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Michael Nolan

Publications and source records attributed to Michael Nolan.

6 recordsLinked to original sources
PubMed2007

Silicon nanowire band gap modification.

Band gap modification for small-diameter (approximately 1 nm) silicon nanowires resulting from the use of different species for surface termination is investigated by density functional theory calculations. Because of quantum confinement, small-diameter wires exhibit a direct band gap that increases as the wire diameter narrows, irrespective of surface termination. This effect has been observed in previous experimental and theoretical studies for hydrogenated wires. For a fixed cross-section, the functional group used to saturate the silicon surface significantly modifies the band gap, resulting in relative energy shifts of up to an electronvolt. The band gap shifts are traced to details of the hybridization between the silicon valence band and the frontier orbitals of the terminating group, which is in competition with quantum confinement.

Molecular Structure↗
PubMed2006

Hole localization in Al doped silica: A DFT + U description.

Despite density functional theory (DFT) being the most widely used ab initio approach for studying the properties of oxide materials, the modeling of localized hole states in doped or defective oxides can be a challenge. The electronic hole formed when silica is doped with aluminum is such a defect, for which a DFT description of the atomic and electronic structures has previously been found to be inconsistent with experiment, while Hartree-Fock provides a consistent description. We have applied the DFT + U approach to this problem and find that the structural distortions around the dopant are consistent with experimental data as well as earlier cluster calculations using Hartree-Fock and perturbation theory. A hole state is found 1.1 eV (1.6 eV experimentally) above the top of the valence band with localization of spin on the oxygen atom which shows the elongated Al-O distance. A formation energy of 5.7 eV is found. We discuss implications for using DFT+U to model defective oxide systems with O 2p holes.

Journal Article↗
PubMed2006

The surface dependence of CO adsorption on Ceria.

An understanding of the interaction between ceria and environmentally sensitive molecules is vital for developing its role in catalysis. We present the structure and energetics of CO adsorbed onto stoichiometric (111), (110), and (100) surfaces of ceria from first principles density functional theory corrected for on-site Coulomb interactions, DFT+U. DFT+U is applied because it can describe consistently the properties of both the stoichiometric and reduced surfaces. Our major finding is that the interaction is strongly surface dependent, consistent with experiment. Upon interaction of CO with the (111) surface, weak binding is found, with little perturbation to the surface or the molecule. For the (110) and (100) surfaces, the most stable adsorbate is that in which the CO molecule bridges two oxygen atoms and pulls these atoms out of their lattice sites, with formation of a (CO(3)) species. This results in a strong modification to the surface structure, consistent with that resulting from mild reduction. The electronic structure also demonstrates reduction of the ceria surface and consequent localization of charge on cerium atoms neighboring the vacancy sites. The surface-bound (CO(3)) species is identified as a carbonate, (CO(3))(2-) group, which is formed along with two reduced surface Ce(III) ions, in good agreement with experimental infrared data. These results provide a detailed investigation of the interactions involved in the adsorption of CO on ceria surfaces, allowing a rationalization of experimental findings and demonstrate further the applicability of the DFT+U approach to the study of systems in which reduced ceria surfaces play a role.

Journal Article↗
PubMed2006

Reduction of NO2 on ceria surfaces.

Cerium dioxide, CeO2, plays an important role in catalysis, due to its ability to store and release oxygen depending on the conditions present in the catalyst environment. To understand the role of ceria in catalytic reactions, we need to study the details of the interaction of ceria surfaces with environmentally sensitive molecules. In this work, we examine the adsorption of the NO2 molecule onto defective (reduced) surfaces of ceria using density functional theory with a correction for on-site Coulomb interactions (DFT+U), which allows for a consistent description of pure and reduced ceria. The interaction of NO2 with defective (111), (110), and (100) surfaces gives an adsorbate-surface structure in which the bond lengths around one Ce(III) ion from the reduced surface shorten, while one N-O bond lengthens. Analysis of the electronic structure and spin density distributions demonstrates that one Ce(III) has been reoxidized to Ce(IV), with the formation of adsorbed NO2-. Finally, we discuss the energetics of the interaction of NO2 with ceria.

Journal Article↗
PubMed2005

CeO2 catalysed conversion of CO, NO2 and NO from first principles energetics.

First principles calculations using density functional theory with corrections for on-site Coulomb interactions (DFT + U) are presented in which we compute the energy for the conversion of CO to CO(2), NO(2) to NO and NO to N(2) over ceria surfaces. The surface sensitivity is discussed on the basis of the vacancy formation energies.

Carbon Monoxide↗