Bonding Evaluation of Graphene-Oxide Layers on Flexible Substrates

Abstract

Flexible electronics are getting interest in development of field effects transducers such as biomedical health screening tools, structural health monitoring in infrastructures, aerospace, vehicular industries and cell phones. As a promising candidate for flexible electronics, graphene-based devices have been developed through exceptional electrochemical and thermomechanical properties of graphene. Reducing the graphene oxide enables creation of large scale devices through its high manufacturability. Same as other types of electronics, the bonding of sensing units to the substrate is significantly dependent to the deposition method used for the fabrication of device. In this study, the mechanical strength of reduced graphene oxide (rGO) layers on the polymeric substrates is evaluated while the rGO layers are deposited by drop casting on the substrate. The tape test is adopted to measure the failure strength at the interface of rGO layers and substrate. To achieve a consistent and repeatable measurement of peel force, a new design of peel test fixture is suggested to control effective parameters on the peel test and keep constant the peel rate and angle. The new design of peel test has shown low coefficient of variation of about 8% for peel force measurement, which is much lower than 37% reported by ASTM standard for the tape test. Employing an image processing technique, a geometric analysis is conducted to identify the contributions of cohesive and adhesive failures in overall peel force. A mathematical method is developed to connect the geometric analysis result from the image processing to the experimental peel force measure. As a result of mathematical method, the magnitude of cohesive and adhesive energies are identified. Performing analysis of variation (ANOVA) on the bonding energies, the significant parameters of thermal processing on the bonding strength of rGO layers and substrates are determined so that the concentration of GO solution has illustrated as the most significant factor. The surface treatment duration for GO and substrates are the next priorities of significant factors. In this study, the mechanical strength and performance of rGO-based electronics were evaluated based on a new methodology for the peel test. The Kapton has demonstrated the best performance and is served as the best candidate for the fabricating of rGO-based electronics based on thermal processing. The PDMS showed high potential for being a suitable candidate for graphene-based electronics. Considering low surface energy of Teflon (PTFE), it would be viable candidate for transfer printing of graphene-based sensors. Despite the rGO layers showed very low adhesion boding to the Teflon substrate, the rGO layers on the Teflon had shown good uniformity itself.