Ultrasensitive Gas Sensors Based on Vertical Graphene Nanowalls/SiC/Si Heterostructure

Electrical Kinetic Model of a Hydroxylated Graphene FET Gas Sensor

Graphene transistor sensors, with advantages such as facile surface functionalization and high sensitivity, have gained extensive research interest in gas detection applications. This study fabricated back-gated graphene transistors and employed a hydroxylation scheme for the surface functionalization of graphene. On the basis of the interaction mechanisms between gas molecules and graphene's electrical properties, a compact electrical kinetics model considering the gas-solid surface reaction of graphene transistors is proposed. The model can accurately predict the electrical kinetic performance and can be used to optimize sensor characteristics. The bias condition of a higher response can be rapidly determined. In addition, the density of hydroxyl groups on graphene is revealed to be the direction of improvement and a key factor of response. Hence, the gas detection capacity of sensors with varying densities of hydroxyl groups was assessed concerning ammonia gas, and design technology co-optimization (DTCO) is realized. Measurement results show that the sensor with 70 s of hydroxylation time has a 7.7% response under 22 ppm ammonia gas.

The Synergistic Effect on the Mimetic Optical Structure of Feline Eyes toward Household Health Monitoring of Acute and Chronic Diseases

A breakthrough in the performance of bionic optical structures will only be achieved if we can obtain an in-depth understanding of the synergy mechanisms operating in natural optical structures and find ways to imitate them. In this work, inspired by feline eyes, an optical substrate that takes advantage of a synergistic effect that occurs between resonant and reflective structures was designed. The synergistic effect between the reflective and resonant components leads to a Raman enhancement factor (EF) of 1.16 × 107, which is much greater than that achieved using the reflective/resonant cavities on their own. Finite-difference time-domain (FDTD) simulations and experimental results together confirm that the mechanism of this synergistic effect is achieved by realizing multiple reflections and repeated absorptions of light, generating a strong local electric field. Thus, a 2-3 order of magnitude increase in sensitivity could be achieved. More importantly, with the homemade centrifugal device, above optical substrates were further used to develop a rapidly highly sensitive household health monitoring system (detection time <3 min). It can thus be used to give early warning of acute diseases with high risk (e.g., acute myocardial infarction (AMI) and cerebral peduncle). Due to the good reusability and storability (9% and 8% reduction in EF after washing 30 times and 9 months of storage, respectively) of the substrates, the substrates thus reduce detection costs (to ∼$1), making them much cheaper to use than the current gold-standard methods (e.g., ∼$16 for gout detection).

Carbon nanowall-based gas sensors for carbon dioxide gas detection

Carbon nanowalls (CNWs) have attracted significant attention for gas sensing applications due to their exceptional material properties such as large specific surface area, electric conductivity, nano- and/or micro-porous structure, and high charge carrier mobility. In this work, CNW films were synthesized and used to fabricate gas sensors for carbon dioxide (CO2) gas sensing. The CNW films were synthesized using an inductively-coupled plasma (ICP) plasma-enhanced chemical vapor deposition (PECVD) method and their structural and morphological properties were characterized using Raman spectroscopy and electron microscopy. The obtained CNW films were used to fabricate gas sensors employing interdigitated gold (Au) microelectrodes. The gas sensors were fabricated using both direct synthesis of CNW films on interdigitated Au microelectrodes on quartz and also transferring presynthesized CNW films onto interdigitated Au microelectrodes on glass. The CO2gas-sensing properties of fabricated devices were investigated for different concentrations of CO2gas and temperature-ranges. The sensitivities of fabricated devices were found to have a linear dependence on the concentration of CO2gas and increase with temperature. It was revealed that devices, in which CNW films have a maze-like structure, perform better compared to the ones that have a petal-like structure. A sensitivity value of 1.18% was obtained at 500 ppm CO2concentration and 100 °C device temperature. The CNW-based gas sensors have the potential for the development of easy-to-manufacture and efficient gas sensors for toxic gas monitoring.

Progress of One-Dimensional SiC Nanomaterials: Design, Fabrication and Sensing Applications

One-dimensional silicon carbide (SiC) nanomaterials hold great promise for a series of applications, such as nanoelectronic devices, sensors, supercapacitors, and catalyst carriers, attributed to their unique electrical, mechanical, and physicochemical properties. Recent progress in their design and fabrication has led to a deep understanding of the structural evolution and structure-property correlation. Several unique attributes, such as high electron mobility, offer SiC nanomaterials an opportunity in the design of SiC-based sensors with high sensitivity. In this review, a brief introduction to the structure and properties of SiC is first presented, and the latest progress in design and fabrication of one-dimensional SiC nanomaterials is summarized. Then, the sensing applications of one-dimensional SiC nanomaterials are reviewed. Finally, our perspectives on the important research direction and future opportunities of one-dimensional SiC nanomaterial for sensors are proposed.

