Depth- and time-resolved polarization reversal in poly(vinylidene fluoride-trifluoroethylene)

Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) is a ferroelectric polymer with applications ranging from electromechanical transducers to nonvolatile ferroelectric random-access memory (FeRAM). In this work, the reversal of electrical polarization in PVDF-TrFE was studied with both time- and depth resolution using a combination of a Sawyer-Tower circuit and a thermal-pulse apparatus. Poling was done by applying a pulsed electric field with field strengths between 60 and 100 MV/m and pulse widths between 1 μs and 1 s. To probe the polarization as a function of depth, a pulsed laser heats the opaque front electrode on the sample. As the thermal pulse diffuses through the ferroelectric polymer, the non-local heating gives rise to a transient pyroelectric current from which the polarization distribution can be reconstructed. After compensating the nonlinear response of the current preamplifier, it was found that a surface layer with a thickness of approximately 250 nm has faster switching dynamics than the bulk, although the remanent polarization (obtained after having applied the poling field for several seconds) is lower than in the bulk. These observations could be explained by impurities that on the one hand lower the remanent polarization, but on the other hand reduce domain sizes, thus allowing faster switching.