Observed Changes In Conductivity Research Articles

An Enzymatic Electrochemical Biosensing Interface Developed by The Laser‐Induced Graphene Electrode

AbstractLaser‐inducing provides a cost‐effective, easily‐manufacturable, and environment‐friendly approach to directly transfer carbon‐rich polymers into graphene materials, which attracts attention from various fields, such as sensors, electrocatalysts, micro‐supercapacitors, etc. Laser‐induced graphene (LIG) benefits from the intrinsic properties of graphene, for example, high conductivity, high electroactivity, and high specific area. In this work the potential of laser‐induced graphene in constructing an enzymatic electrochemical biosensing interface is evaluated. Here, a laminar‐structured laser‐induced graphene material is fabricated by laser engraving with polyimide. After deposition of the electron mediator ferrocene, a conjugated enzyme complex of bovine serum albumin‐glucose oxidase (BSA‐GOx) is modified on the laser‐induced graphene by cross‐linking. The fabricated glucose oxidase/ferrocene/LIG (GOx/Fc/LIG) biosensor achieves high sensitivity of 11.3 µA mM−1 cm−2), wide linear range of 0–11 mM, and low detection of limit of 0.04 µM. The LIG electrodes exhibit high flexibility with bending angle as high as 60° without observed conductivity change. The repeatability and robustness of the developed LIG biosensor in detection of real serum samples empower it with great potential in clinical implementation in the future.

Operando photoelectron spectromicroscopy of nanodevices: Correlating the surface chemistry and transport in SnO2 nanowire chemiresistors

With size reduction of active elements in microelectronics to tens of nanometers and below, the effect of surface and interface properties on overall device performance becomes crucial. High resolution spectroscopic and imaging techniques provide a metrological route for characterization of these properties relevant to device diagnostics and failure analysis. With its roughly 100 nm spatial resolution, superior surface sensitivity, and approximately 200 meV spectral resolution, scanning photoelectron microscopy (SPEM) stands out as a comprehensive tool to access the surface/interface composition of nanodevices, as well to provide chemical state designations and materials property evolutions upon treatment by thermal, electrical, chemical, radiative and other stimuli. Here we present a SPEM-on-device setup that combines X-ray spectromicroscopy with advanced NIST microhotplate technology to demonstrate new combined analytical and electrical measurements capabilities of this metrology platform for operando nanodevice characterization. Using model integrated SnO2 nanowire (NW) chemiresistor devices, the chemically induced alterations in the chemical state of the nanowire surface are correlated to the observed conductance changes, thus directly testing the receptor and transduction mechanisms for SnO2 NW conductometric chemical sensors.

Hemilabile Ligands as Mechanosensitive Electrode Contacts for Molecular Electronics.

Single‐molecule junctions that are sensitive to compression or elongation are an emerging class of nanoelectromechanical systems (NEMS). Although the molecule–electrode interface can be engineered to impart such functionality, most studies to date rely on poorly defined interactions. We focused on this issue by synthesizing molecular wires designed to have chemically defined hemilabile contacts based on (methylthio)thiophene moieties. We measured their conductance as a function of junction size and observed conductance changes of up to two orders of magnitude as junctions were compressed and stretched. Localised interactions between weakly coordinating thienyl sulfurs and the electrodes are responsible for the observed effect and allow reversible monodentate⇄bidentate contact transitions as the junction is modulated in size. We observed an up to ≈100‐fold sensitivity boost of the (methylthio)thiophene‐terminated molecular wire compared with its non‐hemilabile (methylthio)benzene counterpart and demonstrate a previously unexplored application of hemilabile ligands to molecular electronics.

Hemilabile Ligands as Mechanosensitive Electrode Contacts for Molecular Electronics

AbstractSingle‐molecule junctions that are sensitive to compression or elongation are an emerging class of nanoelectromechanical systems (NEMS). Although the molecule–electrode interface can be engineered to impart such functionality, most studies to date rely on poorly defined interactions. We focused on this issue by synthesizing molecular wires designed to have chemically defined hemilabile contacts based on (methylthio)thiophene moieties. We measured their conductance as a function of junction size and observed conductance changes of up to two orders of magnitude as junctions were compressed and stretched. Localised interactions between weakly coordinating thienyl sulfurs and the electrodes are responsible for the observed effect and allow reversible monodentate⇄bidentate contact transitions as the junction is modulated in size. We observed an up to ≈100‐fold sensitivity boost of the (methylthio)thiophene‐terminated molecular wire compared with its non‐hemilabile (methylthio)benzene counterpart and demonstrate a previously unexplored application of hemilabile ligands to molecular electronics.

