Organic Molecules on Insulating Surfaces Investigated by NC-AFM

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Organic Molecules on Insulating Surfaces
Investigated by NC-AFM
June 10th, 2006 ETH Zurich, Switzerland

• Enrico Gnecco
• NCCR Nanoscale Science
• University of Basel, Switzerland

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Motivations
I
molecule
electrodes
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Motivations

• Advantage
• Insulating spacers (porphyrins, landers)

I
molecule
electrodes
metallic substrate

• Disadvantage
• Spacers adaptation to the substrate ? changes in
  the electronic properties

? Insulating surfaces are potentially good
candidates
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UHV atomic force microscope

• Surface preparation in vacuum
• Light-beam adjusted by motorized mirrors

L. Howald et al., APL 63 (1993) 117
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Observing organic molecules with AFMintrinsic
problems

• The vertical resolution is the same but...
• Long range contribution is detrimental for
  lateral resolution
• The tip sharpness is critical

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Observing organic molecules with AFMintrinsic
problems
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Despite the problems...

• Energetics of a single molecule can be studied

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Switching to insulators...

• Atomic resolution on KBr(100)

a 0.66 nm b 0.47 nm
5 nm
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Trapping the molecules...

• How to reduce the mobility of the molecules?

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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
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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?
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Legless molecules in the holes

• The holes are empty or (partially) filled

• No resolution of single molecules

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Towards polar molecules...

• Three fold symmetry
• Charge of the chlorine ?0.42 e

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SubPC molecules on e--irradiated KBr

• 1 ML on KBr(100) at 80 C

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Molecular confinement

• Some details

• Height of the islands 0.6 nm ( hole depth 1
  nm)

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Matching the substrate...

• Potential arrangement of the molecules
• Apparent size 1 nm
• Alignment along the 110 axis

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Understanding the trapping mechanism

• A dipole d 1 debye can be trapped at the
  corner site!
• (U ?dE 8 kBT)

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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 gt kBT ? molecular
  confinement

• Mismatch at edges due to 3-fold symmetry

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Empty vs filled holes

• On larger scale...

150 nm

• Only the holes lt 15 nm in size are filled !

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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

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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 Univ. Aix-Marseille III, France ()
Now at McGill Univ., Montreal, Canada

• This work was supported by
• The Swiss National Science Fundation
• The Swiss National Center of Competence in
  Research on Nanoscale Science