Tip-surface interaction regimes in dynamic force microscopy:
Nonlinear dynamics properties and experimental implications.

Ricardo Garcia and Alvaro San Paulo

Instituto de Microelectronica de Madrid, CSIC, Isaac Newton 8, 28760 Tres Cantos
Madrid, Spain

Numerical and analytical studies of the tip motion in dynamic atomic force microscopy are presented1-2. A force microscope operated with an amplitude modulation AFM may have two steady states, low and high amplitude respectively. Theoretical simulations show that the tipｹs phase space is divided in two basins of attraction1. A phase space diagram dominated by either basin of attraction implies a stable motion while a substantial contribution from both basins is associated with instabilities. The dominance of a given basin of attraction depends on the tip-sample separation, interaction potential, free oscillation amplitude and driving frequency as a consequence stable and unstable motions are intrinsic features of an oscillating tip near or in intermittent contact with a surface.
We study the performance of the low and high amplitude states to imaging single antibody molecules3 and gold nanoparticles. The low amplitude state allows the determination of the basic morphologies of the antibodies on the support. More importantly, operating the instrument in the low amplitude state is able to resolve the characteristic Y-shaped domain structure of the antibodies and the hinge region between domains. Imaging in the high amplitude state is associated with the irreversible deformation of the molecules. Two major physical differences distinguish the imaging in the high amplitude state from the low amplitude: the existence of tip-sample contact and the strength of the forces involved. Similarly high resolution and non-perturbative images of gold nanoparticles are obtained in the low amplitude state.

1. R. Garcia and A. San Paulo, Phys. Rev. B 61, R13381 (2000); Phys. Rev. B60, 4961 (1999).
2. A. San Paulo and R. Garcia, Phys. Rev. B (submitted)
3. A. San Paulo and R. Garcia, Biophys. J. 78, 1599 (2000).