Positioning of Single Co Atoms Steered by a Self-Assembled Organic Molecular Template

The bonding and organization of cobalt atoms on a self-assembled organic molecular template are investigated by low-temperature scanning tunneling microscopy. In a first step, <i>N</i>,<i>N</i>′-diphenyl oxalic amide is deposited on the Ag(111) surface with submonolayer and monolayer coverage, leading to the formation of supramolecular nanogratings and a dense-packed layer, respectively. These templates are exposed to evaporated cobalt at different substrate temperatures in the range of 110 to 240 K. We find that Co always binds on top of the phenyl rings, and thus the realization of Co–phenyl complexes is preferred over metal cluster growth on the bare Ag(111) surface. In the case of the dense-packed template, a large fraction of the provided Co is engaged in the formation of well-defined, uniform monomeric Co-half-sandwich structures. At optimal temperatures in the 180–200 K range, the fraction of monomeric Co species on the template exceeds 80% of the total amount of Co deposited. The temperature-dependent adsorption behavior and monomer fraction are compared with calculations, simulating the site-selective positioning in the diffusionless limit by a hit-and-stick adsorption model. This analysis indicates that the organic template suppresses the clustering tendency inherent to diffusing Co atoms and allows the production of a monomer fraction as high as that for statistical growth in the low-coverage regime

Chemical Transformations Drive Complex Self-Assembly of Uracil on Close-Packed Coinage Metal Surfaces

We address the interplay of adsorption, chemical nature, and self-assembly of uracil on the Ag(111) and Cu(111) surfaces as a function of molecular coverage (0.3 to 1 monolayer) and temperature. We find that both metal surfaces act as templates and the Cu(111) surface acts additionally as a catalyst for the resulting self-assembled structures. With a combination of STM, synchrotron XPS, and NEXAFS studies, we unravel a distinct polymorphism on Cu(111), in stark contrast to what is observed for the case of uracil on the more inert Ag(111) surface. On Ag(111) uracil adsorbs flat and intact and forms close-packed two-dimensional islands. The self-assembly is driven by stable hydrogen-bonded dimers with poor two-dimensional order. On Cu(111) complex structures are observed exhibiting, in addition, a strong annealing temperature dependence. We determine the corresponding structural transformations to be driven by gradual deprotonation of the uracil molecules. Our XPS study reveals unambiguously the tautomeric signature of uracil in the contact layer and on Cu(111) the molecule’s deprotonation sites. The metal-mediated deprotonation of uracil and the subsequent electron localization in the molecule determine important biological reactions. Our data show a dependence between molecular coverage and molecule–metal interaction on Cu(111), as the molecules tilt at higher coverages in order to accommodate a higher packing density. After deprotonation of both uracil N atoms, we observe an adsorption geometry that can be understood as coordinative anchoring with a significant charge redistribution in the molecule. DFT calculations are employed to analyze the surface bonding and accurately describe the pertaining electronic structure

Meta-Positioning of Carbonitrile Functional Groups Induces Interfacial Edge-On Phase of Oligophenyl Derivatives

We present a combined systematic experimental and theoretical study of the supramolecular organization of a carbonitrile-functionalized oligophenyl on Cu(111) and Ag(111) showing multiple self-assembled structures including a novel edge-on phase at room temperature. We were able to follow the coverage dependent assemblies by means of scanning tunneling microscopy, near-edge X-ray absorption-fine-structure spectroscopy, and X-ray photoelectron spectroscopy and supported the analysis of the experimental results by density functional theory calculations. With increasing coverage the molecular orientation of the building blocks changes from nearly coplanar with the substrate to upright standing. With this reorientation, the stabilizing forces behind the respective bonding motives change from noncovalent molecule–molecule attraction governed by the polar functional groups to metal–organic coordination toward the substrate combined with intermolecular π–π interaction

Self-Assembly and Chemical Modifications of Bisphenol A on Cu(111): Interplay Between Ordering and Thermally Activated Stepwise Deprotonation

Bisphenol A (BPA) is a chemical widely used in the synthesis pathway of polycarbonates for the production of many daily used products. Besides other adverse health effects, medical studies have shown that BPA can cause DNA hypomethylation and therefore alters the epigenetic code. In the present work, the reactivity and self-assembly of the molecule was investigated under ultra-high-vacuum conditions on a Cu(111) surface. We show that the surface-confined molecule goes through a series of thermally activated chemical transitions. Scanning tunneling microscopy investigations showed multiple distinct molecular arrangements dependent on the temperature treatment and the formation of polymer-like molecular strings for temperatures above 470 K. X-ray photoelectron spectroscopy measurements revealed the stepwise deprotonation of the hydroxy groups, which allows the molecules to interact strongly with the underlying substrate as well as their neighboring molecules and therefore drive the organization into distinct structural arrangements. On the basis of the combined experimental evidence in conjunction with density functional theory calculations, structural models for the self-assemblies after the thermal treatment were elaborated

Correction to “In Vacuo Porphyrin Metalation on Ag(111) via Chemical Vapor Deposition of Ru₃(CO)₁₂: Mechanistic Insights”

Correction to “In Vacuo Porphyrin Metalation on Ag(111) via Chemical Vapor Deposition of Ru₃(CO)₁₂: Mechanistic Insights

Control of Molecular Organization and Energy Level Alignment by an Electronically Nanopatterned Boron Nitride Template

