Long-term Electrical Stability Research Articles

Study of Electrical and Mechanical Properties of Polypyrrole-Coated Three-Dimensional Spacer Fabric

In this study, the liquid phase oxidative polymerization method was utilized to prepare polypyrrole conductive three-dimensional spacer fabrics. By controlling pyrrole solution, oxidant FeCl3 solution, dopant p-toluene sulfonic acid concentration and reaction time, the optimal process for the preparation of conductive spacer fabrics was obtained. This led to the best preparation process of polypyrrole-coated three-dimensional spacer conductive fabric (polypyrrole/three-dimensional spacer fabrics) being obtained. The results showed that the conductive properties of polypyrrole/three-dimensional spacer fabrics were the best when 0.10 mol/L pyrrole, 0.40 mol/L oxidant FeCl3 solution, and 0.40 mol/L dopant p-toluene sulfonic acid were prepared within a 2-h reaction time. The properties of polypyrrole/three-dimensional spacer fabrics were analyzed using the results from surface resistance, Fourier transform-infrared spectroscopy, mechanical properties, and stability tests. The results showed that polypyrrole was well attached to the three-dimensional spacer fabric surface, and the concentration of polypyrrole in the fabric exhibited an inverse correlation with changes in surface resistance. The mechanical properties of polypyrrole/three-dimensional spacer fabrics after treatment exhibit superior performance, with consistent changes in the meridional and zonal electrical properties during tensile testing and can maintain excellent long-term electrical stability in atmospheric environments.

Room temperature deposition of stable p-type ZnO:N thin films through chemical species modulation using reactive pulsed laser deposition

Low temperature processing as well as long-term electrical stability are required to match p-type ZnO thin films with flexible electronics applications. Herein, ZnO and ZnO:N thin films were deposited at room temperature by reactive pulsed laser deposition of a zinc metallic target. Several ZnO thin films were obtained by varying the oxygen working pressure from 1.2 × 10-5 Torr to 3.5 × 10-2 Torr. Chemical concentration was smoothly tuned as demonstrated by in situ X-ray Photoelectron Spectroscopy. Introducing and varying both N2 and O2 fluxes at a constant pressure, N-doped ZnO films from 0.3 at.% to 5 at.% were deposited. Adjusting the N2/O2 flux ratio, p-type ZnO:N films were successfully achieved, exhibiting a carrier concentration of 5.3 × 1018 cm−3, resistivity of 2 Ωcm, and mobility of 0.5 cm2V-1s−1. Four representative p-type samples were analyzed to confirm chemical and electrical reproducibility. Electrical properties of one of these films were measured again nine months after deposited, and p-type conductivity was still observed. Cathodoluminescence revealed green-dominant emissions in n-ZnO and UV-dominant emissions in p-ZnO:N films as nitrogen doping effect. Finally, a p-n diode was fabricated using photolithography, exhibiting rectification performance even nine months after deposited. P-type ZnO:N presented here are suitable for flexible electronics devices fabrication.

(Invited) bionic Composite Films for Biomechanical Energy Harvesting and Self-Powered Sensing Applications

Triboelectric nanogenerators (TENGs) are novel and sustainable devices for harvesting ambient energy for many technological-based applications. However, extreme friction wear of triboelectric layers severely limits their long-term electrical stability and mechanical reliability, restricting the applicability of TENGs for low-frequency mechanical energy harvesting. Herein, we report novel bionic composite (BC) films, inspired by the structures and compositions of bioskins, as wear-resistant triboelectric contact layers for TENGs. We demonstrated that the proposed BC films has superior charge transfer capability, high wear resistance, and maintained long-term stable outputs compared to commercially available films. These works provide an effective strategy to reduce material abrasion by exploiting unique characteristics of the bionic materials, and lays a foundation for effective environmental energy harvesting toward practical applications. Figure 1

