Spectroscopy of CdTe/CdSe Type-II Nanostructures: Morphology, Lattice Mismatch, and Band-Bowing Effects

We compare the spectroscopy of two different morphologies of CdTe/CdSe type-II nanocrystals. Core/tetrapod and spherical core/shell particles are grown from identical CdTe cores, and both morphologies exhibit type-II, spectroscopic behavior. The two morphologies show very different oscillator strengths for the lowest (luminescent) transition; the core/tetrapod particles exhibit larger oscillator strengths for the same amount of spectral shift. A model is presented that explains this difference and accurately predicts emission wavelengths and relative oscillator strengths for the spherical particles. This model uses an elastic continuum treatment to consider strain induced by lattice mismatch at the core/shell interface. The CdTe-CdSe lattice mismatch results in a calculated core pressure of about 2.9 GPa for the particles with the thickest shells. This pressure causes a change in the CdTe conduction band energy of about 0.24 eV and a change in the CdTe lattice parameter of 2.3%. The change in the lattice parameter is also seen in XRD spectra and HRTEM lattice fringe images. Because of the different morphology, core compression is essentially absent in the core/tetrapod particles. The model also considers radial interdiffusion of the selenium and tellurium. Particle annealing results in an alloyed region at the core shell interface, and the radial composition profile can be calculated from a diffusion treatment. Partial alloying causes the luminescence to shift further to the red, which may be quantitatively understood in terms of calculated radial composition profiles and the known optical band-bowing parameters of CdTe-CdSe alloys.