In an epitaxial growth process, control of the thickness of the shell over the nanoparticle core is another important parameter as thick shells induce surface defects that result in reduced photoluminescence. For example, ZnS is a suitable shell material for over coating of CdSe core QDs because the small lattice mismatch between ZnS and CdSe prevents a change of crystal structure, minimizes surface-defects and leads to increased quantum yields in CdSe/ZnS core/shell QDs. The second step in the formation of core/shell QDs is the coating of QDs with appropriate materials to form the core/shell QDs.Įpitaxial shell growth processes, necessitate the selection of suitable shell materials to allow the formation of core/shell QDs with improved optical properties. QDs glowing under UV light: ZnSe(S) QDs (blue) and CdSe(S) QDs (yellow, red and Orange). Fig 1 shows UV emission of CdSe(S) and ZnSe(S) QDs synthesized using an aqueous hydrothermal synthetic method. In the colloidal synthesis approach, uniform QDs can be produced using organometallic synthesis, solvothermal methods, microwave assisted methods, hydrothermal approaches and direct aqueous synthetic methods. However, the liquid phase colloidal synthesis is a widely used chemical approach to produce various QDs including CdSe, CdSe, ZnSe, ZnS, CdTe, InP and ZnS QDs. Gas phase syntheses are mainly based upon producing QDs by epitaxial growth of thin-films on crystal surfaces to form three dimensional nanoparticles, including QDs. In physical methods, QDs are produced by breaking down bulk semiconductors. QDs can be synthesized using different synthetic methods including physical methods, gas phase syntheses and the liquid phase colloidal synthesis. Ĭore/shell QDs are mostly synthesized in two steps: first, the synthesis of the core QDs and then over-coating of these QDs through shell growth reactions. CdSe/ZnSe, CdTe/CdSe and CdS/ZnSe core/shell QDs are some examples of type-II core/shell QDs. In type-II systems both the valence and conduction band edges of the core are lower or higher than those in the shell and both the hole and the electron are confined to the core. Examples include CdS/HgS, CdS/CdSe and ZnSe/CdSe core/shell QDs. Consequently, the holes and electrons are confined in the shell. In inverse type-I materials the band gap of the core is wider than the band gap of the shell and both the conduction and valence bands of the shell are therefore localized within the band gap of core. CdSe/ZnS, and InAs/CdSe core/shell QDs are other examples for type-I core/shell nanocrystal systems. Therefore, both the holes and electrons are confined to the CdSe core. ![]() For example in CdSe/CdS type-I core/shell QDs, the band gap of the CdSe core is 1.74 eV and the band gap of the CdS shell is 2.42 eV. In this type of nanoparticle, either the conduction or the valance bands of the core align within the band gap of the shell, such that both the electrons and holes are localized in the core. Ĭore/shell QDs can be categorized according to the band gap and energy levels of their components into three broad groups: type I, reverse type I and type II. Coating of QDs with appropriate materials to form core/shell QDs leads them to exhibit higher quantum yields and greater stability than core QDs the shell growth both confines the excitation to the core and protects the core against oxidation and chemical degradation. These under-coordinated atoms make them more active than those in the bulk of the QD materials. A QD has a high surface area to volume ratio with unsaturated bonds, or dangling bonds, existing on the surface. They exhibit improved optical properties over simple QDs due to the shell surrounding the QD core, which improves stability and photoluminescence efficiency. Core/Shell quantum dots are products of further engineering in the structures of quantum dots (QDs).
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