http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

Fiber-based structures are highly desirable for wearable electronics that are expected to be light-weight, long-lasting, flexible, and conformable Many fibrous structures have been manufactured by well-established lost-effective textile processing technologies, normally at ambient conditions The advancement of nanotechnology has made it feasible to build electronic devices directly on the surface or inside of single fibers, which have typical thickness of several to tens microns However, imparting electronic functions to porous, highly deformable and three-dimensional fiber assemblies and maintaining them during wear represent great challenges from both views of fundamental understanding and practical implementation This article attempts to critically review the current state-of-arts with respect to materials, fabrication techniques, and structural design of devices as well as applications of the fiber-based wearable electronic products In addition, this review elaborates the performance requirements of the fiber-based wearable electronic products, especially regarding the correlation among materials, fiber/textile structures and electronic as well as mechanical functionalities of fiber-based electronic devices Finally, discussions will be presented regarding to limitations of current materials, fabrication techniques, devices concerning manufacturability and performance as well as scientific understanding that must be improved prior to their wide adoption

Fiber‐Based Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications

Flexible nanogenerators that efficiently convert mechanical energy into electrical energy have been extensively studied because of their great potential for driving low-power personal electronics and self-powered sensors. Integration of flexibility and stretchability to nanogenerator has important research significance that enables applications in flexible/stretchable electronics, organic optoelectronics, and wearable electronics. Progress in nanogenerators for mechanical energy harvesting is reviewed, mainly including two key technologies: flexible piezoelectric nanogenerators (PENGs) and flexible triboelectric nanogenerators (TENGs). By means of material classification, various approaches of PENGs based on ZnO nanowires, lead zirconate titanate (PZT), poly(vinylidene fluoride) (PVDF), 2D materials, and composite materials are introduced. For flexible TENG, its structural designs and factors determining its output performance are discussed, as well as its integration, fabrication and applications. The latest representative achievements regarding the hybrid nanogenerator are also summarized. Finally, some perspectives and challenges in this field are discussed.

Flexible Nanogenerators for Energy Harvesting and Self-Powered Electronics.

Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging

Perovskite ferroelectric nanowires have rarely been used for the conversion of tiny mechanical vibrations into electricity, in spite of their large piezoelectricity. Here we present a lead-free NaNbO3 nanowire-based piezoelectric device as a high output and cost-effective flexible nanogenerator. The device consists of a NaNbO3 nanowire–poly(dimethylsiloxane) (PDMS) polymer composite and Au/Cr-coated polymer films. High-quality NaNbO3 nanowires can be grown by hydrothermal method at low temperature and can be poled by an electric field at room temperature. The NaNbO3 nanowire–PDMS polymer composite device shows an output voltage of 3.2 V and output current of 72 nA (current density of 16 nA/cm2) under a compressive strain of 0.23%. These results imply that NaNbO3 nanowires should be quite useful for large-scale lead-free piezoelectric nanogenerator applications.

