Department of Materials Science and Engineering
http://digital.library.wisc.edu/1793/290
2024-03-28T18:03:45ZSynthetic and dynamic control in strongly correlated transition metal oxides
http://digital.library.wisc.edu/1793/83659
Synthetic and dynamic control in strongly correlated transition metal oxides
Marks, Samuel David
Binary and mixed oxides incorporating transition metal cations host a broad range of scientifically compelling and technologically significant optical, electronic, and magnetic properties. Transition metal oxides (TMOs) are being explored for applications in energy storage, optoelectronics, sensors, and magnetic storage among many other potential uses. The diverse range of physical properties within this class of materials arises from the large flexibility in chemical compositions and crystal structures. These compositions and structures can be generated during synthesis or tailored after synthesis with external stimuli.
In this thesis, I develop strategies for reaching structural and chemical states in transition metal oxides with technologically important optical and electronic properties. I demonstrate a new synthesis strategy for single-crystal SrVO3 films – a transparent conducting oxide with potential applications in display and photovoltaic technologies – using solid-phase epitaxy. By this technique, epitaxial layers of SrVO3 are crystallized from amorphous precursor films. The electrical conductivity and visible light transmission in these epilayers are comparable with SrVO3 formed through other epitaxial synthesis methods. This synthesis route employs thin film deposition and crystallization techniques that are scalable to m2 surface areas. Scalability is a crucial step for commercial applications of transparent conducting oxide layers.
Crystal growth from amorphous precursor films is a recent development for transition metal oxides that have traditionally been synthesized using vapor-phase epitaxy. As a result, fundamental insight into the amorphous-to-crystalline transformation and defect formation processes in solid-phase epitaxy for transition metal oxides is comparatively rare. In situ synchrotron x-ray characterization is a powerful experimental approach for gathering mechanistic insight for crystal growth processes. I have designed new instrumentation for synchrotron x-ray studies of the amorphous layer deposition, crystallization, and defect formation processes inherent to solid-phase epitaxy. This instrumentation combines a vacuum sample deposition and crystallization environment with x-ray focusing optics for in situ x-ray microbeam diffraction, reflectivity, and scattering studies. Design features and key capabilities are demonstrated through a series of results from experiments performed during the commissioning of the instrument at the Advanced Photon Source.
In a separate ex-situ study, I examine the crystal structure of micrometer-scale regions of SrTiO3 crystallized from nanoscale seeds using lateral solid-phase crystallization. Using a high-energy synchrotron x-ray beam focused to 200 nm, I reveal a continuous rotation in the lattice planes in the laterally crystallized regions. A rotation of nearly 25° per micrometer of lateral crystallization is measured for several SrTiO3 crystals independent of the crystallographic orientation of the growth front. The uniform lattice rotation rate suggests a single defect formation process that is characteristic of lateral crystal growth through an amorphous precursor layer. These findings support a hypothesis that the lattice rotation is driven by dislocations that form in response to mechanical stresses arising from the density difference across the crystal-amorphous interface.
¬Controlling the oxygen environment is crucial to forming specific structural phases during synthesis. Similarly, modifications to oxygen stoichiometry can be used to modify the physical properties in epitaxial thin films of multivalent transition metal oxides. In this project, I use x-ray nanobeam diffraction and scanning near-field optical microscopy to simultaneously probe the structural and optoelectronic features of oxygen-deficient epitaxial monoclinic vanadium dioxide thin-films. In this study, an electrically conductive phase is patterned in insulating vanadium dioxide using intense electric fields delivered from an atomic force microscope probe. Electrical conductivity arises from oxygen vacancies created in the presence of the electric field that modify the electronic band structure. The stability and relaxation of the electrically conducting state are governed by the oxygen vacancy dynamics that can be manipulated with hard x-ray irradiation. This study demonstrates a way to manipulate nanoscale structural and electronic states in vanadium dioxide with local electric fields and focused hard x-rays, bringing new insights into the stability of the oxygen-deficient conductive phase of vanadium dioxide.
2022-07-31T00:00:00ZPhotoinduced Structural Phenomena in Ferroelectric Oxide Electronic Materials Investigated by Synchrotron X-ray Diffraction
http://digital.library.wisc.edu/1793/81678
Photoinduced Structural Phenomena in Ferroelectric Oxide Electronic Materials Investigated by Synchrotron X-ray Diffraction
Ahn, Youngjun
My thesis research investigates photoinduced phenomena in ferroelectric electronic oxide
materials, with a focus on changes that occur on sub-nanosecond timescales and length scales
ranging from single unit cells to the mesoscopic ferroelectric polarization pattern. The
experiments employ time-resolved synchrotron X-ray diffraction and microscopy. A series
of X-ray diffraction experiments and theoretical calculations were employed in order to
understand photoinduced phenomena from a structural perspective. The dynamics of the
photoinduced phenomena were analyzed to obtain insight into the mechanisms of lightinduced
structural effects in ferroelectric materials. Taken together, the results show that
bound charges due to the polarization discontinuity at domain boundaries or interfaces with
non-polar materials have an important role in photoinduced structural phenomena in
ferroelectrics with nanoscale domain patterns. The photoexcitation phenomena reported in
this thesis are excited by ultrashort optical pulse. Optical absorption is followed by a rapid
deformation of the equilibrium structure via a series of pathways that are reported in detail.
