Oxygen reduction activity on perovskite oxide surfaces: a comparative first-principle study of LaMnO3_3, LaFeO3_3 and LaCrO3_3

The understanding of oxygen reduction reaction (ORR) activity on perovskite oxide surfaces is essential for promising future fuel cell applications. We report a comparative study of ORR mechanisms on La$B$O$_3$ ($B$=Mn, Fe, Cr) surfaces by first-principles calculations based on density functional theory (DFT). Results obtained from varied DFT methods such as generalized gradient approximation(GGA), GGA+$U$ and the hybrid Hartree-Fock density functional method are reported for comparative purposes. We find that the results calculated from hybrid-functional method suggest that the order of ORR activity is LaMnO$_3$ $>$ LaCrO$_3$ $>$ LaFeO$_3$, which is in better agreement with recent experimental results (Suntivich \textit{et al.}, Nature Chemistry 3, 546 (2011)) than those using the GGA or GGA+$U$ method.Comment: submitte

First-principles study of multi-control graphene doping using light-switching molecules

The high carrier mobility in graphene promises its utility in electronics applications. Azobenzene is a widely studied organic molecule for switchable optoelectronic devices that can be synthesized with a wide variety of ligands and deposited on graphene. Using first-principles calculations, we investigate graphene doping by physisorbed azobenzene molecules with various electron-donating and -accepting ligands. We confirm previous experimental results that demonstrate greater p-doping of graphene for the trans compared to cis configuration when using a SO$_3$ electron-accepting ligand, however we find that NO$_2$ ligands maximize the p-doping difference between isomers. We also examine how these doping effects change when the graphene monolayer is supported on a silica substrate. We then extend these findings by examining the doping effects of an applied electrical bias and mechanical strain to the graphene, which lead to changes in doping for both the trans and cis isomers. These results demonstrate a new type of multi-control device combining light, electric field, and strain to change carrier concentration in graphene

First-principles Simulations of a Graphene Based Field-Effect Transistor

We improvise a novel approach to carry out first-principles simulations of graphene-based vertical field effect tunneling transistors that consist of a graphene$|${\it h}-BN$|$graphene multilayer structure. Within the density functional theory framework, we exploit the effective screening medium (ESM) method to properly treat boundary conditions for electrostatic potentials and investigate the effect of gate voltage. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated self-consistently. The dielectric properties of {\it h}-BN thin films sandwiched between graphene layers are computed layer-by-layer following the theory of microscopic permittivity. We find that the permittivities of BN layers are very close to that of crystalline {\it h}-BN. The effect of interface with graphene on the dielectric properties of {\it h}-BN is weak, according to an analysis on the interface charge redistribution.Comment: 6 pages, 6 figure

Does Silicene on Ag(111) Have a Dirac Cone?

We investigate the currently debated issue of the existence of the Dirac cone in silicene on an Ag(111) surface, using first-principles calculations based on density functional theory to obtain the band structure. By unfolding the band structure in the Brillouin zone of a supercell to that of a primitive cell, followed by projecting onto Ag and silicene subsystems, we demonstrate that the Dirac cone in silicene on Ag(111) is destroyed. Our results clearly indicate that the linear dispersions observed in both angular-resolved photoemission spectroscopy (ARPES) [P. Vogt et al, Phys. Rev. Lett. 108, 155501 (2012)] and scanning tunneling spectroscopy (STS) [L. Chen et al, Phys. Rev. Lett. 109, 056804 (2012)] come from the Ag substrate and not from silicene.Comment: 5 pages, 3 figure

Preventing rapid energy loss from electron-hole pairs to phonons in graphene quantum dots

In semiconductors, photoexcited electrons and holes (carriers) initially occupy high-energy states, but quickly lose energy to phonons and relax to the band edge within a picosecond [1]. Increasing the lifetime of carriers in light-absorbing materials is necessary to improve open-circuit voltage in photovoltaics [2], charge separation in organic solar cells [3], and charge transfer in photodetection de vices [4]. Here we demonstrate long lifetimes over one hundred picoseconds for electron-hole pairs in graphene quantum dots (GQDs) due to large transition energies and weak coupling to excitonic states below the fundamental band gap. This possibility for a large transition energy to bound excitons is due to graphene's poor screening, illustrating a unique mechanism in this QD to occupy higher-energy states for long timescales. GQD edges can be terminated with either armchair or zigzag carbon patterns, and this edge structure changes excited state lifetimes by orders of magnitude. These results indicate nanoscale control of carrier lifetimes in optoelectronics.Comment: 5 figures, 3 supplementary figure

First-Principles Studies of Photoinduced Charge Transfer in Noncovalently Functionalized Carbon Nanotubes

