Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals.

Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn(2+), Co(2+) and so on) couple to band carriers via strong sp-d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit.

Unrestricted absolutely localized molecular orbitals for energy decomposition analysis: theory and applications to intermolecular interactions involving radicals.

Radical-closed shell and radical-radical intermolecular interactions are less well-understood than those between closed shell species. With the objective of gaining additional insight, this work reports a generalization of the absolutely localized molecular orbital (ALMO) energy decomposition analysis (EDA) to open shell fragments, described by self-consistent field methods, such as standard density functional theory. The ALMO-EDA variationally partitions an intermolecular interaction energy into three separate contributions; frozen orbital interactions, polarization, and charge transfer.

Multiphoton photoelectron emission microscopy of single Au nanorods: combined experimental and theoretical study of rod morphology and dielectric environment on localized surface plasmon resonances.

Multiphoton photoelectron emission from individual Au nanorods deposited on indium tin oxide (ITO) substrates is studied via scanning photoionization microscopy, based on femtosecond laser excitation at frequencies near the rod longitudinal surface plasmon resonance (LSPR). The observed resonances in photoemission correlate strongly with plasmon resonances measured in dark field microscopy (DFM), thus establishing a novel scheme for wavelength-resolved study of plasmons in isolated metallic nanoparticles based on highly sensitive electron counting methods. In this work, we explore experimental and theoretical effects of (i) morphology and (ii) aspect ratio (AR) for longitudinal plasmon resonance behavior in Au nanorods.

Coherent multiphoton photoelectron emission from single au nanorods: the critical role of plasmonic electric near-field enhancement.

Electron emission from individual Au nanorods deposited on indium-tin-oxide (ITO) following excitation with femtosecond laser pulses near the rod longitudinal plasmon resonance is studied via scanning photoionization microscopy. The measured electron signal is observed to strongly depend on the excitation laser polarization and wavelength. Correlated secondary electron microscopy (SEM) and dark-field microscopy (DFM) studies of the same nanorods unambiguously confirm that maximum electron emission results from (i) laser polarization aligned with the rod long axis and (ii) laser wavelength resonant with the localized surface plasmon resonance.

On the nature of electron correlation in C60.

The ground state restricted Hartree Fock (RHF) wave function of C(60) is found to be unstable with respect to spin symmetry breaking, and further minimization leads to a significantly spin contaminated unrestricted Hartree Fock (UHF) solution ( = 7.5, 9.6 for singlet and triplet, respectively).

Polarization-dependent scanning photoionization microscopy: ultrafast plasmon-mediated electron ejection dynamics in single Au nanorods.

This work investigates plasmon-enhanced multiphoton scanning photoelectron emission microscopy (SPIM) of single gold nanorods under vacuum conditions. Striking differences in their photoemission properties are observed for nanorods deposited either on 2 nm thick Pt films or 10 nm thick indium tin oxide (ITO) films. On a Pt support, the Au nanorods display fourth-order photoionization when excited at 800 nm, a wavelength corresponding to their plasmon resonance in aqueous solution.

The mystery of gold's chemical activity: local bonding, morphology and reactivity of atomic oxygen.

Recently, gold has been intensely studied as a catalyst for key synthetic reactions. Gold is an attractive catalyst because, surprisingly, it is highly active and very selective for partial oxidation processes suggesting promise for energy-efficient "green" chemistry. The underlying origin of the high activity of Au is a controversial subject since metallic gold is commonly thought to be inert.

Modeling the charge transfer between alkali metals and polycyclic aromatic hydrocarbons using electronic structure methods.

The interaction of alkali metals-specifically, lithium-with polycyclic aromatic hydrocarbons (PAHs) was studied using a variety of electronic structure methods. Electron transfer from lithium to a PAH depends on the size and structure of the PAH and the electronic structure method used. In some cases, we observe an artificial transfer when using density functional theory (DFT) due to the self-interaction error, whereas Hartree-Fock underestimates the amount of charge transfer due to overlocalization.

Effect of molecular structure on epoxidation of allylic olefins by atomic oxygen on Au.

