Shota Kuwahara

Tip-enhanced nano-Raman analytical imaging of locally induced strain distribution in carbon nanotubes

Tip-enhanced Raman scattering microscopy is a powerful technique for analysing nanomaterials at high spatial resolution far beyond the diffraction limit of light. However, imaging of intrinsic properties of materials such as individual molecules or local structures has not yet been achieved even with a tip-enhanced Raman scattering microscope. Here we demonstrate colour-coded tip-enhanced Raman scattering imaging of strain distribution along the length of a carbon nanotube. The strain is induced by dragging the nanotube with an atomic force microscope tip. A silver-coated nanotip is employed to enhance and detect Raman scattering from specific locations of the nanotube directly under the tip apex, representing deformation of its molecular alignment because of the existence of local strain. Our technique remarkably provides an insight into localized variations of structural properties in nanomaterials, which could prove useful for a variety of applications of carbon nanotubes and other nanomaterials as functional devices and materials.

Effect of electrolyte constituents on the motion of ionic species and recombination kinetics in dye-sensitized solar cells

The dynamic motion of ions in electrolyte solutions and its effect on recombination was investigated by the heterodyne transient grating method in addition to transient absorption and transient photocurrent methods in dye sensitized solar cells. Realignment of ionic species at the electrode/electrolyte interface was observed after the electron injection in TiO2 on the order of μs. The process was affected by the total quantity of ionic species as well as cation species in the electrolyte. The recombination processes of the electrons were also affected by the constituents; the probability of the electron-electrolyte recombination decreased with decrease in I2 concentration; the dominant recombination process changed from the electron-electrolyte to the electron-dye recombination by decreasing I(-) concentration. It is concluded that sufficient I(-) is necessary for the suppression of the electron-dye recombination and that sufficient I2 is necessary for an efficient redox cycle, while low concentration of I3(-) ions at the electrolyte/TiO2 interface is preferable to suppress the electron-electrolyte recombination. The effect of the cation size in an electrolyte solution on the charge dynamics was also investigated, and it was revealed that the steric hindrance of cations changed the penetration of ionic species into the nanoporous dye/TiO2 electrode, causing a change in the electrostatic properties at the interface. The cation dependence indicated that the presence of large-sized cations suppressed the electron-electrolyte recombination by disturbing the approach of I3(-) paired with the cations.

Distinction of electron pathways at titanium oxide/liquid interfaces in photocatalytic processes and co-catalyst effects

Photocatalytic reactions include several different steps and routes for photoexcited carriers, and each dynamic is closely related to the reaction efficiency. Although commonly used time-resolved techniques can reveal the kinetics of photoexcited carriers, the reaction pathways are difficult to distinguish due to decay kinetics extending over many temporal orders and various contributions from the carriers and species involved. Herein, we report the distinction of the electron dynamics in the photocatalytic processes of titanium oxide through the combination of the transient grating method and maximum entropy analysis for the estimation of time constants. We were able to confirm three different carrier responses corresponding to an intrinsic recombination, an interfacial transfer or the decay of surface-trapped electrons, and the decay of polarons. Based on the responses, it appears that both gold and platinum work as good electron acceptors, but that only platinum shortened the lifetime of the polaron state due to the acceleration in the adsorption/desorption exchange of ions, which explains the shorter cycles of the photocatalytic reactions for platinum.

Role of lithium and co-existing cations in electrolyte to improve performance of dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSCs) with an electrolyte including mixed cations was evaluated, and the relevant carrier dynamics were investigated by the heterodyne transient grating method. The performance of the DSSCs showed maximum conversion efficiency for an Li+ cation ratio of 75% with 25% other cations.

Subnanometric stabilization of plasmon-enhanced optical microscopy

We have demonstrated subnanometric stabilization of tip-enhanced optical microscopy under ambient condition. Time-dependent thermal drift of a plasmonic metallic tip was optically sensed at subnanometer scale, and was compensated in real-time. In addition, mechanically induced displacement of the tip, which usually occurs when the amount of tip-applied force varies, was also compensated in situ. The stabilization of tip-enhanced optical microscopy enables us to perform long-time and robust measurement without any degradation of optical signal, resulting in true nanometric optical imaging with high reproducibility and high precision. The technique presented is applicable for AFM-based nanoindentation with subnanometric precision.

