# Poster Session and Reception

Tuesday, May 16, 2017 - 5:45pm - 7:00pm

Lind 400

**Commensurate transition at van der Waals interface in twisted bilayer graphene**

Rebecca Engelke (Harvard University)Hyobin Yoo (Harvard University)

Stacking different two-dimensional (2-D) atomic crystals into heterostructures has proved a fruitful way to create novel materials with multifunctionalities. In addition to material selection, twist angle or lattice constant mismatch between the layers can be manipulated to tune the electronic properties of the system by applying additional periodicity described by a Moiré pattern. Moreover, interlayer interaction at the van der Waals interface favors a commensurate configuration, making it possible to form topological domain structures. Accordingly, understanding the microstructures that result from the van der Waals interaction and their correlation with physical properties is crucial for engineering 2-D heterostructures in various applications. In this presentation, we exploited transmission electron microscopy (TEM) to investigate the microstructures of twisted bilayer graphene with controlled twist angle. Detailed microstructures including commensurate domain structures as well as the domain boundaries were studied by TEM-based analytical techniques such as electron diffraction and dark field imaging.**Nano-optical studies of exciton polaritons in MoSe_2 waveguides**

Fengrui Hu (Iowa State University)

We performed systematic nano-optical studies of exciton-polaritons in MoSe_2 planar waveguides by combining near-field optical microscopy with rigorous numerical modeling. By imaging and analyzing the polariton interference fringes, we found that these polariton modes could travel over tens of microns with a wavelength down to a few hundred nanometers. Furthermore, we were able to map the entire dispersion of polaritons that shows a back-bending behavior close to the exciton energy. All above observations are consistent with our mode calculations and modeling.**Flexible Krylov Subspace Interior Eigensolvers**

Agnieszka Miedlar (University of Kansas)

Determing excited states in quantum physics or calculating the number of valence electrons in the Density Functional Theory (DFT) involve solving eigenvalue problems of very large dimensions. Moreover, very often the interesting features of these complex systems go beyond information contained in the extreme eigenpairs. For this reason, it is important to consider iterative solvers developed to compute a large amount of eigenpairs in the middle of the spectrum of large Hermitian and non-Hermitian matrices. In this talk, we present a newly developed Krylov-type methods and compare them with the well-established techniques in electronic structure calculations. We demonstrate their efficiency and robustness through some numerical examples.**Solvation Effects on MWW-2D Zeolite Framework for Dissociation of β-O-4 Linkage**

Neeraj Rai (Mississippi State University)

Lignin, a major component of plant cell walls is a potential renewable source of biofuels, chemicals, and other value-added products. It consists various aryl ethers, irregularly connected by a variety of linkages (β-O-4, 5-5, β-5, 4-O-5, β-1, dibenzodioxocin, and β-β) which creates a complex structural network; hence, selective bond breaking events are required for the production of other useful products. Out of all these linkages β-O-4 linkage is predominant. Protonated zeolites are the subject of growing interest due to their moderate acid strength. In this poster, properties and behavior of protonated B/Al based MWW-2D zeolites in the presence of different solvents have been investigated by means of periodic density functional theory (DFT) approaches. These substituted B/Al sites can catalytically cleave C-O bond. Here, model acid substituted zeolites in MWW framework are characterized by studied by first principles DFT calculations, with the aim of gaining detailed insights into reaction mechanism for dissociation of β-O-4 linkage**Adaptive Finite Element Modeling of Dynamic Brittle Fracture**

Mallikarjunaiah Muddamallappa (Worcester Polytechnic Institute)

In this work we describe an efficient finite element treatment of a variational, time-discrete model for dynamic brittle fracture. We start by providing an overview of an existing dynamic fracture model that stems from Griffith's theory and based on the Ambrosio-Tortorelli crack regularization. We propose an efficient numerical scheme based on the bilinear finite elements. For the temporal discretization of the equations of motion, we use \emph{Newmark}-$\theta$ time integration algorithm, which is implicit and unconditionally stable. To accommodate the crack irreversibility, we use a primal-dual active set strategy, which can be identified as a semi-smooth Newton's method. It is well known that to resolve the crack-path accurately, the mesh near the crack needs to be very fine, so it is common to use adaptive meshes. We propose a simple, robust, local mesh-refinement criterion to reduce the computational cost. We show that the phase-field based variational approach and adaptive finite-elements provides an efficient procedure for simulating the complex crack propagation including crack-branching.**The Blended-force Based Quasicontinuum Method for Bilayered Multilattices Crystals**

