Chihiro Itoh

Isochronal annealing study of X-ray induced defects in single- and double-walled carbon nanotubes

X-ray induced defects in single-walled (SWCNTs) and double-walled carbon nanotubes (DWCNTs) were characterized by Raman scattering spectroscopy. Frenkel defects, interstitial-vacancy pairs, were revealed to form in both SWCNTs and DWCNTs after X-ray irradiation because these defects were entirely healed by thermal annealing. In order to clarify the structure of the X-ray induced defect in SWCNT and DWCNT, isochronal-annealing experiments were performed on the irradiated samples and the activation energy for defect healing was estimated. The intensity of D band (defect induced band) on Raman spectra was used as a measure of the density of X-ray induced defects. The experimental results were in good agreement with the simulated values using second order reaction model, which indicated that the defect healing was determined by the migration energy of interstitials on the carbon layer. We also found that the activation energy for defect healing of SWCNT and DWCNT were around 0.5 eV and 0.32 eV, respectively. The X-ray induced defects in SWCNTs were more stable than those in DWCNTs. Compared these estimated activation energies to previous theoretical reports, we concluded that bridge and/or dumbbell interstitials are formed in both SWCNT and DWCNT by X-ray irradiation.

Photogeneration of charge carriers in titanium oxides

We have measured action spectra of the photoconductivity of rutile crystal at ∼4 K. The photoconductivity spectrum shows a keen rise at 3.0 eV and shows a peak at 3.2 eV and a shoulder around 3.7 eV. The threshold energy of the photoconductivity excitation well agrees with the onset of the fundamental optical absorption. This result indicates that the carriers contributing to the photoconductivity are generated by the fundamental excitation of the crystal. We have found that the photoconductivity shows remarkable field dependence. Below 5 V/mm, the photoconductivity was almost field independent. On the other hand, increasing the field strength in the range from 5 to 25 V/mm strongly enhanced the photoconductivity and varied the shape of the action spectrum. The origin of the field dependence is discussed.

Diameter-dependent annealing kinetics of X-ray-induced defects in single-walled carbon nanotubes

Diameter-dependent annealing effects of X-ray-induced defects in single-walled carbon nanotubes (SWNTs) were studied by Raman scattering spectroscopy. We found that X- ray-induced defects were formed in thin SWNTs at higher density than that in thick SWNTs. The X-ray-induced defects, distant pairs of interstitials and vacancies (I-V), were eliminated by thermal annealing. The recovery temperature of the X-ray-induced defects in the thin SWNTs were higher than that of the thick ones, indicating the thermal stability of the defects in thinner SWNTs are higher. In order to simulate the annealing behaviours of the X-ray-induced defects in SWNTs with diameters of ~0.9 and ~1.4 nm, we suggested the competitive reactions of diffusion of interstitials, formation of close I-V pairs and their recombination. The simulated results showed that the reaction-rate constant of elimination of close I-V pairs is dependent on tube diameter, which is presumably derived from nano-structure effects of SWNTs.

Structural modification of single-walled carbon nanotube induced by X-ray irradiation and subsequent annealing studied by Raman scattering spectroscopy in radial breathing mode

The formation of X-ray-induced defects changes the spectral shape of the radial breathing mode (RBM) and defect-induced mode (D band) in the Raman spectra of single-walled carbon nanotubes (SWNTs). X-ray-induced defects have been found to be annealed by thermal treatment, indicating that they are Frenkel pairs (vacancy and interstitial pairs). We found that the spectral shape of RBM is not entirely recovered after post-irradiation annealing. The temperatures for the complete annealing of X-ray-induced defects were within the range of 200–600 °C depending on the tube geometry. From these results, we suggest that the stability of X-ray-induced defects depends on the tube geometry and that the combination of X-ray irradiation and post-irradiation annealing causes a chirality change in SWNTs.

Laser induced intrinsic defects: Subpicosecond study of trapping kinetics

Abstract We use a time resolved interferometric method to study the kinetics of carriers trapping and defects formation following electronic excitation by high intensity ultrashort laser pulses in alkali halides. A systematic study of initial excitation density dependence of trapping kinetics, as well as the comparison of pure and doped samples, allows to investigate in detail the mechanism of energy relaxation and carriers localization accompanying the lattice rearrangement. The experimental results concern samples of pure and NO−2 doped KBr, and of pure NaCl. We observe faster trapping in KBrNO−2 than in pure KBr. In NaCl, the trapping rate increases with excitation density. These results show that in both materials the hole trapping is the primary process towards radiation induced defects. Furthermore, in KBr our results give strong evidence for the existence of an intermediate state with a short lifetime in the relaxation pathway between the initial electron–hole pairs and the final F–H pairs. A simple model describing the modification of the refractive index and rate equations governing free and trapped electron and holes populations are used to quantitatively interpret the experimental results.

