Reversible Resistance Change Research Articles

Lubricity of cholesteric liquid-crystal nanomaterials in friction of solids

A physical model of the lubricity of cholesteric liquid-crystal nanomaterials has been proposed in accordance with which the transition from single-crystal layers of a cholesteric liquid-crystal nanomaterial to polymolecular structurally sensitive liquid-crystal films, which occurs with varying temperature and is governed by the effect of molecules of cholesteric liquid-crystal compounds contained in the lubricant, as well as the reverse transition, lead to a reversible change in the shear resistance. This change results from the formation of continuous planar-oriented and helically twisted boundary layers in the zone of dynamic contact, which consist of molecules of the cholesteric liquid-crystal nanomaterials. The pitch distance of the helix and, therefore, the thickness of cholesteric liquid-crystal films of these nanomaterials exhibit a structurally sensitive response to changes in the temperature in the friction zone, which varies the dissipation of the energy during friction.

Extraordinary synergy in the thermal and electric properties of polymer matrix reinforced with hybrid fillers

The effect of the incorporation of different combinations of electric conducting and non‐conducting nanofillers into epoxy resin on the thermal and electric conductivity was studied. Graphite nanoplatelets (GNPs), multi‐wall carbon nanotubes (MWCNTs) and carbon fibers (CFs) were used as the conductive fillers. The non‐conductive ones were boron nitride (BN) or barium hexaferrite (BaFe12O19). The content of nanocarbon in composite materials (CMs) was varied from 1.0 to 6.0 vol%, the content of dielectric filler in composite materials was equal to 15.0 vol% boron nitride and 8.0 vol% BaFe12O19, respectively. The combination of electroconductive nanocarbon and non‐conductive boron nitride particles showed an increase of the thermal conductivity (by a factor of 2 for hybrid composite materials boron nitride/graphite nanoplatelet/epoxy (4.5 vol% of nanocarbon) compared to the thermal conductivity of composite materials with single graphite nanoplatelets filler) and the electric conductivity, whereas the hybrid conductive fillers of various morphology (graphite nanoplatelets/carbon nanotubes or graphite nanoplatelets/carbon fibers) did not show an improvement. On the other hand, the use of graphite nanoplatelets/carbon nanotubes and graphite nanoplatelets/carbon fibers hybrid fillers lead to the anisotropy of the electric conductivity and a reversible change of the electric resistance after a compression up to 50 MPa due to improved elastic properties.

Influence of mechanical exposures on electrical properties of thin and thick-film flexible resistors and conductors

Purpose – The purpose of this paper is to determine the influence of mechanical factors (such as longitudinal elongation or cyclic compressive and tensile stresses) on electrical properties of thin- or thick-film resistors or conductors. Design/methodology/approach – All test samples were made on Kapton foil. Copper foil or silver-based polymer thick-film conductive inks were used for fabrication of conductors. Resistive structures were made with the aid of two polymer thick-film resistive inks or OhmegaPly Ni-P resistive foil. Test structures differ not only in materials applied for resistors or conductors but also in geometrical shape of functional tracks (meanders consisted of many horse-shoes, semicircles, squares or triangles). Findings – Presented results showed significant role of material on range of reversible resistance changes. But shape of test samples also affects relation between relative resistance changes and relative elongation. Originality/value – In general, changes induced by cyclic compressive and stretching stresses were smaller than those caused by substrate elongation.

Flexible Polyaniline/Poly(methyl methacrylate) Composite FibersviaElectrospinning and In Situ Polymerization for Ammonia Gas Sensing and Strain Sensing

Conducting polyaniline (PANI) was in situ polymerized at the surface of electrospun poly(methyl methacrylate) (PMMA) fibers to obtain flexible composite fibers. The electrical conductivity of an individual PANI/PMMA composite fiber was estimated to be 2.0 × 10−1 S cm−1at room temperature. The ammonia sensing properties of the samples were tested by impedance analysis. The PANI/PMMA fibers could obviously respond to low concentration of ammonia at ppb level and could respond to relatively high concentration of ammonia at 10 ppm level quickly. In addition, the sensitivity exhibited a good linear relationship to the ammonia concentration. Particularly, the flexible PANI/PMMA fibers showed a reversible change in electrical resistance with repeated cycles of bending and relaxing, and the electrical resistance decreased with the increase of curvature. These results indicate that the flexible PANI/PMMA composite fibers may be used in toxic ammonia gas detection, strain sensing, and flexible electronic devices.