Xingjie Li (University of North Carolina at Charlotte)

Atomistic-to-continuum (AtC) methods are a vast class of multiphysics models which couple atomistic models of materials, such as molecular statics or molecular dynamics, with continuum mechanics models, such as nonlinear elasticity. The last thirty years has seen a surge of interest in the development of these methods, especially for crystalline materials, and even a thorough mathematical understanding of these methods has begun to emerge in the last five years. However, the applicability of these methods to crystalline materials has often been restricted only to materials comprised of a Bravais lattice. In this poster, we will present the blended force-based quasicontinuum method for modeling bilayered multilattices, which allows for technologically important materials such as alloys and graphene to be modeled. Based on a precise choice of blending mechanism, the optimal error estimates are considered in terms of degrees of freedom. Several benchmarking problems are tested, including the bilayer graphene system. The numerical experiments confirm the theoretical predictions. This is joint work with Dr. Derek Olson, and Dr. Mitchell Luskin.**Imaging the localized plasmon resonance modes in graphene nanoribbons**

Yilong Luan (Iowa State University)

We performed a real-space nano-infrared study of the localized surface plasmon resonance modes of graphene nanoribbons (GNRs) by combining near-field optical microscopy and finite-element plasmonic simulations. From the imaging data, we found symmetric plasmonic interference fringes when excitation laser beam is parallel to GNRs and asymmetric ones in the case of perpendicular excitation. Our analysis and modeling indicate that the observed asymmetric fringes are formed due to the interference between the localized surface plasmon resonance modes and the propagative surface plasmon polariton mode.**Run-away tail (RAT) in SrTiO3 accumulation layers**

Han Fu (University of Minnesota, Twin Cities)

We study the low temperature conductivity of the electron accumulation layer induced by the very strong electric field on the surface of the SrTiO3 sample. Due to the strongly nonlinear lattice dielectric response, the three-dimensional density of electrons n(x) in such a layer decays with the distance from the surface x by the power 12/7. We show that when the mobility is limited by the surface scattering the contribution of such a tail to the conductivity diverges at large x because of growing time electrons need to reach the surface. We explore truncation of this divergence by the finite sample width, by the bulk scattering rate, or by the crossover to the bulk linear dielectric response. As a result we arrive at the anomalously large mobility, Hall factor, thermopower, and magnetoresistance, which depend not only on the rate of the surface scattering, but also on the physics of truncation.**A Discrete-to-Continuum Model of Weakly Interacting Incommensurate Chains**

Malena Espanol (University of Akron)

In this poster, we present a formal discrete-to-continuum procedure to derive a continuum variational model for two chains of atoms with slightly incommensurate lattices. The chains represent a cross-section of a three-dimensional system consisting of a graphene sheet suspended over a substrate. The continuum model recovers both qualitatively and quantitatively the behavior observed in the corresponding discrete model. The numerical solutions for both models demonstrate the presence of large commensurate regions separated by localized incommensurate domain walls.

This is joint work with Dmitry Golovaty and Pat Wilber.**A Multiphase Internal State Variable Model with Rate Equations for Predicting Elastothermoviscoplasticity and Damage of Fiber Reinforced Polymer Composites**

Ge He (Mississippi State University)

This work agglomerates an Internal State Variable (ISV) model for amorphous polymers with damage evolution to a multiphase ISV framework that features a finite strain theoretical framework for capturing the failure and damage mechanisms of Fiber Reinforced Polymer (FRP) composites under various stress states, temperatures, strain rates, and history dependencies. Two new ISVs associated with the fiber orientation in the polymer matrix and the interaction between the fibers and polymer matrix are introduced. The first ISV is used to capture the anisotropic behavior of the FRPs, and the second ISV aims at describing the material's plastic flow behavior and deformation and failure of the fiber-matrix interface region. A scalar damage variable is employed to capture the damage history of such material, which is a result of three damage modes, matrix cracking, fiber breakage, and deterioration in fiber-matrix interface. The present model is developed following a kinematics-thermodynamics-kinetics sequence, whose ISVs can be either calculated from molecular dynamics simulations or calibrated through experimental microscopic observations for specific FRPs.**Properties of In-Plane Graphene/MoS2 Heterojunctions**

