Gated Peak Force Tapping for High-Resolution Electrical, Mechanical and Chemical Property Mapping

Intermittent contact SPM modes such as tapping mode and peak force tapping provide high-resolution maps of topography and properties by analyzing the bending of a cantilever during oscillation near a sample surface. Since the cantilever is not touching the sample for a significant portion of the oscillation cycle, lateral forces remain small, preventing tip and sample wear. If the oscillation occurs well below the cantilever resonance, it is possible to directly extract the details of the tip-sample interaction (without deconvoluting the cantilever transfer function). Force mapping methods such as peak force tapping use this approach to provide nanoscale maps of mechanical properties such as modulus, dissipation, and adhesion.
The lower frequency of the oscillation cycle in peak force tapping also facilitates secondary measurements that can then be localized to a point of time (or Z position) in the oscillation cycle. For example, current can be measured only while the tip is in contact with the surface or during a fraction of this contact time (typically centered around the point in time where the peak force is reached) [1]. The same "gated" measurement concept has been applied to Contact Resonance AFM [2], Scanning Non-linear Dielectric Microscopy [3], and Kelvin Probe Force Microscopy [4] among others. In addition to providing unique new property maps and protecting the tip and sample from damage, these methods benefit from co-located, simultaneous mapping of mechanical properties.
In this talk, we explore the opportunities offered by gated peak force tapping measurements with examples in polymeric materials, semiconductors and 2D heterostructures. The expansion of this approach to other electrical, mechanical & chemical property measurements is reviewed, focusing on properties which are typically collected using contact mode. These operating modes include scanning microwave microscopy (SMIM), scanning capacitance microscopy (SCM), scanning thermal microscopy (SThM), piezoresponse force microscopy (PFM), resonance enhanced AFM-IR, and torsional resonance dynamic friction (TR-DFM also known as torsional force microscopy). We investigate practical aspects of system configuration such as the impact of the position and duration of the ‘gating’ as well as the oscillation frequency on signal-to-noise ratio and signal integrity.

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