Helical graphene oxide fibers as a stretchable sensor and an electrocapillary sucker.

Fibers made from carbon nanotubes or graphene are strong and conductive; encoding helical structures into these fibers may render useful properties such as high stretchability. Here, we directly spin freestanding graphene oxide (GO) films into helical fibers consisting of uniformly arranged loops with tunable diameters, under controlled environmental humidity. Reduced GO fibers with a helical shape are stretched elastically with a reversible electrical resistance change for many strain cycles. Stretchable temperature sensors built on helical fibers work at large strains (up to 50%) and high temperature (up to 300 °C), with a reliable deformation-independent response. The GO fibers also contain through-channels inside with suitable pore size, which can take up an aqueous electrolyte quickly under a low bias, resulting in a fiber-shaped, on-off switchable electrocapillary sucker. Our multifunctional helical and hollow GO fibers have potential applications in stretchable fiber-shaped sensors, actuators and nano-fluid systems.

Novel chemoresistive CH4 sensor with 10 ppm sensitivity based on multiwalled carbon nanotubes functionalized with SnO2 nanocrystals

Chemoresistive sensors based on multiwalled carbon nanotubes (MWCNTs) functionalized with SnO2 nanocrystals (NCs) have great potential for detecting trace gases at low concentrations (single ppm levels) at room temperature, because the SnO2 nanocrystals act as active sites for the chemisorption of gas molecules, and carbon nanotubes (CNTs) act as an excellent current carrying platform, allowing the adsorption of gas on SnO2 to modulate the resistance of the CNTs. However, uniform conjugation of SnO2 NCs with MWCNTs is challenging. An effective atomic layer deposition based approach to functionalize the surface of MWCNTs with SnO2 NCs, resulting in a novel CH4 sensor with 10 ppm sensitivity, is presented in this paper. Scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy, and Raman spectroscopy were implemented to study the morphology, elemental composition, and the crystal quality of SnO2 functionalized MWCNTs. High resolution TEM images showed that the crystal quality of the functionalizing SnO2 NCs was of high quality with clear lattice fringes and the dimension almost three times smaller than shown thus far in literature. A lift-off based photolithography technique comprising bilayer photoresists was optimized to fabricate SnO2 functionalized MWCNTs-based chemoresistor sensor, which at room temperature can reliably sense below 10 ppm of CH4 in air. Such low level gas sensitivity, with significant reversible relative resistance change, is believed to be the direct result of the successful functionalization of the MWCNT surface by SnO2 NCs.

(Invited) Mechanism Study of Reversible Resistivity Change in Oxide Thin Film

Here we present our findings related to the mechanism of reversible resistivity in Pt/TiO2/Pt cells and in Ta2O5 thin films. Our findings for Pt/TiO2/Pt cells indicate that there exists a photovoltaic-like effect, which modulates the resistance reversibly by a few orders of magnitude, depending on the intensity of impinging x-rays. We found that this effect, combined with the xray irradiation induced phase transition confirmed by transmission electron microscopy, triggers a non-volatile reversible resistance change. For Ta2O5 thin films, we found that there are strong correlations among oxygen vacancy number and positions and energy gaps. Ab initio band structure calculations explain the evolution of the electronic excitation spectrum as a function of oxygen vacancy number and positions and importantly provide a predictive description of the oxygen deficient Ta oxide that may improve the desired performance based on atomic level design.

UV-assisted room-temperature chemiresistive NO2 sensor based on TiO2 thin film

TiO2 thin film based, chemiresistive sensors for NO2 gas which operate at room temperature under ultraviolet (UV) illumination have been demonstrated in this work. The rf-sputter deposited and post-annealed TiO2 thin films have been characterized by atomic force microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction to obtain surface morphology, chemical state, and crystal structure, respectively. UV–vis absorption spectroscopy and Tauc plots show the optical properties of the TiO2 films. Under UV illumination, the NO2 sensing performance of the TiO2 films shows a reversible change in resistance at room-temperature. The observed change in electrical resistivity can be explained by the modulation of surface-adsorbed oxygen. This work is the first demonstration of a facile TiO2 sensor for NO2 analyte that operates at room-temperature under UV illumination.

