Tailoring the magnetization behavior of TiO2

TiO2



Dr S.J. Heyes; Structures of Simple Inorganic Solids

http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture4/Lec4.html#anchor14

Tailoring the magnetization behavior of Co-doped titanium dioxide nanobelts

Cobalt-doped titanium dioxide nanobelts which showed room temperature ferromagnetism were prepared from a ferromagnetic rutile TiO2 single crystal. The ferromagnetic ordering in these nanobelts was found to be mediated in the presence of bulk oxygen vacancies. These oxygen vacanciesnext term could be engineered by annealing the as-synthesized Co-doped titanate nanobelts under vacuum at higher temperatures. The sole presence of this type of previous termvacancynext term was achieved when the titanate nanobelts were transformed into the monoclinic TiO2-B phase in which saturated magnetization was also observed. Annealing these nanobelts into the anatase phase caused the formation of other defect and active sites, such as surface and bulk Ti3+ species, which caused the non-saturation of magnetic moments in the magnetization curves. Ferromagnetism was suppressed when the Co-doped titania nanobelts were annealed in air due to the diminishing of the amount of previous termoxygen vacancies.next term

Chong et al, August 2008





Fabrication of low resistivity tin-doped indium oxide films with high electron carrier densities by a plasma sputtering method

Tin-doped indium oxide (ITO) films fabricated on glass substrates using a hot-cathode plasma sputtering method exhibited low resistivity of 9.7 × 10−5 Ω cm, which is due to a high carrier density of 2.1 × 1021 cm−3. The change in the number of carriers, N, as a function of film thickness d, strongly suggests that oxygen extraction in the initial stages of ITO film growth on the glass substrate surface, creates oxygen vacancies as an electron carrier source for improvement in the resistivity of the films.

www.sciencedirect.com, 2008


Band gap tailoring


In polycrystalline TiO2 the presence of shallow traps diminishes and the radiative transitions involve energy levels induced by Ti3+ ions. A red shift of the infrared emission band of titanium ions appears to be determined by oxygen vacancies affecting the band gap energy.

Of great interest is band gap tailoring because of its effects on enhancement of optical absorption in visible range and increase of photocatalytic decomposition rate. On the other hand, the high k property of TiO2 as well as magneto-doping processes were investigated for applications as gate dielectric material and electron spin based nanodevices. All these applications require a profound understanding of quantum processes in which both oxygen vacancies (OV) and titanium ions in various electronic states are involved. Although an impressive number of publications have been devoted to the analysis of OV formation and behavior, an accurate explication of the relationship between defects structure and electronic structure still is not resolved. For instance, there is no broad agreement on whether the intrinsic doping leads to energy levels formation inside the band gap, or the energy states change the band gap width.

Previously reported cathodoluminescence CL results on the emission from a rutile single crystal demonstrated that emission bands in visible region correspond to surface OV with associated shallow energy levels below the conduction band, while infrared emission corresponds to Ti3+ ions with associated energy levels inside the band gap. The infrared emission band shows a very intense peak in single crystal rutile TiO2 as well as in polycrystalline TiO2 investigated herein. However, the peak wavelength position is different. One possible explanation could be the different positions occupied by Ti3+ ions in the basic cell structure. Another possibility could be a band gap energy increase determined by OV in polycrystalline TiO2 treated in argon. The effect of OV concentration on band gap tailoring was recently reported by theoretical studies. (a, b)




Plugaro, Optical properties of Crystalline Titanium Oxide, Journal of Thin surfaces, Elsevier, 2008

a)E. Cho, S. Han, H.-S. Ahn, K.-R. Lee, S.K. Kim, C.S. Hwang, Phys. Rev. B 73 (2006) 193202.
b) F.M. Hossain, G.E. Murch, L. Sheppard, J. Nowotny, Solid State Ionics 178 (2007) 319.