The present work reports the synthesis and the characterization of cobalt oxide thin films obtained by chemical vapor deposition (CVD) on indium tin oxide (ITO) substrates, using a cobalt(II) β-diketonate as precursor. The complex is characterized by electron impact mass spectrometry (EI-MS) and thermal analysis in order to investigate its decomposition pattern. The depositions are carried out in a cold wall reactor in the temperature range 350−500 °C at different oxygen pressures, to tailor film composition from CoO to Co3O4. The crystalline nanostructure is evidenced by X-ray diffraction (XRD), while the surface and in-depth chemical composition is studied by X-ray photoelectron (XPS) and X-ray excited auger electron spectroscopy (XE-AES). Atomic force microscopy (AFM) is employed to analyze the surface morphology of the films and its dependence on the synthesis conditions. Relevant results concerning the control of composition and microstructure of Co−O thin films are presented and discussed.

Composition and Microstructure of Cobalt Oxide Thin Films Obtained from a Novel Cobalt(II) Precursor by Chemical Vapor Deposition

Fabricating complex transition metal oxides with a tunable bandgap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-specific substitution with the Mott insulator lanthanum cobaltite, its bandgap can be narrowed by as much as 1 eV, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the bandgap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the bandgap in complex oxides for use in emerging oxide optoelectronic and energy applications.

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Wide bandgap tunability in complex transition metal oxides by site-specific substitution.

Electrical conductivity of Co3O4 films prepared by chemical vapour deposition

Bulk and surface properties of spinel Co3O4 by density functional calculations

Co3O4 exhibits intriguing physical, chemical and catalytic properties and has demonstrated great potential for next-generation renewable energy applications. These interesting properties and promising applications are underpinned by its electronic structure and optical properties, which are unfortunately poorly understood and the subject of considerable debate over many years. Here, we unveil a consistent electronic structural description of Co3O4 by synergetic infrared optical and in situ photoemission spectroscopy as well as standard density functional theory calculations. In contrast to previous assumptions, we demonstrate a much smaller fundamental band gap, which is directly related to its efficient electro-/photo-activity. The present results may help to advance the fundamental understanding and provide guidance for the use of oxide materials in photocatalysis and solar applications.

Nature of the band gap and origin of the electro-/photo-activity of Co3O4

The optical constants of Co3O4films were determined by analyzing variable angle of incidence spectroscopic ellipsometry data and normal incidence transmittance data, between 3500 and 17 000 A. Absorption was taken to be nonzero for the soda–lime–silica float glass substrate. The absorption of the soda–lime–silica float glass was fitted by an ensemble of four Gaussian oscillators over the entire measured spectral range. Surface roughness of the film was measured with atomic force microscopy and included in the optical model. The films were deposited by spray pyrolysis of cobalt acetylacetonate onto heated soda–lime–silica float glass and fused silica substrates. Fused silica was used to observe film absorption without the effect of substrate absorption. The oxide film phase was identified with thin‐film x‐ray diffraction analysis. Simultaneous fits of ellipsometry and transmittance data from coated samples were computed to obtain the film thickness, optical constants, and the amplitudes of the four Gaussian oscillators representing the glass substrate absorption. This analysis was done by treating the optical constants at each wavelength as simple fitting parameters or as variables with a Kramers–Kronig consistent parametric model. Spectra for the most accurate optical constants of Co3O4films deposited by spray pyrolysis onto both substrates are presented.

Optical properties of cobalt oxide films deposited by spray pyrolysis

Use of artificial neural networks to predict thickness and optical constants of thin films from reflectance data

Development of artificial neural networks for real time, in situ ellipsometry data reduction

The optical properties of a mixed-metal oxide thin film from the Co3−x−yCrxFeyO4 family has been determined from combined analysis of ellipsometry, atomic force microscopy, and transmittance measurements. These types of films are useful as solar absorbing films on a variety of float glass substrates to lower the solar heat gain admitted through a glass pane in a window, skylight, or door. A commercial product with a film composition in this family is sold under its registered trademark SOLARCOOL® glass. Each constituent metal oxide film was analyzed with variable angle of incidence spectroscopic ellipsometry and transmittance (T) measurements to determine their optical constants and film thickness. Surface roughness was measured with atomic force microscopy and included in the optical model as a known variable. The oxide films were optically modeled with a bulk layer and a known surface roughness layer. The mixed-metal oxide film was optically modeled with an effective medium approximation layer consistin...

M.F. Tabet

Papers

Optical properties of cobalt oxide films deposited by spray pyrolysis

Use of artificial neural networks to predict thickness and optical constants of thin films from reflectance data

Development of artificial neural networks for real time, in situ ellipsometry data reduction

Determining the optical properties of a mixed-metal oxide film, Co3−x−yCrxFeyO4, with spectroscopic ellipsometry and atomic force microscopy

Real time, in-situ ellipsometry solutions using artificial neural network pre-processing