(Invited) Mechanism Study of Reversible Resistivity Change in Oxide Thin Film

The field of applied physics is driven by the need to understand materials structure and electronic properties with the ultimate goal of harnessing them in functional devices such as nonvolatile memories. One needs to understand the mechanism, which dictates the behavior of the charged defects and electrons under external stimuli such as electrostatic potential, thermal gradient and photons, to obtain a more microscopic understanding of the system. Then, one arrives at a predictive description of designing functional devices. In the first part of the talk, we report the x-ray-induced reversible resistance change in 40-nm-thick TiO2 films sandwiched by Pt top and bottom electrodes, and propose the physical mechanism behind the emergent phenomenon. Our findings indicate that there exists a photovoltaic-like effect, which modulates the resistance reversibly by a few orders of magnitude, depending on the intensity of impinging x-rays. We found that this effect, combined with the x-ray irradiation induced phase transition confirmed by transmission electron microscopy, triggers a non-volatile reversible resistance change. Understanding x-ray-controlled reversible resistance changes can provide possibilities to control initial resistance states of functional materials, which could be useful for future nonvolatile memories. In the second part, we cover the investigation of Ta2O5 powder and oxygen deficient thin films using synchrotron x-ray studies at the Advanced Photon Source, combining x-ray diffraction, extended x-ray absorption fine structure (EXAFS) and resonant x-ray scattering spectroscopy (RIXS) and Ab initio band structure calculations. Ta-based resistive switching devices have been extensively investigated due to their fast switching and reliable endurance among other materials. Despite extensive recent interests, however, there is still lack of fundamental understanding of electronic structure and local structure of the Ta-based device. Oxygen vacancies play essential roles in the electric functionality of resistive switching devices, which is based on the formation and rupture of conducting paths within the insulating matrix. We found that there are strong correlations among oxygen vacancy number and positions and energy gaps. Ab initio band structure calculations successfully explain the evolution of the electronic excitation spectrum as a function of oxygen vacancy number and positions and importantly provide a predictive description of the oxygen deficient Ta oxide that may improve the desired performance based on atomic level design rather than the traditional trial-error methods.

(Invited) Direct Physical Understanding in Memristive Devices and Corresponding Device Models

Nanoscale devices composed of transition metal-oxide layers exhibiting a prominent resistance change (memristors) have been extensively researched for their technological potential. The mechanism for the reversible resistance change in these systems involves an interaction of many physical effects including electrochemistry, ion motion driven by electric fields and concentration gradients, and interactions with thermal effects. While a complete physical understanding is still in development, I will describe some of the insights gained from direct measurements using structurally and chemically-sensitive techniques with nanoscale resolution such as x-ray spectromicroscopy. Specifically, I will show recent studies utilizing scanning transmission x-ray microscopy (STXM) to observe oxygen migration in a working tantalum oxide memristor device. I will also describe ongoing work utilizing hard x-rays to probe the local bonding of Ta atoms within a memristor device. These studies will aid in developing a microscopic switching model, essential for guiding material selection, device engineering, and effective design of driving circuits. Ultimately, any utilization of this technology depends critically on predictive and microphysically accurate device models, and I will describe our work in this area.

Nonvolatile Electrochemical Metallization Memory Based on Nanocrystalline La<sub>2</sub>O<sub>3</sub> Solid Electrolyte Thin Film

Nanocrystalline La2O3 thin films are deposited on Pt/Ti/SiO2/Si substrate by reactive radio frequency magnetron sputtering. The Ag/La2O3/Pt devices exhibit reversible bipolar resistance change over 1000 cycles with a resistance ratio (high resistance state to low resistance state) of over three orders of magnitude and stable retention for over $10^{6}$ s at room temperature. Analysis indicates that the resistance change originates from the formation/rupture of Ag filaments in the La2O3 thin films, which act as a solid electrolyte. The device shows potential for multilevel storage as well as low power consumption applications.

