Synchrotron X-ray Diffraction Studies on Oxide Surfaces and Interfaces.

Surfaces and interfaces of oxide crystals have gained a burst of attention in recent years due to their importance in technological applications as well as fundamental interest in their exotic behavior. Zinc oxide (ZnO) is one of the oxide materials which has been intensively studied especially for its transport behavior at the Schottky interface, which leads to many electronic device applications. Bismuth ferrite (BiFeO3) is a unique material that exhibits stable magnetoelectric multiferroicity at room temperature, yielding new paradigms in the design of novel electromagnetic devices. Both the Schottky property of ZnO and the multiferroic behavior of BiFeO3 depend critically on the atomic structure of their surfaces and interfaces. Therefore, the accurate determination of their structure is a prerequisite for controlling and optimizing their properties for applications. The atomic surface and interface structures of uncoated and metal-coated ZnO (0001) Zn-polar and (000-1) O-polar wafers are measured with surface x-ray diffraction. All Zn-polar surfaces and Schottky interfaces show the presence of a fully occupied (1x1) overlayer of oxygen atoms on top of the terminating Zn layer, and no significant atomic relaxations are observed. O-polar surfaces are significantly rougher than Zn-polar surfaces, exhibiting Gaussian-shaped roughness profiles with a width of about 1.5 unit cells. They show a decreased layer distance between the topmost oxygen and zinc layers. These findings are important because they are the first results on ZnO Schottky interfaces prepared under typical ambient device processing conditions. The unit-cell symmetry of BiFeO3 thin films is determined via 3-dimensional reciprocal space mapping. The maps clearly show a phase transition from monoclinic to tetragonal symmetry when the film thickness decreases below a critical thickness, both for highly strained and moderately strained films. In the case of moderately strained films, this transition is accompanied by a change in the half-order diffraction peak pattern, which reflects an untilting of the oxygen octahedra. This establishes a definitive connection between the octahedral tilting and the symmetry changes occurring at the structural transition. These results are essential for device applications, since the ferroelectric and magnetic properties are strongly related to the unit-cell symmetry and oxygen octahedral structure.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107250/1/ysyang_1.pd

Multislice Electron Tomography using 4D-STEM

Electron tomography offers important three-dimensional (3D) structural information which cannot be observed by two-dimensional imaging. By combining annular dark field scanning transmission electron microscopy (ADF-STEM) with aberration correction, the resolution of electron tomography has reached atomic resolution. However, tomography based on ADF-STEM inherently suffers from several issues, including a high electron dose requirement, poor contrast for light elements, and artifacts from image contrast nonlinearity. Here, we developed a new method called MultiSlice Electron Tomography (MSET) based on 4D-STEM tilt series. Our simulations show that multislice-based 3D reconstruction can effectively reduce undesirable reconstruction artifacts from the nonlinear contrast, allowing precise determination of atomic structures with improved sensitivity for low-Z elements, at considerably low electron dose conditions. We expect that the MSET method can be applied to a wide variety of materials, including radiation-sensitive samples and materials containing light elements whose 3D atomic structures have never been fully elucidated due to electron dose limitations or nonlinear imaging contrast.Comment: 26 pages, 9 figure

GENFIRE: A generalized Fourier iterative reconstruction algorithm for high-resolution 3D imaging

Tomography has made a radical impact on diverse fields ranging from the study of 3D atomic arrangements in matter to the study of human health in medicine. Despite its very diverse applications, the core of tomography remains the same, that is, a mathematical method must be implemented to reconstruct the 3D structure of an object from a number of 2D projections. In many scientific applications, however, the number of projections that can be measured is limited due to geometric constraints, tolerable radiation dose and/or acquisition speed. Thus it becomes an important problem to obtain the best-possible reconstruction from a limited number of projections. Here, we present the mathematical implementation of a tomographic algorithm, termed GENeralized Fourier Iterative REconstruction (GENFIRE). By iterating between real and reciprocal space, GENFIRE searches for a global solution that is concurrently consistent with the measured data and general physical constraints. The algorithm requires minimal human intervention and also incorporates angular refinement to reduce the tilt angle error. We demonstrate that GENFIRE can produce superior results relative to several other popular tomographic reconstruction techniques by numerical simulations, and by experimentally by reconstructing the 3D structure of a porous material and a frozen-hydrated marine cyanobacterium. Equipped with a graphical user interface, GENFIRE is freely available from our website and is expected to find broad applications across different disciplines.Comment: 18 pages, 6 figure