Comparison of Electronic-Excitation-Induced Structural Modification of Carbon-Based Nanomaterials with that of Semiconductor Surfaces

Modification by electronic excitation of semiconductor surfaces and carbon-related quasi-two-dimensional (2D) nanostructured materials, namely graphene, carbon nanotubes is reviewed. Defect creation in these materials takes place not by low-intensity photoirradiation, but by laser or electron irradiation. The defect creation processes are different from ordinary photochemical processes in molecules or in some solids like alkali halides, which can be modified by a localized exciton. It is pointed out that there are common features in defect creation by electronic excitation in semiconductor surfaces and carbon-related quasi-2D nanomaterials: the yield-intensity relation shows strong superlinearity for laser irradiation near the bandgap energies and linearity or weak superlinearity for higher energy electron or photon irradiation. These results are explained in terms of multi-hole localization, in which bonds are weakened more strongly and more energy is available upon recombination with trapped electrons in comparison with excitons. The multi-hole localized state is considered to be realized by the creation of dense excitons or by cascade excitation for laser irradiation and by multiple excitations or multiple exciton generation by single impacts for electron irradiation. The review includes also polymerization of C60 films by electronic excitation, which is induced by low-intensity photoirradiation as well as by laser or electron irradiation. The experimental observation that laser or electron irradiation polymerize C60 films differently from low-intensity photoirradiation is explained in terms of multi-hole localization similar to the defect formation mechanism. Although fragmentation of C60 is due to electronic excitation of the molecule, it is included in the review because its yield is strongly superlinear for laser irradiation near bandgap energies and weakly superlinear for high-energy electron or photon irradiation as for other cases.

Multi-wavelength Raman scattering spectroscopic study of X-ray-irradiated single-walled carbon nanotube: Possibility of irradiation-induced electronic structure change

The resonant Raman scattering spectra of X-ray-irradiated single-walled carbon nanotubes (SWNTs) have been measured at 532, 561, 568, 647, and 660 nm. We found that X-ray irradiation induced a marked change in the radial breathing mode (RBM). In order to analyze the X-ray-induced change in the RBM spectra, we drew a map of the RBM peaks showing the relation between the peak position and the probe beam energy by interpolating the spectra measured at the probe wavelengths. We found that shifting the transition energy by 0.05 eV reproduced the RBM spectra of the irradiated SWNTs. This model well explains the results of our optical absorption measurements and previous reports on theoretical and scanning tunneling microscopy studies by other groups. On the basis of these results, we suggest that X-ray irradiation is one of the practical methods of controlling the electronic properties of SWNTs.

Laser-induced phase transition of Ti4O7 single crystal

We preset the results of Fourier-transform infrared (FTIR) reflection spectroscopy of the low-temperature insulating phase of Ti4O7 single crystals under the 532-nm CW-laser excitation. We found that CW-laser excitation with the intensity (P) of 7.6 Wcm-2 induced dramatic change of the FTIR reflection spectrum at 112 K. However, the excitation with P > 7.6 Wcm-2 gave no change in the spectrum. The spectrum measured under the excitation was almost identical with that of the high-temperature metallic phase. The result demonstrates that the laser excitation leads to a phase transition from the low-temperature insulating phase into the high-temperature metallic phase.

Exciton self-trapping and formation of diradical intermediates in 5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (TCDU) crystals at low temperatures

The fundamental crystalline excitation of 5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (TCDU) single crystals results in emission of two luminescence bands peaked at 4.28 eV and 2.57 eV, both of which show wide widths and significant degree of natural polarization at low temperatures. Measurements of the decay characteristics indicate that the initial state of the 4.28-eV luminescence is singlet, while that of the 2.57-eV band is triplet in nature. Analysis of the temperature dependence of Urbach tail shows that exciton-phonon coupling is strong enough to form self-trapped state of excitons. Therefore, we identify that the 4.28-eV and 2.57-eV luminescence are due to radiative recombination of the self-trapped excitons (STEs) in TCDU crystals. Excitation of the crystal with intense 4.66-eV laser light results in the formation of dimer diradicals, with expense of the concentration of only the triplet STE. The result shows that the triplet STEs is the metastable precursor responsible for the dimer-diradical formation, which initiates the polymerization of this diacetylene crystal.

Pulsed-ESR Study of Spin Solitons as a Probe of Neutral-to-Ionic Phase Transition of TTF-CA Single Crystal

Ionic phase of charge-transfer crystal composed of tetrathiafulvalene (TTF) and p -chloranil (CA) has been studied by two-pulse electron-spin echo (ESE) at 4 K. By measuring the angular dependence of the field-swept ESE spectrum, at least three kinds of paramagnetic centers have been resolved, which are presumably ascribed to spin solitons formed at domain boundary. With varying the time separation between the microwave pulses, envelope modulations due to anisotropic hyperfine interaction have been detected. By analyzing the modulation, we have firstly obtained ENDOR spectrum of the spin solitons. Response of the ESE intensity for the optical excitation has been also examined.