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Surface Investigations of Novel Materials with Low Energy Ion Scattering

  • Author(s): He, Xiaoxiao
  • Advisor(s): Yarmoff, Jory A
  • et al.
Abstract

Low energy ion scattering (LEIS) is an experimental technique that is of great utility in surface investigations of novel materials. LEIS is uniquely sensitive to the composition of the outermost atomic layer of a solid and can be used to determine the structure of the lattice in the near-surface region of a single crystal. More recently, it has been shown that the neutralization of scattered low energy alkali ions, which occurs via resonant charge transfer (RCT), is sensitive to the localized surface electronic structure.

This thesis further demonstrates that RCT during LEIS reveals correlated-electron behavior at high temperature. The valence electron of a singly charged alkaline-earth ion is a magnetic impurity that interacts with the continuum of many-body excitations in the metal, leading to Kondo and mixed valence resonances near the Fermi energy. The occupation of these resonances is acutely sensitive to the surface temperature, which results in a marked temperature dependence of the ion neutralization. While the neutralization of magnesium (Mg) and gallium (Ga) scattered from polycrystalline gold (Au) shows little dependence on temperature, scattered strontium (Sr) does show an anomalous temperature dependence that provides clear evidence of electron correlations. The difference in neutralization between the alkaline earth Mg and Sr projectiles is discussed in terms of the velocity of the outgoing projectiles.

The Topological Insulator, Bismuth Selenide (Bi2Se3), is comprised of Se-Bi-Se-Bi-Se quintuple layers (QLs), so that Se should terminate the stable surface. After cleaving in ultra-high vacuum at room temperature, however, low energy electron diffraction shows order while low energy Na+ LEIS reveals a Bi-termination. After cleaving at 80 K, the Se:Bi ratio measured by LEIS is enhanced, but slowly decreases to the room temperature value. Density functional theory suggests that a Bi bilayer positioned atop the nominal Se termination is energetically favorable and consistent with ARPES results from the literature. It is thus concluded that Bi2Se3 cleaves between the QLs, but that a thermally activated process leads to the Bi termination. This observation may resolve issues concerning the long-term stability of such materials.

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