Enhanced compositional sensitivity in atomic force microscopy by the excitation of the first two flexural modes*

We demonstrate that the compositional sensitivity of an atomic force microscope is enhanced by the simultaneous excitation of its first two normal eigenmodes$^{1-2}$. The coupling of those modes by the non-linear probe-surface interactions enables to map compositional changes in several conjugated molecular materials with a phase shift sensitivity that is about two orders of magnitude higher than the one achieved in amplitude modulation atomic force microscopy. \begin{enumerate} \item T.R. Rodriguez and R. Garcia, Appl. Phys. Lett. 84, 449 (2004) \item N.F. Martinez, S. Patil, J.R. Lozano and R. Garcia, Appl. Phys. Lett. 89, 153115 (2006) \end{enumerate}