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 (λ_exc) dependence of the photoluminescence (PL) quantum yield (Φ_PL) and decay behavior (τ_PL) 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. λ_exc between 400-565 nm were chosen to cover the first excitonic absorption band of the CdSe core material, the onset of absorption of the CdS shell, and the region of predominant shell absorption. A strong λ_exc dependence of relative and absolutely measured Φ_PL and τ_PL 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 Φ_PL 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 comparability of absolutely measured Φ_PL using an integrating sphere setup and Φ_PL values measured relative to common Φ_PL standards, and underline the need for a correction for particle scattering for QDQRs with high aspect ratios.