Semiconductor Nanoparticle Anchored Single-Walled Carbon Nanotubes for a Bolometer.

The bolometer is developed using single-walled carbon nanotubes (SWCNT) anchored with semiconductor nanoparticles of cadmium sulfide, stannous disulfide, and zinc oxide (ZnO). The bolometric responses were recorded at varying temperatures from 10 K to room temperature. The anchored SWCNTs provided a higher temperature coefficient of resistance (TCR) than pristine SWCNTs. The largest TCR is recorded from SWCNTs/ZnO (-0.11%/K) at room temperature, which is 200% higher than pristine SWCNTs. The largest photoresponsivities of SWCNTs/ZnO under near-infrared (NIR) and wide wavelength source (200-1200 nm) illuminations are 5.06 and 37.51 mV/W, respectively. The extraordinary performance of SWCNTs/ZnO stands out at ∼17 × 104% under IR illumination. The study presents a significant advancement in the development of high-performance SWCNT bolometric materials by anchoring with semiconductor nanoparticles.

Ultrahigh Photo-Responsivity and Detectivity in 2D Bismuth Sulfide Photodetector for Vis-NIR Radiation.

Bismuth sulfide (Bi2S3) exhibits a direct energy bandgap and an exceptional optical absorption capability over a broadband radiation, thus presents a novel class of 2D photodetector material. The field effect transistor (FET) photodetector device is fabricated from 2D Bi2S3. An anomalous variation in the transport characteristics of 2D Bi2S3 is observed with the variation in temperature. The electrical resistance reduces by 99.26% at 10 K compared to the response at 300 K. Defects due to the bismuth and sulfur vacancies play a critical role in the dramatic behavior, which is confirmed using photoluminescence, time-resolved photoluminescence, Hall measurements, and energy dispersive X-ray spectroscopy. The density functional theory calculations provide a significant insight into the thermodynamic properties of intrinsic defects in Bi2S3. Moreover, the effect of gate bias on responsivity additionally confirms its invariance at low temperature. The Bi2S3 based FET photodetector achieves ultrahigh responsivity in the order of ≈106 A W-1 and detectivity of ≈1014 Jones. Moreover, the external quantum efficiency of ≈107% is measured in a wide spectrum of optical illumination (532 to 1064 nm) with a noise-equivalent power of 3.5 × 10-18 W/√Hz at a bias of 0.2 V. The extraordinary performance of Bi2S3 photodetector outstands 2D photodetectors.

Three-dimensional carbon foam-metal oxide-based asymmetric electrodes for high-performance solid-state micro-supercapacitors.

A three-dimensional carbon foam (CF)-based asymmetric planar micro-supercapacitor is fabricated by the direct spray patterning of active materials on an array of interdigital electrodes. The solid-state asymmetric micro-supercapacitor comprises the CF network with pseudocapacitive metal oxides (manganese oxide (MnO), iron oxide (Fe2O3)), where CF-MnO composite as a positive electrode, and CF-Fe2O3 as negative electrode for superior electrochemical performance. The micro-supercapacitor, CF-MnO//CF-Fe2O3, attains an ultrahigh supercapacitance of 18.4 mF cm-2 (2326.8 mF cm-3) at a scan rate of 5 mV s-1. A wider potential window of 1.4 V is achieved with a high energy density of 5 µW h cm-2. The excellent cyclic stability is confirmed by 86.1% capacitance retention after 10 000 electrochemical cycles.

Novel electrode geometry for high performance CF/Fe2O3 based planar solid state micro-electrochemical capacitors.

A novel geometry of sharp-edged electrodes for planar micro-electrochemical capacitors is utilized for an enhanced performance compared to the conventionally used interdigitated electrodes. The sharp-edged electrode geometry achieves a 68% enhancement in the electric field at the sharp-edge of the electrodes as compared to interdigitated electrodes. Moreover, carbon foam with high specific surface area loaded with iron oxide nanoparticles allows a large mass loading for the pseudocapacitance in addition to electric double layer capacitance (EDLC). Thus, an enhancement of 235% was obtained in both the areal specific capacitance and energy density when the performance was compared with the interdigitated electrode based supercapacitors. Moreover, an excellent cycling stability (∼99.5%) over 10 000 charge-discharge cycles was also achieved. The high-performance architecture of sharp-edged electrodes paves a way for smart electrochemical capacitors using an efficient planar structure in combination with high-loading materials for large pseudocapacitance as well as EDLC.

