• 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 [email protected]
• Prof. Dr. R. MöllerTel. +49 203 379 [email protected]
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