Nanoscale impedance and complex properties in energy-related systems
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Nanoscale characterization of complex properties in energy materials and devices Many strategies for energy generation, storage, and efficiency exploit complex properties of materials. Ionic transport and electrochemical reactions dictate the behavior of many electrochemical devices, including solid oxide and polymer electrolyte fuel cells1 and Li-ion batteries.2 Ionic phenomena are equally important in other functional materials and devices, including electroresistive and memristive electronic devices,3,4 molecular electronic devices,5 piezoresistance,6 and ferroelectric resistive switching.7 An important class of these complex properties is based on polarization, represented by the dielectric constant, κ. Dielectric polarization is inherent in the interaction of light with solids, the motion of ions in ionic materials, electrical charges trapped at interfaces, and dipoles in organic compounds and biomolecules. Progress in these applications requires probing complex properties at the nanoscale, establishing the origins of physical behavior, and linking a macroscopic device or material functionality with advanced theoretical studies. Scanning probe microscopy (SPM) has become one of the primary tools to interrogate the local properties of a variety of materials at the nanoscale. Recently, the extension of SPM to probe local ionic and electronic transport, dielectric, optical, ferroelectric, and magnetic properties have made the technique a more
powerful platform to examine the complex behavior of functional surfaces and interfaces at the nanoscale. A number of these SPM techniques are discussed in this issue. This article focuses on the acquisition of nanoscale impedance information using an atomic force microscope (AFM). AFM-based impedance spectroscopy provides localized impedance information for electrical, electrochemical, and dielectric phenomena at the nanoscale by utilizing the conductive AFM tip as a moving electrode to detect current response as a function of time and frequency under controlled environments. In this article, we summarize the principles of AFM-based impedance measurement and review recent examples applying this technique to a variety of functional materials systems, including fuel cells, Li-ion batteries, photoactive biomembranes, polymeric coatings, and semiconducting oxides.
Fundamental principles of impedance Impedance (Z) is a measure of a system’s ability to impede the flow of electrical current, providing an extension of electrical resistance to time or frequency dependent phenomena. Impedance is defined as the ratio between an applied sinusoidal voltage perturbation, V(t) = V0eiωt, and a system’s resultant current response, I(t) = I0eiωt, where V(t) and I(t) are the potential and current at time t, V0 and I0 are the amplitudes of the voltage and current signals, and ω is the radial frequency. Laplace
Wonyoung Lee, Nuclear Science and Engineering Department, Massachusetts Institute of Technology; [email protected] Fritz B. Prinz, Mechanical Engineering and Materials Science and Engineering
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