Soft metal constructs for large strain sensor membrane

Thin gold films on silicone display large reversible change in electrical resistance upon stretching. Eutectic liquid metal conductors maintain bulk metal conductivity, even upon extensive elongation. When integrated together, the soft metals enable multidirectional, large strain sensor skin. Their fabrication process combines thermal evaporation of thin gold film patterns through stencil mask with microplotting of eutectic gallium indium microwires, and packaging in silicone rubber. Using three-element rectangular rosettes, we demonstrate a sensor skin that can reliably and locally quantify the plane strain vector in surfaces subject to stretch (up to 50% strain) and indentation. This hybrid technology will find applications in soft robotics, prosthetics and wearable health monitoring systems.

Development of Electrospun Polyaniline/ZnO Composite Nanofibers for LPG Sensing

In this paper we report the highly sensitive at low temperature and low concentration of liquefied petroleum gas (LPG) sensor based on D-Camphor-10-Sulphonic acid (CSA) doped Polyaniline/Polyethylene Oxide (PANI/PEO) and PANI/ZnO/PEO nanofibers fabricated by electrospinning technique. Nano-sized particles of zinc oxide (ZnO) were synthesized using sol-gel method. The emeraldine base form of polyaniline (PANI) and PANI/ZnO synthesized by chemical oxidative polymerization were treated with CSA for better solubility and conductivity and then blended with PEO solution to get nanofibers using electrospinning. The morphology and structure of electrospun nanofibers of CSA doped PANI/PEO and PANI/ZnO/PEO were investigated by SEM, UV-VIS, FTIR and XRD. The change in electrical resistance of CSA doped PANI/PEO and PANI/ZnO/PEO for 1000ppm of LPG was measured. Both the samples showed a rapid and reversible resistance change upon exposure to LPG gas at 1000ppm concentration. Maximum sensitivity was achieved at 36°C for PANI/ZnO/PEO indicating suitability of nanofibrous PANI/ZnO/PEO towards LPG sensing as compared to pure ZnO and PANI.

TEM and EELS Study on TaOx-based Nanoscale Resistive Switching Devices

ABSTRACTResistive switching, a reversible change in electrical resistance of a dielectric layer through the application of a voltage bias, has propelled a field of research to form improved non-volatile memory device. Tantalum oxide has been investigated as the dielectric component of resistive switching devices as a leading candidate for a few years. Presented here is a structural and chemical investigation of TaOx devices with 55nm in diameter in the virgin, forming on, and switched off (reset) states for comparison using cross sectional TEM techniques including HRTEM, and EELS to gain further understanding of this material system. The nanodevices imaged in this study were switched below 100µA. Unique features found in this study are in agreement with previous hypotheses made by various researchers based on X-ray fluorescence microscopy of micron-scale devices, indicating a variation in oxygen concentration around the switching area.

バルク金属ガラス中の低温緩和挙動

Low temperature structural relaxation in Zr55Cu30Ni5Al10 bulk metallic glass (BMG) that evolved at about 100～150 K under glass transition temperature was investigated by electrical resistance relaxation and volume relaxation. The reversible isochronal change in resistance and the kinetics of relaxation was in agreement with low temperature relaxation behavior previously reported in many amorphous metals. It was checked whether or not the kinetic of relaxation examined obeys kinematical criterions deduced from dynamic mechanical analysis for non-organic glass, polymer glass as well as metallic glass. At least, with regard to a limited glass as pre-annealed Zr55Cu30Ni5Al10 BMG, the equivalency of low temperature relaxation to β-relaxation (Johari-Goldstein relaxation) was not verified.

Structural and Chemical Analysis of Nanoscale Resistive Switching Devices: Assessment on Nonlinear Properties

ABSTRACTInvestigation into the phenomenon of resistive switching, a reversible change in electrical resistance by the application of a voltage bias, has given rise to the device fabrication, DC electrical testing, and cross sectional TEM/EELS characterization of nanoscale resistive switching devices. Typically, resistive switching devices are composed of a thin oxide layer between two conductive electrodes where applied bias can alter the resistance states. In a cross-bar array, nonlinearity of device I-V relation is a highly desirable characteristic that helps to mitigate the sneak path current leakage issue. Negative differential resistance (NDR) switching behavior offers such nonlinearity and has been observed in TaOx nanoscale devices utilizing certain electrode materials. To investigate this phenomenon, nanodevices were fabricated by sputtering TaOx onto TiN nanovias capped Nb electrodes. Cross sectional TEM/EELS were performed to reveal the physical and chemical changes in these devices to explore possible origins of nonlinear behavior when these top electrode materials are utilized with TaOx films.