Adhesive photonic-ionic skins for visualizing wearable strain distributions

Photonic-ionic skins (PI-skins) inspired by natural skins with ion transduction and chromotropic traits have attracted tremendous attention. Although PI-skins with multiple signal synergistic outputs have been developed, incorporating strong adhesion for direct wearing and precise location of deformation remains highly desirable and challenging. Herein, a mechanochromic and electromechanical PI-skin with superior adhesion, sensitivity, stability, and self-healing ability is presented to visualize wearable strain distributions and monitor human motions. The PI-skin is constructed by integrating a close-packed carboxylated polystyrene (PS-COOH) colloidal array with polymerized deep eutectic solvent (PDES) via in-situ photopolymerization. The as-prepared PI-skin simultaneously exhibits sensitive dual-signal sensing of strain (gauge factor: 1.28, response time: 240 ms, mechanochromic sensitivity of 1.87 nm per percent strain) via adjusting the lattice spacing of the colloidal photonic arrays and resistance. Notably, thanks to the multiple hydrogen bond network of PDES, PI-skin has achieved remarkable adhesion (∼1.44 MPa on glass), mechanical strength, and self-healing performance. Furthermore, the excellent resilience of PDES endows the PI-skin with long-term electrical stability and reproducibility. The PDES contributed to high stretchability (∼1197.8 %), guaranteeing the application as a flexible sensor. The PI-skin promises direct and firm wearing as an interactive sensor for motion monitoring and visual mapping strain distribution.

Highly Conductive Flexible Conductor Based on PEDOT:PSS/MWCNTs Nano Composite

Flexible textiles with strong electrical conductivities have enormous potential as active components in wearable electronics. In this study, we fabricated highly flexible electrical conductors based on cotton fabrics using multiwalled carbon nanotubes (MWCNTs) and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) nanocomposites. We propose that mixing and drop-casting with different amounts of MWCNTs and a fixed amount of doped PEDOT:PSS using a cotton fabric provides a wide range of conductivities depending on the amount of MWCNTs in the mixture. Scanning electron microscopy (SEM) confirmed that the distribution of MWCNTs in the PEDOT:PSS films coated the surface of the cotton fabric, thereby increasing its electrical conductivity. We found that the amount of MWCNTs significantly affected the electrical properties of the nanocomposite cotton in two ways. First, the sheet resistance of the nanocomposite cotton decreased from 78.35 Ω/□ to 2.86 Ω/□ when the concentration of the nanocomposite was increased from 9.21 wt% to 60.27 wt%. This implies that the electrical properties of the nanocomposite cotton can be adjusted by controlling the amount of MWCNTs in the blend. Moreover, we found that the relationship between the sheet resistance and nanocomposite concentration obeys the power law with an exponent α ~ 1.676. Second, the study of the effect of temperature on the resistance indicates that the conductive nanocomposite exhibits semiconductor behavior in the temperature range 24–120 °C and obeys the variable range hopping model. The characteristic temperatures, resistance prefactor, and density of localized states and activation energies depend on the concentration of MWCNTs and can be described by power laws with exponents of 0.470, −1.292, −0.470 and 0.118, respectively. The novel nanocomposite cotton fabric developed in this study exhibits suitable electrical and thermal properties and good long-term electrical stability, which make the nanocomposite cotton fabric a potential flexible conductor with a wide range of electrical conductivities, making it suitable for various applications.

Fabrication of wormhole-like YSZ and Ni-YSZ by the novel soft-hard template CTAB/NaCl-assisted route. Suppressing Ni coalescence in SOFC

A novel one-pot synthesis route leading to the formation of a wormhole-like structure was developed for the successful fabrication of porous YSZ and Ni-YSZ systems. This method involved co-precipitation in the presence of the micelle-forming agents CTAB/Pluronic P123 and crystallising NaCl. The obtained skeletons were mechanically stable and presented almost 50% uniform, open porosity without using any additional pore-formers. The fabricated 0.3 M CTAB/NaCl Ni-YSZ showed better long-term electrical stability in hydrogen than a traditional Ni-YSZ cermet. It resulted from the suppression of Ni structural changes throughout the anode scaffold. Moreover, higher electrochemical activity of this novel anode is expected due to the smaller particle sizes of Ni/YSZ, high homogeneity, highly developed TPB, and better interfacial interaction between the Ni and YSZ. Therefore, the novel soft-hard templating method is recognised as a promising route for the fabrication of the YSZ or Ni-YSZ with a highly developed microstructure and improved stability.

