Aligned Insulating Cavities for Metal Atom Binding on Conductive Surfaces

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

The aim of this work is to further the understanding of the important parameters in the formation process of 2D nanostructures and therewith pioneer for novel applications. Such 2D nanostructures can be composed of specially designed organic molecules, which are adsorbed on various surfaces. In order to study true 2D structures, monolayers were deposited. Their properties have been investigated by scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions as well as under ambient conditions. The latter is a highly dynamic environment, where several parameters come into play. Complementary surface analysis techniques such as low-energy electron diffraction (LEED), X-Ray photo-emission spectroscopy (XPS), and Raman spectroscopy were used when necessary to characterize these novel molecular networks. In order to conduct this type of experiments, high technical requirements have to be fulfilled, in particular for UHV experiments. Thus, the focus is on a drift-stable STM, which lays the foundation for high resolution STM topographs. Under ambient conditions, the liquid-solid STM can be easily upgraded by an injection add-on due to the highly flexible design. This special extension allows for adding extra solvent without impairing the high resolution of the STM data. Besides the device, also the quality of the tip is of pivotal importance. In order to meet the high requirements for STM tips, an in vacuo ion-sputtering and electron-beam annealing device was realized for the post-preparation of scanning probes within one device. This two-step cleaning process consists of an ion-sputtering step and subsequent thermal annealing of the probe. One study using this STM setup concerned the incorporation dynamics of coronene (COR) guest molecules into pre-existent pores of a rigid 2D supramolecular host networks of trimesic acid (TMA) as well as the larger analogous benzenetribenzoic acid (BTB) at the liquid-solid interface. By means of the injection add-on the additional solution containing the guest molecules was applied to the surface. At the same time the incorporation process was monitored by the STM. The incorporation dynamics into geometrically perfectly matched pores of trimesic acid as well as into the substantially larger pores of benzentribenzoic acid exhibit a clearly different behavior. For the BTB network instantaneous incorporation within the temporal resolution of the experiment was observed; for the TMA network, however, intermediate adsorption states of COR could be visualized before the final adsorption state was reached. A further issue addressed in this work is the generation of metal-organic frameworks (MOFs) under ultra-high vacuum conditions. A suitable building block therefore is an aromatic trithiol, i.e. 1,3,5-tris(4-mercaptophenyl)benzene (TMB). To understand the specific role of the substrate, the surface-mediated reaction has been studied on Cu(111) as well as on Ag(111). Room temperature deposition on both substrates results in densely packed trigonal structures. Yet, heating the Cu(111) with the TMB molecules to moderate temperature (150 °C) yields two different porous metal coordinated networks, depending on the initial surface coverage. For Ag(111) the first structural change occurs after annealing the sample at 300 °C. Here, several disordered structures with partially covalent disulfur bridges were identified. Proceeding further in the scope of increasing interaction strength between the building blocks, covalent organic frameworks (COFs) were studied under ultra-high vacuum conditions as well as under ambient conditions. For this purpose, a promising strategy is covalent coupling through radical addition reactions of appropriate monomers, i.e. halogenated aromatic molecules such as 1,3,5-tris(4-bromophenyl)benzene (TBPB) and 1,3,5-tris(4- iodophenyl)benzene (TIPB). Besides the correct choice of a catalytic surface, the activation energy for the scission of the carbon-halogen bonds is an essential parameter. In the case of ultra-high vacuum experiments, the influence of substrate temperature, material, and crystallographic orientation on the coupling reaction was studied. For reactive Cu(111) and Ag(110) surfaces room temperature deposition of TBPB already leads to a homolysis of the C-Br bond and subsequent formation of proto-polymers. Applying additional heat facilitates the transformation of proto-polymers into 2D covalent networks. In contrast, for Ag(111) just a variety of self-assembled and rather poorly ordered structures composed of intact molecules has emerged. The deposition onto substrates held at 80 K has never resulted in proto-polymers. For ambient conditions, the polymerization reaction of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) was studied by STM after drop-casting the monomer onto the substrate held either at room temperature or at 100 °C. For room temperature deposition only poorly ordered non-covalent arrangements were observed. In accordance with the established UHV protocol for halogenated coupling reaction, a covalent aryl-aryl coupling was accomplished for high temperature deposition. Interestingly, these covalent aggregates were not directly adsorbed on the Au(111) surface, but attached on top of a chemisorbed monolayer comprised of iodine and partially dehalogenated TIPB molecules. For a detailed analysis of the processes, the temperature dependent dehalogenation reaction was monitored by X-ray photoelectron spectroscopy under ultra-high vacuum conditions.

