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Document Type

Campus-Only Access for Five (5) Years

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Physics

Year Degree Awarded

2018

Month Degree Awarded

May

First Advisor

Dimitrios Maroudas

Second Advisor

Ashwin Ramasubramaniam

Subject Categories

Condensed Matter Physics

Abstract

Graphene has unique mechanical, electronic, and thermal properties, which enable a broad range of technological applications. For example, graphene flakes can be used as filler to enhance the properties of polymer-matrix nanocomposites and graphene derivatives, generated by defect engineering and chemical functionalization of single-layer graphene, have tunable properties that are very promising for engineering electronic and thermomechanical metamaterials. A fundamental understanding of the structure-property relationships that govern the function of such nanocomposites and graphene derivatives is required for designing and developing future graphene-based metamaterials.

Toward this end, we have conducted a systematic study based on extensive molecular-dynamics simulations of mechanical testing of model polymer (high-density polyethylene-HDPE) nanocomposites reinforced by nanocarbon fillers consisting of graphene flakes. In this multiscale atomistic-to-continuum approach for modeling interfacial stress transfer in graphene-reinforced HDPE nanocomposites, via detailed characterization of atomic-level stress profiles in sub-micron graphene fillers, we develop a modified shear-lag model for short fillers. A key feature of our approach lies in the correct accounting of stress concentration at the ends of fillers that exhibits a power-law dependence on filler (“flaw”) size, determined explicitly from atomistic simulations, without any ad hoc modeling assumptions. In addition to two parameters that quantify the end-stress concentration, only one additional shear-lag parameter is required to quantify the atomic-level stress profiles in graphene fillers. This three-parameter model is found to be reliable for fillers with dimensions as small as 10 nm. Our model predicts accurately the elastic response of aligned graphene-HDPE composites and sets appropriate upper bounds for the elastic moduli of nanocomposites with more realistic randomly distributed and oriented fillers. This work provides a systematic approach for developing hierarchical multiscale models of 2D material-based nanocomposites and is of particular relevance for short fillers, which are currently typical of solution-processed 2D materials.

Among various defect-engineered graphene derivatives, we focused on electron- irradiated single-layer graphene and studied its thermomechanical and electronic properties and the effects on such properties of defect passivation by hydrogenation of the irradiated graphene sheets. Specifically, we have conducted a systematic analysis on the effects of hydrogenation on the mechanical behavior of irradiated single-layer graphene sheets, including irradiation-induced amorphous graphene, based on molecular-dynamics (MD) simulations of uniaxial tensile straining tests and using an experimentally validated model of electron-irradiated graphene. We find that hydrogenation has a significant effect on the tensile strength of the irradiated sheets only if it changes the hybridization of the hydrogenated carbon atoms to sp3, causing a reduction in the strength of irradiation-induced amorphous graphene by ∼10 GPa. Hydrogenation also causes a substantial decrease in the failure strain of the defective sheets, regardless of the hybridization of the hydrogenated carbon atoms, and in their fracture toughness, which decreases with increasing hydrogenation for given irradiation dose. We have characterized in detail the fracture mechanisms of the hydrogenated irradiated graphene sheets and elucidated the role of hydrogen and the extent of hydrogenation in the deformation and fracture processes.

Moreover, we have carried out a systematic analysis of thermal transport in these electron-irradiated graphene sheets based on nonequilibrium MD simulations. We focused on the dependence of the thermal conductivity, k, of the irradiated graphene sheets on the inserted irradiation defect density, c, as well as the extent of defect passivation with hydrogen atoms. While the thermal conductivity of irradiated graphene decreases precipitously from that of pristine graphene, k0, upon introducing a low vacancy concentration, c < 1%, in the graphene lattice, further reduction of the thermal conductivity with increasing vacancy concentration exhibits a weaker dependence on c until the amorphization threshold. Beyond the onset of amorphization, the dependence of thermal conductivity on vacancy concentration becomes significantly weaker, and k practically reaches a plateau value. Throughout the range of c and at all hydrogenation levels examined, the correlation k = k0 (1 + αc)1 gives an excellent description of the simulation results. The value of the coefficient α captures the overall strength of the numerous phonon scattering centers in the irradiated graphene sheets, which include monovacancies, vacancy clusters, carbon ring reconstructions, disorder, and a rough nonplanar sheet morphology. Hydrogen passivation increases the value of α but the effect becomes very minor beyond the amorphization threshold.

Finally, based on first-principles density functional theory (DFT) calculations, we have performed systematic studies of the electronic structure of irradiated graphene sheets and the effects of defect passivation on the electronic structure. We found that localized states appear at the Fermi level upon irradiation and the corresponding local density of states increases with increasing inserted vacancy concentration. Through band structure calculations, we have also shown that electron localization in the vicinity of irradiation-induced defects causes band flattening and reduces the charge carrier mobility. This band flattening effect becomes stronger with increasing vacancy concentration inducing an increasing number of flat bands near the Fermi level. In addition, computed frontier orbitals and local charge density distributions provide clear evidence of carrier localization near the unsaturated bonds. Passivating these bonds with hydrogen atoms leads to delocalization of the charge density, hence increasing the carrier mobility, which also results in a reduced density of states at the Fermi level and an increased band dispersion with increasing inserted vacancy concentration. We found these spatially localized states to be spin-polarized, which gives rise to a net local magnetic moment that is removed completely upon hydrogen passivation.

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