Critical Nucleus Size, Composition, and Energetics in the Synthesis of Doped ZnO Quantum Dots

TL;DR

This study investigates how Co2+ ions affect the nucleation and growth of doped ZnO quantum dots, revealing critical nucleus size, energetics, and the inhibitory role of Co2+ in homogeneous nucleation.

Contribution

It provides the first quantitative estimation of the critical nucleus size and activation barrier for Co-doped ZnO nucleation using spectroscopic and calorimetric methods.

Findings

Co2+ ions are excluded from the critical nuclei but incorporated in growth layers.

Critical nucleus size estimated at 25 +/- 4 Zn2+ ions.

Introduction of a single Co2+ impurity reduces nucleation rate by 1.5x10^4.

Abstract

The influence of Co2+ ions on the homogeneous nucleation of ZnO is examined. Using electronic absorption spectroscopy as a dopant-specific in situ spectroscopic probe, Co2+ ions are found to be quantitatively excluded from the ZnO critical nuclei but incorporated nearly statistically in the subsequent growth layers, resulting in crystallites with pure ZnO cores and Zn1-xCoxO shells. Strong inhibition of ZnO nucleation by Co2+ ions is also observed. These results are explained using the classical nucleation model. Statistical analysis of nucleation inhibition data allows estimation of the critical nucleus size as 25 +/- 4 Zn2+ ions. Bulk calorimetric data allow the activation barrier for ZnO nucleation containing a single Co2+ impurity to be estimated as 5.75 kcal/mol cluster greater than that of pure ZnO, corresponding to a 1.5x104-fold reduction in the ZnO nucleation rate constant upon introduction of a single Co2+ impurity. These data and analysis offer a rare view into the role of composition in homogeneous nucleation processes, and specifically address recent experiments targeting formation of semiconductor quantum dots containing single magnetic impurity ions at their precise centers.