Reliability test of fully printed and flexible organic electrolyte-based supercapacitor

As the demand for supercapacitors in various flexible and wearable energy sectors grows, reliability becomes a key aspect to consider. We report the fabrication and reliability study of printed, flexible organic electrolyte-based supercapacitors. The supercapacitor can be operated over a wide temperature range from −40 °C to 100 °C with excellent repeatability and stability. Thermal shock tests led to a defect in the electrode layer’s microstructure, which reduces the supercapacitor performance. Cyclic bending experiments show that the device has excellent robustness, mechanical flexibility, long-term electrical stability, and 100% capacitance retention up to 10 000 bending cycles with a bending radius of 0.41 cm. Thus, the device is suitable for wearable and flexible energy storage applications over a wide temperature range.

Long-term electrical stability of next generation LV Trench IGBT at Hitachi ABB Power Grids

The new 1.2 kV trench IGBT developed at Hitachi ABB Power Grids features not only state-of-the-art protection of the active trench with dummy trenches and a deep p-well implant but also a novel plasma flow control. These features allow for full design freedom along the technology curve, while ensuring the long-term stability of the trench gate oxide. Devices with differently engineered plasma flow controls were stressed with repetitive turn-off pulses (RRBSOA) at elevated voltages and currents. This way, the influence of the plasma flow control feature on the lifetime stability of the IGBT (with different dynamic avalanche characteristics) can be assessed.

Fabrication of a Robust In2O3 Nanolines FET Device as a Biosensor Platform.

Field-effect transistors (FETs) are attractive biosensor platforms for rapid and accurate detection of various analytes through surface immobilization of specific bio-receptors. Since it is difficult to maintain the electrical stability of semiconductors of sensing channel under physiological conditions for long periods, passivation by a stable metal oxide dielectric layer, such as Al2O3 or HfO2, is currently used as a common method to prevent damage. However, protecting the sensing channel by passivation has the disadvantage that the distance between the target and the conductive channel increases, and the sensing signal will be degraded by Debye shielding. Even though many efforts use semiconductor materials directly as channels for biosensors, the electrical stability of semiconductors in the physiological environments has rarely been studied. In this work, an In2O3 nanolines FET device with high robustness in artificial physiological solution of phosphate buffered saline (PBS) was fabricated and used as a platform for biosensors without employing passivation on the sensing channel. The FET device demonstrated reproducibility with an average threshold voltage (VTH) of 5.235 V and a standard deviation (SD) of 0.382 V. We tested the robustness of the In2O3 nanolines FET device in PBS solution and found that the device had a long-term electrical stability in PBS with more than 9 days’ exposure. Finally, we demonstrated its applicability as a biosensor platform by testing the biosensing performance towards miR-21 targets after immobilizing the phosphonic acid terminated DNA probes. Since the surface immobilization of multiple bioreceptors is feasible, we demonstrate that the robust In2O3 FET device can be an excellent biosensor platform for biosensors.

High-strength and highly electrically conductive hydrogels for wearable strain sensor

Conductive hydrogels, important substrates for the manufacture of flexible electronic devices, are often used as components of wearable sensors, soft robots, and health monitoring equipment, due to their excellent conductivity, flexibility, and ease of processing. However, the requirements for the practical application of conductive hydrogels are difficult to satisfy, because a certain balance must be struck between their high mechanical properties and electrical conductivity. A self-healing dual network (DN) ion conductive hydrogel having a high tensile strength (2.7 MPa) and conductivity (13.8 S m−1) was designed through a simple soaking strategy: polyacrylic acid/graphene oxide-ferric cation/chitosan (PAA/GO-Fe3+/CS). This hydrogel exhibits both high sensitivity in a wide linear range and long-term electrical stability. Based on these characteristics, a stretchable strain sensor was elaborated and successfully used in human motion monitoring. The PAA/GO-Fe3+/CS hydrogel shows a broad application prospect in the manufacture of flexible electronic devices.

