Scanning tunneling microscopy and spectroscopy of functional molecules on metal surfaces
This thesis is committed to the study of functional molecules adsorbed on metal surfaces by means of Low Temperature Scanning Tunneling Microscopy. It includes two main parts. In the first part the adsorption geometry of lander molecule (C90H98) on different metal surfaces is discussed. Three conformations as well as chiral structures of lander molecules on the terrace of Cu(100) are observed by STM. Electron scattering quantum chemical calculated results are compared with our experimental data. Lander molecules adsorb at the step edge of Cu(100) with its conducting board parallel to the step, thus preventing good electronic contact between the molecule and the step. The same geometry is found for lander molecules adsorbed at the step of Cu3Au(100) although this substrate yields steps whose height perfectly matches the molecular wire height on terraces. Lander molecules assemble into molecular wires about 10 to 50 nm long on Cu(111) at low coverage. Lander molecules adsorbed at step of Au(788) with specific sites are observed by STM. A further study concentrates on scanning tunneling spectroscopy of lander adsorbed on different substrates. A shift to higher energy for lander empty state peaks on Cu3Au(100), Ag(111), Cu(111) and Au(111) has been found and is due to the fact of work function differences between these substrates. Wider HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) gaps in the spectra of lander molecule on noble (111) substrates compared with Cu3Au(100) are also observed in the spectra data. This is explained as the result of increasing charging energy due to the screening behavior for lander on noble (111) substrates. The second part contains the investigation of Cobalt Phthalocyanine molecule on Cu(111). Coverage dependent supramolecular structures of adsorbed CoPc on Cu(111) has been found by STM. It is due to the delicate balance between local molecule-molecule interaction and molecule-substrate interaction. α-type stacking of CoPc is determined with the help of submolecularly resolved STM images. Evident adsorption-site-dependent STM images of individual CoPc molecule on CoPc monolayer have been found. This could be explained as to the electronic coupling change between the molecule and substrate. And experimentally measured scanning tunneling spectroscopy results of CoPc on Cu(111) are assigned to the molecular orbitals of CoPc.