Excitation wavelength dependence of the photoluminescence quantum yield and decay behavior of CdSe/CdS quantum dot/quantum rods with different aspect ratios
The excitation wavelength (λexc) dependence of the photoluminescence (PL) quantum yield (ΦPL) and decay behavior (τPL) of a series of CdSe/CdS quantum dot/quantum rods (QDQRs), consisting of the same spherical CdSe core and rod-shaped CdS shells, with aspect ratios ranging from 2 to 20 was characterized. λexc was chosen to cover the first excitonic absorption band of the CdSe core materia λexc between 400-565 nm, the onset of absorption of the CdS shell, and the region of predominant shell absorption, respectively. A strong λexc dependence of relativel and absolutely measured ΦPL and τPL was found particularly for the longer QDQRs with higher aspect ratios. This is attributed to combined contributions from a length-dependent shell-to-core exciton localization efficiency, an increasing number of defect states within the shell for the longest QDQRs, and probably also the presence of absorbing, yet non-emitting shell material. Although the ΦPL values of the QDQRs decrease at shorter wavelength, the extremely high extinction coefficients introduced by the shell outweigh this effect, leading to significantly higher brightness values at wavelengths below the absorption onset of the CdS shell compared with direct excitation of the CdSe cores. Moreover, our results present also an interesting example for the comparabability of absolutely measured ΦPL using an integrating sphere setup and ΦPL values measured relative to common ΦPL standards and underline the need for a correction for particle scattering for QDQRs with high aspect ratios.