Electrical and optical properties of thin films consisting of tin-doped indium oxide nanoparticles

Electrical transport and optical properties were investigated in porous thin films consisting of ${\mathrm{In}}_2{\mathrm{O}}_3:\mathrm{Sn}$ (indium tin oxide, ITO) nanoparticles with an initial crystallite size of $\sim 16\hspace{0.5em}\mathrm{nm}$ and a narrow size distribution. Temperature dependent resistivity was measured in the $77<t<\mathrm{}300\mathrm{}\mathrm{K}$ temperature interval for samples annealed at a temperature in the $573<~t_{\mathrm{A}}<~1073\mathrm{K}$ range. Samples annealed at $573<~t_{\mathrm{A}}<~923\mathrm{K}$ exhibited a semiconducting behavior with a negative temperature coefficient of the resistivity (TCR). These data were successfully fitted to a fluctuation induced tunneling model, indicating that the samples comprised large conducting clusters of nanoparticles separated by insulating barriers. Samples annealed at $t_{\mathrm{A}}=1073\mathrm{}\mathrm{K}$ displayed a metallic behavior with no signs of insulating barriers; then the TCR was positive at $t>\mathrm{}130\mathrm{}\mathrm{K}$ and negative at $t<\mathrm{}130\mathrm{}\mathrm{K}.$ Effects of annealing on the ITO nanoparticles were investigated by analyzing the spectral optical reflectance and transmittance using effective medium theory and accounting for ionized impurity scattering. Annealing was found to increase both charge carrier concentration and mobility. The ITO nanoparticles were found to have a resistivity as low as $2\times {10}^{-4}\Omega \mathrm{cm},$ which is comparable to the resistivity of dense high quality ${\mathrm{In}}_2{\mathrm{O}}_3:\mathrm{Sn}$ films. Particulate samples with a luminous transmittance exceeding 90% and a resistivity of $\sim {10}^{-2}\Omega \mathrm{cm}$ were obtained.