M. R. De Guire

Synthesis and characterization of TiO2 thin films on organic self-assembled monolayers: part I. Film formation from aqueous solutions

Self-assembled monolayers (SAMs) bearing sulfonate (-SO 3 H) surface functional groups, on single-crystal Si wafers, were used as substrates for the deposition of TiO 2 thin films from aqueous solutions. Polycrystalline TiO 2 thin films over 50 nm thick formed in 2 h by hydrolysis of TiCl 4 in aqueous HCI solutions at 80 °C. The films were pore-free, showed excellent adherence and uniformity, and consisted of anatase crystallites 2–4 nm in diameter. Annealing at temperatures up to 600 °C caused coarsening of the anatase grains, but no loss of adherence or structural integrity.

Review: deposition of ceramic thin films at low temperatures from aqueous solutions

Many techniques for the synthesis of ceramic thin films from aqueous solutions at low temperatures (25–100°C) have been reported. This paper reviews non-electrochemical, non-hydrothermal, low-temperature aqueous deposition routes, with an emphasis on oxide materials for electronic applications. Originally used for sulfide and selenide thin films, such techniques have also been applied to oxides since the 1970s. Films of single oxides (e.g., transition metal oxides, In2O3, SiO2, SnO2) and multicomponent films (doped ZnO, Cd2SnO4, ZrTiO4, ZrO2-Y2O3, Li-Co-O spinel, ferrites, perovskites) have been produced. The maximum thicknesses of the films obtained have ranged from 100 to 1000 nm, and deposition rates have ranged from 2 to 20,000 nm/h. Compared to vapor-deposition techniques, liquid-deposition routes offer lower capital equipment costs, lower processing temperatures, and flexibility in the choice of substrates with respect to topography and thermal stability. Compared to sol-gel techniques, the routes reviewed here offer lower processing temperatures, lower shrinkage, and (being based on aqueous precursors) lower costs and the potential for reduced environmental impact. This review emphasizes the influence of solution chemistry and process design on the microstructures and growth rates of the films. The current understanding of the mechanisms of film formation is presented, and the advantages and limitations of these techniques are discussed.

Deposition mechanism of oxide thin films on self-assembled organic monolayers

A mechanism for the deposition of oxide thin films from aqueous media at low temperatures onto functionalized organic self-assembled monolayers (SAMs) is proposed. TiO 2 films deposited on sulfonate (SO 3 H-) SAMs on silicon consist of densely packed anatase crystallites, with a small quantity (10-20%) of amorphous TiO 2 . ZrO 2 -containing films deposited on SO 3 H-SAMs on silicon contain nanocrystallites of tetragonal zirconia and amorphous basic zirconium sulfate. Amorphous thin films of basic yttrium carbonate (which crystallize to cubic Y 2 O 3 on subsequent heating) were deposited on SO 3 H-SAMs on silicon and also on bare silicon substrates. The proposed mechanism accounts for the formation of all of these films on the basis of aggregation processes between the deposition surface and the solid particles that are known to exist in the aqueous deposition medium. Estimates, calculated using DLVO theory, of the interaction energies between the SAM and the particles, and between the growing film and the particles, are consistent with the experimental observations.

Synthesis and characterization of TiO 2 thin films on organic self-assembled monolayers: Part II. Film formation via an organometallic route

Crystalline, uniform, adherent, ultrathin films of TiO 2 were deposited onto OH-functionalized organic self-assembled monolayers (SAMs) on single-crystalline Si at low-temperature ( 2 by x-ray photoelectron spectroscopy, electron diffraction, and energy-dispersive x-ray microanalysis. Transmission electron microscopy showed the films to be uniform in thickness (2 ± 0.5 nm) and continuous. On bare Si, in contrast, there was no evidence of TiO 2 deposition under identical conditions. Unlike the anatase films deposited on SAMs from aqueous solutions (described in the preceding paper), the electron diffraction patterns of the films deposited from alkoxide solutions suggest that they were the rutile phase. It is suggested that the functionalized SAMs enable the anchoring of the Ti alkoxide and initiate the formation of an adherent oxide film, and that they are sufficiently uniform that the resulting film is continuous and uniform in thickness.

