Effect of Mn doping on the structural and optical properties of ZrO2 thin films prepared by sol–gel method

Abstract

Homogeneous and transparent Mn doped ZrO2 thin films were prepared by sol–gel dip coating method. The films were annealed in air atmosphere at 500°C. The X-ray diffraction pattern of the undoped ZrO2 thin film revealed a mixed phase of tetragonal and monoclinic ZrO2 with preferred orientations along T(111) and M(−111). Grazing Incidence X-ray Diffraction of Mn doped ZrO2 thin films reveals the introduction of Mn interstitial in ZrO2 which stabilize the mixed phase of ZrO2 into tetragonal phase. Atomic force microscope image shows the addition of catalyst (Mn) which stops isotropic agglomeration of particles, instead of anisotropic agglomeration that occurred resulting in growth of particles in certain direction. Average transmittances of >70% (in UV–vis region) were observed for all samples. The optical band gap decreased from 5.72 to 4.52eV with increase in Mn doping concentration. The reduced band gap is due to the introduction of impurity levels in the band gap, by incorporation of the metal ions into the ZrO2 lattice. The d-electron of Mn (t2g level) can easily overlap with the ZrO2's valence band (VB) because t2g of Mn is very close to VB of ZrO2. This overlap caused a wide VB and consequently decreases the band gap of ZrO2. The photoluminescence (PL) spectrum of undoped zirconia thin film exhibits an intense near band edge emission peak at 392.5nm (3.15eV) and weak emission peaks at 304 (4.07eV), 604nm (2.05eV) and 766nm (1.61eV). Additional PL peaks were observed for Mn doped ZrO2 located at around 420, 447 (blue), 483 (blue) and 529 (green)nm respectively. These peaks were due to the redox properties of various valence state of Mn in ZrO2. The prepared Mn doped ZrO2 thin films can be applied in optical devices.