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Organic Molecules on Insulating Surfaces Investigated by NC-AFM

Organic Molecules on Insulating Surfaces Investigated by NC-AFM. February 26 th , 200 4 MPI Dresden , Germany. Enrico Gnecco Institute of Physics University of Basel, Switzerland. metallic substrate. Motivations. I. molecule. electrodes. Chemistry is important!.

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Organic Molecules on Insulating Surfaces Investigated by NC-AFM

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  1. Organic Molecules on Insulating Surfaces Investigated by NC-AFM February 26th, 2004 MPI Dresden, Germany • Enrico Gnecco • Institute of Physics • University of Basel, Switzerland

  2. metallic substrate Motivations I molecule electrodes

  3. Chemistry is important! The circuit architecture still remains a problem ! Motivations • Advantage: • Insulating spacers (porphyrins, landers) I molecule electrodes metallic substrate • Disadvantage: • Spacers adaptation to the substrate  changes in the electronic properties  Insulating surfacesare potentially good candidates

  4. UHV atomic force microscope • Surface preparation in vacuum • Light-beam adjusted by motorized mirrors L. Howald et al., APL 63 (1993) 117

  5. Cu-tetra porphyrin (Cu-TBPP) on Cu(100): STM NC-AFM Observing organic molecules with AFM:intrinsic problems • The vertical resolution is ~ the same but... • Long range contribution is detrimental for lateral resolution • The tip sharpness is critical

  6. different interaction potentials  different set points ! Observing organic molecules with AFM:intrinsic problems

  7. Comparing force-distance curves: • (i) on the molecule legs and • (ii) on the substrate:  Switching energy: W ~ 0.3 eV Ch. Loppacher et al., Phys. Rev. Lett. 90, 066107 (2003) Despite the problems... • Energetics of a single molecule can be studied:

  8. Stable nanopatterns can be created: E. Gnecco et al., Phys. Rev. Lett. 88, 215501 (2002) Switching to insulators... • “Atomic” resolution on KBr(100): a = 0.66 nm b = 0.47 nm 5 nm 50 nm

  9. Heating at 380 °C  Spiral pattern Step height: 0.35 nm K. Yamamoto et al., J. Cryst. Growth 94 (1989) 629 Trapping the molecules... • How to reduce the mobility of the molecules?

  10. ~ 1.5 nm ~ 3.3 nm • Multi-layered structures L. Nony et al., Nanotechnology 15 (2004) 591 Cu-TBPP on KBr(100) • ½ ML on KBr(100) at room temperature: • The steps are decorated by “molecular wires” • No evidence of internal structures  The mobility of the molecules is still high

  11. R. Bennewitz et al., Surf. Sci. 474, L197 (2001) Lowering the mobility... • KBr(100) irradiated with 1 kV e- at 120 °C: • Rectangular holes (~10 nm wide) • Mono-layer depth (0.33 nm) Holes as molecular traps?

  12. Perylene tetracarboxylic dianhydride (PTCDA): topography 140 nm damping “Legless” molecules in the holes • The holes are empty or (partially) filled • No resolution of single molecules

  13. Molecules with large dipole moment: Sub-phtalocyanine (SubPc) d = 4.8 debye S. Berner et al., Phys. Rev. B 68 (2003) 115410 Towards polar molecules... • Three fold symmetry • Charge of the chlorine: 0.42 e

  14. 18 nm • Single molecules are resolved ! L. Nony, E. Gnecco, R. Bennewitz, A. Baratoff, and E. Meyer et al., in preparation SubPC molecules on e--irradiated KBr • 1 ML on KBr(100) at 80 °C:

  15. 1.4 nm • The molecules are aligned in rows oriented 45° • Along some edges the molecules are mismatched Molecular confinement • Some details: • Height of the islands ~ 0.6 nm (+ hole depth = 1 nm)

  16. • Regular rows: 3b ~ 1.4 nm • Distance between molecules in a row: 2b ~ 0.95 nm Matching the substrate... • Potential arrangement of the molecules : • Apparent size ~1 nm • Alignment along the [110] axis

  17. Electrostatic field inside a hole:  Understanding the trapping mechanism • A dipole d ~ 1 debye can be trapped at the corner site! • (U = d·E ~ 8 kBT)

  18. Interpretation • Expected arrangement of the molecules: • The sign of the corner site selects the growth direction • Dipole-dipole interaction ~ Dipole-substrate interaction • Both interactions are > kBT molecular confinement • Mismatch at edges due to 3-fold symmetry

  19. Empty vs filled holes • On larger scale... 150 nm • Only the holes < 15 nm in size are filled !

  20. Outlook • Molecules with 4-fold symmetry • How to contact electrodes? • Theory of molecular confinement? Conclusions • Holes created by e- irradiation on KBr act as molecular traps • Single organic molecules on insulators have been resolved by AFM • The size of the holes is critical

  21. This work was supported by • The Swiss National Science Fundation • The Swiss National Center of Competence in Research on Nanoscale Science Acknowledgments UNI Basel Ernst Meyer Christoph Gerber Laurent Nony Alexis Baratoff Roland Bennewitz (*) Oliver Pfeiffer Thomas Young University of Tokyo T. Eguchi CNRS Toulouse A. Gourdon C. Joachim (*) Now at McGill University, Montreal

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