Plasmons in strongly coupled metallic nanostructures.

Alkali (Li+, Na+, K+, Rb+, and Cs+) and halide (F−, Cl−, Br−, and I−) ions play an important role in many biological phenomena, roles that range from stabilization of biomolecular structure, to influence on biomolecular dynamics, to key physiological influence on homeostasis and signaling. To properly model ionic interaction and stability in atomistic simulations of biomolecular structure, dynamics, folding, catalysis, and function, an accurate model or representation of the monovalent ions is critically necessary. A good model needs to simultaneously reproduce many properties of ions, including their structure, dynamics, solvation, and moreover both the interactions of these ions with each other in the crystal and in solution and the interactions of ions with other molecules. At present, the best force fields for biomolecules employ a simple additive, nonpolarizable, and pairwise potential for atomic interaction. In this work, we describe our efforts to build better models of the monovalent ions within t...

Determination of Alkali and Halide Monovalent Ion Parameters for Use in Explicitly Solvated Biomolecular Simulations

Molecular conductance junctions are structures in which single molecules or small groups of molecules conduct electrical current between two electrodes. In such junctions, the connection between the molecule and the electrodes greatly affects the current-voltage characteristics. Despite several experimental and theoretical advances, including the understanding of simple systems, there is still limited correspondence between experimental and theoretical studies of these systems.

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Electron transport in molecular wire junctions.

A surface science perspective on TiO2 photocatalysis

Quantum plasmonics is a rapidly growing field of research that involves the study of the quantum properties of light and its interaction with matter at the nanoscale Here, surface plasmons - electromagnetic excitations coupled to electron charge density waves on metal-dielectric interfaces or localized on metallic nanostructures - enable the confinement of light to scales far below that of conventional optics In this article we review recent progress in the experimental and theoretical investigation of the quantum properties of surface plasmons, their role in controlling light-matter interactions at the quantum level and potential applications Quantum plasmonics opens up a new frontier in the study of the fundamental physics of surface plasmons and the realization of quantum-controlled devices, including single-photon sources, transistors and ultra-compact circuitry at the nanoscale

Quantum Plasmonics

The tunneling current from a scanning tunneling microscope was used to image and dissociate single ${\mathrm{O}}_{2}$ molecules on the Pt(111) surface in the temperature range of 40 to 150 K. After dissociation, the two oxygen atoms are found one to three lattice constants apart. The dissociation rate as a function of current was found to vary as ${I}^{0.8\ifmmode\pm\else\textpm\fi{}0.2}$, ${I}^{1.8\ifmmode\pm\else\textpm\fi{}0.2}$, and ${I}^{2.9\ifmmode\pm\else\textpm\fi{}0.3}$ for sample biases of 0.4, 0.3, and 0.2 V, respectively. These rates are explained using a general model for dissociation induced by intramolecular vibrational excitations via resonant inelastic electron tunneling.

Single-Molecule Dissociation by Tunneling Electrons

We present a density functional theory study of water adsorption on metal surfaces. Prototype water structures including monomers, clusters, one-dimensional chains, and overlayers have been investigated in detail on a model system-a Pt(111) surface. The structure, energetics, and vibrational spectra are all obtained and compared with available experimental data. This study is further extended to other metal surfaces including Ru(0001), Rh(111), Pd(111), and Au(111), where adsorption of monomers and bilayers has been investigated. From these studies, a general picture has emerged regarding the water-surface interaction, the interwater hydrogen bonding, and the wetting order of the metal surfaces. The water-surface interaction is dominated by the lone pair-d band coupling through the surface states. It is rather localized in the contacting layer. A simultaneous enhancement of hydrogen bonding is generally observed in many adsorbed structures. Some special issues such as the partial dissociation of water on Ru(0001) and in the RT39 bilayer phase, the H-up and H-down conversion, and the quantum-mechanical motions of H atoms are also discussed.

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Water adsorption on metal surfaces: A general picture from density functional theory studies

The adsorption of water on Pt(111) surface has been studied with ab initio molecular dynamics simulation. Both the energetics and vibrational dynamics indicate the existence of a well-ordered molecular bilayer on this surface. This conclusion is in contrast to the recent result of water on Ru(0001) surface, but agrees with available experiments. In addition, our calculation identifies two different hydrogen bonds in the bilayer. Both can be directly recognized from the vibrational spectra of the OH stretch modes.

Vibrational recognition of hydrogen-bonded water networks on a metal surface

The existence and nature of end and central plasmon resonances in a linear atomic chain, the 1D analog to surface and bulk plasmons in 2D metals, has been predicted by ab initio time-dependent density functional theory. Length dependence of the absorption spectra shows the emergence and development of collectivity of these resonances. It converges to a single resonance in the longitudinal mode, and two transverse resonances, which are localized at the ends and center of the atom chains. These collective modes bridge the gaps, in concept and scale, between the collective excitation of atomic physics and nanoplasmonics. It also outlines a route to atomic-scale engineering of collective excitations.

End and central plasmon resonances in linear atomic chains.

Electronic excitations in linear atomic chains of simple and noble metals (silver) have been studied using time-dependent density-functional theory. The formation and development of collective resonances in the absorption spectra were obtained as functions of the chain length. A longitudinal collective resonance appears in both simple- and noble-metal chains. Its dispersion has been deduced and is compared with that of a one-dimensional electron gas. The transverse excitation generally shows a bimodal structure, which can be assigned as the ``end and central resonances.'' The $d$ electrons of silver atoms reduce both the energies and intensities of the transverse modes but have little effect on its longitudinal resonance. This anisotropic screening is determined by the interband $(d\ensuremath{\rightarrow}p)$ transition, which is involved only in transverse oscillations. Analysis of these results yields a general picture of plasmon resonances in one-dimensional atomic structures. Implications of such atomic-scale plasmons to surface plasmons in larger dimensions are also discussed.

Shiwu Gao

Papers

Single-Molecule Dissociation by Tunneling Electrons

Water adsorption on metal surfaces: A general picture from density functional theory studies

Vibrational recognition of hydrogen-bonded water networks on a metal surface

End and central plasmon resonances in linear atomic chains.

Plasmon resonances in linear atomic chains: Free-electron behavior and anisotropic screening of d electrons