Deterministic Role of Carbon Nanotube-Substrate Coupling for Ultrahigh Actuation in Bilayer Electrothermal Actuators.

Here, the actuation response of an architectured electrothermal actuator comprising a single layer of carbon nanotube (CNT) film and a relatively thicker film of silk, cellulose, or polydimethylsiloxane is studied. An electric current is passed through the CNT film, which generates heat responsible for electrothermal actuation, in all samples, affixed as per doubly clamped beam configuration. All samples, including pure CNT film, show remarkable actuation such that actuation monotonically increases with the applied voltage. Cyclic pulsed electrical loading shows a lag in the electric current stimulus and the actuation. Remarkably, an ultrahigh actuation of ∼2.8%, which was 72 times more than that shown by pure CNT film, is measured in the CNT-cellulose film, that is, the architectured actuator with the natural polymer having the functional property of hygroexpansion and the structural hierarchy of the CNT film, however, at a significantly larger length scale. Overall, the synergetic contribution of the individual layers in these bilayered actuators enabled achieving ultrahigh electrothermal actuation compared to the homogeneous, synthetic polymer-based devices. A detailed discussion, which also includes examination of the role of the hierarchical substructure and the functional properties of the substrate and numerical analysis using the finite element method, is presented to highlight the actuation mechanism in the fabricated actuators.

Tailored viscoelasticity of a polymer cellular structure through nanoscale entanglement of carbon nanotubes.

A three-dimensional carbon nanotube (CNT) cellular structure presents a unique revelation of microstructure dependent mechanical and viscoelastic properties. Tailored CNT-CNT entanglement demonstrated a direct impact on both the strength and viscosity of the structure. Unlike traditional foams, an increase in the CNT-CNT entanglement progressively increases both the strength and the viscosity. The study reveals that an effective load is directly transferred within the structure through the short-range entanglements (nodes) resulting in an enhanced mechanical strength, whereas the long-range entanglements (bundles) regulate the energy absorption capacity. A three-dimensional structure of entangled CNT-CNT shows ∼15 and ∼26 times enhancement in the storage and loss moduli, respectively. The higher peak stress and energy loss are increased by ∼9.2 fold and ∼8.8 fold, respectively, compared to those of the cellular structures without entanglement. The study also revealed that the viscoelastic properties i.e. the Young's modulus, stress relaxation, strain rate sensitivity and fatigue properties can be modulated by tailoring the CNT-CNT entanglements within the cellular structure. A qualitative analysis is performed using finite element simulation to show the impact of CNT-CNT entanglements on the viscoelastic properties. The finding paves a way for designing a new class of meta-cellular materials which are viscous yet strong for shock absorbing or mechanical damping applications.

Spray Coating of Two-Dimensional Suspended Film of Vanadium Oxide-Coated Carbon Nanotubes for Fabrication of a Large Volume Infrared Bolometer.

A novel spray coating and transfer method is developed for fabricating a suspended bolometer of vanadium oxide-coated multiwalled carbon nanotubes (VCNTs). A parametric study was performed to evaluate the effect of the substrate, modulation frequency, and temperature on the bolometric performance and revealed that the performance of the bolometer solely does not depend on the substrate parameter but modulation frequency and bias current as a function of temperature also play a key attribute. The TCR (temperature coefficient of resistance) of the suspended VCNT bolometer is ∼-0.41%/K which is ∼486% higher than the reported suspended multiwalled CNTs at 300 K. Moreover, the suspended bolometer has a voltage responsivity of ∼67.42 ± 5.46 V/W (∼7.68 times that of unsuspended) at 200 K. Thus, the study presents an efficient method to develop the suspended bolometer that has not been realized so far to obtain much higher TCR and responsivity.

