Joel D. Kress

Controlling charge injection in organic electronic devices using self-assembled monolayers

We demonstrate control and improvement of charge injection in organic electronic devices by utilizing self-assembled monolayers (SAMs) to manipulate the Schottky energy barrier between a metal electrode and the organic electronic material. Hole injection from Cu electrodes into the electroluminescent conjugated polymer poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] was varied by using two conjugated-thiol based SAMs. The chemically modified electrodes were incorporated in organic diode structures and changes in the metal/polymer Schottky energy barriers and current–voltage characteristics were measured. Decreasing (increasing) the Schottky energy barrier improves (degrades) charge injection into the polymer.

The Hydration Number of Li+ in Liquid Water

The hydration of ions in water is not only fundamental to physical chemistry, but is also relevant to the current issue of selectivity of biological ion channels. In the context of potassium channels for example, the free energies for replacement of inner shell water ligands with peptide carbonyls donated by proteins of the channel, specifically for the preference of K{sup +} over Na{sup +}. Studies to elucidate the thermodynamic features of such inner shell exchange reactions require prior knowledge of the ion hydration structures and energetics. Simulations have produced a range of results including both four and six inner shell water neighbors with considerable statistical dispersion. Simulations are typically not designed to provide sole determinations of such properties, although they do shed light on the issues determining the hydration number of ions in water. The theoretical scheme used here to address these problems for the Li{sup +}(aq) ion is based upon the quasi-chemical organization of solution theory, which is naturally suited to these problems.

Free energy of liquid water on the basis of quasichemical theory and ab initio molecular dynamics

We use ab initio molecular dynamics as a basis for quasichemical theory evaluation of the free energy of water near conventional liquid thermodynamic states. The Perdew-Wang-91 (PW91), Perdew-Burke-Ernzerhof (PBE), and revised PBE (rPBE) functionals are employed. The oxygen radial density distribution using the rPBE functional is in reasonable agreement with current experiments, whereas the PW91 and PBE functionals predict a more structured oxygen radial density distribution. The diffusion coefficient with the rPBE functional is in reasonable accord with experiments. Using a maximum entropy procedure, we obtain x0 from the coordination number distribution xn for oxygen atoms having n neighbors. Likewise, we obtain p0 from pn, the probability of observing cavities of specified radius containing n water molecules. The probability x0 is a measure of the local chemical interactions and is central to the quasichemical theory of solutions. The probability p0, central to the theory of liquids, is a measure of the free energy required to open cavities of defined sizes in the solvent. Using these values and a reasonable model for electrostatic and dispersion effects, the hydration free energy of water in water at 314 K is calculated to be -5.1 kcal/mole with the rPBE functional, in encouraging agreement with the experimental value of -6.1 kcal/mole.

Quantum reactive scattering in three dimensions using hyperspherical (APH) coordinates. IV : Discrete variable representation (DVR) basis functions and the analysis of accurate results for F+H2

Accurate 3D coupled channel calculations for total angular momentum J=0 for the reaction F+H2→HF+H using a realistic potential energy surface are analyzed. The reactive scattering is formulated using the hyperspherical (APH) coordinates of Pack and Parker. The adiabatic basis functions are generated quite efficiently using the discrete variable representation method. Reaction probabilities for relative collision energies of up to 17.4 kcal/mol are presented. To aid in the interpretation of the resonances and quantum structure observed in the calculated reaction probabilities, we analyze the phases of the S matrix transition elements, Argand diagrams, time delays and eigenlifetimes of the collision lifetime matrix. Collinear (1D) and reduced dimensional 3D bending corrected rotating linear model (BCRLM) calculations are presented and compared with the accurate 3D calculations.

Highly optimized empirical potential model of silicon

We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters to a large database using the force-matching method. Our model provides a reasonable description of energetics for all atomic coordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). It accurately reproduces phonons and elastic constants, as well as point defect energetics. It also provides a good description of reconstruction energetics for both the 30° and 90° partial dislocations. Unlike previous models, our model accurately predicts formation energies and geometries of interstitial complexes - small clusters, interstitial-chain and planar {311} defects.

Molecular dynamics simulation of reactive ion etching of Si by energetic Cl ions

We report results from molecular dynamics simulations of the etching of a Si surface by energetic Cl atoms (15 eV⩽E⩽200 eV). We find that the energy dependence of the Si yield (number of Si atoms desorbed per incident Cl ion) is in reasonable agreement with recent experiments and with previous simulations performed up to 50 eV. We also investigate the variation of the Si yield with the impact angle of incidence, the stoichiometry of the desorbed material, and the effect of a thermal background Cl flux to the surface in the presence of an ion flux at 50 eV. Surface roughening due to etching was observed and the calculated rms roughness is in reasonable agreement with experiments.

Determination of the headgroup-gold(111) potential surface for alkanethiol self-assembled monolayers by ab initio calculation

Abstract We present a realistic empirical potential function to model the head-group interaction for self-assembled monolayers (SAMs) of alkanethiols on Au(111). The potential function is fit to data obtained by ab initio geometry optimization of SCH 3 on Au(111) clusters. The principal result from our calculations is that barriers within the surface corrugation potential are too small to pin S atoms at any particular site. We note that simulations of alkanethiol/gold systems that employ a model precluding lateral movement of S cannot reproduce all dynamical behavior of the SAM.

Accurate three‐dimensional quantum scattering studies of long‐lived resonances for the reaction He+H+2→HeH++H

The adiabatically adjusting principal‐axis hyperspherical (APH) formulation of Pack and Parker for quantum reactive scattering in three dimensions (3D) is used to obtain converged results for the reaction of helium with H+2 (v=1–4) for total angular momentum J=0. The ab initio potential energy surface computed by McLaughlin and Thompson and fitted by Joseph and Sathyamurthy is utilized for the HeH+2 interaction potential. The predicted energy dependence of the accurate 3D state‐to‐state reaction probabilities show clear evidence for quantum resonances. These resonances are even more numerous than those reported earlier for reduced dimensionality studies of this reaction. The calculated time delays for several of these resonances are found to be over 1 ps. Bending corrected rotating linear model (BCRLM) studies of this same reaction are also reported. These results provide useful insight in sorting out the nature and contribution of the resonances found in the 3D studies.

Evolution of ultracold neutral plasmas.

We present the first large-scale simulations of an ultracold neutral plasma, produced by photoionization of laser-cooled xenon atoms, from creation to initial expansion, using classical molecular-dynamics methods with open boundary conditions. We reproduce many of the experimental findings such as the trapping efficiency of electrons with increased ion number, a minimum electron temperature achieved on approach to the photoionization threshold, and recombination into Rydberg states of an anomalously low principal quantum number. In addition, many of these effects establish themselves very early in the plasma evolution ( similar ns) before the present experimental observations begin.

Quantum effects in the F+H2→HF+H reaction. Accurate 3D calculations with a realistic potential energy surface

Abstract We report accurate benchmark 3D coupled channel calculations for total angular momentum J =0 for the reaction F+H 2 →HF+H using a realistic potential energy surface. The adiabatic basis functions are generated using the discrete variable representation method. The resulting reaction probabilities show what appear to be strong quantum resonance features as well as rapid changes in final rotational state distributions.