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}