Optical band edge shift of anatase cobalt-doped titanium dioxide

TL;DR

This study investigates how cobalt doping affects the optical properties of anatase titanium dioxide, revealing a blue shift in the band edge and challenging existing impurity band models.

Contribution

It provides new experimental insights into the optical band edge behavior of cobalt-doped TiO2, highlighting the role of Coulomb interactions and questioning impurity band theories.

Findings

Optical band edge shifts to higher energies with increased Co doping.

Absence of sub-band gap absorption contradicts impurity band models.

Strong Coulomb interactions influence the electronic structure of doped TiO2.

Abstract

We report on the optical properties of magnetic cobalt-doped anatase phase titanium dioxide Ti_{1-x}Co_{x}O_{2-d} films for low doping concentrations, 0 <= x <= 0.02, in the spectral range 0.2 to 5 eV. For well oxygenated films (d << 1) the optical conductivity is characterized by an absence of optical absorption below an onset of interband transitions at 3.6 eV and a blue shift of the optical band edge with increasing Co concentration. The absence of below band gap absorption is inconsistent with theoretical models which contain midgap magnetic impurity bands and suggests that strong on-site Coulomb interactions shift the O-band to Co-level optical transitions to energies above the gap.