Experimental study on interfacial mechanical behavior of single-layer monocrystalline graphene on a stretchable substrate

Abstract

Monocrystalline graphene is expected to become a core material for the next-generation flexible electronic device, owing to its superior mechanical and electrical properties. Therefore, it is essential to analyze the interfacial mechanical property of the composite structure composed of large-scale monocrystalline graphene, prepared by chemical vapor deposition (CVD), and flexible substrate in experiment. Recent years, micro-Raman spectroscopy has become a useful method of micro/nano-mechanics for the experimental investigations on the properties of low-dimensional nanomaterials, such as carbon nanotube (CNT), graphene, molybdenum disulfide (MoS2). Especially, Raman spectroscopy is effectively applied to the investigations on the mechanical behaviors of the interfaces between graphene films and flexible substrates. Among these researches, most of the measured samples are small-scale monocrystalline graphene films which are mechanically exfoliated from highly oriented pyrolytic graphite, a few ones are the large-scale single-layer polycrystalline graphene films prepared by CVD. There is still lack of study of the large-scale single-layer monocrystalline graphene. In this work, micro-Raman spectroscopy is used to quantitatively characterize the behavior of interface between single-layer monocrystalline graphene film prepared by CVD and polyethylene terephthalate (PET) substrate under uniaxial tensile loading. At each loading step from 0 to 2.5% tensile strain on the substrate, the in-plane stress distribution of the graphene is measured directly by using Raman spectroscopy. The interfacial shear stress at the graphene/PET interface is then achieved. The experimental result exhibits that during the whole process of uniaxial tensile loading on the PET substrate, the evolution of the graphene/PET interface includes three states (adhesion, sliding and debonding). Based on these results, the classical shear-lag model is introduced to analyze the interfacial stress transfer from the flexible substrate to the single-layer graphene film. By fitting the experimental data, several mechanical parameters are identified, including the interface strength, the interface stiffness and the interface fracture toughness. The Raman measurements and result analyses are carried out on the samples whose single-layer graphene films have different lengths. It is shown that the stress transfer at the graphene/PET interface controlled by the van der Waals force has obvious scale effect compared with the graphene length. The interface strength, viz. the maximum of the interfacial shear stress, decreases with the increase of the graphene length. While the graphene length has no effect on the debonding strain or the strain transfer limit of graphene/PET interface. Combining with other previous studies of the large-scale single-layer graphene shows that the mechanical parameters of the interface between graphene and flexible substrate have no relation no matter whether the graphene is monocrystalline or polycrystalline.