On the complex refractive index of N-doped TiO2 nanospheres and nanowires in the terahertz spectral region

In this paper, the terahertz (THz) transmission measurement technique was applied to characterize titanium dioxide (TiO2) nanospheres and nanowires subjected to thermal treatments under various conditions. Differences in the spectral features of the nanospheres and nanowires were observed due to treatment and annealing of the samples in different gas atmospheres. The observations made can be explained based on the formation of new phonon bands and/or widening of the phonon bands due to polymorphism. A singly subtractive Kramers Kronig dispersion relation was utilized to estimate the frequency-dependent real refractive index of the various samples, having a priori unknown sample thickness, from the absorbance data.