Abstract
Here, we report large magnetoresistance and magnetocapacitance response of undoped TiO2 quantum dots weighting the contribution of both grain and grain boundaries by means of impedance spectroscopy. We also performed a complete characterization of the TiO2 quantum dots (~5 nm) prepared by sol-gel via water vapor diffusion method, using X-ray diffraction, small angle X-ray scattering, transmission electron microscopy and Raman spectroscopy. In addition, we showed a complete theoretical study on the electronic properties of TiO2 surface and subsurface oxygen and titanium vacancies to shed some light in their electronic and magnetic properties. Based in our study, we can conclude that the presence of defects, mainly at the grain boundary of these undoped TiO2 quantum dots, could be responsible for the large positive magnetoresistance (+1200%) and negative magnetocapacitance (−115%) responses at low applied magnetic fields (1.8 kOe) and room temperature.
Highlights
There is recent interest in the study of diluted magnetic semiconducting oxides after the theoretical prediction of room ferromagnetism in Mn-doped ZnO reported by Dietl et al.[1] and experimentally evidenced for Co-doped ZnO by Ueda et al.[2]
We showed a study on the large magnetoresistance and magnetocapacitance of TiO2 quantum dots prepared by sol-gel via water vapor diffusion method
Our structural characterization revealed the presence of TiO2 anatase quantum dots with a ~5 nm size with n-propoxide residuals possibly bonded to its surface
Summary
There is recent interest in the study of diluted magnetic semiconducting oxides after the theoretical prediction of room ferromagnetism in Mn-doped ZnO reported by Dietl et al.[1] and experimentally evidenced for Co-doped ZnO by Ueda et al.[2]. Sudakar et al showed experimental evidence of room temperature ferromagnetism in vacuum annealed undoped rutile TiO2 films[5]. In this case, ferromagnetism was attributed to the formation of an amorphous thin layer in the nanocrystalline surface, in comparison with the oxygen annealed specimens[5]. Theoretical calculations by means of GGA + U methodology was obtained for bulk titanium-defect anatase with Ti15O32 formula, evidencing the presence of a new band transition above the valence band in the density of states, which explains the higher mobility of the experimentally observed holes[7]. Bansal et al have reported a small negative magnetoresistance response at 9 kOe of ultrathin SnO2 films at low temperatures (T < 40 K)[8]. We show a complete theoretical study on the electronic properties of TiO2 (101) surface and subsurface oxygen and titanium vacancies to shed some light in their electronic and magnetic properties
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