Symposium X—MRS/The Kavli Foundation Frontiers of Materials

Jennifer A. Hollingsworth, Los Alamos National Laboratory

From Flask to Devices—The Making of Exceptionally Functional Colloidal Quantum Dot Emitters

Written by Don Monroe

In her Symposium X talk, Jennifer Hollingsworth discussed designing and making nanocrystal quantum dots that could potentially address the demanding requirements of quantum information technology.

The optical transitions in semiconducting quantum dots are shifted to higher energies by quantum confinement of their electrons and holes. Their optical properties can therefore be tuned by changing their size, typically a few nanometers in diameter. Colloidal synthesis techniques, such as successive ionic layer adsorption and reaction (SILAR) provide “angstrom-level control over size,” notably for CdSe. Quantum dots have many applications involving photon conversion, emission, and photocarrier generation.

For quantum computing, defect states like the “NV” center in diamond feature coupled spin and visible-light degrees of freedom that could be useful for quantum information processing. Hollingsworth asked whether an “artificial NV center” might be made through chemistry, with the spin degree of freedom of a metal-containing molecule coupled to the polarized emission of a separately optimized quantum dot. The strict requirements for the quantum dot are challenging however, including on-demand, tunable emission of near-infrared “telecom” wavelengths with high purity, brightness, and directionality. The devices will also need to be integrated with devices, and indistinguishability of the photons is “probably the most challenging factor that we face,” Hollingsworth said.

CdSe nanoparticles coated with a thin CdS shell historically had problems with “blinking” (going dark and recovering) and “bleaching” (getting progressively dimmer). Hollingsworth attributed these issues to two mechanisms: non-radiative Auger recombination that imparts the energy to a nearby electron instead of a photon and trapping of photoexcited carriers in surface states. More than a decade ago, she and others dramatically reduced these effects using thick CdS shells, 15-20 nm in thickness, to make “giant quantum dots.”

A second important design tool is the band offsets between the core and shell. “Type I” offsets confine both the electrons and holes to the core, while “Type II” only confines one carrier. A related approach uses a graded transition between core and shell rather than an abrupt one. With these new design parameters, Hollingsworth and her colleagues have explored an entire family of giant quantum dots. The improved stability opened a range of applications, such as long-term single-molecule tracking.

She also studied the differences between the “very tedious” monolayer-by-monolayer SILAR method and an alternate “continuous injection” method. Although both methods avoid blinking and bleaching, there are other important differences, where she traced different numbers of stacking fault, chemical terminations, and alloying. An even better hybrid method starts with SILAR but ends with continuous injection and a long anneal. “We have achieved on-demand single-photon sources,” Hollingsworth said.

CdSe quantum dots are limited to the visible, but giant quantum dots with a PbS core emit at room temperature in the near-infrared telecom bands around 1.3 micron and 1.5 micron, which is desirable for optically transmitting quantum information.

Hollingsworth also described advances in the brightness toward the roughly 10⁹ photons per second needed for quantum information. One approach exploits the localized surface plasmon resonance of a nearby conductive particle to enhance the spontaneous emission rates. However, the popular noble-metal particles do not support these resonances in the infrared, so researchers are looking at other materials, including tunably doped semiconductors and the spinel magnetite_.

Unfortunately, this plasmonic technique enhances the nonradiative rates as well as the radiative rates, Hollingsworth said. To enhance only the radiative rate, she collaborated with Maiken Mikkelsen’s group at Duke University to couple quantum dots to nanopatch antennas, reducing the radiative lifetime from microseconds to nanoseconds—yielding nearly a thousandfold brightness increase—or even shorter.

She also collaborated with Ronen Rapaport of the Hebrew University of Jerusalem, using dip-pen lithography to place quantum dots in the center of a bullseye antenna. These techniques enhance the brightness and produce directed emission that enhances collection efficiencies.

Hollingsworth sees paths forward in achieving better photon purity and device integration. To achieve the longer-term challenge of indistinguishable photons, she said, “we need to combine strategies of synthesis and integration.”

Symposium X—MRS/The Kavli Foundation Frontiers of Materials features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.