Effect of pressure on electrical and optical properties of metal doped TiO2

A comparative study of the electrical and optical properties has been done on 3d-doped TiO$_2$. Ti$_1-x$M$_x$O$_2$ (M= Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) powder and its corresponding pellets, with doping concentration $x= 0.05$. The samples were prepared using the solid-state route. Optical and electrical measurements have been performed for all prepared samples and interestingly, it is observed that due to external pressure (i.e. strain) both the properties change significantly. A rigorous theoretical calculation has also been carried out to verify the experimental band gap obtained from optical absorption spectroscopy. In case of pellet sample band gap decreases as compared to the powder sample due to variation of pressure inside the structures. Role of doping has also been investigated both in pellet and powder forms and we found that the band gap decreases as the atomic number of dopants increases. A cross-over behavior is seen in pellet samples on doping with Ni, Cu and Zn (i.e. band gap increases with an increase in the atomic number of dopant). Electrical resistivity measurements have been carried out for both pellet and powder samples and it is found that in the case of strained samples the value of resistivity is smaller while in the case of strain-free samples it is quite large. We believe that the present study suggests a novel approach for tuning the electrical and optical properties of semiconducting oxides either from doping or from applied pressure (or strain).