Size-dependent fluorescence properties of CdSe quantum dots

The optical properties of three different sizes of cadmium selenide (CdSe) quantum dots (QDs) were investigated using absorption, emission, and time-resolved photoluminescence spectroscopy. The absorption and emission spectra disclose the blue shift of a quantum dot due to quantum confinement effects. The contributions of the three individual components of the exciton lifetime of CdSe colloidal QDs were measured and their weighted average exciton life time was determined. A fast lifetime component (${\tau }_1$) related to the band edge to valence band transition, an intermediate lifetime component (${\tau }_2$) associated with the surface-trapped state to valence band or band edge to valence trapped state transition, and a slow lifetime component (${\tau }_3$) associated with the surface-trapped state to valence trapped state transition are included in calculating the average exciton lifetime in CdSe QDs. The study demonstrates that the band-edge transition consistently contributes more significantly to the overall exciton lifetime across all QDs sizes. It reveals that the lifetime increases with size for QDs smaller than Bohr radius. The same trend does not hold for QDs larger than Bohr radius. These findings offer a critical insight into the size-dependent optical properties of CdSe QDs and hold significant implications for the development of advanced optoelectronic devices and other cutting-edge technological applications.