Light emission from ferroelectric barium titanate nanocrystals

Thin laminates (thickness δ = 20–45 nm) of barium titanate, BaTiO₃ (BT), have been synthesised by the sol–gel method followed by heating of the amorphous precursor powder in air. An orthorhombic BT polymorph forms along with a tetragonal phase (t-BT) after 2 h of heating the precursor at 600°C, as evidenced by a well-defined X-ray diffraction pattern. The average crystallite size D ～ 21 nm compares the δ-value obtained from the scanning electron micrograph. On heating at temperatures as high as 750°C, the structure remains mostly t-BT (D ? 44 nm), which emits violet–blue–green light at a wavelength of 380–580 nm with a maximum at around 422 nm. The emission extends to wavelengths shorter than ～300 nm owing to a quantum-size effect in smaller crystals processed at lower temperatures. Electron paramagnetic resonance (EPR), which occurs at a g-value of 1.995 (linewidth ΔH = 1.12 mT) in a sample heated at 400°C, shows an order of magnitude of lower intensity at g = 1.993 (ΔH = 3.69 mT) on annealing out the paramagnetic defects at 600°C in air. No EPR signal arises in t-BT free from such defects in larger crystals.