Figure 3.

(a and b) A scheme of the contactless experiment performed on two different molecules coadsorbed on an insulating substrate. Frequency-shift profiles are taken at different tip-sample distances. In the experiment, the SFM tip never establishes contact with the adsorbed molecules. (c) SFM topographic image showing a SWNT and a DNA molecule adsorbed on mica. (d) This graph shows a frequency-shift profile along the dashed line drawn in c. A clear frequency shift is observed above the SWNT, whereas no measurable change appears above the DNA molecule. This profile was taken with the tip placed at 10 nm above the surface. A phase-locked loop was used to keep the system at resonance. No feedback was applied on the oscillation amplitude.