Wei Chen (Harvard University)

The graphene/MoS2 heterojunction formed by joining the two components laterally in a single plane promises to exhibit a low-resistance contact according to the Schottky-Mott rule. Here we provide an atomic-scale description of the structural, electronic, and magnetic properties of this type of junction. We first identify the energetically favorable structures in which the preference of forming C-S or C-Mo bonds at the boundary depends on the chemical conditions. We find that significant charge transfer between graphene and MoS2 is localized at the boundary. We show that the abundant 1D boundary states substantially pin the Fermi level in the lateral contact between graphene and MoS2, in close analogy to the effect of 2D interfacial states in the contacts between 3D materials. Furthermore, we propose specific ways in which these effects can be exploited to achieve spin-polarized currents.**Lithium Intercalation in Graphene/MoS2 Heterobilayers**

Ioanna Fampiou (Harvard University)Daniel Larson (Harvard University)

We use density functional theory to study the stability, diffusion, and band structure of Li ions intercalating between monolayers of graphene and MoS2. Based on the study of a supercell comprised of a 4x4 MoS2 layer beneath a 5x5 graphene layer, we find that the energetically preferred locations of intercalated Li ions are on top of Mo ions. The energy barriers for hopping between adjacent energy minima are all less than 375 meV. At the maximum lithium concentration allowed by energetic considerations, the intercalants donate nearly 10^15 electrons/cm^2, divided between the graphene and MoS2 layers.**Upper Limits on Mobility in Ionic Liquid Gated Graphene**

Kostantin Reich (University of Minnesota, Twin Cities)

Ionic liquid gating has a number of advantages over solid-state gating, especially for flexible or transparent devices, and applications requiring high carrier densities. However, the large number of charged ions near the channel inevitably results in Coulomb scattering, which limits the carrier mobility in otherwise clean systems. We develop a model for this Coulomb scattering. We validate our model experimentally using ionic liquid gated graphene covered with varying thicknesses of hexagonal boron nitride, demonstrating that disorder in the bulk liquid often dominates the scattering.**Epsilon-near-zero behavior from plasmonic Dirac point: Theory and realization using two-dimensional materials**

Marios Matthaiakis (Harvard University)

The electromagnetic response of a two-dimensional metal embedded in a periodic array of a dielectric host can give rise to a plasmonic Dirac point that emulates Epsilon-Near-Zero (ENZ) behavior. This theoretical result is extremely sensitive to structural features like periodicity of the dielectric medium and thickness imperfections. We propose that such a device can actually be realized by using graphene as the 2D metal and materials like the layered semiconducting transition-metal dichalcogenides or hexagonal boron nitride as the dielectric host. We propose a systematic approach, in terms of design characteristics, for constructing metamaterials with linear, elliptical and hyperbolic dispersion relations which produce ENZ behavior, normal or negative diffraction.**Surface Elasticity in the Steigmann-Ogden Form in Modeling of Fracture**

Anna Zemlyanova (Kansas State University)

A problem of a straight mixed mode non-interface fracture in an infinite plane is treated analytically with the help of complex analysis techniques. The surfaces of the fracture are subjected to surface elasticity in the form proposed by Steigmann and Ogden. The boundary conditions on the banks of the fracture connect the stresses and the derivatives of the displacements. The mechanical problem is reduced to two systems of singular integro-differential equations which are further reduced to the systems of equations with logarithmic singularities. It is shown that modeling of the fracture with the Steigmann-Ogden elasticity produces the stress and strain fields which are bounded at the crack tips. The existence and uniqueness of the solution for almost all the values of the parameters is proved. Additionally, it is shown that introduction of the surface mechanics into the modeling of fracture leads to the size-dependent equations. A numerical scheme of the solution of the systems of singular integro-differential equations is suggested, and the numerical results are presented for different values of the mechanical and the geometric parameters.**Time-domain Modeling of 2D-Material Optics**

Josh Wilson (University of Minnesota, Twin Cities)