Failure prediction in self-sensing nanocomposites via genetic algorithm-enabled piezoresistive inversion

Conductive nanocomposites have been explored extensively for structural health monitoring (SHM) due to their self-sensing nature via the piezoresistive effect. Combined with a non-invasive conductivity imaging modality such as electrical impedance tomography (EIT), piezoresistivity is a powerful tool for SHM. To date, however, the combination of the piezoresistive effect and EIT has been limited to just damage detection. From a SHM perspective, it may be more beneficial to pre-emptively predict failure before it occurs. To that end, we propose a novel methodology for failure prediction in nanocomposites using piezoresistive inversion. Our approach makes use of a genetic algorithm (GA) to determine the mechanical state of the structure using conductivity changes observed via EIT. First, a rectangular nanocomposite specimen with a central hole is manufactured. Second, the specimen is loaded in tension to induce stress concentrations near the hole. Third, EIT is used to image the resulting stress concentration-induced conductivity changes near the hole. Fourth, GA-enabled piezoresistive inversion is implemented to determine the underlying displacements from the observed conductivity changes. The strains are then determined from kinematic relations and the stresses from constitutive relations. Lastly, a failure criterion is used to predict failure. By validating our results with finite element analysis and digital image correlation, we demonstrate that the proposed approach can accurately predict the onset of failure and therefore enable unprecedented SHM capabilities in piezoresistive nanocomposites.

Electrical characterization of molybdenum oxide lamellar crystals irradiated with UV light and proton beams

MoO3 lamellar single crystals were processed into planar metal-semiconductor-metal devices and subjected to irradiation cycles with ultraviolet light and 2 MeV protons in vacuum. In-situ electrical characterization demonstrates that these sensor devices respond well to the radiation with a gain of ~30% upon proton irradiation. However, response times are slow and strong persistent photoconductivity and ion beam induced conductivity are observed when the excitation source is switched off. The current decay is strongly increased when the irradiation chamber is vented with air suggesting that surface processes are at the origin of persistent conductivity. The observed conductivity changes are explained based on a model considering the creation of electron-hole pairs as well as adsorption and desorption processes of oxygen molecules at the surface of the samples.

Effect of (3‐glycidyloxypropyl)trimethoxysilane (GOPS) on the electrical properties of PEDOT:PSS films

ABSTRACTPoly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) has been reported as a successful functional material in a broad variety of applications. One of the most important advantages of PEDOT:PSS is its water‐solubility, which enables simple and environmental friendly manufacturing processes. Unfortunately, this also implies that pristine PEDOT:PSS films are unsuitable for applications in aqueous environments. To reach stability in polar solvents, (3‐glycidyloxypropyl)trimethoxysilane (GOPS) is typically used to cross‐link PEDOT:PSS. Although this strategy is widely used, its mechanism and effect on PEDOT:PSS performance have not been articulated yet. Here, we present a broad study that provides a better understanding of the effect of GOPS on the electrical and electronic properties of PEDOT:PSS. We show that the GOPS reacts with the sulfonic acid group of the excess PSS, causing a change in the PEDOT:PSS film morphology, while the oxidation level of PEDOT remains unaffected. This is at the origin of the observed conductivity changes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 814–820

On the inverse determination of displacements, strains, and stresses in a carbon nanofiber/polyurethane nanocomposite from conductivity data obtained via electrical impedance tomography

Carbon nanofiller-modified composites possess extraordinary potential for structural health monitoring because they are piezoresistive and therefore self-sensing. To date, considerable work has been done to understand how strain affects nanocomposite conductivity and to utilize electrical impedance tomography for detecting strain or damage-induced conductivity changes. Merely detecting the occurrence of mechanical effects, however, does not realize the full potential of piezoresistive nanomaterials. Rather, knowing the mechanical state that results in the observed conductivity changes would be much more valuable from a structural health monitoring perspective. Herein, we make use of an analytical piezoresistivity model to inversely determine the displacement field of a strained carbon nanofiber/polyurethane nanocomposite from conductivity changes obtained via electrical impedance tomography. From the displacements, kinematic and constitutive relations are used to calculate strains and stresses, respectively. A commercial finite element simulation is then used to validate the accuracy of these predictions. These results concretely demonstrate that it is possible to inversely determine displacements, strains, and stresses from conductivity data thereby enabling unprecedented insight into the mechanical response of piezoresistive nanofiller-modified materials and structures.

