Improved Bending Fracture Behavior of Large-Scale Graphene Monolayer-Intervened Flexible Oxide Thin Films

Abstract

This work describes for the first time the quantitative analyses of bending fracture resistance of a flexible transparent conducting oxide (TCO) thin film, here using Al-doped ZnO thin films, which are substantially improved by the intervention of a monolayer graphene layer. Most of the mechanical effects by graphene have focused on the graphene itself and particularly when only the stretched strains are applied. Excessive stretching stress induces buckling and/or flakes with the intervened graphene, which has been the issue for the actual evaluation of flexible cracking performance. This work focuses on the external effect of graphene, namely how the intervention of graphene can influence positively the fracture behaviour of inorganic thin films under bending operation on the basis of quantitative analyses of flexible fracture parameters including fracture energy, film strength and fracture toughness. A graphene monolayer of ~1 cm x 1 cm scale was transferred successfully to a polymer substrate prior to the sputter-deposition of Al-doped ZnO thin films with variable thickness of 50 to 200 nm. Cracking evolution of the ZnO:Al thin films depended largely on the applied bending strain and film thickness. The crack-initiating strain of ~1.64% with the graphene corresponds to a ~61% improvement compared to the reference sample without the graphene. The resultant mechanical parameters, fracture energy, film strength and fracture toughness, support well the enhanced fracture resistance under the bending operation. For example, the fracture energy was identified as enhanced by ~272% with the intervention of graphene.