Hydrogen Defects Research Articles

Theory of Hydrogen Migration in Organic–Inorganic Halide Perovskites

AbstractSolar cells based on organic–inorganic halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current–voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first‐principles calculations based on density functional theory, we study the properties of an important defect in hybrid perovskites—interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization‐enhanced defect migration following the Bourgoin–Corbett mechanism. Our analysis highlights the structural flexibility of organic–inorganic perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells.

Molecular Statics Simulation of Hydrogen Defect Interaction in Tungsten**supported by National Natural Science Foundation of China (Nos. 51171008 and 11405201) and the National Magnetic Confinement Fusion Science Program of China (No. 2013GB1090)

Hydrogen (H) defect interactions have been investigated by molecular statics simulations in tungsten (W), including H-H interactions and interactions between H and W self-interstitial atoms. The interactions between H and small H-vacancy clusters are also demonstrated; the binding energies of an H, a vacancy and a self-interstitial W to an H-vacancy cluster depend on the H-to-vacancy ratio. We conclude that H bubble formation needs a high concentration of H in W for the H bubble nucleation and growth, which are also governed by the H-to-vacancy ratio of the cluster. The vacancy first combines with H atoms and a cluster forms, then the H-vacancy cluster goes through the whole process of vacancy capture, H capture, and vacancy capture again, and as a result the H-vacancy cluster grows larger and larger. Finally, the H bubble forms.

The influence of hydrogen on the seismic properties of solid iron

AbstractWe estimate the implications of having hydrogen dissolved in the solid core from a mineral physics perspective. Based on first‐principles calculations, we observe that hydrogen has a strong effect on decreasing the density of the hexagonal close‐packed structure of Fe. The structural effect is weakly dependent on the ordering pattern of the hydrogen defects. However, hydrogen also increases the elastic moduli of Fe at core pressures. Both the compressional and the shear wave velocities increase with addition even of small amounts of hydrogen, the increase of shear going against the geophysical requirements for light elements. This aspect makes hydrogen an undesirable light element in the Earth's core if we try to match the seismological observations.

Hole trapping at hydrogenic defects in amorphous silicon dioxide

We used ab initio calculations to investigate the hole trapping reactions at a neutral defect generated in amorphous silicon dioxide networks by the interaction of strained Si–O bonds with atomic hydrogen, a so-called hydroxyl E′ center. It was found that the hole trapping at this defect leads to two distinct charged configurations. The first one consists of an H atom bound to a bridging O in a hydronium-like configuration. The second configuration involves relaxation of a Si atom through the plane of its oxygen neighbors facilitated by a weak interaction with a 2-coordinated O atom. The distribution of total energy differences between these two configurations calculated for a number of amorphous network models has a width of about 1.0eV. These hole trapping reactions are discussed in the context of Si complementary metal-oxide-semiconductor device reliability issues.

A unified approach to modelling the charge state of monatomic hydrogen and other defects in crystalline silicon

There are a number of existing models for estimating the charge states of defects in silicon. In order of increasing complexity, these are (a) the Fermi-Dirac distribution, (b) the Shockley-Last model, (c) the Shockley-Read-Hall model, and (d) the Sah-Shockley model. In this work, we demonstrate their consistency with the general occupancy ratio α, and show that this parameter can be universally applied to predict the charge states of both monovalent and multivalent deep levels, under either thermal equilibrium or steady-state conditions with carrier injection. The capture cross section ratio is shown to play an important role in determining the charge state under non-equilibrium conditions. The application of the general occupancy ratio is compared with the quasi-Fermi levels, which are sometimes used to predict the charge states in the literature, and the conditions where the latter can be a good approximation are identified. The general approach is then applied to the prediction of the temperature- and injection level-dependent charge states for the technologically important case of multivalent monatomic hydrogen, and several other key monovalent deep levels including Fe, Cr, and the boron-oxygen complex in silicon solar cells. For the case of hydrogen, we adapt the model of Herring et al., which describes the charge states of hydrogen in thermal equilibrium, and generalize it for non-equilibrium conditions via the inclusion of the general occupancy ratio, while retaining the pre-factors which make the model more complete. Based on these results, the impact of temperature and injection on the hydrogenation of the key monovalent defects, and other pairing reactions, are discussed, demonstrating that the presented model provides a rigorous methodology for understanding the impact of charge states.

