First-principles Investigation of Edged Ferroelectric PbTiO 3 Nanowires and the Role of Axial Strain
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1199-F09-09
First-principles Investigation of Edged Ferroelectric PbTiO3 Nanowires and the Role of Axial Strain Takahiro Shimada1, Shogo Tomoda1 and Takayuki Kitamura1 1 Department of Mechanical Engineering and Science, Kyoto University, Yoshida-hommachi, Sakyo-ku, Kyoto, 606-8501, Japan
ABSTRACT Atomistic and electronic structures of PbTiO3 nanowires with atomically sharp edges consisting of (100)/(010) surfaces using first-principles calculations. Ferroelectricity is enhanced at the PbO-terminated edge in the nanowire because the Pb-O covalent bond that predominates the ferroelectric distortions is partially strengthened. On the other hand, a significant suppression is observed in the TiO2-terminated nanowire. Surprisingly, the smallest (one-unit-cell crosssection) PbO-terminated nanowire can keep ferroelectricity, while ferroelectricity disappears in the TiO2-terminated nanowires with a diameter of smaller than 17 Å. The ferroelectricity is recovered by axial tension, where the thinner nanowire requires the higher critical strain. INTRODUCTION Ferroelectric (FE) nanowires have drawn much attention as one-dimensional (1D) multifunctional materials for their technological applications, e.g., ferroelectric random access memories (FeRAM). In recent years, single-crystalline PbTiO3 nanowires with a diameter of 512 nm [1] have been manufactured, and smaller nanowires with several lattice spacings are anticipated in the near future. The ferroelectric nanowires are surrounded by both surfaces and atomically sharp edges consisting of (100) and (010) surfaces [2]. Since the edge structure is commonly observed in nanostructured perovskite-type oxides, the edges are among the characteristic nanostructures in perovskite oxides. The rapid change in coordination number at the edge can significantly affect the ferroelectric distortions in the nanowires because ferroelectricity originates from the delicate balance between the short-range covalent and longrange Coulomb interactions. In addition, nanowires are normally subjected to axial tension or compression, which sometimes enhances or destabilizes the ferroelectric distortions. In this study, we demonstrate characteristic ferroelectricity at the edges in PbTiO3 nanowires, and the crucial role of mechanical strain from the atomistic and electronic points of view using first-principles (ab initio) density-functional theory calculations. THEORETICAL DETAILS Ab initio calculations based on the projector augmented wave (PAW) method [3] are conducted using the VASP (Vienna Ab-initio Simulation Package) code [4,5]. The plane-wave cutoff energy is set to 500 eV. The local density approximation (LDA) of the Ceperley-Alder
form [6] is used for the evaluation of the exchange correlation energy. The pseudopotentials include the O 2s and 2p, the Ti 3s, 3p, 3d and 4s, the Pb 5d, 6s and 6p electrons in the valence states. We study PbTiO3 nanowires with edges consisting of (100) and (010) surfaces, where there are two possible terminations of the PbO and TiO2 atomic layers. Figures 1(a) an
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