Biotemplate-Assisted Growth of ZnO in Gas Sensors for ppb-Level NO2 Detection.

With the growing concern over the adverse effects of environmental pollution on human health, the combination of environmentally friendly and nontoxic biomaterials with metal oxide semiconductor materials for electronic devices has emerged as a prominent trend in current research. In this study, we utilized 150 mg apple biotemplates to assist in the hydrothermal synthesis of ZnO nanospheres. It successfully achieved high sensitivity for detecting 35 and 350 ppb NO2 at room temperature, with responses of 13.74 and 132.44%, respectively. Simultaneously, the 5-cycle repeatability and multiple-gas selectivity exhibited significant improvements. The ZnO nanospheres demonstrated enhanced sensing performance compared to pure ZnO nanorods, which is attributed to the following mechanisms: reason I, the modified surface morphology increasing the surface-to-volume ratio; reason II, an increase in oxygen vacancies, leading to reduced crystallinity and a higher electron concentration; reason III, incorporation of carbon elements on the nanostructure surface to increase active sites. The novel gas sensor assisted by the apple pectin biotemplate offers a promising solution for NO2 gas detection, featuring low operating temperatures, low concentrations, and high response sensitivity.

In-plane gate graphene transistor with epitaxially grown molybdenum disulfide passivation layers.

We demonstrate in-plane gate transistors based on the molybdenum disulfide (MoS2)/graphene hetero-structure. The graphene works as channels while MoS2 functions as passivation layers. The weak hysteresis of the device suggests that the MoS2 layer can effectively passivate the graphene channel. The characteristics of devices with and without removal of MoS2 between electrodes and graphene are also compared. The device with direct electrode/graphene contact shows a reduced contact resistance, increased drain current, and enhanced field-effect mobility. The higher field-effect mobility than that obtained through Hall measurement indicates that more carriers are present in the channel, rendering it more conductive.

Persistent charge storage and memory operation of top-gate transistors solely based on two-dimensional molybdenum disulfide.

We fabricate top-gate transistors on the three-layer molybdenum disulfide (MoS2) with three, two, and one layers in the source and drain regions using atomic layer etching (ALE). In the presence of ALE, the device at zero gate voltage can exhibit high and low levels of drain current under the forward and reverse gate bias, respectively. The hysteresis loop on the transfer curve of transistor indicates that two distinct charge states exist in the device within a range of gate bias. A long retention time of the charge is observed. Unlike conventional semiconductor memories with transistors and capacitors, the two-dimensional (2D) material itself plays two parts in the current conduction and charge storage. The persistent charge storage and memory operation of the multilayer MoS2transistors with thicknesses of a few atomic layer will further expand the device application of 2D materials with reduced linewidths.

Investigation of Different Oxygen Partial Pressures on MgGa2O4-Resistive Random-Access Memory.

Different oxygen partial-pressure MgGa2O4-resistive RAMs (RRAMs) are fabricated to investigate the resistive switching behaviors. The X-ray photoelectron spectroscopy results, set voltage, reset voltage, cycling endurance, and retention time are drawn for comparison. With the increasing oxygen ratio gas flow, the resistive switching characteristics of MgGa2O4 RRAM are drastically elevated by changing the fabrication conditions of the RS layer. Moreover, we portray a filament model to explain the most likely mechanism associated with the generation and rupture of conductive filaments composed of oxygen vacancies. The formation of the interfacial layer (AlO x ) and the participation of the Joule heating effect are included to explain the highly distributed high-resistance state (HRS). The high randomness among switching cycles for memory application should be prevented, but it is suitable for the physical unclonable function. The relationship between HRS and the next time set voltage shows a strong correlation, and the conduction mechanisms of the low-resistance state (LRS) and HRS correspond to ohmic conduction and space charge-limited conduction, respectively. Meanwhile, the RRAM undergoes 10,000 s retention tests, and the two resistance states can be distinguished without obvious alternation or degradation. A favorable cycling endurance and retention time achieved by optimizing the fabrication parameters of Al/MgGa2O4/Pt RRAM have the potential for nonvolatile memristors and information security applications.

Design of LTE/Sub-6 GHz Dual-Band Transparent Antenna Using Frame-Structured Metal Mesh Conductive Film.

This paper proposes a dual-band transparent antenna using frame-structured metal mesh conductive film (MMCF). The frame-structured metal mesh conductive film is based on the conductive-coated thin film and forms a narrow strip surrounding the edge of the antenna. The frame-structured metal mesh conductive film can resist considerable current leakage on the edge of the conductive strip to improve the antenna's efficiency by 51% at 2.1 GHz and 53% at 3.6 GHz. As a result, the transparent dual-band antenna has an operating bandwidth of 1.9-2.4 GHz and 3.2-4.1 GHz with a high transparency of 80%, which make it valuable to the applications of biomedical electronic components, wearable devices, and automobile vehicles.

