Scanning tunneling spectroscopy on graphene nanoislands, iron nanoislands and phase change materials

This thesis is concerned with scanning tunnelling spectroscopy (STS) of graphene nanoisland on Ir(111), Fe nanoisland on W(110) and phase change materials Ge1Sb2Te4. Low-temperature STS is used to map the local density of states of graphene dots supported on Ir(111). Because of a band gap in the projected Ir band structure around the graphene K-point, the electronic properties of the quantum dots are dominantly graphene-like. Comparison with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory show that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moiré potential and a 10% penetration of an Ir surface resonance into the graphene layer. A further interesting aspect of this system is the pure zigzag edges, which may have a spin-polarized edge state. Comparison of spin polarized scanning tunnelling microscopy and spectroscopy measurements with density functional theory calculations are used to determine the termination of the graphene edges and its spin polarization. Spin polarized scanning tunnelling spectroscopy was also used to study the behaviour of the model system Fe nanoisland on W(110) with and without external magnetic field. The investigation was focused on the movement of vortices in an in-plane magnetic-field and hysteretic behaviour of a single-domain island has been observed in all three spatial dimensions. Another interesting material which has great potential for future generations of storage media are the so-called phase change materials based on Ge-Te-Sb alloys. In this work, the scanning tunnelling microscopy and spectroscopy have been employed to reveal the evolution of the band gap and the Fermi level as a function of the annealing temperature for Ge1Sb2Te4. Moreover, large scaled and atomic resolution topography images were recorded to analyse the morphology and the complex atomic structure of this phase change material