Abstract
The effect of Niobium doping and size of crystallites on highly transparent nano-crystalline Niobium doped Titanium Dioxide (NTO) thin films with stable anatase phase are reported. The Nb doping concentration is varied within the solubility limit in TiO2 lattice. Films were annealed in controlled environment for improving the crystallinity and size of crystallites. Elemental and thickness analysis were carried out using Rutherford backscattering spectrometry and cross sectional field emission scanning electron microscopy. Structural characteristics reveal a substitutional incorporation of Nb+5 in the TiO2 lattice which inhibits the anatase crystallites growth with increasing the doping percentage. The micro-Raman (MR) spectra of films with small size crystallites shows stiffening of about 4 cm−1 for the Eg(1) mode and is ascribed to phonon confinement and non-stoichiometry. In contrast, B1g mode exhibits a large anomalous softening of 20 cm−1 with asymmetrical broadening; which was not reported for the case of pure TiO2 crystallites. This anomalous behaviour is explained by contraction of the apical Ti-O bonds at the surface upon substitutional Nb5+ doping induced reduction of Ti4+ ions also known as hetero-coordination effect. The proposed hypotheses is manifested through studying the electronic structure and phonon dynamics by performing the near edge x-ray absorption fine structure (NEXAFS) and temperature dependent MR down to liquid nitrogen temperature on pure and 2.5 at.% doped NTO films, respectively.
Highlights
Niobium doped Titanium Dioxide (NTO) is a material of current interest for its salient superior properties for potential applications[1] such as improved performance of gas sensors,[2,3] photocatalytic applications,[4,5,6] enhance the electron injection in solar cells,[7] and as alternative conductive electrode.[8,9] The realization of applications depends on their crystal structure like anatase, rutile, and brookite.[10]
Many studies have been focused on the lattice structure and phonon dynamics of anatase TiO2 including the effect of size dependent phonon confinement, stoichiometry defect, lattice strain by aAuthors for correspondence: subodhkgtm@gmail.com
The qualitative comparison of experimental Rutherford Backscattering Spectrometry (RBS) spectra of undoped and doped 5NTO films clearly reveal the existence of Nb doping in the films with systematic increment of Nb concentration, as evidenced by increase in the relative peak intensity of Nb in TiO2 films
Summary
To the best of our knowledge there are very few reports available on the sol gel synthesized doped anatase TiO2 thin films, because most of studies have been carried on nano powders.[14,15] Present study reports the effect of Nb doping; its incorporation in the TiO2 lattice and effects on the structural and optical properties. In this manuscript, the effect of phonon confinement and doping induced non-stoichiometry is investigated by studying the phonon dispersion of Eg(1) mode of NTO films with anatase phase. Phonon dynamics of these modes is further studied by temperature dependent Raman studies
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