Flexible Array of Microsupercapacitor for Additive Energy Storage Performance Over a Large Area.

An on-chip microsupercapacitor (MSC) pattern is obtained by layer-by-layer spray deposition of both manganese dioxide (MnO2) nanoparticle-coated carbon nanotubes (MnO2-CNTs) and MnO2 nanosheet-decorated reduced graphene oxide (MnO2-rGO) on mechanically robust, flexible polyethylene terephthalate. Layer-by-layer patterning of MSC electrodes offers rapid in-plane diffusion of electrolyte ions in electrodes the layered electrode and hence ultrahigh capacitance and energy density of 7.43 mF/cm2 (32300 mF/cm3) and 0.66 µW h/cm2 (2870 µW h/cm3), respectively, are obtained. A robust electrochemical response was measured under multiple bending of the solid-state flexible MSC as well as under repetitive cycles (∼5000).

Doping controlled pyro-phototronic effect in self-powered zinc oxide photodetector for enhancement of photoresponse.

The pyro-phototronic effect can be used in pyroelectric semiconductor materials to significantly contribute in enhancing the self-powered photoresponse of photodetectors (PDs) via modulation of the photogenerated charge density. The pyro-phototronic effect in zinc oxide (ZnO) nanorods (NRs) was exploited thoroughly by doping with halogen elements, such as fluorine, chlorine (Cl), bromine and iodine. Cl-doped ZnO NRs (Cl : ZnO NRs) induces a large number of free charge carriers to enhance the self-powered photoresponse behavior (nearly 333% enhancement in response current) due to the pyro-phototronic effect as compared to pristine ZnO NRs. Moreover, 405% enhancement in pyrocurrent was measured for the Cl : ZnO NRs PD under a ultraviolet illumination intensity of 3 mW cm-2, as compared to 0.3 mW cm-2, in the absence of external bias voltage. Furthermore, other photoresponse parameters such as responsivity, external quantum efficiency and specific detectivity are measured to be higher due to the pyro-phototronic effect. Therefore, this study reveals the direct use of the pyro-phototronic effect to enhance the self-powered photoresponse.

Zn2+ induced molecular responses associated with oxidative stress, DNA damage and histopathological lesions in liver and kidney of the fish, Channa punctatus (Bloch, 1793).

Zn2+ is essential for normal physiological functioning of all organisms in small quantities, but when its concentration enhances in surrounding environment it acts as a toxicant to organisms. Common sources of Zn2+ pollution are electroplating, alloying, mining, and allied industrial operations. The present study aims to assess the biochemical, histopathological and genotoxicological implications under Zn2+ intoxication along with its accumulation patterns in prime biotransformation sites-liver and kidney, of a bottom feeder fish, Channa punctatus. Fish were chronically exposed to two different concentrations of Zn2+i.e., 5mg/L (permissible limit, T1) and 10mg/L (twice the permissible limit, T2). Simultaneous control was maintained. A significant (p<0.05) increment in Zn2+ bioaccumulation, antioxidant enzymes activities of SOD, CAT and GR and induction in micronuclei frequencies along with the significant (p<0.05) decrement in total protein and GSH were observed in all the exposed groups after 28 d. Altered biochemical parameters coupled with enhanced induction in micronuclei and accumulation of Zn2+ in liver and kidney of fish can be regarded as sensitive biomarkers of Zn2+ induced toxicological manifestations and thus, they may be effectively utilized for reliable ecotoxicological biomonitoring of aquatic regimes polluted with Zn2+.

Capacitive behavior of carbon nanotube thin film induced by deformed ZnO microspheres.

Multiwalled carbon nanotubes (CNTs) are uniformly distributed with piezoelectric microspheres. This leads to a large strain gradient due to an induced capacitive response, providing a 250% enhancement in electromechanical response compared with pristine CNTs. The fabricated large-area flexible thin film exhibits excellent pressure sensitivity, which can even detect an arterial pulse with a much faster response time (∼79 ms) in a bendable configuration. In addition, the film shows a rapid relaxation time (∼0.4 s), high stability and excellent durability with a rapid loading-unloading cycle. The dominant contribution of piezoelectric microspheres in a CNT matrix as opposed to nanoparticles showed a much higher sensitivity due to the large change in capacitance. Therefore, hybrid microstructures have various potential applications in wearable smart electronics, including detection of human motion and wrist pulses.