Preparation of self-supporting vertically/horizontally grown graphene microelectrodes for neurotransmitter determination

The development of microelectrodes for the rapid in situ detection of neurotransmitters and their metabolic levels in human biofluids has considerable significance in biomedical research. In this study, self-supported graphene microelectrodes with B-doped, N-doped, and B- and N-co-doped vertical graphene (BVG, NVG, and BNVG, respectively) nanosheets grown on horizontal graphene (HG) were fabricated for the first time. The high electrochemical catalytic activity of BVG/HG on monoamine compounds was explored by investigating the influence of B and N atoms and the VG layer thickness on the response current of neurotransmitters. Quantitative analysis using the BVG/HG electrode in a blood-like environment with pH 7.4 indicated that the linear concentration ranges were 1-400 and 1-350 μM for dopamine (DA) and serotonin (5-HT), with limits of detection (LODs) of 0.271 and 0.361 μM, respectively. For tryptophan (Trp), the sensor measured a wide linear concentration range of 3-1500 μM over a wide pH range of 5.0-9.0, with the LOD fluctuating between 0.58 and 1.04 μM. Furthermore, the BVG/HG microelectrodes could be developed as needle- and pen-type sensors for the detection of DA, 5-HT, and Trp in human blood and gastrointestinal secretion samples.

Hydrogen-Terminated Two-Dimensional Germanane/Silicane Alloys as Self-Powered Photodetectors and Sensors

2D monoelemental materials, particularly germanene and silicene (the single layer of germanium and silicon), which are the base materials for modern electronic devices demonstrated tremendous attraction for their 2D layer structure along with the tuneable electronics and optical band gap. The major shortcoming of synthesized thermodynamically very unstable layered germanene and silicene with their inclination toward oxidation was overcome by topochemical deintercalation of a Zintl phase (CaGe₂, CaGe_1.5Si_0.5, and CaGeSi) in a protic environment. The exfoliated Ge-H, Ge_0.75Si_0.25H, and Ge_0.5Si_0.5H were successfully synthesized and employed as the active layer for photoelectrochemical photodetectors, which showed broad response (420-940 nm), unprecedented responsivity, and detectivity on the order of 168 μA W^-1 and 3.45 × 10⁸ cm Hz^1/2 W^-1, respectively. The sensing capability of exfoliated germanane and silicane composites was explored using electrochemical impedance spectroscopy with ultrafast response and recovery time of less than 1 s. These positive findings serve as the application of exfoliated germanene and silicene composites and can pave a new path to practical applications in efficient future devices.

2D Few-Layered PdPS: Toward High-Efficient Self-Powered Broadband Photodetector and Sensors

Photodetectors and sensors have a prominent role in our lives and cover a wide range of applications, including intelligent systems and the detection of harmful and toxic elements. Although there have been several studies in this direction, their practical applications have been hindered by slow response and low responsiveness. To overcome these problems, we have presented here a self-powered (photoelectrochemical, PEC), ultrasensitive, and ultrafast photodetector platform. For this purpose, a novel few-layered palladium-phosphorus-sulfur (PdPS) was fabricated by shear exfoliation for effective photodetection as a practical assessment. The characterization of this self-powered broadband photodetector demonstrated superior responsivity and specific detectivity in the order of 33 mA W^-1 and 9.87 × 10¹⁰ cm Hz^1/2 W^-1, respectively. The PEC photodetector also exhibits a broadband photodetection capability ranging from UV to IR spectrum, with the ultrafast response (∼40 ms) and recovery time (∼50 ms). In addition, the novel few-layered PdPS showed superior sensing ability to organic vapors with ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity in the form of hydrogen evolution reaction was explored due to the suitable band alignment and pronounced light absorption capability. The self-powered sensing platforms and superior catalytic activity will pave the way for practical applications in efficient future devices.

High-Sensitivity Ammonia Sensors with Carbon Nanowall Active Material via Laser-Induced Transfer

Ammonia sensors with high sensitivity, reproducible response, and low cost are of paramount importance for medicine, i.e., being a biomarker to diagnose lung and renal conditions, and agriculture, given that fertilizer application and livestock manure account for more than 80% of NH₃ emissions. Thus, in this work, we report the fabrication of ultra-sensitive ammonia sensors by a rapid, efficient, and solvent-free laser-based procedure, i.e., laser-induced forward transfer (LIFT). LIFT has been used to transfer carbon nanowalls (CNWs) onto flexible polyimide substrates pre-patterned with metallic electrodes. The feasibility of LIFT is validated by the excellent performance of the laser-printed CNW-based sensors in detecting different concentrations of NH₃ in the air, at room temperature. The sensors prepared by LIFT show reversible responses to ammonia when exposed to 20 ppm, whilst at higher NH₃ concentrations, the responses are quasi-dosimetric. Furthermore, the laser-printed CNW-based sensors have a detection limit as low as 89 ppb and a response time below 10 min for a 20 ppm exposure. In addition, the laser-printed CNW-based sensors are very robust and can withstand more than 200 bending cycles without loss of performance. This work paves the way for the application and integration of laser-based techniques in device fabrication, overcoming the challenges associated with solvent-assisted chemical functionalization.

