Publications

2011
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.

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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-blende GaAs nanowire heterostructures. Physical Review B. 2011;83 :045303.Abstract

A method for the direct correlation at the nanoscale of structural and optical properties of single GaAs nanowires is reported. Nanowires consisting of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are investigated by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence of wurtzite GaAs is consistent with a band gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that are predominantly composed of either zinc-blende or wurtzite phase show photoluminescence emission close to the bulk GaAs band gap, while regions composed of a nonperiodic superlattice of wurtzite and zinc-blende phases exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The dimensions of the quantum heterostructures are correlated with the light emission, allowing us to determine the band alignment between these two crystalline phases. Our first-principles electronic structure calculations within density functional theory, employing a hybrid-exchange functional, predict band offsets and effective masses in good agreement with experimental results.

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Katono M, Bessho T, Meng S, Humphry-Baker R, Rothenberger G, Zakeeruddin SM, Kaxiras E, Graetzel M. D-π-A Dye System Containing Cyano-Benzoic Acid as Anchoring Group for Dye-Sensitized Solar Cells. Langmuir. 2011;27 :14248-14252.Abstract

A D-π-A dye (KM-1) incorporating cyanobenzoic acid as a new acceptor/anchoring group has been synthesized for dye-sensitized solar cells (DSCs) with a high molar extinction coefficient of 66 700 M-1 cm-1 at 437 nm. Theoretical calculations show that the hydrogen bond between CN and surface hydroxyl leads to the most stable configuration on the surface of TiO2. In addition, the adsorption of the dye on TiO2 follows a BrunauerEmmettTeller (BET) isotherm. Multilayer adsorption of KM-1 on TiO2 seems to take place particularly at higher dye concentrations. DSC device using KM-1 reached a maximum incident photon-to-current conversion efficiency (IPCE) of 84%, with a solar to electric power conversion efficiency (PCE) of 3.3% at AM1.5 G illumination (100 mW cm-2 ). This new type of anchoring group paves a way to design new dyes that combine good visible light harvesting with strong binding to the metal oxide surface

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Melchionna S, Kaxiras E, Bernaschi M, Succi S. Endothelial shear stress from large-scale blood flow simulations. Philosophical Transactions of the Royal Society A. 2011;369 :2354-2361.Abstract

We discuss the optimal evaluation of endothelial shear stress for real-life case studies based on anatomic data acquisition. The fluid dynamic simulations require smoothing of the geometric dataset to avoid major artefacts in the flow patterns, especially in the proximity of bifurcations. A systematic series of simulations at different corrugation levels shows that, below a smoothing length of about 0.5 mm, the numerical data are insensitive to further smoothing.

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Mao Y, Wang WL, Wei D, Kaxiras E, Sodroski JG. Graphene Structures at an Extreme Degree of Buckling. ACS Nano. 2011;5 (2) :1395-1400.Abstract

T The distinctive properties of graphene sheets may be significantly influenced by the presence of corrugation structures. Our understanding of these graphene structures has been limited to the mesoscopic scale. Here we characterize angstrom-scale periodic buckling structures in freestanding graphene bilayers produced by liquid-phase processing in the absence of specific substrates. Monochromated, aberration-corrected transmission electron microscopy with sub-angstrom resolution revealed that the unit structures in the major buckling direction consist of only two and three unit cells of graphene's honeycomb lattice, resulting in buckling wavelengths of 3.6±0.5 and 6.4±0.8 Å, respectively. The buckling shows a strong preference of chiral direction and spontaneously chooses the orientation of the lowest deformation energy, governed by simple geometry rules agreeing with Euler buckling theory. Unexpectedly, the overall buckled structures demonstrate geometric complexity with cascaded features. First-principles calculations suggest that significant anisotropic changes in the electronic structure of graphene are induced by the buckling.

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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.Abstract

Organosilicate glasses (OSGs) are used as low-k intermetal dielectrics for advanced integrated circuits. In this application, the material must fulfill two conflicting requirements: It has to have low density to reduce the dielectric constant while being sufficiently mechanically stable to withstand thermomechanical and other stresses during subsequent steps of integrated circuit manufacture. Recent experimental advances in improving the mechanical and electrical properties of these materials have not yet been systematically studied theoretically at the ab initio level due to the large model sizes necessary to realistically describe amorphous materials. In this paper we employ the density-functional-based tight-binding method to achieve an accurate description of OSG properties at different compositions. We analyze the influence of composition and local network defects on the density and bulk modulus of nonporous OSG. We find that the dependence of density and that of mechanical stiffness on chemical composition are of different natures. This difference is traced to a transition between mechanisms of elastic deformation in silica glass and in silicon hydrocarbide, which is also the reason for the two materials’ different sensitivities to network defects.

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Zhao K, Wang WL, Gregoire J, Pharr M, Suo Z, Vlassak JJ, Kaxiras E. Lithium-Assisted Plastic Deformation of Silicon Electrodes in Lithium-Ion Batteries: A First-Principles Theoretical Study. NanoLetters. 2011;11 :2962-2967.Abstract

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries. Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. On the basis of first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation. We find that lithium insertion leads to breaking of SiSi bonds and formation of weaker bonds between neighboring Si and Li atoms, which results in a decrease in Young’s modulus, a reduction in strength, and a brittle-to-ductile transition with increasing Li concentration. The microscopic mechanism of large plastic deformation is attributed to continuous lithium-assisted breaking and re-forming of SiSi bonds and the creation of nanopores.

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