Structural Analysis and Electronic Properties of Negatively Charged TCNQ: 2D Networks (TCNQ)(2)Mn Assembled on Cu(100)

A compound two-dimensional monolayer mixing Mn atoms and 7,7,8,8-tetracyanoquinoclimethane (TCNQ) molecules was synthesized by supramolecular assembly on a Cu(100) surface under ultrahigh-vacuum conditions. The interactions in the Mn(TCNQ)(2) network and in the full system arc analyzed from a molecular orbital perspective and in the light of scanning tunneling microscopy (STM) imaging and simulations. Structural, electronic, and magnetic properties are studied in detail using density functional theory (DFT) calculations. In the absence of Cu and depending on the theoretical method used, the TCNQ species can be formally described as either dianions TCNQ(2-) interacting with Mn4+ cations according to ligand held theory (using GGA calculations) or, alternatively, as radical monoanions interacting with Mn2+ cations in a high spin state (using GGA+U calculations). In the complete system including a Cu substrate, whatever theoretical method is used, TCNQs appear as dianions interacting with both Mn2+ cations in a high spin state (d(5)) and charged copper surface atoms.

Two-dimensional metal-organic coordination networks of Mn-7,7,8,8-tetracyanoquinodimethane assembled on Cu(100): Structural, electronic, and magnetic properties

Klaus Kern, Tzu-Chun Tseng

A compound two-dimensional (2D) monolayer mixing Mn atoms and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules was synthesized by supramolecular assembly on a Cu(100) surface under ultrahigh vacuum conditions. Its structural, electronic, and magnetic properties were analyzed by scanning tunneling microscopy experiment and theory, low-energy electron diffraction, x-ray photoemission spectroscopy, and density-functional theory calculations. The 2D compound has a long-range ordered square planar network structure consisting of fourfold coordinated Mn centers with a Mn: TCNQ ratio of 1:2. The electronic-state analysis revealed a complex charge-transfer scenario indicating strong bonding of TCNQ with both the Mn adsorbates and the Cu surface atoms. The calculations reveal that the Mn centers carry a magnetic moment close to 5 mu(B) with very weak coupling between adjacent Mn centers.

2009

Investigation of low-dimensional supramolecular architectures by low-temperature scanning tunneling microscopy

Sylvain Clair

In this thesis we report investigations on supramolecular architectures assembled at well-defined metal surfaces. Supramolecular chemistry is dedicated to the study and use of non-covalent interactions to build highly organized molecular arrangements, aiming ultimately at creating systems with tailored properties and useful functions. The adaptation of its powerful principles to fabricate molecular systems at surfaces has already been proven to be very promising; the extended possibilities for the choice of on the one hand molecules with defined size, shape, symmetry and function, and on the other hand substrates with controlled composition, symmetry and patterning allow for a quasi-infinite tuning of the structure and properties of the respective assemblies. The Scanning Tunneling Microscope (STM) is a tool particularly efficient for the nanoscale elucidation of supramolecular systems at surfaces, simultaneously providing topographic and spectroscopic information at the single-atom level. In this regard, a new Low-Temperature STM (LT-STM) operational at temperatures in the range 5 to 400 K has been constructed. A detailed description of the instrument is provided and the demonstration of its exquisite performance represents a major achievement. In addition to local STM studies, complementary experiments were performed with a synchrotron radiation source using integral techniques, namely X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD). We present results obtained with the linear organic linker 1,4-benzenedicarboxylic acid (terephthalic acid - TPA) on the Au(111), Cu(111) and Cu(100) surfaces. The three-dimensional design principles of supramolecular chemistry could be successfully adapted to these low-dimensional systems. TPA molecules adsorbed on Au(111) organize in two-dimensional molecular sheets, wherein the typical one-dimensional hydrogen-bonded chain motif is found. We show that the inhomogeneities induced by the surface reconstruction can be used to examine the response of the supramolecular self-assembly. In particular, variations in the hydrogen bond length of up to 20% are encountered. On Cu(111), a strict commensurability of the supramolecular sheet to the substrate is not possible because of the reduced lattice spacing of the latter, and the creation of defects is analyzed. Due to their peculiar reactivity, the elbow sites of the reconstructed Au(111) surface act as nucleation centers in the epitaxial growth of transition metals (Fe, Co, Ni), leading to the formation of regular arrays of nanoscale metallic islands. We take advantage of this patterning to steer the formation of metallosupramolecular nano-architectures. Co-deposited TPA molecules attack both Fe and Co clusters and metal-carboxylate linkages evolve. Depending on the deposition parameters, nanoporous grids, mesoscopically organized stripes or truly one-dimensional linkages are obtained. Furthermore, the dynamics of the formation of the nanogrids are monitored in situ. Finally, we report investigations of the magnetic and electronic properties of surface-supported coordination architectures by means of XAS and XMCD. We studied Fe-terephthalate systems engineered on a Cu(100) substrate. Both mononuclear Fe(TPA)4 compounds and diiron centers structures in 2D Fe-TPA lattices exhibit a paramagnetic behavior. Moreover, we demonstrate the decisive influence of the ligand rather than the substrate on the electronic ground state of the metal centers, thus illustrating new vistas to effectively tailor the valence state and magnetic moment of transition metal atoms at surfaces.

EPFL2004

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

Investigation of low-dimensional supramolecular architectures by low-temperature scanning tunneling microscopy

Sylvain Clair

EPFL2004