Fundamental Properties of Atoms and Molecules on Surfaces: a Combined STM and Shot-Noise Study

In this thesis, two different projects are presented that concentrate on complementary aspects of nanoscopic systems. Not only "conventional" scanning tunneling microscopy (STM) is utilized to study the fundamental properties of atoms and molecules, but in order to detect the fluctuations of the tunneling current, a high-frequency amplifier setup is employed. In the first part, Fe-5,10,15,20-tetra-pyridyl-porphyrin molecules were investigated on a Au(111) surface. Due to the flexibility of this molecule, several different islands of the Fe-TPyP molecule selfassemble on the surface. By means of non-contact atomic force microscopy (nc-AFM), we could show that the molecules in the different arrangements also exhibit differences in the intramolecular distortions. Moreover, using scanning tunneling spectroscopy (STS), the magnetic properties of the molecules in the different structures were compared. We showed that the intramolecular distortions influence the ligand field that is acting on the central Fe atom, such that transverse magnetic anisotropy is introduced in the system. Eventually, a spin crossover was observable, which might be ascribed to large intramolecular distortions. Furthermore, an exceptionally large number of additional inelastic excitations were observable in the differential conductance spectra of the Fe-TPyP molecules between 20 meV and 150 meV in some of the molecular arrangements, whereas no signatures of these inelastic excitations were observable on others. By means of high-resolution electron energy loss spectroscopy (HREELS), we could identify these features as excitations of molecular vibrations. By comparing the energy position of the corresponding electronic orbitals that are involved in the excitation process, the differences in the intensity of the vibrations between the different structures could be explained in a simple co-tunneling picture. In the second part of this thesis, the optimization and testing of a complementary measurement technique is described. Here, not the DC current but the current fluctuations, i.e., the shot noise, allow to gain insight into the transport properties of a system. Due to electron correlations, the shot noise of a system contains information about the effective charge e ∗ of the charge carriers, such that fractional charges, Cooper-pair tunneling and spin-polarized transport can be identified from the current fluctuations. As shot-noise measurements require the signal detection at high frequencies, in the order of several hundreds of kHz, a high-frequency amplifier setup has been included into the STM system. A preamplifier is mounted directly at the STM head, where it is working at cryogenic temperatures. A second amplifier is then installed outside the vacuum chamber to further enhance the signal. The characteristics of these amplifiers and the main steps towards achieving a working shot-noise setup are explained in detail in this thesis. Moreover, test measurements were performed on Au adatoms that were created on the Au(111) surface by tip identation. Comparison of our experimental shot noise data with literature shows that the amplifier setup is fully functional and exhibits the necessary sensitivity to detect the tiny current fluctuations. Finally, an outlook on first measurements of the shot noise of cobalt adatoms on the Au(111) surface will be given. As single Co atoms on Au(111) have shown to exhibit distinct magnetic properties depending on their adsorption site, it is an interesting system for shot-noise measurements.