Magnitude Of Resistance Change Research Articles

Synthesis and electrical behavior of VO2 thin films grown on SrRuO3 electrode layers

VO2 thin films were grown on conducting oxide underlayer SrRuO3 buffered SrTiO3 (111) and Si/SiO2 substrates, respectively, using sputtering. X-ray diffraction phi-scans revealed the epitaxial nature of the VO2 films grown on SrRuO3 buffered SrTiO3 and polycrystalline structure for films grown on SrRuO3 buffered Si/SiO2. X-ray photoelectron spectroscopy confirms a dominant presence of V4+ in both films and establishes a high-quality growth of single-phase VO2 films. Temperature and electric-field driven metal-insulator-transition in both the in-plane and out-of-plane configurations were investigated. Depending on the configuration, the resistance change across the metal-insulator-transition varies from a factor of 1.57–3. The measured resistance in each state as well as the magnitude of resistance change were similar during temperature and electric-field driven metal-insulator-transition. To shed light on the suppressed metal-insulator-transition characteristics due to the current shunting effect from conducting SrRuO3 bottom electrode, a distributed resistance network model is proposed and benchmarked against reports from the literature. The results demonstrate the growth of high-quality VO2 on conducting SrRuO3 layers and their electrical behavior, which is of particular interest for all-oxide electronic devices utilizing phase transitions such as resistive memory and neuromorphic oscillators.

Near-perfect terahertz wave amplitude modulation enabled by impedance matching in VO2 thin films

We present a terahertz (THz) amplitude modulation method with near perfect E-field amplitude modulation depths that is based on impedance matching in VO2 thin films during the thermally induced insulator-metal transition (IMT). It was observed that the impedance matching-induced THz amplitude modulation was sensitive to the resistance switching characteristics of the VO2 thin films. By designing the VO2 thin films to have four orders of magnitude of change in resistance during the IMT, we experimentally achieved an E-field amplitude modulation depth of 94.5% (intensity modulation depth of 99.7%) between the insulator phase of VO2 and the impedance matching state, and an E-field amplitude modulation depth of 97.6% (intensity modulation depth of 99.94%) between the impedance matching state and the metallic phase of VO2 at 0.5 THz. The experimental results were consistent with the results of simulations based on the transmission matrix model.

Assessing the electrical behaviour of MWCNTs/epoxy nanocomposite for strain sensing

Abstract Nanocomposite sensors are gaining importance not only because they have low cost, but also because of the possibility to adjust sensor properties to the requirements of certain applications. In this paper, we focus on the realization of multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposite for strain sensing applications. We propose to use a simple direct mixing process to realize randomly distributed nanocomposite films with different concentration ranging from 0.3 to 0.75 wt.% and thicknesses. Dispersions were firstly examined using scanning electron microscopy (SEM). The results show a good homogeneity and distribution of the MWCNTs in agreement with the reduced electrical resistance when increasing the CNTs content and film thickness. Additionally, it has been demonstrated that the piezoresistive characteristic of nanocomposites depends principally on the CNTs content and film thickness. A higher magnitude of change in resistance is observed for the sample with low volume fraction and thickness leading to a gauge factor 14.19 with a linearity correlation of R 2 = 0.9414. In this work, we address also the effect of environmental changes by characterization of temperature, humidity and stability behaviour. These films show higher sensitivity to temperature and humidity at lower MWCNTs concentration. The film resistance shows a good stability under ambient conditions.

Theoretical Study on Effect of Defective Connection to Reservoirs in an Atomic-Scale Conductor

We theoretically investigate the effect of a defect at the interface between a conductor and reservoirs in an atomic-scale device. Since fabrication of atomic-scale contacts is a complex task, there could be defects at the interface between the conductor and reservoirs. Such defective contacts will make it difficult to measure currents properly. In this paper, we calculate current-voltage characteristics in two-dimensional devices with a defective connection to reservoirs by using the nonequilibrium Green’s function method. Results show that the magnitude of resistance change depends on the amplitude of quantized wave functions at the position of the defect.

Simulation and Fabrication of an Ultra-Low Power Miniature Microbridge Thermal Conductivity Gas Sensor

A miniature, ultra-low power, sensitive, microbridge-based thermal conductivity gas sensor has been developed. The batch fabrication of the sensors was realized by CMOS compatible processes and surface micromachining techniques. Doped polysilicon was used as the structural material of the bridge with critical dimension of 500 nm. A model of the microbridge was simulated in COMSOL that couples electrical and thermal physics together and includes minimal simplifications. Modeling results shows that the majority of heat is transferred via conduction through the gas gap under the bridge. Heat loss from constant voltage application was observed to be a function of the thermal conductivity of the gas ambient, resulting in different magnitude of resistance change. Maximum temperature occurs at the center of the bridge and could be as high as 800 K. Modeling results coincide with experimental data in predicting resistance changes. The sensor was tested with nitrogen, carbon dioxide, and helium. The response of the sensor to the mixtures of helium fractions in nitrogen was tested. We demonstrated that the sensitivity for helium in nitrogen was 0.34 mΩ/ppm when operated at 3.6 V supplies (at power level of 4.3 mW). With a Wheatstone bridge with ac excitation and a lock-in amplifier the sensor limit of detection was ∼700 ppm helium in nitrogen. The stability of the sensor was excellent achieving over 30 billion measurements before failure.

