Optical properties of amorphous and crystalline magnesium nickel hydride films

Magnesium nickel hydride (Mg-Ni-H) is a metal-hydride semiconductor which could find applications in e. g. solar cells. The material has earlier been investigated for the purpose of windows with controllable transparency, but the published literature describing the optical properties of the material is limited.

Thin-films of magnesium nickel hydride can be prepared in two forms; 1. an amorphous metal hydride resulting from hydrogenation of Mg-Ni films or in-situ deposition at room temperature and 2. a cubic crystalline structure resulting from hydrogentation of Mg-Ni films at high temperature (above 240 degrees C). The cubic crystalline structure resembles the well-known high-temperature (HT) structure of Mg2NiH4, but the structure is stable at lower temperatures also for the thin-film Mg-Ni-H.

In a recent paper* published in Thin Solid Films, we demonstrate that the cubic crystalline structure can be obtained by heating the amorphous films to approximately 250 degrees C. This is maybe not so surprising, since this is the temperature at which the HT structure of Mg2NiH4 normally forms. What is facinating, is that the crystallization treatment can be carried out in air, with lots of reactive oxygen present, and the films do not dehydrogenate or oxidize substantially. The films are therefore much more resistant than we believed in the start of our work.

The appearance of a gradient composition sample of amorphous (upper) and crystalline (lower) Mg-Ni-H films of ~500 nm thickness, deposited on glass. The transparent red and transparent yellow region in the amorphous and crystalline samples, respectively, correspond to the composition of Mg~2NiH~4. The samples are more Mg-rich on the left-hand side and more Ni-rich on the right-hand side. The difference in color demonstrate the change in band gap upon crystallization.

In the same paper we show the dielectric functions of both the amorphous and the crystalline films. The literature contains some scattered information on the optical properties of amorphous Mg-Ni-H, but the methods that have been used are not so strong and the reported dielectric functions are a little speculative. On the crystalline Mg-Ni-H, no values for the optical properties have been reported earlier.

The conclusion with respect to the band gap of these materials is 1.6 eV for amorphous Mg2NiH4 and 2.1 eV for cubic crystalline Mg2NiH4.

*Paper: The dielectric functions and optical band gaps of thin films of amorphous and cubic crystalline Mg~2NiH~4
Arxiv: Download article
Published in: Thin Solid Films
DOI: 10.1016/j.tsf.2012.07.044


For the convenience of future research projects working on Mg-Ni-H, I here give the calculated dielectric functions for amorphous and crystalline magnesium nickel hydride in tabulated format (download .xls sheet):

Thesis and defence of thesis

Last Friday I had the final defence of my thesis, at the Physics Department of the University of Oslo. I had a stressing couple of weeks before the defence, but when I was there it was actually quite a nice experience.

My two opponents were Aline Rougier from CNRS in France and Björgvin Hjörvarsson from Uppsala University, Sweden. Both of my opponents did an excellent job in pointing out the weaknesses and the strengths of my work. We also actually had a quite interesting discussion, especially concerning the photochromic effect in yttrium hydride films, but also on the origin and nature of oxygen in the samples.

Discussions with Björgvin Hjörvarsson and me about the origin of oxygen and chemical reactions in the deposition of thin-film metal hydrides.

To quickly summarize my work: I have been working with the deposition and characterization of thin films of metal hydrides, with the purpose of utilization in solar energy technology. Originally the focus was to develop metal hydride semiconductors for solar cell technology (see blogpost), and results with magnesium nickel hydride (Mg2NiH4) showed that this material had quite interesting properties for this purpose (see blogpost). Further, during my work I made the discovery of a strong photochromic effect in yttrium hydride films. This was the first ever demonstration of photochromism in a metal hydride at ambient conditions (see blogpost), and may thus have relevance for technological applications of photochromic materials.

The presentation I used for presenting my thesis in the public defence. Operate via the forward-backward controls in the bottom of the graphic.

My thesis, entiteled "Thin-film metal hydrides for solar energy applications" can be viewed and downloaded from academia.edu.