Origins of phase contrast in the atomic force microscope in liquids

John Melcher, Purdue University
Carolina Carrasco, Univ Autonoma Madrid
Xianfan Xu, Birck Nanotechnology Center, School of Materials Engineering, Purdue University
Jose L. Carrascosa, CSIC, Dept Estructura Macomol, Ctr Natl Biotecnol, E-28049 Madrid, Spain
Julio Gomez-Herrero, Univ Autonoma Madrid
Pedro Jose de Pablo, Univ Autonoma Madrid
Arvind Raman, Birck Nanotechnology Center, School of Materials Engineering, Purdue University

Abstract

We study the physical origins of phase contrast in dynamic atomic force microscopy (dAFM) in liquids where low-stiffness microcantilever probes are often used for nanoscale imaging of soft biological samples with gentle forces. Under these conditions, we show that the phase contrast derives primarily from a unique energy flow channel that opens up in liquids due to the momentary excitation of higher eigenmodes. Contrary to the common assumption, phase-contrast images in liquids using soft microcantilevers are often maps of short-range conservative interactions, such as local elastic response, rather than tip-sample dissipation. The theory is used to demonstrate variations in local elasticity of purple membrane and bacteriophage phi 29 virions in buffer solutions using the phase- contrast images.

Discipline(s)

Nanoscience and Nanotechnology