The influence of contact metals on epitaxially grown molybdenum disulfide for electrical and optical device applications.

Bottom-gate transistors with mono-layer MoS2channels and polycrystalline antimonene source/drain contact electrodes deposited at 75 °C are fabricated. Significant performance enhancement of field-effect mobility 11.80 cm2V-1·s-1and >106ON/OFF ratio are observed for the device. Increasing photocurrents are also observed for the MoS2transistor under light irradiation, which is attributed to the reduced carrier recombination at the metal/2D material interfaces. The results have demonstrated that besides the matching of work function values with the 2D material channel, the crystallinity of the contact electrodes is the other important parameter for the Ohmic contact formation of 2D material devices.

Charge Storage of Isolated Monolayer Molybdenum Disulfide in Epitaxially Grown MoS2/Graphene Heterostructures for Memory Device Applications.

We epitaxially grew bilayer molybdenum disulfide (MoS2) on monolayer graphene by sulfurizing a molybdenum-trioxide film (MoO3) which was deposited with thermal evaporation. The Hall mobilities of graphene before and after the growth of MoS2 are similar, indicating that the underlying 2D layer was little affected during the deposition and sulfurization. Through the atomic-layer etching, the topmost layer of MoS2 is isolated from the source and drain electrodes. The top-gate transistor with the isolated monolayer MoS2 on top of the graphene channel exhibits hysteresis of drain current as the gate voltage varies. This may be due to the weak tunneling through 2D layers bonded by the van der Waals force in the absence of an external electric field. The long retention time of the device features robust charge storage around the isolated MoS2 layer. The one-transistor-zero-capacitor memory module based on this thin heterostructure of 2D materials can be advantageous for applications in dynamic random access memories with reduced thickness.

Stability-Enhanced Resistive Random-Access Memory via Stacked In x Ga1-x O by the RF Sputtering Method.

The stability of a resistive random-access memory (RRAM) device over long-term use has been widely acknowledged as a pertinent concern. For investigating the stability of RRAM devices, a stacked In x Ga1-x O structure is designed as its switching layer in this study. Each stacked structure in the switching layer, formed via sputtering, consists of varying contents of gallium, which is a suppressor of oxygen vacancies; thus, the oxygen vacancies are well controlled in each layer. When a stacked structure with layers of different contents is formed, the original gradients of concentration of oxygen vacancies and mobility influence the set and reset processes. With the stacked structure, an average set voltage of 0.76 V, an average reset voltage of -0.66 V, a coefficient of variation of set voltage of 0.34, and a coefficient of variation of reset voltage of 0.18 are obtained. Additionally, under DC sweeps, the stacked RRAM demonstrates a high operating life of more than 4000 cycles. In conclusion, the performance and stability of the RRAM are enhanced herein by adjusting the concentration of oxygen vacancies via different compositions of elements.

Fabrication of Silicon Dioxide by Photo-Chemical Vapor Deposition to Decrease Detector Current of ZnO Ultraviolet Photodetectors.

Zinc oxide (ZnO)-based semiconductor is a promising application for ultraviolet photodetectors (UV PDs). The performance of ZnO UV PDs can be improved in two orientations: by reduction of the dark current and by increasing the photocurrent. In the study, we used two processes to prepare ZnO UV PDs: photochemical vapor deposition to fabricate silicon dioxide as an insulator layer and a radio frequency sputter system to prepare the ZnO film as an active layer. The results show that the silicon dioxide layer can reduce the dark current. Moreover, a large photo-dark current ratio of the metal-insulator-semiconductor (MIS) structured PD is 200 times than the metal-semiconductor-metal (MSM) structured PD. When the silicon dioxide thickness is 98 nm, we can significantly enhance the rejection ratio. The silicon dioxide layer can reduce the noise effect and enhance the device detectivity. These results indicate that the insertion of a silicon dioxide layer into ZnO PDs is potentially useful for practical applications.

Luminescence enhancement and dual-color emission of stacked mono-layer 2D materials.

With additional precursor soaking, a thin Al2O3 dielectric layer can be grown on mono-layer MoS2 by using atomic layer deposition (ALD). Similar optical characteristics are observed before and after ALD growth for the mono-layer MoS2, which indicates that minor damage to the thin 2D material film is introduced during the growth procedure. With the thin separation layer, luminescence enhancement and dual-color emission are observed by transferring MoS2 and WS2 mono-layer 2D materials to 5 nm Al2O3/mono-layer MoS2 samples, respectively. The results demonstrate that with careful treatment of the interfaces of 2D crystals with other materials, different stacked structures can be established.

