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.

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

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

High performance with lower power consumption is one among the essential features of a sensing device. Minute traces of hazardous gases such as NO2 are difficult to detect. Tin disulfide (SnS2) nanosheets have emerged as a promising NO2 sensor. However, their poor room-temperature conductivity gives rise to inferior sensitivity and sluggish recovery rates, thereby hindering their applications. To mitigate this problem, we present a low-cost ultrasensitive NO2 gas sensor with tin disulfide/multiwalled carbon nanotube (SnS2/MWCNT) nanocomposites, prepared using a single-step hydrothermal method, as sensing elements. Relative to pure SnS2, the conductivity of nanocomposites improved significantly. The sensor displayed a decrease in resistance when exposed to NO2, an oxidizing gas, and exhibited p-type conduction, also confirmed in separate Mott-Schottky measurements. At a temperature of 20 °C, the sensor device has a relative response of about ≈5% (3%) for 25 ppb (1 ppb) of NO2 with complete recovery in air (10 min) and excellent recovery rates with UV activation (0.3 min). A theoretical lower limit of detection (LOD) of 7 ppt implies greater sensitivity than all previously reported SnS2-based gas sensors, to the best of our knowledge. The improved sensing characteristics were attributed to the formation of nano p-n heterojunctions, which enhances the charge transport and gives rise to faster response. The composite sensor also demonstrated good NO2 selectivity against a variety of oxidizing and reducing gases, as well as excellent stability and long-term durability. This work will provide a fresh perspective on SnS2-based composite materials for practical gas sensors.

Ultrasensitive Room-Temperature NO2 Detection Using SnS2/MWCNT Composites and Accelerated Recovery Kinetics by UV Activation.

Two important methods for enhancing gas sensing performance are vacancy/defect and interlayer engineering. Tin sulfide (SnS2) has recently attracted much attention for sensing of the NO2 gas due to its active surface sites and tunable electronic structure. Herein, SnS2 has been synthesized by the chemical vapor deposition (CVD) method followed by nitrogen plasma treatment with different exposure times for fast detection of NO2 molecules. Plasma treatment created a substantial number of surface vacancies on SnS2 flakes, which were controlled by the exposure period to modify the surface of flakes. After 12 min of nitrogen plasma treatment, SnS2 nanoflakes show considerable improvement in NO2 sensing characteristics, including a high sensing response of ∼264% toward 100 ppm NO2 at 120°C. The enhancement in the relative response of the sensor is due to the electronic interaction between NO2 molecules and the S vacancies on the surface of SnS2. Density functional theory (DFT) computations indicate that the S-vacancy defects on the surface dominate the effective NO2 detection and the NO2 adsorption mechanism transition from physisorption to chemisorption. Adsorption kinetics of the NO2 gas over SnS2 nanoflake-based chemiresistor sensors were studied using the Lee and Strano model [ Langmuir 2005, 21(11), 5192-5196]. The irreversible rate of the reaction for various NO2 concentrations exposed to the gas sensor is extracted using this model, which also appropriately describes the response curves. The forward rate constant of the irreversible gas sensor increased with the increase of the N2 plasma treatment time and reached the maximum in the 12 min plasma-treated sample. Through defect engineering, this research may open up new vistas for the design and synthesis of 2D materials with enhanced sensing properties.

Investigations of Vacancy-Assisted Selective Detection of NO2 Molecules in Vertically Aligned SnS2.

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.

Akash Popat Gutal

Papers

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