Intense red photoluminescence emission of sol-gel-derived nanocrystalline Mg ₂ TiO ₄ Thin Films

In this work, undoped Mg₂TiO₄ thin films were fabricated on p-type Si(111) substrates by the sol-gel method, and the red photoluminescence (PL) of the films is introduced and discussed. According to the experimental results, the red emission appears when the films have been thermally treated at higher temperatures, which have a long range and well-organized crystalline arrangement. Furthermore, to have better realization of the red emission mechanism of Mg₂TiO₄ films, the optical band gap of Mg₂TiO₄ (E_g ^Mg2TiO4) was estimated at ∼3.7 eV; furthermore, 325 nm (corresponding energy, hν = 3.82 eV > E_g^Mg2TiO4) and 633 nm (corresponding energy, hν = 1.96 eV < E_g ^Mg2TiO4) excited light sources were used to clarify the position of the defect levels. In addition, the influence of annealing atmospheres (O ₂, air, and vacuum) on the red emission of our samples is also discussed. A significant variety of red emissions can be found between these annealing conditions: the red emission can be effectively enhanced by O ₂ annealing, but weakened by vacuum annealing. Results reveal that the red emission of Mg₂TiO₄ thin films may be highly dependent on the completeness of the O-X-O (X = Mg, Ti) bonds.