Towards understanding the hydrogen molecule in ZnO

Using Raman scattering, molecular hydrogen defects are shown to exist in ZnO, the first II-VI semiconductor in which this has been observed. The rotational-vibrational properties of the defects are shown to depend on the history of the sample and an ortho-para conversion between the nuclear spin states is observed.

The effect of baking time, fillet radius, and hardness on the lifecycles of pole fastening screws in an electric motor with hydrogen embrittlement

In order to protect bolts from corrosion, electroplating such as zinc plating is widely used. However, hydrogen can easily penetrate or diffuse into the vacancies and dislocations between the lattices of bolt steel during electroplating. As the diffused hydrogen defects inside the lattice are in gaseous form, small cracks can easily be produced due to high pressure from the hydrogen gas. In this research, in order to determine the root cause of the fracture in pole fastening screws resulting from hydrogen embrittlement in typical electric motors, additional factors that accelerate hydrogen embrittlement fracture were selectively applied, including a small fillet in the head–shank transition and excessive hardness, and parametric study was performed experimentally.

Identification of hydrogen defects linked to boron substitution in synthetic forsterite and natural olivine

Experimental and theoretical evidence for the coupled substitution of B and H in synthetic forsterite and a natural olivine is presented. The intensities of OH bands at 3704 cm -1 (//z), 3598 cm -1 (//x,y), and 3525 cm -1 (//x) in a heterogeneous B-doped synthetic forsterite crystal matches the zoning of B concentration measured by ion probe. The two anti-symmetric stretching vibrations of BO 3 groups occur at 1301 cm -1 (//x) and 1207 cm -1 (//z) for the 10 B and at 1256 and 1168 cm -1 for the 11 B isotope. A microscopic model of the mixed (B,H) defect that accounts for experimental observations is obtained from quantum mechanical calculations. The BO 3 group lies on the (O3-O1-O3) face of the vacant Si site and the H atom is bonded to the O2 atom at the remaining apex. The occurrence of the same OH bands associated with v 3 BO 3 vibrations in a natural olivine sample from Pakistan confirms the occurrence of this defect in nature. The three diagnostic OH bands can be used as a signature of H associated with boron substitution in olivine and forsterite, leading to a quantitative analysis of their contribution to H-defects.

Inter-cage dynamics in structure I, II, and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations.

Prospective industrial applications of clathrate hydrates as materials for gas separation require further knowledge of cavity distortion, cavity selectivity, and defects induction by guest-host interactions. The results presented in this contribution show that under certain temperature conditions the guest combination of CH3F and a large polar molecule induces defects on the clathrate hydrate framework that allow intercage guest dynamics. (13)C NMR chemical shifts of a CH3F/CH4/TBME sH hydrate and a temperature analysis of the (2)H NMR powder lineshapes of a CD3F/THF sII and CD3F/TBME sH hydrate, displayed evidence that the populations of CH4 and CH3F in the D and D' cages were in a state of rapid exchange. A hydrogen bonding analysis using molecular dynamics simulations on the TBME/CH3F and TBME/CH4 sH hydrates showed that the presence of CH3F enhances the hydrogen bonding probability of the TBME molecule with the water molecules of the cavity. Similar results were obtained for THF/CH3F and THF/CH4 sII hydrates. The enhanced hydrogen bond formation leads to the formation of defects in the water hydrogen bonding lattice and this can enhance the migration of CH3F molecules between adjacent small cages.

Magnetism of hydrogen-irradiated silicon carbide

Spin-polarized density functional theory is used to study two-hydrogen defect complexes in silicon carbide. We find that the magnetism depends on the distances of the two hydrogen atoms. Magnetism appears when the two hydrogen defects are distant from each other, and magnetism cancels out if they are close to each other. The critical distance between the two hydrogen defects is determined. (C) 2014 Elsevier B.V. All rights reserved.

Dominant factor determining the conduction-type of nitrogen-doped ZnO film.

Nitrogen-doped zinc oxide (ZnO) film has been grown by molecular beam epitaxy. The as-grown sample showed p-type conduction with a hole concentration of 3.1 x 10(17) cm(-3). After an annealing process in O2 at 600 degrees C for 30 min, p-type conduction was still remained, and the hole concentration of the film decreased to 6.8 x 10(16) cm(-3). Secondary ion mass spectroscopy revealed that the concentration of both nitrogen and hydrogen decreased after the annealing process. It is demonstrated that the intrinsic compensation source has been decreased after the annealing process. Because the variation trend of the hole concentration in the ZnO:N film is opposite to that of hydrogen and intrinsic defects, but in good accordance with nitrogen, the extrinsically substituted nitrogen (N(o)) should be the dominant factor that determines the conduction-type of the ZnO:N film.

