Charge transfer dynamics between MPA capped CdTe quantum dots and methyl viologen

The understanding of charge carrier dynamics in hybrid materials based on colloidal semiconductor nanocrystals (or quantum dots) and organic moieties is fundamental for the design of efficient photonic and photovoltaic devices. In the present work, we investigate the interactions occurring between CdTe quantum dots, capped with a strong capping agent such as 3-mercaptopropionic acid, and a well known electron acceptor such as methylviologen molecule. The nature of the interactions and of exciton dynamics is investigated by stationary and time-resolved spectroscopies. Luminescence data recorded in presence of increasing methylviologen concentrations, indicate that the organic molecule is able to statically interact with the surface sites of CdTe quantum dots; a biphasic interaction behavior is evidenced by determining the apparent association constants. These latter are obtained through the analysis of luminescence data, and values in the range 10(3)-10(4) are determined. The nature of the interactions is characterized by nanosecond and femtosecond transient absorption spectroscopies, to clarify the dynamics and the conditions able to foster charge mobility. Nanosecond flash photolysis measurements, carried out upon quantum dots excitation, shows the absorption of methylviologen radical cation specie at 605 nm, suggesting the occurrence of electron transfer from CdTe nanocrystals to the organic acceptor; the relatively long decay time of the transient signal (10.3 mu s) indicates that back electron transfer processes are negligible. Ultrafast transient absorption measurements confirm the occurrence of an ultrafast electron transfer process; spectral and kinetic analysis of the transient data show that methylviologen radical cation is formed almost instantaneously on the ps-time scale but mainly when the samples are pumped in the energy continuum at 400 nm. This finding suggests that electron mobility from the nanocrystals to the organic units is achieved mainly when the excitonic states possess an excess of energy. The comparison of the kinetic behaviour of the signals at increasing methylviologen concentrations indicates that the electron transfer process competes with the radiative exciton recombination. In addition, the kinetic data suggests that surface trapping and Auger recombination processes might slow down the charge mobility.