A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges.

Ternary systems are less studied for sensing applications due to complex synthesis procedures. However, they have more sources of resistance modulation, leading to an enhanced gas response. In this study, a ternary system, namely Au-decorated ZnO-polyaniline (PANI) composite nanofibers with different amounts of PANI (10, 25, and 50 wt.%) were synthesized for NO2 gas sensing studies. First, ZnO nanofibers were synthesized by electrospinning, and then an Au layer (9 nm) was coated on the ZnO nanofibers. Finally, PANI was coated onto the prepared Au-decorated ZnO nanofibers. NO2 gas sensing investigations indicated that the sensor with 25 wt.% PANI had the best response to NO2 gas at 300 °C. In addition, the optimized sensor exhibited high selectivity to NO2 gas. The improved performance of the optimal gas sensor was attributed to the role of Au, the formation of ZnO-PANI heterojunctions, and the optimal amount of PANI. The promising effect of this ternary system for NO2 sensing was demonstrated, and it can be extended to other similar systems.

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Au-Decorated Polyaniline-ZnO Electrospun Composite Nanofiber Gas Sensors with Enhanced Response to NO2 Gas

The problem of air pollution and an increasing number of hazardous gases leaking into the atmosphere is of growing concern. To protect human and animal life it is necessary to monitor these toxic gases. Gases such as NH3, CO2, CH4, CO, and SO2 can lead to fatal health risks. Gas sensors have attracted extensive attention from academic and commercial fields to monitor such pollutants. The sensing properties, such as measurement sensitivity, response and recovery time, and selectivity, heavily rely on sensing. In this review, the different groups of the sensing materials are described in detail, including metal oxides, metal sulfides, metal ferrites, perovskites, carbon materials, organic polymers, transition metal dichalcogenides, and chalcogenide nanomaterials. The synthesis methods of these compounds and their basic properties are elaborated. Also, morphology has a very important role to tailor the performance of gas sensors. In addition, this review discusses the gas sensing properties of the aforementioned materials along with the explanation of their sensing mechanisms. Special attention is paid to the detection of hazardous organic vapors and toxic gases. The wide discussion of various materials-based gas sensors in near future can be attached to the Internet of Things to develop more rigid and highly sensitive gas leakage detectors to avoid accident risks as well as health threats. This review provides a comprehensive overview of the recent achievements in the application of sensors for different gas detection and indicates the current challenges and future outlooks in this field. 

Recent advances in multifunctional materials for gas sensing applications

III-nitrides are attracting considerable attention as promising materials for a variety of applications due to their wide bandgap, high electron mobility, high thermal stability and many other exceptional properties. This...

Recent progress on Group III-nitride nanostructures-based Gas Sensors

The nanoflower-like Au/SnS2/SnO2 heterojunctions were synthesized following a combination process. In sequence, the SnS2/SnO2 nanoflowers were obtained by the thermal oxidation of the flower-like SnS2 that was firstly synthesized by a solvothermal method. Then, the nanoflower-like Au/SnS2/SnO2 heterojunctions were achieved by in-situ decoration of SnS2/SnO2 nanoflowers with gold nanoparticles. The as-synthesized Au/SnS2/SnO2 heterojunctions based chemiresistive-type sensor has exhibited a response of 22.3 to 8 ppm NO2 at 80 °C, which is much higher than that of the pristine SnS2 sensor (8.1). The response and recovery rate of the sensor is also increased by 1 ~ 2 times. The theoretical calculation using the density function theory (DFT) based on the first principle was implemented to study the sensing mechanisms. The enhanced sensing performance could be mainly attributed to the formation of Au/SnS2/SnO2 heterojunctions. 

Ultrasensitive NO2 sensors based on Au/SnS2/SnO2 nanoflower-like heterojunctions at low temperature

Two dimensional chalcogenides and metal sulfide materials with their sensitive surfaces are promising and potential postulant for the purpose of chemical gas sensing as a consequence of extraordinary vast surface area to volume ratio and significant band gap. We demonstrate a highly selective and sensitive NO2 gas sensor using vertically aligned 2D SnS2 flakes, grown by a chemical vapor deposition method in controlled gas flow. The vertically aligned SnS2 flakes were confirmed by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. The sensing behaviour of the vertical aligned structures was analyzed for various concentrations of NO2 at different temperatures. The sensor exhibits high gas sensing response (ΔR/R%) of ~164 at 120 °C with 50 ppm of NO2 gas, which is the highest response among the SnS2 based reports. The device shows excellent sensing response at room temperature but poor recovery. The selectivity of the sensor was performed under different gas environments and found highly selective towards NO2 gas, and the results are supported by electronic structure calculations. Based on experimental results and electronic structure calculations, a gas sensing mechanism is proposed. The result indicates that fabricated sensor can be used in air purification industries and in air monitoring.

