Single-step preparation of oxide-oxide nanocomposites: Chemical vapor synthesis of LnAlO(3)/Al2O3 (Ln = Pr, Nd) thin films

Thin films containing manometer-sized PrAlO3 or NdAlO3 crystals evenly disp ersed in an amorphous Al2O3 matrix were grown by chemical vapor deposition (CVD) of heterometal alkoxides, [Ln{Al(OPri)(4)}(3)((PrOH)-O-i)] (Ln = Pr ( 1), Nd (2)). The lanthanoid to aluminum ratio in the precursors, establishe d by single-crystal X-ray diffraction and chemical analyses of the crystall ine products, enables the formation of a stoichiometric 1:1 composite of th e general formula LnAlO(3)/Al2O3 in a single-stage synthesis. The gas-phase thermolysis of 1 and 2 in a cold-wall reactor gave amorphous films of form al composition LnAl(3)O(6), suggesting intact vaporization and decompositio n of the heterometal frameworks on the substrates. On-line mass spectral an alysis of the gaseous products formed during the CVD process revealed isopr opyl alcohol, acetone, propene, and dihydrogen as the main byproducts. The electron spectroscopy for chemical analysis and energy-dispersive X-ray ana lysis confirmed the film compositions to be LnAl(3)O(6), which was shown to be homogeneous through the film bulk by glow discharge mass spectrometry. The surface imaging (scanning electron microscopy/atomic force microscopy) exhibits films with a regular globulated morphology. The annealing of as-de posited films at 800 degreesC shows PrAlO3 and NdAlO3 to be the only crysta lline phases (X-ray diffraction) with average grain sizes of 14 and 12 nm, respectively. The residual Al component (Al2O3) remains amorphous until 120 0 degreesC; however, the evolution of transition aluminas (mixture of kappa -, delta-, and gamma -phases) was observed at higher temperatures (> 1200 d egreesC). The ceramic-glass (crystalline-amorphous) nature of the system wa s established by high-resolution transmission electron microscopy studies, while the compositions of the phases were determined by spatially resolved energy dispersive spectra. The infrared and solid-state Al-27 MAS NMR data of the film material showed spectral patterns formed by an overlapping of t he signals corresponding to LnAlO(3) and Al2O3 phases. The ellipsometric st udies revealed the refractive indices to be 1.76 and 1.67 for the Pr-O-Al a nd Nd-O-Al systems, respectively. The film growth rates were found to be in the range 1.7-1.8 mum/h at 500 degreesC.