Semiconducting single-walled carbon nanotube/graphene van der Waals junctions for highly sensitive all-carbon hybrid humidity sensors

van der Waals (vdW) heterojunctions composed of diverse atomic-thin nanomaterials have great potential as building blocks for innovative nanodevices due to the unique electron coupling effect. However, previous research studies have mainly focused on the light-matter interactions and charge transfer processes, and the interactions between gas/vapor and vdW heterojunctions still remain to be verified. Herein, we first demonstrate the charge transfer based on the interaction between water (H2O) molecules and the all-carbon vdW junctions, which are formed by stacking non-carboxylated (6, 5) single-walled carbon nanotubes (SWCNTs) on the surface of chemical vapor deposition synthesized graphene. Based on the interaction mechanism and charge transfer process, a proof-of-concept all-carbon vdW junction humidity sensor has been fabricated. The device exhibits good reproducibility and high sensitivity of 24% per 1% RH change in a wide relative humidity range of 5-80% with a short response/recovery time of 198/110 ms. Furthermore, the effect of H2O molecule absorption on the charge transfer dynamics at the SWCNT/graphene vdW junctions has been demonstrated via Raman spectra, giving a novel humidity sensing mechanism. Our finding provides a new strategy to fabricate high performance humidity sensors using vdW heterojunctions as sensing layers.