1. Quantum Mechanical Control of Surface Chemical Reactivity
H.H. Weitering, Department of Physics and Astronomy, The University of Tennessee
L.C. Feldman, Department of Physics and Astronomy, Rutgers University
Objective: To identify electronic quantum phenomena in ultrathin Mg films and their effect on hydride formation
The figure above shows STM images at low and high Mg coverage. (a): 100x100 nm2 STM image of a 7 ML Mg film, hydrogenated at low temperature and annealed to room temperature. (b) 400x400 nm2 STM image of a 21 ML film, hydrogenated at low temperature and annealed to room temperature, showing scattered hydride clusters. (c) 3000x3000 nm2 STM image of a 14 ML film, hydrogenated at room temperature.
Summary
Low temperature exposure of ultrathin Mg films to atomic hydrogen results in the formation of a thin MgH2 ‘skin layer.’
The two-dimensional morphology of the hydride layer is stable up to room temperature, only if the Mg film thickness is less than 10 monolayers (ML).
For Mg film thicknesses above 10 ML, the hydride morphology at room temperature is rather complex and strongly depends on the kinetic pathway.
The existence of a ‘critical thickness’ of 10 ML is likely electronic in origin
The figure above shows the Mg 1s core level spectra for various Mg film thicknesses after hydrogenation at ~130 K. The sharp peak on the high kinetic energy side corresponds to the metallic Mg 1s core level; the broader feature next to it corresponds to the Mg 1s core level from the hydride phase. Other features at lower kinetic energy are due to various combinations of hydride and Mg plasmon losses. The surface plasmon losses seen in clean Mg films are absent, meaning that metallic Mg is buried below the hydride layer.