• 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 the molecule (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-AFM SFB 616 • Tobias Kunstmann Tel. +49 203 379 2137 [email protected] • Prof. Dr. R. Möller Tel. +49 203 379 4220 [email protected] University of Duisburg-Essen Physics Department AG Prof. Dr. R. Möller Lotharstr. 1-21 D-47048 Duisburg MF/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: flat lying molecules, herringbone structure Unit cell: 12 x 19 Ų System: PTCDA/Ag(111) a b References Contact Acknowledgement Financial support is granted by the Deutsche Forschungsgemeinschaft (DFG) through SFB 616 “Energy dissipation at surfaces” and Nachwuchsförderung of the University of Duisburg-Essen SFB 616 Dissipation in FM-AFM: • General theory [2,3]: Transition of the tip-sample system between two states of a double-well potential during approach and retraction of the tip • Hysteresis of tip-sample force • Area between force curves corresponds to the dissipated 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 Introduction F N exciter phase shifter 0 sin( ) A t ω ⋅ frequency measurement df variable gain amplifier dissipation amplitude 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 energy [kcal/mol] 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 energy [kcal/mol] Methyle group rotation [degrees] Force Fields AMBER OPLS CHARMM MM+ 0.983 0.690 0.563 0.863 Preliminary 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 nm 0.286 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 Dissipation (eV/cycle) 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 Hz unit 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 functional groups 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 Dissipation [eV/cycle] 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 the sides 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