit an enhanced interaction of light
with excitons. This is what a doped quantum dot with an LSPR mode could
achieve.”
The potential for strongly coupled electronic and photonic modes in doped
quantum dots arises from the fact that semiconductor quantum dots allow for
quantized electronic excitations (excitons), while LSPRs serve to strongly localize
or confine light of specific frequencies within the quantum dot. The result is
an enhanced exciton-light interaction. Since the LSPR frequency can be
controlled by changing the doping level, and excitons can be tuned by quantum
confinement, it should be possible to engineer doped quantum dots for
harvesting the richest frequencies of light in the solar spectrum.
Quantum dot plasmonics also hold intriguing possibilities for future quantum
communication and computation devices.
“The use of single photons, in the form of quantized plasmons, would allow
quantum systems to send information at nearly the speed of light, compared with
the electron speed and resistance in classical systems,” Jain says. “Doped
quantum dots by providing strongly coupled quantized excitons and LSPRs and
within the same nanostructure could serve as a source of single plasmons.”
Jain and others in Alivsatos’ research group are now investigating the
potential of doped quantum dots made from other semiconductors, such as copper
selenide and germanium telluride, which also display tunable plasmonic or
photonic resonances. Germanium telluride is of particular interest because it
has phase change properties that are useful for memory storage devices.
“A long term goal is to generalize plasmonic phenomena to all doped quantum
dots, whether heavily self-doped or extrinsically doped with relatively few
impurities or vacancies,” Jain says.