Synergistic effect in the heterostructure of ZnCo2O4 and hydrogenated zinc oxide nanorods for high capacitive response.

Herein, a novel heterostructure was fabricated by combining electrochemically and optically active materials to achieve a high capacitive response of 896 F g-1 at 5 A g-1. A network of ZnCo2O4 nanorods (NRs) was directly grown on a three-dimensional matrix of H : ZnO NRs (ZnCo2O4/H : ZnO NRs) that offered synergistic advantages by providing an optimum ion/charge transportation path, large electrochemically active surface area, and stable capacitive response during the electrolytic process. Furthermore, the fabricated solid-state asymmetric supercapacitor ZnCo2O4/H : ZnO NRs//activated carbon induced a large potential window of 1.5 V that offered excellent energy and power densities. In addition, optically active ZnCo2O4/H : ZnO NRs were also used for the conversion of optical energy over a broad wavelength range; thus, the as-fabricated asymmetric solid-state supercapacitor could easily provide the required power for the operation of a photodetector. Therefore, the unique heterostructure of ZnCo2O4/H : ZnO NRs not only presents excellent capacitive response but also demonstrates great potential for energy conversion.

Surface photo-charge effect in doped-ZnO nanorods for high-performance self-powered ultraviolet photodetectors.

The unique photo-charge characteristics of chlorine-doped zinc oxide nanorods (Cl-ZnO NRs) are explored for the first time in ultraviolet (UV) photodetector (PD) that offers an outstanding self-powered photoresponse towards low UV illumination signals. A self-powered Cl-ZnO NRs PD exhibits superior photon detection speed of the order of a few ms with high sensitivity and photoelasticity. Therefore, the presented PD opens up a novel route to fabricate highly efficient self-powered PDs on a large scale without employing complex multilayer systems.

Influence of charge traps in carbon nanodots on gas interaction.

The nonlinear electrical characteristic of carbon nanodots (CNDs) has revealed important physical phenomena of charge trapping playing a dominant role in surface interactions. Functional groups on the surface of CNDs attract ambient water molecules which in turn act as charge traps and give rise to electrical hysteresis that plays a dominant role in understanding charge transport in CNDs on surface interactions. Hysteresis in the current-voltage response is further utilized to study the interaction of the CNDs with nitrogen dioxide gas as an external stimuli. The hysteresis area is observed to be dependent on the time of gas interaction with the CNDs, therefore revealing the interaction mechanism of the CNDs with the gas.

Conjugated assembly of colloidal zinc oxide quantum dots and multiwalled carbon nanotubes for an excellent photosensitive ultraviolet photodetector.

Conjugation of highly dense colloidal zinc oxide quantum dots (ZnO QDs) on multiwalled carbon nanotubes (ZnO QDs@MWCNTs) is achieved for high performance ultraviolet (UV) photodetection. Significant improvement in the photoresponse of the ZnO QDs@MWCNTs photodetector (PD) is established as compared to a pristine ZnO QDs PD. The conjugation of two constituents allows the direct transfer of photoinduced charge carriers in ZnO QDs to MWCNTs for an efficient electrical path that considerably reduces charge recombination during UV exposure. Linearity in the response current with both the UV illumination intensity as well as external bias voltage reveals the photoelastic behavior of the ZnO QDs@MWCNTs PD. Moreover, the PD displays faster response and recovery times of 1.6 s and 1.9 s, respectively, than the most conventional PDs. In addition, spectral photoresponse analysis of the PD presents visible-blind behavior. Overall, conjugation of the hybrid heterostructure presented excellent photoelastic, high performance and visible-blind UV photodetection.

Energy-Efficient Hydrogenated Zinc Oxide Nanoflakes for High-Performance Self-Powered Ultraviolet Photodetector.

