FIGURE 4.

Stretch method for measuring molecular spring constants by AFM. The stretch method measures the molecular spring constant from the slope of the linear ascending phase of the force-extension curve before rupture. A dead zone of zero mean force was observed between this ascending phase and another one to the left, which had negative (compressive) forces, indicating decreased indentation of the AFM tip against the bilayer. The dead zone represents the resting length of the molecular complex as it has to be picked up and fully extended before it could resist tensile force (see text). The force on the molecular complex is calculated using f = k_c〈z〉 where the quasistatic tip deflection 〈z〉 is obtained from the mean photodiode signals. Subtracting 〈z〉 from the PZT movement zpzt gives the molecular extension z_m. Unlike the thermal case, here the coupled system behaves as if the cantilever spring and the molecular spring are in series because the same mean force is applied to both the cantilever and the molecular complex.