Unraveling surface enabled phenomena in low-dimensional molecular systems

This thesis focuses on the investigation of on-surface molecular architectures which exhibit extraordinary magnetic and quantum properties originating from the reduced dimensionality at surfaces. Many different combinations of spin-bearing square planar molecules and substrates were used and probed by local techniques as well as by spatial averaging techniques. Probing low-dimensional molecular magnetism by combination of several complementary techniques provides a more complete insight into the subtle interplay of the interactions involved at the surfaces.
The comprehensive study of magnetism of Cr-phthalocyanine molecules supported on several different ferromagnetic and non-magnetic substrates demonstrated how the spin state of such molecules depends on the interaction with the substrate. Also in my work I have shown that the relative orientation of the molecule’s and the substrate’s easy magnetization axes is of great importance, even for molecules which are paramagnetic in the bulk. This is further supported by the example of interactions of Cr-based adsorbates with the Au(111) substrate where, for example, a very strong anisotropy of the Cr magnetic moment is observed. At the same time, the exchange coupling interactions with bare ferromagnetic substrates, Co and Ni are different in both the intensity and sign. These observations indicate that a refinement of the current models describing interface magnetism is needed to understand the peculiar magnetic coupling in these systems. Study of various phthalocyanine molecules on Pb(111) demonstrate the importance of employment of X-ray based techniques to complement the local probe investigations of these spin systems coupled to a superconductor. Although such experiments can drive a system out of the superconductive phase by the presence of a magnetic field, it was shown that some magnetic properties of these molecules won’t depend greatly on whether the system is or is not in the superconducting state. This fact is making X-ray based investigations even more important.
The emergence of interesting magnetic phenomena through intra- and inter-molecular interactions was addressed next. Pilot experiments performed on triply-fused bisporphyrin molecules opened up the field for a new class of molecules containing two spin centers that can be exchanged providing a plethora of possibilities for tuning the molecule’s magnetic properties. Following up on our recent observation of long range 2D ferrimagnetic ordering in heteromolecular checkerboard assemblies of Fe and Mn phthalocyanine molecules supported on Au(111), we performed the experiments with similar binary 2D systems to further glimpse into the role of 3d orbitals, their symmetries and filling in maintaining long range ordering. It was shown that depending on the configuration and filling of their 3d orbitals the metallo-phthalocyanine molecules will interact by the RKKY interaction or not. In addition, I reported on a significant asymmetry in the mixing of hetero molecular layers that is occurring due to the pinning of one of the molecular types to the surface. Surprisingly this process modifies the layer structure of multilayers and therefore needs to be taken into account for on-surface metalation reactions or for the design of spintronic devices.
Further on, different ways of modification of magnetic properties have been investigated. We reported on how spin states of various phthalocyanine molecules can be altered upon exposure to molecular and atomic hydrogen. In the former case, this process is completely reversible, while in the latter case it leads to irreversible changes of both the spin state of the metal center and of the molecule. Also, the ability to induce a Co surface functionalization with both N and Cl adlayers is demonstrated. Here, X-ray Photoelectron Diffraction has been employed to precisely determine interatomic distances in the created functionalized surfaces.
In the last part the importance of development of new preparation/characterization techniques is demonstrated. It is shown how we successfully implemented the technique of deposition of large non-sublimable molecules into the UHV directly from solution, and how we have adapted a detector that is commonly used in time-of-flight mass spectrometry for acquiring fast, time-resolved XAS signal at SIM beamline of the SLS.
In short, this thesis represents a collection of several pieces of a larger scientific puzzle grazing through several aspects of molecular magnetism.