Publications by Year: 2014

Wang WL, Santos EJG, Jiang B, Cubuk ED, Ophus C, Centeno A, Pesquera A, Zurutuza A, Ciston J, Westervelt R, et al. Direct Observation of a Long-Lived Single-Atom Catalyst Chiseling Atomic Structures in Graphene. NanoLetters. 2014;14 :450-455.Abstract

Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberrationcorrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale.

Tritsaris GA, Vinichenko D, Kolesov G, Friend CM, Kaxiras E. Dynamics of the Photogenerated Hole at the Rutile TiO2(110)/Water Interface: A Nonadiabatic Simulation Study. Journal of Physical Chemistry C . 2014;118 :27393-27401.Abstract

Hydrogen production in photoelectrochemical cells constitutes an important avenue toward carbon-free fuel. The most convenient process for hydrogen production is the splitting of water molecules, which necessitates a catalytic
reaction involving a semiconductor. Here, we introduce a framework for the study of photocatalyzed reactions on semiconductor surfaces based on time-dependent density functional theory that explicitly accounts for the evolution of electronically excited states. Within this framework, we investigate the possibility of hole-mediated splitting of molecularly adsorbed water on a representative metal oxide surface the rutile TiO2(110). We find that oxidative dehydrogenation of water is possible in synergy with thermal effects at temperatures between 60 and 100 K only when defects like Ti interstitials are present in the subsurface region. This study presents a general computational strategy for describing photoexcited semiconductor/adsorbate interfaces and also demonstrates that the occurrence of water dissociation on the rutile TiO2(110) surface depends sensitively on the local atomic environment and external parameters such as temperature.

Malone BD, Gali A, Kaxiras E. First principles study of point defects in SnS. Physical Chemistry Chemical Physics. 2014;16 :26176-26183.Abstract

Photovoltaic cells based on SnS as the absorber layer show promise for efficient solar devices containing non-toxic materials that are abundant enough for large scale production. The efficiency of SnS cells has been increasing steadily, but various loss mechanisms in the device, related to the presence of defects in the material, have so far limited it far below its maximal theoretical value. In this work we perform first principles, density-functional-theory calculations to examine the behavior and nature of both intrinsic and extrinsic defects in the SnS absorber layer. We focus on the elements known to exist in the environment of SnS-based photovoltaic devices during growth. In what concerns intrinsic defects, our calculations support the current understanding of the role of the Sn vacancy (VSn) acceptor defect, namely that it is responsible for the p-type conductivity in SnS. We also present calculations for extrinsic defects and make extensive comparison to experimental expectations. Our detailed treatment of electrostatic correction terms for charged defects provides theoretical predictions on both the highfrequency and low-frequency dielectric tensors of SnS.

Yu L, Lee Y-H, Ling X, Santos EJG, Shin YC, Lin Y, Dubey M, Kaxiras E, Kong J, Wang H, et al. Graphene/MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics. NanoLetters. 2014;14 :30553063.Abstract

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated highperformance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

Randles A, Kaxiras E. Parallel in time approximation of the lattice Boltzmann method for laminar flows. Journal of Computational Physics. 2014;270 :577-586.Abstract

Fluid dynamics simulations using grid-based methods, such as the lattice Boltzmann equation, can benefit from parallel-in-space computation. However, for a fixed-size simulation of this type, the efficiency of larger processor counts will saturate when the number of grid points per core becomes too small. To overcome this fundamental strong scaling limit in space-parallel approaches, we present a novel time-parallel version of the lattice Boltzmann method using the parareal algorithm. This method is based on a predictor–corrector scheme combined with mesh refinement to enable the simulation of larger number of time steps. We present results of up to a 32× increase in speed for a model system consisting of a cylinder with conditions for laminar flow. The parallel gain obtainable is predicted with strong accuracy, providing a quantitative understanding of the potential impact of this method.

Zhang G, Li X, Wu G, Wang J, Culcer D, Kaxiras E, Zhang Z. Quantum phase transitions and topological proximity effects in graphene nanoribbon heterostructures. Nanoscale. 2014;6 :3259-3267.Abstract

Topological insulators are bulk insulators that possess robust chiral conducting states along their interfaces with normal insulators. A tremendous research effort has recently been devoted to topological insulatorbased heterostructures, in which conventional proximity effects give rise to a series of exotic physical phenomena. Here we establish the potential existence of topological proximity effects at the interface between a topological insulator and a normal insulator, using graphene-based heterostructures as prototypical systems. Unlike conventional proximity effects in topological insulator based heterostructures, which refer to various phase transitions associated with the symmetry breaking of specific local order parameters, topological proximity effects describe the rich variety of quantum phase transitions associated with the global properties of the system measured by the location of the topological edge states. Specifically, we show that the location of the topological edge states exhibits a versatile tunability as a function of the interface orientation, the strength of the interface tunnel coupling between a topological graphene nanoribbon and a normal graphene nanoribbon, the spin–orbit coupling strength in the normal graphene nanoribbon, and the width of the system. For zigzag and bearded graphene nanoribbons, the topological edge states can be tuned to be either at the interface or outer edge of the normal ribbon. For armchair graphene nanoribbons, the potential location of the topological edge state can be further shifted to the edge of or within the normal ribbon, to the interface, or diving into the topological graphene nanoribbon. We further show that the topological phase diagram established for the prototypical graphene heterostructures can also explain the intriguing quantum phase transition reported recently in other topological-insulator heterostructures. We also discuss potential experimental realizations of the predicted topological proximity effects, which may pave the way for integrating the salient functionality of topological insulators and graphene in future device applications.

