Joshua C. Agar

Bio: Joshua C. Agar is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Ferroelectricity & Dielectric. The author has an hindex of 15, co-authored 31 publications receiving 682 citations. Previous affiliations of Joshua C. Agar include Georgia Institute of Technology & University of Illinois at Urbana–Champaign.

New modalities of strain-control of ferroelectric thin films

Abstract: Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Highly mobile ferroelastic domain walls in compositionally graded ferroelectric thin films

Abstract: Domains and domain walls are critical in determining the response of ferroelectrics, and the ability to controllably create, annihilate, or move domains is essential to enable a range of next-generation devices. Whereas electric-field control has been demonstrated for ferroelectric 180° domain walls, similar control of ferroelastic domains has not been achieved. Here, using controlled composition and strain gradients, we demonstrate deterministic control of ferroelastic domains that are rendered highly mobile in a controlled and reversible manner. Through a combination of thin-film growth, transmission-electron-microscopy-based nanobeam diffraction and nanoscale band-excitation switching spectroscopy, we show that strain gradients in compositionally graded PbZr1-xTixO3 heterostructures stabilize needle-like ferroelastic domains that terminate inside the film. These needle-like domains are highly labile in the out-of-plane direction under applied electric fields, producing a locally enhanced piezoresponse. This work demonstrates the efficacy of novel modes of epitaxy in providing new modalities of domain engineering and potential for as-yet-unrealized nanoscale functional devices.

Unexpected crystal and domain structures and properties in compositionally graded PbZr(1-x)Ti(x)O3 thin films.

Abstract: Synthesis of compositionally graded versions of PbZr(1-x)Ti(x)O3 thin films results in unprecedented strains (as large as ≈4.5 × 10(5) m(-1)) and correspondingly unexpected crystal structures, ferroelectric domain structures, and properties. This includes the observation of built-in electric fields in films as large as 200 kV/cm. Compositional and strain gradients could represent a new direction of strain-control of materials.

Improved Pyroelectric Figures of Merit in Compositionally Graded PbZr1–xTixO3 Thin Films

Abstract: Pyroelectric materials have been widely used for a range of thermal-related applications including thermal imaging/sensing, waste heat energy conversion, and electron emission. In general, the figures of merit for applications of pyroelectric materials are proportional to the pyroelectric coefficient and inversely proportional to the dielectric permittivity. In this context, we explore single-layer and compositionally graded PbZr1–xTixO3 thin-film heterostructures as a way to independently engineer the pyroelectric coefficient and dielectric permittivity of materials and increase overall performance. Compositional gradients in thin films are found to produce large strain gradients which generate large built-in potentials in the films that can reduce the permittivity while maintaining large pyroelectric response. Routes to enhance the figures of merit of pyroelectric materials by 3–12 times are reported, and comparisons to standard materials are made.

Effect of 90° domain walls and thermal expansion mismatch on the pyroelectric properties of epitaxial PbZr0.2Ti0.8O3 thin films.

Abstract: We have investigated the contribution of 90° domain walls and thermal expansion mismatch to pyroelectricity in PbZr(0.2)Ti(0.8)O(3) thin films. The first phenomenological models to include extrinsic and secondary contributions to pyroelectricity in polydomain films predict significant extrinsic contributions (arising from the temperature-dependent motion of domain walls) and large secondary contributions (arising from thermal expansion mismatch between the film and the substrate). Phase-sensitive pyroelectric current measurements are applied to model thin films for the first time and reveal a dramatic increase in the pyroelectric coefficient with increasing fraction of in-plane oriented domains and thermal expansion mismatch.

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

Abstract: The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin ( 5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

A Survey on Wireless Body Area Networks: Technologies and Design Challenges

Abstract: Interest in Wireless Body Area Networks (WBANs) has increased significantly in recent years thanks to the advances in microelectronics and wireless communications. Owing to the very stringent application requirements in terms of reliability, energy efficiency, and low device complexity, the design of these networks requires the definition of new protocols with respect to those used in general purpose wireless sensor networks. This motivates the effort in research activities and in standardisation process of the last years. This survey paper aims at reporting an overview of WBAN main applications, technologies and standards, issues in WBANs design, and evolutions. Some case studies are reported, based on both real implementation and experimentation on the field, and on simulations. These results have the aim of providing useful insights for WBAN designers and of highlighting the main issues affecting the performance of these kind of networks.

Pyroelectric materials and devices for energy harvesting applications

Abstract: This review covers energy harvesting technologies associated with pyroelectric materials and systems. Such materials have the potential to generate electrical power from thermal fluctuations and is a less well explored form of thermal energy harvesting than thermoelectric systems. The pyroelectric effect and potential thermal and electric field cycles for energy harvesting are explored. Materials of interest are discussed and pyroelectric architectures and systems that can be employed to improve device performance, such as frequency and power level, are described. In addition to the solid materials employed, the appropriate pyroelectric harvesting circuits to condition and store the electrical power are discussed.

Thin-film ferroelectric materials and their applications

Abstract: Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. Strain engineering can be used to control the properties of thin-film ferroelectric materials, which are promising for electronic, thermal, photovoltaic and transduction applications. This Review addresses fundamental aspects, novel ways to control materials properties and the development of new ferroelectric-based devices.

Exploring Chemical, Mechanical, and Electrical Functionalities of Binders for Advanced Energy-Storage Devices

Abstract: Tremendous efforts have been devoted to the development of electrode materials, electrolytes, and separators of energy-storage devices to address the fundamental needs of emerging technologies such as electric vehicles, artificial intelligence, and virtual reality. However, binders, as an important component of energy-storage devices, are yet to receive similar attention. Polyvinylidene fluoride (PVDF) has been the dominant binder in the battery industry for decades despite several well-recognized drawbacks, i.e., limited binding strength due to the lack of chemical bonds with electroactive materials, insufficient mechanical properties, and low electronic and lithium-ion conductivities. The limited binding function cannot meet inherent demands of emerging electrode materials with high capacities such as silicon anodes and sulfur cathodes. To address these concerns, in this review we divide the binding between active materials and binders into two major mechanisms: mechanical interlocking and interfacial b...