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978-3-8439-0264-9, Reihe Physik
Jens Herwig Brede
Spin-Polarized Scanning Tunneling Microscopy and Spectroscopy of Phthalocyanine Molecules on Surfaces
187 Seiten, Dissertation Universität Hamburg (2011), Softcover, A5
In this thesis spin polarized scanning tunneling microscopy (SPSTM) and scanning tunneling spectroscopy (SPSTS) are used to experimentally address the electronic and magnetic properties of various phthalocyanine molecules adsorbed on different surfaces.
The interaction of single phthalocyanines and the surface is systematically addressed. The molecular orbitals (MOs) of pristine CoPCs are spatially resolved using scanning tunneling microscopy and compared to simulated orbitals of free PCs. In order to study the undisturbed MOs the hybridization of molecular and substrate states must be reduced. The molecules are therefore electronically decoupled with thin buffer layers of sodium chloride. Nevertheless, the underlying metal support still shows an influence on the PCs, which causes CoPC molecules to be anionic when adsorbed on two monolayers (ML) of NaCl on Cu(111), while they are neutral when deposited on two ML of NaCl on Au(111).
The interaction of PC molecules with ferromagnetic surfaces is studied. The adsorption configuration, i.e. the alignment of the PCs with respect to the underlying atomic lattice is determined and the molecular spin polarization is resolved for all configurations. CoPC and FePC molecules that are deposited on a ferromagnetic Fe film show a spin polarization on their central atom, which changes from majority (up) dominated, at negative bias voltages, to minority (down) dominated at positive bias voltages. More interestingly, the paramagnetic MPCs as well as the diamagnetic metal-free PC molecules show an up dominated spin polarization on the organic molecular ligand close to the Fermi edge. The comparison with first-principles calculations (performed by Nicolae Atodiresei et al.) shows that the hybridization of molecular orbitals with exchange-split 3d states creates spin-polarized states which are relevant for transport experiments and for molecular device applications. Moreover, the calculations indicate that CoPC molecules are no longer magnetic when deposited on the Fe film: This is due to a charge transfer between surface and molecule.
When CoPC molecules are deposited on a ferromagnetic Co surface the situation is different: A spin polarized resonance at an energy of about -0.1 eV is present on the central molecular Co atom. The spin polarization has the same sign as the spin polarization of the underlying Co island. This indicates a ferromagnetic coupling of the molecular spin with the underlying ferromagnet.