Charles H. Patterson

Role of defects in ferromagnetism in Zn1-xCoxO: A hybrid density functional study

Experimental studies of Zn1-xCoxO as thin films or nanocrystals have found ferromagnetism and Curie temperatures above room temperature and that p- or n-type doping of Zn1-xCoxO can change its magnetic state. Bulk Zn1-xCoxO with a low defect density and x in the range used in experimental thin film studies exhibits ferromagnetism only at very low temperatures. Therefore defects in thin film samples or nanocrystals may play an important role in promoting magnetic interactions between Co ions in Zn1-xCoxO. The electronic structures of Co substituted for Zn in ZnO, Zn and O vacancies, substituted N and interstitial Zn in ZnO were calculated using the B3LYP hybrid density functional in a supercell. The B3LYP functional predicts a band gap of 3.34 eV for bulk ZnO, close to the experimental value of 3.47 eV. Occupied minority spin Co 3d levels are at the top of the valence band and unoccupied levels lie above the conduction band minimum. Majority spin Co 3d levels hybridize strongly with bulk ZnO states. The neutral O vacancy and interstitial Zn are deep and shallow donors, respectively. The Zn vacancy is a deep acceptor and the acceptor level for substituted N is at mid gap. The possibility that p- or n-type dopants promote exchange coupling of Co ions was investigated by computing total energies of magnetic states of ZnO supercells containing two Co ions and an oxygen vacancy, substituted N or interstitial Zn in various charge states. The neutral N defect and the singly-positively charged O vacancy are the only defects which strongly promote ferromagnetic exchange coupling of Co ions at intermediate range.

Soft-x-ray spectroscopic investigation of ferromagnetic Co-doped ZnO

The electronic properties of cobalt-doped ZnO were investigated through site-selective and element-sensitive x-ray-absorption spectroscopy in the vicinity of the Co L2,3 edge, the oxygen K edge, and at the Zn L3 edge. The spectroscopic measurements of the ferromagnetic cobalt-doped ZnO films appear to have additional components in the O K edge x-ray-absorption spectrum not observed in the undoped films. The observed features may derive from both hybridization with unoccupied Co 3d states and also from lattice defects such as oxygen vacancies. Only minor changes in the Zn L3 edge spectra were observed. These observations are consistent with a polaron percolation model in which the ferromagnetic coupling is mediated by shallow donor electrons trapped in oxygen vacancies and couples the Co atoms substituted on Zn sites in the hexagonal wurtzite ZnO structure.

Molecular pathways in the cyclotrimerisation of acetylene on Pd(111): Vibrational spectra of the C4H4 intermediate and its thermal decomposition products

Abstract HREELS and XPS have been used to study a hydrocarbon surface species, C 4 H 4 , which can be isolated on the Pd(111) surface at 150 K or 300 K by dissociative chemisorption of C 4 H 4 Cl 4 (cis-3,4-dichlorocyclobutene). The C 4 H 4 hydrocarbon is deduced to be a metallacycle and is a key intermediate in the formation of benzene from acetylene on Pd(111) and can also lead to ethylene formation at higher temperatures: it transforms to another species between 250 and 350 K, possibly vinylidene. Above 400 K, CH is the only hydrocarbon surface species detectable. HREEL spectra of the three hydrocarbon species are discussed in detail and compared to IR spectra of model compounds.

K-CO on Transition Metals: A Local Ionic Interaction.

We present a model for the bonding of CO adsorbed on alkali‐promoted surfaces that accounts for differences in experimental observations between the singly and coadsorbed systems. Theoretical evidence indicates that the interaction between the alkali metal and the CO on the surface is local and dominantly an ionic one, namely, the alkali atom transfers its electron to the CO, resulting in a large Coulombic interaction. This conclusion is supported by the results of generalized valence‐bond calculations and qualitatively accounts for the unusual behavior in thermal desorption, electron energy‐loss spectroscopy, Auger, and core and valence photoemission in the coadsorbed system.

