• Positions of the molecules can be determined using small frequency shifts ∆f=-10 Hz ( top)

• Different topography at large frequency shift (∆f = -15 Hz) (left)

• Dissipation shows two peaks per molecule maxima, occur at the functional groups of themolecule (middle)

• The peak dissipation is 1.1 eV/cycle (right)

The role of functionalized groups in the formation of sub molecular contrast in the damping signal of FM-AFMSFB 616

• Tobias KunstmannTel. +49 203 379 2137tobias.kunstmann@uni-due.de

• Prof. Dr. R. MöllerTel. +49 203 379 4220rolf.moeller@uni-due.de

University of Duisburg-EssenPhysics DepartmentAG Prof. Dr. R. MöllerLotharstr. 1-21D-47048 DuisburgMF/MG Building

[1] S. Morita, R. Wiesendanger and E. Meyer: Non contact Atomic Force Microscopy, Springer (2002)

[2] N. Sasaki and M. Tsukada, Jpn. J. Appl. Phys. 39, L1334 (2000)

[3] L. Kantorovich and T. Trevethan, Phys. Rev. Lett. 93, 236102 (2004)

[4] A. Hauschild et al., Phys. Rev. Lett. 94, 036106 (2005)

[5] R. Temirov, F.S. Tautz, http://arXiv:cond-mat/0612036v1 [cond-mat.str-el] (2006)

[6] K. Glöckler et al., Surf. Sci. 405, 1 (1998)

M. Fendrich, T. Kunstmann, R. Möller

14.2 Å

9.2

Å

PTCDA: 3,4,9,10 perylene-tetracarboxylic-dianhydride

crystallography: flatlying molecules, herringbonestructure

Unit cell: 12 x 19 Ų

System: PTCDA/Ag(111)

aa

bb

References

Contact AcknowledgementFinancial support is granted by the Deutsche Forschungsgemeinschaft(DFG) through SFB 616 “Energy dissipation at surfaces” and Nachwuchsförderungof the University of Duisburg-Essen

SFB 616

Dissipation in FM-AFM:

• General theory [2,3]: Transition of the tip-samplesystem between two states of a double-wellpotential during approach and retraction of the tip

• Hysteresis of tip-sample force

• Area between force curves corresponds to thedissipated energy

This work:

• Dissipation mechanisms within a single molecule: PTCDA / Ag(111) and DiMe-PTCDI / Ag(111) Double-well potential and

hysteresis of tip-sample force (from [2])

Frequency Modulation –AFM[1]

• sample is brought near an oscillating silicon cantilever with tip

• tip-sample forces change the resonace frequency, distance control keeps the frequency shift constant: atomic resolution imaging also on insulating surfaces

• second control loop keeps amplitude constant: external driving energy = dissipated energy

IntroductionFNexciter

phase shifter

0 sin( )A tω⋅

frequency measurement

df

variable gainamplifier

dissipationamplitude set point

RMS DC

distance control

Submolecular resolution in Dissipation

Deformation of the dicarboxylic anhydride group on Ag(111) (from [4] and model for the switching process similar to the model proposed in [5])

System: DiMe-PTCDI/Ag(111)

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50

388.0

388.1

388.2

388.3

ener

gy [k

cal/m

ol]

Methyle group rotation [degrees]

tip-molecule distance 0.470 nm 0.465 nm 0.460 nm 0.455 nm 0.450 nm 0.445 nm

A

B

If a tip (small cluster) approaches the methyl group, the barrier for the rotation is reduced (pink curve); State B becomes more favorable.

0 30 60 90 120 150 180 210 240 270 300 330 360

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

ener

gy [k

cal/m

ol]

Methyle group rotation [degrees]

Force Fields AMBER OPLS CHARMM MM+

0.983

0.690

0.563

0.863Preliminary calculation: Energy barrier for the rotation of a methyl group (in vacuo)

→ small barrier (~40 meV)

barrier increased when molecule adsorbed?

topography, ∆f = -10 Hz

dissipation, ∆f = -15 Hz

unit cell averaged

2.8nm

0.297 nm0.286 nm

0.268 nm

1 2 3

Summary and Conclusion

2.8nm

Motivation• Switching of functional groups: Possible applications in

future molecular electronics

• Damping signal in FM-AFM: Indicates “switching” processes?

• Do functional groups have an influence on the dissipation?

• Does the hysteresis model for dissipation in FM-AFM apply to organic molecules?

0.0 0.5 1.0 1.5 2.0

2.1

2.2

2.3

2.4

2.5

Diss

ipat

ion

(eV/

cycl

e)

distance (nm)

Molecule 1 Molecule 2

(a) (b)(c)(c)

(c)(c)

Important Result:

Two maxima per molecule in dissipation!

topography 10 nm x 10 nm ∆f=-12 Hz topography 10 nm x 10 nm ∆f=-16 Hz

Dissipation ∆f=-16 Hz Dissipation ∆f=-16 Hzunit cell averaged

• Positions of the molecules can be determined using small frequency shifts ∆f = -12 Hz (a)

• Poor resolution in topography at large frequency shift (∆f = -16 Hz) (b)

• Dissipation @ -16 Hz shows two peaks per molecule maxima occur at the functionalgroups of the molecule (c)

• The peak dissipation is 2.4 eV/cycle

Results

topography unit cell averaged dissipation unit cell averaged

17.6 Å

9.2

Å

0,0 0,5 1,0 1,5 2,0 2,5

1,00

1,02

1,04

1,06

1,08

1,10

1,12

Dis

sipa

tion

[eV

/cyc

le]

distance [nm]

molecule 1 molecule 2

linescans dissipation, ∆f = -15 Hz

topography, ∆f = -15 Hz

unit cell averaged

Model

Model calculations:

• Molecular resolution in FM-AFM is achieved for both molecules

• The dissipation signal for perylene derivates shows an increased signal at thesides of the functional groups

• The proposed model of breaking oxygen bonds for PTCDA is in good agreement with the model proposed by Temirov et al.[5] for STM experiments

• Model calculations indicate another possible mechanism for dissipation for DiMe-PTCDI

• Dissipated energy depends on the functional group of the molecule

(d)linescans dissipation, ∆f = -10 Hz

Results:

N-N´-dimethylperylene-

3,4,9,10-dicarboximide

adsorption model proposed by Glöckler [6]

Rotational barrier decreases when tip approaches