Nanoscale Hydrogenography on Single Magnesium Nanoparticles.

Active plasmonics is enabling novel devices such as switchable metasurfaces for active beam steering or dynamic holography. Magnesium with its particle plasmon resonances in the visible spectral range is an ideal material for this technology. Upon hydrogenation, metallic magnesium switches reversibly into dielectric magnesium hydride (MgH2), turning the plasmonic resonances off and on. However, up until now, it has been unknown how exactly the hydrogenation process progresses in the individual plasmonic nanoparticles. Here, we introduce a new method, namely nanoscale hydrogenography, that combines near-field scattering microscopy, atomic force microscopy, and single-particle far-field spectroscopy to visualize the hydrogen absorption process in single Mg nanodisks. Using this method, we reveal that hydrogen progresses along individual single-crystalline nanocrystallites within the nanostructure. We are able to monitor the spatially resolved forward and backward switching of the phase transitions of several individual nanoparticles, demonstrating differences and similarities of that process. Our method lays the foundations for gaining a better understanding of hydrogen diffusion in metal nanoparticles and for improving future active nano-optical switching devices.