First-principles Calculations Of The Ferroelectric Nanoscale Materials

Ferroelectric nanoscale materials have been widely used in microelectronic devices due to their excellent ferroelectric, dielectric, piezoelectric and optoelectronic properties. Strain has a significant influence on the properties of ferroelectric materials at nanoscale. Understanding strain effect on nanoscale ferroelectrics is not only theoretical importance but also practical significance. In this thesis, strain tuning of ferroelectric nanoscale materials is investigated by using the first-principles calculations based on the density-functional. Firstly, the polarization induced by strain gradient, i.e. flexoelectricity, is studied. The flexoelectricity describes the linear response of electrical polarization to a strain gradient which exists extensively among ferroelectric materials at the nanoscale. This linear relationship is described by the flexoelectric tensor. The flexoelectric tensor is fourth rank and it is difficult to measure all the flexoelectric coefficients directly by experiments. The transverse and shear flexoelectric coefficients of BaTiO3are calculated using direct models. To compare the results with different materials, the flexoelectric coefficients of paraelectric material SrTiO3are also calculated. For SrTiO3, the flexoelectric coefficients predicted from our approach are in good agreement with the experimental measurements. For BaTiO3, our predictions have a large discrepancy from the experimental measurements, which may be attributed to the difference in temperature and the extrinsic contribution from surface and defect effects. Secondly, we study the effect of strain on improper ferroelectricity in extremely thin PbTiO3single-wall nanotubes (PTO-SWNTs) folded from (110) nanosheets. The first-principles calculations results show that the tilting of oxygen octahedral, which is originated from tension in the outer shell and the compression in the inner layer, can induce the appearance of improper ferroelectricity in ferroelectric nanotubes and the value is comparable to the bulk value. Strains originating from different curvatures and chiralities play unique roles in the stabilization of the single-wall tubular structure and the strong coupling between ferroelectric and antiferrodistortive distortions. Finally, two kinds of graphene-ferroelectric composite materials are designed and explored on the atomic level to study the interaction between them. The results show that the electric charge in graphene can effectively screen the depolarization field in PbTiO3which stabilizes the ferroelectric distortion, and the interfacial effect can enhance the ferroelectricity in PTO.