Reorientation kinetics of hydroxyl groups in anatase TiO2.

The reorientation kinetics of hydrogen in a variety of complexes in the anatase polymorph of TiO2 was investigated by means of stress-induced dichroism. For the hydrogen-defect resulting in an O-H vibrational mode with a frequency of 3389 cm-1, the energy barrier separating adjacent equivalent in-plane sites of hydrogen was determined to be independent of the isotope and equal to 0.74 ± 0.02 eV, whereas the attempt frequency was found to be (1.10 ± 0.20) × 1012 and (0.75 ± 0.15) × 1012 s-1 for hydrogen and deuterium, respectively. The defect with vibrational modes at 3412 and 3417 cm-1 previously assigned to isolated hydrogen did not reveal alignment under the stress up to room temperature, which indicates that the barrier of hydrogen motion is above 0.9 eV.

Hydrogen motion in rutile TiO2.

Uniaxial-stress experiments have been performed for the 3287- and 2445-cm-1 local vibrational modes assigned to the positive charge state of interstitial hydrogen [Formula: see text] and deuterium [Formula: see text], respectively, occurring in mono-crystalline rutile TiO2. The onset of the defect alignment under the stress applied perpendicular to the [001] axis is detected at 165 K (185 K), which corresponds to the activation energy of 0.53 eV (0.58 eV) for interstitial hydrogen (deuterium). Based on these findings the diffusion constants of [Formula: see text] and [Formula: see text] along the [001] axis of TiO2 are determined. The experimental data are complemented by density-functional theory calculations and compared with the earlier results on the diffusion of [Formula: see text]/[Formula: see text] at elevated temperatures up to 700 °C. It is found that the activation energy value deduced from our low-temperature stress measurements yields a very good agreement with the high-temperature data, covering a dynamic range of 12 orders of magnitude.

Photoconductive detection of tetrahedrally coordinated hydrogen in ZnO.

In this Letter we apply an innovative experimental approach, which allows us to improve the sensitivity of detecting local vibrational modes (LVMs) even in highly absorbing spectral regions. This photoconductive technique allowed us to confirm a recent suggestion of a new multicenter bond for hydrogen in ZnO [A. Janotti and C. G. Van de Walle, Nature Mater. 6, 44 (2007)]. The two LVMs of the hydrogen substituting oxygen in ZnO are identified at 742 and 792 cm(-1). The modes belong to a nondegenerated A(1) and a twofold degenerated E representations of the C(3v) point group. The tetrahedral coordination of the hydrogen atom is the result of a newly detected multicenter bond for defects in solids.

Deep level transient spectroscopy studies of n-type ZnO single crystals grown by different techniques.

In the present study single-crystalline ZnO samples grown from the vapor phase, the melt, and a high-temperature aqueous solution (hydrothermal growth) are investigated before and after hydrogen plasma treatments, by means of deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS. Dominant DLTS peaks are found to appear in the range of 120-350 K for all materials. The DLTS spectra depend on the procedure of growth of the ZnO. The thermal stabilities of the defects in an oxygen atmosphere and in an oxygen-lean atmosphere are analyzed. The origin of the DLTS peaks is discussed.

Identification of hydrogen molecules in ZnO.

Hydrogen molecules in ZnO are identified by their local vibrational modes. In a Raman study, interstitial H2, HD, and D2 species were found to exhibit local vibrational modes at frequencies 4145, 3628, and 2985 cm-1, respectively. After thermal treatment of vapor phase grown ZnO samples in hydrogen atmosphere, most hydrogen forms shallow donors at the bond-centered site (HBC). Subsequently, HBC migrates through the crystal and forms electrically inactive H2. These results imply that the "hidden" hydrogen in ZnO [G. A. Shi et al., Appl. Phys. Lett. 85, 5601 (2004)10.1063/1.1832736] occurs in the form of interstitial H2.

Ortho-para conversion of interstitial H2 in Si.

Ortho-para conversion of isolated interstitial H2 in single-crystalline Si is studied by Raman scattering. This process is suggested to be caused by the interaction of H2 with the nuclear magnetic moment of 29Si. At 77 K the ortho-to-para conversion rate is approximately 0.015 h(-1) for all Si samples employed in the experiments. At 300 K, the reverse para-to-ortho transition is observed. It occurs with a rate of roughly 0.18 h(-1) and results in a thermodynamically nonequilibrium ortho-para ratio.

Ortho and para interstitial H2 in silicon.

A Raman scattering study of H2 trapped at the interstitial T site in Si is presented. Both ortho and para nuclear-spin states of H2 and D2 have been observed. It is shown that the Raman signals of H2 and D2 in the J=0 state, where J is the rotational quantum number, disappear preferentially from the spectra during laser excitation or prolonged storage at room temperature in the dark. This surprising behavior is tentatively explained by different diffusion rates of H2 in the J=0 and J=1 states.