Imidazolium salts (NHC(ewg)HCl) with electronically variable substituents in the 4,5-position (H,H or Cl,Cl or H,NO(2) or CN,CN) and sterically variable substituents in the 1,3-position (Me,Me or Et,Et or iPr,iPr or Me,iPr) were synthesized and converted into the respective [AgI(NHC)(ewg)] complexes. The reactions of [(NHC)RuCl(2)(CHPh)(py)(2)] with the [AgI(NHC(ewg))] complexes provide the respective [(NHC)(NHC(ewg))RuCl(2)(CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring-closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2-0.5 mol % at 80 degrees C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC(ewg) group with 1,3-Me,iPr and 4,5-Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL-NHC)(NHC(ewg))RuCl(2)(CHPh)] and [(FL-NHC(ewg))(NHC)RuCl(2)(CHPh)], with a dansyl fluorophore (FL)-tagged electron-rich NHC ligand (FL-NHC) and an electron-deficient NHC ligand (FL-NHC(ewg)) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore-tagged ligand. In this manner, it was shown for ring-opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC(ewg) ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.

Barrier height of self-assembled monolayers (SAMs) on Au(111) was measured using scanning tunneling microscopy (STM) with a vertical modulation. We obtained nanometer scale STM images of alkanethiol SAMs, and binary component SAM films fabricated by an implantation technique. Local barrier height on the alkanethiol SAM at the depressions and the domain boundaries is larger than that on the domains where the molecular lattice is visible, probably due to the molecular density or orientation. A molecular resolution contrast was obtained at a conjugated molecule embedded in the alkanethiol SAM, reflecting the difference in the molecular species. We found that the tip-sample interaction force affects the barrier height measurements, especially when the tunneling gap is small, e.g., on nonanethiol SAM at the tunneling resistance smaller than 50 GΩ.

This thesis reports on the magnetism of low coordinated Fe and Co atoms in different chemical and electronic environments. The objective of this research was to contribute to the fundamental understanding of the magnetic properties, such as magneto-crystalline anisotropy energy (MAE), orbital and spin magnetic moment, of surface supported transition metal (TM) atoms and one atomic layer thick epitaxial TM films. A detailed knowledge on the interplay of electronic hybridization and magnetic properties is mandatory to elaborate new materials with high perpendicular magnetic anisotropy and acceptable writing field for future use in magnetic recording devices. The magnetic properties of the different experimental systems were investigated by x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the effect of electronic hybridization between isolated 3d TM atoms and different kinds of substrates. First, we considered 4d and 5d TM substrates known for their high magnetic polarizability due to the close onset of ferromagnetism. Deposition of single Fe and Co atoms on Rh(111), Pd(111), and Pt(111) produces an increasing MAE going from Rh to Pd and finally to Pt that we directly correlate to the increasing spin-orbit constant of the substrate interlinking the induced magnetic moment and the MAE. Further we investigated single Fe and Co atoms on two different insulating layers, namely Al2O3/Ni3Al(111) and Cu2N/Cu(001), providing an efficient decoupling of the TM 3d states from the conduction electrons of the substrates. In case of the alumina film the absorption spectra of Fe and Co show a pronounced multiplet structure. On Cu2N the multiplet features of the adatoms are less defined due to the lower thickness of the screening layer. On both insulating layers we find a common out-of-plane easy axis, highly unquenched orbital moments, and giant MAE values of several meV/atom which are due to the formation of strong anisotropic bonds with the O and N atoms, respectively. The second part is dedicated to single atom thick layers of Fe and Co deposited on Pt(111) and Rh(111). We find the MAE to be strongly enhanced compared with bulk and we establish a direct proportionality between the induced magnetic moment of the topmost substrate layer and the MAE. The study of bimetallic Fe1-xCox alloys on platinum reveal a material with high MAE (0.5 meV at the equiatomic composition), large saturation magnetization (3μB to 2μB going from Fe to Co), and perpendicular magnetization easy axis. These characteristics hint at new possibilities for the further miniaturization of the grain size in recording devices. In the last part the theoretical description of the MOKE for ultra-thin films is extended to the transverse geometry and improved analytic formulae for the polar and longitudinal geometry are presented. This finding was used to optimize the Kerr signal for a new MOKE setup which is fully integrated in an ultrahigh vacuum chamber combining in situ MOKE and STM. The setup performance was demonstrated for a continuous film of 0.9 monolayers (ML) Co/Rh(111) with in-plane easy axis and for a superlattice of nanometric double layer Co islands on Au(11,12,12) with out-of-plane easy axis. The detection limit is 0.5 ML for transverse and 0.1 ML for polar Kerr geometry corresponding for island superlattices with the density of Co/Au(11,12,12) to islands composed of about 50 atoms. Moreover, the setup holds two independent ways to measure the MAE which is either by thermally induced magnetization reversal or by applying a torque onto the magnetization by turning the magnetic field. Both ways yield similar results for 1.1 ML Co/Au(11,12,12).