Randles A, Kaxiras E. A Spatio-Temporal Coupling Method to Reduce the Time-to-Solution of Cardiovascular Simulations. 2014 IEEE 28th International Parallel & Distributed Processing Symposium. 2014 :593-602.Abstract

We present a new parallel-in-time method designed to reduce the overall time-to- solution of a patientspecific cardiovascular flow simulation. Using a modified parareal algorithm, our approach extends strong scalability beyond spatial parallelism with fully controllable accuracy and no decrease in stability. We discuss the coupling of spatial and temporal domain decompositions used in our implementation, and showcase the use of the method on a study of blood flow through the aorta. We observe an additional 40% reduction in overall wall clock time with no significant loss of accuracy, in agreement with a predictive performance model.

Cheng J, Wang WL, Mosallaei H, Kaxiras E. Surface Plasmon Engineering in Graphene Functionalized with Organic Molecules: A Multiscale Theoretical Investigation. NanoLetters. 2014;14 :50-56.Abstract

Graphene was recently shown to support deep subwavelength surface plasmons at terahertz frequencies characterized by low energy loss and strong field localization, both highly desirable. The properties of graphene can be locally tuned by applying an external gate voltage or by the adsorption of organic molecules that lead to doping through charge transfer. Local tuning of the electronic features of graphene opens the possibility to realize any desired gradient index profile and thus brings large flexibility to control and manipulate the propagation of surface plasmons. Here, we explore this possibility created by functionalizing graphene with organic molecules. We employ a multiscale theoretical approach that combines firstprinciples electronic structure calculations and finite-difference time-domain simulations coupled by surface conductivity. We show that by patterning two types of organic molecules on graphene, a plasmonic metasurface can be realized with any gradient effective refractive index profile to manipulate surface plasmon beams as desired. The special properties of such devices based on functionalized graphene are compared to the similar metamaterials based on metallic films on top of a gradient index dielectric substrate. Using this idea, we design and analyze an ultrathin broadband THz plasmonic lens as proof-of-concept, while more sophisticated index profiles can also be realized and various plasmonic applications are readily accessible.

Lee C-H, Schiros T, Santos EJG, Kim B, Yager KG, Kang SJ, Lee S, Yu J, Watanabe K, Taniguchi T, et al. Epitaxial Growth of Molecular Crystals on van der Waals Substrates for High-Performance Organic Electronics. Advanced Materials. 2014;26 :2812-2817. 2014_advmater_26_2812.pdf
Monachon C, Schusteritsch G, Kaxiras E, Weber L. Qualitative link between work of adhesion and thermal conductance of metal/diamond interfaces. Journal of Applied Physics. 2014;115 :123509.Abstract

We report Time-Domain ThermoReflectance experiments measuring the Thermal Boundary Conductance (TBC) of interfaces between diamond and metal surfaces, based on samples consisting of [111]-oriented diamond substrates with hydrogen or with sp2 carbon surface terminations created using plasma treatments. In a concurrent theoretical study, we calculate the work of adhesion between Ni, Cu, and diamond interfaces with (111) surface orientation, with or without hydrogen termination of the diamond surface, using first-principles electronic structure calculations based on density functional theory (DFT). We find a positive correlation between the calculated work of adhesion and the measured conductance of these interfaces, suggesting that DFT could be used as a screening tool to identify metal/dielectric systems with high TBC. We also explain the negative effect of hydrogen on the thermal conductance of metal/diamond interfaces.

Tritsaris GA, Malone BD, Kaxiras E. Structural stability and electronic properties of low-index surfaces of SnS. Journal of Applied Physics. 2014;115 :173702.Abstract

Thin film photovoltaic cells are increasingly important for cost-effective solar energy harvesting. Layered SnS is a promising absorber material due to its high optical absorption in the visible and good doping characteristics. We use first-principles calculations based on density functional theory to study structures of low-index surfaces of SnS using stoichiometric and oxygen-containing structural models, in order to elucidate their possible effect on the efficiency of the photovoltaic device. We find that the surface energy is minimized for the surface with orientation parallel to the layer stacking direction. Compared to stoichiometric surfaces, the oxygen-containing surfaces exhibit fewer electronic states near the band gap. This reduction of near-gap surface states by oxygen should reduce recombination losses at grain boundaries and interfaces of the SnS absorber, and should be beneficial to the efficiency of the solar cell.