‘Phase imaging’ in AFM refers to recording the phase shift signal in intermittent-contact AFM. The phase shift can be thought of as a “delay” in the oscillation of the cantilever as is moves up and down in and out of contact with the sample, as illustrated below.
In 1995 for the first time, the phase signal was described as being sensitive to variations in composition, adhesion, friction, viscoelasticity as well as other factors. Then in 1996 Garcia and Tamayo suggested that the phase signal in soft materials is sensitive to viscoelastic properties and adhesion forces, with little participation by elastic properties. It has been a common assumption ever since that phase contrast will show adhesion or viscoelastic properties. In fact, phase contrast from material properties is seen in a wide variety of samples, but also reflects topometric differences (differences in slope). This is because the phase is really a measure of the energy dissipation involved in the contact between the tip and the sample, which depends on a number of factors, including such features as viscoelasticity, adhesion and also contact area. As contact area is dependent on the slope of the sample, the phase image also contains topographic contributions, so unambiguous interpretation of contrast in phase images is best left to flat samples. Even in such cases, understanding of the contribution of the individual factors to the phase shift is not simple. Despite the complications involved in interpretation, phase contrast is one of the most commonly used techniques for ‘mechanical’ and composition characterization of sample surfaces.
Example of phase contrast in IC-AFM : a triblock copolymer topography (left) barely shows height differences for the different phases. The phase image (right) shows clear contrast.
This article was condensed from “Atomic Force Microscopy” by Eaton and West, OUP, 2010. The article comes from Chapter 3, which describes all of the commonly used modes in Atomic Force Microscopes. The Article in the book also contains a full reference list.
Images in this article come from the book, and are Copyright Peter Eaton/ OUP 2010.