The role of grain boundaries in the mechanism of plasma immersion hydrogenation of nanocrystalline magnesium films

In this paper, attention in focused on the nanostructured magnesium films for hydrogen storage. It is shown that 2 μm thick Mg film is transformed into MgH₂ film under high-flux and fluence hydrogen plasma immersion ion implantation at 450 K for 15 min. All hydrogen desorbs at temperature about 530 K, which corresponds to the decomposition of MgH₂ → Mg + H₂↑. The macroscopic and microscopic observations show that magnesium film undergoes a high deformation and restructuring during hydrogenation-dehydrogenation reaction. The suggested hydrogenation model is based upon the incorporation of excess of hydrogen atoms in grain boundaries of nanocrystalline Mg film driven by the increase in surface chemical potential associated with the implantation flux. The results provide new aspects of hydriding of thin nanocrystalline film materials under highly non-equalibrium conditions on the surface.     

Growth and hydrogenation of ultra-thin Mg films on Mo(1 1 1)

The growth and hydrogenation of ultra-thin magnesium overlayers have been investigated on a Mo(1 1 1) single crystal substrate. For increasing magnesium coverages we observe intermediate stages in the TPD and LEISS profiles, which illustrate the transition from one monolayer to multilayer growth. Hydrogen cannot be adsorbed on magnesium films under UHV conditions. However, when evaporating Mg in a hydrogen background, a hydrogen overlayer is seen to adsorb at the Mg surface, due to the catalytic interaction with the Mo(1 1 1) substrate and subsequent spill-over. We show that two monolayers of Mg are necessary to sustain this purely adsorbed state. Using predissociated hydrogen we show that the hydride formation is self-stabilizing and the hydride only decomposes at a temperature where a considerable desorption of magnesium occurs.     

A high-resolution core-level study of Ni-catalyzed absorption and desorption of hydrogen in Mg-films

Ultra-thin magnesium and magnesium-nickel films grown on Mo(1 1 0) under ultra-high vacuum (UHV) conditions are hydrogenated at room temperature with atomic H and studied by high-resolution core-level photoelectron spectroscopy (PES). For the Mg-film a layer of hydride is formed at the Mg/Mo interface and a sub-mono layer of hydride is formed at the surface, while an intermediate region remained non-hydrogenated. For the Mg-Ni-film a thick layer of surface hydride was detected, showing that Ni enhances formation of Mg-hydride at the surface. During the heat-up of the hydrogenated Mg-Ni-film hydrogen desorption started at 400 K.     

Effect of oxygen on the hydrogenation properties of magnesium films

The effect of magnesium oxide on the magnesium and hydrogen desorption properties of magnesium films have been investigated. We find that by capping metallic magnesium films with oxide overlayers the apparent desorption energy of magnesium is increased from 146 kJ/mol to 314 kJ/mol. The results are discussed in light of previous investigations of ball-milled magnesium powders.     

Interaction of hydrogen with InN thin films elaborated on InP(1 0 0)

III-V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 °C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment.     

Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films

H₂ interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes ΔΦ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10⁻³ Pa, increasing the work function. Under higher H₂ pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.     

Electrodeposited metals at conducting polymer electrodes: I—Effect of particle size and film thickness on electrochemical response

Conducting polymers are electrochemically polymerized at platinum electrode substrates. The thickness, porosity and surface morphology of the resulting films are controlled by the charge passing during electropolymerization step and the synthesis conditions. The polymer films are modified by electrochemically depositing platinum particles. The technique of deposition depends on applying a programmed potential pulse at the polymer film from a solution containing platinum complex that resulted in the formation of platinum particles of controlled size and distribution. The effect of changing the size of platinum particles and polymer film thickness on the voltammetric behavior of the resulting hybrid material showed noticeable changes in the electro-catalytic current in acid medium. On the other hand, the electrochemical impedance spectroscopy experiments showed that diffusion and charge-transfer rate increased in the order: unmodified polymer films, thin polymer films containing small size/amount of platinum particles and relatively thick polymer films containing larger size/amount of platinum particles. The morphology of polymer films, size and distribution of platinum particles in the film were studied by scanning electron microscopy. The presence of platinum and its distribution over the film surface was confirmed from the X-ray dispersive analysis and surface mapping. The hybrid materials are good candidates for the application in devices for exchange of hydrogen ions.     

