Discrepancies in the morphology and physical properties of amorphous and crystalline sprayed lanthanum nickel oxide films

摘要

Amorphous LaNiO_3 (a-LNO) and crystalline LaNiO_3 (c-LNO) films were prepared by spraying an aqueous precursor solution of lanthanum and nickel chlorides on hot (450℃) fused silica substrates followed by annealing at high temperatures (550-850℃). Thermal analysis of a dried precursor indicated that a stable oxide phase is formed at 560 °C with no distinct crystallization peak. Scanning electron microscopy (SEM) powered with energy-dispersive X-ray spectroscopy (EDX) of as-sprayed films showed rough surfaces with par-ticulate-like deposits and incomplete pyrolysis chloride composition. No chloride contents were detected in annealed films. X-ray diffraction showed that films annealed at 550 °C and 650 °C were a-LNO and those annealed at 750℃ and 850℃ were c-LNO. The c-LNO phase was indexed as a single-phase perovskite structure with (110) orientation. SEM/EDX showed that a-LNO films have rough surfaces and c-LNO films have uniform crack-free smooth surfaces. Electrical properties measurements showed that c-LNO films have lower resistivity than a-LNO films and both types of LNO films have semiconductor resistant temperature dependence. The activation energy of electric conduction of a-LNO films was found to be much higher than that of c-LNO films. The optical transmittance and reflectance of the films were studied in the UV-visible-near IR range. The optical constants were obtained by modeling the measured transmission and reflection spectra. Because of the discrepancies in the morphology and in the physical properties of a-LNO and c-LNO films, the best fit modeling of transmission and reflection spectra was obtained by using different theoretical models and different geometrical configurations. While the Drude model accounting for larger carrier density was found to be significant for c-LNO, using the Bruggmann model and a configuration of a rough layer on top of a compact film was found to be significant for a-LNO.