Revisiting the inhomogeneity in drop-on-demand printing of graphene: An effective route for overcoming the coffee-ring effect

Graphene patterns by drop-on-demand (DOD) printing are promising for fabricating functional devices in industry. However, uneven solute distribution caused by the coffee-ring effect is an obstacle to the uniform morphology and high performance of graphene patterns. Here, we develop a strategy to suppress and eliminate the coffee ring of graphene droplets based on porous cellulose nanopaper (CNP), without dependence on ink properties and environment. A theoretical droplet model concerning both infiltration and evaporation on porous surfaces with nanoscale pores is proposed to predict deposition patterns and uncover the suppression mechanism of coffee-ring depositions. Coffee-ring patterns on the CNP could be tailored by the time scale ratio of the graphene flake migration (tau f) to the total pinning (tau e+i) of evaporation and infiltration. The efficacy of coffee-ring inhibition using CNP is demonstrated by comparing drying behaviors and deposition patterns of graphene droplets on glass and CNP substrates experimentally. Further, a parameter map relating graphene concentration and droplet size (c-d0) is summarized to identify regions of uniform pattern printing through theoretical and experimental investigations. This work effectively eliminates graphene coffee-ring effect and provides guidance for other functional materials, opening up opportunities for smart devices built through DOD printing.