Controlling photoinduced electron transfer dynamics between molecular adsorbates and semiconductor nanoparticles

Photoinduced electron transfer (ET) dynamics from molecular adsorbates to inorganic semiconductor (SC) nanocrystalline thin films have been investigated by ultrafast transient absorption spectroscopy. The dependence of ET rate on various factors, such as the nature of semiconductors, doping level, the thickness of insulating overlayer, the particle size of quantum dots and the adsorbate anchoring bridge length was examined by choosing proper dyes and semiconductor nanoparticles.; The dependence of ET dynamics on the nature of semiconductors was investigated by using Ru polypyridyl complex-sensitized Nb2O5 and In2O3 films and comparing with previous studies of Ru dyes on SnO2, ZnO, and TiO2. The injection kinetics of Ru dyes on Nb2O5 are biphasic, with an ultrafast component (<100 Es, 37%) and a much slower (ps or longer timescale) component, corresponding to injection from unthermalized and thermalized excited states of dye molecules, respectively. After excitation of the adsorbates at 532 nm, the injection kinetics from RuN3 to In2O3 are similar to those of SnO2. These kinetics traces show a negligible <100 fs injection component and are very different from those to TiO 2. A much faster injection rate from unthermalized excited states is observed for d-type semiconductors (Nb2O5 and TiO 2) than that for s-type semiconductors (ZnO, In2O3, and SnO2), likely due to orders of magnitude higher density of states in the former.; The dependence of ET dynamics on the doping level was investigated by using Re polypyridyl complexes and a conjugated polymer (MEH-PPV) sensitized antimony doped SnO2 (ATO) nanoparticles. In particles with different levels of Sb dopant, the rates of electron injection from adsorbate excited states to ATO nanoparticles are independent of dopant level, and the rates of recombination increase with the doping level. The observed similar forward electron injection rates are attributed to negligible changes of available accepting states in the conduction band at the doping levels studied. The dependence of the recombination rate on conduction band electron density is explained by a continuous-time random walk model.; The dependence of ET dynamics on the thickness of an insulating overlayer was investigated by using Ru and Re polypyridyl complex sensitized Al 2O3-coated TiO2 and SnO2 nanocrystalline thin films. The injection rates from dye to Al2O3-coated semiconductor nanoparticles decrease with an increasing number of Al 2O3 overlayers. The uniformity of Al2O3 is examined by using catechol as a probe molecule. It is found that catechol could still form charge transfer complexes with TiO2 even at three Al2O3 overlayers, indicating that the Al2O 3 overlayer may be inhomogeneously distributed on the semiconductor nanoparticle surface.; The dependence of ET dynamics on the size of quantum dots (qdots) were investigated by monitoring the fluorescence decay of dyes bound to qdots. Electron injection from C343 to 2.4 and 3.4 nm CdS qdots is observed, while no injection is observed from RdB to CdS qdots. This is attributed to the difference of relative energetics between C343/RdB and CdS qdots. The ET rate is faster from C343 to 3.4 nm CdS than to 2.4 nm CdS, corresponding to a lower conduction band edge of large CdS qdots.; The dependence of ET dynamics on anchoring bridge length was investigated by using phenylene-bridged PDI and methylene-bridged C343 on SnO2 or TiO2. The ET rates decrease with increasing PDI loading, which was attributed to the decrease of accepting states due to the formation of PDI excimer. No bridge length dependence is observed on both phenylene-bridged PDI and methylene-bridged C343. We suspect the dyes are sitting on the surface through conjugated pi systems due to the strong interaction of pi rings and semiconductor. Further experiments and simulations may be needed to clarify the results.; Thin films with mixed nanoparticles were prepared by using Layer-by-Layer (LBL) assembly based on...