Frequency-Modulated Atomic Force Microscopy

Atomic force microscopy (AFM)
images were acquired with a Cypher S from Oxford Instruments under
a dry air atmosphere and ambient conditions. To improve the electrostatic
sensitivity, Pt-coated AC240 cantilevers from an Olympus were used.
The scans were performed using frequency-modulated AFM with a net-attractive
feedback (frequency shift of ca. −15 Hz, amplitude of ca. 18
nm) at a scan speed of 2.5 μm per second. The topography images
were leveled and flattened using Gwyddion.^{109 (link)} Simultaneously, we determined the local surface potential of the
sample by using FM-KFM with sideband demodulation. A home-built algorithm
based on a Kalman filter was used to improve the feedback performance.^{77 (link)} Both AFM and KFM controls were performed on
an external device (HF2LI) from Zurich Instruments. Further details
of the KFM setup may be found elsewhere.^{77 (link),78 (link)}