Enhanced dimethyl methylphosphonate detection based on two-dimensional WSe2 nanosheets at room temperature

Dopant Enhanced Conjugated Polymer Thin Film for Low-Power, Flexible and Wearable DMMP Sensor

Conjugated polymer has the potential to be applied on flexible devices as an active layer, but further investigation is still hindered by poor conductivity and mechanical stability. Here, this work demonstrates a dopant-enhanced conductive polymer thin film and its application in dimethyl methylphosphonate (DMMP) sensor. Among five comparable polymers this work employs, poly(bisdodecylthioquaterthiophene) (PQTS12) achieves the highest doping efficiency after doped by FeCl3, with the conductivity increasing by about five orders of magnitude. The changes in Young's modulus are also considered to optimize the conductivity and flexibility of this thin film, and finally the decay of conductivity is only 9.2% after 3000 times of mechanical bending. This work applies this thin film as the active layer of the DMMP gas sensor, which could be operated under 1 mV driving voltage and 28 nW power consumption, with a sustainable durability against bending and compression. In addition, this sensor is provided with alarm capability while exposed to the DMMP atmospheres at different hazard levels. This work expects that this general approach could offer solutions for the fabrication of low-power and flexible gas sensors, and provide guidance for next-generation wearable devices with broader applications.

Low-Temperature Synthesis of WSe2 by the Selenization Process under Ultrahigh Vacuum for BEOL Compatible Reconfigurable Neurons

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 °C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

Two-Dimensional Bimetallic Phthalocyanine Covalent-Organic-Framework-Based Chemiresistive Gas Sensor for ppb-Level NO2 Detection

Two-dimensional (2D) phthalocyanine-based covalent organic frameworks (COFs) provide an ideal platform for efficient and rapid gas sensing-this can be attributed to their regular structure, moderate conductivity, and a large number of scalable metal active centers. However, there remains a need to explore structural modification strategies for optimizing the sluggish desorption process caused by the extensive porosity and strong adsorption effect of metal sites. Herein, we reported a 2D bimetallic phthalocyanine-based COF (COF-CuNiPc) as chemiresistive gas sensors that exhibited a high gas-sensing performance to nitrogen dioxide (NO₂). Bimetallic COF-CuNiPc with an asymmetric synergistic effect achieves a fast adsorption/desorption process to NO₂. It is demonstrated that the COF-CuNiPc can detect 50 ppb NO₂ with a recovery time of 7 s assisted by ultraviolet illumination. Compared with single-metal phthalocyanine-based COFs (COF-CuPc and COF-NiPc), the bimetallic structure of COF-CuNiPc can provide a proper band gap to interact with NO₂ gas molecules. The CuNiPc heterometallic active site expands the overlap of d-orbitals, and the optimized electronic arrangement accelerates the adsorption/desorption processes. The concept of a synergistic effect enabled by bimetallic phthalocyanines in this work can provide an innovative direction to design high-performance chemiresistive gas sensors.

Simultaneous measurement of trace dimethyl methyl phosphate and temperature using all fiber Michaelson interferometer cascaded FBG

All fiber Michaelson interferometer cascaded fiber Bragg grating (FBG) sensor for simultaneous measurement of trace dimethyl methyl phosphate and temperature is proposed. One end of the four-core fiber (FCF) is spliced with a multimode fiber (MMF), the other end is flattened and evaporated with silver film to enhance reflection, and the Michelson interference structure is formed. The grating is engraved in the single-mode fiber (SMF) core and spliced with MMF, then the Michelson interference cascaded FBG, FBG-MMF-FCF sensor is obtained. The sensing film, MnCo₂O₄ is coated on the surface of FCF, and the structure, elemental composition and morphology of MnCo₂O₄ were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The sensitivity and the detection limit of DMMP are 86.44 dB/ppm and 0.1767 ppb, respectively. The response/recovery time is about 14/10 s. the temperature sensitivity can be compensated and calculated as 0.069 nm/°C. The sensor has good selectivity and stability, and has a good application prospect in high sensitivity detection of trace DMMP vapor.

Three-Dimensional Assemblies of Edge-Enriched WSe2 Nanoflowers for Selectively Detecting Ammonia or Nitrogen Dioxide

