Synthesis and Characterization of Ferroelectric Nanomaterials

In this dissertation, BaTiO3 nanocrystals, Bi4Ti3O12 nanostructured microspheres, and cosubstituted Bi4Ti3O12 nanoparticles and ceramics were prepared using solvothermal, hydrothermal and citrate-gel methods. The ferroelectric properties of the prepared cosubstituted Bi4Ti3O12 ceramics were studied using P–E hysteresis loop, leakage, and polarization fatigue measurements. A two-phase solvothermal synthesis approach for the preparation of hydrophobic BaTiO3 nanocrystals was developed. The two-phase method is based on the growth of nanocrystals at the oil/water interface by the reaction between metal surfactant complexes in the oil phase and a mineralizer in the water phase. Three kind of organic solvents, hexadecene, toluene, and heptane were used as the oil phase and compared to each other with respect to the product quality. The BaTiO3 particles are crystalline with a mean size of 3.7 nm and can be dispersed in a variety of organic solvents forming highly transparent dispersions. A hydrothermal method was developed for the synthesis of Bi4Ti3O12 nanostructured microspheres consisting of granular nanoparticles and nano-platelets. The precursor powder was prepared using a diethylene glycol mediated coprecipitation method. Tailoring of the morphology was achieved by changing the precursor quantity, sodium hydroxide concentration, and reaction time. The formation mechanism of the nanostructured microspheres probably involves aggregation, followed by dissolution and recrystallization. Bi3.25Pr0.75Ti2.97V0.03O12 (BPTV) and Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ferroelectric nanoparticles and ceramics were synthesized using a modified citrate-gel method that has a crystallization temperature as low as 450 °C. The synthesized nanoparticles were spherical ranging from 30 to 100 nm. Except Nb5+, other donor cations were introduced using the corresponding oxides that have advantages in terms of high purity, low cost, and availability. The Bi3.25Pr0.75Ti2.97V0.03O12 ceramic is orthorhombic and its 2Pr and 2Ec values measured at 300 kV/cm were 35 μC/cm2 and 148 kV/cm respectively. The texture, microstructure, and ferroelectric properties of the prepared Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ceramics depend on x. The maximum 2Pr (30–32 μC cm−2) was achieved at an optimum cosubstitution level (x = 0.025 for M6+, x = 0.03 for M5+). The high remanent polarization, low leakage current, and low polarization fatigue render the prepared ceramics promising for practical applications

The impact of crystal size and temperature on the adsorption-induced flexibility of the Zr-based metal-organic framework DUT-98

In this contribution we analyze the influence of adsorption cycling, crystal size, and temperature on the switching behavior of the flexible Zr-based metal-organic framework DUT-98. We observe a shift in the gate-opening pressure upon cycling of adsorption experiments for micrometer-sized crystals and assign this to a fragmentation of the crystals. In a series of samples, the average crystal size of DUT-98 crystals was varied from 120 mu m to 50 nm and the obtained solids were characterized by X-ray diffraction, infrared spectroscopy, as well as scanning and transmission electron microscopy. We analyzed the adsorption behavior by nitrogen and water adsorption at 77 K and 298 K, respectively, and show that adsorption-induced flexibility is only observed for micrometer-sized crystals. Nanometer-sized crystals were found to exhibit reversible type I adsorption behavior upon adsorption of nitrogen and exhibit a crystal-size-dependent steep water uptake of up to 20 mmol g(-1) at 0.5 p/p(0) with potential for water harvesting and heat pump applications. We furthermore investigate the temperature-induced structural transition by in situ powder X-ray diffraction. At temperatures beyond 110 degrees C, the open-pore state of the nanometer-sized DUT-98 crystals is found to irreversibly transform to a closed-pore state. The connection of crystal fragmentation upon adsorption cycling and the crystal size dependence of the adsorption-induced flexibility is an important finding for evaluation of these materials in future adsorption-based applications. This work thus extends the limited amount of studies on crystal size effects in flexible MOFs and hopefully motivates further investigations in this field.</p

Formation mechanism of Ruddlesden-Popper-type antiphase boundaries during the kinetically limited growth of Sr rich SrTiO3_{3} thin films

We elucidated the formation process for Ruddlesden-Popper-type defects during pulsed laser deposition of Sr rich SrTiO3 thin films by a combined analysis of in-situ atomic force microscopy, low energy electron diffraction and high resolution scanning transmission electron microscopy. At the early growth stage of 1.5 unit cells, the excess Sr results in the formation of SrO on the surface, resulting in a local termination change from TiO2 to SrO, thereby forming a Sr rich (2 × 2) surface reconstruction. With progressive SrTiO3 growth, islands with thermodynamically stable SrO rock-salt structure are formed, coexisting with TiO2 terminated islands. During the overgrowth of these thermodynamically stable islands, both lateral as well as vertical Ruddlesden-Popper-type anti-phase boundaries are formed, accommodating the Sr excess of the SrTiO3 film. We suggest the formation of thermodynamically stable SrO rock-salt structures as origin for the formation of Ruddlesden-Popper-type antiphase boundaries, which are as a result of kinetic limitations confined to certain regions on the surface

Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices

AbstractResistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure

DMPFIT: A Tool for Atomic-Scale Metrology via Nonlinear Least-Squares Fitting of Peaks in Atomic-Resolution TEM Images

Despite the wide availability and usage of Gatan’s DigitalMicrograph software in the electron microscopy community for image recording and analysis, nonlinear least-squares fitting in DigitalMicrograph is less straightforward. This work presents a ready-to-use tool, the DMPFIT software package, written in DigitalMicrograph script and C++ language, for nonlinear least-squares fitting of the intensity distribution of atomic columns in atomic-resolution transmission electron microscopy (TEM) images with a general two-dimensional (2D) Gaussian model. Applications of the DMPFIT software are demonstrated both in atomic-resolution conventional coherent TEM (CTEM) images recorded by the negative spherical aberration imaging technique and in high angle annular dark field (HAADF) scanning TEM (STEM) images. The implemented peak-finding algorithm based on the periodicity of 2D lattices enables reliable and convenient atomic-scale metrology as well as intuitive presentation of the resolved atomic structures

A nonlinear filtering algorithm for denoising HR(S)TEM micrographs

Noise reduction of micrographs is often an essential task in high resolution (scanning) transmission electron microscopy (HR(S)TEM) either for a higher visual quality or for a more accurate quantification. Since HR(S)TEM studies are often aimed at resolving periodic atomistic columns and their non-periodic deviation at defects, it is important to develop a noise reduction algorithm that can simultaneously handle both periodic and non-periodic features properly. In this work, a nonlinear filtering algorithm is developed based on widely used techniques of low-pass filter and Wiener filter, which can efficiently reduce noise without noticeable artifacts even in HR(S)TEM micrographs with contrast of variation of background and defects. The developed nonlinear filtering algorithm is particularly suitable for quantitative electron microscopy, and is also of great interest for beam sensitive samples, in situ analyses, and atomic resolution EFTEM

Synthesis and Characterization of Ferroelectric Nanomaterials

In this dissertation, BaTiO3 nanocrystals, Bi4Ti3O12 nanostructured microspheres, and cosubstituted Bi4Ti3O12 nanoparticles and ceramics were prepared using solvothermal, hydrothermal and citrate-gel methods. The ferroelectric properties of the prepared cosubstituted Bi4Ti3O12 ceramics were studied using P–E hysteresis loop, leakage, and polarization fatigue measurements. A two-phase solvothermal synthesis approach for the preparation of hydrophobic BaTiO3 nanocrystals was developed. The two-phase method is based on the growth of nanocrystals at the oil/water interface by the reaction between metal surfactant complexes in the oil phase and a mineralizer in the water phase. Three kind of organic solvents, hexadecene, toluene, and heptane were used as the oil phase and compared to each other with respect to the product quality. The BaTiO3 particles are crystalline with a mean size of 3.7 nm and can be dispersed in a variety of organic solvents forming highly transparent dispersions. A hydrothermal method was developed for the synthesis of Bi4Ti3O12 nanostructured microspheres consisting of granular nanoparticles and nano-platelets. The precursor powder was prepared using a diethylene glycol mediated coprecipitation method. Tailoring of the morphology was achieved by changing the precursor quantity, sodium hydroxide concentration, and reaction time. The formation mechanism of the nanostructured microspheres probably involves aggregation, followed by dissolution and recrystallization. Bi3.25Pr0.75Ti2.97V0.03O12 (BPTV) and Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ferroelectric nanoparticles and ceramics were synthesized using a modified citrate-gel method that has a crystallization temperature as low as 450 °C. The synthesized nanoparticles were spherical ranging from 30 to 100 nm. Except Nb5+, other donor cations were introduced using the corresponding oxides that have advantages in terms of high purity, low cost, and availability. The Bi3.25Pr0.75Ti2.97V0.03O12 ceramic is orthorhombic and its 2Pr and 2Ec values measured at 300 kV/cm were 35 μC/cm2 and 148 kV/cm respectively. The texture, microstructure, and ferroelectric properties of the prepared Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ceramics depend on x. The maximum 2Pr (30–32 μC cm−2) was achieved at an optimum cosubstitution level (x = 0.025 for M6+, x = 0.03 for M5+). The high remanent polarization, low leakage current, and low polarization fatigue render the prepared ceramics promising for practical applications

Surface Atomic Structure and Growth Mechanism of Monodisperse {1 0 0}-Faceted Strontium Titanate Zirconate Nanocubes

The highly sensitive and selective properties of monodisperse faceted nanocrystals inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is merely determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, atomic details on the faceted surfaces need to be studied on the atomic level. Here, we demonstrate that the surface atomic structure of faceted nanocrystals of complex oxides, {1 0 0}-faceted strontium titanate zirconate nanocubes, can be unambiguously resolved by aberration-corrected scanning transmission electron microscopy. The resolved surface atomic details reveal a layerwise growth process of the nanocubes, thereby allowing an in-depth understanding of the growth mechanism