The adatoms, dimers and rest atoms in the three outermost atomic layers of the Si(111)-(7 x 7) surface are directly imaged with glancing Kikuchi electron holography. The applicability of Kikuchi electron holography to complicated multiple-emitter surfaces is evident. The three-dimensional relative positions of atoms on the Si(111)-(7 x 7) surface are in good accordance with the LEED-optimized DAS model.
Three-dimensional atomic images of a Pt(111) surface are obtained by direct inversion of multiple low-energy Kikuchi electron-diffraction patterns. The images are in the backscattering direction. and the positions of the images are consistent with those expected from the atomic structure near the Pt(111) surface. The strong electron scattering of the Pt atoms causes no observable problems in the Kikuchi electron holography.
The structural bases on the metal/semiconductor interfaces, such as gold trimers on the Au/Si (111)(root 3 x root 3)R30 degrees surface and antimony trimers on the Sb/Si(111)(root 3 x root 3)R30 degrees surface, can be imaged directly with a simple inversion of low-energy (<600 eV) Kikuchi-electron patterns (Kikuchi-electron holography-KEH). The relative positions of the building blocks (trimers) on the adsorbates to the substrate atoms are also determined. This short-range-order KEH tool, which provides the 3D Patterson function, can be viewed as a twin of grazing-incidence X-ray diffraction. Using direct structural information obtained by KEH, one can greatly reduce the tested models in a complete trial-and-error structural-determination process.
Hydrogenation-induced metal-semiconductor transitions usually occur in simple systems based on rare earths and/or magnesium, accompanied by major reconstructions of the metal host (atom shifts >2 Angstrom). We report on the first such transition in a quaternary system based on a transition element. Metallic LaMg2Ni absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 which has a nearly unchanged metal host structure (atom shifts <0.7 Angstrom). The transition is induced by a charge transfer of conduction electrons into tetrahedral [NiH4](4-) complexes having closed-shell electron configurations.
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