Surface properties of complex intermetallics at the nanoscale : from fundamentals to applications

Abstract : Complex metallic alloys (CMAs) are intermetallic compounds possessing a large unit cell containing several tens to hundreds of atoms. Their structure can be described alternatively by the packing of highly symmetric atomic clusters. Clathrate (or cage) compounds are a new class of CMAs having a crystal structure described by a complex arrangement of covalently-bonded cages. The Ba8Au5.25Ge40.75 type-I clathrate is one such cage compound, whose bulk properties have been (and still are) extensively explored for thermoelectric applications. In fact, it is possible to tune the compound electronic structure by a fine control of its bulk composition. Regarding the properties of the Ba8Au5.25Ge40.75 surface, information remains scarce if not inexistent. However, it is known that the surfaces of CMAs often exhibit interesting surface properties. To this end, we have studied two low-index surfaces: BaAuGe(100) and BaAuGe(110) by a combination of experimental (XPS; LEED; STM) and computational (DFT) methods. Experimental results show no evidence for surface segregation and LEED patterns are consistent with (1x1) bulk terminations with no surface reconstruction. The interplay between the 3D nano-caged structure and 2D surfaces is investigated. We demonstrate that the surface structures of the two surfaces considered preserve the bulk structure cages in addition to an ordered arrangement of surface Ba atoms. The two surfaces are formed by a breakage of highly directional covalent bonds present within the framework, hence leading to destabilizing dangling bonds. Ab initio calculations show that the surface structure is stabilized through electron charge transfer from protruding Ba to surface Ge and Au atoms, saturating the dangling bonds. This charge-balance mechanism lifts the possible surface reconstruction envisaged. We reveal how the surface nanostructuration is surface orientation dependent. The results indicate that the surface electronic structure of BaAuGe(110) is impacted by the Au surface concentration. The surface models for BaAuGe(100) and BaAuGe(110) present a metallic character and low work function values, useful for further applications. Such structurally complex surfaces may also be used as templates for novel nanoscale architectures. Further in this work, we also applied the state-of-the-art surface science techniques to investigate the wetting properties of Al-based CMAs. In these experiments, chemically inert Pb element was used as a metal probe. Systematic analysis is done to find the correlation between the wetting properties and the electronic structure properties of these CMAs. Interfacial energy calculations have been performed to model the Pb/CMA interface based on few approaches reported in literature. We have tested these approaches on a moiré patterned Pb(111)/Al(111) interface. This interface is found to be controlled by geometric factors. Hence, an acquired understanding was applied to Pb deposited on Al13Co4(100) (Al-rich side) interface
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Kanika Anand. Surface properties of complex intermetallics at the nanoscale : from fundamentals to applications. Materials Science [cond-mat.mtrl-sci]. Université de Lorraine, 2018. English. ⟨NNT : 2018LORR0239⟩. ⟨tel-02103063⟩

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