Monatomic Hydrogen Research Articles

On: X-ray diffraction from the electron gas in monatomic metallic hydrogen

Solid hydrogen is expected to become a monatomic metal under sufficiently high compression. With hydrogen having only a single valence electron and no ion core, the nature of x-ray diffraction patterns from the electron gas of monatomic metallic hydrogen is uncertain, and it is unclear whether they may yield enough information for a crystal structure determination. With emphasis on the Cs-IV-type ( I41/amd ) structure predicted for hydrogen at ∼500 GPa, the electron density distributions, zero-point and thermal atomic motion, and x-ray diffraction intensities are determined from first-principles calculations for several candidate phases of metallic hydrogen. It is shown that the electron distribution is much more structured than might be expected from the commonly employed free-electron-gas picture, and in fact more modulated than what is obtained from the superposition of free-atom charge densities. We demonstrate that an identification of the crystal structure of monatomic metallic hydrogen from x-ray diffraction is fundamentally possible and discuss the possibility of single-crystal diffraction from metallic hydrogen. An atomic scattering factor for the hydrogen atom in monatomic metallic hydrogen is constructed to aid the quantitative analysis of diffraction intensities from future x-ray diffraction experiments.

The magnetic field design of a solenoid for the cold-cathode Penning ion source of a miniature neutron tube

A high-yield and beam-stable neutron tube can be applied in many fields. It is of great significance to the optimal external magnetic field intensity of the cold-cathode Penning ion source (PIS) and precisely controls the movement of deuterium (D), tritium (T) ions and electrons in the source of the neutron tubes. A cold-cathode PIS is designed based on the solenoidal magnetic field to obtain better uniformity of the magnetic field and higher yield of the neutron tube. The degree of magnetic field uniformity among the magnetic block, double magnetic rings and solenoidal ion sources is compared using finite element simulation methods. Using drift diffusion approximation and a magnetic field coupling method, the plasma distribution of hydrogen and the relationship between plasma density and magnetic field intensity at 0.06 Pa pressure and a solenoid magnetic field are obtained. The results show that the solenoidal ion source has the most uniform magnetic field distribution. The optimum magnetic field strength of about 0.1 T is obtained in the ion source at an excitation voltage of 1 V. The maximum average number density of monatomic hydrogen ions (H+) is 1 × 108 m−3, and an ion-beam current of about 14.51 μA is formed under the −5000 V extraction field. The study of the solenoidal magnetic field contributes to the understanding of the particle dynamics within the PIS and provides a reference for the further improvement of the source performance of the neutron tube in the future.

Diastereoselective Electrocatalytic Hydrogenation of Cyclic Ketones Using a Proton-Exchange Membrane Reactor: A Step toward the Electrification of Fine-Chemical Production

We report the diastereoselective electrocatalytic hydrogenation of cyclic ketones using a proton-exchange membrane (PEM) reactor. The adsorbed monatomic hydrogen species (Hads) generated from protons at the triple-phase boundary of the cathode were found to reduce cyclic ketones with high diastereoselectivity. As high as 94% cis-selectivity was realized under optimal conditions upon using a Rh catalyst. Operando infrared spectroscopy enabled the direct observation of the adsorbed ketone involved in the reaction. We also demonstrate 5 g scale electrolysis of 4-tert-butylcyclohexanone by coupling the hydrogenation process with water oxidation as an anodic reaction. This reaction successfully produced cis-4-tert-butylcyclohexanol, which is 52 times more expensive than the starting material, according to commercial prices, uses only electrical energy and water as the reagent, and, remarkably, does not require H2 gas. This study demonstrates the potential of PEM reactors as reliable, robust, and green systems for the electrochemical production of fine chemicals.

Excitonic-insulator instability and Peierls distortion in one-dimensional semimetals

The charge density wave instability in one-dimensional semimetals is usually explained through a Peierls-like mechanism, where the coupling of electrons and phonons induces a periodic lattice distortion along certain modes of vibration, leading to a gap opening in the electronic band structure and to a lowering of the symmetry of the lattice. In this work, we study two prototypical Peierls systems: the one-dimensional carbon chain and the monatomic hydrogen chain with accurate ab initio calculations based on quantum Monte Carlo and hybrid density functional theory. We demonstrate that in one-dimensional semimetals at $T=0$, a purely electronic instability can exist independently of a lattice distortion. It is induced by spontaneous formation of low energy electron-hole pairs resulting in the electronic band gap opening, i.e., the destabilization of the semimetallic phase is due to an excitonic mechanism.

