Piezoresponse Force Microscopy Studies of Ferroelectric Domain Structures in Co-Polymer PVDF-TrFE Thin Films*

We report an in situ piezo-response force microscopy (PFM) study of the thermal effect on ferroelectric domain structures in ultrathin ferroelectric co-polymer polycrystalline poly(vinylidene-fluoride-trifluroethylene) (PVDF-TrFE) films. We prepared 9 monolayers (18 nm) PVDF-TrFE using the Langmuir-Blodgett approach, with a surface roughness of 1.2 nm. Stripe-shape domains have been created by applying 9 V to a conductive AFM tip while it is scanning the film surface at room temperature. The sample has then been subjected to progressively higher temperatures up to the ferroelectric Curie temperature T_C of about 90 C. We then image the domain structures using PFM, from which we can examine quantitatively the evolution of domain wall roughness and thermal induced spontaneous polarization reversal. Our study has shed light on the energy competition among thermal roughening, elastic energy and the disorder on the domain stability in layered ferroelectric polymer thin films. We compare the results obtained on these layered polymers with those on perovskite ferroelectric oxides, which exhibits 3D ferroelectricity.