Photoluminescence Spectroscopy of CdSe/Cd[0.5]Zn[0.5]S Core/Shell Colloidal Quantum Dots

Abstract

The photoluminescence (PL) of dense aggregates of CdSe/Cd0.5Zn0.5S core/shell colloidal quantum dots (cQDs) was investigated to gain insight into bleaching processes, quantum efficiency behaviour, band gap energy temperature dependence, and the phonon-coupling contribution to thermal broadening. This work also encompassed a study of various sample preparation techniques. These newly investigated strain-engineered core/shell cQDs, having an effective lattice constant in between that of CdS and ZnS, fosters the study of novel properties not present in the previous two systems. The bleaching behaviour, studied subject to strong optical pumping for different external cQD environments, showed effective shell passivation, thus limiting non-radiative processes mainly to the core or shell structures. The relative quantum efficiency was determined as a function of temperature, showing two distinct regimes, with activation of a phonon process above ~ 180 K. The band gap energy was found to follow the empirical Varshni relation describing the temperature dependence of a semiconductor material, dominating any thermally induced changes of the cQD volume and confinement potential. The PL spectral width change with temperature showed highly coupled carrier-phonon scattering behaviour.