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Graduate Seminar (G.R.A.S.S). – Shuo Sui (Perry) and Mengxi Chen (Maroudas)


Tuesday, December 12, 2017 - 11:30am


LGRT 201


Mengxi Chen – Dimitrios Maroudas Lab

Atomic-scale Computational Studies of Graphene-Based Nanomaterials with Tunable Properties

Mengxi Chen and Dimitrios Maroudas

Graphene-based nanomaterials have exceptional electronic, mechanical, and thermal properties that can be tuned by precise control of their nanostructural features. Such tunable properties are responsible for the unique function of these nanomaterials that have potential to enable numerous technological applications. Toward this end, in this seminar, we report a systematic computational analysis of the mechanical behavior of two classes of two-dimensional (2D) carbon-based nanostructures, namely, graphene nanomeshes (GNMs) and interlayer-bonded twisted bilayer graphene (IB-TBG) superstructures. In both cases, the analysis is based on molecular-dynamics simulations of uniaxial tensile straining tests according to a reliable interatomic bond-order potential.

Graphene nanomeshes (GNMs) are ordered, defect-engineered graphene nanostructures consisting of periodic arrays of nanopores in the graphene lattice with neck widths less than 10 nm. The electronic, transport, and mechanical properties of GNMs can be tuned by varying the structural, chemical, and architectural parameters of the nanomeshes, namely, their porosity, as well their pore lattice structure, pore morphology, and pore edge passivation. Here, we study the mechanical response of GNMs to uniaxial tensile straining and determine their mechanical properties. We establish the dependences of the elastic modulus, fracture strain, ultimate tensile strength, and toughness on the nanomesh porosity and derive scaling laws for GNM modulus-density and strength-density relations. We also establish the dependence of the above properties on pore morphology, for GNMs with circular and elliptical pores over a range of aspect ratios, and on pore edge hydrogen passivation that causes elastic stiffening and strength reduction. The underlying mechanisms of crack initiation and propagation leading to nanomesh failure also are characterized.

We also report a systematic computational analysis of the mechanical behavior of interlayer-bonded twisted bilayer graphene (IB-TBG) superstructures, which constitute graphene-diamond nanocomposites formed through interlayer covalent bonding of twisted bilayer graphene (TBG) with commensurate bilayers. The interlayer bonding is induced by patterned hydrogenation that leads to formation of superlattices of 2D nanodiamond domains embedded between the two graphene layers and having the periodicity of the underlying Moiré pattern. The mechanical response of these carbon nanocomposite structures is explored systematically as a function of their structural parameters, which include the commensurate bilayer’s twist angle, the stacking type of the nanodomains where the interlayer bonds are formed, the interlayer bond pattern and density, and the concentration of sp3-bonded C atoms in these superstructures. We determine the mechanical properties of these 2D materials and identify a range of structural parameters over which their fracture is ductile, mediated by void formation, growth, and coalescence, in contrast to the typical brittle fracture of graphene. We introduce a ductility metric, demonstrate its direct dependence on the concentration of sp3-bonded C atoms, and show that increasing the concentration of sp3-bonded C atoms beyond a critical level induces ductile mechanical response. We analyze the ductile fracture mechanisms and probe the brittle-to-ductile transition. Our study sets the stage for designing few-layer-graphene-based nanocomposites with unique mechanical properties and functionality.



Shuo Sui – Sarah Perry Lab

Graphene-Integrated Microfluidics for Advanced Crystallography

Shuo Sui and Sarah Perry

X-ray crystallography has long been the main workhorse technique for the structure determiantion of proteins and other biomacromolecules. The increasing briliance of X-ray sources has enabled the use of ever smaller and more weakly diffracting crystals and has accelerated the quality and rate of structure determination. However, this push towards high-throughput crystallography is limited by traditional manual harvesting and manipulation of crystals. Microfluidic strategies have been used to enable the growth and subsequent in situ crystallographic analysis of large numbers of crystals, avoid the challenge of harvesting micro-crystals, and have the potential to facilitate the structural characterization of protein targets that have been resistant to single-crystal strategies. We have developed a robust strategy for the incorporation of single-layer graphene as an ultra-thin window material and vapor-diffusion barrier into microfluidic devices. This architecture allows for a total material thickness of less than 1 µm, facilitating on-chip X-ray diffraction analysis while creating a sample environment that is stable against significant water loss over several weeks. These devices enable the collection of high quality, room-temperature diffraction data with excellent signal-to-noise. Furthermore, these devices have significant potential to enable continuous diffraction/diffuse scattering experiments, as well as the analysis of oxygen-sensitive targets because of the low background and barrier properties of the graphene layers. We are also exploring the possibility of utilizing the conductivity of graphene as an X-ray compatible integrated electrode for the application of an electric field for voltage-jump triggering of protein structural dynamics. Although this work is focused on the use of graphene for protein crystallography, we anticipate that this technology should find utility in a wide range of both X-ray and other lab-on-a-chip applications.

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