Miki Murai

Dynamics of efficient electron–hole separation in TiO2 nanoparticles revealed by femtosecond transient absorption spectroscopy under the weak-excitation condition

The transient absorption of nanocrystalline TiO(2) films in the visible and IR wavelength regions was measured under the weak-excitation condition, where the second-order electron-hole recombination process can be ignored. The intrinsic dynamics of the electron-hole pairs in the femtosecond to picosecond time range was elucidated. Surface-trapped electrons and surface-trapped holes were generated within approximately 200 fs (time resolution). Surface-trapped electrons, which gave an absorption peak at around 800 nm, and bulk electrons, which absorbed in the IR wavelength region, decayed with a 500-ps time constant due to relaxation into deep bulk trapping sites. It is already known that, after this relaxation, electrons and holes survive for microseconds. We interpreted these long lifetimes in terms of the prompt spatial charge separation of electrons in the bulk and holes at the surface.

Femtosecond visible-to-IR spectroscopy of TiO2 nanocrystalline films: dynamics of UV-generated charge carrier relaxation at different excitation wavelengths

The transient absorption of nanocrystalline TiO2 films in the visible-to-IR wavelength region was measured under UV excitation conditions at different wavelengths of 266 nm and 355 nm. Under weak 355 nm excitation, the generated charge carrier density could be reduced as low as the second-order electron-hole recombination process could be ignored as we reported previously (Y. Tamaki et al. Phys. Chem. Chem. Phys. 9, 1453-1460 (2007)). The result was compared with data obtained under 266 nm excitation, where the band-gap exaction was strong and efficient electron-hole recombination occurred due to the high charge carrier density. Taking into account the dynamics of the electrons and holes in the femtosecond to picosecond time range, such as ultrafast charge carrier trapping and slow deep trapping of electrons, intra-band relaxation in the conduction and the valence bands and intra-particle diffusion of electrons in the shallow trap levels were revealed.

Chapter 7 Time-resolved spectroscopy and microscopy on nanocrystalline TiO2 and ZnO films

6. Summary We have investigated electron-hole dynamics in nanocrystalline TiO2 film by means of femtosecond and nanosecond transient absorption spectroscopy in the visible nd near-IR wavelength regions. Electron and hole spectra were assigned, and charge-carrier dynamics such as trapping and recombination were revealed. Free electrons showed a very broad absorption band whose intensity increased monotonously with increasing wavelength. Trapped electrons and holes were found to have absorption bands around 800 nm and 500 nm, respectively. The trapping and thermalization processes occurred in a complex fashion over a wide temporal range from a few hundred femtoseconds to several hundred picoseconds. Electron-injection processes in dye-sensitized ZnO films were also investigated with a newly developed transient absorption microscope and the femtosecond spectroscopic system. Microcrystals formed by aggregation of the Ru complex dye (N3) molecules and Zn ions were observed in the fluorescence image, and these were found not to function as sensitizers, whereas adsorbed N3 dyes in monomeric form seem to inject electrons efficiently into the film at any position. Femtosecond spectroscopy of ZnO films with an adsorbed coumarin derivative revealed that part of the electron-injection process proceeds stepwise through exciplex states as intermediates. The rate of electron injection for the stepwise process was slower than that for direct injection (without an intermediate). The overall injection rate was slower for the dye having a lower LUMO energy.