Shozo Tero-Kubota

Electron paramagnetic resonance studies on microcrystalline silicon prepared by sputtering method

Dangling bond (DB) defects in unhydrogenated microcrystalline silicon (μc-Si) prepared by rf sputtering have been studied. Raman spectra and x-ray diffraction indicate that the μc-Si fraction has been formed at the Ar sputtering pressure higher than 26.6 Pa while only amorphous silicon (a-Si) has been produced at the lower pressure. The electron paramagnetic resonance (EPR) spectrum in the μc-Si film is broad and unsymmetrical with the average g value of g=2.006 compared with that of a-Si (g=2.0055). The X- and Q-band EPR measurements suggest that the line shape is mainly governed by the inhomogeneous broadening due to the g anisotropy, indicating relatively large distribution of the structure of the DB defects.

Theoretical analysis of singlet–triplet energy splitting generated by charge-transfer interaction in electron donor–acceptor radical pair systems

A theoretical analysis was performed for a prediction of the singlet–triplet energy splitting (J) of electron donor–acceptor radical pairs on the basis of the electron transfer reaction theory. It has been strongly indicated that charge-transfer interaction dominates the J in condensed phase photoinduced electron transfer systems.

Spin-orbit coupling induced electron spin polarization in photoinduced electron transfer reactions

The observation of net-absorptive chemically induced dynamic electron polarization (CIDEP) spectra induced by spin-orbit coupling (SOC) interactions due to heavy atoms gives proof of the generation of a contact radical pair (RP) or triplet exciplex as the intermediate. Recent time-resolved EPR and pulsed EPR studies on the spin dynamics are reviewed. The SOC interaction causes simultaneous changes in the orbital and spin operators, leading to sublevel-selective back electron transfer from the short-lived intermediate to the singlet ground state. The intrinsic enhancement factor of the SOC-induced polarization increases with increasing atomic number of the heavy atom substituent. On the other hand, the radical yield decreases remarkably with increasingly heavy atoms. The build-up rate of the SOC-induced CIDEP signal gives the formation rate of the RP-type triplet exciplex. Quantitative analyses of the spin dynamics provide kinetic parameters such as the back electron transfer rate, escape rate, relaxation times of the precursor of the excited triplet state and free radicals. It has been shown that the experiments on the magnetic field effect on the radical yield complement the spin polarization dynamics studies.

Twist conformational effects on the excited triplet states of aromatic ketones studied by multifrequency TREPR and pulsed EPR spectroscopy

Employing X- and Q-band time-resolved EPR and pulsed EPR techniques, we have investigated effects of twist conformation on the electronic structures and magnetic properties of the lowest excited triplet states of alkyl phenyl ketones. The conformational variation due to steric hindrance of a bulky substituent on the α-position drastically leads to a decrease of the zerofield splitting constant of |D| and an increase of the energy gap between 3nπ* and 3ππ* states. Hydrogen bonding with H2O destabilized significantly the 3nπ* state and switched the T1 state to the 3ππ* state. t-Butyl phenyl ketone gave unusually broad EPR spectra in rigid glassy matrices. The multifrequency and pulsed EPR measurements clarified that the inhomogeneously broadening spectra were ascribable to the distribution of the dihedral angle about a twist conformation. Asymmetric distribution of the ZFS parameters was interpreted in terms of the perturbation of spin—orbit interactions to the spin sublevels.

Multi-band electron paramagnetic resonance study of the defects in microcrystalline silicon

Abstract We report electron paramagnetic resonance (EPR) studies in dangling bond (DB) defects in the hydrogenated microcrystalline silicon (μc-Si:H) film prepared by a plasma enhanced chemical vapor deposition (PECVD), unhydrogenated microcrystalline silicon (μc-Si) film prepared by RF sputtering, and μc-Si embedded in SiO2 film formed by co-sputtering followed by annealing. Multi-band EPR measurements showed that their line widths originate primarily from a distribution of the g-values. We have detected a signal at g=2.006 in the microcrystalline fraction of the μc-Si, μc-Si:H and μc-Si embedded SiO2 films. The signal has a greater width than the DB signal in amorphous silicon (a-Si), indicating a larger distribution of g-values of the defects in the microcrystalline material. The DB signal in the μc-Si embedded in SiO2 was reproduced by the computer simulation taking into account the distribution of the g-values.

