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Scholarly Works (11 results)

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Peer Reviewed

Atomic-scale study of low-dimensional features in ferroelectric thin films

Huyan, Huaixun
Advisor(s): Pan, Xiaoqing

UC Irvine Electronic Theses and Dissertations (2022)

Due to their spontaneous polarization, ferroelectric materials have a variety of applications in electronic, optical, and electromechanical devices. In addition, recent advances in atomic-level control of thin film growth techniques, including molecular beam epitaxy and pulsed laser deposition, have made it possible to grow ferroelectric thin films with low-dimensional features leading to the discovery of many new phenomena and functional properties stemming from the coupling between the ferroelectricity and the low-dimensional features. However, since the features are usually in nano- or atomic- size, conventional characterization techniques have low spatial resolutions cannot accurately resolve these tiny features. Herein, I reported a comprehensive atomic-scale characterization method to study the low-dimensional features in ferroelectric materials using advanced transmission electron microscopy (TEM) techniques, including in situ TEM, scanning TEM (STEM), 4-dimensional STEM (4D STEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). This opens new opportunities for studying both the mechanisms for how macroscopic properties can emerge from atomic-scale phenomena and the fundamental physics of ferroelectricity.First, we systematically study the spontaneous polarization induced positive/negative charge accumulation at ferroelectric-insulator interface. Second, we show dislocation-assisted BiMnOx nanochannels embedded in SrTiO3/La0.67Sr0.33MnO3 thin films having diode-like behavior. Finally, we demonstrate superelasticity and domain switching of freestanding BiFeO3 films using in situ straining and biasing TEM, respectively. During the in situ straining experiment, the film shows an unprecedented tremendous lattice strain up to 34.4%, which is two orders of magnitude higher than their bulk counterparts and comparable to ductile metals.

Cover page: Atomic-scale study of low-dimensional features in ferroelectric thin films

Article
Peer Reviewed

Structures and Electronic Properties of Domain Walls in BiFeO3 Thin Films

UC Irvine Previously Published Works (2019)

Domain walls (DWs) in ferroelectrics are atomically sharp and can be created, erased, and reconfigured within the same physical volume of ferroelectric matrix by external electric fields. They possess a myriad of novel properties and functionalities that are absent in the bulk of the domains, and thus could become an essential element in next-generation nanodevices based on ferroelectrics. The knowledge about the structure and properties of ferroelectric DWs not only advances the fundamental understanding of ferroelectrics, but also provides guidance for the design of ferroelectric-based devices. In this article, we provide a review of structures and properties of DWs in one of the most widely studied ferroelectric systems, BiFeO₃ thin films. We correlate their conductivity and photovoltaic properties to the atomic-scale structure and dynamic behaviors of DWs.

Cover page: Structures and Electronic Properties of Domain Walls in BiFeO3 Thin Films

Creative Commons 'BY' version 4.0 license

Article
Peer Reviewed

Anisotropic and hierarchical SiC@SiO2 nanowire aerogel with exceptional stiffness and stability for thermal superinsulation

UC Irvine Previously Published Works (2020)

Ceramic aerogels are promising lightweight and high-efficient thermal insulators for applications in buildings, industry, and aerospace vehicles but are usually limited by their brittleness and structural collapse at high temperatures. In recent years, fabricating nanostructure-based ultralight materials has been proved to be an effective way to realize the resilience of ceramic aerogels. However, the randomly distributed macroscale pores in these architectures usually lead to low stiffness and reduced thermal insulation performance. Here, to overcome these obstacles, a SiC@SiO2 nanowire aerogel with a nanowire-assembled anisotropic and hierarchical microstructure was prepared by using directional freeze casting and subsequent heat treatment. The aerogel exhibits an ultralow thermal conductivity of ~14 mW/m·K, an exceptional high stiffness (a specific modulus of ~24.7 kN·m/kg), and excellent thermal and chemical stabilities even under heating at 1200°C by a butane blow torch, which makes it an ideal thermally superinsulating material for applications under extreme conditions.