An inverse methodology for calculating strains from conductivity changes in piezoresistive nanocomposites

Carbon nanofiller-modified composites have incredible potential for self-sensing, structural health monitoring (SHM), and damage identification because they are piezoresistive. This means that conductivity changes can be monitored to detect mechanical perturbations such as damage and strain. Recently, considerable research effort has been dedicated to understanding the effect of strain on nanocomposite conductivity. However, from a SHM perspective, it is more desirable to know the strain state giving rise to an observed conductivity change. Therefore, we herein develop an inverse relation for nanocomposite piezoresistivity. That is, given a conductivity change, we seek to determine the underlying strain state of the nanocomposite. This is done by formulating an inverse problem wherein we seek the strain state that minimizes the difference between the observed conductivity and the conductivity predicted by a piezoresistivity model. This approach is verified through simulations by making use of a piezoresistivity model published in the literature. Our results show that strains can indeed be reconstructed from known conductivity changes. When coupled with conductivity-based SHM such as electrical impedance tomography, these results have the potential to provide unprecedented insight into the mechanical response of nanofiller-modified materials and structures.

Probing near-interface ferroelectricity by conductance modulation of a nano-granular metal

The electronic functionality of thin films is governed by their interfaces. This is very important for the ferroelectric (FE) state which depends on thin-film clamping and interfacial charge transfer. Here we show that in a heterostructure consisting of a nano-granular metal and an organic FE layer of [tetrathiafulvalene][p-chloranil] the nano-granular layerʼs conductance provides a sensitive and non-invasive probe of the temperature-dependent dielectric properties of the FE layer. We provide a theoretical framework that is able to qualitatively reproduce the observed conductance changes taking the anisotropy of the dielectric anomaly at the paraelectric–FE phase transition into account. The approach is also suitable for observing dynamical effects close to the phase transition. Focused electron beam induced deposition as fabrication method for the nano-granular metal guarantees excellent down-scaling capabilities, so that monitoring the FE state on the lateral scale down to 20–30 nm can be envisioned.

Real-time electrical impedance variations in women with and without breast cancer.

The chaotic vascular network surrounding malignant tumors leads to pulsatile blood flow patterns that differ from those in benign regions of the breast. This study aimed to determine if high-speed electrical impedance tomography (EIT) is able to detect conductivity changes associated with cyclic blood-volume changes and to gauge the potential of using these signatures to differentiate malignant from benign regions within the breast. EIT imaging of pulsating latex membranes submerged in saline baths provided initial validation of its use for tracking temporally varying conductivities. Nineteen women (10 with cancer, nine without) were imaged with EIT over the course of several heartbeats in synchrony with pulse-oximetry acquisition. Eight parameters ( rs, ϕ(rt,max), rt,max, Plow:full, Phigh:full, Plow:high) relating the conductivity images and pulse-oximeter signatures were extracted and used as a means of comparing malignant and benign regions of the breast. Significant differences between malignant and benign regions of interest were noted in seven of the eight parameters. The maximum correlation between conductivity and pulse-oximeter signals, rt,max , was observed to be the optimal discriminating parameter with a receiver operating characteristic area under the curve of 0.8 and a specificity of 81% at a sensitivity of 77%. Assessing the dynamic conductivity of breast may provide additional clinical utility to that of standard imaging modalities, but further investigation is necessary to better understand the biophysical mechanisms leading to the observed conductivity changes.

Dynamic Control Over Electronic Transport in 3D Bulk Nanographene via Interfacial Charging

Electrochemical surface charge‐induced variation of physical properties in interface‐dominated bulk materials is a rapidly emerging field in material science. The recently developed three‐dimensional bulk nanographene (3D‐BNG) macro‐assemblies with ultra‐high surface area and chemical inertness offer new opportunities in this area. Here, the electronic transport in centimeter‐sized 3D‐BNG monoliths can be dynamically controlled via electrochemically induced surface charge density. Specifically, a fully reversible variation in macroscopic conductance up to several hundred percent is observed with ≤1 V applied gate potential. The observed conductivity change can be explained in the light of the electrochemically‐induced accumulation or depletion of charge carriers in combination with a large variation in the carrier mobility; the latter, being highly affected by the defect density modulations resulting from the interfacial charge injection, sharply decreases with an increase in defect concentrations. The phenomenon presented in this study is believed to open the door to novel applications of bulk graphene materials such as, for example, low voltage and high power tunable resistors.

