Lattice distortion and luminescence of CdSe/ZnSe nanocrystals

Colloidal CdSe/ZnSe core/shell nanocrystals were prepared via the conventional TOP/TOPO process and their shell thickness was controlled varying the ZnSe precursor concentration. X-ray diffraction (XRD) on the nanocrystals showed gradual peak shifts to high two-theta angles due to a decrease in the lattice parameter of CdSe cores with an increase in the ZnSe shell thickness. High-resolution transmission electron microscopy (HRTEM) images of the CdSe/ZnSe nanocrystals show a c-axis elongation with an increase in ZnSe precursor concentration and crystalline defect formation in some of the nanocrystals with 2.0 mM ZnSe precursor concentration, possibly due to high compressive strain. Photoluminescence (PL) spectra of the CdSe/ZnSe nanocrystals showed an increase in the peak intensity up to 1.0 mM ZnSe precursor concentration due to the surface passivation effect, and a decrease beyond this value, possibly due to the formation of crystalline defects such as stacking faults. The PL wavelength showed red-shifts up to 1.0 mM ZnSe precursor concentration due to the partial leakage of an electron wavefunction of CdSe cores into the ZnSe shells and blue-shifts beyond this value due to the high compressive strain from the ZnSe shells.