(Invited) Chemical Modulation of Precursor Reactivity for Systematic Synthesis of High-Quality Quantum Dots

Abstract

With their size-dependent electronic properties, solution processability, and synthetic tunability, quantum dots (QDs) have been heavily studied for fundamental research and commercial applications. The performance of QD-based devices and QD probes are greatly affected by their size distribution and structural quality. To date, extensive studies have investigated size control, allowing us to grow QDs with less than a single monolayer difference in their size variation. In contrast, high quality QD growth is achieved mostly by trial-and-error and a reaction condition optimized for a specific QD system cannot be adapted to QDs of different sizes or materials. To address this critical gap in QD synthesis, we first identified the key parameters contributing to the structural quality of QDs and examined how they affect QD growth. Our study shows that the reaction temperature governs the instability of the QD surface and the level of lattice relaxation, while precursor reactivity impacts the reaction kinetics at the surface as well as satellite particle formation. Systematic optimization of growth conditions requires independent tuning of these two parameters. Unfortunately, conventional precursors do not allow temperature-independent modulation of their reactivity, requiring new precursors to be synthesized for different reaction conditions. This approach is inherently low throughput and labor-intensive.To enable temperature-independent modulation of precursor reactivity, we designed a new sulfur precursor that has a reactivity that can be chemically tuned in a predictable manner. The additives used for this reaction are commercially available, making the new chemistry highly accessible. The new precursor chemistry represents a new paradigm for QD growth: chemically-induced precursor conversion. We show that the new precursor allows systematic growth of high quality QDs of various sizes and materials for both core-only and core-shell QDs. Moreover, a close examination of the growth results provided invaluable insight into the nanoparticle growth process, enabling logical tuning of the reaction condition. We also show that this new precursor chemistry employs completely different reaction mechanisms for nucleation and growth. By effectively separating growth and nucleation even at a high concentration of precursors, the new precursor allows heat-up-based growth of high quality shells that are comparable to those created by the injection method. Our study inspires the design of other precursors to systematically grow high quality nanocrystals and achieve the heat-up-based growth of high quality shells, a highly economical and scalable approach for large-scale synthesis.