Cover page: Anisotropic and hierarchical SiC@SiO2 nanowire aerogel with exceptional stiffness and stability for thermal superinsulation

Article
Peer Reviewed

Direct observation of elemental fluctuation and oxygen octahedral distortion-dependent charge distribution in high entropy oxides

UC Irvine Previously Published Works (2022)

The enhanced compositional flexibility to incorporate multiple-principal cations in high entropy oxides (HEOs) offers the opportunity to expand boundaries for accessible compositions and unconventional properties in oxides. Attractive functionalities have been reported in some bulk HEOs, which are attributed to the long-range compositional homogeneity, lattice distortion, and local chemical bonding characteristics in materials. However, the intricate details of local composition fluctuation, metal-oxygen bond distortion and covalency are difficult to visualize experimentally, especially on the atomic scale. Here, we study the atomic structure-chemical bonding-property correlations in a series of perovskite-HEOs utilizing the recently developed four-dimensional scanning transmission electron microscopy techniques which enables to determine the structure, chemical bonding, electric field, and charge density on the atomic scale. The existence of compositional fluctuations along with significant composition-dependent distortion of metal-oxygen bonds is observed. Consequently, distinct variations of metal-oxygen bonding covalency are shown by the real-space charge-density distribution maps with sub-ångström resolution. The observed atomic features not only provide a realistic picture of the local physico-chemistry of chemically complex HEOs but can also be directly correlated to their distinctive magneto-electronic properties.

Cover page: Direct observation of elemental fluctuation and oxygen octahedral distortion-dependent charge distribution in high entropy oxides

Article
Peer Reviewed

Dislocation-Assisted Quasi-Two-Dimensional Semiconducting Nanochannels Embedded in Perovskite Thin Films

UC Irvine Previously Published Works (2023)

Defect engineering in perovskite thin films has attracted extensive attention recently due to the films' atomic-scale modification, allowing for remarkable flexibility to design novel nanostructures for next generation nanodevices. However, the defect-assisted three-dimensional nanostructures in thin film matrices usually has large misfit strain and thus causes unstable thin film structures. In contrast, defect-assisted one- or two-dimensional nanostructures embedded in thin films can sustain large misfit strains without relaxation, which make them suitable for defect engineering in perovskite thin films. Here, we reported the fabrication and characterization of edge-type misfit dislocation-assisted two-dimensional BiMnO_x nanochannels embedded in SrTiO₃/La_0.7Sr_0.3MnO₃/TbScO₃ perovskite thin films. The nanochannels are epitaxially grown from the surrounding films without noticeable misfit strain. Diode-like current rectification was spatially observed at nanochannels due to the formation of Schottky junctions between BiMnO_x nanochannels and conducting La_0.7Sr_0.3MnO₃ thin films. Such atomically scaled heterostructures constitute more flexible ultimate functional units for nanoscale electronic devices.

Cover page: Dislocation-Assisted Quasi-Two-Dimensional Semiconducting Nanochannels Embedded in Perovskite Thin Films

Article
Peer Reviewed

Nanoscale imaging of phonon dynamics by electron microscopy

UC Irvine Previously Published Works (2022)

Spatially resolved vibrational mapping of nanostructures is indispensable to the development and understanding of thermal nanodevices¹, modulation of thermal transport² and novel nanostructured thermoelectric materials^3-5. Through the engineering of complex structures, such as alloys, nanostructures and superlattice interfaces, one can significantly alter the propagation of phonons and suppress material thermal conductivity while maintaining electrical conductivity². There have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. Here we demonstrate two-dimensional spatial mapping of phonons in a single silicon-germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy in the transmission electron microscope. Tracking the variation of the Si optical mode in and around the QD, we observe the nanoscale modification of the composition-induced red shift. We observe non-equilibrium phonons that only exist near the interface and, furthermore, develop a novel technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure: a major advancement in the field. Our work unveils the non-equilibrium phonon dynamics at nanoscale interfaces and can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.

Cover page: Nanoscale imaging of phonon dynamics by electron microscopy

Article
Peer Reviewed

Manipulation of the Ferromagnetism in LaCoO3 Thin Films Through Cation‐Stoichiometric Engineering

UC Berkeley Previously Published Works (2023)

Spin-state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low-spin to high-spin transition in LaCoO3 thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen-vacancy formation, spin-state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3 thin films is systematically modulated by varying the oxygen pressure during thin-film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen-vacancy and spin-ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low-spin to high-spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation-off-stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3 thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation-stoichiometry engineering.

Cover page: Manipulation of the Ferromagnetism in LaCoO3 Thin Films Through Cation‐Stoichiometric Engineering

Creative Commons 'BY-NC-SA' version 4.0 license

Article
Peer Reviewed

Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolyte Li_0.375Sr_0.4375Ta_0.75Zr_0.25O₃.