This thesis reports studies of photoinduced phenomena in three ferroelectric materials
system. The first results involve the dynamics of ferroelectric nanodomain patterns within
PbTiO3/SrTiO3 superlattices. Previous studies have found that the steady-state domain
pattern can be modified by changing mechanical and electrostatic boundary conditions. In a
series of in-situ X-ray diffraction experiments, we have discovered a photoinduced
transformation to uniform polarization state. Thermodynamic calculations reveal that the
uniform polarization state is energetically stabilized by the screening of bound charges. An
analysis of the relaxation dynamics indicates that trapped charge carriers have an important
role in setting the concentration of mobile charge carriers. The results are reported in Ahn et
al., Phys. Rev. Lett. 119, 057601 (2017).
A second study involves low-strain BaTiO3 thin films. Photoexcitation leads to a
reorientation of the domain walls in this system on a sub-nanosecond timescale. The
reorientation is observed only in the room-temperature regime in which two phases of the
domain pattern coexist. Electrostatic calculations show that the domain wall reorientation
results from the screening of the bound charges at domain walls. An alternative model based
on an elastic response to optically induced expansion is not consistent with the experimental
results. The bound charges can arise due to roughness and disorder of the domain walls and
from weak in-plane polarization components, both of which are reduced in the hightemperature
single-domain phase.
Finally, we report an optically induced transformation between structural phases of a
compressively strained BiFeO3 film. These structural phases have distinct electronic and
magnetic properties coupled to the crystal structures and thus the system has a potential to
enable properties to be manipulated significantly via this phase transformation. Timeresolved
synchrotron X-ray diffraction microscopy showed the photoinduced phase
transformation on nanosecond timescale. Thermodynamic free energy calculations provide
insight into the phase transformation [Ahn et al., Phys. Rev. Lett. 123, 045703 (2019)].
2020-07-01T00:00:00ZEpitaxial and Lateral Solid-Phase Crystallization of Complex Oxides
http://digital.library.wisc.edu/1793/79896
Epitaxial and Lateral Solid-Phase Crystallization of Complex Oxides
Chen, Yajin
The crystallization of complex-oxide materials through a transformation from the amorphous to crystalline forms presents a range of new opportunities to synthesize new materials, and simultaneously poses important scientific challenges. New crystallization method complements more conventional vapor-phase epitaxy techniques for epitaxial complex-oxide thin film growth that involve long-range surface diffusion on 2D planar crystal surfaces. The vapor-phase techniques are not readily adaptable to creating nanoscale epitaxial complex-oxide crystals. The alternative synthesis method described in this thesis is solid-phase crystallization, which is the crystallization of amorphous oxides, often in the form of thin films, by post-deposition heating. The creation of epitaxial complex-oxide nanostructures can facilitate their integration in 3D electronic, optoelectronic and ionic devices.
Epitaxial complex-oxide crystals in intricate geometries can be created by solid-phase crystallization employing patterned substrates with a distribution of isolated crystalline seeds. This method requires the study of distinct crystal growth and nucleation kinetics on epitaxial and non-epitaxial surfaces. Nanoscale seeded crystallization can be achieved by understanding the relative rates of nucleation and lateral crystal growth processes, and the role of seeds in determining the overall orientation of the resulting crystals. Epitaxial complex-oxide thin films in intricate geometries with an expanded range of compositions can be created by combining the use of atomic layer deposition (ALD) and solid-phase crystallization, with the development of new ALD procedures to deposit amorphous oxide films and the study of the subsequent crystallization processes to select the crystalline structures of the crystallized film. ALD itself allows for the conformal deposition of thin films over non-planar surfaces.
Solid-phase crystallization can also be used to deposit epitaxial complex-oxide thin films with a wider range of compositions, including those that cannot be deposited from the vapor phase at high temperatures. Such oxides include the oxides that have complex compositions and volatile components. The different kinetic constraints of solid-phase crystallization allow the epitaxial growth of those oxide thin films because of the slow diffusion in the solid state at relatively low crystallization temperatures.
