Radiative and nonradiative pathways in multiexciton recombination in giant nanocrystal quantum dots

Recently,\footnote{Y. Chen et al., \textit{JACS} \textbf{130}, 5026 (2008)} we developed ``giant'' nanocrystal quantum dots (g-NQDs), in which a small emitting core of CdSe is overcoated with a thick shell of a wider-gap CdS. We conduct room-temp measurements of photoluminescence (PL) lifetimes in such g-NQDs as a function of excitation power and a number of shell monolayers. At low pump levels, corresponding to excitation of less than 1 exciton per dot on average ($\langle $\textit{${\rm N}$}$\rangle <$ 1), we observed excitonic radiative lifetimes of $\sim $100 ns and a linear scaling of the PL signal with pump intensity. At powers corresponding to $\langle $\textit{${\rm N}$}$\rangle >$1, fast ($\sim $1ns) PL component appeared, accompanied by a transition to a sub-linear scaling of PL intensity with $\langle $\textit{${\rm N}$}$\rangle $.~Our findings indicate that while g-NQDs indeed produce suppression of nonradiative Auger recombination,\footnote{F. Garcia-Santamaria et al., \textit{Nanoletters} \textbf{9}, 3482 (2009)} this suppression is incomplete. We conduct systematic studies of relative efficiencies of nonradiative and radiative processes in these nanostructures.