FIRST-PRINCIPLES CALCULATIONS AND THERMODYNAMIC STUDY ON SURFACE REACTIVITY OF LIGHT METAL OXIDES AND FLUORIDES

Yu. Zhukovskii, E. Kotomin, and R. Eglitis,
D. Fuks (Ben Gurion University of the Negev, Ber Sheeva, Israel),
D.E. Ellis (Northwestern University, Evanston, Illinois, USA),
P. Balaya and J. Maier (MPI for Solid State Research, Stuttgart, Germany),
G. Borstel (Osnabrück University, Germany).


Understanding the metal adhesion and growth mode of thin metallic films is important for micro- and nanoelectronics. In collaboration with Northwestern University, Evanston and Ben Gurion University, Ber Sheeva, we have completed ab initio calculations (using hybrid B3LYP method) and thermodynamic study of copper and silver adhesion onto defective magnesia substrate. We have observed a strong change of the bonding between the metal adatoms and substrate in the vicinity of the surface Fs centers (neutral O vacancies), which affects the thermodynamic conditions and the morphology of the growing metallic layer. For a perfect magnesia surface we have confirmed the experimentally observed submonolayer growth of metallic islands (Ag possesses a higher trend to such 3D adatom aggregation than Cu). However, the surface Fs centers weaken the trend toward metal atom aggregation and above some critical s urface concentration lead to formation of disordered 2D metallic films; i.e., the island formation mode is changed for the layer-by-layer growth mode. For example, for cF = 0.06 ML, copper adatoms form the continuous series of disordered solid solutions (uniform metallic film) at all atomic fractions of Cu and the temperatures T ≥ 846 K, whereas for cF = 0.09 ML the same occurs already at the room temperature. For silver films, the effect of the Fs centers is much less pronounced: substantially higher atomic fraction of defects (at least 35-40 per cent) is needed for the growth of uniform Ag film.

Ab initio calculations on H2O and O2 molecules adsorbed on different Al2O3 substrates, namely the Al terminated (0001) surface of crystalline corundum, and amorphous-like (Al2O3)n clusters with n=2-7 formula units, have been performed in collaboration with Osnabrück University. Two types of first-principles computer codes, CRYSTAL and SIESTA, have been used for the calculations on periodic slabs and clusters, respectively. We have also performed complementary research of adsorption and dissociation of water on (Al2O3)n amorphous-like clusters with n = 2-7. As a next point, we have calculated the binding energy of an O2 molecule on a-Al2O3 (0001) substrate. Using Hartree-Fock and Kohn-Sham approximation, the O2 adsorption energy is found to be 0.12 and 0.38 eV, respectively, at equilibrium separation distance of 2.46 Å or 1.81 Å between surface and O2. Allowing full relaxation of atoms by means of SIESTA code, the Kohn-Sham adsorption energy of O2 calculated within the GGA is 0.58 eV at the equilibrium distance of 1.98 Å. Our results point to a large contribution of Coulomb correlations and relaxation effects in the adsorption processes on alumina surfaces and clusters.

To clarify the mechanism of lithium storage anomaly in LiF nanocomposites in the context of lithium batteries, we have performed comparative DFT hybrid calculations on the atomic and electronic structure of the non-polar Cu/LiF(001) and model Li/LiF(001) interfaces, in collaboration with Max Planck Institute for Solid State Research, Stuttgart. For this aim, we have modeled the extra Li atoms incorporated at several possible sites of the Me/LiF interface, including the free surface of the substrate slab and interstitial sites inside a slab. Increase of Li concentration at the substrate side of interfaces is accompanied by an increased electron charge transfer from the extra Li atoms towards the transition metal adlayers, in agreement with a proposed mechanism of interfacial charge storage. However, interfacial excess of lithium atoms inside the Me/LiF interface is limited by a single Li monolayer, unlike at least additional Li bilayer possible inside the Ti/Li2O(111) observed by us earlier. Interfacial stability and charge transfer depends on the number of extra Li atoms and Me adatoms per LiF(001) surface unit cell. The Li diffusion on the interface is found to be energetically much easier than Li penetration into the bulk.

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