Investigations on nanoscale domain dynamics and surface potential behavior in ferroelectric poly(vinylidenefluoride-co-trifluoroethylene) thin films using scanning probe microscopy

Abstract

Poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] is the most widely known and studied ferroelectric polymer material due to its strong electromechanical activity and intrinsic polarization originated from carbon-fluorine dipoles. In order to understand the ferroelectric properties of the P(VDF-TrFE) thin films at the nanoscale, and to enhance the insight to domain dynamics, retention loss, several groups performed plenty of studies with various analysis methods. However, there is still scarce information on nanoscale domain dynamics, retention loss properties, and surface potential distribution and its relaxation behavior. In this study, therefore, nanoscale domain switching dynamics, retention loss properties and surface potential distribution and its relaxation behavior of ferroelectric P(VDF-TrFE) thin films, are investigated using scanning probe microscopy. Firstly, we investigate the nanoscale domain switching dynamics of P(VDF-TrFE) thin films using piezoresponse force microscopy (PFM). The nanoscale dot domains are formed by applying voltage pulses to the bottom electrode. The domain size is linearly proportional to voltage pulse and logarithmic value of the pulse width. However, there is a significant asymmetry in the dot domain switching, probably due to the self-aligned dipole (SAD) region near the interface. The obtained activation field for the domain wall motion is smaller than that for the domain nucleation, indicative of the domain nucleation-limited switching dynamics in ferroelectric P(VDF-TrFE) thin films. Secondly, we study the retention loss of the ferroelectric domains in P(VDF-TrFE) thin films using PFM. We confirm that the retention loss occurred by nucleation of opposite domains at the regions between the peak and valley with morphological gradients range between 0.079 and 0.146. In addition, there are the collective decreases in piezoresponse amplitude of the opposite domains after $0.8 \times 106$ sec even though each opposite domain shows different growth rate as evidenced by different threshold time for phase reversal in PFM images. These results can suggest the methodology for enhancement of the retention property of P(VDF-TrFE) thin films by controlling the optimized surface morphology. In addition, the retention loss of the box patterned domain shows the stretched exponential relaxation behavior, which suggest that the loss occurs by random walk process with not only good stretched exponential factor n and time constant $\tau$ but also superior polarization retention capability with the time. Finally, we investigate the local surface potential distribution and its relaxation behavior in P(VDF-TrFE) ultrathin films by using Kelvin-probe force microscopy (KFM). All surface potentials are negative regardless of the sign of the applied voltage because of the intrinsic negative charges that originated from the SAD region at the ferroelectric/electrode interface. In addition, we found that the effects of the intrinsic negative charges increase as the film thickness decreases, because the polarization components in the thin films decrease as the film thickness decreases, whereas the thickness of the SAD regions does not experience significant changes. The intrinsic charges in the P(VDF-TrFE) films would have a remarkable influence on the reliability of written data. These results would provide enhanced fundamental basis for understanding of nanoscale domain dynamics and local surface potential behavior of ferroelectric P(VDF-TrFE) thin films. Furthermore, this study would suggest a scientific guide vector which can facilitate P(VDF-TrFE) to be applied to more various fields.