Tight-binding simulation of current-carrying nanostructures

The tight-binding (TB) approach to the modelling of electrical conduction in small structures is introduced. Different equivalent forms of the TB expression for the electrical current in a nanoscale junction are derived. The use of the formalism to calculate the current density and local potential is illustrated by model examples. A first-principles time-dependent TB formalism for calculating current-induced forces and the dynamical response of atoms is presented. An earlier expression for current-induced forces under steady-state conditions is generalized beyond local charge neutrality and beyond orthogonal TB. Future directions in the modelling of power dissipation and local heating in nanoscale conductors are discussed.

Molecular Plasmonics with Tunable Exciton-Plasmon Coupling Strength in J-Aggregate Hybridized Au Nanorod Assemblies

Controlling coherent electromagnetic interactions in molecular systems is a problem of both fundamental interest and important applicative potential in the development of photonic and opto-electronic devices. The strength of these interactions determines both the absorption and emission properties of molecules coupled to nanostructures, effectively governing the optical properties of such a composite metamaterial. Here we report on the observation of strong coupling between a plasmon supported by an assembly of oriented gold nanorods (ANR) and a molecular exciton. We show that the coupling is easily engineered and is deterministic as both spatial and spectral overlap between the plasmonic structure and molecular aggregates are controlled. We think that these results in conjunction with the flexible geometry of the ANR are of potential significance to the development of plasmonic molecular devices.

Domains in three-dimensional ferroelectric nanostructures

We report the experimental measurement of domains in single- crystal nanocolumns of ferroelectric BaTiO3, together with a theory of domain size scaling in three- dimensional structures which explains the observations.

Recent advances in understanding CO oxidation on gold nanoparticles using density functional theory

Since the discovery of a series of Au-based catalysts by Haruta et al. considerable progress has been made in understanding the active role of Au in CO oxidation catalysis. This review provides a summary of recent theoretical work performed in this field; in particular it addresses DFT studies of CO oxidation catalysis over free and supported gold nanoparticles. Several properties of the Au particles have been found to contribute to their unique catalytic activity. Of these properties, the low-coordination state of the Au atoms is arguably the most pertinent, although other properties of the Au cluster atoms, such as electronic charge, cannot be ignored. The current consensuses regarding the mechanism for CO oxidation over Au-based catalysts is also discussed. Finally, water-enhanced catalysis of CO oxidation on Au clusters is summarized.

Voltammetric studies of gold, protons, and [HCl2](-) in ionic liquids

The comparative study of the voltammetry of H[NTf2], HCl and H[AuCl4] in [C(4)mim][NTf2] has provided an insight into the influence of protons on the reduction of [AuCl4](-) at Au, Pt or glassy carbon (GC) electrodes, and has allowed the identification of an unprecedented proton-induced electroless deposition of Au on relatively inert GC surfaces. For the first time, clear evidence of the quantitative formation of [HCl2](-) has been obtained in HCl/[C(4)mim][NTf2] mixtures, and the electrochemical behavior of these mixtures analyzed. In particular, a significant shift of the dissociation equilibrium toward the formation of chloride and the solvated proton (H-IL(+)), following electrochemical reduction of H-IL(+) has been observed in the time-scale of the experiments.

Electrochemical studies of gold and chloride in ionic liquids

For the first time, the electrochemistry of gold has been studied in detail in a 'second-generation', non-haloaluminate, ionic liquid. In particular, the electrochemical behaviour of Na[AuCl4] has been investigated in 1-butyl-3-methylimidazolium bis{(tifluoromethyl)sulfonyl} imide, [C(4)mim][NTf2], over gold, platinum and glassy carbon working electrodes. The reduction of [AuCl4](-) initially forms [AuCl2](-) before deposition on the electrode as Au(0). To enable stripping of deposited gold or electrodissolution of bulk gold, the presence of chloride, trichloride or chlorine is required. Specifically trichloride and chlorine have been identified as the active species which preferentially form Au(I) and Au(III), respectively.

Catalytic role of gold in gold-based catalysts: A density functional theory study on the CO oxidation on gold

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO2 has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O-2 dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O-2 dissociation on Au (O-2-->20(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O-2 on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O-2 can adsorb on the interface of Au/TiO2 with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O-2 dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O-2-->CO2+O, and CO+O-->CO2; and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.