Lead-free NaNbO3 nanowires for a high output piezoelectric nanogenerator

A ${\mathrm{VO}}_{2}$ film was grown on a sapphire(0001) substrate using pulsed laser deposition. The film showed a first-order metal-insulator (MI) transition and its dc conductivity started to increase drastically near 68 \ifmmode^\circ\else\textdegree\fi{}C and changed by three orders of magnitude. Mid-infrared transmittance and reflectance spectra of the ${\mathrm{VO}}_{2}$ film were measured between 20 \ifmmode^\circ\else\textdegree\fi{}C and 90 \ifmmode^\circ\else\textdegree\fi{}C. Using the intensity transfer-matrix method, the frequency-dependent dielectric constant ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and the conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) of the film were obtained between 1600 and 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the measured transmittance and reflectance spectra. With the ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) spectra, mid-infrared properties of the ${\mathrm{VO}}_{2}$ film near the MI transition region were investigated in detail. Above 78 \ifmmode^\circ\else\textdegree\fi{}C, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega})0 and d${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$/d\ensuremath{\omega}g0, which is a typical metallic behavior. In particular, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) at 88 \ifmmode^\circ\else\textdegree\fi{}C were analyzed in terms of extended Drude model in which the frequency-dependent scattering rate and the effective mass could be obtained. The mean free path of charge carriers in the dc limit was estimated to be larger by an order of magnitude than the previously reported value, i.e., 4 \AA{}. Below 74 \ifmmode^\circ\else\textdegree\fi{}C, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega})g0 and d${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$/d\ensuremath{\omega}\ensuremath{\approxeq}0, which is characteristic of an insulator. Interestingly, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$ in the insulating region increased as the temperature approached the MI transition temperature. To explain this anomalous behavior, the MI transition of the ${\mathrm{VO}}_{2}$ film was modeled with coexistence of metallic and insulating domains and their dynamic evolution. Then the behaviors of ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) were explained using the effective medium approximation, which is a mean-field theory predicting a percolation transition. This work clearly demonstrates that the transport and optical properties near the MI transition region are strongly influenced by the connectivity of the metallic domains. \textcopyright{} 1996 The American Physical Society.

Mid-infrared properties of a VO 2 film near the metal-insulator transition

Pyroelectric nanogenerators fabricated using a lead-free KNbO(3) nanowire-PDMS polymer composite are reported for the first time. The voltage/current output of the nanogenerators can be controlled by electric fields and enhanced by increasing the rate of change in temperature. The fabricated nanogenerators can be used to harvest energy from sunlight illumination and have potential applications in self-powered nanodevices and nanosystems.

https://www.nanoscience.gatech.edu/paper/2012/12_AM_10.pdf

Flexible pyroelectric nanogenerators using a composite structure of lead-free KNbO(3) nanowires.

The optical magnetoelectric (ME) effect, i.e., the change of optical absorption upon the reversal of the light propagation direction, has been investigated for a polar ferrimagnet GaFeO3. For dipole- and spin-forbidden d-d transition bands located at 1.2-2.3 eV, a clear signal of the optical ME effect (Deltaalphat approximately 3x10(-3)) is observed with an applied magnetic field as low as 500 Oe and a sample thickness (t) of 50 microm. The observation of a large ME effect in the present compound suggests a possible route to magnification of this novel phenomenon for application.

Optical magnetoelectric effect in the polar GaFeO3 ferrimagnet.

Reflectivity spectra of ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were measured in a wide photon energy region between 5 meV and 30 eV at room temperature. Using the conductivity spectra obtained from the Kramers-Kronig analysis, electronic structures of the manganese oxides were investigated. In particular, the states near the Fermi energy ${\mathrm{E}}_{\mathrm{F}}$, O 2p and Mn 3d (${\mathrm{e}}_{\mathrm{g}}$,${\mathrm{t}}_{2\mathrm{g}}$), were studied in detail. It was found that the O 2p band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than the Mn ${\mathrm{t}}_{2\mathrm{g}}$ band. For ${\mathrm{LaMnO}}_{3}$, the filled Mn ${\mathrm{e}}_{\mathrm{g}\mathrm{\ensuremath{\uparrow}}}^{1}$ band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than any other valence band. The ${\mathrm{t}}_{2\mathrm{g}}$-${\mathrm{e}}_{\mathrm{g}}$ Hund exchange energy was found to be about 3.4 eV and the Jahn-Teller stabilization energy ${\mathrm{E}}_{0}$ was estimated to be less than 0.5 eV.

Jong Hoon Jung

Papers

Lead-free NaNbO3 nanowires for a high output piezoelectric nanogenerator

Mid-infrared properties of a VO 2 film near the metal-insulator transition

Flexible pyroelectric nanogenerators using a composite structure of lead-free KNbO(3) nanowires.

Optical magnetoelectric effect in the polar GaFeO3 ferrimagnet.

Determination of electronic band structures of CaMnO 3 and LaMnO 3 using optical-conductivity analyses