In collaboration with researchers at Purdue University, US, the team measured the dynamics of the AFM cantilever during HSAFM imaging using a scanning laser Doppler vibrometer. Using the data they reconstructed a 3D representation of the cantilever with sub-nanometre resolution and nanosecond temporal resolution. The analysis allows measurements of deflection, torsion and displacement to be taken at any location along the cantilever and led to a surprising discovery. After the tip encountered a surface feature, the deflection signal would contain strong noise signals (ringing), whereas the displacement signal measured directly above the tip of the cantilever would be free of these noise sources.

Adapted beam equation

The researchers determined that the cantilever remained in contact with the surface even over comparatively large surface features at scan speeds in excess of 1 mm/s. When the data were observed in the frequency domain, it became clear that the noise was due to the oscillation of higher eigenmodes of the cantilever. These modes were being misinterpreted by the optical beam deflection mechanism as height changes at the tip of the cantilever. These eigenmodes have since been modelled by the team using an adapted Euler-Bernoulli beam equation and modal shapes and frequencies can now be predicted for many common cantilever designs.

Back in the lab, the team has applied its new understanding of the system and combined the HSAFM with a displacement detection system to collect the most accurate topography images from its apparatus to date.

Full details can be found in the following papers:

Nanotechnology 23 205704
Nanotechnology 23 265702