An Ultra-Low Power Microbridge Gas Sensor

A miniature, ultra-low power, sensitive, microbridge-based thermal conductivity gas sensor has been developed. The batch fabrication of the sensors was realized by CMOS compatible processes and surface micromachining techniques. Doped polysilicon was used as the structural material of the bridge with critical dimension of 1µm. The sensor has been tested with nitrogen, carbon dioxide, and helium. Heat loss from constant voltage application was observed to be a function of the thermal conductivity of the gas ambient, resulting in different magnitude of resistance change. The response time to a voltage pulse was found to be less than 0.5µs and stability of the sensor excellent. We demonstrated that the sensitivity for helium in nitrogen was 2.05mΩ/ppm when operated at 3.6V dc supplies (a power level in the order of 4 milliwatts). With a Wheatstone bridge with ac excitation and a Lock-in amplifier the sensor limit of detection was ~100ppm helium in nitrogen.

Epitaxial Nature and Transport Properties in (LaBa)Co2O5+δ Thin Films

Epitaxial (LaBa)Co2O5+δ thin films were grown on (001) LaAlO3 single-crystal substrates using pulsed laser deposition. Microstructure characterizations from X-ray diffraction and electron microscopy indicate that the films are highly c-axis oriented with cube-on-cube epitaxy. Transport property measurements indicate that the films have typical semiconductor behavior with a novel phase transition and hysteresis phenomena at 540 K. The chemical dynamic studies reveals that the resistance of the film changes drastically with the change of redox environment, i.e., the magnitude of resistance changes, ΔR = 1 × 102 ⇔ 1 × 106 Ω, is found within a short response time (∼700 ms). These phenomena suggest that the as-grown (LaBa)Co2O5+δ film have extraordinary sensitivity to reducing-oxidizing environment and the exceedingly fast surface exchange rate.

Polyaniline nanofiber composites with amines: Novel materials for phosgene detection

Several orders of magnitude of change in resistance are observed upon chemical doping and dedoping of the conducting polymer polyaniline. This large conductivity range can be utilized to make sensitive chemical sensors. Polyaniline, in its nanofiber form, has even greater sensing capabilities due to the small fiber diameters, high surface area, and porous nanofiber network that enhances gas diffusion into the fibers. Polyaniline nanofibers have been synthesized using a rapid mixing method and dispersed in water allowing them to be easily modified with water soluble agents, making new composite materials. Polyaniline nanofiber composite materials can be used to enhance detection of analytes that unmodified polyaniline would not otherwise be able to detect. The detection mechanism involves the reaction of an additive with the analyte to generate a strong acid that is easily detected by polyaniline, resulting in orders of magnitude changes in resistance. The reaction of the additive alone with the analyte produces no electrical response, however. In this paper, an array of amine-polyaniline nanofiber composite materials is investigated for the detection of phosgene gas. The influence of environmental conditions such as humidity and temperature are examined and a detection mechanism is presented.

The study of H-absorption-desorption process in Ni67-X MXZr33 glassy alloys monitored by in situ resistance measurements

The catalytic potency and the existence of irreversible H-sorption induced by the addition of various 3d transition elements and noble metals in Ni67Zr33 glassy alloy are investigated. The cyclic H-absorption and desorption are monitored by in situ resistance measurements. While the periods of H-absorption and desorption are significantly shortened in the case of Pd and Cr addition, this effect is not detectable in the case Pt or Cu alloying. The resulting resistance increase due to H-absorption is highly composition specific, and no reasonable explanation was found for the correlation between the magnitude of resistance change and the chemical properties of alloying element. Opposite to the variety of catalytic effects, the irreversible H-trapping can be detected in each alloy by the resistance measurements, which indicates the dominant role of Zr host metal in this phenomenon.

A vapor selectivity study of microsensor arrays employing various functionalized ligand protected gold nanoclusters

We synthesized and tested four different monolayer protected gold nanoclusters (MPCs) as chemically selective interfaces for an organic vapor sensor array. The ligands chosen for capping the nano-Au particles and for selective organic vapor sorption were octanethiol, 2-naphthalenethiol, 2-benzothiazolethiol and 4-methoxythiolphenol. The same set of gold nanoclusters were tested on two different types of sensor platforms, a chemiresistor (CR) and a quartz crystal microbalance (QCM). The sensing properties of both sensor arrays were investigated with 10 organic vapors of various functional groups. Vapor sensing selectivity, dominated by the shell ligand structure of MPC, was demonstrated. The sensitivities of MPC coated CR are better than those of QCM sensors coated with the same material. The average CR/QCM amplification factors are range from 1.9 for 4-methoxythiolphenol MPC to 16.9 for octanethiol MPC. These differences in amplification factors indicate the functional group specific mechanisms for each vapor–MPC pair. The shell penetration mechanism of hydrogen-bonding vapor molecules into the 2-benzothiazolethiol capped MPC reduced the CR/QCM amplification factors. Strong attraction between MPC shell ligands can also reduce the magnitude of resistance changes during vapor sorption.

