INTERFACIAL CARRIERS DYNAMICS OF CDS NANOPARTICLES

The relaxation dynamics of charge carriers in 4 nm CdS colloidal quant um 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 accepter s on the surface of these particles. From these experimental measureme nts, we reached a model of the electron-hole dynamics in these nanopar ticles consistent with previous proposals. In particular, we have conf irmed that the electron trapping in these particles is slower than the hole trapping (30 ps versus a few picoseconds). After excitation, rap id formation of an optical hole (bleach) within the lowest energy exci ton (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 rec overs 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 re covery time is reduced to a few picoseconds. This leads to the followi ng conclusions: (1) the shift in the bleach band results from hole tra pping dynamics, and (2) the bleach recovery is rate limited by the ele ctron 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 Klimo v et al., that the rate of the recovery in CdS nanoparticles is determ ined by the electron surface trapping process. The electron transfer t o the viologen accepters is found to be very efficient and takes place in 200-300 fs, which efficiently competes with surface trapping and e lectron-hole recombination processes and thus quenches both the band g ap and the deep trap emissions.