Space charge limited conduction in anatase and mixed-phase (anatase/rutile) single TiO2 nanotubes

TiO2 nanotube arrays are used for several applications; however, in-depth analysis of charge conduction through an individual nanotube is necessary to predict the behavior of devices and to achieve higher efficiency. Here we report successful isolation and charge transport study through individual nanotubes annealed at different conditions. Anatase or anatase/rutile mixed-phase nanotubes are obtained by annealing in air or nitrogen at temperatures 450 and 650 °C, respectively. Current-voltage measurements in the range of 213 K–413 K indicate that the charge transport through single nanotubes follows Ohmic conduction in lower voltage and trap-assisted space charge limited conduction (SCLC) at higher voltage. Charge transport is controlled by two thermal activation processes resulting in two different values of activation energy, which changes with applied voltage. The activation energy in mixed-phase sample is higher than in the anatase nanotube. Calculated values of trap density vary from 9.28 × 1013/cm3 for anatase nanotubes to 6.60 × 1015/cm3 for mixed-phase (∼52% rutile) and 3.34 × 1016/cm3 in mixed-phase nanotubes (∼76% rutile). The variation of trap density with an increase in rutile phase is attributed to electron trapping by rutile regions arising from a corresponding band alignment. This is also reflected in room temperature conductivity which decreases with increase in rutile content.