▎ 摘　　要

In this work, a surfactant-free graphene ink is prepared in a mixture of terpineol (T) and cyclohexanone (C) and optimized to yield rheologies appropriate for ink-jet printing on both rigid SiO2/Si and flexible polyimide substrates. The surfactant-free ink optimized here, clearly demonstrates its enhanced electrical transport characteristics, where resistivity rho values are 7x lower, i.e. 1.1 m Omega m, compared to 7.1 m Omega m for the surfactant-assisted formulations derived from N-methyl-2-pyrrolidone (NMP) and ethyl cellulose (EC) reported earlier. The C:T surfactant-free ink is stable with aging, exhibiting minimal signs of graphene nano-membrane re-agglomeration. The mechanical elasticity and robustness of the printed structures is evaluated through strain-dependent bending tests that reveal minimal variations in resistance (similar to 8%) with bending radii of curvature up to 0.16 cm(-1). Finally, the thermal behavior of the printed features formed using the surfactant-free ink is deciphered from the Resistance-Temperature data obtained from 6K to 350 K, where the temperature coefficient of resistivity (TCR) is calculated to be very low (e.g. 1 Omega/K in the range of 6-80 K, or -2.7 x 10(-4) ppm/K), comparable to other low TCR materials such as polymer/carbon composites. In summary, the resistive structures designed using the surfactant-free, environmentally friendly inks formulated here, exhibit attributes that are extremely desirable for flexible electronics, such as enhanced electronic transport, good mechanical robustness and a TCR that varies minimally with temperature. (C) 2016 Elsevier Ltd. All rights reserved.