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Open Access Publications from the University of California

Scholarly Works (14 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

This dissertation makes a tactful attempt at theoretically guiding materials’ design using first-principles approach with known and/or underexplored material systems as thin-film heterostructures for a myriad of functionalities. In order to do so, I use a two-pronged approach, with one involving high-throughput materials discovery and the other involving careful manipulation of an individual material structure to comprehend its thermodynamic (meta)stability and functional behavior as a polar compound further. First, I focused on two relatively new complex oxide-based material systems BiAlO3 and BiInO3 which were chosen from a high-throughput database for ferroelectric materials. While I was unable to obtain the desired polar functionalities therein, this study highlighted the importance of threshold energy values associated with thermodynamic (meta)stability and that corresponding to the polar structural distortion in bridging the gap between theoretical and experimental approaches towards materials’ design. Next, I focused on a more targeted approach with first-principles calculations performed on Sr2Nb2O7 (an unexplored layer perovskite-based ferroelectric oxide) to design a high-performance piezoelectric material by checking for the disorder tolerance of the soft optical phonon mode with substitutional alloying at niobium site.With the valuable lessons learnt about materials’ design, synthesis, and characterization, I proceeded to choose an antiferroelectric material system PbHfO3 again from the high-throughput ferroelectric database, which was expected to satisfy the determined criteria for thermodynamic stability and polar distortion. I started with exploring the structural, chemical, and electrical properties of antiferroelectric PbHfO3 as epitaxial thin-film heterostructures. The PbHfO3 heterostructures exhibited characteristic double-hysteresis loops with a saturation polarization ≈ 53 µC cm−2 at 1.6 MV cm−1. Further, using chemical modification at both the lead and hafnium sites, I was able to explore two very different functionalities in the resultant PbHfO3-based solid solutions, pointing to the versatility of this material system. First, in Pb1-xSrxHfO3 I observed a strong increase in the electric-breakdown strength and decrease in hysteretic loss, thus enhancing the capacitive energy storage density (Ur) and efficiency (η) with values as high as 77 ± 5 J cm−3 (Ur) and 97 ± 2% (η); well out-performing many known antiferroelectric materials. From there, I explored the PbHf1-xTixO3 system wherein titanium substitution resulted in the evolution of both the structure and electrical properties. For all compositions studied herein, ferroelectric behavior was observed with a rhombohedral-to-tetragonal phase transition with increasing titanium content. Further, intermediate PbHf1-xTixO3 compositions (i.e., x = 0.4-0.55) exhibited a mixed-phase (rhombohedral + monoclinic + tetragonal) structure akin to the morphotropic phase boundary (MPB) observed in the well-studied PbZr1-xTixO3 system. Owing to the structural evolution, an enhancement in the dielectric response was observed in the intermediate compositions pointing to the presence of an MPB-like behavior therein at par with that for PbZr0.52Ti0.48O3 films in the same geometry. With a substantial understanding about piezoelectric material systems and the need to accurately assess their piezoelectric response, especially as thin-film heterostructures where the associated response can be significantly diminished due to substrate-induced clamping, I delved deeper into developing an easy and efficient methodology for the same. This involved measuring the electromechanical surface displacement for films < 1 μm-thick so as to quantify their piezoelectric coefficients using laser Doppler vibrometry while exploring the intricate details about sample geometry, measurement reproducibility and accuracy, error-minimization, and ultimately, finite-element modeling to calculate the direct piezoelectric coefficients. At the end, this dissertation has been concluded by going to back to studying the antiferroelectric PbHfO3 system, however, for its unique electromechanical behavior where the single-layer PbHfO3 heterostructures were found to be exempt from the diminishing effects of substrate-induced clamping while exhibiting electromechanical strain values as high as 1.1-1.2%. Going a step further