Fabrication of ZnO thin films from nanocrystal inks

Zinc oxide nanocrystals were prepared in ethanol and spin-cast to form semiconductor nanocrystal thin films that were thermally annealed at temperatures between 100 and 800 °C. Particle size, monodispersity, and film porosity were determined by X-ray diffraction, ultraviolet-visible absorption spectroscopy, and spectroscopic ellipsometry, respectively. Film porosity rapidly decreased above 400 °C, from 32% to 26%, which coincided with a change in electronic properties. Above 400 °C, the ZnO electron mobility, determined from FET transfer characteristics, increased from 10^-3 to 10^-1 cm² V s^-1, while the surface resistivity, determined from electrical impedance, decreased from 10⁷ to 10 ³ ω m over the same temperature range. Below the densification point, nanoparticle core resistivity was found to increase from 10⁴ to 10⁶ ω m, which is caused by the increasing polydispersity in the quantized energy levels of the nanocrystals. From 100 to 800 °C, crystallite size was found to increase from 5 to 18 nm in diameter. The surface resistance was decreased dramatically by passivation with butane thiol.