Hierarchical Assembly and Reticulation of Two-Dimensional Mn- and Ni-TCNQ(x) (x=1, 2,4) Coordination Structures on a Metal Surface

We report a scanning tunneling microscopy (STM) investigation of the self-assembly of two-dimensional metal-organic coordination networks comprised of Mn or Ni atoms (M) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on a Ag(100) surface. The growth is dominated by the coupling of the cyano ligands to the substrate, and this interaction is reduced by lateral coordination of the molecules to metal adatoms. Initially mononuclear M-TCNQ(4) complexes form at room temperature, which hierarchically assemble into reticulated M-TCNQ(2) networks at sufficiently high metal concentrations. M-TCNQ(1) networks with fully Coordinated molecules can be obtained at elevated substrate temperatures and sufficient metal concentration. A commensurate a and an incommensurate beta phase can be distinguished that are unrelated to the antedecent M-TCNQ(4) complexes. The coordination of the cyano groups to metal adatoms alters the balance between lateral interactions and coupling of the molecules to the substrate. This effect is accompanied by distinct changes in the apparent heights of metal centers in the STM data indicating changes in their electronic configuration.

Bonding and Charge Transfer in Metal-Organic Coordination Networks on Au(111) with Strong Acceptor Molecules

Nan Jiang, Klaus Kern, Alexander Langner, Sebastian Stepanow

The geometric and electronic structure of two structurally similar metal organic networks grown on the Au(111) surface is investigated by scanning tunnelling microscopy (STM) and spectroscopy (STS) combined with density functional theory (DFT) calculations. The networks are composed of (i) F4TCNQ (C12F4N4, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane) molecules and Au adatoms segregated from the pristine metal surface, and (ii) TCNQ (C12H4N4, 7,7,8,8-tetracyanoquinodimethane) and codeposited Mn atoms. In both cases, the strong electron acceptor character of the molecules results in metal to-ligand charge transfer to the lowest unoccupied molecular orbital (LUMO). The amount of electrons donated from the 4-fold coordinated Mn atoms to TCNQ is higher compared to the 2-fold coordinated Au adatoms to F4TCNQ This behavior is reflected in the appearance of distinct spectral features in STS data in the energy region close to the Fermi level resulting from the intricate interplay between surface states, adatom states, and molecular orbitals. These observations are consistent with a picture in which the LUMO of the TCNQ acceptor molecule hybridizes with Mn and Au substrate metal states becoming practically filled, while the LUMO of F4TCNQ is only partially filled despite being the stronger electron acceptor. Our results reveal the importance of the type of bonding between the strong acceptor and the metal center (Au or Mn) as well as its coordination in the determination of the charge transfer to the adlayer, which is important for its electronic properties.

Amer Chemical Soc2012

Single-molecule-level characterization of supramolecular systems at surfaces

Dietmar Payer

This thesis deals with the self-organization of individual building blocks, specifically organic molecules and metal atoms, into supramolecular structures on metal surfaces under ultra high vacuum conditions (UHV). Self-organization of supramolecular systems is a promising candidate for a bottom-up approach in the fabrication of complex structures with specific properties. This process is steered by interactions between functional groups of the individual building blocks and is of apparent interest to gain a detailed understanding of the influence and the behavior of these functional groups enabling intermolecular binding. In the first part of the thesis, we investigate hydrogen bonded structures on metal surfaces. Of especial interest is the possibility of the formation of ionic hydrogen bonds, a special class of hydrogen bond which connects a charged donor (acceptor) and a neutral acceptor (donor) and is important in biological systems. By combined Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) measurement we show the deprotonation of carboxylic functions in the molecules, forming carboxylate groups involved in intermolecular binding. With our complementary experimental and theoretical examinations we clearly show that the carboxylate groups are indeed still charged although adsorbed onto a conducting metal surface and that the observed structures can be computationally reproduced by using these charges in classical molecular dynamic calculations. Our investigations clearly reveal that all requirements for the formation of ionic hydrogen bonds in these samples at metal surfaces under solvent free conditions are fulfilled. In the second part, the confinement of surface state electrons in self-assembled hydrogen-bonded structures is investigated. The examined structure contains two-dimensional cavities, with different inner diameters in the periodic arrangement and in domain boundaries. Inside the cavities we detect a spatially localized modification in the local density of states (LDOS) of the Ag(111) surface state electrons. The energy of the peak correlates with the cavity area as predicted by a simple "particle in a box" model. This clearly shows that the surface state electrons form a confined state inside the cavities. Furthermore we examine self-assembled periodic structures as templates for binding single macrocyclic molecules. The macrocyclic molecules can be deposited by sublimation, as STM-topographs with submolecular resolution reflect the shape of the intact molecules and reveal a coverage dependent aggregation behavior. We show that it is possible to fabricate a structure in which at most one macrocyclic molecule per pore can be found in a defined binding position. In the last part of the thesis we investigate the influence of individual metal atoms on the chemical activity and the self-assembly process of molecules on metal surfaces. First, we demonstrate the high chemical reactivity of these metal atoms. In our experiments we address the chemical reactivity of static active sites, like kinks and step edges, and of dynamic active sites, like the metal atoms, separately. Doing this we show that a non-negligible amount of the chemical reactivity of a metal surface is due to this dynamic adatom gas. Furthermore the influence of metal atoms on the structure formation of organic molecules is investigated. We show that two-dimensional structures based on a complex formation between metal centers and ligands can be formed and that the coordination number is independent of the substrate symmetry. Furthermore, we show the possibility that on metal surfaces one can prepare three-fold coordinated Fe-centers, a coordination number which is rarely seen in three dimensional chemistry. In the last part we study catenanes, which consist of two interlocked macrocyclic molecules. Main interest is on the confirmation of a structural change of the catenanes adsorbed on a metal substrate. For our experiments we use the fact that the catenanes are designed to "trap" a Cu-atom by complex formation. As this reaction is associated with a structural change and a change in intermolecular interaction, we can verify a reaction of catenanes adsorbed onto a surface with Cu adatoms and therefore a structural alteration by simply imaging the formed structures. Our measurements show the possibility of depositing large molecules like catenanes by sublimation and of their potential for use as molecular machines adsorbed onto metal surfaces.

