title: Optimization of phase contrast in bimodal amplitude modulation AFM.
abstract: Bimodal force microscopy has expanded the capabilities of atomic force microscopy (AFM) by providing high spatial resolution images, compositional contrast and quantitative mapping of material properties without compromising the data acquisition speed. In the first bimodal AFM configuration, an amplitude feedback loop keeps constant the amplitude of the first mode while the observables of the second mode have not feedback restrictions (bimodal AM). Here we study the conditions to enhance the compositional contrast in bimodal AM while imaging heterogeneous materials. The contrast has a maximum by decreasing the amplitude of the second mode. We demonstrate that the roles of the excited modes are asymmetric. The operational range of bimodal AM is maximized when the second mode is free to follow changes in the force. We also study the contrast in trimodal AFM by analyzing the kinetic energy ratios. The phase contrast improves by decreasing the energy of second mode relative to those of the first and third modes.
authors.affiliation: Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain ; Permanent address: Department of Mechanical Engineering, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran.
authors.forename: Mehrnoosh
authors.initials: M
authors.lastname: Damircheli
authors.affiliation: Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain.
authors.forename: Amir F
authors.initials: AF
authors.lastname: Payam
authors.affiliation: Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain.
authors.forename: Ricardo
authors.initials: R
authors.lastname: Garcia
keywords: bimodal AFM; dynamic AFM; tapping mode