Study Of The Effect Of Crystal Orientation On The Physical Properties Of Low Dimensional Ferroelectric Materials

Recently, low-dimensional ferroelectric materials, such as ferroelectric thin film, nanowire, nanopillar and nanoparticle, are widely used in micro/nano devices like ferroelectric memories, infrared detectors, high frequency capacitors, nano-sensors and nano-generators due to their excellent dielectric, piezoelectric, pyroelectric and photoelectric properties. Recent experiments show that, in addition to the electric field and mechanical load, the preparation of preferred crystal orientation becomes an effective way to tune the properties of ferroelectric materials. In particular, it is more significant in micro/nano ferroelectric structures such as thin films and nanowires. For(001)-oriented low-dimensional ferroelectric materials, there are a lot of experimental and theoretical studies, whereas investigations on the low-dimensional ferroelectric materials with other crystal orientations are still very scarce. Most of previous studies are based on the qualitative analysis of experimental results, and the investigations on the underlying mechanism are rather lacking.In this paper, a nonlinear Landau-Ginsburg-Devonshire(LGD) thermodynamic theory combined with the mechanical boundary conditions of low-dimensional ferroelectric materials is used to construct the effective energy expressions of ferroelectric thin films with different orientations. The effects of crystal orientations on the physical properties of ferroelectric thin films are theoretically predicted. Meanwhile, a phase field model is employed to explore the effects of different crystal orientations on physical properties of ferroelectric single-crystal thin film, polycrystal thin film and ferroelectric nanopillar, respectively. The main research works are as follows:(1)For the epitaxial(110)- and(111)-oriented single-crystal Ba Ti O3(BTO) and Pb Ti O3(PTO) thin films with single domain, the effects of crystal orientations, strain and temperature on the polarization, phase transition temperature, phase diagrams, dielectric properties and piezoelectric properties of the films are investigated and analyzed based on the single-domain thermodynamic model.(2)Considering the dense domain structures, the effective energy expressions of(110)- and(111)-oriented single-crystal polydomain BTO thin films are established according to LGD thermodynamic theory. The effects of crystal orientations, strain and temperature on the polarization, phase transition temperature, phase diagrams, dielectric properties and piezoelectric properties of single-crystal polydomain ferroelectric thin films are studied and analyzed. At the same time, the phase field model is employed to explore the domain evolution of different oriented single-crystal BTO thin films under different strain and temperature conditions. By comparing the simulated data with the results of LGD thermodynamic theory, the rationality and consistency of the results are analyzed both qualitatively and quantitatively.(3)By constructing the phase field model of the polycrystal polydomain(110)- and(111)-oriented ferroelectric thin films, the effects of orientations, strains and electric fields on domain structure, electric polarization, dielectric and piezoelectric properties of polycrystal(110)- and(111)-oriented PTO thin films are investigated and analyzed.(4)For the single-crystal(001)-,(110)-, and(111)- oriented ferroelectric nanopillars, the phase field model is employed to investigate the effects of orientations, strains and electric fields on the domain structure, electric polarization, pyroelectric properties of single-crystal(001)-,(110)-, and(111)-oriented PTO thin films.In summary, the crystal orientations have important effects on the physical properties of ferroelectric materials, such as ferroelectric thin films, ferroelectric nanopillars and so on. The crystal orientations combined with strain, temperature and electric fields, can effectively optimize the physical properties of ferroelectric materials, which provides an effective way to enhance the performance of ferroelectric materials. The present work provides a theoretical instruction to optimize the design and performance of novel micro/nano devices.