Light absorption efficiency and doping induced charge carrier density play a vital role in self-powered optoelectronic devices. Unique vanadium-doped zinc oxide nanoflake array (VZnO NFs) is fabricated for self-powered ultraviolet (UV) photodetection. The light harvesting efficiency drastically improved from 84% in ZnO NRs to 98% in VZnO NFs. Moreover, the hydrogenation of as-synthesized VZnO (H:VZnO) NFs displayed an outstanding increase in response current as compared to pristine structures. The H:VZnO NFs device presents an extraordinary photoelastic behavior with faster photodetection speed in the order of ms under a low UV illumination signal. Excellent responsivity and external quantum efficiency with larger value of specific detectivity of H:VZnO NFs device promises an outstanding sensitivity for UV signal and self-powered high-performance visible-blind photodetector.

Sandwiched assembly of ZnO nanowires between graphene layers for a self-powered and fast responsive ultraviolet photodetector.

A heterostructure of graphene and zinc oxide (ZnO) nanowires (NWs) is fabricated by sandwiching an array of ZnO NWs between two graphene layers for an ultraviolet (UV) photodetector. This unique structure allows NWs to be in direct contact with the graphene layers, minimizing the effect of the substrate or metal electrodes. In this device, graphene layers act as highly conducting electrodes with a high mobility of the generated charge carriers. An excellent sensitivity is demonstrated towards UV illumination, with a reversible photoresponse even for a short period of UV illumination. Response and recovery times of a few milliseconds demonstrated a much faster photoresponse than most of the conventional ZnO nanostructure-based photodetectors. It is shown that the generation of a built-in electric field between the interface of graphene and ZnO NWs effectively contributes to the separation of photogenerated electron-hole pairs for photocurrent generation without applying any external bias. Upon application of external bias voltage, the electric field further increases the drift velocity of photogenerated electrons by reducing the charge recombination rates, and results in an enhancement of the photocurrent. Therefore, the graphene-based heterostructure (G/ZnO NW/G) opens avenues to constructing a novel heterostructure with a combination of two functionally dissimilar materials.

Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc Oxide Nanorods.

Cobalt integrated zinc oxide nanorod (Co-ZnO NR) array is presented as a novel heterostructure for ultraviolet (UV) photodetector (PD). Defect states in Co-ZnO NRs surface induces an enhancement in photocurrent as compared to pristine ZnO NRs PD. Presented Co-ZnO NRs PD is highly sensitive to external magnetic field that demonstrated 185.7% enhancement in response current. It is concluded that the opposite polarizations of electron and holes in the presence of external magnetic field contribute to effective separation of electron-hole pairs that have drifted upon UV illumination. Moreover, Co-ZnO NRs PD shows a faster photodetection speed (1.2 s response time and 7.4 s recovery time) as compared to the pristine ZnO NRs where the response and recovery times are observed as 38 and 195 s, respectively.

Photoresponse of double-stacked graphene to Infrared radiation.

We report the photoresponse of stacked graphene layers towards infrared radiation. Graphene is stacked in two configurations, namely, crossed and parallel layers. Raman analysis demonstrated a strong interaction among the stacked graphene layers. Graphene in the crossed configuration exhibited the presence of both negative and positive conductivities; however, other configurations of graphene exhibited positive conductivity only. The presence of negative photoconductivity is proposed to be due to oxygen or oxygen-related functional group absorbents that are trapped in between two monolayers of graphene and act as scattering centers for free carriers. An interesting trend is reported in differential conductivity when stacked layers are compared with multilayers and parallel-stacked graphene layers.

Few-layer graphene/ZnO nanowires based high performance UV photodetector.

A graphene and zinc oxide nanowires (G/ZnO NWs) based ultraviolet (UV) photodetector presents excellent responsivity and photocurrent gain with detectivity. Graphene due to higher charge carrier transport mobility induces faster response to UV illumination at the interface between ZnO and graphene with improved response and decay times as compared to a ZnO NWs device alone. A linear increase is revealed for both the responsivity and photocurrent gain of the G/ZnO NWs device with the applied bias. These results suggest that the G/ZnO NWs device exhibits great promise for highly efficient UV photodetectors.