Figure 4. Theoretical force and frequency shift curves.

(a) Theoretical F(z) curves over the centre of a PTCDA (light blue) and a corner adatom (orange). The model of an Si dimer tip shown in Fig. 3c is used. The corresponding experimental curves from Fig. 3c are shown: F(z) on PTCDA (blue) and Si adatom (red). The Van der Waals (vdW) force is taken into account for the calculations on the molecule. The theoretically derived maximum attractive force is around −0.29 nN, which matches the value obtained experimentally. Since the attractive force in the F(z) calculated without the vdW contribution is too small to reproduce the experimental results (not shown), the vdW interaction force dominates the attractive force on the molecule. (b) Theoretical F(z) curves over the centre of a PTCDA (blue), a carbon atom (green) and an oxygen atom (red). (c) A model of the PTCDA molecule showing the three sites used for the force calculations. (d) Δf(z) curves converted from the F(z) curves shown in b. For the conversion, we used the same parameters as used for the experiments shown in Fig. 3a.