Harnessing Environmental Sensitivity in SnSe-Based Metal-Semiconductor-Metal Devices: Unveiling Negative Photoconductivity for Enhanced Photodetector Performance and Humidity Sensing.

The extreme sensitivity of 2D-layered materials to environmental adsorbates, which is typically seen as a challenge, is harnessed in this study to fine-tune the material properties. This work investigates the impact of environmental adsorbates on electrical properties by studying metal-semiconductor-metal (MSM) devices fabricated on CVD-synthesized SnSe flakes. The freshly prepared devices exhibit positive photoconductivity (PPC), whereas they gradually develop negative photoconductivity (NPC) after being exposed to an ambient environment for ∼1 day. While the photodetectors based on positive photoconductivity exhibit a responsivity and detectivity of 6.1 A/W and 5.06 × 10 ⁸ Jones, the same for the negative photoconductivity-based photodetector reaches up to 36.3 A/W and 1.49 × 10 ⁹ Jones, respectively. In addition, the noise-equivalent power of the NPC photodetector decreases by 300 times as compared to the PPC device, which implies a prominent detection capability of the NPC device against weak photo signals. To substantiate the hypothesis that negative photoconductivity stems from the photodesorption of water and oxygen molecules on the dangling bonds of SnSe flakes, the flakes are etched along the most active planes (010) with a focused laser beam in an inert environment, which enhances responsivity by 43%, supporting negative photoconductivity linked to photodesorption. Furthermore, the humidity-dependent dark current variation of the NPC photodetectors is used to design a humidity sensor for human respiration monitoring with faster response and recovery times of 0.72 and 0.68 s, respectively. These findings open up the possibility of tuning the photoelectrical response of layered materials in a facile manner to develop future sensors and optoelectronic multifunctional devices.