Introduction Inorganic perovskite quantum dots have attracted extensive attention due to their excellent optoelectronic properties,but the poor stability limits their practical applications. In recent years, perovskite quantum dots in situ crystallization inside robust inorganic glass network (CsPbX3@glass, X=Cl, Br, I) have been evidenced to have superior stability. However, most studies have focused on optimizing the optical properties, while few investigations aiming at carrier dynamics. What’s more, we don't know if interesting phenomenon will happen to the carrier dynamics of perovskite quantum dot glasses, which obtain rare study of mechanism of carrier dynamics, and no halogen mixing are involved before. In this work, the influence of defects and halogen ratios induced by the elevation of crystallization temperature on the carrier dynamics of CsPb(Cl/Br)3@glass nanocomposites are investigated by means of femtosecond laser ultrafast transient absorption spectroscopy (fs-TAS).
Methods We synthesized perovskite quantum dot glass by high temperature melting-heat treatment, showing final composition with proportions of (%, in mole) 30SiO-25B2O3-5Al2O3-5ZnO-5CaF2-2PbBr2-4PbCl2-5NaBr-10NaCl-9Cs2CO3. The raw materials are silica (SiO2, Bodie, analytical grade), alumina (Al2O3, Hengxing, 99%), calcium fluoride (CaF2, McLean, 99%), boron oxide (B2O3,McLean, 98%), zinc oxide (ZnO, McLean, 99%), cesium carbonate (Cs2CO3, McLean, 98%), sodium bromide (NaBr, McLean, 99%) and sodium iodide (NaCl, McLean, 99.5%), which are used as raw materials to weigh, mix and grind in agate mortar. The resulting mixture was then placed in a sealed alumina crucible and melted at 1 200 ℃ for 30 min in ambient atmosphere. Subsequently, the glass melt was poured into a copper mold at room temperature. Finally, the obtained bulk glass was treated at 480, 500 ℃ and 520 ℃ respectively for 10 h to induce the precipitation of perovskite quantum dots in the glass, thus obtaining glass ceramics containing perovskite quantum dots.
Results and discussion TEM shows that perovskite quantum dots are precipitated in glass. With the increase of temperature of thermal treatment, XRD appears attenuated diffraction peak of perovskite quantum dots and shifts to a small angle, while PL spectrum and absorption peak red shift, which prove that Br– has entered into the lattice. Subsequently, PLQY proved that high thermal treatment temperature could weaken radiation recombination, and the fluorescence attenuation curve shows an unusually long life. The transient absorption spectrum explores the whole carrier dynamics process. First, it is obtained that the hot carrier relaxation lifetimes of perovskite quantum dot glasses treated at 480, 500 ℃ and 520℃ are 140, 190 fs and 210 fs, respectively. Thetemperature related spectrum proves that the hot carrier relaxation becomes slower due to the weakening of electro-acoustic coupling. In the following process of exciton recombination, a new defect state can be observed on the transient absorption spectrum, which may be the reason of the abnormal life of high thermal treatment temperature.
Conclusions In summary, with the increase of thermal treatment temperature, the electro-acoustic coupling strength decreases due to the entry of Br– into the lattice. Meanwhile, the hot carrier relaxation takes longer period of time. In addition, due to the high treatment temperature , new shallow level defects that can capture electrons appear, and some electrons can be released back to the conduction band for radiation recombination. This eventually leads to a great improvement of fluorescence lifetime. Understanding these internal mechanisms may provide new guidance for synthesis and application of the investigated material.