Herein, we present, for the first time, a chemoresistive-type gas sensor composed of two-dimensional WSe₂, fabricated by a simple selenization of tungsten trioxide (WO₃) nanowires at atmospheric pressure. The morphological, structural, and chemical composition investigation shows the growth of vertically oriented three-dimensional (3D) assemblies of edge-enriched WSe₂ nanoplatelets arrayed in a nanoflower shape. The gas sensing properties of flowered nanoplatelets (2H-WSe₂) are investigated thoroughly toward specific gases (NH₃ and NO₂) at different operating temperatures. The integration of 3D WSe₂ with unique structural arrangements resulted in exceptional gas sensing characteristics with dual selectivity toward NH₃ and NO₂ gases. Selectivity can be tuned by selecting its operating temperature (150 °C for NH₃ and 100 °C for NO₂). For instance, the sensor has shown stable and reproducible responses (24.5%) toward 40 ppm NH₃ vapor detection with an experimental LoD < 2 ppm at moderate temperatures. The gas detecting capabilities for CO, H₂, C₆H₆, and NO₂ were also investigated to better comprehend the selectivity of the nanoflower sensor. Sensors showed repeatable responses with high sensitivity to NO₂ molecules at a substantially lower operating temperature (100 °C) (even at room temperature) and LoD < 0.1 ppm. However, the gas sensing properties reveal high selectivity toward NH₃ gas at moderate operating temperatures. Moreover, the sensor demonstrated high resilience against ambient humidity (Rh = 50%), demonstrating its remarkable stability toward NH₃ gas detection. Considering the detection of NO₂ in a humid ambient atmosphere, there was a modest increase in the sensor response (5.5%). Furthermore, four-month long-term stability assessments were also taken toward NH₃ gas detection, and sensors showed excellent response stability. Therefore, this study highlights the practical application of the 2H variant of WSe₂ nanoflower gas sensors for NH₃ vapor detection.

Homogeneously niobium-doped MoS2 for rapid and high-sensitive detection of typical chemical warfare agents

Rapid detection of Chemical Warfare Agents (CWAs) is of great significance in protecting civilians in public places and military personnel on the battlefield. Two-dimensional (2D) molybdenum disulfide (MoS2) nanosheets (NSs) can be integrated as a gas sensor at room temperature (25°C) due to their large specific surface area and excellent semiconductor properties. However, low sensitivity and long response-recovery time hinder the pure MoS2 application in CWAs gas sensors. In this work, we developed a CWAs sensor based on in-situ niobium-doped MoS2 NSs (Nb-MoS2 NSs) via direct chemical-vapor-deposition (CVD) growth. Characterization results show that the high content of Nb elements (7.8 at%) are homogeneously dispersed on the large-area 2D structure of MoS2. The Nb-MoS2 NSs-based CWAs sensor exhibits higher sensitivity (−2.09% and −3.95% to 0.05 mg/m3 sarin and sulfur mustard, respectively) and faster response speed (78 s and 30 s to 0.05 mg/m3 sarin and sulfur mustard, respectively) than MoS2 and other 2D materials at room temperature. And the sensor has certain specificity for sarin and sulfur mustard and is especially sensitive to sulfur mustard. This can be attributed to the improvement of adsorption properties via electronic regulation of Nb doping. This is the first report about CWAs detection based on two-dimensional (2D) transition metal dichalcogenides (TMDs) sensing materials, which demonstrates that the high sensitivity, rapid response, and low limit of detection of 2D TMDs-based CWAs sensor can meet the monitoring needs of many scenarios, thus showing a strong application potential.

Gas sensing devices based on two-dimensional materials: a review

Gas sensors have been widely utilized penetrating every aspect of our daily lives, such as medical industry, environmental safety testing, and the food industry. In recent years, two-dimensional (2D) materials have shown promising potential and prominent advantages in gas sensing technology, due to their unique physical and chemical properties. In addition, the ultra-high surface-to-volume ratio and surface activity of the 2D materials with atomic-level thickness enables enhanced absorption and sensitivity. Till now, different gas sensing techniques have been developed to further boost the performance of 2D materials-based gas sensors, such as various surface functionalization and Van der Waals heterojunction formation. In this article, a comprehensive review of advanced gas sensing devices is provided based on 2D materials, focusing on two sensing principles of charge-exchange and surface oxygen ion adsorption. Six types of typical gas sensor devices based on 2D materials are introduced with discussion of latest research progress and future perspectives.

A prototype portable instrument employing micro-preconcentrator and FBAR sensor for the detection of chemical warfare agents

The presence of chemical warfare agents (CWAs) in the environment is a serious threat to human safety, but there are many problems with the currently available detection methods for CWAs. For example, gas chromatography–mass spectrometry cannot be used for in-field detection owing to the rather large size of the equipment required, while commercial sensors have the disadvantages of low sensitivity and poor selectivity. Here, we develop a portable gas sensing instrument for CWA detection that consists of a MEMS-fabricated micro-preconcentrator (μPC) and a film bulk acoustic resonator (FBAR) gas sensor. The μPC is coated with a nanoporous metal–organic framework material to enrich the target, while the FBAR provides rapid detection without the need for extra carrier gas. Dimethyl methylphosphonate (DMMP), a simulant of the chemical warfare agent sarin, is used to test the performance of the instrument. Experimental results show that the μPC provides effective sample pretreatment, while the FBAR gas sensor has good sensitivity to DMMP vapor. The combination of μPC and FBAR in one instrument gives full play to their respective advantages, reducing the limit of detection of the analyte. Moreover, both the μPC and the FBAR are fabricated using a CMOS-compatible approach, and the prototype instrument is compact in size with high portability and thus has potential for application to in-field detection of CWAs.

