Fast Hydrogen-induced Optical And Electrical Transitions Of Mg And Mg-based Alloys Films With Amorphous Structure

Switchable mirrors based on magnesium alloys that can be reversibly switched between a mirror and low-impedance state and a transparent and high-impedance one have attracted great interests in exploring new photoelectric devices, such as H2 sensor and "smart" windows. However, the reversible optical and electrical transitions of Mg film are rather sluggish owing to the low hydrogen diffusion rate inside film. In this paper, we promoted the sluggish hydrogen diffusion inside Mg and Mg-based alloys thin films by amorphization. In addition, the optical and electrical transitions of amorphous Mg and Mg-TM thin films upon hydrogen loading/unloading at room temperature were investigated.200 nm amorphous and crystalline Mg films were deposited onto the substrates without heating process in a 3-gun DC/RF magnetron co-sputtering system. The capped Pd layer was set to be 15 run. The optical and electrical response characteristics of amorphous and crystalline Mg films were compared upon hydrogen loading/unloading. For amorphous Mg film, it takes about 110 s to switch from opaque state to transparent one with an increase of resistivity from 0.81 to 7.14 mΩ·cm and 85 min to return to the opaque state with a decrease of the resistivity from 7.14 to 2.00 mΩ·cm. These response times are much shorter than those of crystalline Mg films. It demonstrates that the optical and electrical response time of Mg film can be greatly reduced by amorphization.After adding Ni to form amorphous MgNix film (x=0.03-0.29), the response times are further reduced as compared to that of amorphous Mg film. With increasing Ni content from 0.03 to 0.3, the response time of MgNix film is generally observed to first decrease, pass through a minimum, and then increase. Among the prepared MgNix films, MgNi0.09 film shows the quickest response, it takes 26 s to switch from opaque state to transparent one with an increase of resistivity from 1.65 to 8.69 mΩ·cm and 22 min to return to the opaque state. On the other hand, with increasing Ni content, the transmittance is gradually reduced and the films have a pale yellow color. To understand the reaction mechanism, the amount of absorbed hydrogen and the optical transmittance of MgNix film were simultaneously monitored during hydrogen loading/unloading. In hydrogen loading process, hydrogen solid solution phase is first formed and then the hydrides phase done, suggesting that amorphous structure prevents the formation of blocking hydride layer at the interface of Pd layer and MgNix layers. For hydrogen unloading, the synchronous decrease of the amount of absorbed hydrogen and transmittance indicates that hydrogen desorbs from the film once the hydrides decompose. Amorphous structure greatly facilitates hydrogen diffusion, resulting in not only preventing the formation of blocking hydrides layers but also changing the rate-controlling step from hydrogen diffusion for crystalline film to the reaction between hydrogen and Mg and/or Mg-Ni phases.Amorphization improves the response characteristics of the films, but MgNix films have a pale yellow color in transparent state. It reports that Mg-Ti films have a color-neutral transparent state and Ti can catalyze the formation and decomposition of MgH2. Thus, we add Ti to amorphous Mg film to accelerate the rate-controlling step-the formation and decomposition of MgH2. The results indicate that:i) the absorption edges of all loaded MgTix films are lower than 300 nm, leading to a color-neutral transparent state, ii) Ti addition shows little influence on hydrogen diffusion but a strong catalytic effect on MgH2 formation and decomposition. Since MgH2 formation and decomposition are the rate controlling step when Ti content is lower than 0.3, amorphous MgTix films show faster optical and electrical responses with increasing Ti content. Among the prepared MgTix films, MgTio.29 film shows the quickest response, it takes 11 s to switch from opaque state to transparent one with an increase of resistivity from 12.4 to 46.8 Ω/□ and 12 min to return to the opaque state, iii) the hydrogen diffusion inside MgTix films is dramatically accelerated by amorphization to form a hydrogen distribution throughout the films before hydrides formation, leading to a transparent state after hydrogen loading.