Atomic Force Microscopy Based Viscoelastic Property Measurements with AM-FM Viscoelastic Mapping Mode

Mechanical property data is crucial in a number of applications. Viscoelastic properties such as loss tangent and storage modulus can be quickly visualized using AM-FM Viscoelastic Mapping Mode with spatial resolution on the nanoscale. The new AM-FM Viscoelastic Mapping Mode from Asylum Research is highly versatile, thanks to wide operating range between below 1MPa and hundreds of GPa. AM-FM is an optional mode on all Asylum Atomic Force Microscopes (AFMs), which is also one of the several features available in the NanomechPro™ Toolkit of Asylum for nanomechanical measurements.

Key Features of AM-FM Viscoelastic Mapping Mode

The AM-FM Viscoelastic Mapping Mode from Asylum can be used for nanomechanical characterization of almost all materials, from ceramics and metals to polymers and biomaterials. The versatile tool yields elastic information such as contact stiffness, Young’s modulus, and storage modulus, and viscoelastic information such as viscoelastic loss modulus and loss tangent.

The AM-FM Mode simultaneously operates at two cantilever resonances. The amplitude modulation (AM) resonance is applied for tapping mode imaging, whereas a higher resonance mode runs in frequency modulation (FM). At resonance, the phase and frequency of the cantilever are highly sensitive to variations in sample properties, providing highly accurate and precise measurements of slight phase and frequency shifts with reduced uncertainty.

Besides rapidly imaging relative contrast, the AM-FM Mode determines constituents of samples. The mechanical properties can be quantitatively measured using the observed frequency, phase and amplitude data. The AM-FM Mode is user-friendly as it operates like tapping mode in the repulsive regime and has other features of the tapping mode such as gentle forces, high spatial resolution and fast scanning.

Using Asylum’s low-noise systems such as Cypher S and ES AFMs, modulus mapping in AM-FM mode can run at forces down to 50pN and scan at rates up to 20Hz on high speed. Low forces indicate less deformation of samples, about a few nanometers, thus optimizing spatial resolution and reducing damage. Since the FM amplitude is a tiny proportion of the AM amplitude at a different frequency, topographic imaging is performed identically to in the standard tapping mode. Hence, the AM-FM Mode is more reliable and consistent to operate.

Working Principle of AM-FM Mode

Figure 1 illustrates the fundamental principles of operation of the AM-FM Mode. Two oscillators emit signals, which are added and transmitted to excite the cantilever. This results in the simultaneous excitation of two different vibrational modes of the cantilever. The first and the second flexural resonances are generally applied. However, it is also possible to use the higher resonances.

Figure 1. In AM-FM Mode, two separate excitation signals (blue and red curves on right) are combined to excite two cantilever resonances simultaneously (purple curve, center).

There are two ways to analyze the response of the cantilever. The lower cantilever resonance runs in AM mode. A lock-in amplifier measures the amplitude and phase at a fixed frequency. The loss tangent is determined from these signals. The AFM also applies the amplitude for feedback control, fine-tuning the vertical position of the cantilever to maintain the amplitude at the setpoint value. Same as in tapping mode, topography data can be obtained from this signal.

At the same time, the amplitude and phase are measured by a second lock-in amplifier in FM mode (higher resonance mode). The amplitude is maintained constant by an automatic gain control circuit through adjustment of drive voltage. The phase is kept at 90º by a phase-locked loop by adjusting the drive frequency. The output drive voltage signal provides information about viscous or dissipative forces, while the output resonance frequency delineates the elastic interaction between the sample and tip. Higher frequency indicates greater modulus or stiffness.

Using a contact mechanics model, elastic modulus is quantified from phase, amplitude and frequency of the two modes. Using a reference sample of known modulus or by quantifying the tip shape using a tip-check sample, it is possible to determine the model parameters such as the radius of the cantilever tip. Here, model parameters are fine-tuned in the software during the scanning of the reference sample to get the expected modulus value.

The imaging of the test sample is then performed with the parameter values. Highly accurate results are obtained when the modulus of the reference and test samples is similar. The loss tangent and storage modulus values can be used to calculate the loss modulus. Using the cantilever spring constant, it is possible to determine the tip-sample contact stiffness. Moreover, maps of sample indentation or deformation are collected for further analysis. Illustrative examples for AM-FM Mode imaging are presented in Figures 2, 3, and 4.

Figure 2. AM-FM Mode images for a polystyrene-polycaprolactone (PS-PCL) polymer film on mica: elastic storage modulus (a and c), viscoelastic loss tangent (b), and indentation depth (d). Scan sizes 5 µm (a and b) and 1.5 µm (c and d).

Figure 3. AM-FM Mode images of dissipation (top) and elastic modulus (bottom) for mouse prion amyloid fibrils on a polystyrene (PS) substrate with a (LDPE) low-density polyethylene domain. Values for Young’s modulus in the image and histogram (far right) were obtained by referencing to the PS-LDPE substrate. Scan size 3µm.

Figure 4. (top) Loss tangent and (bottom) frequency for a commercial coffee packaging bag in cross section. Both images distinguish between the different sample components (vapor barriers, “tie” layers, and metal layer). Scan size 15µm. Acquired with a Cypher S.

Conclusion

AM-FM mode is available on all Aslyum AFMs. The AM-FM Mode operation is simplified by ModeMaster™, which facilitates automatic configuration of the software and guides users through the experiment. GetStarted™ software further reduces configuration time by automatically setting tapping mode parameters. It also calibrates cantilever spring constants for measurements of absolute contact stiffness using the AM-FM Mode. The requirement for a cantilever excitation source with comparatively flat response over a broad frequency range for the AM-FM Mode is achieved with the AM-FM Probe Holder available on Cypher S and MFP-3D AFMs. The highly clean and consistent photothermal actuation of blueDrive is another option available on Cypher S and ES AFMs. The AM-FM Mode does not require specialized cantilevers.

About Asylum Research, an Oxford Instruments Company

Asylum Research is the technology leader in atomic force probe microscopy (AFM) for both materials and bioscience applications. Founded in 1999, we are dedicated to innovative instrumentation for nanoscience and nanotechnology. Our instruments are used for a variety of nanoscience applications in material science, physics, data storage and semiconductors, polymers, chemistry, biomaterials, and bioscience, including single molecule mechanical experiments on DNA, protein unfolding and polymer elasticity, as well as force measurements for biomaterials, chemical sensing, polymers, colloidal forces, adhesion, and more.

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

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