All-carbon stretchable and cavity-free white lasers

Flexible, stretchable, and bendable electronics and optoelectronics have a great potential for wide applications in smart life. An environmentally friendly, cost effective and wide-angle emission laser is indispensable for the emerging technology. In this work, circumvent the challenge issue, cavity-free and stretchable white light lasers based on all carbon materials have been demonstrated by integration of fluorescent carbon quantum dots (CQDs) and crumpled graphene. The typical emission spectrum of the cavity-free laser based on all-carbon materials has a CIE chromaticity coordinate of (0.30, 0.38) exhibiting an intriguing broadband white-light emission. The unprecedented and non-toxic stretchable and white light cavity-free lasers based on all-carbon materials can serve as next-generation optoelectronic devices for a wide range application covering solid-state lighting and future wearable technologies.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

Effect of GNWs/NiO-WO3/GNWs Heterostructure for NO2 Gas Sensing at Room Temperature

Recently, as air pollution and particulate matter worsen, the importance of a platform that can monitor the air environment is emerging. Especially, among air pollutants, nitrogen dioxide (NO₂) is a toxic gas that can not only generate secondary particulate matter, but can also derive numerous toxic gases. To detect such NO₂ gas at low concentration, we fabricated a GNWs/NiO-WO₃/GNWs heterostructure-based gas sensor using microwave plasma-enhanced chemical vapor deposition (MPECVD) and sputter, and we confirmed the NO₂ detection characteristics between 10 and 50 ppm at room temperature. The morphology and carbon lattice characteristics of the sensing layer were investigated using field emission scanning electron microscopy (FESEM) and Raman spectroscopy. In the gas detection measurement, the resistance negative change according to the NO₂ gas concentration was recorded. Moreover, it reacted even at low concentrations such as 5–7 ppm, and showed excellent recovery characteristics of more than 98%. Furthermore, it also showed a change in which the reactivity decreased with respect to humidity of 33% and 66%.

Self-Powered Broadband Photodetector and Sensor Based on Novel Few-Layered Pd3(PS4)2 Nanosheets

Optoelectronics and sensing devices are of enormous importance in our modern lives, which has propelled the scientific community to explore new two-dimensional (2D) nanomaterials to meet the requirements of future devices. Herein, we present the exfoliation of palladium thiophosphate (Pd₃(PS₄)₂) by mechanical shear force exfoliation. The Pd₃(PS₄)₂-based photoelectrochemical (PEC) device demonstrated self-powered broadband photodetection in the range of 385-940 nm with an unprecedented responsivity of 2 A W^-1 and a specific detectivity of about 8.67 × 10¹¹ cm Hz^1/2 W^-1 under the illumination of 420 nm LED light. The crucial parameters such as photoresponsivity, response, and recovery time of the device can be controlled by an externally applied voltage and the analyte concentration. Moreover, Pd₃(PS₄)₂-based vapor-sensing devices exhibited frequency-dependent selective acetone sensing in the presence of other organic vapors with an ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity of Pd₃(PS₄)₂ was revealed, which can be attributed to the presence of an appropriate band alignment with the catalytic activity of Pd. This novel material with the aforementioned fascinating phenomenon will pave the way toward practical future applications in optoelectronics and sensing.

Sensitive, Reusable, Surface-Enhanced Raman Scattering Sensors Constructed with a 3D Graphene/Si Hybrid

Surface-enhanced Raman scattering (SERS) substrates based on graphene and its derivatives have recently attracted attention among those interested in the detection of trace molecules; however, these substrates generally show poor uniformity, an unsatisfactory enhancement factor, and require a complex fabrication process. Herein, we design and fabricate three-dimensional (3D) graphene/silicon (3D-Gr/Si) heterojunction SERS substrates to detect various types of molecules. Notably, the detection limit of 3D-Gr/Si can reach 10^-10 M for rhodamine 6G (R6G) and rhodamine B (RB), 10^-7 M for crystal violet (CRV), copper(II) phthalocyanine (CuPc), and methylene blue (MB), 10^-8 M for dopamine (DA), 10^-6 M for bovine serum albumin (BSA), and 10^-5 M for melamine (Mel), which is superior to most reported graphene-based SERS substrates. Besides, the proposed 3D-Gr/Si heterojunction SERS substrates can achieve a high uniformity with relative standard deviations (RSDs) of less than 5%. Moreover, the 3D-Gr/Si SERS substrates are reusable after washing with ethyl alcohol to remove the adsorbed molecules. These excellent SERS performances are attributed to the novel 3D structure and abundantly exposed atomically thin edges, which facilitate charge transfer between 3D-Gr and probe molecules. We believe that the 3D-Gr/Si heterojunction SERS substrates offer potential for practical applications in biochemical molecule detection and provide insight into the design of high-performance SERS substrates.

