B. Domenichini

Effect of the surface stoichiometry on the interaction of Mo with TiO2 (110)

Abstract Molydenum has been deposited at room temperature on (110) TiO2 surfaces with different stoichiometries, roughnesses and crystallinities. Whatever the substrate preparation is, in-situ Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) studies as well as ex-situ atomic force microscopy (AFM) and reflexion high-energy electron diffraction (RHEED) studies reveal a Stranski–Krastanov growth mode: the completion of three monolayers followed by islands growth is observed in every case. The three monolayers are always composed of amorphous molybdenum oxide with an oxidation state of molybdenum less than IV. The oxidation of the molybdenum layers generates Ti3+ and Ti2+ in the substrate, and induces a reconstruction of the surface: during the layer formation, the roughness of the material strongly decreases. Moreover, if the substrate is prereduced prior to the deposition, the reduction induced by molybdenum oxidation can easily migrate in the TiO2 bulk. In the case of an initial stoichiometric surface, the reduction is more located in the interfacial layers. After the growth of three layers, metallic BC islands without preferential orientation appear.

Iron deposition on TiO2(110): effect of the surface stoichiometry and roughness

Abstract Characterizations of ultra-thin iron films deposited on TiO 2 (110) surfaces with different stoichiometries, roughnesses and crystallinities have been carried out by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). For a high initial roughness of the substrate, a 2D growth mode is observed up to three monolayers. But, if the initial roughness is low, clusters grow on the TiO 2 surface. Whatever the initial surface stoichiometry, electronic exchanges occur between titanium and iron leading to a reduction of titanium and an oxidation of iron. This interaction between iron and titanium dioxide surface takes place only at the interface between the metal and the oxide surface: during the completion of the first layer for deposition on a non-stoichiometric and rough surface (2D growth) and at the base and periphery of the islands for 3D growth (in the case of deposition on a flat and stoichiometric surface). Moreover, the amount of electrons exchanged between titanium and iron is lower when the substrate oxide surface is prereduced. The interaction between the deposited metal and the oxide surface is related to the amount of available oxygen.

Non-hindered ansasamarocenes, versatile catalysts for diene/olefin/polar monomer copolymerisations. What is really the active species?

Abstract Catalytic systems containing an ansabiscyclopentadienyllanthanide core and lithium and/or magnesium salts are obtained by reaction of the chloride precursors with allyllithium. These allyl complexes lead to the same active species which polymerises 1,3-dienes, copolymerises 1,3-dienes and α-olefin or α,ω-dienes or allows the controlled diblock polyisoprene/polycaprolactone copolymerisation. The exact nature of this active species and of the allyl precursors is investigated here.

Molybdenum deposition on TiO2 (110) surfaces with different stoichiometries

Abstract The deposition of ultra thin molybdenum films has been carried out on three different TiO 2 surfaces: a stoichiometric and flat one obtained after annealing, a non stoichiometric and rough surface made by Ar + bombardment and a stoichiometric and rough surface obtained by oxygen bombardment. Whatever the substrate preparation, in situ AES and XPS studies and ex situ AFM and RHEED characterizations have revealed a Stranski–Krastanov growth mode: the completion of three monolayers followed by island growth is observed in any case. The three monolayers are composed of amorphous molybdenum oxide with a molybdenum oxidation state between III and IV. The oxidation of the molybdenum layers generates a reduction of the substrate with the formation of Ti 3+ and Ti 2+ and induces a reconstruction of the surface: during the formation of the molybdenum oxide layers the roughness of the surface strongly decreases. After the growth of the three layers, the surface is flat whatever the initial roughness. Then, the molybdenum atoms can diffuse on the surface and generate clusters. The resulting islands are metallic (BC structure) but without preferential orientation.

Coexistence of several structural phases in MOCVD TiO2 layers: evolution from nanometre to micrometre thick films

The morphology and the structure of TiO2 films, grown on Si (1 0 0) substrates by metal organic chemical vapour deposition (MOCVD) was investigated in 5–500 nm thick films. It was shown that the TiO2 layer is mainly amorphous at the first stages of deposition. The growth of nanocrystallites begins inside the amorphous TiO2 layer, and it continues at the expense of the amorphous phase until the crystallized grains occupy the whole layer. Then, the film growth continues with a columnar structure. The coexistence of anatase and rutile phases was evidenced from the beginning of the growth by high resolution transmission electron microscopy and grazing incidence x-ray diffraction. However, the anatase growth overcomes that of rutile, leading to an inhomogeneous phase distribution as a function of the film thickness.

