Structure and Ferroelectric Properties of Nanoimprinted Poly(vinylidene fluoride-ran-trifluoroethylene)

Abstract

Nanoimprint lithography (NIL) was used to shape thin films of a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (PVDF-TRFE), using a variety of molding shapes and imprinting conditions. The morphology of the layers was characterized by atomic force microscopy (AFM), and preferential orientation of the crystallographic axes was monitored by infrared microspectroscopy; in addition, the local ferroelectric properties were obtained by piezoresponse force microscopy (PFM). When the sample is imprinted in its paraelectric phase in conditions leading to complete confinement, in cavities of size lower than the natural lamellar length observed in a continuous thin film, the crystallographic a axis aligns preferentially parallel to the substrate, and the crystalline lamellae are of significantly reduced length. These characteristics translate in a strongly decreased coercive field and accelerated ferroelectric switching, which is in part ascribed to the improved coupling between the electric field and the properly oriented dipole moments. When decreasing the confinement either by leaving a residual film connecting the nanopillars, or by increasing the lateral size of the nanopillars above the natural lamellar length, or by using line molds where confinement only exists in one direction, or by using continuous films, the preferential orientation becomes less visible and the lamellar length increases, resulting in increased coercive and switching fields. Interestingly, the average length of the crystalline lamellae tends to correlate with the value of the coercive field. Finally, if the sample is imprinted in the melt, a flat-on setting of the crystalline lamellae ensues, with a vertical chain axis which is unfavorable for ferroelectric properties probed with a vertical electric field.