Effect of Substrate Types on the Structure of Vertical Graphene Prepared by Plasma-Enhanced Chemical Vapor Deposition

Although the structure of vertical graphene (VG) is important for various applications, the growth mechanism of VG is not yet fully clear. Here, the impacts of electrical conductivity of substrate on the morphology and structure of VG prepared by plasma-enhanced chemical vapor deposition are studied by scanning electron microscopy and Raman spectroscopy. The results show that VG with greater thickness can be grown on substrate with better electrical conductivity in the same growth time. Even though longer deposition time leads to more VG, more defects might develop in VG, especially at the position furthest away from the substrates. The change of morphology and structure of VG is closely correlated with strength of electric field near the substrate surface, which offers a new approach for orderly growing of VG. The discoveries not only shed light on the growth mechanism of VG, but also are beneficial for promoting the applications of VG.

Review of Vertical Graphene and its Applications

Vertical graphene (VG) is a thin-film complex material featuring hierarchical microstructures: graphene-containing carbon nanosheets growing vertically on its deposition substrate, few-layer graphene basal layers, and chemically active atomistic defect sites and edges. Thanks to the fundamental characteristics of graphene materials, e.g. excellent electrical conductivity, thermal conductivity, chemical stability, and large specific surface area, VG materials have been successfully implemented into various niche applications which are strongly associated with their unique morphology. The microstructure of VG materials can be tuned by modifying growth methods and the parameters of growth processes. Multiple growth processes have been developed to address faster, safer, and mass production methods of VG materials, as well as accommodating various applications. VG's successful applications include field emission, supercapacitors, fuel cells, batteries, gas sensors, biochemical sensors, electrochemical analysis, strain sensors, wearable electronics, photo trapping, terahertz emission, etc. Research topics on VG have been more diversified in recent years, indicating extensive attention from the research community and great commercial value. In this review article, VG's morphology is briefly reviewed, and then various growth processes are discussed from the perspective of plasma science. After that, the most recent progress in its applications and related sciences and technologies are discussed.

Correlation between Microstructure and Chemical Composition of Zinc Oxide Gas Sensor Layers and Their Gas-Sensitive Properties in Chlorine Atmosphere

In this article, we present results concerning the impact of structural and chemical properties of zinc oxide in various morphological forms and its gas-sensitive properties, tested in an atmosphere containing a very aggressive gas such as chlorine. The aim of this research was to understand the mechanism of chlorine detection using a resistive gas sensor with an active layer made of zinc oxide with a different structure and morphology. Two types of ZnO sensor layers obtained by two different technological methods were used in sensor construction. Their morphology, crystal structure, specific surface area, porosity, surface chemistry and structural defects were characterized, and then compared with gas-sensitive properties in a chlorine-containing atmosphere. To achieve this goal, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL) methods were used. The sensing properties of obtained active layers were tested by the temperature stimulated conductance method (TSC). We have noticed that their response in a chlorine atmosphere is not determined by the size of the specific surface or porosity. The obtained results showed that the structural defects of ZnO crystals play the most important role in chlorine detection. We demonstrated that Cl2 adsorption is a concurrent process to oxygen adsorption. Both of them occur on the same active species (oxygen vacancies). Their concentration is higher on the side planes of the zinc oxide crystal than the others. Additionally, ZnO sublimation process plays an important role in the chlorine detection mechanism.

Emerging pnictogen-based 2D semiconductors: sensing and electronic devices

Pnictogens are an intensively studied group of monoelemental two-dimensional materials. This group of elements consists of phosphorus, arsenic, antimony, and bismuth. In this group, the elements adopt two different layered structural allotropes, orthorhombic structure with true van der Waals layered interactions and rhombohedral structure, where covalent interactions between layers are also present. The orthorhombic structure is well known for phosphorus and arsenic, and the rhombohedral structure is the most thermodynamically stable allotropic modification of arsenic, antimony, and bismuth. Due to the electronic structure of pnictogen layers and their semiconducting character, these materials have huge application potential for electronic devices such as transistors and sensors including photosensitive devices as well as gas and electrochemical sensors. While photodetection and gas sensing applications are often related to lithography processed materials, chemical sensing proceeds in a liquid environment (either aqueous or non-aqueous) and can be influenced by surface oxidation of these materials. In this review, we explore the current state of pnictogen applications in sensing and electronic devices including transistors, photodetectors, gas sensors, and chemical/electrochemical sensors.