High-Q Microcavity Enhanced Optical Properties of CuInS₂/ZnS Colloidal Quantum Dots toward Non-Photodegradation

We report on a temporal evolution of photoluminescence (PL) spectroscopy of CuInS₂/ZnS colloidal quantum dots (QDs) by drop-casting on SiO₂/Si substrates and high quality factor microdisks (MDS) under different atmospheric conditions. Fast PL decay, peak blue shift, and line width broadening due to photooxidation have been observed at low excitation power. With further increasing of the excitation power, the PL peak position shows a red shift and the line width becomes narrow, which is ascribed to the enhanced Förster resonant energy transfer between different QDs by photoinduced agglomeration. The oxygen plays an important role in optically induced PL decay, which is verified by a reduced photobleaching effect under vacuum. When the QDs are drop-casted on MDS, photooxidation and photobleaching are accelerated because the excitation efficiency is greatly enhanced with coupling the pumping laser with the cavity modes. However, when the emitted photons couple with cavity modes, a PL enhancement by more than 20 times is achieved because of the increased extraction efficiency and Purcell effects of MDS at room temperature (RT) and 35 times at 20 K. The photobleaching can be avoided with a small excitation power but with a strong PL intensity by taking advantage of high quality factor cavities. The highly efficient PL emission without photodegradation is very promising for using CuInS₂ QDs as highly efficient photon emitters at RT, where the photodegradation has always limited the practical applications of colloidal quantum dots.