Hot and Relaxed Electron Transfer from the CdSe Core and Core/Shell Nanorods

Transient absorption spectroscopy has been used to study the rates of electron transfer (ET) from CdSe and CdSe/ZnS core/shell nanorods to adsorbed methyl viologen, MV2+. The nanorods are excited with 387 nm light, producing electrons 7700 cm(-1) above the conduction band edge. Kinetics are measured in particles without adsorbed MV2+, giving electron cooling and electron hole recombination times. The kinetics obtained with and without adsorbed MV2+ are compared to infer the ET rates. The results indicate that electron cooling occurs on the 0.7-1.8 ps time scale, with the fastest cooling occurring from the highest energy states. Hot electron transfer from the highest energy levels competes with electron cooling, occurring on the 0.5 ps time scale. Bare particle (relaxed) electron transfer occurs on the time scale of less than or about 4 ps. This is faster than biexciton Auger recombination which occurs on the 50 ps time scale. The energy dependence of the ET times can be semiquantitatively understood in terms of penetration of the conduction band wave function past the particle surface and overlap with the adsorbed MV2+. In CdSe/ZnS particles, ET to adsorbed MV2+ is slower than electron cooling, and hot electron transfer does not occur. For a 1.0 nm thick ZnS shell, the ET from the bottom of the conduction band occurs on a range of time scales, with the fastest component of about 45 ps.