Investigation of Photoexcited Carrier Dynamics in Hematite and the Effect of Surface Modifications by an Advanced Transient Grating Technique

Photoexcited carrier dynamics in a hematite film with and without amorphous NiFeOx on the surface was investigated using the heterodyne transient grating method. We found that two different electron/hole dynamics took place in the micro- and millisecond time regions and successfully assigned each component to the decay processes of electrons and holes trapped at surface states, respectively. It was also demonstrated that the amorphous NiFeOx coating plays a crucial role in increasing the survival of the holes at the surface trap states, which was caused by the decrease in the surface recombination rate.

Local extraction and condensation under a microscope using the optically controlled phase separation of a thermoresponsive polymer.

In-situ extraction and condensation of various dyes were carried out in a phase-separation region of a thermoresponsive polymer aqueous solution generated by near infrared (NIR) laser heating under a microscope. The NIR laser irradiation was directed at a chromium line deposited on a glass substrate, thereby causing local heating of the solution due to the photothermal effect. A phase-separation region was formed by dehydration of the thermoresponsive polymer followed by ejection of water outside of the phase-separation region. When various dyes were included in the solution, some dye molecules were extracted into the phase-separation region, where they condensed. In the case of poly(N-isopropylacrylamide) (PNIPAM, 10 wt% in an aqueous solution) as the thermoresponsive polymer and crystal violet (CV) as the dye (0.1 mM), CV condensed by about 25 times. It was found that one of the necessary conditions for the extraction/condensation is the hydrophobicity of the dye molecule; however, the dominant cause for accumulating inside the PNIPAM chain is the molecular interaction between the amide group in the side chain of PNIPAM and the functional groups such as carbonyl or amino groups in the dye molecules.

Host-guest molecular interactions in the phase transition of liquid crystals

ABSTRACT Phase transition dynamics of liquid crystal (LC) was measured by the heterodyne transient grating (HD-TG) method to clarify the difference between the photochemical and photothermal phase transitions. The phase transition was induced by the photo-excitation of different guest dyes doped in the host liquid crystals; azobenzene and solvent red were used as photochemical and photothermal dyes as guest dyes. The photochemical phase transition was partly induced even at much lower temperature than the nematic – isotropic phase transition temperature (TNI) and the region was gradually increased as the temperature, while photothermal phase transition was induced in the whole area just when reaching TNI. Furthermore, the anisotropic re-alignment of molecules was observed in the phase recovery process.

Optimization of Experimental Parameters for the Performance of Solid-state Dye-sensitized Solar Cells

The effects of various sample parameters for solid-state dye sensitized solar cells were studied with carrier dynamics measurements and electrochemical measurements. Although many parameters and processes have been decided based on the experience of researchers, the chemical and physical reasons for the selections have not been clarified. We studied the effect of the generally utilized materials and processing such as the blocking layer, titanium oxide thickness, surface treatment, and the selection of dyes and hole transfer materials. Based on our findings, we were able to rationally optimize the structure of the solid-state dye sensitized solar cells in terms of cell performance or the lifetime of charge carriers.

Direct evidence of the molecular interaction propagation in the phase transition of liquid crystals

The molecular interaction sometimes propagates in a collective manner, reaching for a long distance on the order of millimeters. Such interactions have been well known in the field of strongly-correlated electron systems in a beautiful crystal interleaved by donor and acceptor layers, induced by photo-stimulus. The other examples can be found in liquid crystals (LCs), which could be found in many places in nature such as bio-membrane. Different from crystals, LCs features “softness”, which enables it to be a curved structure such as a cell. In LCs, even a small molecular change would trigger the overall structural change by the propagation of the molecular interaction. Here we will show, for the first time, how long and how fast the molecular interaction propagates in LCs. The patterned phase transition was induced in a LC, causing the phase transition propagation in a controlled way and the propagation was measured with an time-resolved optical technique, called the transient grating. A LC sample doped with azobenzene was put into a thermally controlled LC cell. A grating pattern of a pulse light with 355 nm was impinged to the LC cell, and the light was absorbed by the dyes, releasing heat or photomechanical motion. We could observe the fringe spacing dependence on the phase transition response, which indicates that phase transition was delayed as the fringe spacing due to the delay by the phase transition propagation. This is the first direct evidence of the molecular interaction propagation of the LC molecules.