Positive exchange interaction in the radical ion pair of benzophenone anion and 1,4-diazabicyclo[2,2,2]octane cation radicals studied by FT-EPR spectroscopy

Abstract Electron spin polarization generated from the photoreduction of benzophenone (BP) and its derivatives in the presence of 1,4-diazabicyclo[2,2,2]octane (DABCO) was studied in various solvents. The DABCO cation radical obtained showed a CIDEP spectrum with A/E (absorption/emission) polarization by RPM, while other neutral radicals gaee an E/A pattern. Using triplet quenchers, it was confirmed that these RPM signals were generated through the triplet reaction process. The present results revealed that the radical ion pair including the BP anion and DABCO +. cation radicals has a positive J , while the neutral radical pairs generated under the same condition have a negative J . The sign of J is independent of the polarity of the organic solvents used.

Magnetic field effects on the triplet exciplex dynamics in the duroquinone- N,N -dimethylaniline derivative systems

Magnetic field effects (MFEs) on the radical yield in the photoinduced electron transfer reaction from the p-halogen derivatives (4XDMA) of N,N-dimethylaniline to the excited triplet state of duroquinone (DQ) have been investigated in alcoholic solutions at room temperature. In 1-propanol and 1-butanol solutions, the radical yields decreased as the magnetic field increased and became nearly constant at 1-1.8 T in the DQ-4BrDMA and DQ-4IDMA systems, suggesting that the spin-orbit coupling interaction due to the heavy atoms governs the radical yield. On the other hand, in the methanol solution MFE due to a radical pair mechanism was observed. We concluded that the key intermediate to determine the radical yield is the triplet exciplex or contact radical ion pair in the 1-propanol and 1-butanol solutions, while it is the solvent-separated radical ion pair in the methanol solution.

Nitrogen-Doping Effects on Electrical Properties of Hydrogenated Microcrystalline Silicon as Studied by Electron Paramagnetic Resonance and Conductivity

We have examined Raman scattering, X-ray diffraction, electron paramagnetic resonance (EPR) spectra and the conductivity of nitrogen-doped hydrogenated microcrystalline silicon. The EPR signals due to conduction electrons have been observed in the doped films, except for highly doped samples that have no microcrystalline fraction. The result indicates that the doped nitrogen atom acts as an electron donor in the microcrystalline silicon. The temperature dependence of the conductivity clarify that the activation energy depends on the doping level. The influence of the doping level on the conductivity can be interpreted in terms of the balance of the effective electron donation and the decrease of carrier mobility due to a decrease of the microcrystalline phase volume ratio. At temperatures lower than approximately 180 K, the conductivity shows little variation. This is explained using a model of the hopping conduction, in terms of defect states for all samples.

Electronic structure of the excited triplet state of nonphosphorescent tropolone studied by time-resolved EPR

Time-resolved EPR measurements and semi-empirical MO calculations have been carried out to clarify the electronic structure of the T1 state of nonphosphorescent tropolone. The observed zero-field splitting parameters indicate that the T1 state has a pure ππ* character and the energy order among the spin sublevels was TY (in-plane)>TX (in-plane)>0>TZ (out-of-plane), which is the same as tropone for a parent molecule. However, the principal X-axis slightly turns out of the carbonyl bond direction so that the intersystem crossing from the S1 state to the TY sublevel becomes more active. The spin densities obtained from hyperfine splittings of protons and MO calculation of AM1 method show an asymmetric distribution of unpaired π-electrons over two oxygens as well as the seven-membered ring. This unbalanced distribution makes the deviation of the X-axis from the carbonyl bond clear. We found two sites that can exchange each other by intramolecular proton transfer in a durene single crystal.

TIME-RESOLVED AND FOURIER TRANSFORM EPR STUDIES OF TYPE I PHOTOCHEMICAL REACTION. DIRECT DETERMINATION OF KINETIC PARAMETERS OF C-C BOND CLEAVAGE

Continuous wave time-resolved EPR (CW-TREPR) and pulsed Fourier transform EPR (FT-EPR) experiments with a resolution of ≈10 ns were carried out to investigate the Type I photocleavage. A two-pulse echo detection method was used to eliminate the dead-time problem in the FID measurements. An analysis of the buildup kinetics of the FT-EPR signal intensity due to t-butyl radicals provided the rates of the α-cleavage of t-butylphenyl ketone at various temperatures. We obtained two kinetic parameters at low and high temperatures which correspond to the non-radiative decay rate of the excited triplet state of the ketone and the rate of the Type I photocleavage, respectively.