Preparation And Luminescence Properties Of Rare-earth Doped MSi₂O₂N₂ (M=Ca,Sr,Ba) Phosphors

White light-emitting diodes (LEDs) have drawn much attention owing to their excellent properties, such as high luminous efficiency, low power consumption, environment friendly, reliability, long life and so on. White LEDs show high potential for replacement of conventional lighting like incandescent and fluorescent lamps, and are considered as the fourth generation solid-state lighting. Today, commercial white LEDs are phosphor-converted-LEDs, which connect a blue LED chip and the phosphor. The phosphor plays an important role in white LEDs for improving the color rendering index and luminescence efficiency. So it is necessary to develop new phosphors for white LEDs.Silicon-based (oxy)nitrides are generally built up of networks of crosslinking Si(O,N)4 tetrahedra. The excited state of the 5d electrons of rare-earth elements is significantly lowered to low energy due to large crystal-field splitting and a strong nephelauxetic effect as a result of a high degree of crosslinking Si(O,N)4 tetrahedra in the structure of silicon-based (oxy)nitrides. This enables silicon-based (oxy)nitride to be excited efficiently by UV or blue-light irradiation. The structural versatility of (oxy)nitride phosphors makes it possible to attain all the emission colors of blue, green, yellow, and red; thus, they are suitable for using in white LEDs. This novel class of phosphors has been seemingly the most promising materials nowadays because of their high thermal and chemical stability and excellent photoluminescence properties. In present work, a systematic research was carried out on the processing methods, luminescence properties, spectral tuning of the rare earth doped MSi₂O₂N₂(M=Ca,Sr,Ba). The main work contents and achievements can be summarized as the following:1, MSi₂O₂N₂:Eu²⁺(M=Ca,Sr,Ba) phosphors were prepared through a conventional solid state reaction method and a two step method with M2SiO4 as a precursor. The effect of formation processing on phase type and luminescence properties of samples was investigated. Low firing temperature leads to the sample including impurities and low luminescence intensity. Due to forming low content of the intermediate phase, the sample by the two step method shows luminescence intensity 1.5 times higher than that by the conventional method.2, Luminescence properties of Eu²⁺ or Ce3+ doped MSi₂O₂N₂(M=Ca,Sr,Ba) were systematically investigated. The excitation bands of MSi₂O₂N₂:Eu²⁺ (M=Ca,Sr,Ba) cover the spectral region from UV to the visible part(250-500nm), the emission spectra show a single intense board emission band centered at 560nm,535nm and 490nm for M=Ca, Sr, Ba, respectively, which is ascribed to the allowed 4f65d→4f7 transitions of Eu²⁺. The excitation spectra of MSi₂O₂N₂:Ce3+(M=Ca,Sr,Ba) cover a broad band from 250 to 370nm. The emission bands of MSi₂O₂N₂:Ce³⁺ center at 390,395 and 396nm for M=Ca, Sr, Ba, respectively. With increasing the concentration of Eu²⁺or Ce³⁺, red-shift and concentration quenching of emission spectra were observed.3, The effect of co-activator (Mn²⁺, Ce³⁺, and Dy³⁺) on the luminescence behavior of Eu²⁺ activated MSi₂O₂N₂(M=Ca,Sr,Ba) phosphor was discussed in detail. The emission intensities of Eu²⁺ can be enhanced by co-doping with Mn²⁺, Ce³⁺, and Dy³⁺ in MSi₂O₂N₂(M=Ca,Sr,Ba). There is an energy transfer between Ce³⁺ and Eu²⁺. The calculations of the efficiency of energy transfer from Ce³⁺ to Eu²⁺ and the critical distance between Ce³⁺ and Eu²⁺ suggest resonance-type energy transfer mechanism from Ce³⁺ to Eu²⁺ is due to dipole-dipole interactions.4, The intersolubility in the CaSi₂O₂N₂-SrSi₂O₂N₂-BaSi₂O₂N₂ system and the tenability of the emission of Eu²⁺ in these hosts were systematically investigated. Well solid solution can be formed throughout the whole composition range between CaSi₂O₂N₂ and SrSi₂O₂N₂, which have the same crystal structure, and therefore the emission of Eu²⁺ in these hosts can be continuously tuned. In the SrSi₂O₂N₂-BaSi₂O₂N₂ and CaSi₂O₂N₂-BaSi₂O₂N₂ system, solid solution can only be formed in some particular composition range due to different crystal structures. Therefore, the emission of Eu²⁺ in these hosts can only be tuned in composition ranges under which solid solution is formed.