MgHx thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented.

The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH2 at 5.6 eV. The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgHx thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.

The optical switching from shiny metallic to "black" absorbing states of 30 nm Mg2Ni/50 nm TM/10 nm Pd (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Pd) trilayer thin films upon hydrogenation is used to determine the hydrogen absorption/desorption kinetics at moderate H2 pressure and room temperature. A strong correlation is observed between these kinetics and the enthalpies of solution of H in TM. A diffusion model based on chemical potentials of hydrogen in metals has been developed that agrees qualitatively with the experimental data, both from kinetics and hydrogen concentration profile points of view. The influences of the solution enthalpy and the diffusion coefficient of H in TM and that of the TM layer thickness are discussed with respect to this model. Finally, we conclude with potential applications of these trilayers, from optical hydrogen detection to reinvestigation of diffusion coefficients of H in TM.

Mg_y Ni (2 ≤ y ≤ 10) thin films covered with a Pd cap layer are hydrogenated in 1 0⁵ Pa H₂ between room temperature and 80 ° C and their dielectric function over(ε, ̃) is determined from reflection and transmission measurements. The hydrogenated Mg_y NiH_x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg₂ NiH₄ host takes place at least for y ≤ 3.5 while no signature of MgH₂ is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg₂ NiH₄ (energy gap E_g = 1.6 eV) and MgH₂ (E_g = 5.6 eV) is expected.

The hydrogenation of metallic Mg2 Ni films was shown to proceed via a self-organized double layering of transparent Mg2 NiH4 and metallic Mg2 NiH0.3. For stoichiometries departing from Mg2 Ni we conclude from optical reflection, transmission and electrical measurements that the hydrogenation process involves two competing effects: (i) the nucleation of the initial Mg2 NiH4 layer near the substrate and the ensuing growth of this layer and (ii) the slow random nucleation of the same phase within the remaining part of the film. Which process dominates depends critically on the stoichiometry of the parent metal alloy.

A triple layer thin film (30 nm Mg2Ni/100 nm Ti/10 nm Pd sputtered on glass) switches reversibly from a shiny metallic to a "black" state upon exposure to moderate hydrogen pressure (≈5.103 Pa). This black state resembles that obtained in thick Mg2NiHx layers and has the great advantage of being stable and easily controlled. Both the reversible high optical contrast (R reflective/Rblack ≈ 10 in the red wavelength range) and the fast kinetics of hydrogen absorption and desorption make this material interesting for applications as optical hydrogen sensors.

Hydrogen absorption by a thin Mg2Ni film capped with Pd results in the nucleation of the Mg2NiH4 phase at the film/substrate interface. On further hydrogenation, a self-organized two-layer system consisting of a Mg2NiH0.3/Mg2NiH4 bottom-layer and a Mg2NiH0.3 top-layer is formed. This leads to an intermediate optical black state in Mg2Ni thin films, which transforms from metallic/reflective to semiconducting/transparent upon hydrogenation. This hydrogen absorption behavior is completely unexpected, since the hydrogen enters the film through the Pd-capped film surface. To explain the preferential nucleation of Mg2NiH4 at the substrate/film interface, we determine the chemical homogeneity of these thin films by RBS and SIMS. Furthermore by STM, TEM and SEM, we analyze the microstructure. We find that up to a film thickness of 50 nm, the film consists of small grains and clusters of small grains. On further growth, a columnar structure develops. We propose that the nucleation barrier for the formation of the Mg2NiH4 phase is smaller for the small loosely packed grains at the interface, while the columnar grain boundaries promote the hydrogen diffusion to the substrate.

Mg2NiH4 thin films have been prepared by activated reactive evaporation in a molecular beam epitaxy system equipped with an atomic hydrogen source. The optical reflection spectra and the resistivity of the films are measured in situ during deposition. In situ grown Mg2NiH 4 appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg2NiH4 phase is kinetically blocked. Hydrogen desorption only takes place when a Pd cap layer is added. The optical band gap of the in situ deposited Mg2NiH4 hydride, 1.75 eV, is in good agreement with that of Mg2NiH4 which has been formed ex situ by hydrogenation of metallic Pd capped Mg2Ni films. The microstructure of these in situ grown films is characterized by a homogeneous layer with very small grain sizes. This microstructure suppresses the preferred hydride nucleation at the film/substrate interface which was found in as-grown Mg2Ni thin films that are hydrogenated after deposition.

Mg-Al thin films with a compositional gradient are co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. We study their hydride formation by monitoring their optical transmission during hydrogenation under defined pressure and temperature conditions. We find that Mg(AlH 4)2 is already formed from the elements at p(H2)=1 bar and T=100 ° C. A thin layer of Ti acts as a catalyst. Doping of Mg-Al with Ti has a negative influence on hydrogen absorption.

A novel optical technique to measure catalytic activity for hydrogen sorption is presented. The catalytic activity of the noble metals of Pd, Ni, Cu and Ag is studied as function of pressure and temperature. The uptake rate is limited by chemisorption as deduced by modelling kinetics. An empirical relation between uptake rate and the measured activation energy is given. The catalytic activity of NbOx is studied as a function of the oxygen concentration determined from electron spectroscopy. The measured activation energy depends strongly on the oxygen concentration and reaches a minimum of 0.6 eV at the highest oxygen content (x=2.5, i.e. Nb2O5). Pure Nb does not display a significant catalytic effect.

Hydrogen absorption by a Pd capped thin Mg2Ni film results in the nucleation of the Mg2NiH4 phase at the film/substrate interface and thus induces a self-organized two-layer system. This leads to the optical black state in Mg2Ni thin films upon hydrogenation. This unusual hydrogenation behaviour is completely unexpected since the hydrogen enters through the top film surface. To explain the nucleation of Mg2NiH4 close to the substrate/film interface we performed scanning tunneling microscopy (STM) on as-prepared Mg2Ni films with various thicknesses (20–150 nm). For films thinner than 50 nm, the film consists of small grains and clusters of small grains whereas on further growth the grain size increases and a columnar microstructure develops. We propose, therefore, that close to the substrate, the relatively porous structure of the film with small Mg2Ni grains locally reduces the nucleation barrier for Mg2NiH4 formation.

Thin films of Mg2NiHx which exhibit a remarkable black state with low reflection over the entire visible spectrum were investigated. The films show zero transmission and a low electrical resistivity. It was shown that such a black state is not explicable for a homogeneous layer. The results show that a large absorption coefficient yields substantial reflection.