CdSe/ZnS Quantum Dots

Quantum dots are semiconductor nanocrystals that have tunable emission through changes in their size. Producing bright, efficient quantum dots with stable fluorescence is important for using them in applications in lighting, photovoltaics, and biological imaging.In order to increase the fluorescence and stability of CdSe quantum dots were passivated with ZnS shell through the pyrolysis of organometallic precursors.

The inorganic passivation of nanocrystal with higher band gap materials, resulting in core/shell structure nanocrystals such as CdSe/ZnS and CdSe/CdS, showed high quantum efficiency up to 50% because of the robust passivation of the surface defects and also the quantum confinement effect which enhances exciton recombination in the core. The shell-protectedCdSe/ZnSQDs exhibited higher photoluminescent (PL) efficiency and stability than their corresponding CdSe coreQDs. The proper passivation of the nanocrystal surfaces is necessary to achieve a high quantum yield (QY).

Quantum yield was observed to increase with increasing shell thickness until 3 monolayers, after which quantum yield decreased and the likelihood of flocculation of the colloid increased. The quantum yield also increased with increasing Zn:S ratio, possibly indicating that zinc atoms may substitute for missing cadmium atoms at the CdSe surface.

With increasing time and temperatures the nanoparticles become slowly disordered hence the excitonenergy decreases whereas the nanoparticle size increases slowly. It is due to size effect which depends on thethermodynamic properties of nanoparticles.

CdSe/ZnS core-shell quantum dots (QDs) have received special interest from due to their high photoluminescence (PL) quantum yield (QY) with narrow bandwidth, large band gap tunability across the visible spectrum and robustness. Composite materials containing CdSe/ZnSnanocrystals are promising for a wide range of high-performance applications, as biosensors high efficiency quantum-LEDs  photovoltaic devices and lasers.