Organic Molecules on Insulating Surfaces Organic Molecules on Insulating Surfaces Investigated by NC-AFMInvestigated by NC-AFM

February 26February 26thth, 200, 20044

MPI DresdenMPI Dresden, , GermanyGermany

Enrico GneccoEnrico Gnecco

Institute of Physics Institute of Physics University of Basel, SwitzerlandUniversity of Basel, Switzerland

Motivations

electrodes

metallicsubstrate

moleculeI

Motivations

metallicsubstrate

electrodes

moleculeI

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, 066107 (2003)

Switching energy: W ~ 0.3 eV

Switching to insulators...

• “Atomic” resolution on KBr(100):

5 nm

a = 0.66 nmb = 0.47 nm

• Stable nanopatterns can be created:

E. Gnecco et al., Phys. Rev. Lett. 88, 215501 (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

0 100 200 300

-2

-1

0

1

hei

gh

t (n

m)

distance (nm)

~ 1.5 nm

~ 3.3 nm

• Multi-layered structures

• No evidence of internal structures

• ½ ML on KBr(100) at room temperature:

L. Nony 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

4.55

4.6

4.65

4.7

4.75

4.8

4.85

4.9

4.95

0 20 40 60 80 100 120 140

Da

mp

ing

(eV

/cyc

le)

Cross section (nm)

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 20 40 60 80 100 120 140

He

igh

t (

nm

)

Cross section (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) 115410

d = 4.8 debye

SubPC molecules on e-irradiated KBr

• 1 ML on KBr(100) at 80 °C:

• Single molecules are resolved !

L. Nony, E. Gnecco, R. Bennewitz, A. Baratoff, and E. Meyer et al., in preparation

18 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: 3b ~ 1.4 nm

• Distance between molecules in a row: 2b ~ 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 kBT)

Interpretation

• Both interactions are > kBT 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?

AcknowledgmentsUNI 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 McGill University, Montreal