Microstructural, electrical, and mechanical properties of graphene films on flexible substrate determined by cyclic bending test.

Three kinds of graphene/polyimide specimen were prepared via transfer with 3, 6, and 9 graphene layers, respectively. A self-designed bending tester was applied to carry out cyclic bending tests with various bending cycles and bending frequencies. The variations of electrical resistance of the specimens during the bending process and the rate of increase of electrical resistance with the number of bending cycles and bending frequency for various total graphene thicknesses were determined. The voids that form at the interfaces between any two adjacent layers increase in size, leading to a disconnection between graphene layers after a number of bending cycles. A reduction in the graphene thickness and increases in the number of bending cycles and bending frequency increase the rate of increase of electrical resistance. For specimens with a given graphene thickness, the ID/IG value of the Raman shift increases exponentially with increasing number of bending cycles and bending frequency. An increase in ID/IG is accompanied by increases in both the rate of increase of electrical resistance and the aspect ratio L1/L2 (where L1 and L2 are the half lengths of the long and short axes, respectively, of the selected-area electron diffraction pattern of graphene). The tilt angle formed in the top graphene layer of the specimen after bending tests increases with increasing graphene thickness for a given bending frequency. The rate of increase of the tilt angle is affected by the bending frequency.