Freie Universität Berlin
Structural, Electronic and Switching Properties of Metalloporphyrin Molecules on Metal Surfaces
Abstract
dc:description.abstractAs a prototype species, metalloporphyrin molecules are ideal candidates for studying organic/metal interface properties for the sake of future applications of molecular electronics and spintronics. In this thesis, combined scanning tunneling microscopy (STM) and atomic force microscopy (AFM) techniques are utilized for investigating the structural, electronic and magnetic properties of three different metalloporphyrins on noble metal surfaces. We studied the adsorption and self-assembly behavior of manganese- tetra-pyridine-porphy-rin (Mn-TPyP) molecules on a Cu(111) surface. The Mn- TPyP molecules align along the <110> directions and arrange into linear chains. Depending on the annealing temperature of Mn-TPyP/Cu(111) samples and their molecular coverage, three types of molecular chains containing two different molecular conformers with respectively flat and tilted pyridine groups are found on the surface. Within these chains, the molecules with flat pyridine groups are discerned by the number of their coordination bonds to Cu adatoms. Scanning tunneling spectroscopy reveals broad orbital resonances for molecules having flat pyridine groups, indicating strong molecule-surface coupling. In contrast, the molecules with tilted pyridine groups show narrower orbital features due to weaker coupling to the surface. Furthermore, we found both, irreversible and reversible rotation of the pyridine groups of molecules in chains, which is likely activated by resonant electron tunneling through the molecular orbital at +2.7 eV. Magnetic properties of iron octaethylporphyrin chloride (FeOEP-Cl) and iron octaethylporphyrin (FeOEP) molecules on Au(111) surface are tuned by their interaction with Au tips and chlorine (Cl) functionalized tips, respectively. Simultaneously force and (differential) conductance measurements show that the short range repulsive force between a Au tip and a FeOEP-Cl molecule pushes the Fe center toward the Au surface and enhances the Fe-surface hybridization, thus giving rise to a new Kondo peak at the Fermi energy. The repulsive interaction between Cl- terminated tips and FeOEP molecules changes the molecular spin state (oxidation state) from S = 1 (+2) into S = 5/2 (+3), indicated by the Kondo resonance in differential conductance (dI/dV) spectra. The attractive and repulsive interaction between Au tips and FeOEPs can enhance spin excitation and Kondo effect for the molecules, respectively. Xenon (Xe) functionalized tips change the asymmetric dip shape of the dI/dV spectra of FeOEP into spin excitation related step features due to the attractive tip-molecule interaction. Moreover, we confirm the chemical structure of iron tetrabenzoporphyrin generated by temperature induced ring closure reaction of FeOEP on Au(111) surface by NC-AFM measurement. The magnetic state of individual manganese-tetra-phenyl-tert-butyl-porphyrin (Mn-TPTBP) molecules on a Au(111) surface is modified when the Mn(II) center coordinates to CO molecules adsorbed on top. dI/dV spectra show that the asymmetric Kondo peak of Mn-TPTBP changes into a zero-bias anomaly with a superposed line shape consisting of a peak and a dip when the CO-Mn-TPTBP complex is formed. Based on density functional calculation we conclude that the CO molecule reduces the spin of the adsorbed Mn-TPTBP from S = 5/2 to S = 1/2. The new spectral feature on Mn-TPTBP-CO is probably originating from a mixed-valence state of the Mn atom and a vibration of the molecule. The original magnetic state of Mn-TPTBP can be recovered by selective desorption of individual CO molecules from the CO-Mn-TPTBP complex.
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Chen, Xianwen
Subjects
dc:subject × 4Rights
- Licence dc:rights.uri
- Language dc:language
- eng
Identifiers
dc:identifier.*- Identifier URI
- http://dx.doi.org/10.17169/refubium-7161