Influence of the Core Size on Biexciton Quantum Yield of Giant CdSe/CdS Nanocrystals

We present a systematic study of photoluminescence (PL) emission intensity and biexciton (BX) quantum yields (QY_{BX}) in individual "giant" CdSe/CdS nanocrystals (g-NCs) as a function of g-NC core size and shell thickness. We show that g-NC core size significantly affects QY_{BX} and can be utilized as an effective tuning parameter towards higher QY_{BX} while keeping the total volume of the g-NC constant. Specifically, we observe that small-core (2.2 nm diameter) CdSe/CdS NCs with a volume of ~200 nm³ (shell comprises 4 CdS monolayers) show very low average and maximum QY_{BX}'s of ~3 and 7%, respectively. In contrast, same-volume medium-core (3 nm diameter) NCs afford higher average values of ~10%, while QY_{BX}'s of ~30% are achieved for same-volume large-core (5.5 nm diameter) CdSe/CdS NCs, with some approaching ~80%. These observations underline the influence of the g-NC core size on the evolution of PL emissive states in multi-shell NCs. Moreover, our study also reveals that the use of long anneal times in the growth of CdS shells plays a critical role in achieving high QY_{BX}.