Recent progress of flexible NO2 and NH3 gas sensors based on transition metal dichalcogenides for room temperature sensing

Wearable sensors have gained enormous attention because of their capability to do real-time monitoring of health as well as the environment. A wearable sensing device mainly relies on flexible sensors incorporated within it that undergo mechanical deformations during utilization. The applications of wearable sensors in various sectors like homeland security, defense, regular monitoring of environment and human health, etc. are motivating researchers to develop the most reliable and highly efficient flexible sensors. In recent years, two-dimensional (2D) transition metal dichalcogenide (TMDC)-based flexible sensors have been rigorously developed for gas detection due to their special attributes like high surface-to-volume ratio, tunable bandgap, robust mechanical strength, high conformability on different substrates, and low power consumption. The most important is the ability to operate at room temperature, which is not easily attainable in metal oxides and other gas-sensing materials. The room temperature detection of gases is essential in some vital applications, like continuous health monitoring and medical diagnosis. Therefore, the key focus of this review is on the recent progress and future aspects of room temperature operable energy-efficient flexible gas sensors fabricated by utilizing 2D-TMDCs. The properties of 2D-TMDCs favorable for a flexible sensor, sensing mechanism, and different approaches to optimize the performance parameters of flexible gas sensors are reviewed. In addition, various strategies and underlying challenges encountered through the integration of the sensing material into e-skin/textile-oriented gas sensors along with promising solutions for these challenges are discussed. Future prospects of flexible gas sensors with respect to their performance, biocompatibility, and durability for realization at the commercial level have also been deliberated.