Electronic structure and stability of hydrogen defects in diamond and boron doped diamond: A density functional theory study

Isolated hydrogen and hydrogen pairs in bulk diamond matrix have been studied using density functional theory calculations. The electronic structure and stability of isolated and paired hydrogen defects are investigated at different possible lattice sites in pure diamond and boron doped diamond. Calculations revealed that isolated hydrogen defect is stable at bond center sites for pure diamond and bond center puckered site for boron doped diamond. In case of hydrogen pairs, H2* defect (one hydrogen at bond center and second at anti-bonding site) is stable for pure diamond, while for boron doped diamond B–H2BC complex (one H atom at the B–C bond centered puckered position and the other one at the puckered position of one of the C–C bond first neighbor of the B atom) is most stable. Multiple hydrogen trapping sites in boron doped diamond has also been studied. Calculated results are discussed and compared with previously reported theoretical results in detailed.

Hydrogen induced optically-active defects in silicon photonic nanocavities.

We demonstrate intense room temperature photoluminescence (PL) from optically active hydrogen- related defects incorporated into crystalline silicon. Hydrogen was incorporated into the device layer of a silicon on insulator (SOI) wafer by two methods: hydrogen plasma treatment and ion implantation. The room temperature PL spectra show two broad PL bands centered at 1300 and 1500 nm wavelengths: the first one relates to implanted defects while the other band mainly relates to the plasma treatment. Structural characterization reveals the presence of nanometric platelets and bubbles and we attribute different features of the emission spectrum to the presence of these different kind of defects. The emission is further enhanced by introducing defects into photonic crystal (PhC) nanocavities. Transmission electron microscopy analyses revealed that the isotropicity of plasma treatment causes the formation of a higher defects density around the whole cavity compared to the ion implantation technique, while ion implantation creates a lower density of defects embedded in the Si layer, resulting in a higher PL enhancement. These results further increase the understanding of the nature of optically active hydrogen defects and their relation with the observed photoluminescence, which will ultimately lead to the development of intense and tunable crystalline silicon light sources at room temperature.

Accumulation features of radiation-induced hydrogen defects in LiF

Infrared absorption and electron paramagnetic resonance spectra have been investigated for LiF:OH crystals irradiated in a nuclear reactor at low and normal reactor temperatures. The influence of irradiation temperature on accumulation of interstitial hydrogen ions Hi- (U1-centers) was studied. It was observed that these centers were predominantly accumulated under the low-temperature irradiation. The maximum frequency of local vibrations of Hi- ions in LiF depended on the irradiation dose. The shift of vibration frequency of U1-centers to low energy with the increase in the irradiation dose was studied and analyzed. Negative hydrogen ions [H−] in anionic sites (U-centers) were registered in irradiated LiF:OH crystals without their preliminary thermal treatment after the neutron irradiation both at reactor and low temperatures. Accumulation of hydrogen atoms (U2-centers) in LiF for different irradiation fluxes was studied by the thermal treatment. After the irradiation of LiF:OH at 20K the H0 atoms were observed without annealing of the crystal.

Hydrogen defects in tetragonal ZrO2studied using density functional theory

In the energy-structure paradigm, we analyzed the defects that can arise in tetragonal zirconium oxide (T-ZrO2) involving the hydrogen atom or the hydrogen molecule using density functional theory. Our results indicate that the dominant hydrogen defect under reducing conditions is H(·)(0), a complex formed between the hydride ion and a doubly charged oxygen vacancy. This result is consistent with the experimental observation that under reducing conditions, the solubility of hydrogen is proportional to the degree of hypostoichiometry of T-ZrO2. Under oxidizing conditions we found three different hydrogen defects, each predominating in a specific range of the chemical potential of electrons. Starting from the valence band top toward the conduction band bottom, these defects are the interstitial proton, H(·)(i), a complex formed between two hydrogen species and a zirconium vacancy with a net effective charge of (2-), (2H)"(Zr), and finally a complex similar to the latter but with a net effective charge of (4-), (H(2))'''(Zr). In (2H)"(Zr)the two hydrogens exist in the form of hydroxyl groups, while in (H(2))" " (Zr) they exist in the form of a hydrogen molecule. In addition, we found that up to three hydrogen species can favorably accumulate in a zirconium vacancy under oxidizing conditions. The clustering of hydrogen in cation vacancies can be a precursor for the deleterious effects of hydrogen on the mechanical properties and stability of metal oxides, in analogy with hydrogen embrittlement in metals. Finally we observed a red-shift and a blue-shift for the vibrational frequencies of all the hydroxyl groups and all the hydrogen molecules, respectively, in T-ZrO2 when compared to the gas phase frequencies. This is an important characteristic for guiding future experimental efforts to detect and identify hydrogen defects in T-ZrO2. The insights presented in this work advance our predictive understanding of the degradation behavior of T-ZrO2 as a corrosion resistant passive layer, as a gate dielectric and in biomedical applications.

