ABSTRACT
Monodisperse 11 nm indium tin oxide (ITO) nanocrystals (NCs) were synthesized by thermal decomposition of indium acetylacetonate, In(acac)(3), and tin bis(acetylacetonate)dichloride, Sn(acac)(2)Cl(2), at 270 °C in 1-octadecene with oleylamine and oleic acid as surfactants. Dispersed in hexane, these ITO NCs were spin-cast on centimeter-wide glass substrates, forming uniform ITO NC assemblies with root-mean-square roughness of 2.9 nm. The assembly thickness was controlled by ITO NC concentrations in hexane and rotation speeds of the spin coater. Via controlled thermal annealing at 300 °C for 6 h under Ar and 5% H(2), the ITO NC assemblies became conductive and transparent with the 146 nm-thick assembly showing 5.2 × 10(-3) Ω·cm (R(s) = 356 Ω/sq) resistivity and 93% transparency in the visible spectral range--the best values ever reported for ITO NC assemblies prepared from solution phase processes. The stable hexane dispersion of ITO NCs was also readily spin-cast on polyimide (T(g) ~360 °C), and the resultant ITO assembly exhibited a comparable conductivity and transparency to the assembly on a glass substrate. The reported synthesis and assembly provide a promising solution to the fabrication of transparent and conducting ITO NCs on flexible substrates for optoelectronic applications.
ABSTRACT
The effect of the microstructure of tin-doped indium oxide (ITO) films on their electrochemical performance was studied using three redox probes, tris(2,2'-bipyridyl ruthenium(II) chloride (Ru(bpy)3(2+/3+)), ferrocyanide (Fe(CN)6(4-/3-)), and ferrocenemethanol (FcCH2H(0/+)). ITO films were deposited using dc magnetron sputtering under a variety of conditions that resulted in films having different degrees of crystallinity, crystallographic texture, sheet resistance, surface roughness, and percent tin. It was found that the electron transfer for all three redox probes used in this study was more efficient at polycrystalline films than at amorphous ITO films. This effect is more pronounced at faster scan rates. The crystallographic texture of the ITO films, surface roughness, and a change in sheet resistance from 7.9 to 13.7 ohms/square did not have an effect on electron-transfer kinetics. ITO films deposited using a 1 wt % SnO2 target and having sheet resistance comparable to films deposited using a 10 wt % SnO2 target had dramatically different microstructure from the films with higher weight percent Sn and were shown to perform poorly when used as electrode materials. We believe that the dramatic differences in electron-transfer kinetics observed at the various ITO films can be attributed to either the different density of defect sites along the grain boundaries or defect sites caused by substitutional Sn in the film.
