Electrical and humidity-sensing characterization of inkjet-printed multi-walled carbon nanotubes for smart packaging

Printing is considered a cost-effective way to fabricate electronics on unconventional substrates enabling, for example, smart packaging. Functionalized multi-walled carbon nanotubes (f-MWCNTs) having carboxylic groups on their surfaces possess great potential as flexible resistive humidity sensor. In this paper, we report on the inkjet printing and characterization of f-MWCNTs in terms of sheet resistance and humidity-sensitivity. Stable f-MWCNTs ink is formulated using aqueous ethylene glycol as solvent. Sheet resistance of printed f-MWCNTs films on polyimide foil reduces by increasing the number of printed layers as well as post-printing annealing temperature. Meanwhile, the raised annealing temperature degrades the films' humidity-sensitivity, which could be explained by the loss of the carboxylic groups. The electrical and sensing properties of f-MWCNTs also have a negative temperature coefficient regarding ambient temperature, which should be considered in practical application.