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The electronic properties of Mg-O divacancy defects at the MgO surface obtained by removing of a pair of O and Mg ions from terrace, step, or comer sites have been investigated using an embedded cluster model. Long-range polarization and lattice relaxation effects have been included through a shell model approach. It is demonstrated that all these defects are electron traps: an addition of one electron to a neutral precursor results in a stable paramagnetic center. We calculate relaxed electron affinities, vertical ionization energies, formation energies, and hyperfine coupling constants of these defects and discuss their relevance for the interpretation of experimental results on the nature of paramagnetic electronic defects at the surface of MgO. These results further extend a concept of surface electron traps beyond simple anion vacancies to more general structural features. .... [ABSTRACT FROM AUTHOR]

We have studied the optical absorptions of a series of point defects in SiO[sub 2] by means of time-dependent density functional response theory (TD-DFT). The structure of the defects has been described with cluster models and atomic orbital basis functions. For each center the lowest singlet→singlet and singlet→triplet (for diamagnetic defects) and doublet→doublet (for paramagnetic defects) transitions have been considered. The results have been compared with accurate ab initio calculations based on explicit treatment of correlation effects and, when possible, with experimental data. Defects with localized wave functions and low excitation energies (<5 eV) are well described by TD-DFT and the predicted transition energies are within a few tenths of an eV from the experimental or ab initio values. For defects which give rise to transition energies higher than 5 eV the TD-DFT values are 10%–20% too low. The comparison of various exchange-correlation functionals show that the hybrid B3LYP method provides more accurate answers than other gradient-corrected DFT approaches. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The authors present an electrochemically controlled, drug releasing neural interface composed of a glassy carbon (GC) microelectrode array combined with a multilayer poly(3,4-ethylenedioxythiophene) (PEDOT) coating. The system integrates the high stability of the GC electrode substrate, ideal for electrical stimulation and electrochemical detection of neurotransmitters, with the on-demand drug-releasing capabilities of PEDOT-dexamethasone compound, through a mechanically stable interlayer of PEDOT-polystyrene sulfonate (PSS)-carbon nanotubes (CNT). The authors demonstrate that such interlayer improves both the mechanical and electrochemical properties of the neural interface, when compared with a single PEDOT-dexamethasone coating. Moreover, the multilayer coating is able to withstand 10×106 biphasic pulses and delamination test with negligible change to the impedance spectra. Cross-section scanning electron microscopy images support that the PEDOT-PSS-CNT interlayer significantly improves the adhesion between the GC substrate and PEDOT-dexamethasone coating, showing no discontinuities between the three well-interconnected layers. Furthermore, the multilayer coating has superior electrochemical properties, in terms of impedance and charge transfer capabilities as compared to a single layer of either PEDOT coating or the GC substrate alone. The authors verified the drug releasing capabilities of the PEDOT-dexamethasone layer when integrated into the multilayer interface through repeated stimulation protocols in vitro, and found a pharmacologically relevant release of dexamethasone. [ABSTRACT FROM AUTHOR]

Embedding methods specifically designed to treat large molecules with bulky ligands or in polar solvents are used to describe the electronic structure of point defects in the covalently bonded solids SiO[sub 2], Si[sub 3]N[sub 4], and Si[sub 2]N[sub 2]O. The mechanical relaxation of the lattice around a given defect, in particular an anion vacancy or interstitial, is described using the ONIOM approach where the system is partitioned in two regions, the local defect treated at the gradient corrected DFT level, and the surrounding matrix treated with a semiempirical Hamiltonian. In this way clusters of 100 atoms and more are used to describe a portion of the solid of 10-15 Å of diameter. The long-range lattice polarization induced by a charged defect, a charged oxygen vacancy or a proton bound to O or N atoms, is estimated by means of the isodensity polarized continuum model, IPCM, and compared with the approximate Born's formula. The two simplified embedding schemes provide a simple way to improve cluster models of neutral and charged defects in covalent materials. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]