Project Technical Summary. Due to their distinct electronic properties, AB-stacked bilayer and trilayer graphene have shown extraordinary potential for next-generation optoelectronic and microprocessor applications. These promising materials, nevertheless, require new synthesis methods to effectively control the number of AB-stacked layers through graphite exfoliation and advanced processing techniques. Solution graphene dispersions are promising raw materials for printable electronics and nanocomposites, and most importantly, this approach represents the only possible route at this time for the mass-production of AB stacked bi- and tri-layer graphene. However, since the first graphene solution was reported in 2008, there are still many unanswered questions and technical bottlenecks that hinder the progress of this field. Although experiments have shown that the distribution of graphene layer numbers highly depends on the choice of solvents or surfactants used, very little is known about the molecular origin of the interactions between graphene and solvent/surfactant molecules, including correlating these interactions with the colloidal stability of the graphene solution. In addition, regardless of the methods used to produce solution-phase graphene, its size, shape, and number of stacked layers are all poly-disperse. Engineering approaches that can directly control the size and the stacking geometry (by narrowing their distributions) of the exfoliated graphene flakes have to be developed.
This project seeks to understand the origin of the interactions between suspended graphene with surrounding solvent/surfactant molecules via large-scale molecular dynamics simulations. These interactions will be quantified by calculating the potential of mean force between graphene sheets in solvents or aqueous surfactant solutions to establish a library of thermodynamic properties for dispersed graphene sheets. A theoretical framework that combines molecular dynamics simulations and kinetic theories of colloid aggregation will be developed to further design optimal solvent/surfactant molecules that can stabilize graphene dispersions efficiently. Experimentally, layer-controlled graphene dispersions will be produced using ionic graphite intercalation compounds (GICs). By carefully controlling the intercalation kinetics, high-quality Stage-2 and Stage-3 GICs will be synthesized, which are expected to be excellent precursors for bilayer and trilayer graphene dispersions. Systematic characterizations of graphene dispersions will be carried out to optimize the exfoliation process such that the produced dispersions are sufficiently concentrated, and that the graphene flakes are large enough, for conventional photolithography in which they will be deposited on a target substrate. Advanced separation techniques will also be developed to produce monodisperse bilayer and trilayer graphene solutions. The novel fundamental and practical insights gained from the proposed modeling and experiments will be used to guide the fabrication of electronic devices. The combined expertise of the two PIs across colloid science, engineering nanotechnology, computer simulations, and molecular modeling will enable rapid progress towards engineering graphene solutions for device manufacturing.
Broader Impact and Outreach:
The development of a novel advanced technique to exfoliate graphene in liquid phases will greatly increase the capability to produce AB-stacked bilayer and trilayer graphene for practical applications, such as fabricating electronic devices. In addition, novel fundamental insights into the interactions between graphene and other molecules will contribute to the overall theoretical and practical understanding of the kinetic behavior of graphene in the liquid phase. Ultimately, such knowledge can lead to the rational design of better media for graphene dispersions.
The modeling and experimental advances made will be incorporated into courses (MIT course 10.55 - Colloid and Surfactant Science by Professor Blankschtein and 10.585 - Engineering Nanotechnology by Professor Strano) and workshops at MIT that will expose a larger scientific audience to the fundamentals of graphene dispersion and stabilization in liquid phases, as well as to modeling these phenomena at the molecular level. It is noteworthy that the raw material used to produce the graphene dispersions is graphite, which is readily available in large quantities. New materials/devices based on graphite represent a viable and economical alternative to existing technologies. The production of the highly-conductive, two-dimensional dispersions proposed here will be potentially useful for the production of conducting inks, printable electronics, and biological sensors. The technological impact to society extends from the development of rollable displays to the creation of novel glucose detectors for diabetics. The students involved in the proposed research at both the graduate an undergraduate level will gain intellectually and professionally from the integrated experimental/modeling research proposed here. The PIs and students involved will participate in the Saturday Engineering Enrichment and Discovery (SEED) Academy at MIT, as well as in the Minority Introduction to Engineering and Science (MITES) program, both of which target high school age students and influence minority recruitment.