New Insight into Nanoscale Identification of the Polar Axis Direction in Organic Ferroelectric Films

Ferroelectric poly(vinylidene fluoride- co -trifluoroethylene) [P(VDF- co -TrFE)] thin films have been deposited by spin-coating onto the Bi 0.5 Na 0.5 TiO 3 (BNT)/LNO/SiO 2 /Si heterostructure. The copolymer microstructure investigated by using grazing-incidence wide-angle X-ray diffraction (GIWAXD) and deduced from the (200)/(110) reflections demonstrates that the b -axis in the P(VDF- co -TrFE) orthorhombic unit cell is either in the plane or out of the plane, depending on the face-on or on the two types of edge-on (called I and II ) lamellar structures locally identified by atomic force microscopy (AFM). For edge-on I lamellae regions, the electroactivity ( d zz eff ∼ −50.3 pm/V) is found to be twice as high as that measured for both edge-on II or face-on crystalline domains, as probed by piezoresponse force microscopy (PFM). This result is directly correlated to the direction of the ferroelectric polarization vector in the P(VDF- co -TrFE) orthorhombic cell: larger nanoscale piezoactivity is related to the b -axis which lies along the normal to the substrate plane in the case of the edge-on I domains. Here, the ability to thoroughly gain access to the as-grown polar axis direction within the edge-on crystal lamellae of the ferroelectric organic layers is evidenced by combining the nanometric resolution of the PFM technique with a statistical approach based on its spectroscopic tool. By the gathering of information at the nanoscale, two orientations for the polar b -axis are identified in edge-on lamellar structures. These findings contribute to a better understanding of the structure–property relationships in P(VDF- co -TrFE) films, which is a key issue for the design of future advanced organic electronic devices.