Watching the in situ hydrogen diffusion dynamics in magnesium on the nanoscale

Switchable materials that have extreme material contrast and short switching times with negligible degradation can contribute to active plasmonic and nanophotonic systems. In order to understand their supreme properties, researchers must gather in-depth knowledge about nanoscopic processes. In a new study now published on Science Advances, Julian Karst and a team of scientists at the University of Stuttgart, Germany, investigated nanoscopic details of the phase transition dynamics of metallic magnesium (Mg) to dielectric magnesium hydride (MgH₂) using free-standing films to conduct nanoimaging in the lab. The team used characteristic MgH₂ phonon resonance to obtain unprecedented chemical specificity between the material states. The results revealed the nucleation process that occurred during nanocrystalline formation. They measured a faster hydride phase propagation at the nanoscale, compared to macroscopic propagation dynamics. The innovative method offers an engineering strategy to overcome limited diffusion coefficients with substantial impact in order to design, develop and analyze switchable phase transition, hydrogen storage and generation materials.

Materials that maintain prominent metal to insulator phase transitions are prime candidates for switchable optical and nanophotonic systems and have undergone extensive research. Such materials can undergo extreme change of optical properties during transition from a metallic to a dielectric phase to form highly relevant switchable optical and active plasmonic systems. In this work, Karst et al. selected Magnesium (Mg) as the archetypical material system, since it has received wide-spread research mainly in the context of hydrogen storage. In its initial metallic state, magnesium is an excellent plasmonic material. When the element is exposed to hydrogen (H₂), a phase transition occurs from metallic Mg to the dielectric magnesium (di)hydride (MgH₂) to form a highly transparent dielectric material. The MgH₂ phase is reversible to the metallic Mg state in a fully cyclic transition. The concept allows researchers to control and reversibly switch the plasmonic resonances of magnesium nanostructures on and off, for applications in switchable metasurfaces (as Mg-to-MgH₂), dynamic holography or in plasmonic color displays.

During the experiments, the scientists used gold grids precoated with a 2 to 3-nm film of palladium (Pd). The Pd acted as a catalytic layer to split the hydrogen molecules and enable diffusion into the Mg film. The team used Titanium (Ti) to prevent alloying between Mg and Pd, which could have formed a hydrogen diffusion barrier. In the experimental setup, hydrogen gas accessed the free-standing thin films, while Mg remained accessible for scattering-type scanning near-field optical microscopy (s-SNOM) measurements. Karst et al. scanned the tip of the s-SNOM across the exposed Mg surface to observe and investigate time dynamics of hydride formation and diffusion of hydrogen into the film at nanometer resolution. When they exposed the film to hydrogen concentration in two percent nitrogen (N₂), the highly reflective metallic Mg film switched to dielectric MgH₂, which appeared black in color.

The s-SNOM measurement delivered two main quantities, topological information and information on local optical properties relative to the complex dielectric function. The team then raster scanned the atomic force microscopy cantilever within the s-SNOM setup across the sample surface to deliver surface topography. Demodulation and detection techniques allowed them to obtain information on local properties at nanoscale resolution. In order to probe the local properties of the material, Karst et al. illuminated the tip with a strong light field and noted the scattering amplitude to be influenced by changes in the film topography and local properties. However, the scattering phase detected for Mg (blue) and MgH₂ (red) regions showed strong phase contrast due to the characteristic infrared phonon of MgH₂, to depict a distinct signature of hydrogenated areas compared to metallic regions. Based on the findings, Karst et al. further studied hydrogenation of free-standing Mg films by inspecting the scattering phase maps by overlaying the phase maps with grain boundary maps to visualize in situ hydrogen absorption in Mg at selected time steps.

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Further analysis allowed the team to distinguish between nanoscopic and macroscopic hydride phase propagation dynamics in Mg to provide insight to hydrogenation at the scale of the individual grain. Hydrogen diffusion in Mg films depended on the morphology of the material. After each individual grain, hydrogenation of the film stopped, allowing for new nucleation before the next grain transformed. However, even after 60 minutes of hydrogenation, the team observed substantial amounts of pristine metallic Mg on the film surface, which contradicted with previous literature on Mg. Karst et al. credited the behavior to several factors, including the blocking layer formed to halt the vertical hydrogen front progression in the setup, which may have left the surface in a pristine state. They also noted the changing film morphology and film expansion on hydrogen exposure as possible contributing factors.

In this way, Julian Karst and colleagues investigated nanoscale hydrogen diffusion dynamics in the lab using s-SNOM. Based on a characteristic IR phonon resonance of MgH₂, they allowed chemical specificity to track hydride formation, nucleation and lateral growth. The process was highly influenced by the nanoscale morphology of the Mg film, which was also responsible for the slow diffusion of hydrogen throughout the entire film. The team noted how the process of hydrogenation stopped before the entire film switched, leaving areas of metallic Mg in the dielectric MgH₂. The findings have immediate impact for a range of active optical and plasmonic systems using Mg and other transition materials. The work forms an important step forward to enhance and understand the diffusion kinetics, dynamics, and efficiency of phase change across switchable materials.

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