Alexander Timmer

Ballbot-type motion of N-heterocyclic carbenes on gold surfaces

Recently, N-heterocyclic carbenes (NHCs) were introduced as alternative anchors for surface modifications and so offered many attractive features, which might render them superior to thiol-based systems. However, little effort has been made to investigate the self-organization process of NHCs on surfaces, an important aspect for the formation of self-assembled monolayers (SAMs), which requires molecular mobility. Based on investigations with scanning tunnelling microscopy and first-principles calculations, we provide an understanding of the microscopic mechanism behind the high mobility observed for NHCs. These NHCs extract a gold atom from the surface, which leads to the formation of an NHC-gold adatom complex that displays a high surface mobility by a ballbot-type motion. Together with their high desorption barrier this enables the formation of ordered and strongly bound SAMs. In addition, this mechanism allows a complementary surface-assisted synthesis of dimeric and hitherto unknown trimeric NHC gold complexes on the surface.

Surface Supported Gold–Organic Hybrids: On‐Surface Synthesis and Surface Directed Orientation

The surface-assisted synthesis of gold-organic hybrids on Au (111) and Au (100) surfaces is repotred by thermally initiated dehalogenation of chloro-substituted perylene-3,4,9,10-tetracarboxylic acid bisimides (PBIs). Structures and surface-directed alignment of the Au-PBI chains are investigated by scanning tunnelling microscopy in ultra high vacuum conditions. Using dichloro-PBI as a model system, the mechanism for the formation of Au-PBI dimer is revealed with scanning tunnelling microscopy studies and density functional theory calculations. A PBI radical generated from the homolytic C-Cl bond dissociation can covalently bind a surface gold atom and partially pull it out of the surface to form stable PBI-Au hybrid species, which also gives rise to the surface-directed alignment of the Au-PBI chains on reconstructed Au (100) surfaces.

Submolecular Imaging by Noncontact Atomic Force Microscopy with an Oxygen Atom Rigidly Connected to a Metallic Probe.

In scanning probe microscopy, the imaging characteristics in the various interaction channels crucially depend on the chemical termination of the probe tip. Here we analyze the contrast signatures of an oxygen-terminated copper tip with a tetrahedral configuration of the covalently bound terminal O atom. Supported by first-principles calculations we show how this tip termination can be identified by contrast analysis in noncontact atomic force and scanning tunneling microscopy (NC-AFM, STM) on a partially oxidized Cu(110) surface. After controlled tip functionalization by soft indentations of only a few angstroms in an oxide nanodomain, we demonstrate that this tip allows imaging an organic molecule adsorbed on Cu(110) by constant-height NC-AFM in the repulsive force regime, revealing its internal bond structure. In established tip functionalization approaches where, for example, CO or Xe is deliberately picked up from a surface, these probe particles are only weakly bound to the metallic tip, leading to lateral deflections during scanning. Therefore, the contrast mechanism is subject to image distortions, artifacts, and related controversies. In contrast, our simulations for the O-terminated Cu tip show that lateral deflections of the terminating O atom are negligible. This allows a detailed discussion of the fundamental imaging mechanisms in high-resolution NC-AFM experiments. With its structural rigidity, its chemically passivated state, and a high electron density at the apex, we identify the main characteristics of the O-terminated Cu tip, making it a highly attractive complementary probe for the characterization of organic nanostructures on surfaces.

Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces

The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.

Intermolecular On-Surface σ-Bond Metathesis

Silylation and desilylation are important functional group manipulations in solution-phase organic chemistry that are heavily used to protect/deprotect different functionalities. Herein, we disclose the first examples of the σ-bond metathesis of silylated alkynes with aromatic carboxylic acids on the Ag(111) and Au(111) surfaces to give the corresponding terminal alkynes and silyl esters, which is supported by density functional theory calculations and further confirmed by X-ray photoelectron spectroscopy analysis. Such a protecting group strategy applied to on-surface chemistry allows self-assembly structures to be generated from molecules that are inherently unstable in solution and in the solid state. This is shown by the successful formation of self-assembled hexaethynylbenzene at Ag(111). Furthermore, it is also shown that on the Au(111) surface this σ-bond metathesis can be combined with Glaser coupling to fabricate covalent polymers via a cascade process.

