Humidity Sensing\nProperties of Different Atomic Layers\nof Graphene on the SiO<sub>2</sub>/Si Substrate

Graphene has great potential to be used for humidity\nsensing due\nto its ultrahigh surface area and conductivity. However, the impact\nof different atomic layers of graphene on the SiO<sub>2</sub>/Si substrate\non humidity sensing has not been studied yet. In this paper, we fabricated\nthree types of humidity sensors on the SiO<sub>2</sub>/Si substrate\nbased on one to three atomic layers of graphene, in which the sensing\nareas of graphene are 75 μm × 72 μm and 45 μm\n× 72 μm, respectively. We studied the impact of both the\nnumber of atomic layers of graphene and the sensing areas of graphene\non the responsivity and response/recovery time of the prepared graphene-based\nhumidity sensors. We found that the relative resistance change of\nthe prepared devices decreased with the increase of number of atomic\nlayers of graphene under the same change of relative humidity. Further,\ndevices based on tri-layer graphene showed the fastest response/recovery\ntime, while devices based on double-layer graphene showed the slowest\nresponse/recovery time. Finally, we chose devices based on double-layer\ngraphene that have relatively good responsivity and stability for\napplication in respiration monitoring and contact-free finger monitoring.