A simulation study of the kinetics of passage of CO2 and N2 through the liquid/vapor interface of water

The rate of passage of molecules of carbon dioxide and nitrogen through the vapor–liquid interface of water at 300 K is studied by simulation. Previous work has established the form of the free energy profile which has a minimum when the solute molecule is on the surface and a barrier between this state and solution in the bulk liquid. In one set of simulations, trajectories were initiated in the gas phase. From these, the average lifetime of molecules in the surface is determined to be considerably longer than the inverse of the energy relaxation rate, so that the sticking coefficient is one and exiting molecules have no memory of their original velocities. However, most molecules do return to the gas phase rather than entering the bulk solution. The rate of passage of molecules over the free energy barrier is studied using the reactive flux method with trajectories initiated near the top of the barrier. The results for nitrogen, in particular, give a good plateau in the time-dependent transmission coeff...

The passage of gases through the liquid water/vapour interface: a simulation study

We have used atomistic simulation to measure the free energy profiles for a number of molecules crossing the air/liquid water interface at room temperature. The principal molecules studied were CO2 and N2, both common molecules with quadrupole moments; in addition some results were obtained for CH3CN, an example of a dipolar molecule, and Ar which has no electrostatic interaction with water. Our aim is to establish these profiles in order to provide a foundation for understanding the kinetics of gas uptake across the vapour/liquid interface and to understand them in terms of local structure at the molecular level. We found that there is a free energy minimum corresponding to a surface-adsorbed site in each case and that for N2 and CO2 there is a free energy barrier to passage from the bulk solution to the surface-adsorbed site. We discuss these results in terms of the local structure and in relation to some theoretical models of gas uptake.

Long bonds in silicon clusters: a failure of conventional Møller—Plesset perturbation theory?

Abstract On the basis of generalized valence bond calculations we propose a new type of structure for the Si 10 cluster. This structure is found to have large intra-pair correlation effects, suggesting the inadequacy of a single determinant reference wave function in conjunction with Moller—Plesset perturbation theory for determining the relative energetics of silicon clusters. The bonding in Si 10 is analyzed and the origin of the large intra-pair correlation effects is found to be four “long bonds” in the new Si 10 geometry.

The binding site of CO on NiAl(110) determined by low energy ion scattering

Abstract Using low energy ion scattering (LEIS) we have found that CO adsorbs in on-top Ni sites of the ordered NiAl(110) surface at 100 K. On the clean surface an outward relaxation of Al atoms relative to Ni atoms in the first layer of the solid, which had previously been measured by LEED, MEIS and LEIS was observed using LEIS in the impact collision ion scattering spectroscopy (ICISS) mode. This relaxation is attenuated when CO is adsorbed. 1.9 keV He + and 5 keV Ne + beams were employed in the time-of-flight (TOF) technique which acquires a spectrum with a sufficiently small ion dose to damage only a negligible fraction of the surface. Experimental requirements for He scattering in the TOP mode are discussed, viz., the use of channeling or low-incidence angles to suppress the deep layer scattering yield.

Transition levels of defects in ZnO: Total energy and Janak's theorem methods

Transition levels of defects are commonly calculated using either methods based on total energies of defects in relevant charge states or energy band single particle eigenvalues. The former method requires calculation of total energies of charged, perfect bulk supercells, as well as charged defect supercells, to obtain defect formation energies for various charge states. The latter method depends on Janaks theorem to obtain differences in defect formation energies for various charge states. Transition levels of V(Zn), V(O), and V(ZnO) vacancy defects in ZnO are calculated using both methods. The mean absolute deviation in transition level calculated using either method is 0.3 eV. Relative computational costs and accuracies of the methods are discussed.

Competing crystal structures in La0.5Ca0.5MnO3 : Conventional charge order versus Zener polarons

Equilibrium crystal structures for