Hydrogen-Anion-Induced Carrier Recombination in MAPbI3 Perovskite Solar Cells.

Identification and passivation of defect-induced electron-hole recombination centers are currently crucial for improving the efficiency of hybrid perovskite solar cells. Besides general intrinsic defects, experimental reports have indicated that hydrogen interstitials are also abundant in hybrid perovskite layers; however, few reports have evaluated the effect of such defects on the charge carrier recombination and device efficiencies. Here, we reveal that under I-poor synthesis conditions, the negatively charged monatomic hydrogen interstitial, Hi-, will form in the prototypical CH3NH3PbI3 perovskite layer, acting as a detrimental deep-level defect, which leads to efficient electron-hole recombination and lowers the cell performance. We further rationalize that Br doping can mitigate the large atomic displacement caused by the presence of Hi- and hence suppress the formation of the deep localized state. The results advance the knowledge of the deep-level defects in hybrid perovskites and provide useful information for enhancing solar cell performance by defect engineering.

Modeling the Charge State of Monatomic Hydrogen and Other Defects with Arbitrary Concentrations in Crystalline Silicon

A rigorous physical‐mathematical model for predicting the charge states of both mono‐ and multivalent defects with arbitrary concentrations in Si in both thermal equilibrium and non‐equilibrium steady‐state conditions is presented. The model avoids the assumption that the defect concentration is much lower than the doping level, which is common in previous approaches. It is shown that the general occupancy ratio given by α = (kn1 + p)/(kn + p1) is still valid under these more general conditions. However, more caution needs to be taken when calculating the equilibrium and excess carrier densities in the presence of larger defect concentrations, because of: a) the non‐negligible contribution to n0 and p0 from defect ionisation, and b) unequal Δn and Δp caused by the change in the occupied states at the defect level from equilibrium to non‐equilibrium. Modeled examples of monatomic H and interstitial Fe in Si are shown and the effects of defect concentration, temperature, and carrier injection are discussed.

Temporary Recovery of the Defect Responsible for Light- and Elevated Temperature-Induced Degradation: Insights Into the Physical Mechanisms Behind LeTID

The effect of light- and elevated temperature-induced degradation (LeTID) can be nonpermanently reversed by charge carrier injection below the degradation temperature (commonly used degradation temperatures are above ~70 °C). In this study, we show that the rate of temporary recovery depends strongly on the excess carrier density. We observe that the order of the reaction changes from pseudo-zero to first with increasing injection. The rate decreases slightly with increasing temperature. Since the samples can go through multiple degradation/recovery cycles without distinct changes in the degradation kinetics, the experimentally accessible recovered and degraded states are interpreted as manifestations of the equilibrium concentrations of the defect responsible for LeTID at different temperatures. Based on our observations, we argue that the process underlying LeTID degradation is the dissociation of a precursor rather than an association of two or more components. In light of the relation between LeTID susceptibility and bulk hydrogen concentration, we hypothesize that the LeTID precursor dissociates into the LeTID defect and monatomic hydrogen. Numerical simulations of the coupled rate equations including hydrogen interactions well reproduce the experimental observations; according to these results, the presence of a sink for the atomic hydrogen such as dopant atoms is paramount for the LeTID degradation.

DEMS and RAMAN study of the monatomic hydrogen adsorption during electro-reduction of NO3- and NO2- at Pt nanoparticles supported at W18O49-ZrO2-C nanocomposite