MgH2(110)表面Pd单原子催化氢脱附反应的第一性原理研究

本文采用第一性原理密度泛函理论计算研究了MgH₂(110)表面吸附单原子Pd后的氢脱附反应. 计算发现,在吸附一个Pd单原子后,MgH₂(110)表面氢脱附反应的能垒可以从1.802 eV显著地降低到1.154 eV,表明Pd单原子对于氢脱附具有很强的催化效应. 并且,Pd单原子催化还可以将氢脱附的温度从573 K显著地降低到了367 K,从而使MgH₂(110)表面的氢脱附反应更加容易和快速地发生. 此外,通过MgH₂(110)表面氢溢出机制的反向过程来讨论了氢脱附反应的微观过程. 该研究表明Pd/MgH₂薄膜在未来的实验中可作为良好的储氢材料.     

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H₂) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current-voltage (I-V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I-V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V_N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H₂-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.     

Structure transformations and hydrogen storage properties of co-sputtered MgNi films

The quantity of accommodated and distribution profiles of hydrogen in 1.5-μm thick co-sputtered MgNi films after uptake of hydrogen at 800 kPa pressure within the temperature range 200-250 °C during 1 h, 3 h, 6 h and 72 h are measured. The occurring phase changes are followed by X-ray diffraction measurements at room temperature. We conclude that the hydrogenation process involves two stages: (i) the fast nucleation of the initial Mg₂NiH₄ layer near the substrate interface and (ii) the slow random nucleation of the same phase within the remaining part of the film. The growth of the initial hydride layer may be blocked by the surface oxide barrier layer formed during hydrogenation. We find that hydrogen-induced structural transformations are correlated with oxygen contamination and modify hydrogen storage properties.     

Surface and bulk phenomena in the process of hydride formation in thin films of rare earth metals: Comparison between terbium and europium

Work function changes ΔΦ caused by H₂ interaction with thin terbium and europium films deposited on glass under UHV conditions were correlated with hydrogen uptake and electrical resistance R, measured in situ. For both metals, the course of ΔΦ(H/Me) at room temperature confirms the change in charge-transfer direction on the surface during hydride formation. As a result, the hydrogen adsorbate's nature is changed from positively polarized (precursor state) to negatively polarized adspecies. The hydrogen behavior is significantly different at low temperature due to the formation of the surface ordered low-temperature phase (α′) with positively polarized hydrogen adspecies. This phase, strongly inhibiting penetration of hydrogen into the bulk, is stable up to 100 K for terbium and 160 K for europium. Increasing temperature above these values resulted in additional large absorption of hydrogen. Moreover, differences in the course of R(H/Me) are clearly noticeable between the investigated metals. The resistance of thin TbH_x (x ∼ 3) films in our experiments did not exceed 1 kΩ, however transition of thin metallic europium film into EuH_y (y ∼ 2) increased the resistance up to 10 MΩ. This dissimilarity in electrical behavior can be explained by the coexistence of two factors which are different for the two metals in question: the phase relation of hydrides and the response of thin film to stress generated during hydride formation.     

表面修饰纳米TiO2的贮氢合金电极的光充电行为

采用水解-沉淀法制备了锐钛矿结构的纳米级TiO2,研究了表面修饰TiO2的贮氢合金电极的光充电、循环伏安及交流阻抗特性.结果表明,表面未修饰TiO2的贮氢合金电极在光照下电极电位基本无变化,而表面修饰TiO2的贮氢合金电极在光照下,电极电位向负方向偏移,可达-0.835V,表明在光照射条件下电极表面有氢原子形成.电化学阻抗谱的结果也表明,表面修饰电极在光照时表面有吸附氢存在,并存在氢原子向贮氢合金内部的扩散过程.扫描电镜观察表明,表面修饰TiO2的贮氢合金电极在光充电后产生的氢原子被贮氢合金吸收引起膨胀,导致表面出现大量微裂纹.     

