Liquid Organic Hydrogen Carrier (LOHC) systems represent a promising option for hydrogen storage and transport in chemically bound and infrastructure-compatible form. For multiple use of the carrier liquid in repetitive...
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 and deuterium , 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 and 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 / 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.
Enhanced indirect ferromagnetic p-d exchange coupling of Mn in oxygen rich ZnO:Mn nanoparticles synthesized by wet chemical method J. Appl. Phys. 111, 033503 (2012); 10.1063/1.3679129Time-delayed transformation of defects in zinc oxide layers grown along the zinc-face using a hydrothermal technique
Hydrothermally grown n-type ZnO samples, implanted with helium (He + ) at a sample temperature of ∼40 K and fluences of 5 × 10 9 and 5 × 10 10 cm −2 , have been studied in situ by capacitance voltage (CV) and junction spectroscopy measurements. The results are complemented by data from secondary ion mass spectrometry and Fourier transform infrared absorption measurements and first-principles calculations. Removal/passivation of an implantation-induced shallow donor center or alternatively growth of a deep acceptor defect are observed after annealing, monitored via charge carrier concentration (N d ) versus depth profiles extracted from CV data. Isothermal anneals in the temperature range of 290-325 K were performed to study the evolution in N d , revealing a first-order kinetics with an activation energy, E a ≈ 0.7 eV and frequency factor, c 0 ∼ 10 6 s −1 . Two models are discussed in order to explain these annealing results. One relies on transition of oxygen interstitials (O i ) from a split configuration (neutral state) to an octahedral configuration (deep double acceptor state) as a key feature. The other one is based on the migration of Zn interstitials (double donor) and trapping by neutral Zn-vacancy-hydrogen complexes as the core ingredient. In particular, the latter model exhibits good quantitative agreement with the experimental data and gives an activation energy of ∼0.75 eV for the migration of Zn interstitials.
Electronic states in the upper part of the bandgap of reduced and/or hydrogenated n-type rutile TiO2 single crystals have been studied by means of thermal admittance and deep-level transient spectroscopy measurements. The studies were performed at sample temperatures between 28 and 300 K. The results reveal limited charge carrier freeze-out even at 28 K and evidence the existence of dominant shallow donors with ionization energies below 25 meV. Interstitial atomic hydrogen is considered to be a major contributor to these shallow donors, substantiated by infrared absorption measurements. Three defect energy levels with positions of about 70 meV, 95 meV, and 120 meV below the conduction band edge occur in all the studied samples, irrespective of the sample production batch and the post-growth heat treatment used. The origin of these levels is discussed in terms of electron polarons, intrinsic point defects, and/or common residual impurities, where especially interstitial titanium atoms, oxygen vacancies, and complexes involving Al atoms appear as likely candidates. In contrast, no common deep-level defect, exhibiting a charge state transition in the 200–700 meV range below the conduction band edge, is found in different samples. This may possibly indicate a strong influence on deep-level defects by the post-growth heat treatments employed.
Hydrothermally grown n-type ZnO bulk samples have been implanted with protons and deuterium ions to fluences in the range of 8 Â 10 10 to 8 Â 10 11 cm À2 . The implantations were performed at the temperature of 285 K, and the samples were then analyzed in-situ by deep level transient spectroscopy (DLTS) using a setup connected to the implanter beam line. The concentration of the socalled E4 center, having an apparent energy level at $0.57 eV below the conduction edge, is found to increase linearly with the ion fluence and the generation rate is proportional to the elastic energy deposition, as expected for a primary defect. Isothermal annealing of the E4 center at temperatures between 290 and 315 K reveals first-order kinetics with the activation energy of $0.6 eV. The annealing rate is strongly enhanced with increasing hydrogen fluence, and a model invoking migration of interstitial hydrogen and subsequent reaction with E4 exhibits close agreement with the experimental data. The rate of electron capture by E4 during the DLTS filling pulse depends on temperature, and it displays a thermal barrier of $0.15 eV. Most of these experimental results for E4 conform to the theoretically predicted properties of the oxygen vacancy (V O ) and a tentative assignment of E4 to V O is made, corroborating previous suggestions in the literature. In particular, the 0.57 eV level is ascribed to the double donor state of V O . Published by AIP Publishing.
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