Structural and surface property study of sputter deposited transparent conductive Nb-doped titanium oxide films

We have investigated structural and surface property of transparent conductive Nb-doped titanium oxides (TNO) thin film with high conductivity of 10⁻⁴ Ω cm order which were made by RF-magnetron sputtering at high deposition rates followed by an annealing in vacuum. The grain sizes of TNO evaluated by atomic force microscope were found to become larger by annealing at temperature higher than 500 °C. The measured work functions of the TNO films using ultra-violet light photoelectron spectroscopy were 5.02-5.47 eV, and depended on TNO grain size and on the amount of surface weakly bound oxygen that was estimated from peak area intensities of O(1 s) X-ray photoelectron spectra.     

Ca-vacancy effect on the stability of substitutional divalent cations in calcium-deficient hydroxyapatite

First-principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M²⁺ (M = Mg, Zn, Sr) in Ca-deficient hydroxyapatite (dHAp). M²⁺ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M²⁺ are lower in dHAp than in stoichiometric HAp (sHAp) at all M²⁺ concentrations. This indicates that these M²⁺ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca–O bond lengths over a wide area so that Ca–O polyhedrons with various sizes are produced. As a result, M²⁺ ions can predominantly occupy Ca sites at which M²⁺ fits better, depending on the ionic radii of M²⁺. For Zn²⁺ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp.