Wang WL, Bhandari S, Yi W, Bell DC, Westervelt R, Kaxiras E. Direct Imaging of Atomic-Scale Ripples in Few-Layer Graphene. NanoLetters. 2012;12 :2278-2282.Abstract

Graphene has been touted as the prototypical two-dimensional solid of extraordinary stability and strength. However, its very existence relies on out-of-plane ripples as predicted by theory and confirmed by experiments. Evidence of the intrinsic ripples has been reported in the form of broadened diffraction spots in reciprocal space, in which all spatial information is lost. Here we show direct real-space images of the ripples in a few-layer graphene (FLG) membrane resolved at the atomic scale using monochromated aberrationcorrected transmission electron microscopy (TEM). The thickness of FLG amplifies the weak local effects of the ripples, resulting in spatially varying TEM contrast that is unique up to inversion symmetry. We compare the characteristic TEM contrast with simulated images based on accurate first-principles calculations of the scattering potential. Our results characterize the ripples in real space and suggest that such features are likely common in ultrathin materials, even in the nanometer-thickness range.

Zhao K, Tritsaris GA, Pharr M, Wang WL, Okeke O, Suo Z, Vlassak JJ, Kaxiras E. Reactive Flow in Silicon Electrodes Assisted by the Insertion of Lithium. NanoLetters. 2012;12 :4397-4403.Abstract

In the search for high-energy density materials for Li-ion batteries, silicon has emerged as a promising candidate for anodes due to its ability to absorb a large number of Li atoms. Lithiation of Si leads to large deformation and concurrent changes in its mechanical properties, from a brittle material in its pure form to a material that can sustain large inelastic deformation in the lithiated form. These remarkable changes in behavior pose a challenge to theoretical treatment of the material properties. Here, we provide a detailed picture of the origin of changes in the mechanical properties, based on first-principles calculations of the atomic-scale structural and electronic properties in a model amorphous silicon (a-Si) structure. We regard the reactive flow of lithiated silicon as a nonequilibrium process consisting of concurrent Li insertion driven by unbalanced chemical potential and flow driven by deviatoric stress. The reaction enables the material to flow at a lower level of stress. Our theoretical model is in excellent quantitative agreement with experimental measurements of lithiation-induced stress on a Si thin film.

Ren J, Meng S, Kaxiras E. Theoretical Investigation of the C60/Copper Phthalocyanine Organic Photovoltaic Heterojunction. Nano Research. 2012;5 :248-257.Abstract

Molecular heterojunctions, such as the one based on copper phthalocyanine (CuPc) and carbon fullerene (C60) molecules, are commonly employed in organic photovoltaic cells as electron donor–acceptor pairs. We have investigated the different atomic structures and electronic and optical properties of the C60/CuPc heterojunction through first-principles calculations based on density functional theory (DFT) and time-dependent DFT. In general, configurations with the CuPc molecule “lying down” on C60 are energetically more favorable than configurations with the CuPc molecule “standing up”. The lying-down configurations also facilitate charge transfer between the two molecules, due to the stronger interaction and the larger overlap between electronic wavefunctions at the interface. The energetically preferred structure consists of CuPc placed so that the Cu atom is above a bridge site of C60, with one N–Cu–N bond of CuPc being parallel to a C–C bond of C60. We also considered the structure of a periodic CuPc monolayer deposited on the (001) surface of a face-centered cubic (fcc) crystal of C60 molecules with the lying-down orientation and on the (111) surface with the standing-up configuration. We find that the first arrangement can lead to larger open circuit voltage due to an enhanced electronic interaction between CuPc and C60 molecules. 

