• Infrared Spectroscopy of (Nb+In) Co-Doped Rutile

      Yee, Susan; Department of Physics
      This work studied rutile TiO2 in pure form and co-doped with In (e􀀀 acceptor) and Nb (e􀀀 donor) at 5% and 10% to explore the effect of co-doping on the infrared active (IR) modes and the complex dielectric response function between 50 and 8000 cm􀀀1 (1.5 - 240 THz, 0.00620 - 0.993 eV). Ceramic pellets of pure, 5% and 10% co-doped TiO2 were prepared using a standard technique. Infrared reflectance (IR) measurements were taken and these data are supplemented with data from the literature to extend the range of frequencies beyond infrared. The dielectric function was determined two ways: (i) by fits of the reflectance to the factorized model of the dielectric function and (ii) by Kramers- Kronig analysis. Co-doping rutile appears to decrease the permittivity at frequencies just below the mode that softens. It is possible that this is due to an increase in porosity resulting from codoping. It appears that the increase in permittivity recently observed elsewhere [1] is not caused by doping induced changes to the phonon modes. The overall effect of co-doping is to make the sample less reflective. The spectrum is composed of three wide, high-reflectance bands. For all levels of co-doping the first band is a mode that softens. The amount of doping does not affect the frequency of the mode that softens. The second and third bands are hard modes. Co-doping appears to introduce four new, impurity, phonon modes that increase in oscillator strength with increasing co-doping level. These modes are centered near w 136, 447, 654 and 793 cm􀀀1 which are close to four, previously observed, Raman active modes in rutile. It is possible that the co-doping process causes the Raman modes to develop a dipole moment and become weakly IR active.
    • Optical Study of (Nb0.5In0.5)0.02 Ti0.98O2 Crystals

      Cosco, Mike; Department of Physics
      This work was a study of pure TiO2Rutile crystals, as well as Rutile crystals 2% co-doped with Indium and Niobium (2-NITO). There is much interest surrounding co-doped TiO2re-cently, with several papers published on ’colossal permittivity’ in the lower frequency ranges (10^2-10^6Hz range). The aim of this work was to study the optical and Raman modes of pure and co-doped crystals to determine the effects co-doping has on these modes. Infrared reflectance along with Raman Spectroscopy were used for this purpose. In order to determine the dielectric function from the infrared data, the Factorized Model and Kramers-Kronig analysis were used. Since TiO2has a tetragonal unit cell, infrared measurements of both the a and c axes of both doped and undoped crystals were done. Thea-axis is known to have 3 optical modes, whereas the c-axis only has one. However an additional mode was seen in all spectra, believed to be caused by anharmonicity. In addition, the 136cm−1mode observed in polycrystalline conductivity spectra of 5 and 10-NITO lines up directly with the A2u mode and the 793cm−1 mode also appears in single crystal TiO2, meaning these are not new modes. However the 447cm−1 and 654cm−1 modes do not appear in our data, and are likely a result of higher percentage co-doping. The effect of co-doping was observed to be an overall decrease in the reflectance of TiO2. We also observed sizable increases inγtofor all modes in 2-NITO. In addition, the dielectric permittivity decreases below the first phonon mode; suggesting that the enhanced permittivity observed at lower frequencies is not caused by co-doped changes to phonon modes. All expected Raman-active modes were observed, however due to poor data resolution some of the peak positions appear to be slightly different than previously measured. Our Raman spectra showed new structures at around 300cm−1and 700cm−1 in the (100) surface spectra, it is possible these are combination lines.