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

Magnetic exchange coupling of a synthetic Co(II)-complex to a ferromagnetic Ni substrate

On-surface assembly of a spin-bearing and non-aromatic porphyrin-related synthetic Co(II)-complex on a ferromagnetic Ni thin film substrate and subsequent magnetic exchange interaction across the interface were studied by scanning tunnelling microscopy (STM), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and density functional theory +U (DFT + U) calculations

Two-Dimensional Calix[4]arene-based Metal-Organic Coordination Networks of Tunable Crystallinity

A flexible and versatile method to fabricate two-dimensional metal-organic coordination networks (MOCNs) by bottom-up self-assembly is described. 2D crystalline layers were formed at the air-water interface, coordinated by ions from the liquid phase, and transferred onto a solid substrate with their crystallinity preserved. By using an inherently three-dimensional amphiphile, namely 25,26,27,28-tetrapropoxycalix[4] arene-5,11,17,23-tetracarboxylic acid, and a copper metal node, large and monocrystalline dendritic MOCN domains were formed. The method described allows for the fabrication of monolayers of tunable crystallinity on liquid and solid substrates. It can be applied to a large range of differently functionalized organic building blocks, also beyond macrocycles, which can be interconnected by diverse metal nodes

Probing the Reactivity of Functionalized Surfaces by Porphyrin Metalation

The presence of N- and Cl-induced superstructures is shown to drastically alter the physicochemical properties of the Cu(001) substrate. We present coherent evidence that N- and Cl-c(2x2) superstructures on Cu(001) decisively impact the metalation reaction of 5,10,15,20-tetraphenylporphyrin (2HTPP) as well as the on-surface diffusion and assembly of this molecule. The N superstructure facilitates the metalation reaction and self-assembled molecular domains of CuTPP are formed at room temperature (RT). In contrast, the Cl superstructure completely inhibits the self-metalation reaction requiring metal atoms to be deposited from the top and causes 2HTPP to assemble into small clusters. A spectro-microscopy correlation approach combining X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM) has been utilized in this study

Ti, Zr, and Hf-based molecular hybrid materials as EUV photoresists

Metal oxoclusters are hybrid inorganic-organic molecular compounds with a well-defined number of metal and oxygen atoms in their cores. This type of materials is a promising platform for extreme ultraviolet (EUV) photoresists: their inorganic cores provide them with tunable EUV absorptivity and their molecular nature might favour smaller resolution and roughness while it also renders specific spectroscopic fingerprints that allow to monitor the chemical changes induced by EUV light. In this work, we compare the EUV photochemistry of metal oxoclusters based on Ti, Zr, and Hf and methacrylate ligands (Mc) and their sensitivity as resist materials for EUV lithography. Decarboxylation processes upon EUV exposure are detected in all cases with ex-situ X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR). However, the structural changes after film deposition and after exposure differed among the three compounds. Higher sensitivity was detected for the Hf-based material than for the Zr-based analogue, in line with its higher absorptivity. XPS analyses suggest that only a small fraction of the carboxylate ligands is lost at the dose-to-gel. This change in the chemical composition is accompanied by an increased structural disorder in the layer and a rather small degree of aggregation, according to grazing incidence X-ray scattering (GIXS). These results indicate that neither a drastic loss of organic shell nor a high degree of aggregation of the naked inorganic cores are required for this type of molecular thin film to reliably operate as a resist material

The fate of bromine after temperature-induced dehydrogenation of on-surface synthesized bisheptahelicene

International audienc

Structural characterization of a covalent monolayer sheet obtained by two-dimensional polymerization at an air/water interface

This work describes a two-dimensional polymerization at an air/water interface and provides, for the first time, direct spectroscopic evidence for the kind of crosslinks formed and for the conversion reached in a covalently bonded monolayer sheet. This evidence was obtained through a combination of a variety of monolayer characterization techniques before and after transfer onto solid substrates, in particular by tip-enhanced Raman spectroscopy (TERS) and TERS mapping after transfer of both the monomer and polymer monolayer onto Au(111). This work is a major advance for the field of 2D polymers synthesized at the air/water interface as it, in principle, allows estimation of the crystallinity by percolation theory and the location of regions with defects.M.M. and T.A.J. acknowledge financial support of the Swiss Nanoscience Institute (SNI) through the Oracle grant (P1308). M.B. and T.A.J. acknowledge the Swiss National Science Foundation (Grant no. 200020-153549 and 206021-113149). Acknowledgement is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. F.S. thanks the Chinese Scholarship Council for a Ph.D. student fellowship.Peer Reviewe

Supramolecular architectures of molecularly thin yet robust free-standing layers.

Stable, single-nanometer thin, and free-standing two-dimensional layers with controlled molecular architectures are desired for several applications ranging from (opto-)electronic devices to nanoparticle and single-biomolecule characterization. It is, however, challenging to construct these stable single molecular layers via self-assembly, as the cohesion of those systems is ensured only by in-plane bonds. We herein demonstrate that relatively weak noncovalent bonds of limited directionality such as dipole-dipole (-CN⋅⋅⋅NC-) interactions act in a synergistic fashion to stabilize crystalline monomolecular layers of tetrafunctional calixarenes. The monolayers produced, demonstrated to be free-standing, display a well-defined atomic structure on the single-nanometer scale and are robust under a wide range of conditions including photon and electron radiation. This work opens up new avenues for the fabrication of robust, single-component, and free-standing layers via bottom-up self-assembly