Core shell quantum dot
![core shell quantum dot core shell quantum dot](https://pubs.rsc.org/image/article/2018/RA/c8ra03652g/c8ra03652g-f1_hi-res.gif)
synthesized high-quality QDs using the successive ionic layer adsorption and reaction (SILAR) approach and achieved precise control of the shell thickness. Although descriptions of the structure have been reported in the literature, most of the information was derived from the size change of the nanocrystals compared with core QDs and the synthetic process. Identifying the interface of QDs would help to determine the shell thickness, the shape, and position of the core, which play an important role in the optical properties of the QDs. Imaging the interface in QDs is a long-standing unresolved challenge, which limits in-depth research on the relationship between the band alignment and properties, and their effective exploitation in optoelectronic devices. However, due to the good lattice match between CdSe and CdS and the same cation of the two materials, it is very challenging to identify the interface between the core and shell by using traditional transmission electron microscopy (TEM). reported the detailed growth of QDs and emphasized the effect of reaction parameters on their shape and crystalline phase, and explored the relationship between the structure and optical properties. reported high-quality QDs with narrow emission linewidths and suppressed blinking by using octanethiol and cadmium oleate as precursors for the growth of the CdS shell. QDs have been successfully synthesized with control of size and shape by using several synthetic approaches, revealing unique structure/property relationships. Therefore, the CdS shell can grow epitaxially along the CdSe core. Among various QDs, is one of the most studied and best developed systems.įor QDs, CdSe core and CdS shell have the same crystalline structure and a small lattice mismatch (3.9%). Compared with bare QDs, QDs exhibit an enhanced photoluminescence quantum yield, higher efficiency, and stability within solar cells and other optoelectronic devices. Coating the core QDs with a shell layer is a typical approach to passivate surface defects, which can reduce the surface effects on the optical properties of the QDs. In addition, for traditional bare QDs, surface trap states/defects are unavoidable due to the small size and large specific surface area, which influence their optical properties. This structural difference is subtle, thus it is difficult to synthesize nanocrystals with perfect crystallinity. In the ZB structure, the lattice plane follows an ABCABC stacking along the direction, while in the WZ structure, it follows an ABABAB stacking along the direction. In these two structures, each cation is coordinated with four counter-ions in the tetrahedral configuration and the local coordination and bond length are identical, while the stacking sequence of the two structures is different. As the most studied QDs, those with CdX (X = Se, S, Te) composition may crystallize in two different structures, that is, the cubic zincblende (ZB) and the hexagonal wurtzite (WZ) structure. The results define an approach to characterize the heterostructure of two materials with the same crystalline structure and cations.Ĭolloidal semiconductor quantum dots (QDs) have been widely studied for applications in solar cells, luminescent solar concentrators, photocatalysis, and other optoelectronic devices due to their high quantum yield, size/chemical composition tunable absorption and emission spectra, and solution processability. As the shell thickness further increases, a sharp interface appears. For thick-shelled QDs, the lattice spacing is different at the core and shell regions, while the heterostructured interface is still coherent and cannot be clearly imaged. For thin-shelled QDs, an ideal coherent interface forms between core and shell due to the small lattice mismatch, and the lattice spacing remains unchanged at the core and shell regions. By examining changes in lattice spacing in an individual quantum dot, the atomic interface is identified.
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Herein, high-resolution transmission electron microscopy, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy elemental mapping are combined to characterize the structure and identify the interface in the QDs with different CdS shell thicknesses. However, since CdSe and CdS have the same crystal structure, same cations, and similar lattice parameters, it is very challenging to image the interface. Quantum dots (QDs) have been widely studied in recent years, due to their architecture which allows to tailor properties by controlling structure and composition.