Asylum Research’s Contact Resonance Viscoelastic Mapping Mode option forthe MFP-3D™ and Cypher™ S atomic force microscopes (AFMs) enables high resolution, quantitative imaging of both elastic storage modulus and viscoelastic loss modulus. It is just one of the many anomechanical tools in Asylum’s NanomechPro™ Toolkit. The contact resonance technique is particularly well suited for characterizing moderate to high modulus materials in the range of about 1GPa to 200GPa. Thanks to recent advances by Asylum and our collaborators, Contact Resonance Viscoelastic Mapping Mode is now faster, more quantitative, and easier to use than earlier implementations.
How it works
The contact resonance principle
The contact resonance technique, first developed in the 1990s by the Yamanaka and Arnold groups,1,2 is based on the principle that the resonance of an AFM cantilever changes when it is in contact with a sample. As shown in Figure 1, the cantilever and sample in contact can be thought of as one spring coupled in series to a second spring and dashpot in parallel. Here, the first spring represents the elastic response of the cantilever and the second spring and dashpot represent the viscoelastic response of the sample. Therefore, as the stiffness of the sample contact changes, the frequency of the contact resonance changes (higher stiffness = higher frequency). Changes in the viscous response of the sample are reflected in the Quality factor (Q) of the contact resonance (more viscous = lower Q). Standard contact mechanics models can be used to then convert these stiffness and dissipation measurements to elastic modulus and loss modulus.
Contact resonance imaging
The contact resonance technique is based on contact mode imaging. This means that the AFM cantilever scans along the sample surface at a constant force (the setpoint), measured by the cantilever deflection. As the tip encounters higher (or lower) features, the tip-sample force begins to increase (or decrease). A feedback loop continuously adjusts the height up (or down) to keep the force at a constant value (the setpoint). This motion is recorded as the sample topography.
While the tip scans the sample in contact mode, the contact resonance is continuously changing with the sample mechanical properties. In order to measure the contact resonance, a very low amplitude vertical modulation is introduced by driving either the cantilever or the sample. The drive is at a relatively high frequency, so this modulation does not affect the contact mode feedback loop, but the modulation nevertheless couples to the cantilever deflection and can be measured using a lock-in amplifier.