Taishi Nishihara

Nanoparticles of iron(II) spin-crossover

We report the synthesis of spin crossover 69 nm spherical nanoparticles of [Fe(NH2-trz)3](Br)2.3H2O.0.03(surfactant) (NH2trz = 4-amino-1,2,4-triazole, surfactant = Lauropal), prepared by the reverse micelle technique, which exhibit at room temperature a thermal hysteresis characterized by magnetic, diffuse reflectivity and Raman studies.

Exciton recombination dynamics in nanoring cycloparaphenylenes

We have studied the photoluminescence (PL) properties of [n]cycloparaphenylenes ([n]CPPs), which have simple string structures consisting of varying numbers of benzene rings, using steady-state and time-resolved PL spectroscopy. The PL lifetime decreased dramatically as the number of benzene rings increased, and increased with an increase in temperature. These results reveal that the exciton spreads beyond the benzene rings and that the temperature dependence of the PL lifetime is determined by thermal distribution of the excitons.

Trion formation and recombination dynamics in hole-doped single-walled carbon nanotubes

We studied the trion (charged exciton) formation and recombination dynamics in hole-doped (7,5) single-walled carbon nanotubes (SWCNTs) by performing femtosecond transient absorption spectroscopy. The doping of SWCNTs with holes leads to a fast decay component from an exciton to a trion, and the trion decays with a lifetime of a few picoseconds. The experimental results can be explained by a quantized model accounting for the dark exciton and trion states and the hole number distribution in a SWCNT. Our findings show that the optical responses of SWCNTs can be manipulated by doping of SWCNTs with a small number of holes.

Fast Dissociation and Reduced Auger Recombination of Multiple Excitons in Closely Packed PbS Nanocrystal Thin Films

Exciton decay dynamics in chemically treated PbS quantum-dot (QD) films have been studied using femtosecond transient-absorption (TA) spectroscopy. In photoconductive QD films, a decay component with a lifetime of a few nanoseconds appeared in the TA signals because of exciton dissociation under weak excitation. Increasing excitation fluence resulted in additional fast-decay components corresponding to the lifetimes of multiple excitons, which decreased with increasing photoconductivity of the closely packed QD films. Auger recombination in photoexcited QDs was suppressed in highly photoconductive films. Our findings clearly show that the carrier transfer between the QDs dominates the lifetimes of single and multiple excitons.

Impact of surface ligands on the photocurrent enhancement due to multiple exciton generation in close-packed nanocrystal thin films

Semiconducting nanocrystal quantum dot (QD) films with high multiple exciton generation (MEG) efficiencies and high exciton dissociation efficiencies are appealing as solar cell materials. Here, we report the strong enhancement of the photocurrent in close-packed QD films, and discuss the importance of surface ligands on MEG and charge transport in QD films. Both exciton-recombination and charge-transport properties are influenced by the phenomenon of carrier tunnelling between neighboring QDs in thin films. These results can have important implications in the design of chemical treatments to control the electronic interactions between QDs in solution-processed solar cells. We demonstrate that the inorganic- and small sized-ligand passivation of PbS QDs by potassium thiocyanate causes a significant photocurrent enhancement.

The light-induced spin transition of tetranuclear spin crossover complex [Fe4(CN)4(bpy)4(tpa)2](PF6)4

We report on the light induced spin transition in the tetranuclear spin crossover complex [Fe4(CN)4(bpy)4(tpa)2](PF6)4. The photo-conversion occurs at the specific site (Fe2) of four FeII ions. The red light irradiation (1.79 eV) gives rise to full conversion of Fe2 into the high spin state from the low spin state. The green light irradiation (2.33 eV) can convert only the half of Fe2 into the high spin state, though the photo-conversion rate in the beginning is much higher than that with the red light. We present a simple model in which the photo-conversion kinetics is controlled by a large background absorption due to remaining three FeII ions (Fe1, Fe3 and Fe4).

Quantized exciton?exciton recombination in undoped and hole-doped single-walled carbon nanotubes

We studied the quantized exciton Auger recombination in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) by means of transient absorption spectroscopy and theoretical calculations. In undoped SWCNTs, a fast decay component appears under strong photoexcitation owing to two-exciton Auger recombination. The exciton decay dynamics is well explained by the quantized exciton Auger recombination model that takes into consideration the dark-exciton state. In hole-doped SWCNTs, the fast decay component is drastically reduced even under strong photoexcitation. We calculated the temporal evolution of the exciton population in hole-doped samples by considering exciton?hole interactions and the hole-number distribution in SWCNTs, and found it to be in good agreement with the experimental results.

Dynamics of excitons and trions in semiconducting carbon nanotubes

We report the optical properties and exciton dynamics of undoped and hole-doped single-walled carbon nanotubes (SWCNTs). In the one-dimensional structures of SWCNTs, an electron and a hole form an exciton with a huge exciton binding energy. Stable excitons govern the optical responses of SWCNTs even at room temperature. With hole doping of SWCNTs, new peaks due to positive trions (positively charged excitons) appear below the E11 exciton peaks in the absorption and photoluminescence spectra. Trions are also stable at room temperature. Using femtosecond pump-probe transient absorption spectroscopy, we revealed that the exciton decay dynamics depends on the number of holes in SWCNTs. The exciton lifetime of hole-doped SWCNTs is much shorter than that of undoped SWCNTs. Fast decay components with lifetimes of a few picoseconds are attributed to trion formation and exciton–hole scattering in holedoped SWCNTs.

Dissipative structure in the photo-induced phase under steady light irradiation in the spin crossover complex

We report the spatial and temporal dynamics of the photo-induced phase in the iron (II) spin crossover complex Fe(ptz)(6)(BF(4))(2) studied by image measurement under steady light irradiation and transient absorption measurement. The dynamic factors are derived from the spatial and temporal fluctuation of the image in the steady state under light irradiation between 65 and 100 K. The dynamic factors clearly indicate that the fluctuation has a resonant frequency that strongly depends on the temperature, and is proportional to the relaxation rate of the photo-induced phase. This oscillation of the speckle pattern under steady light irradiation is ascribed to the nonlinear interaction between the spin state and the lattice volume at the surface.

Chemical doping-induced changes in optical properties of single-walled carbon nanotubes

We studied the changes in the optical properties of single-walled carbon nanotubes (SWCNTs) induced by chemical doping. An enhancement in the photoluminescence (PL) of the SWCNTs was observed after doping with reducing agents, whereas a reduction in the PL intensity and the appearance of fast exciton decay resulted from doping with oxidizing agents. Comparisons of the measured PL and transient absorption obtained with the two different types of dopants demonstrated that the PL properties of SWCNTs are strongly linked to the non-radiative exciton recombination processes. Chemical doping strongly modulates the exciton dynamics in SWCNTs.