The adsorption and layering of chloro[subphthalocyaninato]boron(III) (SubPc) on Ag(111)
\nhas been studied in detail by means of Scanning Tunneling Microscopy (STM) and photoelectron
\nspectroscopy at room temperature. SubPc is a polar molecule with an aromatic
\n14-�-electron system. An interesting phase behavior is observed for the �rst molecular layer
\nof SubPc growing on Ag(111). At a coverage below � 0:2 monolayer (ML), only a twodimensional
\n(2D) lattice gas is present, whereas at a coverage in the order of 0.2{0.5 ML
\ntwo-dimensional condensed molecular islands are in coexistence with the 2D lattice gas. In
\nthese condensed islands, the molecules self-organize into well ordered overlayers with a honeycomb
\npattern which has a low packing density. At a coverage of 0.6{0.9 ML the molecules
\nself-organize into a 2D hexagonal close packed (hcp) pattern. This hcp pattern is observed in
\ncoexistence with a dense 2D gas phase. For the honeycomb and the hcp pattern, individual
\nmolecules are imaged with sub-molecular resolution. Typically the molecules are imaged as
\ncharacteristic three lobes. The coexistence of the honeycomb pattern with the 2D lattice gas
\nis studied in detail. A dynamic equilibrium of the condensed honeycomb phase with its gas
\nphase by exchange of molecules is observed. The energy barrier for surface di�usion as well
\nas the condensation energy to form 2D islands is estimated and discussed.
\nInteresting and unusual features of the ordered molecular overlayers are observed. For
\neach pattern, the honeycomb and the hcp, only two di�erent orientations with respect to
\nthe Ag(111) substrate exist. For the honeycomb pattern, the two di�erent orientations have
\ndi�erent chirality, although the SubPc molecule itself is achiral. Not only single SubPc
\nmolecules show di�usion but also whole ordered islands exhibit mobility. This becomes evident
\nin the observed 
\nipping process of overlayers, where whole condensed islands change
\ntheir orientation between the two stable orientations of the pattern. In the case of the honeycomb
\npattern, this constitutes a transition between the two chiral layers. Phase transitions
\nfrom the gas phase to the condensed hcp phase and vice versa of whole overlayer islands
\ncould be controlled by the STM tip. In vacancy islands of the Ag(111) surface produced
\nby sputter defects, it is possible to reversible change the phase of the con�ned molecular
\noverlayer by tip induced experiments. The two phases, the hcp phase and the gas phase,
\nexist at the same conditions at room temperature. Therefore one of these two phases is
\nthermodynamically not favored and in a metastable state. A microscopic model based on
\nthe electric �eld between tip and sample is proposed for this tip-controlled phase transition.
\nIn addition to the STM experiments the adsorption geometry and the electronic structure
\nof SubPc on Ag(111) is studied by means of photoelectron spectroscopy. It is observed that
\nthe intact SubPc molecule is mainly physisorbed on the Ag(111) with the Cl towards the
\nAg surface. The �-electron system of the molecule undergoes only slight changes upon
\nadsorption on the Ag surface. In general, the observed features of the molecular overlayers
\nare attributed to the charge distribution of SubPc and to its resulting dipole moment.

