Bright, efficient red, green and blue quantum-dot LEDs are realized by customizing the nanostructure of the quantum dots.

High-efficiency light-emitting devices based on quantum dots with tailored nanostructures

Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. In this review, we provide an up to date overview of the complex colloidal nanostructures that could be prepared so far by cation exchange. At the same time, the review gives an account of the fundamental thermodynamic and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.

Forging Colloidal Nanostructures via Cation Exchange Reactions

Quantum dot light-emitting diodes are promising light sources for applications in displays. However, to date, there have been no reports of devices that simultaneously offer both high brightness and high external quantum efficiency. Here, we report red, green and blue quantum dot light-emitting diodes based on CdSe/ZnSe core/shell structures that have these attributes. We demonstrate devices with maximum external quantum efficiencies of 21.6%, 22.9% and 8.05% for red, green and blue colours with corresponding brightness of 13,300 cd m–2, 52,500 cd m–2 and 10,100 cd m–2. The devices also offer peak luminance of 356,000 cd m–2, 614,000 cd m–2 and 62,600 cd m–2, respectively. We postulate that this high performance is due to the use of Se throughout the core/shell regions and the existence of alloyed bridging layers at the core/shell interfaces. This study suggests that in the future visible quantum dot light-emitting diodes will also be suitable for lighting applications. Red, green and blue CdSe/ZnSe quantum dot light-emitting diodes combine efficient operation with high brightness.

Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency

We review the synthesis of semiconductor nanocrystals/colloidal quantum dots in organic solvents with special emphasis on earth-abundant and toxic heavy metal free compounds. Following the Introduction, section 2 defines the terms related to the toxicity of nanocrystals and gives a comprehensive overview on toxicity studies concerning all types of quantum dots. Section 3 aims at providing the reader with the basic concepts of nanocrystal synthesis. It starts with the concepts currently used to describe the nucleation and growth of monodisperse particles and next takes a closer look at the chemistry of the inorganic core and its interactions with surface ligands. Section 4 reviews in more detail the synthesis of different families of semiconductor nanocrystals, namely elemental group IV compounds (carbon nanodots, Si, Ge), III–V compounds (e.g., InP, InAs), and binary and multinary metal chalcogenides. Finally, the authors’ view on the perspectives in this field is given.

Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials.

Synthesis and applications of metal-organic framework–quantum dot (QD@MOF) composites

Highly monodisperse zinc blende CdSe nanocrystals have been synthesized by using different phosphine-free Se precursors successfully. To understand the reaction mechanism and obtain high quality CdSe nanocrystals, the effects on the use of different Se and/or Cd precursors, the adjustment of the molar ratios between Cd and Se precursors, and the selection of suitable reaction and growth temperatures have been studied in details. Absorption spectrum, fluorescence spectrum, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used for the characterization of synthesized CdSe nanocrystals. The quality of as-prepared CdSe nanocrystals was reached the same high level compared with the method using phosphine selenium precursors, its quantum yields were among 30 to 60% and photoluminescence (PL) full width at half-maximum (FWHM) was well controlled between 22 and 28 nm. As core, such zinc blende CdSe nanocrystals were also used to synthesize CdSe/ZnS, CdSe/CdS, and CdSe/CdS/ZnS core-shell nanocrystals. The quantum yields of as-prepared core-shell nanocrystals were among 50 to 80%. Large-scale syntheses of such core-shell nanocrystals have been successfully demonstrated and as many as 3 g of high quality CdSe/ZnS nanocrystals were easily synthesized with the use of only low-cost, green, and environmentally friendlier reagents.

High quality synthesis of monodisperse zinc-blende CdSe and CdSe/ZnS nanocrystals with a phosphine-free method

In this paper, we report the synthesis of dot- and branch-shaped HgxCd1−xTe nanocrystals (NCs) with good stability and a high quantum yield of about 30% through an elaborate cation exchange reaction at room temperature. The large red-shifts in both absorption spectra and photoluminescence (PL) spectra confirmed the substitution of mercury ions for cadmium ions and the formation of HgxCd1−xTe NCs. Interesting periodical XRD peaks observed in as-obtained HgxCd1−xTe NCs indicated the formation of layered metal thiolates, which not only played a key role in introducing mercury ions during the cation exchange process, but also acted as ligands to maintain the emission stability of newly formed HgxCd1−xTe NCs. The results indicate that the red-shift in PL emission has close-correlation with several parameters (such as the amounts of thiols and mercury ions, the sample store time, etc.).

Zhaojun Tang

Papers

High quality synthesis of monodisperse zinc-blende CdSe and CdSe/ZnS nanocrystals with a phosphine-free method

Thiolate-assisted cation exchange reaction for the synthesis of near-infrared photoluminescent HgxCd1−xTe nanocrystals