Yoshiaki Tamaki

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.

Solvent-Dependent Size and Phase of Vanadyl Phthalocyanine Nanoparticles Formed by Laser Ablation of VOPc Crystal-Dispersed Solution

Intense excimer laser irradiation of vanadyl phthalocyanine (VOPc) microcrystals dispersed in poor solvents yields VOPc nanoparticles, which was confirmed to be common among some representative solvents. The mean diameters of the nanoparticles formed in water, methanol, ethanol, 1-propanol, and ethyl acetate were determined to be 50 nm, 46 nm, 37 nm, 32 nm, and 38 nm, respectively. Absorption spectral analysis of the nanoparticles indicates that the VOPc nanoparticles from ethanol, 1-propanol, and ethyl acetate exhibit stable phase II, while those from water exhibit phase I and those from methanol cannot be identified. To understand the solvent effect, we attempted to correlate the mean diameter and the absorption spectrum with various solvent parameters such as density, viscosity, thermal diffusivity, dipole moment, dielectric constant, and solubility. Experimentally, the thermal diffusivity of a solvent can be correlated with nanoparticle formation, which is considered in relation to the ablation mechanism.

Imaging of Exciton Absorption in Perylene Crystals by Femtosecond-Laser Scanning Microscopy

The density of excitons, which depends on the local structure of perylene organic crystals, was imaged by femtosecond-laser scanning microscopy. By a two-photon process, a desired position of a single perylene crystal was excited selectively in a three-dimensional space. The resulting excitons were subsequently detected by probing the transient absorption. By using two-dimensional transient absorption images constructed with thousands of data points, we have visualized the density of the excitons generated in perylene crystals with a micrometer spatial resolution and a time resolution of several hundred femtoseconds.

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 14 Preparation of phthalocyanine nanoparticles by laser ablation of its microcrystal dispersed in poor solvents

Publisher Summary Organic nanoparticles consisting of aromatic and dye molecules have attracted increasing attention, because they are expected to be useful as pigments, cosmetics, drags, and new materials for optical and electronic devices. In addition, there are scientific interests in nanoparticles owing to their specific dimension between an isolated molecule and the corresponding bulk materials. This chapter presents a novel preparation method of organic nanoparticles by applying laser ablation technique. In this method, bulk crystals, a poor solvent, are fragmentized into nanoparticles by intense laser excitation. This method is simpler as compared to precipitation methods and provides a novel way to control size and the molecular aggregation structure of nanoparticles by adjusting optical parameters and chemical conditions. The nanoparticle formation of vanadyl phthalocyanine (VOPc) is described along with the size and crystalline phase dependence of the poor solvent and laser pulse duration. The formation processes are also discussed in the chapter that are based on the photothermal laser ablation mechanism of organic solids.

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.