This thesis describes the discovery that, at low crystallization temperatures, epitaxial crystal growth of the model perovskite SrTiO3 on single-crystal SrTiO3 propagates over long distances without nucleation of SrTiO3 on Si with a native oxide. Two kinds of isolated nanoscale seed crystals are employed to study the seeded lateral crystallization of SrTiO3, yielding highly similar results. Micron-scale crystalline regions form surrounding the seeds before encountering separately nucleated crystals away from the seeds. Seed crystals play an important role in determining the orientations of the resulting crystals. New chemical precursors and ALD procedures were developed to grow amorphous PrAlO3 films. An epitaxial γ-Al2O3 layer formed at the interface between the PrAlO3 film and (001) SrTiO3 substrate during the deposition. Epitaxial PrAlO3 films were achieved on (001) γ-Al2O3/SrTiO3 by solid-phase epitaxy. The study of SrTiO3 and PrAlO3 is also applicable to a series of chemically and structurally similar functional ABO3 compounds.
The concepts of solid-phase crystallization also apply to oxides with multiple metal ions and more complex crystal structure. The kinetic processes occurring during the crystallization of ScAlMgO4, on (0001) sapphire substrates are quite different at two different temperatures. Epitaxial ScAlMgO4 crystals grow through the film thickness at a crystallization temperature of 950 ºC. Solid-state reaction and evaporation of the component Sc prohibits the formation of large ScAlMgO4 crystals at a crystallization temperature of 1400 ºC. Low-temperature crystallization can be used to create epitaxial oxide thin films with complex compositions and volatile components.
2019-01-01T00:00:00ZAtomic Scale Medium Range Order and Relaxation Dynamics in Metallic Glass
http://digital.library.wisc.edu/1793/79222
Atomic Scale Medium Range Order and Relaxation Dynamics in Metallic Glass
Zhang, Pei
We studied the atomic scale structure of bulk metallic glass (BMG) with the combination of fluctuation electron microscopy (FEM) and hybrid reverse Monte Carlo (HRMC) simulation. Medium range order (MRO), which occupies the length scale between short range order (SRO) and long-range order, plays an important role on the properties of metallic glass, but the characterization of MRO in experiment is difficult because conventional techniques are not sensitive to the structure at MRO scale. Compared with the X-ray and neutron which can measure SRO by two-body correlation functions, FEM is an effective way to detect MRO structure through three and four-body correlation functions, providing information about the size, distribution, and internal structure of MRO combing HRMC modeling. Thickness estimation is necessary in FEM experiment and HRMC calculation, so in Chapter 3, we measured the elastic and inelastic mean free paths of metallic glass alloys based on focused ion beam prepared thin samples with measured thickness gradients. We developed a model based on the Wentzel atomic model to predict the elastic mean free path for other amorphous materials. In Chapter 4, we studied the correlation of MRO and glass forming ability ZrCuAl alloy. Results from Variable resolution fluctuation microscopy show that in Zr50Cu35Al15 the crystal-like clusters shrink but become more ordered, while icosahedral-like clusters grow. Compared with Zr50Cu45Al5, Zr50Cu35Al15 with poorer glass forming ability exhibits more stable crystal-like structure under annealing, indicating that destabilizing crystal-like structures is important to achieve better glass forming ability in this alloy. In Chapter 5, we studied the crystallization and MRO structural in deformed and quenched Ni60Nb40 metallic glass. The deformed Ni60Nb40 contains fewer icosahedral-like Voronoi clusters and more crystal-like and bcc-like Voronoi clusters. The crystal-like and bcc-like medium range order clusters may be the structural origin for its lower crystallization temperature compared with quenched alloy.
Dynamics heterogeneity is proposed to be the microscopic origin of the dynamic nature of glass transition. Some experimental evidences and simulation have indicated that different regions of materials indeed relax at fast or slow rate. However, the spatial distribution of relaxation time visualized from experiment as the direct evidence of heterogeneous dynamics is still challenging. We proposed to measure the structural dynamics of supercooled metallic glasses with electron correlation microscopy (ECM) technique at the nanometer scale. ECM was developed as a way to measure structural relaxation times of liquids with nanometer-scale spatial resolution using the coherent electron scattering equivalent of photon correlation spectroscopy. In chapter 6, we studied the experimental requirements of ECM to obtain reliable results. For example, the trajectory length must be at least 40 times the relaxation time to obtain a well-converged g2(t), and the time per frame must be less than 0.1 time the relaxation time to obtain sufficient sampling. ECM experiment was firstly realized in scanning transmission electron microscopy (STEM) mode and applied to measure the structural relaxation time of Pd based metallic glass. In order to overcome the drift problem and capture the spatial information, we developed ECM experiment in dark field (DF) mode. In Chapter 7, through DF-ECM, we visualized the spatially heterogeneous dynamics by in-situ heating Pt57.5Cu14.7Ni5.3P22.5 nanowire into supercooled liquid state, and quantify the size of the heterogeneity by four point correlation function. The thickness effect and temporal evolution of the heterogeneous domain was also discussed. Additionally, a fast near surface dynamics was discovered, providing an effective mechanism for surface crystallization of liquids by homogeneous nucleation.
2017-01-01T00:00:00Z