Capturing Nucleation at 4D Atomic Resolution

Nucleation plays a critical role in many physical and biological phenomena ranging from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases. However, nucleation is a challenging process to study in experiments especially in the early stage when several atoms/molecules start to form a new phase from its parent phase. Here, we advance atomic electron tomography to study early stage nucleation at 4D atomic resolution. Using FePt nanoparticles as a model system, we reveal that early stage nuclei are irregularly shaped, each has a core of one to few atoms with the maximum order parameter, and the order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter distribution and its gradient. These experimental observations differ from classical nucleation theory (CNT) and to explain them we propose the order parameter gradient (OPG) model. We show the OPG model generalizes CNT and energetically favours diffuse interfaces for small nuclei and sharp interfaces for large nuclei. We further corroborate this model using molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. We anticipate that the OPG model is applicable to different nucleation processes and our experimental method opens the door to study the structure and dynamics of materials with 4D atomic resolution.Comment: 42 pages, 5 figures, 12 supplementary figures and one supplementary tabl

Profiling the educational value of computer games

There are currently a number of suggestions for educators to include computer games in formal teaching and learning contexts. Educational value is based on claims that games promote the development of complex learning. Very little research, however, has explored what features should be present in a computer game to make it valuable or conducive to learning. We present a list of required features for an educational game to be of value, informed by two studies, which integrated theories of Learning Environments and Learning Styles. A user survey showed that some requirements were typical of games in a particular genre, while other features were present across all genres. The paper concludes with a proposed framework of games and features within and across genres to assist in the design and selection of games for a given educational scenari

The presence of a (1 × 1) oxygen overlayer on ZnO(0001) surfaces and at Schottky interfaces

The atomic surface and interface structures of uncoated and metal-coated epi-polished ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All uncoated samples showed the presence of a fully occupied (1 × 1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer was clearly seen at the interface of ZnO with both elemental and oxidized metal Schottky contact layers. No significant atomic relaxations were observed at surfaces and interfaces processed under typical device fabrication conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98601/1/0953-8984_24_9_095007.pd

Understanding Strain‐Induced Phase Transformations in BiFeO3 Thin Films

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113160/1/advs201500041.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/113160/2/advs201500041-sup-0001-S1.pd

Single-shot 3D coherent diffractive imaging of core-shell nanoparticles with elemental specificity.

We report 3D coherent diffractive imaging (CDI) of Au/Pd core-shell nanoparticles with 6.1 nm spatial resolution with elemental specificity. We measured single-shot diffraction patterns of the nanoparticles using intense x-ray free electron laser pulses. By exploiting the curvature of the Ewald sphere and the symmetry of the nanoparticle, we reconstructed the 3D electron density of 34 core-shell structures from these diffraction patterns. To extract 3D structural information beyond the diffraction signal, we implemented a super-resolution technique by taking advantage of CDI's quantitative reconstruction capabilities. We used high-resolution model fitting to determine the Au core size and the Pd shell thickness to be 65.0 ± 1.0 nm and 4.0 ± 0.5 nm, respectively. We also identified the 3D elemental distribution inside the nanoparticles with an accuracy of 3%. To further examine the model fitting procedure, we simulated noisy diffraction patterns from a Au/Pd core-shell model and a solid Au model and confirmed the validity of the method. We anticipate this super-resolution CDI method can be generally used for quantitative 3D imaging of symmetrical nanostructures with elemental specificity