High photostability and luminescent efficiency of quantum dots: ultrathin epitaxial Al self-passivation layer with a homogeneous ligand

Semiconductor quantum dots (QDs) have aroused extensive interest in various applications because of their excellent optical and electronic properties. However, long-term photostability of the CdSe QDs has always posed application challenges. This study discusses the high-quality QDs synthesized using an uncomplicated coating method. The ultrathin epitaxial Al self-passivation layer is coated with a homogeneous ligand to maintain high luminescent efficiency and photostability of the QDs. The time-dependent intensity responses of the CdSe/CdS, CdSe/CdS/Al, and CdSe/CdS/CdS/AlQDs were verified under continuous irradiation using a single-particle one-watt blue light-emitting diode (LED). The CdSe/CdS photoluminescence (PL) intensity rapidly decreased under the blue LED illumination, whereas that of the CdSe/CdS/CdS/AlQDs remained almost unchanged for 300 min. The PL spectra of the three samples in 10-300 K were verified by comparing the temperature-dependent full-width at half-maximum of their emission energies and luminescent intensities. The mechanism of the temperature-dependent non-radiative relaxation of CdSe QDs coated with CdS layers of different thicknesses evolved from the thermal activation of carrier trapping by surface defects/traps in the CdSe QDs to the thermal escape of carriers in the core/shell QDs, assisted by multiple longitudinal-optical phonons. The luminescence mechanism was almost independent of the Al epitaxial growth layer.