AM-FM and Loss Tangent for Viscoelastic Mapping of Sample Properties

AM-FM Viscoelastic Mapping (AM-FM) combines the features and benefits of normal tapping mode (also called Amplitude Modulation, AM) with quantitative, high sensitivity Frequency Modulation (FM) mode, as well as fast scanning. The topographic feedback operates in normal tapping mode, providing non-invasive, high quality imaging. The second mode drive frequency is adjusted to keep the phase at 90 degrees, on resonance. This resonant frequency is a sensitive measure of the tip-sample interaction. Simply put, a stiffer sample shifts the second resonance to higher value while a softer sample shifts it to a lower value.

Advantages of AM-FM in Visoelastic Mapping

Examples of AM-FM Images

Figure 1 shows an AM-FM image of a commercial coffee packaging bag. A piece of the bag was first coated with a thin layer (about 0.5 mm) of epoxy and the cross-section was cut with a Leica microtome at -160°C, 1 mm/sec with a diamond knife. The bright yellow band shows the aluminum layer attached to two vapor barrier layers (orange layers) by a “tie” layer (dark purple). The resonance frequency shifts range ~2kHz around the second mode resonance of ~1.8MHz. 30µm scan.

Figure 1. AM-FM image of commercial coffee packaging bag

Figure 2 shows an AM-FM image differentiating two types of polymers. The polymer on the left is a relatively soft latex. The other polymer (right) is a ‘VITON’ plastic which is a harder material. The two materials were first roughly polished with a 100 grit sand paper and washed with ethanol. We used a thin layer of epoxy (center) to glue the two materials together. The sample was then cut with a Leica microtome at -160°C, 1mm/sec with a diamond knife, to provide a flat surface for imaging. 13µm scan.

Figure 2. AM-FM image differentiating two polymer types

Figure 3 shows data from an AM-FM scan of graphene on SiO2. Topography is shown in (a) and (e) and second mode frequency is shown in (b) and (f). The topography shows what appears to be a depressed region between two putative single layer sheets of graphene. The graphene sheets show step heights of ~0.3nm, consistent with the expected graphene single atomic layer size. The frequency channel shows clear contrast between the SiO2 and graphene layers, with the softer graphene layer showing a lowered resonance (roughly 500Hz lower). Sections from the topographic and frequency images are shown in figures (c) and (d) respectively. An interesting feature is outlined in blue in (e) and (f) - in this region the topography shows no apparent step contrast; however, the frequency channel shows the same depressed value as over the graphene sheet. We interpret this as indicating the presence of an additional graphene layer not observable in the topographic image. Additionally, it implies that the putative single graphene layer sheets may actually be double layers. Sample courtesy of Fereshte Ghahari, Philip Kim at Columbia University and Dan Dahlberg at the University of Minnesota.

Figure 3. Topography and second mode frequency from an AM-FM scan of graphene on silica.

About Asylum Research

Asylum Research is the technology leader in atomic force and scanning probe microscopy (AFM/SPM) for both materials and bioscience applications.Founded in 1999, they are an employee owned company dedicated to innovative instrumentation for nanoscience and nanotechnology, with over 250 years combined AFM/SPM experience among our staff.

Their instruments are used for a variety of nanoscience applications in material science, physics, 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.

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