Molecular level interactions of large area 2D materials
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Two-dimensional materials such as self-assembled monolayers (SAMs), graphene, etc. are candidate materials for improving the performance of microelectronics components and MEMS/NEMS devices. In view of their relatively large in-plane dimensions, surface forces are likely to dominate their behavior. The purpose of the current work was to extract not only the adhesion energy (or steady state fracture toughness) but also the traction-separation relation associated with interactions between various two-dimensional materials and substrates. In particular, interactions between SAMs terminated by carboxyl and diamine (COOH/NMe2) groups, hydroxylated silicon surfaces, graphene and silicon, graphene and its seed copper and graphene and epoxy over large areas was considered. Traction-separation relations, which are a continuum description of such molecular interactions, were determined by a direct method, which makes use of measurements of crack tip opening displacements; an inverse approach where the key parameters are extracted by comparing measured global parameters with finite element solutions and a hybrid approach in which the direct method was supplemented by finite element analysis. Furthermore, the surface free energy of graphene was measured by contact angle measurements. The most striking observation across all the interactions that were considered is that the interaction ranges were much larger than those attributed to van der Waals forces. While van der Waals models might have been at play between graphene and its seed copper foil and graphene and epoxy, the adhesion energies were surprisingly high. This coupled with the long interaction range suggests that roughness effects modulated the basic force field. Interactions between graphene and silicon and hydroxylated silicon surfaces may have been due to capillary and/or electrostatic again possibly modulated by roughness. The interactions between COOH and NMe2 SAMs became stronger under vacuum, which may have induced chemical bonding, and tougher under mixed-mode loading.