We analyze the mechanism
of seeded growth reactions used to synthesize
colloidal core/shell nanocrystals. Looking at the formation of CdSe/CdS
and CdSe/ZnSe using both zinc blende and wurtzite CdSe seeds with
a different surface termination, we show that the formation rate of
the shell material does not depend on the presence of the seed nanocrystals.
This suggests that shells grow by inclusion of CdS or ZnSe initially
formed in the reaction mixture, possibly under the form of reactive
monomers, and not by successive adsorption and reaction of metal and
chalcogen precursors. This insight makes balancing homogeneous nucleation
and heterogeneous growth of the shell material key to suppressing
spurious secondary nucleation. Through a combination of experimental
work and reaction simulations, we argue that this can be effectively
achieved by raising the monomer solubility through the concentration
of carboxylic acid used in the seeded growth reaction.
We evaluated the effect of high-temperature treatment of Cd 0.9 Zn 0.1 Te:In single crystals using Hall-effect measurements, medium-and high-temperature annealing under various deviations from stoichiometry, and infra-red (IR) transmission microscopy Annealing at ~730 K sharply increased the electrical conductivity (by ~1-2 orders-of-magnitude). Plots of the temperature-and cadmium-pressure dependences of the electrical conductivity, carrier concentration, and mobility were obtained. Treating previously annealed Cd-samples under a Te overpressure at 1070 K allowed us to restore their resistance to its initial high values. The main difference in comparing this material with CdTe was its lowered electron density. We explained our results within the framework of Kröger's theory of quasi-chemical reactions between point defects in solids.
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