A multifunctional hybrid material composed of W18O49 nanobelts (NBs), zirconium oxide, and Vulcan carbon (W18O49-ZrO2-C) has been used as support for platinum nanoparticles (PtWZC). For comparison, platinum supported on Vulcan carbon (PtC) was also synthesized. These catalysts were evaluated in the NO3- and NO2- (NOx) reduction reactions. The addition of W18O49 and ZrO2 to the carbon improved the size and distribution of the platinum nanoparticles. The electrochemical results showed that enhanced electrocatalytic activity for the NOx reduction was obtained on the PtWZC catalyst compared to PtC synthesized by the same technique and PtEtek from a commercial source. Our findings suggest that the adsorbed monatomic hydrogen (Hads) over platinum inhibits the reduction of nitrates and nitrites. Therefore, these reactions need a material that removes Hads from the surface or modifies it for the NOx reduction. The WZC support adsorbed monatomic hydrogen from water to form active bronze species (Hx-W18O49) that interacted with the NO3- and NO2- ions enabling their reduction, in accord with the results from in-situ Raman spectroscopy and Tafel slopes. N2, NO, NH3, and N2H4 products were detected in nitrite ion reduction by the Differential electrochemical mass spectrometry (DEMS) technique on the PtWZC catalyst. The production of N2 dominated in the cathodic and anodic scan. Hence, the novel catalyst can follow the indirect pathway, where the reduction process involves adsorption of NHx and adsorption of NO. The higher electrocatalytic activity obtained by the PtWZC catalyst was attributed to the uniform distribution of platinum on the support, the increased electrical conductivity produced by the W18O49 nanobelts, and the modulation of Hads on the surface.

On the proposal of an Eddington ratio of natural energies, ε

Eddington in 1923, first identified four dimensionless numbers, derived from combinations of the basic physical constants, which are known as the “Eddington constants.” In formulating these dimensionless numbers, Eddington, a leading physicist of his time, claimed that they are characteristic of the structure and dynamics of the Universe at large, on the microscopical scale, and at the macroscopical scale. These four dimensionless ratios are labeled here, and elsewhere, as α (also called the fine structure constant), and β (the electron-proton mass ratio), and γ, (the ratio of electrical-to-gravitational force of the proton on the electron), and δ (a ratio involving the cosmological constant and other constants). Here, in this communication, is defined a 5th Eddington ratio, labeled as e (a ratio of characteristic energies of diatomic and monatomic hydrogen). The uncanny fitting of these 5 fundamental ratios to simple formulas involving the mathematical constants e, the base of natural logarithms, π, the familiar circular constant, ϕ, the golden ratio and “2,” the only even prime number, is described to some degree along with a tabulation of characteristics of these 5 ratios.

Copper atomic contacts exposed to water molecules.

Monatomic and molecular hydrogen and also oxygen, as well as water molecules and OH that are exposed to atomic copper in intimate contact, have been studied theoretically using computational methods. The authors optimized moderately large structures of Cu/H/Cu, Cu/HCuH/Cu, Cu/H2/Cu, Cu/H2O/Cu, Cu/OH/Cu, Cu/O/Cu, and Cu/O2/Cu and calculated appropriate values for conductance and inelastic tunneling spectroscopy (IETS) properties of the contact junctions, elucidating them as being a possible outcome resulting from the exposure of copper electrodes to the atomic/molecular contaminant species. Here we also demonstrate the IETS properties, by means of ab initio calculations, which can determine the form of the junction geometries. Furthermore, we identify the bonding geometries at the interfaces of the copper electrodes that directly give rise to the specific IETS signatures that have been observed in recent experiments. Based on low-bias conductance and IETS calculations, for the specific case of water exposure of copper electrodes, it was concluded that a single hydrogen or a single oxygen atom bridging the copper electrodes is not responsible for the high conductance peak measurements. Regarding Model 4, where an individual water molecule is considered to be the bridging constituent, our computational results suggest that it has a relatively low probability of being an appropriate candidate. Based upon current computational results, the two hydrogens in Model 3 appear to be in molecular form, although they still form a bond with the adjacent copper atoms. Comparing computational with experimental results indicates that Model 3 is in acceptable agreement with available data.

