Organic Molecules on Insulating Surfaces Investigated by NC-AFM June 10 th, 2006 ETH Zurich, Switzerland Enrico Gnecco NCCR Nanoscale Science University of Basel, Switzerland

Motivations electrodes metallic substrate molecule I

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

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

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 NC-AFMSTM Cu-tetra porphyrin (Cu-TBPP) on Cu(100):

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

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

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

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

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

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

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

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

SubPC molecules on e  -irradiated KBr 1 ML on KBr(100) at 80 °C: Single molecules are resolved ! L. Nony, EG et al., Nano Lett. 4 (2004) nm

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

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

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

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

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

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 Outlook Molecules with 4-fold symmetry How to contact electrodes? Theory of molecular confinement?

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 This work was supported by The Swiss National Science Fundation The Swiss National Center of Competence in Research on Nanoscale Science (*) Now at Univ. Aix-Marseille III, France (**) Now at McGill Univ., Montreal, Canada