Suitable templates to steer the formation of nanostructure arrays on surfaces are indispensable in nanoscience. Recently, atomically thin sp²-bonded layers such as graphene or boron nitride (BN) grown on metal supports have attracted considerable interest due to their potential geometric corrugation guiding the positioning of atoms, metallic clusters or molecules. Here, we demonstrate three specific functions of a geometrically smooth, but electronically corrugated, sp²/metal interface, namely, BN/Cu(111), qualifying it as a unique nanoscale template. As functional adsorbates we employed free-base porphine (2H–P), a prototype tetrapyrrole compound, and tetracyanoquinodimethane (TCNQ), a well-known electron acceptor. (i) The electronic moirons of the BN/Cu(111) interface trap both 2H–P and TCNQ, steering self-organized growth of arrays with extended molecular assemblies. (ii) We report an effective decoupling of the trapped molecules from the underlying metal support by the BN, which allows for a direct visualization of frontier orbitals by scanning tunneling microscopy (STM). (iii) The lateral molecular positioning in the superstructured surface determines the energetic level alignment; <i>i.e.</i>, the energy of the frontier orbitals, and the electronic gap are tunable

In Vacuo Porphyrin Metalation on Ag(111) via Chemical Vapor Deposition of Ru₃(CO)₁₂: Mechanistic Insights

Porphyrin molecules offer a very stable molecular environment for the incorporation of numerous metal ions inside their cavity, which enables a plethora of applications. The fabrication and characterization of surface confined metal–organic architectures by employing porphyrins are of particular interest. Here, we report on a comprehensive study of chemical vapor deposition (CVD) of triruthenium dodecacarbonyl as metal precursor for the on-surface metalation of different porphyrin species with Ru under ultrahigh vacuum conditions. By employing synchrotron radiation X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM), we investigated the metalation process and particularly the role of the support: the close packed Ag(111) surface. It was found that the surface is active in the metalation process under the employed conditions: it decomposes the metal precursor and delivers metal centers to the porphyrin macrocycles. The generality of the metalation process is illustrated for tetraphenylporphyrin, its high temperature derivatives, and porphine

Surface-assisted Dehydrogenative Homocoupling of Porphine Molecules

The templated synthesis of porphyrin dimers, oligomers, and tapes has recently attracted considerable interest. Here, we introduce a clean, temperature-induced covalent dehydrogenative coupling mechanism between unsubstituted free-base porphine units yielding dimers, trimers, and larger oligomers directly on a Ag(111) support under ultrahigh-vacuum conditions. Our multitechnique approach, including scanning tunneling microscopy, near-edge X-ray absorption fine structure and photoelectron spectroscopy complemented by theoretical modeling, allows a comprehensive characterization of the resulting nanostructures and sheds light on the coupling mechanism. We identify distinct coupling motifs and report a decrease of the electronic gap and a modification of the frontier orbitals directly associated with the formation of triply fused dimeric species. This new on-surface homocoupling protocol yields covalent porphyrin nanostructures addressable with submolecular resolution and provides prospective model systems towards the exploration of extended oligomers with tailored chemical and physical properties

How Surface Bonding and Repulsive Interactions Cause Phase Transformations: Ordering of a Prototype Macrocyclic Compound on Ag(111)

We investigated the surface bonding and ordering of free-base porphine (2H-P), the parent compound of all porphyrins, on a smooth noble metal support. Our multitechnique investigation reveals a surprisingly rich and complex behavior, including intramolecular proton switching, repulsive intermolecular interactions, and density-driven phase transformations. For small concentrations, molecular-level observations using low-temperature scanning tunneling microscopy clearly show the operation of repulsive interactions between 2H-P molecules in direct contact with the employed Ag(111) surface, preventing the formation of islands. An increase of the molecular coverage results in a continuous decrease of the average intermolecular distance, correlated with multiple phase transformations: the system evolves from an isotropic, gas-like configuration <i>via</i> a fluid-like phase to a crystalline structure, which finally gives way to a disordered layer. Herein, considerable site-specific molecule–substrate interactions, favoring an exclusive adsorption on bridge positions of the Ag(111) lattice, play an important role. Accordingly, the 2D assembly of 2H-P/Ag(111) layers is dictated by the balance between adsorption energy maximization while retaining a single adsorption site counteracted by the repulsive molecule–molecule interactions. The long-range repulsion is associated with a charge redistribution at the 2H-P/Ag(111) interface comprising a partial filling of the lowest unoccupied molecular orbital, resulting in long-range electrostatic interactions between the adsorbates. Indeed, 2H-P molecules in the second layer that are electronically only weakly coupled to the Ag substrate show no repulsive behavior, but form dense-packed islands

Self-Terminating Protocol for an Interfacial Complexation Reaction <i>in Vacuo</i> by Metal–Organic Chemical Vapor Deposition

The fabrication and control of coordination compounds or architectures at well-defined interfaces is a thriving research domain with promise for various research areas, including single-site catalysis, molecular magnetism, light-harvesting, and molecular rotors and machines. To date, such systems have been realized either by grafting or depositing prefabricated metal–organic complexes or by protocols combining molecular linkers and single metal atoms at the interface. Here we report a different pathway employing metal–organic chemical vapor deposition, as exemplified by the reaction of <i>meso</i>-tetraphenylporphyrin derivatives on atomistically clean Ag(111) with a metal carbonyl precursor (Ru₃(CO)₁₂) under vacuum conditions. Scanning tunneling microscopy and X-ray spectroscopy reveal the formation of a <i>meso</i>-tetraphenylporphyrin cyclodehydrogenation product that readily undergoes metalation after exposure to the Ru-carbonyl precursor vapor and thermal treatment. The self-terminating porphyrin metalation protocol proceeds without additional surface-bound byproducts, yielding a single and thermally robust layer of Ru metalloporphyrins. The introduced fabrication scheme presents a new approach toward the realization of complex metal–organic interfaces incorporating metal centers in unique coordination environments