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Super sensitive Electrostatic Force Microscopy

Last Updated Aug 2015
By: Michael Nystås

One of the most popular and useful methods of Electrostatic Force Microscopy (EFM) is Kelvin Probe Force Microscopy (KPFM),  which provides a measurement of the contact potential difference VCPD  (sometimes referred to as the surface potential). KPFM is widely used for advanced imaging of composite polymeric materials, for imaging of the local work function on the surface of organic photovoltaic materials, and for mapping doping concentrations in electronic devices. Although KPFM is a useful technique to investigate electric properties of surfaces at the nanoscale, the signal-to-noise ratio, accuracy, and speed are limited by the additional feed-back loops commonly used in its implementations. In this paper, researchers at KTH Royal Institute of Technology propose and demonstrate an open-loop technique that exploits the intermodulation (frequency mixing) of an electrostatic drive force and a mechanical drive force, to up-convert the electrostatic drive frequency to the first flexural resonance where the high quality factor allows for a more sensitive force measurement. The contact potential difference can be imaged in a single-pass, allowing for shorter imaging times with higher resolution.

Borgani, R. ; Forchheimer,  D. ; Bergqvist, J. ; Thoren, P-A. ; Inganäs, O. ; and Haviland D. B., Intermodulation electrostatic force microscopy for imaging surface photo-voltage (2014). Applied Physics Letters 105, 143113 (2014); DOI: 10.1063/1.4897966.

Caption: (a)–(e) ImEFM on a TQ1:PCBM:C60 sample, 500 nm scan size with 256 × 256 pixels resolution. The total acquisition time is 5 min. Despite the very flat topography and limited contrast in the phase image, different domains are clearly visible in the contact potential difference images in dark and under illumination, and especially in the surface photo-voltage image, which shows domains with size of order 50–100 nm. The domains appear to be regular across the surface as shown with a bigger scan size of 2 μm (f).