Electron transport in nanoparticulate ZnO films

Electron transport in a nanocrystalline thin film was studied using an electrode geometry in which the resistance was measured between two contacts which are separated by a narrow gap bridged by the material of interest. This setup, which allows the direct study of electron transport in a wet nanocrystalline thin film, was used to investigate in situ the electron mobility in ZnO films in contact with liquid electrolyte solutions under accumulation conditions. The setup is also suitable for widely different experimental conditions and materials. Nanocrystalline ZnO films with 5 nm particle size were prepared from a washed and concentrated ethanolic dispersion. Characterization by optical transmission and scanning electron microscopy revealed a porous homogeneous structure with the same particle size as in the original sol. The mobility ranged from 10(-3) to 10(-1) cm(2)/Vs for an electron density between 5 x 10(18) and 1 x 10(20) cm(-3). The mobility depended on the electron density and the solvent (aqueous or nonaqueous), but was not affected by the type or the concentration of the electrolyte ions, or the dielectric constant of the solvent. Interparticle electron transfer is very likely the slow step in electron transport. The importance of the surface chemistry of the particles and the film porosity and heterogeneity is outlined.