A biocompatible polypyrrole membrane for biomedical applications.

Polypyrrole (PPy) is the most widely investigated electrically conductive biomaterial. However, because of its intrinsic rigidity, PPy has only been used either in the form of a composite or a thin coating. This work presents a pure and soft PPy membrane that is synergically reinforced with the electrospun polyurethane (PU) and poly-l-lactic acid (PLLA) fibers. This particular reinforcement not only renders the originally rather fragile PPy membrane easy to manipulate, it also prevents the membrane from deformation in an aqueous environment. Peel and mechanical tests confirmed the strong adhesion of the fibers and the significantly increased tensile strength of the reinforced membrane. Surface electrical conductivity and long-term electrical stability were tested, showing that these properties were not affected by the reinforcement. Surface morphology and chemistry were analyzed with scanning electron spectroscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Material thermal stability was investigated with thermogravimetric analysis (TGA). Finally, the adhesion and proliferation of human skin keratinocytes on the membrane were assessed by Hoechst staining and the methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay. In conclusion, this membrane proves to be the first PPy-based soft conductive biomaterial that can be practically used. Its electrical conductivity and cytocompatibility promise a wide range of biomedical applications.

Chronic neural interfacing with cerebral cortex using single-walled carbon nanotube-polymer grids

Objective. The development of electrode arrays able to reliably record brain electrical activity is a critical issue in brain machine interface (BMI) technology. In the present study we undertook a comprehensive physico-chemical, physiological, histological and immunohistochemical characterization of new single-walled carbon nanotubes (SWCNT)-based electrode arrays grafted onto medium-density polyethylene (MD-PE) films. Approach. The long-term electrical stability, flexibility, and biocompatibility of the SWCNT arrays were investigated in vivo in laboratory rats by two-months recording and analysis of subdural electrocorticogram (ECoG). Ex-vivo characterization of a thin flexible and single probe SWCNT/polymer electrode is also provided. Main results. The SWCNT arrays were able to capture high quality and very stable ECoG signals across 8 weeks. The histological and immunohistochemical analyses demonstrated that SWCNT arrays show promising biocompatibility properties and may be used in chronic conditions. The SWCNT-based arrays are flexible and stretchable, providing low electrode-tissue impedance, and, therefore, high compliance with the irregular topography of the cortical surface. Finally, reliable evoked synaptic local field potentials in rat brain slices were recorded using a special SWCNT-polymer-based flexible electrode. Significance. The results demonstrate that the SWCNT arrays grafted in MD-PE are suitable for manufacturing flexible devices for subdural ECoG recording and might represent promising candidates for long-term neural implants for epilepsy monitoring or neuroprosthetic BMI.

High-performance ZnO nanowires-based glucose biosensor modified by graphene nanoplates

ZnO nanowires (NWs) are known to be highly sensitive to glucose due to their strong electron transfer and adsorption as well as stability and high surface to volume ratio. On the other hand, thanks to its suitable conductivity, and chemical stability, Graphene is the most rampant nanomaterial used in electrochemical sensors. We have previously reported the fabrication of a glucose biosensor based on ZnO NWs synthesis on an Au interdigitated electrode. In this paper, we present the modification of the sensor by incorporation of Graphene Nanoplates (GNPs) beneath the ZnO NWs to achieve a glucose biosensor with high proficiency. The experimental results show excellent improvement in the sensor performance in glucose detection due to this modification. The synergetic effects of ZnO NWs and GNPs have resulted in a desirable performance by the proposed sensor. The response time of 5 s, the detection range of 0.003–30000 mg/dL and long-term electrical stability are three outstanding characteristics of the sensor.