from single-layer to multilayer heterostructures, I combined PbHfO3 (a non-traditional piezoelectric) and with PbHf1–xTixO3 (a traditional MPB-based) i.e., PbHfO3/PbHf1–xTixO3 as multilayer heterostructures while studying the structural, electrical, and electromechanical behavior with compositional modification (0.3 ≤ x ≤ 0.6) of the PbHf1-xTixO3 layer and as a function of the interface density (0.008-3.1 nm-1). With the optimum composition (i.e., PbHfO3/PbHf0.6Ti0.4O3) and interface density in the multilayer heterostructure (i.e., 1.5 nm-1), the electric-breakdown strength (EB) was found to be enhanced to as high as 6.89 MV/cm, while revealing colossal values for electromechanical strain as high as ~4.6% with a reasonably low threshold value for the antiferroelectric-to-ferroelectric phase transition. As such this dissertation starts with theory-guided synthesis and characterization of different perovskite-based complex oxides as thin-film heterostructures while exploring a multitude of their ferroelectric functionalities. At the end, however, based on the different systems explored, I chose to delve deeper into the antiferroelectric materials systems and by systematically designing multilayer thin-film heterostructures which can surpass the traditional limitations of piezoelectric thin-film heterostructures.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2022)
    Ab initio methods provide a powerful tool in the search for novel polar materials. In particular, there has been a surge to identify lead-free piezoelectric materials to replace PbZr0.52Ti0.48O3. This study examines a computational strategy to identify increased piezoelectric and dielectric responses of alloy systems based on the linear interpolation of force constants, Born effective charges, and internal strain tensors from their end-point compounds. We choose the ferroelectric layered perovskite Sr2Nb2O7as a parent structure and employ this alloying strategy for 19 potential cation substitutions, targeting thermodynamically metastable alloys with high piezoelectric response. From this screening, we identify Sr2Nb2-2xV2xO7as a promising polar system. We conduct large-unit-cell calculations of Sr2Nb2-2xV2xO7at x = 0.0625, 0.125 for multiple cation orderings and find a significant 184% enhanced piezoelectric response. The solid solution system is synthesized as single-crystalline thin-film heterostructures using pulsed-laser deposition, and an enhanced dielectric response is observed at x = 0.05 and at x = 0.1. We present the Sr2Nb2-2xV2xO7alloy system designed through high-throughput computational screening methods with a large calculated piezoelectric response and experimentally verified increased dielectric response. Our methodology is provided as a high-throughput screening tool for novel materials with enhanced polarizability and alloy systems with potential morphotropic phase boundaries.
      Cover page: Theory-Guided Exploration of the Sr2Nb2O7 System for Increased Dielectric and Piezoelectric Properties and Synthesis of Vanadium-Alloyed Sr2Nb2O7Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2022)
      Epitaxial PbHf1−xTixO3/SrTiO3(001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr1−xTixO3 system. End members, PbHfO3 and PbTiO3, were found to possess orthorhombic (Pbam) and tetragonal (P4mm) crystal structures and antiferroelectric and ferroelectric (∼87 μC/cm2) behavior, respectively. PbHf0.75Ti0.25O3 and PbHf0.25Ti0.75O3 solid solutions were both found to be ferroelectric with rhombohedral (R3c, ∼22 μC/cm2) and tetragonal (P4mm, ∼46 μC/ cm2) structures, respectively. For intermediate PbHf1−xTixO3 compositions (e.g., x = 0.4, 0.45, 0.5, and 0.55), a structural transition was observed from rhombohedral (hafnium-rich) to tetragonal (titanium-rich) phases. These intermediate compositions also exhibited mixed-phase structures including R3c, monoclinic (Cm), and P4mm symmetries and, in all cases, were ferroelectric with remanent (5−22 μC/cm2) and saturation (18.5−36 μC/cm2) polarization and coercive field (24−34.5 kV/cm) values increasing with x. While the dielectric constant was the largest for PbHf0.6Ti0.4O3, the MPB is thought to be near x = 0.5 after separation of the intrinsic and extrinsic contributions to the dielectric response. Furthermore, piezoelectric displacement−voltage hysteresis loops were obtained for all chemistries revealing displacement values as good as PbZr0.52Ti0.48O3 films in the same geometry. Thereby, the PbHf1−xTixO3 system is a viable alternative to the PbZr1−xTixO3 system offering comparable performance.