Photoresponses of Zinc Tin Oxide Thin-Film Transistor.

In this study, the optical and electrical properties of a zinc tin oxide (ZTO) thin-film transistor (TFT) were investigated. The TFT was fabricated using ZTO as the active layer, which was deposited by a radio frequency magnetron sputtering system, to form an ultraviolet (UV) photodetector. The device has a threshold voltage of 0.48 V, field-effect mobility of 1.47 cm²/Vs in the saturation region, on/off drain current ratio of 2×106, and subthreshold swing of 0.45 V/decade in a dark environment. Moreover, as a UV photodetector, the device has a long photoresponse time, responsivity of 0.329 A/W, and rejection ratio of 3.19×104 at a gate voltage of -15 V under illumination of wavelength 300 nm.

Fast Detection and Flexible Microfluidic pH Sensors Based on Al-Doped ZnO Nanosheets with a Novel Morphology.

In this study, a flexible and stable pH sensor based on aluminum-doped zinc oxide nanosheets (Al-doped ZnO NSs) was developed by a low-cost hydrothermal method. The results obtained from this study indicated that Al ions could be doped successfully into the ZnO nanostructure, which could change the morphology and improve the pH-sensing properties. The pH sensitivity of Al-doped ZnO nanosheets reached 50.2 mV/pH with a correlation coefficient of around 0.99468 when compared with that of ZnO film (34.13 mV/pH) and pure ZnO nanowires (45.89 mV/pH). The test range of pH values was widened by Al-doping, and the Al-doped ZnO NS sensor could detect the pH value ranging from 2 to 12. It was observed that in a more acidic environment, especially at pH 2, the sensor, Al-doped ZnO nanosheet, was strongly stable over 12 weeks of testing. It was noted that the response time was utterly fast and the response time of the sensors for each pH standard buffer solutions was around 0.3 s. Thus, the response time and performance were quite stable. The microchannel provided a novel testing method for the pH sensor, where the liquid to be tested was just 5 mL. Hence, it was suggested to be useful for many medical diagnoses and treatments. The benefits of Al-doped ZnO nanosheet pH sensor were high sensitivity, good long-term usage, good flexible property, and requirement of a small amount of test liquid, which could make the sensors viable candidates for practical applications.

Visible Illumination Enhanced Nonenzymatic Glucose Photobiosensor Based on TiO2 Nanorods Decorated With Au Nanoparticles.

A nonenzymatic glucose photobiosensor was developed based on Au-nanoparticle-decorated TiO2 nanorods (NRs) under visible illumination. Au nanoparticles (NPs) absorbed the visible illumination, resulting in surface plasmon resonance (SPR). The SPR of the Au NPs indicated that there was a strong electric field around them, which promoted the transport of more electrons to the TiO2 NRs and enhanced the glucose sensing properties. The sensing current under visible illumination was five times higher than in the dark when in 0.1 M NaOH solution at a potential of 0.17 V. Moreover, the Michaelis-Menten constant (Km) of the Au NPs/TiO2 NRs/FTO under visible illumination was 0.52 mM, which is much smaller than that reported previously. Moreover, these results indicate that the Au NPs/TiO2 NRs/FTO under visible illumination feature outstanding properties as a nonenzymatic glucose photobiosensor.

Bandgap-Engineered Zinc-Tin-Oxide Thin Films for Ultraviolet Sensors.

Zinc-tin-oxide thin-film transistors were prepared by radio frequency magnetron co-sputtering, while an identical zinc-tin-oxide thin film was deposited simultaneously on a clear glass substrate to facilitate measurements of the optical properties. When we adjusted the deposition power of ZnO and SnO2, the bandgap of the amorphous thin film was dominated by the deposition power of SnO2. Since the thin-film transistor has obvious absorption in the ultraviolet region owing to the wide bandgap, the drain current increases with the generation of electron-hole pairs. As part of these investigations, a zinc-tin-oxide thin-film transistor has been fabricated that appears to be very promising for ultraviolet applications.

Fabrication of Zinc Oxide-Based Thin-Film Transistors by Radio Frequency Sputtering for Ultraviolet Sensing Applications.

In this study, zinc indium tin oxide thin-film transistors (ZITO TFTs) were fabricated by the radio frequency (RF) sputtering deposition method. Adding indium cations to ZnO by co-sputtering allows the development of ZITO TFTs with improved performance. Material characterization revealed that ZITO TFTs have a threshold voltage of 0.9 V, a subthreshold swing of 0.294 V/decade, a field-effect mobility of 5.32 cm2/Vs, and an on-off ratio of 4.7 × 105. Furthermore, an investigation of the photosensitivity of the fabricated devices was conducted by an illumination test. The responsivity of ZITO TFTs was 26 mA/W, with 330-nm illumination and a gate bias of -1 V. The UV-to-visible rejection ratio for ZITO TFTs was 2706. ZITO TFTs were observed to have greater UV light sensitivity than that of ZnO TFTs. We believe that these results suggest a significant step toward achieving high photosensitivity. In addition, the ZITO semiconductor system could be a promising candidate for use in high performance transparent TFTs, as well as further sensing applications.

