Incorporation of a Binary Alloy in an Oxide Matrix via Single Source Precursor CVD Process

Using a Ni−Sn heterometal alkoxide, [Ni 2 Sn 2 (O t Bu) 8 ], in a chemical vapor deposition (CVD) process, thin films of biphasic composite, Ni 3 Sn 4 /SnO 2, have been obtained. Due to the presence of two metal atoms in a single molecule, the decomposition occurs at a molecular level resulting in homogeneous incorporation of intermetallic Ni 3 Sn 4 in an SnO 2 matrix. The CVD experiments performed at different temperatures (450−550 °C) show that the obtained composite results from two chemical processes: (i) disproportionation of Sn(II) species and (ii) the redox reactions ocurring between Sn(II) and Ni(II) species. Fragmentation of the precursor and disproportionation of the tin(II) component dominate up to 500 °C, resulting in the formation of NiO, Sn(0), and SnO 2 . Redox reactions are favored at higher temperature (550 °C) which lead to the formation of the Ni 3 Sn 4 alloy. This alloy−metal oxide composite has been deposited on different substrates (steel, copper, silicon wafer), and no heterogeneity was observed on a micrometer level (energy-dispersive X-ray analysis). Powder X-ray diffraction patterns of the deposits obtained at 550 °C show Ni 3 Sn 4 and SnO 2 as the only crystalline phases. The scanning electron micrograph images reveal a microstructured surface with a fibrous morphology. High-resolution transmission electron microscope investigations show a bimodal mixture where the Ni 3 Sn 4 crystallites (ca. 60−80 nm) are uniformly dispersed in a SnO 2 matrix (30−45 nm). Well-developed lattice fringes, for both particle types, corroborate the crystalline nature of the two phases. The isomeric shift in the Mössbauer spectrum of the CVD deposit, when compared with the Ni 3 Sn 4 and SnO 2 standards, confirms the biphasic nature of the obtained material and shows the composition to be Ni 3 Sn 4 /SnO 2 . Electron spectroscopy for chemical analysis (ESCA) studies performed on both (i) as obtained and (ii) argon sputtered samples established the elemental composition, the oxidation states of the Ni and Sn atoms, and the effect of atmospheric oxidation on the metal atoms located on the surface of the layers. Further characterization of the Ni−Sn intermetallic phase was achieved by detailed ESCA and high-resolution transmission electron microscopy (HR-TEM) studies.