Interfacial carriers dynamics of CdS nanoparticles

The relaxation dynamics of charge carriers in 4 nm CdS colloidal quantum dots are studied by means of picosecond time-resolved fluorescence and femtosecond transient absorption experiments. We also studied the effects of the adsorption of viologen derivatives as electron accepters on the surface of these particles. From these experimental measurements, we reached a model of the electron-hole dynamics in these nanoparticles consistent with previous proposals. In particular, we have confirmed that the electron trapping in these particles is slower than the hole trapping (30 ps versus a few picoseconds). After excitation, rapid formation of an optical hole (bleach) within the lowest energy exciton (band gap) absorption region appears. The maximum of the bleaching band is red-shifted by 20 meV in 2.5 ps, and the bleach intensity recovers in 30 ps. Upon the adsorption of electron accepters, the rate of the red shift of the optical hole is not affected while the bleach recovery time is reduced to a few picoseconds. This leads to the following conclusions: (1) the shift in the bleach band results from hole trapping dynamics, and (2) the bleach recovery is rate limited by the electron trapping process in the CdS nanoparticles (30 ps) or by the hole trapping process (a few picoseconds) in the presence of the electron accepters. The latter conclusion supports a previous proposal by Klimov et al., that the rate of the recovery in CdS nanoparticles is determined by the electron surface trapping process. The electron transfer to the viologen accepters is found to be very efficient and takes place in 200-300 fs, which efficiently competes with surface trapping and electron-hole recombination processes and thus quenches both the band gap and the deep trap emissions.