Terahertz depolarization effects in colloidal TiO2 films reveal particle morphology

Films of colloidal TiO₂ nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO₂ films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report. ultrafast photoconductivity studies of films consisting of sintered TiO₂ particles of very different sizes performed using time-resolved terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nanometers, respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO₂ films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the dielectric function inherent to these types of films. Modeling of the influence Of depolarization fields on the terahertz conductivity allows us to explain the measured data and gain insights into the morphology of the film. Specifically, we show that the observed photoconductivity spectra reflect percolated pathways in the colloidal TiO₂ nanoparticles films, through which charge carrier diffusion can occur over macroscopic length scales.