ABSTRACT
The pyrolysis of organometallic reagents in a coordinating solvent can be exploited to prepare nearly monodisperse CdSe particles
with nanoscale diameters [36, 37]. The resulting quantum dots emit radiations in the visible region of the electromagnetic spectrum (Fig. 2.1), albeit with poor quantum yields. Indeed, defects on the surface of these particles facilitate the nonradiative deactivation of their excited state and are predominantly responsible for the low emission intensities [38]. Nonetheless, the epitaxial growth of a protective ZnS shell around a preformed CdSe core can eliminate defects on the CdSe surface and enhance the luminescence quantum yield significantly [39]. As a result, quantum dots with this particular elemental composition are one of the most studied classes of nanoparticles [40]. In fact, reliable synthetic protocols for the efficient preparation of CdSe-ZnS core-shell quantum dots
Table 2.1 Photophysical parameters of typical organic dyes and quantum dots e[a]
have already been established. In a typical procedure (Fig. 2.2) [41], CdO and Se are heated at high temperature in a mixture of tetra-n-decylphosphonic acid (TDPA), tri-n-octylphosphine oxide (TOPO) and tri-n-octylphosphine (TOP). This treatment encourages the nucleation of CdSe seeds and their subsequent growth into nanoparticles with TDPA, TOPO and TOP molecules adsorbed on their surface. Once a desired particle size is reached, the temperature can be lowered to arrest growth and the resulting quantum dots can be isolated on the basis of reiterative sequences of precipitation, centrifugation and filtration steps. Then, the assembled CdSe nanoparticles can again be heated at high temperature in a mixture of TOPO and TOP and in the presence of stoichiometric amounts of diethyl zinc and hexamethyldisilathiane [42]. Under these conditions, ZnS layers grow spontaneously around the preformed CdSe particles to generate CdSe-ZnS core-shell quantum dots with TOP and TOPO molecules adsorbed on their surface. After lowering the temperature down to ambient conditions, the final nanostructured assemblies can be isolated, once again, relying on reiterative sequences of precipitation, centrifugation and filtration steps.
