Temperature and thickness dependence of the sensitivity of nitrogen dioxide graphene gas sensors modified by atomic layer deposited zinc oxide films

The Chemical Vapor Deposition (CVD) grown graphene nitrogen dioxide (NO2) gas sensors modified by zinc oxide (ZnO) thin films via atomic layer deposition (ALD) were fabricated and their sensitivity dependence on the temperature and ZnO film thickness was investigated. The anomalous p-type response of the ALD ZnO modified graphene sensors (ZnO/graphene) to NO2 at room temperature was found which might be attributed to the n to p conductance transition due to the reaction between oxygen vacancies and oxygen in the ZnO films. At elevated temperature, ZnO/graphene sensors exhibited p to n conductance transition and the transition temperature increased from 200 degrees C for pristine graphene, 300 degrees C for 5 ALD cycles and 1 nm ZnO/graphene, to 350 degrees C for 3, 5 and 10 nm ZnO/graphene. Meanwhile, the sensors' sensitivity revealed strong temperature dependence with the optimal temperature of 100 degrees C for ultra thin 5 ALD cycles ZnO/graphene (including pristine graphene) and 200 degrees C for other ZnO/graphene sensors (ZnO films thickness >= 1 nm). Such transition and sensitivity dependence on temperature could be ascribed to the change of carrier type and concentration in the ZnO/graphene with the variation of the temperature. Besides, strong sensitivity dependence on the ZnO film thickness at various temperatures was also demonstrated with the optimal ZnO film thicknesses of 5 ALD cycles for relatively low temperature of 100 degrees C and 3 nm for higher temperatures of 200 degrees C and 300 degrees C. The mechanism responsible for the sensors' sensitivity dependence on the temperature and ZnO film thickness was discussed.