Current-perpendicular-to-plane giant magnetoresistance in submicron pseudo-spin-valve devices

We measured the transport behavior of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}(\mathrm{Py})∕\mathrm{Cu}∕\mathrm{Py}$ current-perpendicular-to-plane (CPP) submicron spin-valve devices with Cu spacer thickness up to 530 nm, fabricated directly from as-grown heterostructures using a three-dimensional focused-ion beam etching technique. The work bridged the gap between existing conventional CPP giant magnetoresistance measurements and lateral type spin-valve experiments. By fitting the data with the Valet-Fert model, we determined the spin asymmetry ratio of $0.80\ifmmode\pm\else\textpm\fi{}0.01$ and spin diffusion length of $600\ifmmode\pm\else\textpm\fi{}245$ nm in Cu at 77 K. The magnitude of resistance change obtained in this work was comparable with conventional CPP measurements, but was about two orders of magnitude larger than those of lateral spin-valve devices. The results implied that any success in exploiting the lateral type spintronic devices for practical uses would rely critically on improving the size of the measured signals.

Study on optical and electrical switching properties and phase transition mechanism of Mo6+-doped vanadium dioxide thin films

In the present work using V2O5 and MoO3 powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo6+ doped vanadium dioxide (VO2) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO2, the structure of MoO3 in the films didn't change, and the phase transition temperature of the VO2 was obviously lowered with increasing MoO3 doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO3 concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO2 doped films formed local energy level, and then reduced the forbidden band gap of VO2 as the donor defect changing its optical and electrical properties and lowering the phase transition temperature.

Switching Properties and Phase Transition Mechanism ofMo6+-Doped Vanadium Dioxide Thin Films

Using V2O5 and MoO3 powders as precursors,a novel preparation method, i.e., the so-called inorganic sol-gel, isdeveloped to synthesize Mo6+-doped vanadium dioxide (VO2)thin films. The structure, valence state, phase transition temperatureand magnitude of resistivity change are characterized by x-raydiffraction, x-ray photoelectron spectroscopy and the four-pointequipment. The results show that the main chemical composition of dopedthin films was VO2, the structure of MoO3 in doped thinfilms did not change, and the phase transition temperature of dopedthin films was obviously lowered with the increasing MoO3 dopedconcentration, but the magnitude of resistivity change was alsodecreased. However, so long as MoO3 doped concentration was notmore than 5 wt.%, the magnitude of resistivity change of doped thinfilms still reached more 2 orders. The analysis show that MoO3dissolved in crystal structure of VO2 formed the donor defectMOvx and then reduced the forbidden-band width, which lowered thephase transition temperature. Consequently it was widened applicationsof the VO2 thin films.

Polyaniline and its substituted derivatives as sensor for aliphatic alcohols

Polyaniline (PAni) as well as its substituted derivatives such as poly( o-toluidine) (Po-Tol), poly( o-anisidine) (Po-Anis), poly( N-methyl aniline) (PNMA), poly( N-ethyl aniline) (PNEA), poly(2,3 dimethyl aniline) (P2,3-DMA), poly(2,5 dimethyl aniline) (P2,5-DMA) and poly(diphenyl amine) (PDPA) were found to be sensitive to different alcohols such as methanol, ethanol, propanol, butanol and heptanol vapours. A negative change in resistance was observed upon exposing the polymers to methanol, ethanol or propanol vapours, whereas, a reverse trend has been observed for butanol and heptanol vapours. Although, the magnitude of change in resistance was found to be very high in many cases: poor response time was observed for most of the polymers. Rapid responses were exhibited only by P2,3-DMA and PAni for methanol and ethanol, respectively. High sensitivity value (>80%) have been obtained for saturated methanol vapours compared to other alcohols in all polymers. Further, measurable response (sensitivity ∼60%) has been obtained at lowest alcohol concentration of ∼3000 ppm with extended switching time. The results are explained on the basis of vapour induced change in the crystallinity of the polymer. The extent of change was found to be governed by the chain length of the alcohol and its chemical nature.

Domain-Wall Motion under Mechanical Stresses and Giant Piezoresistivity in Thin Barium Titanate Ceramic Bars

Extremely large piezoresistance (stress-sensitive resistance), equivalent to a value of the gage factor (the magnitude of resistance change per strain) of > 107 was observed in single grain boundaries formed in thin semiconducting barium titanate ceramic bars at room temperature. The samples were produced to have a grain structure consisting of single grains joined together in series and a diameter in the range 10-20,um. In this study, observation of ferroelectric domain structures and measurements of resistance were simultaneously carried out for some samples under cyclic mechanical stresses to closer look at the influence of the morphological change of ferroelectric domain structures on the grain boundary resistance in the materials. In a bar sample with a single grain boundary, a 90° domain wall was observed to appear and move from one end of the sample to the other during one cycle of the stress applied. It has been found that the observed large piezoresistance is closely connected with a drastic change of the domain structure in the vicinity of grain boundaries in the samples.