Authors Y. Qi, J. M. P. Martirez, Wissam A. Saidi, J. J. Urban, W. S. Yun, J. E.Spanier, and A. M. Rappe
We investigate the origin of the depolarization rates in ultrathin adsorbate-stabilized ferroelectric wires. By applying density functional theory calculations and analytic modeling, we demonstrate that the depolarization results from the leakage of charges stored at the surface adsorbates, which play an important role in the polarization stabilization. The depolarization speed varies with thickness and temperature, following several complex trends. A comprehensive physical model is presented, in which quantum tunneling, Schottky emission, and temperature-dependent electron mobility are taken into consideration. This model simulates experimental results, validating the physical mechanism. We also expect that this improved tunneling-Schottky emission model could be applied to predict the retention time of polarization and the leakage current for various ferroelectric materials with different thicknesses and temperatures.
Authors Wissam A. Saidi, John Mark P. Martirez, and Andrew M. Rappe
We present a systematic evaluation of the effects of polarization switchability on surface structure and stoichiometry in BaTiO3 and PbTiO3 ferroelectric oxides. We show that charge passivation, mostly by ionic surface reconstructions, is the driving force for the stability of the surfaces, which suggests that varying the substrate polarization offers a new mechanism for controlling surface reconstructions in polar systems and inducing highly nonstoichiometric structures. Conversely, for thin-films the chemical environment can drive polarization switching via induced compositional changes on the surface. We find that the value of the oxygen partial pressure for the positive-to-negative polar transition is in good agreement with the recent experimental value for thin-film PbTiO3. For BaTiO3, we show that it is harder for oxygen control to drive polar transition because it is more difficult to reduce. This study opens up the possibility of real-time control of structure and composition of oxide surfaces.
We present an ab initio density functional study of ferroelectricity in single-domain PbTiO3-based nanocapacitors. We used density functional theory with the recently introduced PBEsol generalized-gradient exchange-correlation functional, which we found to give accurate properties of bulk ferroelectric (FE) materials. Pt and Au electrodes are used in our study to gain a thorough understanding of the electrode-oxide interfaces, and the role of the interfacial chemical bonding and charge transfer in stabilizing the FE polar phase. We found that the FE properties of the thin films depend not only on the electrode and the FE material but also on the electrode-perovskite termination (TiO2 vs PbO), exemplifying the key role of the interface in these systems. The critical thickness was found to be 24â€“28?Å. In addition, a Löwdin orbital analysis gives a detailed description of the distribution of charges in the system, and shows the importance of charge passivation by the electrodes in stabilizing the FE polar phase.