Publications

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Experimental studies have been performed on the velocity-field characteristics of AlGaN/GaN heterostructures. A pulsed voltage input in combination with a four-point measurement was used in a 50 {Omega} environment to determinethe drift velocity of electrons in the two-dimensional electron gas as a function of the applied electric field. These measurements show an apparent saturation velocity near 3.1 x 10{sub 7} cm/s, at a field of 140 kV/cm. A comparison of these studies shows that the experimental velocities are close to previously published simulations based upon Monte Carlo techniques.

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Nonuniformities in both sheath electric field and plasma excitation were observed around dissimilar metals placed on a rf electrode. Spatial maps of the rf sheath electric field obtained by laser-induced fluorescence-dip (LIF-dip) spectroscopy show that the sheath structure was a function of the electrode metal. In addition to the electric-field measurements, LIF, optical emission, and Langmuir probe measurements show nonuniform excitation around the dissimilar metals. The degree and spatial extent of the discharge nonuniformities were dependent on discharge conditions and the history of the metal surfaces.

We use quantum mechanics to characterize the structure and current-voltage performance of the Stoddart-Heath rotaxane-based programmable electronic switch. We find that the current when the ring is on the DNP is 37?58 times the current when the ring is on the TTF, in agreement with experiment (ratio of 10?100). This establishes the basis for iterative experimental?theoretical efforts to optimize systems for molecule-based electronics which we illustrate by predicting the effect of adding a group such as CN to the rotaxane.

The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac){sub 2}(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated {Delta}H(0 K){sub {+-}} = 35.1 kcal/mol and {Delta}G(298 K){sub {+-}} = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C?H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated {Delta}H(0 K){sub {+-}} = 27.0 kcal/mol and {Delta}G(298 K){sub {+-}} = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C?H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir{sup V}, with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of {beta}-hydride products is due to kinetic factors, although {beta}-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.

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