We have studied the energetics, electronic structure, optical excitation, and electron relaxation of dinitromethane molecules (CH$_{2}$N$_{2}$O$_{4}$) adsorbed on semiconducting carbon nanotubes (CNTs) of chiral index (n,0) (n=7, 10, 13, 16, 19). Using first-principles density functional theory (DFT) with generalized gradient approximations and van der Waals corrections, we have calculated adsorption energies of dinitropentylpyrene, in which the dinitromethane is linked to the pyrene via an aliphatic chain, on a CNT. A 75.26 kJ/mol binding energy has been found, which explains why such aliphatic chain-pyrene units can be and have been used in experiments to bind functional molecules to CNTs. The calculated electronic structures show that the dinitromethane introduces a localized state inside the band gap of CNT systems of n=10, 13, 16 and 19; such a state can trap an electron when the CNT is photoexcited. We have therefore investigated the dynamics of intra-band relaxations using the reduced density matrix formalism in conjunction with DFT. For pristine CNTs, we have found that the calculated charge relaxation constants agree well with the experimental time scales. Upon adsorption, these constants are modified, but there is not a clear trend for the direction and magnitude of the change. Nevertheless, our calculations predict that electron relaxation in the conduction band is faster than hole relaxation in the valence band for CNTs with and without molecular adsorbates.Comment: 30 pages, 7 figures, 3 tables, submitte

Electron Transport Through Ag-Silicene-Ag Junctions

For several years the electronic structure properties of the novel two-dimensional system silicene have been studied extensively. Electron transport across metal-silicence junctions, however, remains relatively unexplored. To address this issue, we developed and implemented a theoretical framework that utilizes the tight-binding Fisher-Lee relation to span non-equilibrium Green's function (NEGF) techniques, the scattering method, and semiclassical Boltzmann transport theory. Within this hybrid quantum-classical, two-scale framework, we calculated transmission and reflection coefficients of monolayer and bilayer Ag-silicene-Ag junctions using the NEGF method in conjunction with density functional theory; derived and calculated the group velocities; and computed resistance using the semi-classical Boltzmann equation. We found that resistances of these junctions are $\sim${} 0.08 \fom for monolayer silicene junctions and $\sim${} 0.3 \fom for bilayer ones, factors of $\sim$8 and $\sim$2, respectively, smaller than Sharvin resistances estimated via the Landauer formalism.Comment: 5 pages, 4 figure

Electronic and transport properties of azobenzene monolayer junctions as molecular switches

We investigate from first-principles the change in transport properties of a two-dimensional azobenzene monolayer sandwiched between two Au electrodes that undergoes molecular switching. We focus on transport differences between a chemisorbed and physisorbed top monolayer-electrode contact. The conductance of the monolayer junction with a chemisorbed top contact is higher in trans configuration, in agreement with the previous theoretical predictions of one-dimensional single molecule junctions. However, with a physisorbed top contact, the "ON" state with larger conductance is associated with the cis configuration due to a reduced effective tunneling pathway by switching from trans to cis, which successfully explains recently experimental measurements of azobenzene monolayer junctions. A simple model is developed to explain electron transmission across subsystems in the molecular junction. We also discuss the effects of monolayer packing density, molecule tilt angle, and contact geometry on the calculated transmission functions. In particular, we find that a tip-like contact with chemisorption significantly affects the electric current through the cis monolayer, leading to highly asymmetric current-voltage characteristics as well as large negative differential resistance behavior.Comment: 10 pages, 11 figures, publishe

Multiscale Modeling of Materials - Concepts and Illustration

The approximate representation of a quantum solid as an equivalent composite semi-classical solid is considered for insulating materials. The composite is comprised of point ions moving on a potential energy surface. In the classical bulk domain this potential energy is represented by pair potentials constructed to give the same structure and elastic properties as the underlying quantum solid. In a small local quantum domain the potential is determined from a detailed quantum calculation of the electronic structure. The primary new ingredients are 1) a determination of the pair potential from quantum data for equilibrium and strained structures, 2) development of pseudo-atoms for a realistic treatment of charge densities where bonds have been broken to define the quantum domain, and 3) inclusion of polarization effects on the quantum domain due to its environment. This formal structure is illustrated in detail for an silica nanorod. For each configuration considered, the charge density of the entire solid is calculated quantum mechanically to provide the reference by which to judge the accuracy of the modeling.It is then shown that the quantum rod, the rod constructed from the classical pair potentials, and the composite classical/quantum rod all have the same equilibrium structure and response to elastic strain. The accuracy of the modeling is shown to apply for two quite different quantum chemical methods for the underlying quantum mechanics: transfer Hamiltonian and density functional methods.Comment: 19 pages, 9 figures, submitted to Physical Review

Constructing A Small Strain Potential for Multi-Scale Modeling

For problems relating to fracture, a consistent embedding of a quantum (QM) domain in its classical (CM) environment requires that the classical system should yield the same structure and elastic properties as the QM domain for states near equilibrium. It is proposed that an appropriate classical potential can be constructed using ab initio data on the equilibrium and weakly strained configurations calculated from the quantum description, rather than the more usual approach of fitting to a wide range of empirical data. The scheme is illustrated in detail for a model system, silica nanorod that has the proper stiochiometric ratio of Si:O as observed in real silica. The potential is chosen to be pairwise additive, with the same pair potential functional form as familiar phenomenological TTAM potential. Here, the parameters are determined using a genetic algorithm with force data obtained directly from a quantum calculation. The resulting potential gives excellent agreement with properties of the reference quantum calculations both for structure (bond lengths, bond angles) and elasticity (Young's modulus). The proposed method for constructing the classical potential is carried out for two different choices for the quantum mechanical description: a transfer Hamiltonian method (NDDO with coupled cluster parameterization) and density functional theory (with plane wave basis set and PBE exchange correlation functional). The quality of the potentials obtained in both cases is quite good, although the two quantum rods have significant differences.Comment: 24 pages, 7 figures (submitted to Journal of Molecular Simulations