Intense pulsed light-induced structure-transformed ultrathin Ni shell for improving the chemical, thermal, and electrical reliability of metal nanowire electrodes without transmittance loss

Metal nanowires (NWs) boost the development of flexible transparent conductive electrodes (TCEs), but they are hardly applied in industrial productions due to poor reliability. Although various coating technologies have been used to resolve this issue, most of them deteriorate the transmittance of TCEs due to the thick coating shell. Herein, we propose a facile and highly compatible structural transformation process for fabricating core-shell NW TCEs with highly improved reliability. Instead of increasing the thickness of the coating shell to guarantee sufficient coverage around NWs, an instantaneous and surface heating technology of intense pulsed light is creatively applied to realize the structural transformation of scattered Ni crumbs into an ultrathin and uniform Ni coating shell. Results show that without the decline of the transmittance, the reliability of structure-transformed Ag@Ni (STAN) core-shell NW TCEs has been increased to 10 times higher than that of pristine Ag@Ni NW TCEs, which are much better than most of the state-of-the-art publications on core-shell NW TCEs. Flexible transparent heaters fabricated by the STAN NW TCEs exhibit both rapid response and long-term electrical stability. It is believed that the proposed structure-transformed coating method provides a novel and highly versatile way to generate ultrathin protective shells around NWs, which breaks the contradiction between achieving the high transmittance and obtaining the long-term reliability during the fabrication of NW TCEs.

Mimicking Sensory Adaptation with Dielectric Engineered Organic Transistors.

Mimicking sensory adaptation with transistors is essential for developing next-generation smart circuits. A key challenge is how to obtain controllable and reversible short-term signal decay while simultaneously maintaining long-term electrical stability. By introducing a buried dynamic-trapping interface within the dielectric layer, an organic adaptive transistor (OAT) with sensory adaptation functionality is developed. The device induces self-adaptive interfacial trapping to enable volatile shielding of the gating field, thereby leading to rapid and temporary carrier concentration decay in the conductive channel without diminishing the mobility upon a fixed voltage bias. More importantly, the device exhibits a fine-tuned decay constant ranging from 50 ms to 5 s, accurately matching the adaptation timescales in bio-systems. This not only suggests promising applications of OATs in flexible artificial intelligent elements, but also provides a strategy for engineering organic devices toward novel biomimetic functions.

Nanoenabled Direct Contact Interfacing of Syringe-Injectable Mesh Electronics.

Polymer-based electronics with low bending stiffnesses and high flexibility, including recently reported macroporous syringe-injectable mesh electronics, have shown substantial promise for chronic studies of neural circuitry in the brains of live animals. A central challenge for exploiting these highly flexible materials for in vivo studies has centered on the development of efficient input/output (I/O) connections to an external interface with high yield, low bonding resistance, and long-term stability. Here we report a new paradigm applied to the challenging case of injectable mesh electronics that exploits the high flexibility of nanoscale thickness two-sided metal I/O pads that can deform and contact standard interface cables in high yield with long-term electrical stability. First, we describe the design and facile fabrication of two-sided metal I/O pads that allow for contact without regard to probe orientation. Second, systematic studies of the contact resistance as a function of I/O pad design and mechanical properties demonstrate the key role of the I/O pad bending stiffness in achieving low-resistance stable contacts. Additionally, computational studies provide design rules for achieving high-yield multiplexed contact interfacing in the case of angular misalignment such that adjacent channels are not shorted. Third, the in vitro measurement of 32-channel mesh electronics probes bonded to interface cables using the direct contact method shows a reproducibly high yield of electrical connectivity. Finally, in vivo experiments with 32-channel mesh electronics probes implanted in live mice demonstrate the chronic stability of the direct contact interface, enabling consistent tracking of single-unit neural activity over at least 2 months without a loss of channel recording. The direct contact interfacing methodology paves the way for scalable long-term connections of multiplexed mesh electronics neural probes for neural recording and modulation and moreover could be used to facilitate a scalable interconnection of other flexible electronics in biological studies and therapeutic applications.

Strong, Conductive, Foldable Graphene Sheets by Sequential Ionic and π Bridging.