Three-Dimensional ZnO Nanostructure Based Gas and Humidity Sensors.

The ZnO nanostructure environmental sensors were prepared via the three-dimensional through silicon via (3D-TSV) technique. For 3D-TSV, the diameter and length of the Si via were about 200 and 400 µm, respectively. For nitrogen oxide (NO), the measured responses were around ~12, ~16, and ~20% when the concentrations of the injected NO gas were 20, 40 and 60 ppm, respectively. For humidity and temperature sensing, the measured nanowire current increased logarithmically with increasing chamber temperature. The response to relative humidity increased with increasing temperature.

High Sensitivity of NO Gas Sensors Based on Novel Ag-Doped ZnO Nanoflowers Enhanced with a UV Light-Emitting Diode.

An ultraviolet-enhanced (UV-enhanced) nitric oxide (NO) sensor based on silver-doped zinc oxide (ZnO) nanoflowers is developed using a low-cost hydrothermal method. The results indicate that silver (Ag) ions were doped into the ZnO nanostructure successfully, thus changing the morphology. In the high-resolution transmission electron microscopy images, we also found that some Ag ions were separated out onto the surface of the ZnO nanoflowers and that the Ag-doped and Ag nanoparticles improved the sensing property. The NO sensing property increased from 73.91 to 89.04% through the use of a UV light-emitting diode (UV-LED). The response time was approximately 120 s without the UV-LED, and the UV-enhanced Ag-doped ZnO nanoflower sensor exhibited a reduced response time (60 s). The best working temperature could be reduced from 200 to 150 °C using UV light illumination, and it was found that the NO response increased by 15.13% at 150 °C. The UV photoresponse of the Ag-doped ZnO nanoflowers and the mechanisms by which the improvement of NO sensing property occurred through the use of UV light illumination are discussed. The property of the gas sensor can be calibrated using a self-photoelectric effect under UV light illumination. These interesting UV-enhanced Ag-doped ZnO nanoflowers are viable candidates for practical applications.

Through-silicon via submount for the CuO/Cu2O nanostructured field emission display.

A three dimensional (3D) field emission display structure was prepared using CuO/Cu2O composite nanowires (NWs) and a three dimensional through silicon via (3D-TSV) technique. The experimental results indicated that the diameter and length of the Si via were about 100 µm and 200 µm, respectively. For the 3D field emission structure, high-density CuO/Cu2O composite nanowires (NWs) were grown on the concave TSV structure using thermal oxidation. The field emission turn-on field and enhancement factor of the CuO/Cu2O composite NWs were 15 V µm-1 and ∼1748, respectively. With regard to field emission displays, we successfully used the 3D field emission structure to excite the orange phosphors.

Highly stable ITO/Zn2TiO4/Pt resistive random access memory and its application in two-bit-per-cell.

We discuss the fabrication procedure and device characteristics of ITO/Zn2TiO4/Pt resistive random-access memory (RRAM) at room temperature. Four different resistive states were obtained by applying different current compliances, all of which showed good retention characteristics with no obvious degradation and were individually distinguished after 10 000 s at a read voltage of 100 mV. The multilevel memory effect can be attributed to the combination of the radial growth of filaments and the formation of conductive filaments when applying different compliance current values during the set process. The set and reset voltages of the ITO/Zn2TiO4/Pt RRAM device were maintained within ±1 V. The device performed well at low operation voltages. The mechanisms of multilevel resistive switching characteristics were investigated to illustrate the multilevel carrier conduction phenomenon associated with Zn2TiO4-based RRAM devices. In this study, our group illustrated the application of zinc titanate (Zn2TiO4) in non-volatile memories for the first time.

High Responsivity MgZnO Ultraviolet Thin-Film Phototransistor Developed Using Radio Frequency Sputtering.

We investigated the electrical and optoelectronic properties of a magnesium zinc oxide thin-film phototransistor. We fabricate an ultraviolet phototransistor by using a wide-bandgap MgZnO thin film as the active layer material of the thin film transistor (TFT). The fabricated device demonstrated a threshold voltage of 3.1 V, on-off current ratio of 105, subthreshold swing of 0.8 V/decade, and mobility of 5 cm²/V·s in a dark environment. As a UV photodetector, the responsivity of the device was 3.12 A/W, and the rejection ratio was 6.55 × 105 at a gate bias of -5 V under 290 nm illumination.