The synergistic effects of oxygen vacancy engineering and surface gold decoration on commercial SnO2 for ppb-level DMMP sensing

The metal oxides-based chemiresitive gas sensors have attracted enormous interest because of their exellent sensing perforamcnes, which have emerged as very promising candidates for gas monitoring. However, from the view of organophosphorus compounds detection, a unique combination of low detection limit and fast respons/recovery rate remainschallenging. Herein, the synersgitic effects of oxygen vacancy engineering and surface gold decoration enabling excellent sensing performances for detection of dimethyl methyl phosphonate (DMMP, a typical organophosphorus) is reported. To demonstrate the proof of concept, Au nanoparticles (NPs) decorated oxygen vacancy-enriched SnO₂ hybrids (designated as Au-O-SnO₂) were designedas sensing materials, where the O-SnO₂ samples were fabricated by introduction of oxygen vacancies onto commercial SnO₂ through organometallic chemistry-assisted approach using (CH₃)₂SnCl₂ as precursor, followed by deposition of Au NPs by an in-situ reduction routine. After optimizing Au NPs content in hybrids (1 wt%, 3 wt%, 5 wt% and 7 wt%), O-SnO₂ decorated with 5 wt% Au NPs (designated as Au-O-SnO₂-5) exhibits excellent DMMP sensing performances, such as, an enhanced recoverable response of 1.67 to 680 ppb DMMP, low detection limit of 4.8 ppb, shortresponse time of 26 s and recoverytime of 32 s, as well as good selectivity, which are much better than that of commercial SnO₂ (C-SnO₂) and O-SnO₂, and Au NPs decorated C-SnO₂. Based on the detailed investigation, the enhanced DMMP sensing performances of Au-O-SnO₂ hybrids can be mainly ascribed to the synergistic effect of increasing surface active sites induced by oxygen vacancies, the chemical and electronic sensitization of Au NPs. As a result, Au-O-SnO₂-5 hybrids display relatively low activation energy of 24.11 kJ/mol for DMMP oxidization, which is lower than that of O-SnO₂ (35.54 kJ/mol). Our results provide a feasible method for boosting sensing performances for DMMP detection, paving new way for fabrication of metal oxides-based gas sensors for rapid detection of trace organic compounds with complexed structures.

Noble metal (Ag, Au, Pd and Pt) doped TaS2 monolayer for gas sensing: a first-principles investigation

Two-dimensional (2D) layered nanomaterials have attracted increasing attention in gas sensing due to their graphene-like properties. Although the gas sensing performances of 2D layered semiconductor transition metal dichalcogenides (TMDs), including MoS₂, WS₂, MoSe₂ and WSe₂, have been extensively studied, it has remained a grand challenge to develop a high-performance gas sensing material that can meet practical applications. Tantalum disulfide (TaS₂), as a metallic TMD with low resistance and high current signal, has great promise in high-performance gas sensing. In stark contrast with Mo and W, Ta has a stronger positive charge, which contributes to a higher surface energy to capture gas molecules. Herein, through calculating the adsorption energy, charge transfer, electronic structure, and work function of the adsorption system with first-principles calculations, we first systematically studied the performance of noble metal atom substitution doping on a TaS₂ monolayer for toxic nitrogen-containing gas (NH₃, NO and NO₂) sensing. We found that the TaS₂ monolayer exhibits excellent NO sensing performance with an adsorption energy of 0.49 eV and a charge transfer of 0.17 e. However, it has a considerable adsorption energy (-0.22 and -0.39 eV) to NH₃ and NO₂ molecules, but a low charge transfer (-0.03 and 0.04 e) between the gas molecules and the TaS₂ monolayer. To further enhance the gas-sensing performance of the TaS₂ monolayer, noble metal atoms (Ag, Au, Pd and Pt) were substitutionally doped into the lattice of the TaS₂ monolayer. The results showed that the values of adsorption energy and charge transfer can be significantly improved, and the electronic structure and work function of the doping system has also greatly changed, which makes it much easier to detect the changes in electrical signal due to gas adsorption. Our work indicates that the intrinsic as well as the noble metal doped TaS₂ monolayers are promising candidates for high-performance gas sensors.

Room temperature DMMP gas sensing based on cobalt phthalocyanine derivative/graphene quantum dot hybrid materials

In this study, two kinds of cobalt phthalocyanine (CoPc) derivatives containing hexafluoroisopropanol (HFIP) and hexafluorbisphenol A (6FBPA) substituents have been obtained. Graphene quantum dots (GQDs) were anchored to CoPc derivatives by π-π bonding, forming hybrid materials. They were employed to detect dimethyl methylphosphonate (DMMP) gas, an ideal simulant gas for sarin nerve gas, and achieved good gas response performance at room temperature. There are strong hydrogen bonds between the two functional group molecules (HFIP and 6FBPA) and the DMMP molecule, leading to their excellent response performance to DMMP molecules. GQDs can effectively increase the electrical conductivity of hybrid materials by π-π bonding with CoPc derivatives. Therefore, the response speed of the hybrid materials to DMMP gas has been significantly improved, and the minimum detection limit is 500 ppb, while maintaining excellent repeatability, stability and selectivity. Laser-assisted irradiation was used to solve the problem of the slow recovery of CoPc derivatives. This result demonstrates that these CoPc derivative/GQD hybrid materials are expected to be the raw materials of the sarin gas sensor.