Phase Imaging of Proton Exchange Membranes under Attractive and Repulsive Tip-Sample Interaction Forces

The nature of tip−sample interaction forces in atomic force microscopy (AFM) phase imaging strongly affects the resolution of proton conducting domains mapped at the surface of Nafion membranes. Images acquired in repulsive mode overestimated the area of individual proton conducting domains by a factor of 4 (360 vs 90 nm2) and underestimated the number of these domains by a factor of 3 (0.9 domains per 1000 nm2 vs 2.7 domains per 1000 nm2) compared to attractive mode. When the cantilever was driven above resonance or when the combination of scan parameters resulted in an AFM feedback loop that was not fully optimized, phase contrast arose not from proton conducting domains but instead from changes in topography. In attractive mode, phase contrast did not correlate with either topography or changes in topography, and the resulting images most accurately represent the fluorocarbon and aqueous domains at the surface of Nafion membranes.