The goal of this work is to develop an inexpensive low-temperature process that provides polymer-free, high-strength, high-toughness, electrically conducting sheets of reduced graphene oxide (rGO). To develop this process, we have evaluated the mechanical and electrical properties resulting from the application of an ionic bonding agent (Cr3+ ), a π-π bonding agent comprising pyrene end groups, and their combinations for enhancing the performance of rGO sheets. When only one bonding agent was used, the π-π bonding agent is much more effective than the ionic bonding agent for improving both the mechanical and electrical properties of rGO sheets. However, the successive application of ionic bonding and π-π bonding agents maximizes tensile strength, toughness, long-term electrical stability in various corrosive solutions, and resistance to mechanical abuse and ultrasonic dissolution. Using a combination of ionic bonding and π-π bonding agents, high tensile strength (821 MPa), high toughness (20 MJ m-3 ), and electrical conductivity (416 S cm-1 ) were obtained, as well as remarkable retention of mechanical and electrical properties during ultrasonication and mechanical cycling by both sheet stretch and sheet folding, suggesting high potential for applications in aerospace and flexible electronics.

Experimental research on time-resolved evolution of cathode plasma expansion velocity in a long pulsed magnetically insulated coaxial diode

Unlike planar diodes, separate research of the axial and radial plasma expansion velocities is difficult for magnetically insulated coaxial diodes. Time-resolved electrical diagnostic which is based on the voltage-ampere characteristics has been employed to study the temporal evolution of the axial and radial cathode plasma expansion velocities in a long pulsed magnetically insulated coaxial diode. Different from a planar diode with a “U” shaped profile of temporal velocity evolution, the temporal evolution trend of the axial expansion velocity is proved to be a “V” shaped profile. Apart from the suppression on the radial expansion velocity, the strong magnetic field is also conducive to slowing down the axial expansion velocity. Compared with the ordinary graphite cathode, the carbon velvet and graphite composite cathode showed superior characteristics as judged by the low plasma expansion velocity and long-term electrical stability as a promising result for applications where long-pulsed and reliable operation at high power is required.

Long-Term Stability of Oxide Nanowire Sensors via Heavily Doped Oxide Contact.

Long-term stability of a chemical sensor is an essential quality for long-term collection of data related to exhaled breath, environmental air, and other sources in the Internet of things (IoT) era. Although an oxide nanowire sensor has shown great potential as a chemical sensor, the long-term stability of sensitivity has not been realized yet due to electrical degradation under harsh sensing conditions. Here, we report a rational concept to accomplish long-term electrical stability of metal oxide nanowire sensors via introduction of a heavily doped metal oxide contact layer. Antimony-doped SnO2 (ATO) contacts on SnO2 nanowires show much more stable and lower electrical contact resistance than conventional Ti contacts for high temperature (200 °C) conditions, which are required to operate chemical sensors. The stable and low contact resistance of ATO was confirmed for at least 1960 h under 200 °C in open air. This heavily doped oxide contact enables us to realize the long-term stability of SnO2 nanowire sensors while maintaining the sensitivity for both NO2 gas and light (photo) detections. The applicability of our method is confirmed for sensors on a flexible polyethylene naphthalate (PEN) substrate. Since the proposed fundamental concept can be applied to various oxide nanostructures, it will give a foundation for designing long-term stable oxide nanomaterial-based IoT sensors.

Two-stage unified stretched-exponential model for time-dependence of threshold voltage shift under positive-bias-stresses in amorphous indium–gallium–zinc oxide thin-film transistors

In this study, we show that the two-stage unified stretched-exponential model can more exactly describe the time-dependence of threshold voltage shift (ΔVTH) under long-term positive-bias-stresses compared to the traditional stretched-exponential model in amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs). ΔVTH is mainly dominated by electron trapping at short stress times, and the contribution of trap state generation becomes significant with an increase in the stress time. The two-stage unified stretched-exponential model can provide useful information not only for evaluating the long-term electrical stability and lifetime of the a-IGZO TFT but also for understanding the stress-induced degradation mechanism in a-IGZO TFTs.