AM-FM Viscoelastic Mapping of Sample Mechanical 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.

• Provides quantitative information on sample stiffness, as well as loss tangent and tip-sample dissipation.
  
  
• Combines the stability and ease of tapping with the quantitative, high sensitivity of FM imaging.
  
  
• Frequency feedback and topographic feedback are decoupled, allowing much more stable, robust operation.
  
  
• The FM image returns a quantitative value of the frequency shift which depends on the sample stiffness.
  
  
• Tapping mode feedback is proven, reliable, non-invasive and gentle.
  
  
• The tapping phase signal is simultaneously available, providing information on the sample loss tangent.
  
  
• The second mode amplitude also contains information on the tip-sample dissipation.
  
  
• Because it is a carry-along signal on top of normal tapping, one can also scan rapidly. Quantitative frequency shifts have been demonstrated at over 300um/second scan rates.
  
  
• Exclusively available from Asylum Research, US patents 8,024,963, 7,937,991, 7,603,891, 7,921,466 and 7,958,563 with others pending.
  
  
• This technique makes use of new high frequency actuator technology, exclusive to Asylum Research.

Download AM-FM Data Sheet
(5.2 MB PDF)

(205 KB PDF)

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.

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.

AM-FM 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.