Synthesis of ZnO Nanowires via Hotwire Thermal Evaporation of Brass (CuZn) Assisted by Vapor Phase Transport of Methanol

Zinc oxide (ZnO) nanowires (NWs) were synthesized using vapor phase transport (VPT) and thermal evaporation of Zn from CuZn. Time dependence of ZnO NWs growth was investigated for 5, 10, 15, 20, 25, and 30 minutes. Significant changes were observed from the field electron scanning electron microscopy (FESEM) images as well as from the X‐ray diffraction (XRD) profile. The photoluminescence (PL) profile was attributed to the contribution of oxygen vacancy, zinc interstitials, and hydrogen defects in the ZnO NWs. Raman scattering results show a significant peak at 143 cm−1 and possible functionalization on the wall of ZnO NWs. Growth of ZnO NWs in (0002) with an estimated distance between adjacent lattice planes 0.26 nm was determined from transmission electron microscopy (TEM) analysis.

Ionic conductivity and the formation of cubic CaH2 in the LiBH4–Ca(BH4)2 composite

LiBH4–Ca(BH4)2 composites were prepared by ball milling. Their crystal structures and phase composition were investigated using synchrotron X-ray diffraction and Rietveld refinement, and their ionic conductivity was measured using impedance spectroscopy. The materials were found to form a physical mixture. The composites were composed of α-Ca(BH4)2, γ-Ca(BH4)2 and orthorhombic LiBH4, and the relative phase quantities of the Ca(BH4)2 polymorphs varied significantly with LiBH4 content. The formation of small amounts of orthorhombic CaH2 and cubic CaH2 in a CaF2-like structure was observed upon heat treatment. Concurrent formation of elemental boron may also occur. The ionic conductivity of the composites was measured using impedance spectroscopy, and was found to be lower than that of ball milled LiBH4. Electronic band structure calculations indicate that cubic CaH2 with hydrogen defects is electronically conducting. Its formation along with the possible precipitation of boron therefore has an effect on the measured conductivity of the LiBH4–Ca(BH4)2 composites and may increase the risk of an internal short-circuit in the cells.

Physical Properties of Nano-Structured Carbon Nitride Film for Integrated Humidity Sensors

Carbon-nitride films with -20 nm grain size have been deposited on silicon substrate for applying to integrated humidity sensors. Crystal structure, surface morphology and bonding structures of the deposited films were investigated by XRD, SEM and FTIR. Carbon nitride films have hydrogen defect, this offer the adsorption sites to water molecules. An integrated humidity FET that was fabricated by 0.8 microm CMOS technology was sensitive to humidity changes. A gas adsorption model was proposed in gas-carbon nitride interface.

Bulk and Surface Tunneling Hydrogen Defects in Alumina

We perform abinitio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al- and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz frequency TLSs in alumina.

On the cooperative formation of non-hydrogen-bonded water at molecular hydrophobic interfaces

The unique structural, dynamical and chemical properties of air/water and oil/water interfaces are thought to play a key role in various biological, geological and environmental processes. For example, non-hydrogen-bonded ('dangling') OH groups--which create surface defects in water's hydrogen bonding network and are experimentally detected at both macroscopic (air/water or oil/water) and microscopic (dissolved hydrophobic molecule) interfaces--are thought to catalyse some chemical reactions. However, how the size, curvature or charge of the exposed hydrophobic surface influences water's propensity to form dangling OH defects has not yet been established quantitatively. Here we use Raman multivariate curve resolution to probe spectroscopically the hydrophobic hydration shell and, using a statistical multisite analysis, we show that such interfacial dangling OH structures are entropically stabilized and their formation is cooperative (the probability that a non-hydrogen-bonded OH group will form depends nonlinearly on the hydrophobic surface area). We thus expose an important difference between the chemical properties of molecular and macroscopic oil/water interfaces.