The reaction of the amine-stabilized alane (NMe3) ⋅ AlH3 1 with the backbone-saturated N-heterocyclic carbene (NHC) SIDipp (SIDipp=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolidin-2-ylidene) at 0 °C yielded the NHC alane adduct (SIDipp) ⋅ AlH3 2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe3) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {=1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene}, and IiPr {=1,3-di-iso-propyl-imidazolin-2-ylidene}) afforded the NHC-stabilized RER-products (NHC) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NHC) (NHC=IMeMe, IiPrMe, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolin-2-ylidene), SIDipp and ItBu (=1,3-di-tert-butyl-imidazolin-2-ylidene). The compounds 3 ⋅ (NHC) were also obtained starting from (SIDipp) ⋅ AlH3 2 and NHC at room temperature. Heating solutions of (SIDipp) ⋅ AlH3 2 without additional base to 95 °C resulted in decarbonization of the NHC and substitution of the carbene carbon atom with aluminum hydride under loss of ethene. Subsequent dimerization afforded cis-[AlH{μ-N(Dipp)CH2CH2N(Dipp)}]2 4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp) ⋅ AlR3 2R (R=Me, Et) to 145 °C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR2 5R (R=Me, Et).

Reflectance anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM) are used to study the growth of indium (In) on the anisotropic Si(111)−(4 × 1)−In surface at room temperature. RAS shows that epitaxial growth of In is accompanied by the disappearance of the surface optical anisotropy at 1.9 eV which is the fingerprint for the (4 × 1)-In surface reconstruction and the appearance of a large minimum at 1.4 eV which is at the same energy as interband transitions observed on bulk crystalline In. Subsequent spectra taken on the surface, over 3 h, show that this minimum decreases and eventually disappears along with the reappearance of the original RAS signature of the (4 × 1)-In surface. STM of this surface shows elongated, anisotropic In crystal islands on top of a (4 × 1)-In reconstructed surface. Upon annealing the surface to 720 K, the surface reconstruction changes with STM showing regions covered with a phase that resembles the (√7 × √3)-In reconstruction and RAS showing a large positive anisotropy at about 1.5 eV. The epitaxial In islands now show a hexagonal shape, unlike on the (4 × 1) surface. Thus, the growth morphology of the islands is shown to be dependent on the initial surface reconstruction. The authors attribute these findings to Ostwald ripening of the In islands mediated by diffusion, which is dependent on the structurally different In wetting layers on the Si substrate.

Growth of Mn on Cu(100) studied by STM: the c(2 × 2) and pgg(4 × 2) ordered surface alloys

The structural and spectroscopic properties of a Cu(I) complex bearing a methylene-linked bis(N-heterocyclic carbene) ligand, [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) were investigated. X-ray single crystal structure analysis revealed that the complex is binuclear similar to the corresponding silver(I) complex. In [Cu(2)(mu-Me-mbim)(2)](PF(6))(2), cation-pi interaction between copper and the adjacent carbene carbon is observed. On the other hand, the copper-copper interaction is very weak in the crystal and almost negligible in solution. The absorption spectrum of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) in methanol shows a strong absorption band (epsilon = 23 000 dm(3) mol(-1) cm(-1)) and a weaker shoulder (epsilon = 6200 dm(3) mol(-1) cm(-1)) at 261 nm and 300 nm, respectively. From molecular orbital calculations using TD-DFT, these absorption bands are assigned to the metal-centered transitions with some contribution from the NHC orbitals. The powdered sample of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) shows bright blue-green phosphorescence with a high quantum yield (43%). The phosphorescence is of dual-emission character at room temperature with peak maxima at 374 nm and 482 nm whereas it changes to a single emission band centered around 500 nm at 77 K. Molecular orbital calculations indicate that the luminescence derives from the triplet MC and MLCT mixed excited states. A methanolic solution of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) shows yellow-green phosphorescence with a peak maximum at 542 nm. Unlike in the solid state, no dual-emission was observed. These results suggest that the dual emission is caused by differences in the contribution of metal-metal interactions at room temperature in the solid state. The differences in the absorption and emission properties between [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) and the related Cu(I)-diphosphine complex, [Cu(2)(mu-dcpm)(2)](BF(4))(2) are discussed.

