DFRT-PFM Workshop

Earlier in November 2014, Zurich Instruments participated in a 2-day workshop on Piezoresponse Force Microscopy (PFM) organized by the Institut des Nanotechnologies de Lyon (INL) and the CNRS network RéMiSoL (Réseau des Microscopies à Sondes Locales). Starting with some theoretical introduction by Prof Brice Gautier, 2 parallel sessions on practical work allowed every participant to get some hand-on experience with DFRT-PFM as well as Switching Spectroscopy-PFM on a NT-MDT Ntegra and Bruker Dimension microscope both equipped with the Zurich Instruments HF2LI Lock-in Amplifier.

After some initial tests at 2 fixed frequencies for measurements of out-of-plane and in-plane polarization, we turned on bimodal excitation to use the DFRT method but this time on the two contact resonances simultaneously. The principle of the method is the same, but instead of using 3 demodulators, we used all 6 demodulators with both MOD1 & MOD2, taking full advantage of the HF2LI-MOD option. In a frequency sweep, the two contact resonance can be measured and the difference of sideband amplitude used as input for two separate PID feedback. The figure below illustrates the DFRT principle applied to two peaks.

Both contact resonance needs to be driven electrostatically, which was possible using the Add feature on the HF2LI front panel in order to physically add Signal Output 2 to Signal Output 1 for AC bias modulation and generate corresponding bimodal excitation. The overall connection to the HF2LI Lock-in Amplifier would look like this:

Within one single scan, both piezoresponse phase and amplitude as well as both frequency shift (from contact resonance in- and out-of-plane) could all be acquired simultaneously, thus allowing for the complete vector reconstruction of the piezoresponse force. Here is an example of both phases:

Once the contact resonance is locked (frequency tracking) it can be measured while sweeping the bias voltage in the so-called Switching Spectroscopy PFM (SS-PFM) mode to polarize (DC bias on) and measure (DC bias off) in the same sweep, the results over 2 averaged cycle look like this:

These short measuring days was accompanied with a friendly dinner in a Bouchon Lyonnais not far from the Traboule in the old town. The next January edition is already fully booked and might provide Zurich Instruments ideas on how to organize local User Meetings in the future!

Acknowledgements: I would like to thank David Albertini and Simon Martin for providing excellent measuring conditions and processing of the images and spectroscopy curves as shown in this blog post. The SS-PFM curves were obtained using the LabVIEW API and customized software.