Synthesis Of High-quality Quantum Dots Research Articles

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

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.

Controllable modulation of precursor reactivity using chemical additives for systematic synthesis of high-quality quantum dots

The optical and electronic performance of quantum dots (QDs) are affected by their size distribution and structural quality. Although the synthetic strategies for size control are well established and widely applicable to various QD systems, the structural characteristics of QDs, such as morphology and crystallinity, are tuned mostly by trial and error in a material-specific manner. Here, we show that reaction temperature and precursor reactivity, the two parameters governing the surface-reaction kinetics during growth, govern the structural quality of QDs. For conventional precursors, their reactivity is determined by their chemical structure. Therefore, a variation of precursor reactivity requires the synthesis of different precursor molecules. As a result, existing precursor selections often have significant gaps in reactivity or require synthesis of precursor libraries comprising a large number of variants. We designed a sulfur precursor employing a boron-sulfur bond, which enables controllable modulation of their reactivity using commercially available Lewis bases. This precursor chemistry allows systematic optimization of the reaction temperature and precursor reactivity using a single precursor and grows high-quality QDs from cores of various sizes and materials. This work provides critical insights into the nanoparticle growth process and precursor designs, enabling the systematic preparation of high-quality QD of any sizes and materials.

Microfluidic Synthesis of Semiconducting Colloidal Quantum Dots and Their Applications

The large-scale synthesis of high-quality quantum dots (QDs) for commercial applications, such as lighting, displays, and biomedical devices, is an urgent necessity. Batch reactor systems present a number of problems, such as improper mixing, heating, and reagent addition. Hence, controlling the growth and size of nanocrystals is difficult in this type of system. A number of microfluidic techniques have been developed to enable semiconductor colloidal QD synthesis. The reaction parameters of these techniques are controlled precisely during synthesis. Over the last 16 years, many advancements have been introduced to achieve products similar to those obtained from batch systems. Multiphase flow reactors reduce reactor fouling by using immiscible carrier liquids, which decrease the contact between reagents and the channel wall. Online monitoring of the nanocrystal growth through absorbance and fluorescence spectrometry provides detailed information on the reaction parameters. Chip-based reactors with subchan...

Effective improvement in optical properties of colloidal CdTe@ZnS quantum dots synthesized from aqueous solution

Efficient synthesis of high-quality quantum dots (QDs) with excellent optical properties by aqueous synthesis is still of great significance for extended optical applications. Herein we highlight the advantages in optical properties of colloidal CdTe@ZnS QDs prepared by a facile and highly effective aqueous synthesis method. These achievements were realized by delicate manipulation of the conditions involved in nucleation and the growth process. Transmission electron microscopy (TEM) images indicated the QDs were uniform size and well dispersible. The emission peaks of the as-prepared QDs could shift from 496 to 698 nm with narrow full width at half maximum (FWHM), and the corresponding fluorescent color changed from green to red. Moreover, the emission could even reach to the near-infrared (NIR) region (706–796 nm) by extending the reaction time. The highest photoluminescence (PL) quantum yield (QY) of the QDs could reach to 60%, and the average of FWHM was about 55 nm. To address the problem of wide size-distribution in PL QY decrease and FWHM broadening, the colloids of QDs prepared at long reaction time (above 3 h) were centrifuged (12 000 r min−1). In addition, the assessment of QD cytotoxicity indicated the CdTe@ZnS QDs were much less cytotoxic and showed good biocompatibility. Compared with organic synthesis, our aqueous synthesis of QDs could be carried out efficiently on a large scale and showed good batch-to-batch reproducibility. The as-prepared CdTe@ZnS QDs exhibited excellent optical properties and hold a good potential to be applied in optoelectronic and biological applications.

Stability Investigation of Ligand-Exchanged CdSe/ZnS-Y (Y = 3-Mercaptopropionic Acid or Mercaptosuccinic Acid) through Zeta Potential Measurements

Quantum dots have been considered to be promising candidates for bioapplications because of their high sensitivity, rapid response, and reliability. The synthesis of high-quality quantum dots that can be dissolved in water and other biological media is a crucial step toward their further application in biology. Starting with a one-pot reaction and the successive ionic layer adsorption and reaction (SILAR) method, we produced the CdSe/ZnS core/shell structure. Through a ligand-exchange mechanism, we coated the as-made CdSe/ZnS structure with 3-mercaptopropionic acid (MPA) or mercaptosuccinic acid (MSA). Various techniques, including photoluminescence (PL), ultraviolet-visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy, were utilized to characterize the ligand-coated CdSe/ZnS structure. The results show enhanced luminescence intensity, CdSe surface passivation by ZnS, and successful coating with MPA and MSA. The stability of quantum dots in solutions with different pH values was investigated by performing zeta potential measurements. The results revealed that the quantum dots shifted from displaying hydrophobic to hydrophilic behavior and could be connected with bioagents.

Quantum dots tailored with poly(para-phenylene vinylene).

In polymernanoparticle composites, uniform dispersion of the nanoparticles carries advantages over cases where nanoparticle aggregation dominates. Such dispersion has been particularly difficult to obtain in the case of composites prepared from nanoparticles and conjugated polymers. Here, we show that cadmium selenide nanocrystals, or quantum dots, can be integrated into thin films of poly(para-phenylene vinylene) (PPV) without aggregation. The two key departures from previous studies of quantum-dot/electronic polymer composites are (1) the synthesis of high-quality quantum dots directly in novel, functional ligands, thus eliminating the need for ligand exchange, and (2) polymerization chemistry that grafts PPV to the quantum dot surface. Solid-state photoluminescence spectra of composite materials prepared by these novel techniques reveal the critical importance of the quantum dot-polymer interface that will enable new investigations in nanoparticle-based light-emitting devices.