Stranski-Krastanov Shell Growth in ZnTe/CdSe Core/Shell Nanocrystals

ZnTe/CdSe core/shell nanoparticles are synthesized in noncoordinating solvents at different temperatures. The experimental results show that CdSe shell deposition at 215 degrees C on spherical ZnTe core particles is analogous to Stranski-Kranstanov growth of 2D epitaxial films. The shell thickness inhomogeneity is determined by measuring the inhomogeneity in interfacial hole transfer rates to an adsorbed hole acceptor, phenothiazine. We find that the first approximately three layers of CdSe are deposited uniformly and that subsequent layers produce a rough shell surface. The origin of the shell thickness inhomogeneity is investigated. ZnTe and CdSe have very close to the same lattice constants, and the interface therefore has very little lattice strain. However, cation interdiffusion changes the radial composition profile of the ZnTe-CdSe interface, leading to a large amount of lattice strain. The extent of cation interdiffusion and hence the surface morphology can be controlled by varying the deposition temperature and the subsequent annealing time and temperature, The particle spectroscopy and the shell thickness inhomogeneity are consistent with calculations based on an elastic continuum model with a cation interdiffusion constant of 1.3 X 10(-2) nm(2) min(-1) in the ZnTe/CdSe particles at 250 degrees C. The comparison of the energetics involved in S-K growth of thin films and nanocrystal shells is discussed.