| Abstrakt: |
Deposition of continuous, dense WOxfilms and nanorods was accomplished by aerosol-assisted chemical vapor deposition (AACVD) using the recently synthesized precursors WO(OCH3)3(acac) (1), WO(OCH2C(CH3)3)3(tbac) (2), WO(OCH2C(CH3)3)3(dpm) (3), WO(OC(CH3)3)3(tbac) (4), and WO(OCH2C(CH3)3)3(tbpa) (5). This works seeks to define the deposition conditions and precursors that yield C-free tungsten oxide and the potential to control the stoichiometry and phase of deposited WOx. In addition, the systematic variation of the ligand chemistry provides insight into precursor design. Variation of the precursor and growth temperature during deposition revealed a window where C-free WOxwas deposited using 3, 4, and 5. The surface morphology of the WOxvaried from amorphous thin film to crystalline nanorods to dendrites as temperature was increased. Films grown between 150 and 350°C in pure N2atmosphere are sub-stoichiometric, amorphous and contaminated with carbide species (3–9 at.%). As the deposition temperature increased (400–550°C), the tungsten became more oxidized, the sub-stoichiometric crystalline W18O49monoclinic phase formed, and increased surface bound C was detected. Material was also deposited under oxidizing conditions (1–2% O2in N2) as well as annealing in air at the deposition temperature. The material grown in N2/O2carrier gas at low temperatures (200 and 300°C) is amorphous WOx, similar to that grown in N2. At higher temperature (350 and 550°C), however, GIXRD results reveal WOxcorresponding to a sub-stoichiometric tetragonal phase transitioning to the monoclinic WO3phase for samples grown at 550°C. This demonstrates that crystalline structure of WOxis affected by the growth temperature and introducing O2in the carrier gas. Air annealing samples grown in pure N2also produced structural and compositional changes, but not identical to those grown in a N2/O2carrier gas. Notably, annealing samples grown at 250 and 350°C at the same temperature, produced C-free material with unchanged amorphous morphology. The measured stoichiometry and crystallinity showed a dependence on the precursor structure. The growth rate of deposited material was measured as a function of temperature and activation energies were estimated for growth of amorphous and nanostructured material. The systematic variation in activation energies is consistent with initial dissociation of the alkoxide C-O bonds and modifications of the steric bulk of the β-diketonate ligand. |