        Cover page: Exploring the Morphotropic Phase Boundary in Epitaxial PbHf1–x Ti x O3 Thin FilmsCreative Commons 'BY-NC-ND' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2020)
        Dielectric capacitors can store and release electric energy at ultrafast rates and are extensively studied for applications in electronics and electric power systems. Among various candidates, thin films based on relaxor ferroelectrics, a special kind of ferroelectric with nanometer-sized domains, have attracted special attention because of their high energy densities and efficiencies. We show that high-energy ion bombardment improves the energy storage performance of relaxor ferroelectric thin films. Intrinsic point defects created by ion bombardment reduce leakage, delay low-field polarization saturation, enhance high-field polarizability, and improve breakdown strength. We demonstrate energy storage densities as high as ~133 joules per cubic centimeter with efficiencies exceeding 75%. Deterministic control of defects by means of postsynthesis processing methods such as ion bombardment can be used to overcome the trade-off between high polarizability and breakdown strength.
          Cover page: Ultrahigh capacitive energy density in ion-bombarded relaxor ferroelectric films
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2022)
          Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length scales. This review traces the evolution of ferroelectric thin-film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high-throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand-in-hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.
            Cover page: Thin‐Film Ferroelectrics
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            Ferroelectrics and related materials (e.g., non-traditional ferroelectrics such as relaxors) have long been used in a range of applications, but with the advent of new ways of modeling, synthesizing, and characterizing these materials, continued access to astonishing breakthroughs in our fundamental understanding come each year. While we still rely on these materials in a range of applications, we continue to re-write what is possible to be done with them. In turn, assumptions that have underpinned the use and design of certain materials are progressively being revisited. This perspective aims to provide an overview of the field of ferroelectric/relaxor/polar-oxide thin films in recent years, with an emphasis on emergent structure and function enabled by advanced synthesis, processing, and computational modeling.
              Cover page: Beyond Expectation: Advanced Materials Design, Synthesis, and Processing to Enable Novel Ferroelectric Properties and Applications
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2022)
              The hafnate perovskites PbHfO3 (antiferroelectric) and SrHfO3 ("potential" ferroelectric) are studied as epitaxial thin films on SrTiO3 (001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb1- x Srx HfO3 system. The resulting (240)-oriented PbHfO3 (Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm-2 at 1.6 MV cm-1 , weak-field dielectric constant ≈186 at 298 K, and an antiferroelectric-to-paraelectric phase transition at ≈518 K. (002)-oriented SrHfO3 films exhibited neither ferroelectric behavior nor evidence of a polar P4mm phase . Instead, the SrHfO3 films exhibited a weak-field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77-623 K) and electric field (-3 to 3 MV cm-1 ). While the lack of ferroelectric order in SrHfO3 removes the potential for MPB, structural and property evolution of the Pb1- x Srx HfO3 (0 ≤ x < 1) system is explored. Strontium alloying increased the electric-breakdown strength (EB ) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (Ur ) and efficiency (η). The composition, Pb0.5 Sr0.5 HfO3 produced the best combination of EB  = 5.12 ± 0.5 MV cm-1 , Ur  = 77 ± 5 J cm-3 , and η = 97 ± 2%, well out-performing PbHfO3 and other antiferroelectric oxides.
