Annealing effect on UV-illuminated recovery in gas response of graphene-based NO₂ sensors

The response and recovery of a graphene-based sensor for nitrogen dioxide (NO₂) sensing is improved by a combination of two treatments including rapid thermal annealing (RTA) of graphene and UV illumination during the pump down period. A two-dimensional monolayer graphene grown by chemical vapor deposition was transferred to an arc-shape electrode and subsequently heated at temperatures from 200 to 400 °C for 1 min in N₂ atmosphere by RTA to eliminate the chemical residues on the graphene generated in the transfer process. The effect of RTA and poly(methyl methacrylate) (PMMA) residues was investigated using Raman spectroscopy. The shift of the G and 2D bands could be due to graphene suffering from compressive strain and hole doping from the substrate enhanced by the RTA treatment. The hole doping effect was also observed from Hall measurements. Atomic force microscopy images confirm the PMMA residues and surface roughness reduction by the RTA treatment. Annealing at 300 °C enhances the NO₂ sensing response at 1 ppm by 4 times compared to the pristine graphene without RTA. Full recovery of the sensor to the initial baseline could be achieved by the adjustment of UV illumination time.