Seeding the vertical growth of laterally coherent coordination polymers on the rutile-TiO2(110) surface

Coordination polymers may be synthesized by linear bridging ligands to metal ions with conventional chemistry methods (e.g. in solution). Such complexes can be hardly brought onto a substrate with the chemical, spatial and geometrical homogeneity required for device integration. Instead, we follow an in situ synthesis approach, where the anchoring points are provided by a monolayer of metal(II)-tetraphenylporphyrin (M-TPP, M = Cu, Zn, Co) grown in vacuum on the rutile-TiO2(110) surface. We probed the metal affinity to axial coordination by further deposition of symmetric dipyridyl-naphthalenediimide (DPNDI). By NEXAFS linear polarization dichroism, we show that DPNDI stands up on Zn- and Co-TPP thanks to axial coordination, whereas it lies down on the substrate for Cu-TPP. Calculations for a model pyridine ligand predict strong binding to Zn and Co cations, whose interaction with the O anions underneath is disrupted by surface trans effect. The weaker interactions between pyridine and Cu-TPP are then overcome by the strong attraction between TiO2 and DPNDI. The binding sites exposed by the homeotropic alignment of the ditopic DPNDI ligand on Zn- and Co-TPP are the foundations to grow coordination polymers preserving the lateral coherence of the basal layer

Ultrafast Bidirectional Charge Transport and Electron Decoherence at Molecule/Surface Interfaces: A Comparison of Gold, Graphene, and Graphene Nanoribbon Surfaces

We investigate bidirectional femtosecond charge transfer dynamics using the core-hole clock implementation of resonant photoemission spectroscopy from 4,4'-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core-hole creation in all systems, opening an additional decay channel for the core-hole, involving electron donation from substrate to the molecule. Furthermore, using the core-hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface

Planar growth of pentacene on the dielectric TiO2(110) surface

We have studied the growth of pentacene molecules on the unreconstructed and stoichiometric surface of TiO2(110). At variance with its characteristic homeotropic growth mode, pentacene is found to be physisorbed on this dielectric substrate with its long molecular axis oriented parallel to the surface and aligned along the [001] direction. Pentacene molecules couple side-by-side into long stripes running along the [11̅0] direction, where the overlayer preserves the substrate lattice periodicity (6.5 Å). In the opposite direction, head-to-head pentacene repulsion drives the ordering of the stripes, whose spacing simply depends on the surface coverage. By near-edge X-ray absorption, NEXAFS, we have determined the pentacene molecules to be tilted by ∼25° off the surface around their long axis. At the monolayer coverage, the pentacene orientation and spacing are very close to that of the (010) bulk planes (also called a−c planes) of pentacene crystals. We have observed that at least two additional layers can be grown on top of the monolayer following a planar configuration. Both the strong side-by-side intermolecular attraction and the full development of the bulklike electronic states, as probed by NEXAFS, suggest an optimal charge transport along the monolayer stripes of lying-down moleculesFinancial support from the Spanish CYCIT (MAT2008-1497) and the Ministry of Science and Innovation (CSD2007-41 NANOSELECT) is greatly acknowledged. C.S.-S. is grateful to Ministerio de Educación for the AP2005-0433 FPU grant. Funding from the European Community Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 226716 is acknowledged.Peer Reviewe

TiO2(110) charge donation to an extended π-conjugated molecule

The surface reduction of rutile TiO2(110) generates a state in the band gap whose excess electrons are spread among multiple sites, making the surface conductive and reactive. The charge extraction, hence the surface catalytic properties, depends critically on the spatial extent of the charge redistribution, which has been hitherto probed by small molecules that recombine at oxygen vacancy (O-vac) sites. We demonstrate by valence band resonant photoemission (RESPES) a very general charge extraction mechanism from a reduced TiO2(110) surface to an extended electron-acceptor organic molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped at Ovac sites and forms a closely packed, planar layer on TiO2(110). In this configuration, the perylene core spills out the substrate excess electrons, filling the lowest unoccupied molecular orbital (LUMO). The charge transfer from the reduced surface to an extended pi-conjugated system demonstrates the universality of the injection/extraction mechanism, opening new perspectives for the coupling of reducible oxides to organic semiconductors and supported catalysts

Methylamine terminated molecules on Ni(111): A path to low temperature synthesis of nitrogen-doped graphene

The thermal treatment of a monolayer of 1-naphthylmethylamine (NMA) on Ni(1 1 1) surface reveals a complex chemistry promoted by the high surface reactivity. Both the amino-terminated functional group and the naphthalene body of NMA undergo a progressive dehydrogenation, which eventually leads to the coalescence of the molecules. As a result, 2D structures of increasing size are synthesized on the surface, characterized by a graphene-like lattice in which nitrogen atoms are mainly incorporated as pyridinic substitutional defects. In this paper, we show the evidence of the formation of extended nitrogen-doped graphene (N-GR) domains at temperatures as low as 300 degrees C, with band dispersion measurements that reveal p-type doping

Planar Growth of Pentacene on the Dielectric TiO2(110) Surface

We have studied the growth of pentacene molecules on the unreconstructed and stoichiometric surface of TiO2(110). At variance with its characteristic homeotropic growth mode, pentacene is found to be physisorbed on this dielectric substrate with its long molecular axis oriented parallel to the surface and aligned along the [001] direction. Pentacene molecules couple side-by-side into long stripes running along the [1 (1) over bar0] direction, where the overlayer preserves the substrate lattice periodicity (similar to 6.5 angstrom). In the opposite direction, head-to-head pentacene repulsion drives the ordering of the stripes, whose spacing simply depends on the surface coverage. By near-edge X-ray absorption, NEXAFS, we have determined the pentacene molecules to be tilted by similar to 25 degrees off the surface around their long axis. At the monolayer coverage, the pentacene orientation and spacing are very close to that of the (010) bulk planes (also called a-c planes) of pentacene crystals. We have observed that at least two additional layers can be grown on top of the monolayer following a planar configuration. Both the strong side-by-side intermolecular attraction and the full development of the bulklike electronic states, as probed by NEXAFS, suggest an optimal charge transport along the monolayer stripes of lying-down molecules

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is similar to 4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces

Methylamine terminated molecules on Ni(1 1 1): A path to low temperature synthesis of nitrogen-doped graphene

The thermal treatment of a monolayer of 1-naphthylmethylamine (NMA) on Ni(1 1 1) surface reveals a complex chemistry promoted by the high surface reactivity. Both the amino-terminated functional group and the naphthalene body of NMA undergo a progressive dehydrogenation, which eventually leads to the coalescence of the molecules. As a result, 2D structures of increasing size are synthesized on the surface, characterized by a graphene-like lattice in which nitrogen atoms are mainly incorporated as pyridinic substitutional defects. In this paper, we show the evidence of the formation of extended nitrogen-doped graphene (N-GR) domains at temperatures as low as 300 \ub0C, with band dispersion measurements that reveal p-type doping