EPFL2007

TCNQ-based Supramolecular Architectures at Metal Surfaces

Tzu-Chun Tseng

Molecular self-assembly at metal surfaces has been recognized as an efficient strategy to create supramolecular nanoarchitectures with promising functionalities. The focus of this thesis lies on the strong electron accepting molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ). The self-assembly of TCNQ and metal-TCNQ coordination networks on metal surfaces were investigated. The structural formation is directed by non-covalent interactions. By controlling fabrication parameters, surface coordination nanostructures with different chemical composition or molecular packing have been synthesized and characterized by scanning tunneling microscopy (STM). The electronic and magnetic properties of the surface organic or metal-organic nanostructures were studied by a combination of x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). The thesis is organized into three parts. In the first part, the self-assembly of TCNQ molecules on Cu(100) and Ag(100) surfaces will be discussed. The two substrates provide different chemical environments as exhibited by the surface mobility of TCNQ. Charge transfer from either Cu(100) or Ag(100) substrate induces conformational bending of the -C≡N cyano groups of TCNQ, which consequently facilitates the bonding of molecules to surfaces via nitrogen lone pairs. As shown by density functional theory (DFT) calculations, on Cu(100), the substrate-cyano interaction leads to local substrate reconstructions at the proximity of cyano groups. These charge-transfer induced structural modifications at the TCNQ/Cu(100) interface and the electrostatic interaction between molecules determine the self-assembly of TCNQ into long-range ordered domains. The adsorption and self-assembly of TCNQ on Ag(100) is presumably governed by a similar mechanism as concluded from the molecular packing and charge transfer phenomenon. In the second part of this thesis, the fabrication of MTCNQx (M = Mn, Fe, Co, and Ni) coordination complexes on metal surfaces is presented. The molecular adsorption geometry in all structures indicates the participation of the cyano-substrate interaction in determining the final structures, except the x = 1 case in which all cyano groups are coordinated to metal adatoms. The competition between the coordination of TCNQ to metal adatom and TCNQ to substrate is shown by the growth phenomena on the Cu(100) and Ag(100) substrates. The weaker substrate interaction provided by the Ag(100) substrate results in multiple ordered phases (x = 1, 2, or 4), in contrast to the single ordered x = 2 phase on Cu(100). The MTCNQ2 networks synthesized on both substrates are isostructural apart from a rotation of the network orientation to adapt the substrate periodicities. This highlights the importance of the substrate lattice in guiding the growth of supramolecular networks as well as the robust intra-network interaction to obtain a stable structure. The inherent chemical reactivity of Mn, Fe, Co and Ni atoms, which can be further modified upon adsorption due to substrate hybridization, strongly influences the resultant coordination structures. In addition to metal coordination, hydrogen bonding also plays a role in the stabilization of metal-TCNQ networks. Adjusting the substrate temperature has been an important and effective method to tune the interplay between adsorbate-adsorbate and adsorbate-substrate interactions. These experiments provide a systematic investigation on the formation of metal-TCNQ coordination structures on metal surfaces. In the last part of this thesis, the electronic and magnetic properties of the MTCNQx complexes on metal surfaces will be discussed. XPS measurements and DFT calculations show that both the substrate and metal coordination atoms contribute to the electron donation to TCNQ molecules. The magnetic properties of the metal coordination centers were studied in x-ray magnetic circular dichroism (XMCD) measurements. The Mn centers are found to be paramagnetic from 300 K to 8 K. The Mn-coordinated networks represent ordered arrays of magnetic atoms carrying a high spin moment of S = 5/2. In the case of Ni-coordinated networks, the magnetism of Ni centers is induced by TCNQ coordination and dehybridization from the substrate. These results show the interesting physical phenomena emerging in two-dimensional metal-organic coordination systems.