Thomas Gemming

Growth of platinum on TiO2- and SrO-terminated SrTiO3(100)

Abstract Growth by molecular-beam epitaxy of platinum on TiO 2 -terminated and SrO-terminated (100) SrTiO 3 (STO) single-crystal substrates was investigated. TiO 2 -terminated surfaces were produced by wet chemical etching and SrO-terminated surfaces by reactive molecular-beam epitaxy of SrO on TiO 2 -terminated substrates. Growth of both the platinum and SrO layers was monitored in vacuum by reflection high-energy electron diffraction and Auger electron spectroscopy, and the resulting films were characterized by X-ray diffraction, atomic force microscopy, and cross-sectional conventional and high-resolution transmission electron microscopies. The orientation of the platinum films was found to be strongly dependent on the surface termination: cube-on-cube Pt(100) was predominant for the TiO 2 -terminated surfaces, while the SrO-terminated showed a mixture of Pt(100) and Pt(111). The change in film orientation observed is attributed to a smaller degree of bonding across the interface for the SrO-terminated surface.

Iterative structure retrieval techniques in HREM: a comparative study and a modular program package

A retrieval technique for crystal structures using high‐resolution electron microscopy is presented. The inversion of the complex structure‐to‐image relation is performed by numerical optimization of the configuration space of object models. Unlike structure refinement in X‐ray crystallography, the method operates on unknown defect structures on a nanometre scale. The diversity of crystal defects examined and the differences in microscope types and alignment conditions makes it necessary to adapt the basic algorithm to a broad variety of needs resulting in a modular program package for general use. New developments, such as a real space slice function calculation, problem adapted optimization strategies, and finally an examination of iterative matching of image series and exit wavefunctions are presented.

Prismatic ς 3 (1 0 -1 0) twin boundary in α- Al2O3 investigated by density functional theory and transmission electron microscopy

The microscopic structure of a prismatic Σ3 (1010) twin boundary in α-Al 2 O 3 is characterized by combining ab initio local-density-functional theory, electron energy-loss spectroscopy measuring energy-loss near-edge structures (ELNES) of the oxygen K-ionization edge, and high-resolution transmission electron microscopy (HRTEM). Theoretically, two distinct microscopic boundary variants with very low interface energies are derived and analyzed. The interface-projected densities of states (PDOS) calculated for the two variants agree equally well with ELNES, therefore the comparison between experimental ELNES and theoretical PDOS cannot discriminate the one or the other boundary structure. The analysis reveals that the distinction between the metastable interfaces from ELNES is limited by the spatial resolution of the scanning transmission electron microscope used to measure ELNES, not by its energetical resolution. The quantitative analysis of experimental HRTEM images obtained with an atomic-resolution microscope yields that the experimentally observed interface corresponds to the boundary variant with the lowest energy.

Core-hole effect in the ELNES of α-Al2O3: experiment and theory

The occurrence of the core-hole effect at the Al-K and Al-L1 edge in alpha-Al2O3 was studied by comparing experimental electron energy-loss near-edge structures (ELNES) and results of band-structure calculations with and without accounting for core-hole effects by the Z + 1 approximation. It was found that the theoretically calculated unoccupied p-like projected densities of states (PDOS) without Z + 1 approximation matches better to the experimental Al-L1 ELNES, whereas the PDOS with Z + 1 approximation matches better to the experimental Al-K ELNES. We conclude that the localisation of the initial state is an important prerequisite for the observability of the core-hole effect in the ELNES.

Morphology and microstructure of the Ar+ion sputtered (0001)α-Al2O3 surface

Abstract The morphology and microstructure of Ar+-ion bombarded (0001) α-Al2O3 surfaces were studied by employing analytical electron microscopy (AEM) and high-resolution transmission electron microscopy (HRTEM). Surface bombardment with 1 keV Ar+-ions resulted in the formation of a ca. 3-nm thick γ-Al2O3 layer with a high density of structural defects. A well-defined epitaxial orientation relationship between the γ-Al2O3 layer and the substrate was observed: (0001)α ∥(111)γ, [10 1 0]α ∥[110]γ, and [11 2 0]α ∥±[11 2 ]γ.

Validity of the dipole-selection rule for the Al-L2,3 edge of α-Al2O3 under channeling conditions

The validity of the dipole-selection rule for the Al-L2,3 electron energy-loss edge of alpha-Al2O3 is investigated. Dipole forbidden transitions can be observed in a transmission electron microscope operated with large collection apertures. In addition, it is shown that channeling along a highly symmetrical zone axis in alpha-Al2O3 can lead to the detection of dipole forbidden transitions even with small collection apertures. For an incident electron direction parallel to the <0001> orientation it is observed experimentally that the fine structure of the Al-L2,3 edge shows additional features compared to measurements with the electron beam parallel to <1100>. This effect is due to the occurrence of channeling conditions, indicated by the dependence of the additional dipole forbidden features on the sample thickness. These additional features disappear when tilting the crystal by 1.8 degrees (0.84 A(-1)) or even into the less-symmetrical <1100> zone axis. It is suggested that these observations are explained by differences in local symmetry at the excited center with respect to the incident beam directions.