Tungsten oxo-alkoxides β-ketoesterates WO(OCH2C(CH3)3)3(tbac), WO(OC(CH3)3)3(tbac), and WO(OCH2C(CH3)3)3(tbpa)) (and for comparison tungsten oxo-alkoxides β-diketonates WO(OCH3)3(acac), WO(OCH2C(CH3)3)3(thd) ) were tested as precursors for the AACVD deposition of WOx films and nanorods. Precursors ligand chemistry and deposition conditions were varied to control the stoichiometry and crystalline phase and C contamination content of deposited WOx (where C-free WOx were obtained at certain deposition temperature window for WO(OC(CH3)3)3(tbac) and WO(OCH2C(CH3)3)3(tbpa)) (as well as WO(OCH2C(CH3)3)3(thd) ); the WOx surface morphology changed from amorphous layer to crystalline nanorods to dendrites at higher growth temperatures: at 150-350°C in pure N2 atmosphere sub-stoichiometric amorphous carbide-contaminated (3–9 at.%) WOx films were obtained, whereas more oxidized but still sub-stoichiometric crystalline W18O49 monoclinic phase (with increased surface bound C) was obtained for higher growth temperatures (400–550°C). The increase of WOx crystallinity by increasing growth temperature and addition of O2 to the carrier gas was observed: under oxidizing conditions (1–2% O2 in N2) amorphous WOx was obtained at low temperatures (200-300°C) (similar to growth in N2); at higher temperatures (350-550°C) a sub-stoichiometric tetragonal phase and at 550°C monoclinic WO3 was obtained according to GIXRD. Annealing in air of N2 carrier-grown WOx films induced structural and compositional changes, which were not identical to N2/O2-grown WOx films: annealing the samples grown at 250 and 350°C at the same temperature, produced C-free material but the morphology remained amorphous. Growth rate as a function of temperature and activation energies were estimated for growth of amorphous and nanostructured WOx layers; the activation energies variation was consistent with initial dissociation of the alkoxide C-O bonds and change of steric bulk of the ligand.[i]
[i] H. Kim, R.O. Bonsu, D. C. Bock, N.C. Ou, R.Y. Korotkov, L. McElwee-White, T. Anderson, ECS J. Solid State Sci. Techn., 2016, Vol. 5, 11 , Q3095, https://iopscience.iop.org/article/10.1149/2.0171611jss/meta , « Tungsten Oxide Film and Nanorods Grown by Aerosol-Assisted Chemical Vapor Deposition Using κ2-β-Diketonate and β-Ketoesterate Tungsten (VI) Oxo-Alkoxide Precursors »