Channel-Length Dependent Performance Degradation of Thermally Stressed IGZO TFTs

The focus of this work is on the performance degradation of thermally stressed IGZO TFTs with SiO2 for both the gate dielectric and back-channel passivation material. The TFT fabrication process included a post-passivation 400 °C O2 anneal to adjust the IGZO semiconductor properties. I-V characteristics of TFTs with bottom-gate (BG) and double-gate (DG) electrode configurations were observed to left-shift and degrade with 200 °C hotplate treatments as shown in Fig. 1. The mechanism causing the instability is not completely understood, however experimental results indicate the instability occurs either directly or indirectly due to the influence of H2O within the passivation oxide above the IGZO channel region. The instability of DG devices was consistently more pronounced, suggesting a reaction between H2O and the aluminum top-gate electrode resulting in the liberation of monatomic hydrogen. This interpretation is supported by complementary experiments on MOS capacitors fabricated on bulk silicon substrates. The application of an alumina capping layer deposited by atomic layer deposition (ALD) was successful in improving thermal stability, with representative results shown in Fig. 2. While most BG capped devices remained stable over several successive thermal treatments, DG devices demonstrated the onset of degradation with noted variation in both the time required and occurrence frequency. The analysis revealed an unanticipated dependence on channel length, with long-channel devices (L = 48 µm) being much more prone to an early onset of degradation compared to shorter devices (L = 12 µm) as shown in Fig. 2. With H2O remaining as the expected origin of thermal instability, a hypothesis has been developed to explain the observed channel length dependence. The results are consistent with the molybdenum source/drain contact regions acting as gettering sites for H2O molecules during the 400 °C annealing process. The removal of H2O from the back-channel region is a two dimensional process limited by the diffusivity of water, with the process more complete in short channel devices. Remaining H2O molecules at or near the back-channel interface in long-channel devices are able to migrate during thermal stress treatments, establishing complete coverage on devices that experience degradation. Experiments which explore the addition of getter features on or adjacent to the back-channel region, as illustrated in Fig. 3, in addition to localized materials analysis techniques such as secondary ion mass spectroscopy (SIMS) will provide additional evidence either in support or denial of the proposed hypothesis. Figure 1

Hydrogenation of polycyclic aromatic hydrocarbons over a solid superacid

Hydrogenation reactions of some polycyclic aromatic hydrocarbons (PAHs), including anthracene, 9-phenylanthracene (9-PA) and 9,10-diphenylanthracene (9,10-DPA), were carried out under moderate reaction conditions (with the initial hydrogen pressure of 5MPa at room temperature, reaction temperature was 300°C) over a solid superacid (SSA). The results showed that the reactivity of the PAHs decreased in the order of anthracene >9-PA>9,10-DPA. It was found that not only the hydrogenation and the reversible dehydrogenation through hydrogen atom transfer, but also disproportionation reaction by intermolecular hydrogen transfer took place in the process of PAHs reactions. The catalyst proved to be versatile for the hydrogenation of anthracene and 9-PA through biatomic hydrogen transfer (BAHT) and for the hydrocraking of 9,10-DPA by monatomic hydrogen transfer (MAHT). The SSA is able to catalyze the cleavage of the CarCar bridge bond in 9,10-DPA to produce 9-PA and benzene, in which the key step is the generation of the mobile H+ as well as the H+ transferring to the ipso-position of anthracene ring in 9,10-DPA. The SSA has the selectivity to catalyze the hydrogenation or hydrocracking of different PAHs.

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.

Influence of Gas Phase Equilibria on the Chemical Vapor Deposition of Graphene

We have investigated the influence of gas phase chemistry on the chemical vapor deposition of graphene in a hot wall reactor. A new extended parameter space for graphene growth was defined through literature review and experimentation at low pressures (≥0.001 mbar). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy, and dark field optical microscopy, with the latter showing promise as a rapid and nondestructive characterization technique for graphene films. The equilibrium gas compositions have been calculated across this parameter space. Correlations between the graphene films grown and prevalent species in the equilibrium gas phase revealed that deposition conditions associated with a high acetylene equilibrium concentration lead to good quality graphene deposition, and conditions that stabilize large hydrocarbon molecules in the gas phase result in films with multiple defects. The transition between lobed and hexagonal graphene islands was found to be linked to the concentration of the monatomic hydrogen radical, with low concentrations associated with hexagonal islands.

Efficient generation of electron-hole pairs in a selenium p-n junction exposed to atomic hydrogen

It is found that the chemical energy released in the recombination of monatomic hydrogen on the surface of selenium is efficiently transferred to the electron subsystem of the crystal, which is manifested in the generation of electron-hole pairs and excitation of steady-state chemicurrent. Both the stage of adsorption and the stage of association of the hydrogen atoms on the surface contribute to the effect. The atomic hydrogen-selenium system is a very simple one and can be used as a model system in the search for semiconductors and chemical reactions suitable for direct conversion of the chemical energy into electricity.

