Exploring structural, electronic, magnetic, and vibrationalproperties of nanostructures with a scanning tunneling microscope

In this thesis spin-resolved and spin-integrated scanning tunneling microscopy (STM) and spectroscopy (STS) are used to explore structural, electronic, magnetic, and vibrational properties of nanostructures on surfaces. STS of the differential conductance is performed at constant current and at constant distance. These modes of operation significantly affect peak positions and line shapes in spectra as well as patterns in spatial maps of the differential conductance. A normalization procedure for constant-current data, which relies on experimental current-distance data, is shown to yield spectral information on the local density of states. Using spin-resolved STM the conductance of single-atom contacts comprised of Co and Cr atoms, a Fe-covered W(110) substrate, and Cr-covered tips was investigated. Spin-resolved spectroscopy suggests that Co (Cr) atoms couple ferromagnetically (antiferromagnetically) to the substrate. The single-atom contacts exhibit a clear spin valve effect with higher conductances for parallel alignment of tip and atom magnetizations. Moreover, the magnetization of the chromium-covered tungsten tip was modified by controlled material transfer from the tip to the sample. Using double-layer iron islands on W(110) the spin polarization of the probe was monitored by spectra and maps of the differential conductance and varied between $0.23$ and $-0.21$. Using spin-integrated STM electronic and vibrational properties of single magnetic Co atoms adsorbed on Au(110) were investigated. Cobalt atoms reside in the missing row of the $(2 \times 1)$ surface reconstruction and exhibit a Fano-type resonance in spectra of the differential conductance at the Fermi level. Most likely this resonance is due to the Kondo effect with a Kondo temperature of $(85 \pm 5)$K. Vibrational spectroscopy with a Au-covered W tip, whose apex was modified by a Co atom showed the spectroscopic signature of the symmetric stretch mode at $\pm 31$\,meV. For clean Au-covered tips the vibrational signal stayed below the detection limit. One of the scientific quest among exploring and understanding magnetic materials at the nanometer scale is to fabricate them. For this reason, the missing row reconstructed Au(110) surface is employed as a template to guide cobalt deposition. Scanning tunnelling microscopy reveals chain-like Co assemblies oriented along the missing row direction. While deposition at room temperature leads to chains with a variety of widths and lengths, deposition at $78\,{\rm K}$ gives rise to assemblies with a narrow size distribution. At $78\,{\rm K}$ single cobalt atoms incorporated in the reconstruction rows were resolved providing evidence for an initial stage of surface alloying. Additionally, the experimentally obtained results are modeled with second moment Tight-binding simulations revealing the energetics of various diffusion processes. Moreover, Cesium adsorption structures on Ag(111) were characterized in a scanning tunneling microscopy experiment. At low coverages, atomic resolution of individual Cs atoms is occasionally suppressed in regions of an otherwise hexagonally ordered adsorbate film on terraces. Close to step edges Cs atoms appear as elongated protrusions along the step edge direction. At higher coverages, Cs superstructures with atomically resolved hexagonal lattices are observed. Kinetic Monte Carlo simulations model the observed adsorbate structures on a qualitative level. Finally, it is shown that single Cs atoms induce a resonance appearing just below the threshold of the quasi two-dimensional Shockley-type surface-state continuum. Scanning tunneling spectroscopy analysis and theoretical modeling are used to identify the importance of various contributions to the linewidth and to the decay of this resonance: resonant charge transfer, inelastic transitions, and adsorbate vibrations perpendicular to the surface.