Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires

ARTICLE Received 16 Jul 2015 | Accepted 20 Nov 2015 | Published 4 Jan 2016 DOI: 10.1038/ncomms10235 OPEN Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires Guillaume Vasseur1, Yannick Fagot-Revurat1, Muriel Sicot1, Bertrand Kierren1, Luc Moreau1, Daniel Malterre1, Luis Cardenas2,3, Gianluca Galeotti2, Josh Lipton-Duffin2,4, Federico Rosei2,5, Marco Di Giovannantonio6, Giorgio Contini6,7, Patrick Le Fèvre8, Franc¸ois Bertran8, Liangbo Liang9,10, Vincent Meunier9 & Dmitrii F. Perepichka11 On-surface covalent self-assembly of organic molecules is a very promising bottom–up approach for producing atomically controlled nanostructures. Due to their highly tuneable properties, these structures may be used as building blocks in electronic carbon-based molecular devices. Following this idea, here we report on the electronic structure of an ordered array of poly(para-phenylene) nanowires produced by surface-catalysed dehalogenative reaction. By scanning tunnelling spectroscopy we follow the quantization of unoccupied molecular states as a function of oligomer length, with Fermi level crossing observed for long chains. Angle-resolved photoelectron spectroscopy reveals a quasi-1D valence band as well as a direct gap of 1.15 eV, as the conduction band is partially filled through adsorption on the surface. Tight-binding modelling and ab initio density functional theory calculations lead to a full description of the band structure, including the gap size and charge transfer mechanisms, highlighting a strong substrate–molecule interaction that drives the system into a metallic behaviour. 1 Institut Jean Lamour, UMR 7198, Université de Lorraine/CNRS, BP 70239, F-54506 Vandoeuvre-les-Nancy, France. 2 Centre Énergie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique, 1650 Boulevard Lionel-Boulet, Varennes, Quebec, Canada J3X 1S2. 3 IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Villeurbanne 69626, France. 4 Institute for Future Environments, Queensland University of Technology (QUT), 2 George Street, Brisbane, Queensland 4001, Australia. 5 Institute for Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, China. 6 Instituto di Struttura della Materia, CNR, Via Fosso del Cavaliere 100, 00133 Roma, Italy. 7 Physics Department, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica 1, I-00133 Roma, Italy. 8 Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif sur Yvette, France. 9 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. 10 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. 11 Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 0B8. Correspondence and requests for materials should be addressed to Y.F.-R. (email: ). NATURE COMMUNICATIONS | 7:10235 | DOI: 10.1038/ncomms10235 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10235 A major challenge in modern surface science is to create ordered arrays of covalently linked organic nanostructures. By doping molecular electronic bands into highly conductive states, these structures may be promising for use as elementary building blocks in electronic carbon-based molecular devices such as organic field-effect transistors1, lightemitting diodes2,3, photovoltaics4 and sensors5. Despite its exceptional physical properties, graphene’s lack of a bandgap severely limits its potential for creating such devices. Engineering the gap in graphene by using nanostructuring, for example, creating graphene nanoribbons (GNRs) of narrow width, has been proposed as a feasible route towards carbon-based electronics. Thus, the GNRs’ bandgap can be tuned by altering their lateral size or by modifying their edge termination (armchair versus zigzag)6–8. An emerging bottom–up approach for producing such carbon nanostructures, exploits covalent linking (polymerization) of precursor molecules on metal surfaces9–20. In these materials, functional properties, including the geometry and the bandgap, can be tailored by means of a judicious choice of monomer and supporting surfaces21–24. The on-surface polymerization is typically demonstrated by measuring the periodicity of polymeric architectures using scanning tunnelling microscopy (STM)25. Evidence of p-conjugation was shown by combining X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure26–29 (NEXAFS). Bandgaps can be deduced by scanning tunnelling spectroscopy (STS) and/or angle-resolved photoelectron spectroscopy (ARPES) and supported by theoretical calculations30–35. However, a fullband dispersion in polymeric chains has not been reported to date, due to the difficulty in obtaining ordered phases at sufficiently long range. In this work, we unambiguously establish the full-band structure of a surface-confined p-conjugated organic polymer, as well as the impact of the substrate on its electronic properties. A long-range ordered array of poly(para-phenylene) (PPP) chains was produced through the surface-catalysed dehalogenative polymerization of 1,4-dibromobenzene (dBB) on copper (110). The high structural quality of the molecular layer, combined with the large extent of the individual PPP oligomers permitted both local and surface-averaged studies. Energy-dependent standing wave patterns observed by STS in finite-size PPP oligomers allowed the determination of the k-resolved conduction band dispersion. The conduction band is observed to cross the Fermi level, conferring to the polymer a metallic character. Using ARPES, we measured the valence band structure along the chains spread over 6.7 eV. As the conduction band is partially occupied, a 1.15 eV bandgap was directly observed. A Hückel/tight-binding (TB) model provides understanding of both ARPES and STS measurements, allowing the estimation of both effective intraand interchains resonance integrals and establishes the quasi-onedimensional (1D) nature of the dispersion. First-principles density functional theory (DFT) calculations fully reproduce the band structure and point out a strong hybridization at the organic/metal interface, which is responsible for filling the polymer’s unoccupied states. Results An ordered and commensurate polymeric phase on Cu(110). A systematic investigation of the dBB/Cu(110) interface as a function of coverage and annealing temperature allows us to identify an unreported structural arrangement, which was used as a starting point for the formation of an ordered polymer phase via thermal activation. In previous work, Di Giovannantonio et al.26 demonstrated that the thermal treatment of vacuum-deposited dBB on Cu(110) leads to the formation of PPP chains. This 2 process is understood to be an Ullmann coup (...truncated)