J. Fraxedas 1 , J. Langer 2 , I. Díez 3 and F. Sanz 3 1 ICMAB-CSIC, Barcelona (E)

1. Measuring energies at the nanometer-scale of molecular organic materials with an AFM: TTF-TCNQ as a case study J. Fraxedas 1, J. Langer 2, I. Díez 3 and F. Sanz 3 1 ICMAB-CSIC, Barcelona (E) 2 Freie Universität Berlin (D) 3 CBEN-Universitat Barcelona (E) fraxedas@icmab.es ISCOM2005, Key West

2. B A D C z y x AFM is an extremely versatile tool working principle operation modes topography contact intermitent lithography electrochemical force spectroscopy photodiode laser beam cantilever sample environments air, liquid, vacuum piezoelectric scanner (A+B)-(C+D) z (indentation) (A+C)-(B+D) x (friction)

3. Force spectroscopy: z-motion Force curve A approach C jump to contact D indentation F pull-off (adhesion) BCDE nanoindentation pull-off  adhesion unfolding of proteins

4. D d z Nanoindentation cantilever sample piezoelectric scanner F = kc× D d = z – D F (d) F applied force kc cantilever force constant D cantilever deflection z piezo displacement d deformation/penetration

5. F ds ds F d ks ks R < 10 nm point force e e  d s = Ee Young’s modulus Lateral interactions play a key role F = k  (1- ds / (2 + ds2)) k Nm-1 stiffness ds nm contact radius F  kdd»ds Hooke’s law

6. PbS NaCl-type structure a = 0.59 nm No dislocations are generated

7. layered material: HOPG F

8. Nanoindentation of lipid bilayers S. Garcia-Manyes et al. Biophys. J. 89, 1812 (2005)

9. solids k  kD = m ωD2 = m (kBQD/ ħ)2 Debye frequency! confined liquids k = m ω2 surface + compressional modes J. Fraxedas et al. PNAS 99, 5228 (2002); Surf. Sci. 588, 41 (2005)

10. Which are the energies involved? Elastic energyEeF (d)dd NaCl V~pds2dY=6.4 nm3 m ~ 1.3×10-20 gr  N ~140 anion-cation pairs Ee / N ~ 150 attoJ / 140 = 1.1 attoJ / pair Cohesion energy Uc=1.5 attoJ … a bit of information requires a little cube of atoms 5 × 5 × 5-that is, 125 atoms. Perhaps we need a hundred and some odd atoms to make sure that the information is not lost through diffusion, or through some other process. There’ s Plenty of Room at the Bottom, R. P. Feynman, 1959

11. Layered system: TTF-TCNQ thin films (thickness < 1 mm) highly oriented (ab-plane) 5 mm  5 mm

12. kc 25 Nm-1, R < 10 nm ambient conditions k 38.5 Nm-1

13. Maximum accumulated energy  2.0  10-15 J in ca. 2250 nm3  the estimated maximum mean energy per TTF-TCNQ pair is 2.3 eV. Enthalpy of sublimation of TTF-TCNQ is 2.7 eV The 2.3 eV value thus gives the energy required to sublimate TTF-TCNQ but obtained by a mechanical method and only locally invasive. The cohesive energy of TTF-TCNQ is 4.9 eV, which essentially accounts for the Coulomb (Madelung term), polarization (charge-induced dipole), dispersion (van der Waals) and repulsion energies. Nanoindentation performed on the surface  sublimation. The estimated QD value for TTF-TCNQ is about 90 K, in agreement with previous heat capacity determinations.

14. Critical issues • Calibration kc thermal noise, cal. weights D diamond surface • Ideally kc k uncertainty is reduced • Orientation applied force ┴ surface

15. Conclusions • Characteristic energies can be determined with AFMs • TTF-TCNQ: enthalpy of sublimation • New generation of AFMs with pN resolution • 1D systems: carbon nanotubes

16. spring model  lateral interactions play a key role!! z(x) upon nanoindentation d=z(x=0)