Gas phase fabrication of morphology-controlled ITO nanoparticles and their assembled conductive films

Abstract

ITO nanoparticles were generated in the gas phase with a magnetron plasma gas aggregation cluster source. Their morphologies were modified by modulating the discharging power of magnetron sputtering. The shape of the nanoparticles changed from rough spheroid formed with a higher discharging power to multi-branch formed with a lower discharging power. With a discharging power of 25 W, the ITO nanoparticles were enriched with tripod and tetrapod-shaped nanoparticles. The formation mechanism of multi-branch nanoparticles was attributed to the oriented attachment of the initially nucleated smaller nanocrystallites. Transparent conductive ITO nanoparticle films were fabricated by depositing the preformed nanoparticles with controlled thickness. The electron conduction in the film was dominated by electron tunnelling and/or hopping in the percolative channels comprised of closely spaced ITO nanoparticle assemblies and could be tuned from highly resistive nonmetal-like to highly conductive metal-like by changing the deposition thickness. The film also displayed a SPR band in the near-IR region. The conductivity of the multi-branch ITO nanoparticle film was significantly superior to that of the spheroidal nanoparticle film. For a 46 nm thick multi-branch ITO nanoparticle film, a surprisingly low specific resistance of 3.09 × 10⁻⁴ Ω cm, which is comparable to the top-class conductivity of bulk ITO films, was obtained after annealing at a mild temperature of 250 °C, with a transmittance larger than 85%.