Ultrafast Photoluminescence Quenching of Initially Excited States in CdSe Quantum Dots Functionalized with a Charge Acceptor Dye

Abstract

CdSe quantum dots have interesting carrier transfer characteristics and can be used as photon collectors in certain kinds of hybrid photovoltaic devices. Some of these systems work through a charge transfer process from an excitonic state to a surface-adsorbed organic dye. In this article, we explore carrier transfer time scales through the characterization of the ultrafast photoluminescence behavior of nanocrystal excitonic states in the presence of adsorbed molecular charge acceptors. We show that upon physisorption of the cyanine dye Indocyanine Green, significant emission quenching due to carrier transfer can take place in a direct way from the initially pumped states in 5.7 nm diameter CdSe dots. We show that such transfer takes place independently of the excess energy above the band gap. Importantly, this near-instantaneous quenching is responsible for the loss of an important fraction of the excitonic population on a time scale much faster than intraband (hole and electron) excitonic relaxation. The time scales for the excitonic quenching and relaxation were addressed by femtosecond photoluminescence up-conversion experiments. These experiments showed that the time constants associated with the accumulation of the band-edge excitons remain unchanged upon dye physisorption; however, the signal amplitude is significantly reduced as a function of the addition of Indocyanine Green. The transient photoluminescence from the spectral region associated with states that act as intermediaries during excitonic relaxation (like the 1P3/21P and the 2S1/21S states) shows a significant reduction in the amplitude of the exponential components, but there was no difference in the transient’s time constants. These features indicate that the yield of accumulation into these transiently populated states is diminished by the presence of the cyanine dye due to near instantaneous exciton quenching of the initially formed states.

Mechanistic Insights into Selective Sensing of Pb2+ in Water by Photoluminescent CdS Quantum Dots

“…PL kinetics, governed by exciton recombination pathways, are extremely sensitive to surface states; hence, such an effect on PL kinetics even in nanosecond domain is imperative. This effect has been widely observed in a variety of QD–acceptor systems where acceptors are surface-adsorbed, even though the actual carrier transfer timescales are ultrafast (as in the present system). , …”

“…This effect has been widely observed in a variety of QD−acceptor systems where acceptors are surface-adsorbed, even though the actual carrier transfer timescales are ultrafast (as in the present system). 25,26 To gain further insight into the quenching mechanism, the PL spectra of CdS QDs in the absence and presence of Pb 2+ have been fitted to sums of three Gaussian functions (Figure 1C,D), having maxima at 2.71, 2.39, and 2.09 eV (components 1, 2, and 3 respectively). While all PL components are quenched in the presence of Pb 2+ , the relative contribution of the 2.09 eV component decreases drastically, thus leading to the narrowing of the overall PL spectrum and thus apparent blueshift.…”

“…Hot carrier transfer (vide infra) can play a significant role in such ultrafast quenching, which would not be reflected in nanosecond TCSPC transients and would lead to a considerable mismatch in Stern−Volmer plots constructed using PL intensities and lifetimes. 26,31,32 Hence, while a traditional Stern−Volmer analysis is less than adequate to explain the quenching mechanism, it provides important Ultrafast Transient Absorption: Hot Electron Transfer. A systematic investigation using transient absorption of QDs in the presence of varying concentration of Pb 2+ is performed to further elucidate the charge transfer process.…”

“…The nonlinearity of the Stern–Volmer plot and the excessively large value of k q indicates a close association of Pb 2+ ions with the QDs leading to “instantaneous” PL quenching post photoexcitation. Hot carrier transfer (vide infra) can play a significant role in such ultrafast quenching, which would not be reflected in nanosecond TCSPC transients and would lead to a considerable mismatch in Stern–Volmer plots constructed using PL intensities and lifetimes. ,, Hence, while a traditional Stern–Volmer analysis is less than adequate to explain the quenching mechanism, it provides important pointers, prompting a careful analysis of the femtosecond transient absorption data.…”