Invisible Surface Oxygen Vacancies in a Thin MgO Film: Impacts on the Chemical Activity and Secondary Electron Emission.

Today, Fs defects in MgO as isolated surface neutral oxygen vacancies are in the focus of surface science, catalysis research, and emission coating of microchannel plates. With the 10-4 atom % content at 750 K and under pO2 = 10-9 Torr, estimated by us from the known equilibrium T-x and p-T-x diagrams of MgO, Fs defects remain invisible or difficult-to-detect objects. The MgO(100) → MgO(100) + Fs + 1/2O2 phase transition was studied in MgO films deposited by the metal-organic chemical vapor deposition (MOCVD) procedure from the mixed-ligand Mg precursor on Si substrates at 725 K in the O2 flow where the nonstoichiometric phase (MgO/Fs) is formed in the gas medium containing simultaneous H2, CO, H2O, CO, and O2 species in unbalanced concentrations. Realization of the above transition was proven theoretically and experimentally through kinetic-thermodynamic analysis of the nonequilibrium system with revealing thermodynamic motive forces, i.e., the positive enthalpy and entropy, as well as through a new combination of diagnostic methods including the original differential dissolution method, due to which separate determination of the point and morphological defects was achieved. It was found that Fs defects occur when oxygen in the immediate vicinity to the substrate surface is replaced practically completely by the oxidized products of the precursor and the resulting oxygen pressure becomes enough for this process. The 90 mass % of the as-deposited MgO-film-involved (MgO/Fs) phase; its chemical activity is demonstrated through dissolution in hot water, while the electron donor activity is through 9 at 750 eV secondary electron yield. A good understanding of gas-phase reactions between the precursors and oxygen provides the fundamental basis of the MOCVD process to deposit MgO films that are dense, free from carbon, and of homogeneous texture. This makes the MOCVD process suitable also for use as coatings of microchannel plates.

Volatile heterometallics: structural diversity of Pd-Pb ß-diketonates and correlation with thermal properties.

Volatile heterobimetallics based on Pb(ii) hexafluoroacetylacetonate as a host material and a range of Pd(ii) complexes containing both various donor atoms and terminal substituents in the ß-diketonate ligand have been studied to trace the influence of the nature of the initial monometallic complexes on the structure, composition and thermal properties of heterobimetallic compounds. The structural variety and stoichiometry of the heterobimetallic compounds are caused by various combinations of the competing donor-acceptor interactions between the constituting monometallic moieties. The major structure forming factor is the metallophilic interaction between Pd and Pb atoms resulting in polynuclear structures with a specific topology. The structural motif (discrete or polymeric) for the compounds significantly depends on the structure and composition of the initial monometallic complexes. The approach for explaining and predicting the thermal stability of these compounds is proposed by using their structural data and the results of quantum chemistry calculations.

Volatile Heterobimetallic Complexes from PdII and CuII ß-Diketonates: Structure, Magnetic Anisotropy, and Thermal Properties Related to the Chemical Vapor Deposition of CuPd Thin Films.

A novel approach for preparing volatile heterometallic complexes for use as precursors for the chemical vapor deposition of various materials is reported. New CuPd complexes based on ß-diketonate units were prepared, and their structures and compositions were determined. [PdL2 *CuL2 ] (1) and [PdL2 *Cu(tmhd)2 ] (2) (L=2-methoxy-2,6,6-trimethylheptane-3,5-dionate; tmhd=2,2,6,6- tetramethylheptane-3,5-dionate) are 1D coordination polymers with alternating metal complexes, which are connected through weak interactions between the Cu atoms and the OCH3 groups from the ligand of the Pd complexes. The volatility and thermal stability were studied using thermogravimetric and differential thermal analyses and mass spectrometry. Compound 1 vaporizes without decomposition into monometallic complexes. It exhibits magnetic anisotropy, which was revealed from the angular variations in the EPR spectrum of a single crystal. The vapor thermolysis process for 1 was investigated using mass spectrometry, allowing the process to be framed within the temperature range of 200-350 °C. The experimental data, supported by QTAIM calculations of the allowed intermolecular interactions, suggest that 1 likely exists in the gas phase as bimetallic molecules. Compound 1 proved to be suitable as a single-source precursor for the efficient preparation of CuPd alloy films with tunable Cu/Pd ratio. A possible mechanism for the film growth is proposed based on the reported data.