| 1. |
- Azofeifa, D. E., et al.
(författare)
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Hydrogen induced changes in the optical properties of Pd capped V thin films
- 2013
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 580:Suppl. 1, s. S114-S118
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Tidskriftsartikel (refereegranskat)abstract
- Optical properties of 50 nm thick single crystal vanadium films deposited on double side polished MgO substrates have been obtained from spectrophotometric measurements. The films were coated with a polycrystalline Pd layer (5 nm thick) to protect them from oxidation and to favor absorption of atomic hydrogen. Electrical resistance was recorded while hydrogen pressure was increased slowly up to 750 mbar keeping temperature constant. Simultaneously, under normal incidence of non-polarized radiation [350-950 nm], transmittance spectra of this Pd/V/MgO system were measured. These were numerically inverted to obtain the spectral behavior of the Pd, V. PdHy, and VHx dielectric functions at 22 and 140 degrees C, and at 750 mbar. Hydrogen concentration in V film was first determined from a resisto-optical method. Finally, we demonstrate the possibility to determine the concentration in the Pd and the V layers independently, solely using the changes in the optical transmission.
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| 2. |
- Baldi, A., et al.
(författare)
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Mg/Ti multilayers : Structural and hydrogen absorption properties
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:22, s. 224203-
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Tidskriftsartikel (refereegranskat)abstract
- Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a "spinodal-like" microstructure with a small degree of chemical short-range order in the atomic distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities. Notwithstanding the large lattice mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. On exposure to H-2 gas a two-step hydrogenation process occurs with the Ti layers forming the hydride before Mg. From in situ measurements of the bilayer thickness Lambda at different hydrogen pressures, we observe large out-of-plane expansions of Mg and Ti layers on hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. On unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces.
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| 3. |
- Blomqvist, Andreas, et al.
(författare)
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Significance of self-trapping on hydrogen diffusion
- 2010
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 105:18, s. 185901-
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Tidskriftsartikel (refereegranskat)abstract
- The diffusion rate of hydrogen in Nb was calculated using ab initio molecular dynamics simulations. At low temperatures the hydrogen is strongly trapped in a local strain field which is caused by the elastic response of the lattice. At elevated temperatures, the residence time (τ) of hydrogen in an interstitial site is not sufficient for fully developing the local strain field. This unbinding of the interstitial hydrogen and the strain field increases the hopping rate (1/τ) at elevated temperatures (>400 K). These results call for a revision of the conceptual framework of diffusion of hydrogen in transition metals at elevated temperatures.
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| 4. |
- Conlon, C. S., et al.
(författare)
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Hard x-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction
- 2019
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 126:7
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Tidskriftsartikel (refereegranskat)abstract
- The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, nondestructive, in-situ measurement ideal for this system. We have thus applied hard x-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 +/- 4 angstrom, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and x-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 angstrom. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision.
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| 5. |
- Droulias, Sotirios A., et al.
(författare)
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Beating effects in multilayer systems studied with neutron reflectometry
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We demonstrate the limitations of a partial, phenomenological tting approach comparedto full simulations, including reection and refraction at all interfaces on theexample of hydrogen loading in ultra thin vanadium layers. Fe/V superlattices areloaded with deuterium and the lattice expansion and deuterium concentration isextracted from neutron reectivity data. We nd a noticeable dierence betweenthe direct extraction of concentrations and bilayer thicknesses from the superlatticepeaks and ts of the full density prole using the Parratt formalism. Our results underlinethe importance of carefully considering the limitations of phenomenologicalapproaches, in order to obtain robust results.
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| 6. |
- Droulias, Sotirios A., et al.
(författare)
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Crystal perfection by strain engineering : The case of Fe/V (001)
- 2017
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 636, s. 608-614
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Tidskriftsartikel (refereegranskat)abstract
- We study the effect of bilayer thickness at fixed volume fraction on the structural quality of Fe/V (001)superlattices. We find that such artificial metallic superlattices can be manufactured with excellent crystalquality and layering up to at least 50 Å in repeat distance (K = LFe +LV). For an intended fixed ratio of theconstituents: LFe/LV= 1/7, out-of-plane coherence lengths comparable to the thicknesses of the sampleswere obtained. We evaluate the strain in- and out-of-plane of both layers as a function of the bilayer thicknessand comment on the growth using the framework of linear elasticity theory. We interpret the stabilityof the superlattice against crystal degradation due to the alternating compressive and tensile strain, yieldingclose to ideal lattice matching to the substrate.
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| 7. |
- Droulias, S. A., et al.