Coprecipitation synthesis of doped lanthanum chromite

Two coprecipitation methods were used to synthesize powder precursors of doped lanthanum chromite (La, Ca)(Cr, Co)O 3 . The effects of synthesis method and calcination temperature on the composition, sintered density, and microstructure of pressed compacts of (La, Ca)(Cr, Co)O 3 were studied by differential thermal analysis/thermogravimetric analysis, x-ray diffraction, scanning electron microscopy, and density measurement. The cation ratios in the precipitated solids were, with few exceptions, within experimental error of the desired compositions for all four components. Powders obtained by both techniques could be sintered to densities exceeding 93% at 1400 °C. The highest densities were obtained with calcining temperatures from 450 to 700 °C. The sintered microstructures exhibited uniform grain sizes averaging 3–5 μm. The Cr(vi) compounds, CaCrO 4 and La 2 CrO 6 , were observed in all of the calcined powders. The possible role of these phases on chromite densification is discussed.

Growth and characterization of BiSrCaCuO superconducting fibers

Abstract With the laser-heated floating zone method, fibers of nominal overall composition Bi 2 Sr 2 CaCu 2 O 8±Z and Bi 2 Sr 2 Ca 2 Cu 3 O 10±Z have been grown. Fibers of uniform diameter up to ∼1 cm long have been produced. The influence of the growth rates and feed rod compositions on the microstructure of grown fibers were determined with XRD, SEM, EDS, and resistive T c measurements. At 5 mm/h growth rate, compositionally homogeneous material can be grown from the 2212 composition over a length of several hundred μm. Fibers grown at higher rates consist predominantly of Bi 2+x (Sr,Ca) 2−X CuO 6±Z (the 25Ȧ phase) with smaller amounts of Bi 2 (Sr,Ca) 3 Cu 2 O 8±Z (the 31Ȧ phase) and (Sr,Ca)CuO 2 phases. These multiphase fibers show a resistive superconducting transition at 80K. After annealing at 800°C for 12 hrs in air, the proportion of the 31Ȧ phase is increased, the normal state resistivity is decreased, and the T c rises to 90K.

Thermosetting epoxy as a moisture-resistant coating for YBa2Cu3O7

Composites of YBa2Cu307−x and a thermosetting epoxy have been fabricated to study the effectiveness of the epoxy as a protective coating against aqueous decomposition of the oxide, and to determine the effects of the polymer on the oxides superconducting properties. Reaction with water depletes the ceramic of barium, causes cracks perpendicular to the surface, and decreases the volume fraction of superconducting phase. Microstructural studies show that the epoxy coating is an effective barrier against the decomposition of YBa2Cu307−x immersed in 70 ° C water. In addition, resistivity and AC susceptibility measurements indicate that the epoxy has no deleterious effect on the superconducting transition temperature of the ceramic.

Ceramic Thin Films on Organic Self-Assembled Monolayers: Synthesis and the Mechanism of Formation

In recent years, several research groups have pursued biomimetic or bio-inspired techniques for the synthesis of ceramic thin films from aqueous solutions at low temperatures. The substrates range from inorganic materials (metals, glass, single-crystal silicon) without special surface preparation, to functionalized organic surfaces such as self-assembled organic monolayers (SAMs). Our results on the deposition of tin (IV) oxide (SnO 2 , cassiterite) and hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) thin films on SAMs will be reviewed. The former system forms films via assembly of nano-scale particles on the substrate, while the latter system appears to form films via heterogeneous nucleation. In both cases, the role of the substrate in film formation is discussed.

Synthesis and Characterization of Titania and Vanadia Thin Films at Organic Self-Assembled Monolayers

Functionalized self-assembled monolayers (SAMs) on single-crystal Si wafers have been used as substrates for the deposition of titania and vanadia thin films. The formation of a titanium chelate was used to stabilize an otherwise spontaneously precipitating aqueous titanium solution. Uniform titania films have been synthesized from Ti(O 2 ) 2+ in aqueous HCl solutions at 80°C on sulfonated SAMs. Vanadium oxide hydrate films, V 2 O 5 ·0.7 H 2 O, have been directly formed from aqueous vanadate solutions on NH 2 -terminated SAMs at 45°C. In the as-deposited films, water molecules were intercalated between the vanadium oxide layers. Subsequent annealing at 350°C in air led to nanocrystalline V 2 O 5 .