Simultaneously measured photoluminescence lifetime and quantum yield of two-photon cascade emission on single CdSe/ZnS nanocrystals

The photoluminescence (PL) lifetime and quantum yield (QY) of the two-photon cascade emission (TPCE) provide critical information for assessing the quality of optoelectronic devices. The QY of TPCE has been conventionally evaluated by second-order photon correlation, ${g}^{(2)}$, measurements under weak-excitation conditions in single nanocrystals (NCs). Here, we report that by measuring simultaneously both the ${g}^{(2)}$ and PL-decay curves, the QY of the TPCE can be determined even under strong-excitation conditions. With this technique, the effect of ionization on the QY of the TPCEs in single CdSe/ZnS NCs, which appears under strong-excitation condition, can be revealed. The obtained QYs were compared with the PL lifetime of the first and second photon emissions. We demonstrate that even for ionized NCs, the radiative recombination rates during TPCE are almost proportional to the product of the numbers of electrons and holes.