Research Challenge 2: Quantum Dots and Phosphors

 Research Challenge 2: Quantum Dots and Phosphors

This Research Challenge is aimed at studying materials architectures suitable for wavelength down-conversion in solid-state lighting. Particular materials we have focused on in this Research Challenge are Eu³⁺-doped phosphors and II-VI quantum dots (QDs).

Among the principal criteria for such wavelength down-conversion materials are: a high quantum yield (QY); narrow-linewidth emission (particularly in the range of 610-620 nm); broad blue absorption that is well matched to  the emission of the blue pump LED; low thermal quenching; photo and thermal stability; and potential for directionality and chromaticity tuning desirable for higher functionality light.

The Eu³⁺ phosphors are based on tantalate and niobate oxides, and take advantage of the narrow red emission lines characteristic of electronic transitions in the Eu³⁺ f-shell. The QDs are based on CdZn-VI materials, synthesized currently using colloidal methods.

QDs have numerous long-term and fundamental advantages over rare-earth phosphors, including: a broad and a five-orders-of-magnitude-greater blue absorbance; a relatively good accommodation of strain, which can enable a wide range of alloy compositions and emission wavelengths; the possibility of active chromaticity tuning (through Stark-effect-based modification of absorption oscillator strengths) for higher functionality light; and synergy with the light-emission-physics Research Challenges 3, 4, 5, and 6.

We initially focused on a low-temperature inverse-micellar-solution method for QD synthesis. We now believe, however, that luminescent quantum yields of QDs synthesized in this low-temperature manner will always be relatively low. We are now pursuing high-temperature colloidal methods for QD synthesis, bringing in a collaborator and expert at these methods, Professor David Kelley of UC Merced.

Research Participants

• Dr. Jim Martin (SNL) – Principal Investigator, primarily responsible for QD synthesis; participates in characterization.
• Prof. David Kelley (UC Merced) – Guides the synthesis of the QD core/shell heterostructures, especially through developing simulation codes for computing the QD strain energy, and performs electronic structure simulations.
• Dr. May Nyman (SNL) – Hydrothermal synthesis of phosphors and their structural characterization.
• Dr. Lauren Rohwer (SNL) – Photophysical characterization of both the Eu³⁺ phosphors and the quantum dots; also guides synthesis and development of the phosphors.

Research Challenge Publications

Bleier, Grant C.; Nyman, May; Rohwer, Lauren E. S.; and Rodriguez, Mark A. Seeking the optimal LaTaO4:Eu phosphor, Journal of Solid State Chemistry, 184, 3221 (2011). [10.1016/j.jssc.2011.10.008]

Forbes, Tori Z.; Nyman, May; Rodriguez, Mark A.; and Navrotsky, Alex The Energetics of Lanthanum Tantalate Materials, Journal of Solid State Chemistry, 183, 2516 (2010). [10.1016/j.jssc.2010.08.024]