Optical properties and stability of Bi doped La2O2S

Luminescent materials activated by bismuth (Bi) ions exhibit interesting optical properties due to a strong interaction with the surrounding host lattice. The host lanthanum oxide (La2O3) has previously been reported to be hygroscopic and transforms to a hydroxide within days when exposed to the atmosphere. Therefore Bi3+ doped lanthanum oxysulphide (La2O2S) phosphor was synthesized to compare its stability and suitability as a blue emitting phosphor material. Synthesis was performed via the ethanol-assisted solution combustion method, followed by annealing for 2 h at 900 degrees C in a reducing atmosphere (5% H-2 in Ar gas). X-ray diffraction data confirmed that all samples crystallized in the La2O2S hexagonal lattice. Scanning electron microscopy data showed that the particles aggregated and had irregular shapes. Energy dispersive X-ray spectroscopy confirmed the chemical composition, although the Bi dopant could not be identified since its expected peak position overlapped that of S. The samples, measured by diffuse reflectance spectroscopy, were absorbing in the ultraviolet range between 220 and 350 nm. The band gap of the pure host, La2O2S, was found to be 4.90 eV. Excitation at wavelengths of 260 nm and 344 nm resulted in a single broad blue luminescence emission band, centred at 456 nm, which was attributed to transitions between the P-3(1) excited state of Bi3+ ions and the S-1(0) ground state. La2O2S:Bi phosphor was found to have a similar emission colour as La2O3:Bi, although less pure and closer to the centre of the Commission intemationale de l'eclairage (CIE) diagram. Although the emission intensity of the La2O2S:Bi phosphor was initially less than the La2O2:Bi phosphor, it was found to be stable and therefore superior for applications where the phosphor will be exposed to the atmosphere. The La2O2S:Bi phosphor also exhibited persistent luminescence which was attributed to the Bi3+ ions acting as hole traps and host defects acting as electron traps.