(författare)
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Influence of deuterium-induced volume changes on optical transmission in Fe/V (001) and Cr/V (001) superlattices
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:19
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Tidskriftsartikel (refereegranskat)abstract
- The deuterium-induced changes of the optical transmission in Fe/V (001) and Cr/V (001) superlattices are found experimentally to be dominated by the volume changes of the vanadium layers and thus indirectly linked to concentration. The deuterium-induced expansion is 67% larger in Cr/V 2/14 monolayers (ML) as compared to Fe/V 2/14 ML. This large difference can be explained by a difference in the site of deuterium from tetrahedral in Fe/V to octahedral in Cr/V. First-principles calculations based on this assumption give quantitative agreement with both the measured optical transmission and the deuterium-induced expansion coefficient. Placing hydrogen in the middle of the vanadium layers results in total energies at 0 K that favor tetrahedral occupancy at low concentrations, although the energy difference is of the order of the thermal energy available in the experiments. Hence small changes in strain, defect concentration, and/or vibrational spectrum of the superlattices may tip the balance to octahedral occupancy at low concentrations. Given this link to concentration and the linear scaling, optical transmission can, therefore, be used in a straightforward way to obtain pressure-composition isotherms also in thin metal films that do not undergo metal-insulator transitions upon hydrogenation.
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| 8. |
- Droulias, Sotirios A., et al.
(författare)
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Limitations of the kinematic approximation in neutron reflectivity measurements for the analysis of bilayers
- 2018
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Ingår i: Journal of applied crystallography. - : INT UNION CRYSTALLOGRAPHY. - 0021-8898 .- 1600-5767. ; 51, s. 1556-1563
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Tidskriftsartikel (refereegranskat)abstract
- The limitations of a phenomenological fitting approach compared to simulations of the optical model including reflection and refraction at all interfaces are demonstrated using the example of hydrogen loading in ultra-thin vanadium layers. Fe/V superlattices are loaded with deuterium and the lattice expansion and deuterium concentration are extracted from neutron reflectivity data. A noticeable difference is found between the extraction of concentrations and bilayer thicknesses directly from the superlattice peaks and fits of the density profile using the Parratt formalism. The results underline the importance of carefully considering the limitations of phenomenological approaches, in order to obtain robust results. The limitations of the kinematic approximation for the analysis are discussed in detail.
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| 9. |
- Droulias, Sotirios A., et al.
(författare)
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Tailoring thermodynamic properties of metal hydrides with interfaces
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We report profound changes in the enthalpy of solution and critical temperature of nano-sizedmetal hydrides in proximity to dierent non-absorbing metals. We use Cr/V and Fe/V (001) superlatticesas model systems to map out these eects since the superlattices can be made singlecrystalline,have identical strain states and are fully reversible. The thermodynamic propertiesare determined using isothermal pressure and optical transmission measurements. We determinethe mechanism underlying the changes in the thermodynamic properties as well as the ecacyof optical transmission by utilizing simultaneous neutron reectometry and optical transmissionmeasurements, combined with rst principle calculations. The underlying mechanism, which issupported by quantitative agreement between theory and experiment is found to be rooted in aradically dierent volume expansion in the two systems. The commonly used optical transmissionmethod is found to be linear with concentration in both types of superlattices and is attributed tochanges in electron density, rather than a direct inuence from a redistribution of electronic states.The conclusions are generalized to other metal hydride systems and can open up for exploration ofproximity induced eects in metal hydrides.
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| 10. |
- Droulias, Sotirios A.
(författare)
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The effect of nano-confinement on hydrogen uptake in metallic superlattices
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The absorption of hydrogen is exothermic in vanadium whereas it is endothermic in iron and chromium. Investigations of the hydrogen uptake within Fe/V(001) and Cr/V(001) superlattices allow therefore a detailed exploration of finite size effects and the influence of boundaries on hydrogen absorption. Fe/V(001) and Cr/V(001) superlattices can be grown as single crystal structures with a small mosaic spread, as determined by X-ray reflectometry and diffraction. Furthermore when the thickness ratio of the constituents is kept constant the crystal quality can be retained in the range from a few up to 40 monolayer repeat distances (Λ). Neutron reflectometry was used to simultaneously determine the volume expansion and concentration of hydrogen in the vanadium layers. Large differences are found in the expansion of Fe/V(001) and Cr/V(001) superlattices, in good agreement with density functional theory (DFT) calculations. The findings are consistent with tetrahedral and octahedral site occupancy in Cr/V(001) and Fe/V(001) superlattices, respectively. Full fitting of the reflectivity pattern is required to obtainan accurate measure of expansion if the number of repeats is small. Under these conditions, the shift of the first order superlattice peak can be an inaccurate measure of the volume changes. By using a specially designed neutron scattering chamber, allowing simultaneous neutron and optical transmission measurements, it is found that the optical transmission scales linearly with hydrogen concentration. By comparing the experimental results to ab-initio DFT calculations, it is shown that optical transmission scales with electron density changes in the samples, explaining the linearity with concentration. This change is dominated by the hydrogen induced expansion of the lattices and depends therefore strongly on the site occupancy of the hydrogen. Finally, X-ray diffraction was used to address the local strain fields and the α to β phase transition, typically observed in bulk vanadium. Below 448 K the results are consistent with an α to β phase co-existence, separated along the surface normal of the samples.
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