Growth and NO2 gas sensing mechanisms of vertically aligned 2D SnS2 flakes by CVD: Experimental and DFT studies

Two dimensional chalcogenides and metal sulfide materials with their sensitive surfaces are promising and potential postulant for the purpose of chemical gas sensing as a consequence of extraordinary vast surface area to volume ratio and significant band gap. We demonstrate a highly selective and sensitive NO2 gas sensor using vertically aligned 2D SnS2 flakes, grown by a chemical vapor deposition method in controlled gas flow. The vertically aligned SnS2 flakes were confirmed by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. The sensing behaviour of the vertical aligned structures was analyzed for various concentrations of NO2 at different temperatures. The sensor exhibits high gas sensing response (ΔR/R%) of ~164 at 120 °C with 50 ppm of NO2 gas, which is the highest response among the SnS2 based reports. The device shows excellent sensing response at room temperature but poor recovery. The selectivity of the sensor was performed under different gas environments and found highly selective towards NO2 gas, and the results are supported by electronic structure calculations. Based on experimental results and electronic structure calculations, a gas sensing mechanism is proposed. The result indicates that fabricated sensor can be used in air purification industries and in air monitoring. 

The ability to monitor toxic gases under room-temperature conditions, with enhanced response and selectivity present in the atmosphere, is still considered as a technical challenge. In this context, we have fabricated AlGaN/GaN high electron mobility transistors (HEMTs) based sensors incorporating molybdenum disulphide (MoS2) functionalization for very sensitive, selective, and quick measurement of even trace amounts of hazardous NO2 gas in the ambient under room-temperature conditions. MoS2 structures with vertically aligned flower-like structure were synthesised using a simple hydrothermal technique and applied to the gate region of AlGaN/GaN HEMTs. The electrical characterisations of MoS2 functionalized AlGaN/GaN HEMTs are then used to detect the presence of NO2 gas. The fabricated sensor showed an enhanced relative sensing response in the range of 40.5–56.7% for 1–100 ppm NO2 gas with complete recovery to 1 ppm NO2 concentration under room-temperature conditions (25ᵒC) without applying any external thermal or optical stimuli. Furthermore, the cyclic and selectivity tests were performed and we found our sensor to be highly selective towards NO2 gas among various other gases. The experimental results showed that MoS2 had excellent properties for NO2 gas detection when used on such GaN-based sensing platform. These findings may be attributed to the exposed edge sites of MoS2 which compliment with configurations with sulphur and the chemisorption phenomenon on its surface that results in altering the drain to source current (IDS) of the HEMT at a constant drain to source voltage (VDS) of 0.5 V. These findings suggest that gas sensors based on the AlGaN/GaN HEMT structure appear to be a promising candidate for the advancement of application potentials of nitride-based integrated electronics. 

MoS2 functionalized AlGaN/GaN transistor based room temperature NO2 gas sensor

In this chapter, the author presents the control techniques for operation and power management of the DC microgrid (DCMG). The proposed smart DCMG consists of various distributed generations (DGs), various DC and AC loads, and energy storage systems. In this chapter, a control technique for coordinated control of DGs for energy management in the DCMG has been proposed, under various operating modes. Another control technique has also been developed for connecting the one-phase AC loads to the DCMG through the one-phase voltage source inverter. The performance analyses of the proposed controllers along with the DCMG have been carried out in this chapter. 

Ilya Lensky

Papers

Growth and NO2 gas sensing mechanisms of vertically aligned 2D SnS2 flakes by CVD: Experimental and DFT studies

Growth and NO2 gas sensing mechanisms of vertically aligned 2D SnS2 flakes by CVD: Experimental and DFT studies

MoS2 functionalized AlGaN/GaN transistor based room temperature NO2 gas sensor

Control Techniques for the Operation and Power Management of Smart DC Microgrids