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Potential and Limitations of Luminescent Quantum Dots in Biology
Published in Victor I. Klimov, Nanocrystal Quantum Dots, 2017
In 1993, Bawendi and coworkers showed that high quality nanocrystals of CdSe QDs with a narrow size distribution (~5–10%) and relatively high quantum yields can be prepared using high temperature reaction of organometallic precursors [1,31]. They also demonstrated that size distribution can be further improved during postreaction processing. This reaction scheme initially employed dimethylcadmium (CdMe2) and trioctylphosphine selenide (TOP:Se), diluted in trioctylphosphine (TOP) and their rapid injection into a hot (280–300°C) coordinating solution of trioctylphosphine oxide (TOPO) [31]. Subsequently, Peng and coworkers further refined the reaction scheme and showed that additional precursors that are less volatile and less pyrophoric than CdMe2 could effectively be employed [32,33]. In those studies, they and other groups have eventually outlined the importance of impurities (usually acids coordinating the metal precursors, e.g., hexylphosphonic acid [HPA] and tetradecylphosphonic acid [TDPA]) to the reaction progress, and showed that these impurities can be externally controlled. They also applied this rationale to make other types of colloidal QDs. In this route, high purity TOPO and controlled amounts of metal coordinating ligands and metal precursors (such as CdO, Cd-acetate (Cd(Ac)2), and Cd-acetylacetonate (Cd(acac)2) for Cd-based nanocrystals and lead(II) acetate trihydrate for PbSe QDs) are used in the reaction; the selenium precursor still used TOP:Se in most cases [23,32–35]. Following the initial progress made in high temperature QD synthesis, it was demonstrated that passivating the native QD cores with a layer of wider band gap material(s) could dramatically enhance the fluorescence quantum yield of the resulting core–shell nanocrystals. The reaction conditions for preparing a series of core–shell nanoparticles that are strongly fluorescent and stable was detailed in a series of reports by Hines and Guyot-Sionnest [36], Dabbousi et al. [37], and Peng et al. [38] To reduce the particle size distribution (polydispersity) of the made cores or core–shell nanocrystals, growth can be followed by size selective precipitation using a “bad-solvent” (such as methanol or ethanol). In addition to reducing size dispersion, this procedure also removes impurities and precipitated metals from the reaction solution [1,31]. This cleaning step is crucial for nanocrystals made using less reactive precursors, since larger amounts of unreacted metals, acids, and amines are left in the final QD samples. Characterization of these nanocrystals include high and low resolution transmission electron microscopy (TEM), wide angle x-ray diffraction (XRD), small angle x-ray scattering (SAXS), and absorption and fluorescence spectroscopy to extract information such as size, distribution width, crystal structure, band edge value, and emission location [1,2,4,31–39]. Additional details on the synthesis routes, structural characterization, the physics of confinements effects, and their implications on the electronic and spectroscopic properties of colloidal QDs is provided in Chapters 1, 2, 6, and 7.
Facile synthesis of CsPbBr3/PbSe composite clusters
Published in Science and Technology of Advanced Materials, 2018
Thang Phan Nguyen, Abdullah Ozturk, Jongee Park, Woonbae Sohn, Tae Hyung Lee, Ho Won Jang, Soo Young Kim
A nanocomposite material of CsPbBr3 and PbSe was synthesized to protect perovskite material from self-enlargement during reaction. As a core material, CsPbBr3 quantum dots were chosen because CsPbBr3 is the most stable among cesium lead halides. It is reported that materials with low lattice mismatch could easily form the core/shell structure such as CdSe/CdS (3.9% lattice mismatch) [28] or CdSe/ZnS (12% lattice mismatch) [29]. The lattice constant of cubic phase CsPbBr3 is known to be 5.87 Å and that of PbSe is reported to be 6.1 Å [30,31]. Therefore, the lattice mismatch between CsPbBr3 and PbSe is approximately 4%. The same source of Pb2+ in an organic solvent could be used to produce CsPbBr3 and PbSe. Furthermore, trioctylphosphine selenide can be used as an easy source for the PbSe shell layer. It is therefore expected that CsPbBr3/PbSe composites could be synthesized to improve the stability of inorganic perovskite quantum dots.