Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Journal name:
Nature Chemistry
Volume:
2,
Pages:
374–379
Year published:
DOI:
doi:10.1038/nchem.591
Received
Accepted
Published online

Abstract

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

At a glance

Figures

  1. Chemical structure of TCNQ, TCNQ•− and TCNQ2−.
    Figure 1: Chemical structure of TCNQ, TCNQ•− and TCNQ2−.

    In the neutral form the bond B3 in TCNQ is double and the central ring is not aromatic. The uptake of at least one electron, however, aromatizes the central ring, changing B3 from double to single and enhancing the conformational freedom of the molecule24.

  2. Elongated self-assembled islands of TCNQ on Cu(100).
    Figure 2: Elongated self-assembled islands of TCNQ on Cu(100).

    a, Representative STM image (80 × 80 nm2) showing the elongated TCNQ islands on Cu(100). The sample was prepared by thermally evaporating TCNQ (crucible temperature of ∼340 K) onto the Cu(100) crystal held at room temperature. The STM image was recorded at room temperature, because investigation at lower temperatures did not lead to any significant changes in sample morphology. b, Details of the molecular arrangement. Constant charge density isosurfaces for TCNQ molecules, calculated in the gas phase, have been superimposed on the STM image. The projection of the electrostatic potential over the surface has been colour-coded onto it, in such a way that the red areas correspond to areas in which the electrostatic potential is negative and the blue-coded areas represent positive potential.

  3. Highly ordered self-assembled monolayer of TCNQ on Cu(100).
    Figure 3: Highly ordered self-assembled monolayer of TCNQ on Cu(100).

    STM close-up image of TCNQ monolayer island structure on Cu(100). Some dislocation lines (described in the text) are marked by arrows. Inset: LEED pattern recorded at 32 eV. The unit cell dimensions and symmetry as determined by STM are in good agreement with those obtained by LEED (see text).

  4. Charge transfer in a self-assembled monolayer of TCNQ on Cu(100).
    Figure 4: Charge transfer in a self-assembled monolayer of TCNQ on Cu(100).

    XPS spectra of nitrogen 1s core levels for TCNQ/Cu(100) and a bulk sample (powder) of TCNQ. The spectra show raw experimental data (blue), the fit to the experimental data (green) and the decomposition of the fit into their individual components (red). In each spectrum, a high-energy shoulder is visible, this being the molecular ‘shake-up’ peak (see main text for further details). On adsorption at the Cu(100) surface, the nitrogen 1s core level of the TCNQ shifts by 0.9 eV to a lower binding energy, indicative of electron transfer from the copper substrate to the TCNQ.

  5. NEXAFS measurements of TCNQ / Cu(100).
    Figure 5: NEXAFS measurements of TCNQ / Cu(100).

    a, NEXAFS spectra for TCNQ powder (top), exhibiting three principal peaks. The bottom spectra are for a TCNQ monolayer on Cu(100) at various incident beam angles (ϕ). The absence of peak 1 is an indication of charge transfer from copper to TCNQ, as discussed in the text. b, Fits of the peak intensity versus beam angle for peaks 2′a and 2′b. Best fits to these trends indicate a tilting of the cyano groups towards the surface on adsorption by 10.0–19.7°.

  6. Structural rearrangements at both sides of the TCNQ/Cu(100) interface from DFT calculations.
    Figure 6: Structural rearrangements at both sides of the TCNQ/Cu(100) interface from DFT calculations.

    a,b, Top (a) and side (b) views of the calculated relaxed conformation for a single TCNQ molecule adsorbed on Cu(100) (where light blue corresponds to carbon atoms, dark blue corresponds to nitrogen atoms, red corresponds to the copper atoms of the substrate and white corresponds to hydrogen atoms). Bending of the B3 bond and surface reconstruction are clearly observed. c, Top view of the calculated relaxed conformation for the self-assembled TCNQ monolayer. Different shades of red of the copper atoms represent different distances between the copper atom and the unperturbed, unreconstructed equilibrium position (the brightest red shade corresponds to copper atoms separated by 0.22 Å above their equilibrium positions, whereas the darkest shade corresponds to copper atoms placed 0.16 Å below the equilibrium position). Reconstructed atoms act as a glue to link molecular rows along b1.

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Author information

Affiliations

  1. Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany

    • Tzu-Chun Tseng,
    • Steven L. Tait,
    • Nian Lin,
    • Mitsuharu Konuma,
    • Ulrich Starke,
    • Alexander Langner &
    • Klaus Kern
  2. Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain

    • Christian Urban,
    • Roberto Otero,
    • David Écija,
    • Marta Trelka &
    • Rodolfo Miranda
  3. Departamento de Química, Universidad Autónoma de Madrid, 28049 Madrid, Spain

    • Yang Wang,
    • Manuel Alcamí &
    • Fernando Martín
  4. Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Cantoblanco, 28049 Madrid, Spain

    • Roberto Otero,
    • Nazario Martín &
    • Rodolfo Miranda
  5. Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA

    • Steven L. Tait
  6. Instituto de Ciencia de Materiales de Madrid - CSIC, Cantoblanco, 28049 Madrid, Spain

    • José María Gallego
  7. Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

    • Nian Lin
  8. Lehrstuhl für Physikalische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany

    • Alexei Nefedov
  9. Institute of Functional Interfaces, Karlsruhe Institute of Technology (KIT), 76012 Karlsruhe, Germany

    • Christof Wöll
  10. Departamento de Química Orgánica, Universidad Complutense de Madrid, 28040 Madrid, Spain

    • María Ángeles Herranz &
    • Nazario Martín
  11. Institut de Physique de la Matière Condensée, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

    • Klaus Kern

Contributions

STM experiments were carried out by T.-C.T., C.U. and M.T. T.-C.T. and C.U. contributed equally to this work. T.-C.T., C.U. and D.E. were involved in STM data analysis. Y.W., M.A. and F.M. carried out the DFT calculations shown in this paper. T.-C.T., S. L.T., M.K. and U.S. performed the XPS and LEED measurements and analysis, and T.-C.T., A.L., S.L.T. and A.N. performed the NEXAFS experiments and analysis, supervised by C.W. The molecules were provided by M.A.H. and N.M. and they supervised the chemical discussions. R.O., J.M.G. and S.L.T. wrote the paper and coordinated all the experimental work. Experiments were planned and designed by R.O., J.M.G., S.L.T., U.S. and N.L. under the supervision of K.K. and R.M.

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