Schottky diodes based on 2D materials for environmental gas monitoring: a review on emerging trends, recent developments and future perspectives

Ultrathin layered 2D materials and their heterojunctions are reported to show enhanced gas sensing properties because of their tremendous surface-to-volume ratio, active edges with atomic thickness, and tunable electronic and mechanical properties. However, the traditional chemi-resistive-type gas sensors suffer significantly due to their low selectivity and operation at high working temperatures, leading to poor stability and short-term use. Accordingly, gas sensors based on Schottky junctions have been explored in recent years to achieve improved gas sensing performances with low working temperatures, high sensitivity, selectivity, etc. Schottky-contacted gas sensors based on 2D materials are fabricated by creating a heterojunction in semiconductor materials with metals or metal-like materials, in which either the semiconductor or metal-like material is a 2D material. The Schottky contact between the semiconductor and metal can greatly enhance the selectivity and sensitivity of gas sensors because the Schottky barrier acts as a "gate" controller for the current passing through the barrier. Since the current value of sensors is mainly dependent on the Schottky barrier height (SBH), a minute change in the SBH can lead to a large difference in current, which is the basis for the enhanced sensing performance of the Schottky barrier. Herein, the fundamentals, working principles and recent developments on Schottky-contacted gas sensors based on 2D layered materials are reviewed. Furthermore, the strategies and tunable approaches to achieve improved sensing performances and future directions are discussed.