Thesis defense of Lukas Kesper

In the past decade, research on 2D materials has expanded massively due to the popularity of graphene. Its superb electronic and mechanical properties have attracted attention, which opened a new field in solid-state physics. Various elements of the carbon group IVA and beyond have been proven to be stable in a single monolayer (ML) formed in a honeycomb structure. Although the chemical engineering of twodimensional elemental materials and heterostructures has been extensively pursued, the fundamental understanding of the synthesis of 2D materials is not yet complete. Structural parameters, such as the corrugation of the honeycomb lattice, called buckling, or the interface structure of a 2D material to the substrate, directly influence its electronic characteristics. In order to proceed with understanding the element-specific growth, this work presents a study on the structural evolution of the promising 2D material germanene on Ag(111). It provides a survey of germanium formations discovered at different layer thicknesses, like the Ag2Ge surface alloy (SAP), the striped phase (SP), and the mixed phase (MP), right up to the arising of quasi-freestanding germanene (QFG). Using robust surface analysis tools like low-energy electron diffraction (LEED), the long-range and high order of the epitaxially grown Ge structures are demonstrated. The internal and interfacial structure of all germanene phases is revealed by utilizing high-resolution x-ray photoelectron spectroscopy (XPS) with synchrotron radiation. Furthermore, a clear distinction between all phases can be drawn by their electronic structure, the same as collecting evidence for chemically freestanding germanene synthesized on Ag(111). The additionally performed x-ray photoelectron diffraction (XPD) allows to present specific models of the atomic and chemical structure of selected germanene phases, focusing on the structural parameters and electronic interaction at the interfaces. Moreover, the SAP is encapsulated with a capping layer of Al2O3 in order to examine its structural development as a buried interlayer upon the encapsulation. This work offers a perspective on synthesizing promising germanene phases on Ag(111) and provides a prospect to protect the structures from ambient conditions.

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