                Cover page: Exploring the Pb1−xSrxHfO3 System and Potential for High Capacitive Energy Storage Density and Efficiency
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2020)
                Recent advances in high-throughput computational workflows are expanding the realm of materials for a range of applications. Here, we report the experimental evaluation of two such predicted candidate ferroelectric perovskite oxides: BiAlO3 and BiInO3. Attempts were made to synthesize polar BiAlO3 and BiInO3 using pulsed-laser deposition. Despite exploring a wide range of temperatures, pressures, substrates, laser fluences, and so on, attempts to grow BiAlO3 with this approach resulted in no perovskite phase and decomposition to Bi2O3 and Bi24Al2O40. Various orientations of BiInO3 films were synthesized on multiple substrates, with the best crystallinity demonstrated for (200)-oriented films on MgO (001). Density-functional theory predicts two energetically competitive ground-state structures for BiInO3: Pnma (nonpolar) and Pna21 (polar). BiInO3 films were studied by using X-ray diffraction and second-harmonic generation (SHG) and found to exhibit the nonpolar Pnma structure. Temperature-dependent SHG and dielectric measurements revealed no transition to the polar structure. Optical transmission-absorption studies suggest a direct bandgap of ∼4.5 eV for BiInO3. Our study underscores the need for additional descriptors for synthesizability in assessing the potential of ferroelectric candidate materials identified from high-throughput materials databases.
                  Cover page: Searching for New Ferroelectric Materials Using High-Throughput Databases: An Experimental Perspective on BiAlO3 and BiInO3
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2024)
                  This work presents air-coupled piezoelectric micromachined ultrasonic transducers (pMUTs) with high sound pressure level (SPL) under low-driving voltages by utilizing sputtered potassium sodium niobate K0.34Na0.66NbO3 (KNN) films. A prototype single KNN pMUT has been tested to show a resonant frequency at 106.3 kHz under 4 Vp-p with outstanding characteristics: (1) a large vibration amplitude of 3.74 μm/V, and (2) a high acoustic root mean square (RMS) sound pressure level of 105.5 dB/V at 10 cm, which is 5-10 times higher than those of AlN-based pMUTs at a similar frequency. There are various potential sensing and actuating applications, such as fingerprint sensing, touch point, and gesture recognition. In this work, we present demonstrations in three fields: haptics, loudspeakers, and rangefinders. For haptics, an array of 15 × 15 KNN pMUTs is used as a non-contact actuator to provide a focal pressure of around 160.3 dB RMS SPL at a distance of 15 mm. This represents the highest output pressure achieved by an airborne pMUT for haptic sensation on human palms. When used as a loudspeaker, a single pMUT element with a resonant frequency close to the audible range at 22.8 kHz is characterized. It is shown to be able to generate a uniform acoustic output with an amplitude modulation scheme. In the rangefinder application, pulse-echo measurements using a single pMUT element demonstrate good transceiving results, capable of detecting objects up to 2.82 m away. As such, this new class of high-SPL and low-driving-voltage pMUTs could be further extended to other applications requiring high acoustic pressure and a small form factor.
                    Cover page: High sound pressure piezoelectric micromachined ultrasonic transducers using sputtered potassium sodium niobate.
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2023)
                    Antiferroelectrics, which undergo a field-induced phase transition to ferroelectric order that manifests as double-hysteresis polarization switching, exhibit great potential for dielectric, electromechanical, and electrothermal applications. Compared to their ferroelectric cousins, however, considerably fewer efforts have been made to understand and control antiferroelectrics. Here, it is demonstrated that the polarization switching behavior of an antiferroelectric can be strongly influenced and effectively regulated by point defects. In films of the canonical antiferroelectric PbZrO3 , decreasing oxygen pressure during deposition (and thus increasing adatom kinetic energy) causes unexpected "ferroelectric-like" polarization switching although the films remain in the expected antiferroelectric orthorhombic phase. This "ferroelectric-like" switching is correlated with the creation of bombardment-induced point-defect complexes which pin the antiferroelectric-ferroelectric phase boundaries, and thus effectively delay the phase transition under changing field. The effective pinning energy is extracted via temperature-dependent switching-kinetics studies. In turn, by controlling the concentration of defect complexes, the dielectric tunability of the PbZrO3 can be adjusted, including being able to convert between "positive" and "negative" tunability near zero field. This work reveals the important role and strong capability of defects to engineer antiferroelectrics for new performance and functionalities.
                      Cover page: Defect‐Induced, Ferroelectric‐Like Switching and Adjustable Dielectric Tunability in AntiferroelectricsCreative Commons 'BY-NC-SA' version 4.0 license
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