Low-temperature crystal growth of aluminium-doped zinc oxide nanoparticles in a melted viscous liquid of alkylammonium nitrates for fabrication of their transparent crystal films

Abstract

We fabricated conductive Al-doped ZnO (AZO) films on glass substrates via a simple drop-coating process of alcoholic dispersion solutions of AZO nanoparticles less than 10 nm in size, which were prepared by hydrolysis reactions of Zn(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O with an excess amount of isopropylamine. After heating at 150 °C to completely remove the alcoholic solvents, a by-product that remained, isopropylammonium nitrate, was melted and functioned as a low-temperature medium for the AZO nanoparticles. Even in the low-temperature medium at 150 °C, the AZO nanoparticles could readily grow up to ~100 nm, based on Ostwald ripening as a plausible crystal growth mechanism. The medium was evaporated at 240 °C, and a highly transparent AZO film appeared on the glass with a transmittance of ~83% in the visible region. The electrical conductivity of the AZO films was improved by sintering at 450 °C and post-annealing at 450 °C in a stream of a mixed gas of N₂ and H₂. The resistivity of the AZO film reached 4.3 × 10⁻² Ω cm in an Al/Zn molar ratio of 2.0%.