Effects of substrate temperature on the efficiency of hydrogen incorporation on the properties of Al-doped ZnO films

Al-doped ZnO (AZO) transparent conducting films were successfully prepared on glass substrates by RF magnetron sputtering at different substrate temperatures in Ar and H₂ + Ar sputtering ambient. The effects of substrate temperature on the effectiveness of hydrogen incorporation in Al-doped ZnO films were investigated. The microstructural, electrical and optical properties of AZO films were systematically analyzed by surface profiler, X-ray diffractometry, scanning electron microscope, four-point probe measurement and UV/vis spectrophotometer. The XRD patterns and SEM pictures indicate that the crystallinity of AZO thin films was markedly improved with hydrogen incorporation at low substrate temperature, while the improvement of crystallinity was not an obvious change at high substrate temperature. The results also indicate that hydrogen incorporation has the stronger effectiveness on the transparent conductive properties of AZO films with the substrate temperature decreasing. The resistivity of the films decreases, especially for lower substrate temperatures, due to the incorporation of hydrogen atoms. These results suggest that substrate temperature should be controlled to the lower level to effectively reduce resistivity without detriment to transmittance of AZO thin films when hydrogen is incorporated.     

Investigation of the influence of low-concentration hydrogen on the surface potential of thin metallic films for sensor applications

In this paper, a study of the influence of hydrogen (concentrations 6 ppm − 1%) on the work function of thin metallic films at moderately elevated temperatures is presented. The work function was measured indirectly by the observation of the surface potential of dedicated test structures using scanning surface potential microscope. Metallic layers with thicknesses of 10, 20, 30, and 50 nm were deposited on semiconductor substrates as well as on a thick gold layer. The investigations were focused on palladium thin films although a comparison to results obtained for platinum layers was also discussed.     

Two-dimensional hydrogen behaviors on solid surfaces studied by hydrogen microscope

Recently, much work has been done to study hydrogen behavior on solid surfaces for applications in fuel cells, semiconductor devices, and diamond-like carbon films. We have developed a hydrogen microscope making use of electron stimulated desorption (ESD) spectroscopy. A thermal-field emission type electron gun set to a low-energy range (<1 keV) is used to obtain a beam size less than 100 nm in diameter. A pulsed beam has been used to measure the time-of-flight (TOF) to detect desorbed ions from specimen surfaces. Scanning the pulsed beams across solid surfaces, a two-dimensional distribution image of hydrogen atoms can be obtained. This paper reviews some capabilities of the hydrogen microscope and a chemical state analysis for H and O adsorbed by different elements on a surface.     

Effect of hydrogen dilution on the silicon cluster volume fraction of a hydrogenated amorphous silicon film prepared using plasma-enhanced chemical vapor deposition

We investigated the effect of hydrogen dilution on the Si cluster volume fraction of hydrogenated amorphous films by varying the hydrogen dilution ratio at 0.5 Torr and compared it to that obtained at pure silane discharge at 0.3, 0.4, and 0.5 Torr. The correlation between the plasma emission characteristic, deposition rate, and cluster volume fraction in the hydrogen dilution plasma was described. The cluster volume fractions of films under hydrogen dilution conditions were similar to those of the pure silane but showed a higher deposition rate. The results suggest that under hydrogen dilution conditions, it is possible to maintain a higher deposition rate with a lower cluster incorporation rate.     

氮气水合物储氢的激光拉曼光谱研究

近年来,笼型水合物储氢已成为储氢研究的热点之一。采用激光拉曼光谱开展了以氮气水合物为载体的储氢实验研究。在较为温和的条件下(15 MPa, -18 ℃),使合成的氮气水合物与氢气发生反应,对反应产物的拉曼光谱分析结果显示,氢气分子进入到水合物的笼型结构中,并且呈现出多分子的笼占有状态;氮气水合物与氢气的反应时间是影响储氢效果的重要因素。研究结果表明,氮气水合物有希望成为一种有效的储氢介质。     

The microstructure, mechanical and friction properties of protective diamond like carbon films on magnesium alloy