Sintering of Fe2NiO4 with an internal binder: a way to obtain a very dense material

Abstract The coupled synthesis and sintering of Fe2NiO4 can be carried out from the calcination under air at high temperatures (>1200 °C) of precompacted (under 12 MPa) pellets of different mixtures: NiO/α-Fe2O3; NiO/α-Fe2O3/Fe; NiO/α-Fe2O3/Ni. The densest material is obtained at 1200 °C only from the following mixture: NiO (40 mol%), α-Fe2O3 (50 mol%) and Ni (10 mol%). Because the metallic nickel is very ductile, it is used as an internal binder in order to enhance the precompacting of the samples. Moreover, the role of nickel is to enhance the sintering reaction. This route leads to a final material of relative density close to 98±2%.

Molybdenum thin-film growth on rutile titanium dioxide (1 1 0)

Molybdenum films were deposited at room temperature on rutile TiO2(1 1 0) surfaces having different stoichiometries, surface roughnesses and crystallinities. The film structures and compositions and the substrate–film interfaces were investigated by X-ray diffraction, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. Different substrate pretreatments resulted in markedly different film and interface structures. Under the growth conditions studied, no amorphous molybdenum oxide interlayers were formed upon deposition in contrast to previous studies. Preferred (1 1 0) textured Mo films grew on both air-annealed and oxygen-bombarded substrates. While sharp substrate–film interfaces were observed in the air-annealed samples, oxygen bombardment led to a rough interface. Epitaxial growth was achieved on argon-bombarded substrates, and a single crystal TiO interlayer was present as a result of the substrate pretreatment. The orientation relationship among three crystalline layers was: Mo(2 0 0)[0 0 1]//TiO(2 0 0)[0 1 1]//TiO2(1 1 0)[0 0 1]. Even though the growth was epitaxial, the argon bombardment resulted in a rough interface between the substrate and the TiO interlayer and between TiO and the Mo film. The results are compared with previous data on thin Mo film growth ( 6 3 ML) on rutile TiO2(1 1 0), and the structural evolution is discussed. � 2002 Published by Elsevier Science B.V.

Dynamic segregation phenomena during oxidation of titanium ferrites

The cationic composition of three types of titanium ferrite Fe 2.5 Ti 0.5 O 4 has been analyzed by XPS during their oxidation in order to reveal dynamic segregation phenomena. These samples included two pulverised materials obtained by high energy ball milling followed by a thermal treatment under a well controlled reducing atmosphere (I) and by a ceramic process followed by grinding (II), as well as a compact material obtained by a ceramic process (III). In each case, under pure oxygen and under a linear increase of the temperature, the material was subject to oxidation in the cation deficient phase i.e. without phase transformation below 350°C. During this reaction, an important modification of the chemical composition of the near surface layers has been revealed: the titanium ferrite surface becomes richer in iron and poorer in titanium. For pulverised compounds, if the heating is extended above 400°C, the oxidation in a cation deficient phase can proceed and some titanium can move back to the surface. Then, from this temperature, the amount of titanium detected by XPS increases. For samples obtained by high energy ball milling, this phenomenon can lead to a homogeneous compound. This is not so for the samples obtained by the ceramic process. For these, a phase transformation of the compound appears which generates α-Fe 2 O 3 at the surface of the material. The segregation phenomenon has been interpreted on the basis of the different mobilities of the species Fe 2+ , Fe 3+ , Ti 4+ and cation vacancies present in the material.

Elaboration and characterization of barium silicate thin films

Room temperature depositions of barium on a thermal silicon oxide layer were performed in ultra high vacuum (UHV). In-situ X-ray photoelectron spectroscopy (XPS) analyses were carried out as well after exposure to air as after subsequent annealings. These analyses were ex-situ completed by secondary ion mass spectrometry (SIMS) profiles and transmission electron microscopy (TEM) cross-sectional images. The results showed that after air exposure, the barium went carbonated. Annealing at sufficient temperature permitted to decompose the carbonate to benefit of a barium silicate. The silicate layer was formed by interdiffusion of barium with the initial SiO2 layer.

Structural and in depth characterization of newly designed conducting/insulating TiNxOy/TiO2 multilayers obtained by one step LP-MOCVD growth

Abstract TiNxOy/TiO2 multilayers have been grown by LP-MOCVD using titanium isopropoxide (TIP) precursor during the whole growth, but with an ammonia flow interrupted for the TiO2 layers. The one step growth process used to grow these structures allowed to stack the conducting and insulating layers without any growth breakdown. SIMS and TEM analyses showed the presence of an alternated insulating/conducting layers structure. Moreover, electrical measurements allowed to measure the dielectric part of insulating TiO2 stacked in these structures, whose permittivity was found to be about 80 for a MOS structure. Thus, such multilayers may lead to very promising applications in the microelectronics field.