UC San Diego Previously Published Works (2023)

Oxide solid electrolytes (OSEs) have the potential to achieve improved safety and energy density for lithium-ion batteries, but their high grain-boundary (GB) resistance generally is a bottleneck. In the well-studied perovskite oxide solid electrolyte, Li_3xLa_2/3-xTiO₃ (LLTO), the ionic conductivity of grain boundaries is about three orders of magnitude lower than that of the bulk. In contrast, the related Li_0.375Sr_0.4375Ta_0.75Zr_0.25O₃ (LSTZ0.75) perovskite exhibits low grain boundary resistance for reasons yet unknown. Here, we use aberration-corrected scanning transmission electron microscopy and spectroscopy, along with an active learning moment tensor potential, to reveal the atomic scale structure and composition of LSTZ0.75 grain boundaries. Vibrational electron energy loss spectroscopy is applied for the first time to reveal atomically resolved vibrations at grain boundaries of LSTZ0.75 and to characterize the otherwise unmeasurable Li distribution therein. We find that Li depletion, which is a major reason for the low grain boundary ionic conductivity of LLTO, is absent for the grain boundaries of LSTZ0.75. Instead, the low grain boundary resistivity of LSTZ0.75 is attributed to the formation of a nanoscale defective cubic perovskite interfacial structure that contained abundant vacancies. Our study provides new insights into the atomic scale mechanisms of low grain boundary resistivity.

Cover page: Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolyte Li<sub>0.375</sub>Sr<sub>0.4375</sub>Ta<sub>0.75</sub>Zr<sub>0.25</sub>O<sub>3</sub>.

Article
Peer Reviewed

Spontaneous Hall effect enhanced by local Ir moments in epitaxial Pr2Ir2O7 thin films

UC Irvine Previously Published Works (2020)

Rare-earth pyrochlore iridates (RE2Ir2O7) consist of two interpenetrating cation sublattices, the RE with highly frustrated magnetic moments, and the iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. While bulk Pr2Ir2O7 shows a spontaneous hysteretic Hall effect below 1.5 K, we observe the effect at elevated temperatures up to 15 K in epitaxial thin films on (111) yttria-stabilized zirconia (YSZ) substrates synthesized via solid-phase epitaxy. Similar to the bulk, the lack of observable long-range magnetic order in the thin films points to a topological origin. We use synchrotron-based element-specific x-ray diffraction and x-ray magnetic circular dichroism to compare powders and thin films to attribute the spontaneous Hall effect in the films to localization of the Ir moments. We link the thin-film Ir local moments to lattice distortions absent in the bulklike powders. We conclude that the elevated-Temperature spontaneous Hall effect is caused by the topological effect originating either from the Ir or Pr sublattice, with interaction strength enhanced by the Ir local moments. This spontaneous Hall effect with weak net moment highlights the effect of vanishingly small lattice distortions as a means to discover topological phenomena in metallic frustrated magnetic materials.

Cover page: Spontaneous Hall effect enhanced by local Ir moments in epitaxial Pr2Ir2O7 thin films

Article
Peer Reviewed

Ferroelectricity in a semiconducting all-inorganic halide perovskite

UC Berkeley Previously Published Works (2022)

Ferroelectric semiconductors are rare materials with both spontaneous polarizations and visible light absorptions that are promising for designing functional photoferroelectrics, such as optical switches and ferroelectric photovoltaics. The emerging halide perovskites with remarkable semiconducting properties also have the potential of being ferroelectric, yet the evidence of robust ferroelectricity in the typical three-dimensional hybrid halide perovskites has been elusive. Here, we report on the investigation of ferroelectricity in all-inorganic halide perovskites, CsGeX₃, with bandgaps of 1.6 to 3.3 eV. Their ferroelectricity originates from the lone pair stereochemical activity in Ge (II) that promotes the ion displacement. This gives rise to their spontaneous polarizations of ~10 to 20 μC/cm², evidenced by both ab initio calculations and key experiments including atomic-level ionic displacement vector mapping and ferroelectric hysteresis loop measurement. Furthermore, characteristic ferroelectric domain patterns on the well-defined CsGeBr₃ nanoplates are imaged with both piezo-response force microscopy and nonlinear optical microscopic method.

Cover page: Ferroelectricity in a semiconducting all-inorganic halide perovskite