Mao Y, Wang WL, Wei D, Kaxiras E, Sodroski JG. Graphene Structures at an ExtremeDegree of Buckling. ACSNano. 2011;5 :1395.
Ren J, Meng S, Wang Y-L, Ma X-C, Xue Q-K, Kaxiras E. Properties of copper (fluoro-)phthalocyanine layers depositedon epitaxial graphene. J. Chem. Phys. 2011;134 :194706.
Baker TA, Xu B, Jensen SC, andEfthimios Kaxiras CMF. Role of defects in propene adsorption and reaction on a partiallyO-covered Au(111) surface. Catal. Sci. Technol. 2011;1 :1166.
Haubrich J, Kaxiras E, Friend CM. The Role of Surface and Subsurface Point Defects for Chemical ModelStudies on TiO2: A First-Principles Theoretical Study of FormaldehydeBonding on Rutile TiO2(110). Chem. Eur. J. 2011;17 :4496.
Li H, Knaup JM, Kaxiras E, Vlassak JJ. Stiffening of organosilicate glasses by organic cross-linking. Acta Mater. 2011;59 :44.
Lin M-W, Ling C, Agapito LA, Kioussis N, Zhang Y, Cheng MM-C, Wang WL, Kaxiras E, Zhixian Zhou1. Approaching the intrinsic band gap in suspended high-mobility graphene nanoribbons. PHYSICAL REVIEW B. 2011;84 :125411.
Meng S, Kaxiras E, Nazeeruddin MK, Gratzel M. Design of Dye Acceptors for Photovoltaics from First-PrinciplesCalculations. J. Phys. Chem. C. 2011;115 :9276.
Heiss M, Conesa-Boj S, Ren J, Tseng H-H, Gali A, Rudolph A, Uccelli E, Peiro F, Morante JR, Schuh D, et al. Direct correlation of crystal structure and optical properties in wurtzite/zinc-blendeGaAs nanowire heterostructures. PHYSICAL REVIEW B. 2011;83 :045303.
Gali JA, Kaxiras E, Zimanyi GT, Meng S. Effect of symmetry breaking on the optical absorption of semiconductor nanoparticles. PHYSICAL REVIEW B. 2011;84 :035325.
Melchionna S, Kaxiras E, Bernaschi M, Succi S. Endothelial shear stress from large-scale blood flow simulations. Phil. Trans. R. Soc. A. 2011;369 :2354.
Knaup JM, Li H, Vlassak JJ, Kaxiras E. Influence of CH2 content and network defects on the elastic properties of organosilicate glasses. PHYSICAL REVIEW B. 2011;83 :054204.
Zhao K, Wang WL, Gregoire J, Pharr M, Suo Z, Vlassak JJ, Kaxiras E. Lithium-Assisted Plastic Deformation of Silicon Electrodes inLithium-Ion Batteries: A First-Principles Theoretical Study. Nano Lett. 2011;11 :2962.
Kuhne TD, Pascal TA, Kaxiras E, Jung Y. New Insights into the Structure of the Vapor/WaterInterface from Large-Scale First-Principles Simulations. J. Phys. Chem. Lett. 2011;2 :105.
Maze JR, Gali A, Togan E, Chu Y, Trifonov A, Kaxiras E, Lukin MD. Properties of nitrogen-vacancy centers in diamond:the group theoretic approach. New Journal of Physics. 2011;13 :025025.
Xu B, Haubrich J, Baker TA, Kaxiras E, Friend CM. Theoretical Study of O-Assisted Selective Coupling of Methanol onAu(111). J. Phys. Chem. C. 2011;115 :3703.
Katono M, Bessho T, Meng S, Humphry-Baker R, Rothenberger G, Zakeeruddin SM, Kaxiras E, Gratzel M. D-π-A Dye System Containing Cyano-Benzoic Acid as Anchoring Group for Dye-Sensitized Solar Cells. Langmuir. 2011;27 (23) :14248–14252.
Meng S, Kaxiras E, Nazeeruddin MK, Graetzel M. Design of Dye Acceptors for Photovoltaics from First-Principles Calculations. Journal of Physical Chemistry C. 2011;115 :9276-9282.Abstract

We investigate a set of donor-π-acceptor (D-π-A) dyes with new acceptor groups for dye-sensitized solar cells, using time-dependent density-functional-theory calculations of the electronic structure and optical absorption. We considered three types of modifications on existing dye structures: (i) replacement of the side cyano group (CN) on the molecular anchor, (ii) insertion and alteration of the intermediate spacer groups, and (iii) modification of the number and positions of cyano CN groups on a phenyl-ring spacer. We find that with these modifications, the dye electronic levels and corresponding optical absorption properties can be gradually tuned, rendering possible the identification of dyes with desirable structural, electronic, and optical properties. For example, dyes with phenyl and CN-substituted phenyl groups are promising candidates for red light absorption and high molar extinction coefficients.