Strukturelle und Spektroskopische Charakterisierung Monomolekularer Schichten von Einzelmolekülmagneten der Mn12-Klasse

In the course of this thesis, the possibility to deposit intact Mn12 single molecule magnets on conductive surfaces has been investigated which is required for a potential application of this class of molecules in future spintronic devices. A comparison of a variety of established or newly developed preparation methods indicates that many of the previous results on Mn12 monolayers may have to be reassessed. On the other hand, a fully reproducible deposition technique based on a ligand exchange reaction was developed. Many of the experimental techniques commonly used for the characterization of Mn12 monolayers, like scanning probe techniques, photoelectron spectroscopy, and synchrotron-based spectroscopic measurements have been investigated with respect to their suitability for a profound corroboration of the integrity of the molecules. The results demonstrate that none of the previously applied investigation techniques is capable of unambiguously clarifying the ability to obtain monolayers of intact Mn12 molecules. In particular, X-ray based investigation techniques were found to be inappropriate for studies on Mn12 monolayers. Nevertheless, the comparison of all results provides strong indications that the structural integrity of Mn12 molecules deposited via a ligand exchange reaction can be preserved. Advanced scanning tunneling spectroscopy modes were used to derive electronic as well as geometric properties of individual Mn12 molecules. The perfect agreement with theoretical calculations and consistent chemical properties substantiate the integrity of the molecules. The results obtained in this thesis lay the foundation for a variety of future magnetic field dependent cryogenic scanning tunneling spectroscopy measurements on Mn12 molecules and also provide novel concepts for the study of individual single molecule magnets in general

X-Ray Spectroscopic Investigations of Zn 0.94 Co0.06 O Thin Films

We investigated Zn0.94Co0.06O thin films on sapphire (0001) substrates with respect to their structural and magnetic properties. X-ray diffraction shows a axis oriented growth and no secondary phases within its resolution. A clear improvement of the crystalline quality was obtained by post annealing under vacuum conditions. Further information about the local electronic structure is obtained by X-ray absorption spectroscopy at the Co L2,3 and the O edge. Magnetic properties were investigated with a superconducting quantum interference device (SQUID) and by X-ray magnetic circular dichroism at the Co L edg2,3e. Both techniques yield mainly paramagnetic behavior of the samples. For low temperatures, an additional small ferromagnetic contribution was observed in SQUID measurements. Several possible origins of this ferromagnetic contribution are discussed

A comparative study on the deposition of Mn12 single molecule magnets on the Au(111) surface

Different approaches to the deposition of Mn12 single molecule magnets on the Au(111) surface and their characterization by a broad variety of techniques are investigated with respect to their suitability for a profound corroboration of the integrity of the Mn12 core. In this context, the most recent improvements in the experimental approaches are presented and the latest results on the electronic properties of Mn12 are linked to each other. The results confirm the high instability of Mn12 single molecule magnets on surfaces and reveal the need for an amendment of the requirements to define the structural integrity of Mn12 molecules on surfaces

Experimental observation of a band gap in individual Mn12 molecules on Au(111)

The authors report on the electronic properties of individual molecules of two Mn12 derivatives chemically grafted on the functionalized Au(111) surface studied by means of ultrahigh vacuum scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from both Mn12 molecules, showing a well defined wide band gap. In agreement with the tunneling spectroscopy results, the bias voltage variation upon scanning leads to apparent height changes of the Mn12 clusters. The authors discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn12 molecules

Electronic transport properties and orientation of individual Mn12 single-molecule magnets

Individual Mn12 single-molecule magnets have been investigated by means of scanning tunneling spectroscopy at room temperature. Current-voltage characteristics of a Mn12 derivative are studied in detail and compared with simulations. A few-parameter scalar model for ballistic current flow through a single energy level is sufficient to describe the main features observed in scanning tunneling spectra of individual Mn12 molecules and offers a deeper insight into the electronic transport properties of this class of single-molecule magnets. In addition, distance-voltage spectroscopy performed on individual Mn12 molecules reveals a possibility to identify the orientation of the molecular easy axis. The results indicate a preferential orientation of the easy axis of the molecules nearly perpendicular to the surface

A possible approach towards spin-polarized transport through single molecule magnets : Mn12 on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0)

The possibility to use the Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) system as a substrate for future spin-polarized transport measurements on Mn12 single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn12 monolayers was studied. The results demonstrate that Mn12 can be deposited on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies

Single-molecule magnets : a new approach to investigate the electronic structure of Mn12 molecules by scanning tunneling spectroscopy

A new approach to the deposition of Mn12 single-molecule magnet monolayers on the functionalized Au(111) surface optimized for the investigation by means of scanning tunneling spectroscopy was developed. To demonstrate this method, the new Mn12 complex [Mn12O12(O2CC6H4F)16(EtOH)4]·4.4CHCl3 was synthesized and characterized. In MALDI-TOF mass spectra the isotopic distribution of the molecular ion peak of the latter complex was revealed. The complex was grafted to Au(111) surfaces via two different short conducting linker molecules. The Mn12 molecules deposited on the functionalized surface were characterized by means of scanning tunneling microscopy showing homogeneous monolayers of highest quality. Scanning tunneling spectroscopy measurements over a wider energy range compared with previous results could be performed because of the optimized Au(111) surface functionalization. Furthermore, the results substantiate the general suitability of short acidic linker molecules for the preparation of Mn12 monolayers via ligand exchange and represent a crucial step toward addressing the magnetic properties of individual Mn12 single-molecule magnets

Identification of linker molecules suited for deposition and study of Mn12 single molecule magnets on Au surfaces

The authors report on a scanning tunneling microscopy/spectroscopy investigation of the possibility to influence the assembly of monolayers of Mn12 single molecule magnets on a functionalized Au(111) surface by using flexible linker molecules. The results corroborate the general suitability of the deposition via ligand exchange reaction but, on the other hand, reveal the need for a compromise between conductivity and flexibility of the linker molecules. The results are discussed with respect to previous attempts [A. Naitabdi et al., Adv. Mater. (Weinheim, Ger.) 17, 1612 (2005)] to deposit ordered monolayers of Mn12 molecules on Au(111)

Scanning tunneling spectroscopy on Mn12 single molecule magnets grafted on Au(111)

We report on the electronic properties of Mn12 molecules chemically grafted on the functionalized Au(111) surface studied by means of scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from Mn12 molecules showing a large region of low conductance around the Fermi energy. In agreement with the tunneling spectroscopy results the bias voltage variation upon scanning leads to apparent height changes of the Mn12 clusters. We discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn12 molecules