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†
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%.