Luminescence Properties of CdSe Quantum Dots: Role of Crystal Structure and Surface Composition

The crystal-structure-dependent luminescence properties of cadmium selenide quantum dots (QDs) in their cubic zincblende (Zb-CdSe) and hexagonal wurtzite (Wz-CdSe) phases have been investigated by maintaining their optical band gaps as well as the relative ratios of capping ligands the same. The Zb-CdSe QDs exhibited excellent photostability and high photoluminescence quantum yield (phi(L) = 0.375) compared with Wz-CdSe QDs (phi(L) = 0.046). Detailed X-ray photoelectron spectroscopic (XPS) investigation revealed an important finding that the surface of Zb-CdSe is rich with Cd2+ ions, which leads to the formation of CdO layer. This was further confirmed by analyzing the (i) Cd-to-Se stoichiometric ratio of zincblende QDs (2:1 on surface and 1:1 after sputtering) and (ii) core-level XPS spectra of oxygen. In contrast, the Cd-to-Se stoichiometric ratio was found to be same (1:1) for Wz-CdSe QDs throughout the crystal. Thus, the high luminescence of Zb-CdSe is attributed to the formation of a thin layer of CdO, leading to a type-I core-shell structure, which passivates the surface defects and confines the charge carriers. On the basis of the present investigation it is clear that the crystal structure plays a decisive role in modulating the surface properties of QDs, which determine the luminescence properties.