The reactions of three isomers of phenyl-substituted propene, trans-beta-methylstyrene, alpha-methylstyrene, and allylbenzene, with atomic oxygen adsorbed on Au(111) have been investigated in order to probe the effect of molecular structure on the selectivity for competing oxidation pathways. Temperature programmed reaction experiments show that these three isomers share common features in their reaction patterns, providing direct observation that both allylic C-H activation and epoxidation are important reaction pathways that compete with each other. The pathways observed for reaction are (1) oxygen addition to the C=C bond to yield the respective epoxides; (2) allylic C-H activation including oxygen insertion into C-H bonds to form ketone, aldehyde, or acid and C-H dissociation to initiate combustion and carbon deposition; and, (3) nucleophilic attack of the electron-deficient carbon that is adjacent to the phenyl ring to produce benzoic acid.

Local Bonding Effects in the Oxidation of CO on Oxygen-Covered Au(111) from Ab Initio Molecular Dynamics Simulations.

A fully dynamical approach using ab initio molecular dynamics (AIMD) simulations is applied to the investigation of CO oxidation on O-covered Au(111). We investigate how the activity of gold depends upon temperature, oxygen coverage, and surface structure. On clean Au(111) at 500 K, CO binds transiently on top of Au atoms, spending a small fraction (∼7%) of the total simulation time adsorbed on the surface.

Effects of chlorine and oxygen coverage on the structure of the Au(111) surface.

We investigate the effects of Cl and O coverage on the atomic structure of the Au(111) surface using density functional theory calculations. We find that the release and incorporation of gold atoms in the adsorbate layer becomes energetically favorable only at high coverages of either O or Cl (>0.66 ML (monolayer) for O and >0.

Chlorine interaction with defects on the Au(111) surface: a first-principles theoretical investigation.

Chlorine is an important element in promoting oxidation on noble metal surfaces. Here, we report a comprehensive theoretical study of chlorine interaction with defects on the Au(111) surface, using density functional theory calculations and periodic slabs to model the surface. We find that chlorine binds preferentially on steps, vacancies, and gold adatoms.

Polyethylene glycol-based homologated ligands for nicotinic acetylcholine receptors.

A homologous series of polyethylene glycol (PEG) monomethyl ethers were conjugated with three ligand series for nicotinic acetylcholine receptors. Conjugates of acetylaminocholine, the cyclic analog 1-acetyl-4,4-dimethylpiperazinium, and pyridyl ether A-84543 were prepared. Each series was found to retain significant affinity at nicotinic receptors in rat cerebral cortex with tethers of up to six PEG units.

Nature of Cl bonding on the Au(111) surface: evidence of a mainly covalent interaction.

We report theoretical evidence from first-principles density functional theory (DFT) calculations that the bonding between Cl and the Au(111) surface is primarily covalent in character, which is in contrast to the generally held view that the bonding of halogens to metal surfaces is ionic. We observe the transfer of charge density into the region between interacting Au and Cl atoms, which would not be expected in the case of Cl- anion formation (symmetric charge accumulation on Cl). Importantly, we also find a clear directionality of dz2 orbitals of the Au atoms pointing to the adsorbed Cl and the mixing of electronic states between the gold surface and the adsorbed chlorine.

Chlorine adsorption on Au(111): chlorine overlayer or surface chloride?

We report the first scanning tunneling microscope (STM) investigation, combined with density functional theory calculations, to resolve controversy regarding the bonding and structure of chlorine adsorbed on Au(111). STM experiments are carried out at 120 K to overcome instability caused by mobile species upon chlorine adsorption at room temperature. Chlorine adsorption initially lifts the herringbone reconstruction.

Imaging nanostructures with scanning photoionization microscopy.

We report detailed studies of local electronic properties in nanostructured thin metallic films using scanning photoionization microscopy. This novel form of microscopy combines the advantages of diffraction-limited optical excitation with the ability to detect both photons and low kinetic energy photoelectrons, permitting sensitive characterization of heterogeneous surfaces under vacuum conditions. Using this technique, correlated measurements of multiphoton photoemission cross section and optical penetration depth are reported for Au films supported on Pt.

Geometry of simple molecules. 2. Modeling the geometry of AX3E and AX2E2 molecules through the nonbonded interaction (NBI) model.

A new conceptual model of molecular geometry is presented, called the nonbonded interaction (NBI) model. This model is applied to the geometries of the AX3E and AX2E2 (A = N, O, P, S, As, Se, or Te; X = H, F, Cl, Br, I, CH(3), tBu, CF3, SiH3, Sn(tBu)3, or SnPh3) molecule types. For these molecules, the NBI model can be quantified on the basis of a balance between terminal atom-terminal atom (X-X) interactions and lone pair-terminal atom (E-X) interactions.