Submonolayer and near-monolayer coverages of cyanogen (C2N2) and carbon disulfide (CS2) were independently dosed and studied on Au{111} at 4 K. Low-temperature scanning tunneling microscopy was used to investigate the intermolecular and substrate−adsorbate interactions of each of the two chemical species. Both molecules were found to be weakly physisorbed and to form incommensurate structures with respect to the Au{111} lattice. The superlattice formed by the Au{111} reconstruction was determined to have a pronounced effect on the aggregation of the adsorbates. C2N2 with a high quadrupole moment does not form an ordered structure and is easily moved by the scanning tunneling microscope tip at 4 K. In contrast, CS2, having a smaller quadrupole moment, forms a molecular herringbone lattice structure. The Au{111} reconstruction also influences the direction of the CS2 ordered domains, aligning the CS2 molecular herringbone lattice unit cell in a 3-fold symmetry. These observations are discussed in terms of the current understanding of the dynamics of 2D solid/liquid−gas interfaces.

This thesis deals with self-organized supramolecular structures on metal surfaces and their magnetic and electronic properties. To study the systems, various experimental methods such as Scanning Tunneling Microscopy (STM), X-Ray Absorption Spectroscopy (XAS), and X-Ray Magnetic Circular Dichroism (XMCD) combined with Density Functional Theory (DFT) are applied. We focus on several aspects of supramolecular structures on metal surfaces. First, the role of intermolecular and molecule-substrate interactions are discussed regarding their effect on the self-assembling, charge state, and charge transfer. Second, we are interested in the magnetic state of metal atoms embedded in metal-organic networks and the possibility of producing magnetically coupled supramolecular structures. In the first part, three different systems of charge-transfer materials on a Au(111) surface were examined. In the first case, Na atoms and TCNQ molecules organize into ordered structures accompanied with a charge transfer from the Na atoms to the TCNQ molecules. Thus, the TCNQ molecules are negatively charged with exactly one electron. The unpaired spin can be detected by its interaction with Au(111) surface conduction electrons by a Kondo resonance. Moreover, it could be shown that the singly occupied TCNQ orbital is deformed by the presence of the positively charged Na cations affecting the spatial distribution of the Kondo resonance. When we exchange TCNQ with TNAP molecules we observe that the underlying metal surface plays a significant role on the electronic properties of the TNAP molecules. The TNAP molecules within the Na-TNAP layer are charged with one electron. But only specific TNAP molecules show a spin-Kondo resonance depending on their adsorption site on the Au(111) herringbone reconstruction. This is attributed to adsorption-site dependent molecule- substrate interactions. Replacing the Na atoms by organic TTF molecules also leads to the formation of ordered structures. By the interplay of molecule- molecule and moleculesubstrate interactions the TNAP molecules in the TTF-TNAP system are charged with more than one electron enhancing the charge compared to the bulk compound. The second part of the thesis is focused on supramolecular structures consisting of molecules and transition metal atoms. We show that the network architecture is determined by the choice of the molecules and metal atoms and can be selectively altered. The different network architectures also imply different electronic and magnetic properties. The first described network consists of Cu atoms and T4PT molecules and exhibits a hexagonal structure on both the Ag(111) and Cu(111) surfaces. The identical structural and electronic properties on both surfaces indicate that the formation of the system is mainly ruled by the metal-ligand bonds. Furthermore, the Cu atoms within the network have no magnetic moment. By replacing the Cu with Fe atoms an ordered Fe-T4PT network with a bilayer structure is formed on a Au(111) surface. The first layer exhibits a three- fold bonding node consisting of three pyridine end groups and a central Fe atom. An additional T4PT molecule is located on top of each Fe site. Thus, the Fe atoms are placed in a three-dimensional coordination cavity. The Fe atoms are in a high spin state and exhibit a magnetic anisotropy with an easy-axis out-of-plane orientation. Moreover, for the first time, a ferromagnetic coupling between the Fe atoms in such a system can be observed. The exchange coupling mechanism is presumably based on super-exchange through the T4PT molecules. In the final part the dependence of the supramolecular architecture with respect to the molecular functional end groups is investigated. In this case we could change the network structure from bilayer to single-layer by the adequate by the adequate choice of ending groups. This has also an impact on the network electronic properties.

Molecular arrangement and electrical conduction of self-assembled monolayers made from terphenyl thiols

Recovery of self-assembled monolayer on Au(111)