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.

Workshop on metal hydride films for optoelectronic applications

Last week we had a workshop in our department, bringing together international experts within the field of metal hydrides, thin films and electrochromism in metal hydrides and oxides:

Participants at the workshop, from left: Dag Noreus (Stockholm University), Darius Milcius (LEI, Lithuania), Henrik Fahlquist (Stockholm University), Bernard Dam (TU Delft, The Netherlands), Smagul Karazhanov (IFE), Arve Holt (IFE), Kazuki Yoshimura (AIST, Japan), Sean Erik Foss (IFE), Aline Rougier (CNRS, France), Stefano Deledda (IFE), Trygve Mongstad (IFE), Yasusei Yamada (AIST, Japan), Jan Petter Mæhlen (IFE) and Volodomyr Yartys (IFE).

It was interesting to be able to discuss the opportunities with regards to these relatively new applications of metal hydride films. The two main points of interest was the recent advances in Mg-based metal films for electrochromic and gasochromic windows and the opportunities and questions that the recent discovery of photochromic effect in yttrium hydride films present.

Electrochromic oxides

Aline Rougier from CNRS in France set the background of the workshop with a survey of the status of electrochromism in metal oxide films. Metal oxides are more well-established than metal hydrides as chromogenic materials, and has been applied e. g. in automatic dimming of rear-view mirrors in cars, a technology commercialized by Gentex. Rougier has long experience with WO3 as an optically active element, and emphasized also that the synthesis of transparent conducting films is an integral part of the research challenges for electrochromics. The main challenge for implementation of electrochromics in smart window applications is the durability after thousands of switching cycles and the challenge of producing films that are uniform over a large area. Currently, a large research project under the 7th framework programme in the EU, Innoshade, is working on the development of oxides and organic materials for smart window applications.

Smart windows and hydrogen sensing

Kazuki Yoshimura and Yasusei Yamada from the AIST in Japan represent the currently most active research group on metal hydride films. Metal hydride films have the advantage that they become reflective rather than absorbing in the opaque state, which is favorable for windows that control the indoor solar irradiation and energy flow. In recent reports, the group of Yoshimura has demonstrated several impressive advances on gasochromic and electrochromic metal hydride-based devices. Among them are development of a "4th generation" of metal hydride films (Mg-alkaline earth) for switchable windows, durability tests and enhancement of cyclability from a few hundred cycles to several thousand and a full-scale experiment to test the energy performance of metal-hydride-based switchable mirrors with respect to static windows. The latter demonstrated 34% reduction on energy use for a room with electrochromic windows during a typical Japanese summer day. Several spin-off activities are on the way to be commercialized, as optical hydrogen sensors and a simple but innovative hydrogen detection sheet.

Phase transitions in Mg2HiH4

Dag Noreus from Stockholm university has long experience with Mg-based metal hydrides, working on structure determination of hydrides. In a collaboration with Darius Milcius from the Lithuanian Energy Institute, he has done work with films of Mg2NiH4, with the intention of demonstrating a pressure-sensitive metal-hydride switchable window. The idea was that the allotropes of Mg2NiH4 has very different optical properties, and experiments on powders have shown that phase transitions can be driven by applying mechanical pressure. However, no such response to mechanical pressure has yet been demonstrated for metal hydride films.

From switchable windows to nano-thermodynamics

Bernard Dam from TU Delft in the Netherlands has long experience with optical properties of thin-film metal hydrides, coming from the research group of Ronald Griessen who initiated this activity in 1996. Currently, there are several researchers in his group working on metal hydride films for optical hydrogen sensors. The activity directly related to switchable windows has been relatively low in the recent years. Work of Dam's group has further refined the thin-film metal hydride systems as a smart way to investigate nano-scale effects on the thermodynamics and stability of metal hydrides.

Photochromism in metal hydride films

As mentioned, the photochromic effect of yttrium hydride films was one of the main points of interest at the workshop. This effect was discovered in our group, reported in 2011. The effect is interesting on many levels: Firstly, it demonstratrates a new effect in metal hydrides which is of fundamental interest, something we should definitely learn more about. The properties of the effect suggests that the physical mechanism is different from what is found for photochromic metal oxides. Secondly, there are many reasons why this reaction is relevant for technological applications. It is probable that further research will lead to technological innovation. We hope to be able to further study this effect. Bernard Dam, who has been involved from the start of the work on photochromic yttrium hydride is also very eager to continue the studies.

Thin films of semiconducting magnesium nickel hydride

In a paper that became available online this week*, I describe our experience with making thin films of magnesium nickel hydride by using the method reactive sputtering. I am now getting closer to the objective of my thesis work (making "Scrap metal solar cells").

Magnesium nickel hydride (Mg2NiH4) is long known to be a semiconductor, but nobody has ever really tried to take advantage of that in technological applications. The band gap of this semiconductor is known to be a bit above 1.5 eV, which is quite close to the ideal band gap for a semiconductor in a solar cell.

One of the issues of why magnesium nickel hydride has never really been investigated for this purpose, is that most research on this material has been done on powders. It's quite intuitive that you cannot make a solar cell of a powder (although an innovative company actually is trying). So, it is crucial to be able to make the compound in a suitable form for solar cells; namely in the form of a film. That has been done earlier, but it is usually done by hydrogenation of a metallic magnesium nickel layer through a palladium cap. That type of film is, however, not so easy to work with, as the hydrogen will release from the film if you keep it in air, and you will be left with two layers of metals. Using reactive sputtering, on the other hand, we show that we can make films efficiently and that the films are quite easy to handle afterwards. This is really promising with regards to the actual use of this semiconductor in technology.

A Mg-Ni-H film with Al electrodes for electrical measurements.

* Paper: Magnesium nickel (hydride) thin films deposited by magnetron co-sputtering
Published in Journal of Alloys and Compounds
DOI: 10.1016/j.jallcom.2012.02.155


By the way, see also this nice article about our project that was recently published on the popular science news-site ScienceNordic: http://sciencenordic.com/new-material-solar-cells