Single-Step Preparation of Oxide−Oxide Nanocomposites: Chemical Vapor Synthesis of LnAlO3/Al2O3 (Ln = Pr, Nd) Thin Films

Thin films containing nanometer-sized PrAlO 3 or NdAlO 3 crystals evenly dispersed in an amorphous Al 2 O 3 matrix were grown by chemical vapor deposition (CVD) of heterometal alkoxides, [Ln{Al(O Pr i ) 4 } 3 ( Pr i OH)] (Ln = Pr ( 1 ), Nd ( 2 )). The lanthanoid to aluminum ratio in the precursors, established by single-crystal X-ray diffraction and chemical analyses of the crystalline products, enables the formation of a stoichiometric 1:1 composite of the general formula LnAlO 3 /Al 2 O 3 in a single-stage synthesis. The gas-phase thermolysis of 1 and 2 in a cold-wall reactor gave amorphous films of formal composition LnAl 3 O 6, suggesting intact vaporization and decomposition of the heterometal frameworks on the substrates. On-line mass spectral analysis of the gaseous products formed during the CVD process revealed isopropyl alcohol, acetone, propene, and dihydrogen as the main byproducts. The electron spectroscopy for chemical analysis and energy-dispersive X-ray analysis 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-deposited films at 800 °C shows PrAlO 3 and NdAlO 3 to be the only crystalline phases (X-ray diffraction) with average grain sizes of 14 and 12 nm, respectively. The residual Al component (Al 2 O 3 ) remains amorphous until 1200 °C; however, the evolution of transition aluminas (mixture of κ-, δ-, and γ-phases) was observed at higher temperatures (>1200 °C). The ceramic−glass (crystalline−amorphous) nature of the system was 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 27 Al MAS NMR data of the film material showed spectral patterns formed by an overlapping of the signals corresponding to LnAlO 3 and Al 2 O 3 phases. The ellipsometric studies revealed the refractive indices to be 1.76 and 1.67 for the Pr−O−Al and Nd−O−Al systems, respectively. The film growth rates were found to be in the range 1.7−1.8 μm/h at 500 °C.