Annealing dynamics of irradiation-induced defects in high-purity silicon in the presence of hydrogen

A reaction model explaining (i) the hydrogen-mediated transformation of the vacancy-oxygen ($V$O) center into a vacancy--oxygen-hydrogen center ($V$OH${}^{*}$), with an energy level at $0.37\phantom{\rule{0.16em}{0ex}}\mathrm{e}\mathrm{V}$ below the conduction-band edge (${E}_{\mathrm{c}}$), and (ii) the passivation of the divacancy center is presented. $V$OH${}^{*}$ dissociates with a rate of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at $195{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{C}$, causing $V$O to recover after long duration ($>$${10}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{min}$), while a similar evolution occurs at ${300}^{\ensuremath{\circ}}\mathrm{C}$ on a time scale of the order of $10\phantom{\rule{0.16em}{0ex}}\mathrm{min}$. The diffusivity of the monatomic hydrogen used in the model agrees closely with the established values for the diffusivity of protons. After the recovery of $V$O, further annealing at higher temperatures and/or longer durations transforms $V$O into the ``ordinary'' vacancy--oxygen-hydrogen center with an energy level at ${E}_{\mathrm{c}}\ensuremath{-}0.32\phantom{\rule{0.16em}{0ex}}\mathrm{e}\mathrm{V}$ ($V$OH). $V$OH is subsequently transformed into $V$OH${}_{2}$. For temperatures above $250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C},$ two additional hydrogen-related levels occur ($\ensuremath{\sim}0.17$ and $\ensuremath{\sim}$$0.58\phantom{\rule{0.16em}{0ex}}\mathrm{e}\mathrm{V}$ below ${E}_{\mathrm{c}}$) with a one-to-one ratio and a possible association with different charge states of a ${V}_{2}$OH center is discussed.

A fresh look at dense hydrogen under pressure. IV. Two structural models on the road from paired to monatomic hydrogen, via a possible non-crystalline phase

In this paper, we examine the transition from a molecular to monatomic solid in hydrogen over a wide pressure range. This is achieved by setting up two models in which a single parameter δ allows the evolution from a molecular structure to a monatomic one of high coordination. Both models are based on a cubic Bravais lattice with eight atoms in the unit cell; one belongs to space group Pa3, the other to space group R3m. In Pa3 one moves from effective 1-coordination, a molecule, to a simple cubic 6-coordinated structure but through a very special point (the golden mean is involved) of 7-coordination. In R3m, the evolution is from 1 to 4 and then to 3 to 6-coordinate. If one studies the enthalpy as a function of pressure as these two structures evolve (δ increases), one sees the expected stabilization of minima with increased coordination (moving from 1 to 6 to 7 in the Pa3 structure, for instance). Interestingly, at some specific pressures, there are in both structures relatively large regions of phase space where the enthalpy remains roughly the same. Although the structures studied are always higher in enthalpy than the computationally best structures for solid hydrogen - those emerging from the Pickard and Needs or McMahon and Ceperley numerical laboratories - this result is suggestive of the possibility of a microscopically non-crystalline or "soft" phase of hydrogen at elevated pressures, one in which there is a substantial range of roughly equi-enthalpic geometries available to the system. A scaling argument for potential dynamic stabilization of such a phase is presented.

Laser controlled magnetism in hydrogenated fullerene films

Room temperature ferromagnetic-like behavior in fullerene photopolymerized films treated with monatomic hydrogen is reported. The hydrogen treatment controllably varies the paramagnetic spin concentration and laser induced polymerization transforms the paramagnetic phase to a ferromagnetic-like one. Excess laser irradiation destroys magnetic ordering, presumably due to structural changes, which was continuously monitored by Raman spectroscopy. We suggest an interpretation of the data based on first-principles density-functional spin-unrestricted calculations which show that the excess spin from mono-atomic hydrogen is delocalized within the host fullerene and the laser-induced polymerization promotes spin exchange interaction and spin alignment in the polymerized phase.

Ab initio studies of superconductivity in monatomic metallic hydrogen under high pressure

A detailed study of the structural, electronic, dynamical, and superconducting properties in the monatomic metallic hydrogen is presented. At least up to 802 GPa, this phase is stable. Moreover, we find that a strong electron–phonon interaction drives a very high superconducting transition temperature T C ≈ 291.40 K at 539 GPa.

Oxygen vacancies and donor impurities in β-Ga2O3

Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.