Thesis defense of Julian Andreas Hochhaus

The rise of two-dimensional materials, triggered by the discovery of graphene, has sparked intense interest in Xenes, monoelemental 2D lattices composed of heavier group-IV elements. Among these, stanene (2D tin) is a particularly promising candidate for next-generation nanoelectronics and spintronics due to its strong spin-orbit coupling and predicted topological properties, such as the quantum spin Hall effect. However, the synthesis of high-quality stanene remains challenging, as the structural evolution of tin (Sn) is highly sensitive to the substrate and growth conditions. The Sn/Au(111) system, while promising, has been the subject of conflicting reports in the literature, with disputed structural models for the observed submonolayer Sn arrangements.

This thesis presents a comprehensive structural and chemical analysis of submonolayer Sn growth on Au(111), combining canning Tunneling Microscopy (STM), Low-Energy Electron Diffraction (LEED), X-ray Photoelectron Spectroscopy (XPS), and X-ray Photoelectron Diffraction (XPD). By correlating chemical state analysis with detailed structural characterization, the complex interplay between surface ordering and interface alloying is resolved. The structural evolution is categorized into two distinct regimes. At coverages below 0.33 ML, Sn adsorption is characterized by weak substrate interactions. A previously unreported, chemically freestanding (2x2) phase is identified at a coverage of 0.28 ML, representing a precursor state for buckled alpha-stanene. Increasing the coverage leads to the formation of a long-range-ordered Au2Sn surface alloy. Using XPD combined with genetic algorithm optimization, this phase is definitively identified as a substitutional alloy with a Rec(26xsqrt(3)) unit cell, resolving long-standing discrepancies in its atomic structure. At higher coverages (up to 0.66 ML), the growth is driven by the interplay between the interface alloy and the Sn adlayer. This work clarifies the nature of the X-phase, previously interpreted as honeycomb stanene or AuSn alloy. Atomically resolved STM reveals that the X-phase is, in fact, a substrate-symmetry-breaking, square-like Sn arrangement growing atop the Au2Sn alloy, interpreted as the onset of beta-Sn (001)-like growth. Furthermore, a novel striped phase was discovered, featuring alternating stripes of ultraflat honeycomb stanene and the square-like Sn arrangement. These nanoribbon-like structures represent the first experimental realization of ultraflat stanene on Au(111) and the first experimental evidence of nanoribbon-like stanene structures. Collectively, these findings provide a comprehensive framework for the submonolayer Sn/Au(111) system, demonstrating its versatility as a platform for realizing diverse low-dimensional structures and laying the groundwork for future topological investigations.