Protective hard coatings deposited on magnesium alloys are believed to be effective for overcoming their poor wear properties. In this work, diamond-like carbon (DLC) films as hard protective films were deposited on AZ91 magnesium alloy by arc ion plating under negative pulse bias voltages ranging from 0 to −200 V. The microstructure, composition and mechanical properties of the DLC films were analyzed by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation. The tribological behavior of uncoated and coated AZ91 magnesium alloy was investigated using a ball-on-disk tribotester. The results show that the negative pulse bias voltage used for film deposition has a significant effect on the sp³ carbon content and mechanical properties of the deposited DLC films. A maximum sp³ content of 33.3% was obtained at −100 V, resulting in a high hardness of 28.6 GPa and elastic modulus of 300.0 GPa. The DLC films showed very good adhesion to the AZ91 magnesium alloy with no observable cracks and delamination even during friction testing. Compared with the uncoated AZ91 magnesium alloy, the magnesium alloy coated with DLC films exhibits a low friction coefficient and a narrow, shallow wear track. The wear resistance and surface hardness of AZ91 magnesium alloy can be significantly improved by coating a layer of DLC protective film due to its high hardness and low friction coefficient.     

Hydrogen Permeability of a Foil of Pd–Ag Alloy Modified with a Nanoporous Palladium Coating

Thin films with the composition Pd–23% Ag are obtained via magnetron sputtering. The magnetron sputtering of Pd–50% Zn films with subsequent diffusion annealing and etching of the active component is used to modify the surfaces of palladium–silver films to improve their hydrogen permeability. Modifying the surfaces of the resulting Pd–Ag films using a nanoporous palladium coating with a predominant distribution of particles ranging from 0 to 50 nm allows a hydrogen flux density of up to 0.4 mmol s^?1 m^?2 to be achieved for sufficiently thin palladium membranes (&lt;10 μm) under conditions of low temperature (&lt;90°C) and pressure (&lt;0.6 MPa). Experimental evidence is gathered that under these conditions, the velocity of hydrogen transport is limited by dissociative–associative processes at membrane boundaries and can be greatly (by an order of magnitude) increased, due to acceleration of the limiting stage of the process via the formation of a palladium nanoporous coating on the film’s surface.     

SrBi2Ta2O9 ferroelectric thin film capacitors: degradation in a hydrogen ambient

The effects of annealing in forming gas 5% hydrogen, 95% nitrogen; FGA) are studied on spin-coated SrBi₂Ta₂O₉ (SBT) thin films. SBT films on a platinum bottom electrode are characterized with and without a platinum top electrode. Films are characterized by residual stress measurements, scanning electron microscopy (SEM), Auger electron spectroscopy (AES), high-temperature X-ray diffraction (HT-XRD) and secondary ion mass spectrometry (SIMS). To determine the degree of strain, lattice constants of Pt are measured by X-ray diffraction (XRD). HT-XRD of blanket SBT/Pt/Ti films in forming gas revealed that the bismuth-layered perovskite structure of SBT is stable up to approximately 500 °C. After formation of an intermediate phase between 550 °C and 700 °C, SBT changes its structure to an amorphous phase. SIMS analysis of Pt/SBT/Pt samples annealed in deuterated forming gas (5% D₂, 95% N₂) showed that hydrogen accumulates in the SBT layer and at the platinum interfaces next to the SBT. After FGA of blanket SBT films, tall platinum–bismuth whiskers are seen on the SBT surface. It is confirmed that these whiskers originate from the platinum bottom electrode and grow through the SBT layer. FGA of the entire Pt/SBT/Pt/Ti stack shows two different results. For the samples with a high-temperature annealing (HTA) step in oxygen after top electrode patterning, peeling of the top electrode is observed after FGA. For the samples without a HTA step, no peeling is observed after FGA. The residual stress at room temperature is measured for blanket platinum wafers deposited at different temperatures. It is found that an increase in tensile stress caused by the HTA step in oxygen is followed by a decrease in stress caused by the hydrogen in the forming gas. Without HTA, however, an increase of stress is observed after FGA. PACS 77.84.-s; 81.40.-z; 77.55.+f