Surface Potential Imaging of Soft Structures via Sideband KPFM

Semi fluorinated alkanes comprise of two chain segments with one (CF₂)_xF block and one (CH₂)_yH block. F_xH_y is reported to self-assemble on solid substrates and in water in a range of morphologies. The investigation of semi fluorinated alkanes such as F14H20 contributes to the established knowledge of self-assembly.¹

Kelvin probe force microscopy (KPFM) is the ideal solution for the measurement of soft, self-assembled F14H20 structures on solid substrates because a substantial surface potential difference between the substrate and F14H20 is caused by the electric dipole of F14H20.²

A conductive cantilever inspects the sample in KPFM, while a DC and an AC voltage is simultaneously applied to identify variations in the electrostatic force between the sample and tip as a result of local deviations of the surface potential.³

This article compares two imaging methods: off-resonance and Park’s newly deployed sideband KPFM, for performing KPFM measurements of self-assembled F14H20 structures. A strong enhancement in the potential resolution was identified for sideband KPFM.

Figure 1. The same F14H20 aggregate was imaged using off-resonance and sideband KPFM. Cross-sections (red) show improved lateral and potential resolution of sideband KPFM.

KPFM on F14H20

The detection technique of the electrostatic signal is crucial for the accuracy and resolution of the surface potential in KPFM. In off-resonance KPFM, the electrostatic force is regulated by the AC voltage at a frequency far from the cantilever resonance (approximately 17 kHz).

The oscillation amplitude at the AC frequency is used to detect the force, which is nullified with the application of a DC bias matching the potential difference between the sample and tip. The sensitivity can be lowered by the identification of the long-ranged force.³

In sideband KPFM, a low-frequency AC voltage (2-5 kHz) is applied to regulate the electrostatic force. The regulated electrostatic force gradient creates frequency sidebands to the right and left of the resonance of the cantilever.

These sidebands are nullified by sideband KPFM through the application of a DC bias which matches the potential difference. The long-range crosstalk is decreased and the potential sensitivity and lateral resolution extensively improve by detecting the force gradient rather than the force.3

Evaluating the sideband KPFM measurement on an F14H20 aggregate on a silicon substrate, a potential contrast of 700 to 750 mV between F14H20 and substrate was observed, along with a clear lateral resolution where even small gaps in the aggregate were imaged.

On the other hand, off-resonance KPFM displayed a lower lateral resolution and a potential difference of less than 300 mV.

Figure 2. Park NX research product line of AFMs equipped with sideband KPFM. 

Summary

A key feature of all NX research tools from Park Systems, sideband KPFM provides precise surface potential quantification on soft samples such as F14H20 along with metallic and semiconducting materials.

The dependence on the electrostatic force gradient substantially enhances the potential sensitivity and lateral resolution, meaning sideband KPFM is the superior solution for quantitative surface potential measurements on the nanoscale.

Acknowledgments

Produced from materials originally authored by Ilka Hermes and Dr. Andrea Cerreta from Park Systems.

References and Further Reading

1. G. M. C. Silva, et al, Proc. Natl. Acad. Sci., 2019, 116, 14868.
2. A. El Abed, et al, Phys. Rev. E, 2002, 65, 51603.
3. U. Zerweck, et al, Phys. Rev. B, 2005, 71, 125424.

This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.

For more information on this source, please visit Park Systems Europe.