Conductance quantization and synaptic behavior in a Ta2O5-based atomic switch

Quantized conductance was observed in a cation-migration-based resistive switching memory cell with a simple metal–insulator–metal (MIM) structure using a thin Ta2O5 layer. The observed conductance changes are attributed to the formation and dissolution of a metal filament with an atomic point contact of different integer multiples in the Ta2O5 layer. The results demonstrate that atomic point contacts can be realized in an oxide-based MIM structure that functions as a nanogap-based atomic switch (Terabe et al 2005 Nature 433 47). By applying consecutive voltage pulses at periodic intervals of different times, we also observed an effect analogous to the long-term potentiation of biological synapses, which shows that the oxide-based atomic switch has potential for use as an essential building block of neural computing systems.

Effects of Microstructure on Eddy Current Residual Stress Characterization of Shot-Peened Inconel 718

Recent studies have shown the possibility of exploiting the eddy current (EC) method for nondestructive characterization of near-surface residual stresses in surface-treated nickel-base superalloy components. The method involves measuring swept frequency EC signals to obtain near-surface electrical conductivity depth profiles, which can be converted into residual stress profiles using the empirical piezoresistivity relationship. However, the relationship between EC responses and residual stress profiles appears to depend on the microstructure. This paper reports on a systematic study of microstructural effects on the EC responses and near-surface conductivity deviation profiles induced by shot peening, using a series of heat-treated Inconel 718 samples with different secondary phase contents and thus a variety of hardness levels. Calibrated EC signals from 100 kHz to 50 MHz were measured from aged and unaged samples, before and after shot peening at various Almen intensities. Strong dependence of the EC responses on the sample hardness and microstructure has been observed. A procedure is introduced to convert the EC signals into shot-peening-induced conductivity deviation profiles where the microstructure dependency is suppressed. The resulting estimates of the conductivity changes are larger than expected, indicating that characterization of shot-peening-induced residual stresses by EC requires further understanding of the material responses beyond the empirical piezoresistivity relationship. Other mechanisms that could contribute to the observed conductivity changes are discussed.

Tuning the conductivity of vanadium dioxide films on silicon by swift heavy ion irradiation

We demonstrate the generation of a persistent conductivity increase in vanadium dioxide thin films grown on single crystal silicon by irradiation with 1 GeV 238U swift heavy ions at room temperature. VO2 undergoes a temperature driven metal-insulator-transition (MIT) at 67 °C. After room temperature ion irradiation with high electronic energy loss of 50 keV/nm the conductivity of the films below the transition temperature is strongly increased proportional to the ion fluence of 5·109 U/cm2 and 1·1010 U/cm2. At high temperatures the conductivity decreases slightly. The ion irradiation slightly reduces the MIT temperature. This observed conductivity change is persistent and remains after heating the samples above the transition temperature and subsequent cooling. Low temperature measurements down to 15 K show no further MIT below room temperature. Although the conductivity increase after irradiation at such low fluences is due to single ion track effects, atomic force microscopy (AFM) measurements do not show surface hillocks, which are characteristic for ion tracks in other materials. Conductive AFM gives no evidence for conducting ion tracks but rather suggests the existence of conducting regions around poorly conducting ion tracks, possible due to stress generation. Another explanation of the persistent conductivity change could be the ion-induced modification of a high resistivity interface layer formed during film growth between the vanadium dioxide film and the n-Silicon substrate. The swift heavy ions may generate conducting filaments through this layer, thus increasing the effective contact area. Swift heavy ion irradiation can thus be used to tune the conductivity of VO2 films on silicon substrates.

Variations in electrical conductivity of rocks above metamorphic conditions

This study of the electrical conductivity of metamorphic rocks as a function of temperature highlights the importance of conductivity variations associated with dehydration. We observed conductivity changes for anhydrous rocks and hydrous amphibolites up to 1100 K at 0.5 GPa. Our experimental results revealed that dehydration takes place and that there is a non-linear variation above metamorphic temperatures. To evaluate the observed changes in conductivity after dehydration, numerical calculations were conducted. The calculation results showed that the bulk conductivity of rock plus pure water cannot account for the conductivity values of reacted rocks if the crust dehydrates. However, the data for 0.1 m NaCl solutions are consistent with those for amphibolites between 530 and 1100 K. In this temperature range, the estimated volume fractions of dewatered saline solutions vary between 0.04 and 0.36 vol.%. These results show that although dehydration is insignificant, and only small amounts of high-salinity fluids are formed in the crust, the total conductivity of amphibolites is high after dehydration.