Bias voltage induced resistance switching effect in single-molecule magnets’ tunneling junction

An electric-pulse-induced reversible resistance change effect in a molecular magnetic tunneling junction, consisting of a single-molecule magnet (SMM) sandwiched in one nonmagnetic and one ferromagnetic electrode, is theoretically investigated. By applying a time-varying bias voltage, the SMMʼs spin orientation can be manipulated with large bias voltage pulses. Moreover, the different magnetic configuration at high-resistance/low-resistance states can be ‘read out’ by utilizing relative low bias voltage. This device scheme can be implemented with current technologies (Khajetoorians et al 2013 Science 339 55) and has potential application in molecular spintronics and high-density nonvolatile memory devices.

Heavy duty piezoresistivity induced strain sensing natural rubber/carbon black nanocomposites reinforced with different carbon nanofillers

Durable piezoresistive effects of natural rubber nanocomposites have been demonstrated, i.e., with stable and reversible electrical resistance change within the tested 3000 cycles upon applying a small compressive strain (∼16.7%) under a relatively high frequency (0.5 Hz, 2 s/cycle). This unique function was achieved for the first time by combining carbon nanotubes and carbon nanofibers with natural rubber composites pretreated with carbon black. Even though the combination of different carbon nanomaterials, such as graphene nanosheets and carbon nanotubes, can improve the dispersion quality of both the nanostructures in solution or in polymer matrices, this type of synergistic effect between carbon nanotubes and carbon nanofibers in producing stable and reversible piezoresistive effect has been rarely reported. Besides, the strong reinforcement (compressive stress at a maximum strain of 16.7% was increased from 12.6 for untreated to 18.5 MPa for the natural rubber/carbon black composites treated with a combination of 1.0 wt% carbon nanotubes and 1.0 wt% carbon nanofibers) makes the as-prepared composites promising for heavy duty pressure sensors, i.e., healthy motion monitoring of industrial machinery vibrations.

Bipolar resistance switching in Pt/CuOx/Pt via local electrochemical reduction

The local changes in copper oxidation state and the corresponding resistance changes in Pt/CuOx/Pt nanoscale heterostructures have been investigated using x-ray nanoprobe spectro-microscopy and current-voltage characterization. After gentle electroforming, during which the current-voltage behavior remains non-linear, the low resistance state was reached, and we observed regions of 160 nm width that show an increase in Cu K-alpha fluorescence intensity, indicative of partial reduction of the CuOx. Analysis of the current voltage curves showed that the dominant conduction mechanism is Schottky emission and that the resistance state is correlated with the Schottky barrier height. We propose that the reversible resistivity change in these Pt/CuOx/Pt heterostructures occurs through local electrochemical reduction leading to change of the Schottky barrier height at the interface between Pt and the reduced CuOx layers and to change of the CuOx resistivity within laterally confined portions of the CuOx layer. These experiments reveal important insights into the mechanism of resistance switching of Pt/CuOx/Pt performed in a current and voltage regime that does not create a metallic conduction path.

Pencil Drawn Strain Gauges and Chemiresistors on Paper

Pencil traces drawn on print papers are shown to function as strain gauges and chemiresistors. Regular graphite/clay pencils can leave traces composed of percolated networks of fine graphite powders, which exhibit reversible resistance changes upon compressive or tensile deflections. Flexible toy pencils can leave traces that are essentially thin films of graphite/polymer composites, which show reversible changes in resistance upon exposure to volatile organic compounds due to absorption/desorption induced swelling/recovery of the polymer binders. Pencil-on-paper devices are low-cost, extremely simple and rapid to fabricate. They are light, flexible, portable, disposable, and do not generate potentially negative environmental impact during processing and device fabrication. One can envision many other types of pencil drawn paper electronic devices that can take on a great variety of form factors. Hand drawn devices could be useful in resource-limited or emergency situations. They could also lead to new applications integrating art and electronics.