The Nanoscale World

Harmonyx and PeakForce QNM

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BrukerApplications posted on Mon, Jan 11 2010 7:11 PM

At a technical level,  what is the difference between Harmonyx and PeakForce QNM?

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Answered (Not Verified) replied on Fri, Jul 22 2011 3:04 AM

It's easier to start by describing the only common point between those two techniques: they both allow measuring the samples' mechanical properties in a quantitative manner.

1) In HarmoniX, special cantilevers (hammer-head shapes and a tip offset) must be used because we simultaneously record the vertical motion (exactly like in regular tapping mode) and the torsional motion of the cantilever while it's tapping on the surface. By this mean can be monitored up to 25-30 harmonics. The user can filter the first one whic are not sample-specific and record height, adhesion, dissipation, Young's modulus, deformation,... This requires finding the resonance frequency (easy) and the torsional frequency (more challenging, especially in liquid).

2) In PeakForce QNM, the probe is oscillated far below its resonance frequency (1-2 kHz) and a force curve is recorded each time the tip hits the surface, the feedback being based on the maximum peak force value. All the mechanical information is extracted from those force curves. For a clearer description, see:

Figure 5 gives a comparison between different AFM modes (speed of acquisition, resolution, typical loading force on the sample,...).

On a purely ease-of-use point of view, PFQNM is more interesting...

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Hartmut replied on Sun, Jul 24 2011 12:51 PM

Hi Alex,

interesting question and remarks...

Besides the hardware differences (choice of probe, operational mode), the two methods have as well very different probing frequencies. At 1-2kHz probing frequency, a material might have different elastic properties than at 60kHz (viscoelastic materials are typically stiffer at higher frequencies). As we are operating HarmoniX in tapping mode, the loading force changes with the stiffness of the sample surface. That could lead to different values of mechanical properties (change of contact area [whose value is not so easily dynamically adjustable], maybe gradients of properties with different sampling depths etc.). A difference is as well the contact time with the sample (at 1-2kHz this time is longer, at 60kHz this is shorter), which might be relevant for tip contamination by more fluid-like materials.

All of these aspects are of course very sample dependent (and there are papers showing equivalence in results of HarmoniX and Peak Force QNM for specific types of samples).

However, I think, it is good to have several possibilities (HarmoniX based on tapping, and Peak Force QNM based on kHz-force spectroscopy, and standard Hz-force spectroscopy or force-volume) to really be able to cover all types of samples and applications. I would call this as well a "bandwidth advantage" - mechanical measurements from Hz to 10's of kHz...



Hartmut Stadler, Bruker Nano Surfaces Division, Mannheim (Germany)

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replied on Mon, Jul 25 2011 3:52 AM


I totally agree on your points. Moreover I would like to cite this article where pople used HMX and PFQNM to probe the mechanical properties of copolymers. By using the 2 different modes, the find exactly the same Young's moduli. This shows how accurate are the 2 techniques, despite the fact that theya re based on different principles :

Quantitative mapping of elastic moduli at the nanoscale in phase separated polyurethanes by AFM  Original Research Article
European Polymer Journal, Volume 47, Issue 4, April 2011, Pages 692-698
Peter Schön, Kristóf Bagdi, Kinga Molnár, Patrick Markus, Béla Pukánszky, G. Julius Vancso

Graphical abstract

Nanoscale elastic modulus map and elastic modulus distribution of a phase separated polyurethane.

Nanoscale mechanical properties of segmented polyurethanes have been investigated by two novel dynamic imaging modes of AFM. Varying stoichiometric ratio of isocyanate and hydroxyl groups resulted in polyurethanes samples with characteristic fingerprint AFM phase images. AFM resolves the elastic moduli of stiff and soft segments of the phase separated polyurethane samples at nanoscale resolution. Mechanical mappings were in excellent agreement for both modes, opening novel avenues for nanoscale mechanical characterization of heterogeneous polymers.

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