Electrodialysis in whey desalting process

The utilization of whey is limited by its high content of salts. We used electrodialysis unit ED-Z mini (Mega a.s., Straž pod Ralskem, Czech Republic) to remove ions from sweet whey and whey with added salt (1% w/w of NaCl) and observed conductivity changes during the process. Samples were taken every 10 min and the drop in K+, Na+, Ca2+, Mg2+ was measured by capillary electrophoresis PrinCE-C 750 (Prince Technologies B.V., Emmen, The Netherlands). Ions were detected with the DAD detector at 206 nm and the quantity was determined according to calibration. The negative detection with imidazole in the basic electrolyte was used. Normal whey was desalted in 50 min from the conductivity of 4.83 mS/cm to 0.32 mS/cm. Concentration of Na+ decreased from 0.43 g/L to 0.02 g/L, Ca2+ from 0.45 g/L to 0.07 g/L. Salted whey was demineralized in 65 min from 18.41 mS/cm to 0.34 mS/cm. The Na+ concentration decreased from 3.92 g/ L to 0.08 g/L, Ca2+ from 0.33 g/L to 0.03 g/L. These results demonstrate that electrodialysi...

Influence of physisorbed water on the conductivity of hydrogen terminated silicon-on-insulator surfaces

The sheet resistance of hydrogen terminated silicon-on-insulator substrates increases significantly with time in air due to depletion of free carriers, attributed to the growth of electrically active defects as the surface oxidizes. Surprisingly, physisorbed water (via adsorption from ambient or controlled exposure in vacuum) causes an increase in the conductivity. This effect is largely reversible when the water layer is displaced by inert gas purging, heating, or pumping. The observed conductivity changes are correlated with Hall voltage changes, indicating that the adsorbed water layer induces accumulation of majority carriers on n-doped substrates.

In-line application of electric field in capillary separation systems: Joule heating, pH and conductivity

This study concerns the technique electric field-assisted capillary liquid chromatography. In this technique, an electric field is applied over the separation capillary in order to provide an additional selectivity. In this technique, the electric field is applied in- line in the separation capillary and here the electric current is the factor limiting the magnitude of applied electric field. The influence of Joule heating and other factors on the current in such systems has been investigated. The temperature in the capillary was first measured within a standard CE set-up, as function of effect per unit of length. Then the same cooling system was applied to an in-line set-up, to replicate the conditions between the two systems, and thus the temperature. Thus Joule heating effects could then be calculated within the in-line system. It was found that for systems applying an electric field in line, the direct influence from Joule heating was only relatively small. The pH in the capillary was measured in the in-line set-up using cresol red/TRIS solutions as pH probe. Significant changes in pH were observed and the results suggested that electrolysis of water is the dominant electrode reaction in the in-line system. In summary, the observed conductivity change in in-line systems was found to be mainly due to the pH change by hydrolysis of water, but primarily not due the temperature change in the capillary column.

Adsorption of 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.1 3,7]decane on ultra-thin CdS films

The adsorption mechanism for the new compound, 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.1 3,7]decane (7ETTD), on ultra-thin films (∼3 nm) of CdS is investigated. Multiple reflection absorption IR spectroscopy, in conjunction with inelastic electron tunneling spectroscopy, indicates that this compound forms a self-assembled monolayer adsorbed on the CdS surface via each molecule’s trithia-adamantane anchor. Conductance–voltage data are recorded for tunnel junctions of the type Al/CdS/7ETTD/Pb over a temperature range of 4 K to room temperature and they indicate that the presence of the 7ETTD layer on the CdS dramatically modifies the conductance–voltage behavior of the junctions. These measurements show that different conduction mechanisms, including tunneling and possibly hopping, are responsible for charge transfer through the junctions depending on current, temperature, and voltage. WKB fits to the data are used to determine barrier parameters (height and width) for Al/CdS/Pb junctions with and without adsorbed 7ETTD layers on the CdS. Analysis of the fits shows that tunneling occurs at low bias (less than ∼0.2 V) but, at higher bias voltages, modification of the barrier parameters alone is insufficient to account for the observed conductance changes. A frontier orbital model is invoked which does offer a plausible explanation for these conductance changes. The model assumes bias-dependent coupling between HOMO and LUMO states of the adsorbed 7ETTD and the surface states on the CdS. The present work suggests that, because of the marked effect on the conductance of CdS ultra-thin films, 7ETTD and other similar compounds may be candidates for use in molecular electronic device fabrication.