The effect of Cl- and N-doped MoS2 and WS2 coated on epitaxial graphene in gas-sensing applications

Abstract

Abstract In this study, epitaxial graphene (EG) was grown on a 6H-SiC (0001) substrate via the thermal decomposition of SiC. Undoped and Cl- or N-doped molybdenum disulfide (MoS2) and tungsten disulfide (WS2) ultrathin films were spin-coated on the graphene surface. The scanning electron microscopy (SEM) images and topological atomic force microscopy (AFM) analysis showed good distribution of thin MoS2 and WS2 flakes on the EG surface. The X-ray photoelectron spectroscopy (XPS) confirmed the presence of Mo-related peaks of 3d5/2 and 3d3/2 at ~ 232.2\xa0eV and 235.1\xa0eV, respectively. It also represented peaks of W 4f7/2 and 5p5/2 at around 36.1\xa0eV and 37.9\xa0eV, respectively. Moreover, XPS results showed peaks at around 167.4\xa0eV and 168.4\xa0eV corresponding to S 2p for MoS2 and WS2, respectively. The XPS results also confirmed the presence of dopant elements in MoS2 and WS2 flakes. We fabricated sensors using undoped and chlorine- or nitrogen-doped MoS2 and WS2 ultrathin films for gas-sensing applications. These sensors were surveyed for ammonia (NH3) and nitrogen dioxide (NO2) gas sensing. As in NO2, both undoped sensors react with a decrease in relative sensor responses to NH3, hence showing n-type behavior. Doping MoS2 and WS2 with chlorine led to a higher response vis-a-vis the nitrogen-doped sensors. The absolute relative response of Cl-doped WS2 and MoS2 was about 3.5 and 1.8 times more than that of their undoped counterparts toward NH3. A change of direction with a slightly smaller response (approximately\xa0×\xa00.8), however, could also be observed in the doping of MoS2 and WS2 with nitrogen. When exposed to NO2, the Cl-doped WS2 sensor response was 1.2 more than the N-doped one, while for MoS2 these values changed in the range of 1.2 – 1.6 for different flows of gas.