Graphene--a single atomic layer of carbon atoms--is a two-dimensional material with many exceptional properties. For example, pristine defect-free graphene has the highest electrical conductivity as well as the greatest mechanical strength of any known material. To harness these unique properties, scientists and engineers have fabricated many different electrical, optical and magnetic devices from graphene using nanofabrication methods. Many of these devices are much superior to their microscale counterparts in terms of performance and/or energy usage. Thus graphene has tremendous potential to impact society positively. However pristine defect-free graphene is extremely expensive because manual methods must be used to isolate and manipulate it. To address this, several methods to grow graphene in an industrially scalable manner have been demonstrated recently. Graphene grown by such methods would benefit from an economy of scale that could translate to the mass production of nanofabricated devices that take advantage of graphene's unique set of properties. However graphene grown by these methods contains defects that degrade the properties, especially the mechanical properties. Our project addresses the fundamental challenge of quantifying the strength and reliability of graphene grown by industrially scalable methods. The outcome of the project is expected to be: (1) an experimental method to quantify the strength and reliability of as-grown graphene; (2) an understanding of how the growth process can be optimized to maximize the strength and reliability of as-grown graphene; (3) an experimentally validated multiscale theoretical and computational tool to predict the strength and reliability of graphene; and, (4) demonstration that as-grown graphene can be used as the backbone for ultrahigh strength laminate composites. The availability of large area CVD graphene with well-understood properties will make possible the mass production of graphene-based devices such as ever smaller and faster electronic devices and ultra high strength composite materials. In addition, the PIs and supported student will interact with high school teachers in the public New York City school system to host student visits and to develop laboratory experiments to measure the mechanical response of materials fitting high school science projects.
The objective is to quantify CVD graphene's probability of failure at a given stress as well as its mean strength. The Weibull probability distribution for a heterogeneous stress state will be employed. The experimental methodology will be via nanoindentation and pressure loading of free-standing circular films of CVD graphene. In order to rationalize the experimental results, a multiple length constitutive model of grain boundaries in graphene will be developed. Molecular dynamics simulations will predict the strength of individual grain boundaries that were previously characterized at the atomic length scale using Transmission Electron Microscopy (TEM). This information will be transferred to continuum cohesive zone models that will be incorporated into detailed finite element computational models. Thereby, the model will also account rigorously for the non-linear and anisotropic properties of the individual grains in the polycrystalline CVD graphene. The experimentally validated physics-based predictive capability of the model will serve as a tool to optimize the CVD growth of graphene and other two-dimensional materials.