Substrate-Mediated C–C and C–H Coupling after Dehalogenation

Intermolecular C-C coupling after cleavage of C-X (mostly, X = Br or I) bonds has been extensively studied for facilitating the synthesis of polymeric nanostructures. However, the accidental appearance of C-H coupling at the terminal carbon atoms would limit the successive extension of covalent polymers. To our knowledge, the selective C-H coupling after dehalogenation has not so far been reported, which may illuminate another interesting field of chemical synthesis on surfaces besides in situ fabrication of polymers, i.e., synthesis of novel organic molecules. By combining STM imaging, XPS analysis, and DFT calculations, we have achieved predominant C-C coupling on Au(111) and more interestingly selective C-H coupling on Ag(111), which in turn leads to selective synthesis of polymeric chains or new organic molecules.

Quantitative assessment of intermolecular interactions by atomic force microscopy imaging using copper oxide tips

Atomic force microscopy is an impressive tool with which to directly resolve the bonding structure of organic compounds1–5. The methodology usually involves chemical passivation of the probe-tip termination by attaching single molecules or atoms such as CO or Xe (refs 1,6–9). However, these probe particles are only weakly connected to the metallic apex, which results in considerable dynamic deflection. This probe particle deflection leads to pronounced image distortions, systematic overestimation of bond lengths, and in some cases even spurious bond-like contrast features, thus inhibiting reliable data interpretation8–12. Recently, an alternative approach to tip passivation has been used in which slightly indenting a tip into oxidized copper substrates and subsequent contrast analysis allows for the verification of an oxygen-terminated Cu tip13–15. Here we show that, due to the covalently bound configuration of the terminal oxygen atom, this copper oxide tip (CuOx tip) has a high structural stability, allowing not only a quantitative determination of individual bond lengths and access to bond order effects, but also reliable intermolecular bond characterization. In particular, by removing the previous limitations of flexible probe particles, we are able to provide conclusive experimental evidence for an unusual intermolecular N–Au–N three-centre bond. Furthermore, we demonstrate that CuOx tips allow the characterization of the strength and configuration of individual hydrogen bonds within a molecular assembly.Using a rigid tip removes artefacts associated with imaging the strongly varying tip–sample potential of intermolecular sites by atomic force microscopy.

α-Diazo Ketones in On-Surface Chemistry

Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-para-phenylenes is reported. Polymerization on Cu(111) occurs via initial N2 fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,β-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cu-biscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers.

Site-specific adsorption of aromatic molecules on a metal/ metal oxide phase boundary

Nanostructured surfaces are ideal templates to control the self-assembly of molecular structures toward well-defined functional materials. To understand the initial adsorption process, we have investigated the arrangement and configuration of aromatic hydrocarbon molecules on nanostructured substrates composed of an alternating arrangement of Cu(110) and oxygen-reconstructed stripes. Scanning tunneling microscopy reveals a preferential adsorption of molecules at oxide phase boundaries. Noncontact atomic force microscopy experiments provide a detailed insight into the preferred adsorption site. By combining submolecular resolution imaging with density functional theory calculations, the interaction of the molecule with the phase boundary was elucidated excluding a classical hydrogen bonding. Instead, a complex balance of different interactions is revealed. Our results provide an atomistic picture for the driving forces of the adsorption process. This comprehensive understanding enables developing strategies for the bottom-up growth of functional molecular systems using nanotemplates.

Elucidating the Binding Modes of N-Heterocyclic Carbenes on a Gold Surface

Tuning the binding mode of N-heterocyclic carbenes on metal surfaces is crucial for the development of new functional materials. To understand the impact of alkyl side groups on the formation of NHC species at the Au(111) surface, we combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We reveal two significantly different binding modes depending on the alkyl chain length. In the case of a short alkyl substituent, an up-standing configuration with one Au adatom is preferred, whereas the longer alkyl groups result exclusively in NHC-Au-NHC complexes lying flat on the surface. Our study highlights how well-defined structural modifications of NHCs allow for controlling the local binding motif on surfaces, which is important to design designated catalytic sites at interfaces.