A Review on Synthesis of Nanophosphors – Future Luminescent Materials

Nanophosphors have been extensively investigated during the last decade due to their application potential for various high-performance and novel displays and devices. Synthesis of nanophosphors is generally done by chemical methods. Under chemical methods, different routes such as colloidal, capping, cluster formation, sol-gel, electrochemical etc., are being followed. Physical methods widely used are molecular beam epitaxy, ionised cluster beam, liquid metal ion source, consolidation, sputtering and gas aggregation of monomers are much sophisticated. Chemical precipitation in presence of capping agents, reaction in microemulsions, sol gel reaction and autocombustion are commonly used techniques for synthesis of nanophosphors. Particle size must be less than twice of Bohr radii of exciton as quantum confinement regime is limited to that size. A brief review of different synthesis techniques employed all over the world for the development of industrially important nanophosphors and extent of particle size reduction achieved is discussed. The approach being followed by us at NPL shall also be given. Introduction Semi-conductors in nano-crystallized form exhibit markedly different electrical, optical and structural properties as compared to those in the bulk form. Out of these, the ones suited as phosphor host material show considerable size dependent luminescence properties when an impurity is doped in a quantum-confined structure. The impurity incorporation transfers the dominant recombination route from the surface states to impurity states. If the impurity-induced transition can be localized as in the case of the transition metals or the rare earth elements, the radiative efficiency of the impurityinduced emission increases significantly. The emission and decay characteristics of the phosphors are, therefore, modified in nanocrystallized form. Also, the continuous shift of the absorption edge to higher energy due to quantum confinement effect, imparts these materials a degree of tailorability. Obviously, all these attributes of a doped nanocrystalline phosphor material are very attractive for optoelectronic device applications. Nanoparticles, in general, are supposed to have nearly half of their atoms contained in top two monolayers, which make optical properties highly sensitive to surface morphology. Blue shift of band gap and strong non-linear response of nanoparticles of CdS and CdSe in glass samples were first reported in the early 1980s. Enhanced quantum properties were further confirmed with study of other semiconductor nanoparticles of ZnS, PbS, ZnSe and CdSe. Metal nanoparticles were also synthesized with a view to prepare better catalysts. The size has to be less than twice of Bohr radii of exciton (3-5 nm) for quantum confinement regime. In the present talk, I shall review different nano luminescent materials, their preparation strategy, crystalline phase, size and luminescent property. What is the future of nanophosphors and related devices? Are nanoscale display devices, detectors and light sources going to be reality? Preparation Strategies and Results Chemical Precipitation In this strategy we control the size by arrested precipitation technique. The basic trick has been to synthesis and studies the nanomaterial in situ i.e. in the same liquid medium avoiding the physical changes and aggregation of tiny crystallites. Thermal coagulation and Oswald ripening were controlled by double layer repulsion of crystallites using non-aqueous solvents at lower temperatures for synthesis. The first report on nanosize luminescent materials was by Bhargava et al in 1994. They synthesized manganese-doped nanocrystals of zinc sulfide. The nanomaterials had external photoluminescence quantum efficiency of 18 %. The synthesis involved reaction of diethyl zinc with hydrogen sulfide in toluene. The dopant manganese is added as ethylmanganese in tetrahydrfuran solvent to the parent solution of zinc salt before precipitation reaction. Surfactant methacrylic acid was used to maintain separation between the particles formed. The dried material was further subjected to UV curing for possible polymerization of surfactant methacrylate capping film on the surface of Mn doped ZnS nano cluster for imparting true quantum confinement. The enhancement of efficiency has been explained on the basis of surface passivation of the nanocrystals due to photopolymerization of the surfactant. The photoluminescent (PL) and photoluminescence excitation (PLE) spectra of the nanophosphor have been compared with bulk ZnS:Mn. The PL is slightly shifted and there is a larger linewidth in the nanophosphor as compared to bulk. Following the encouraging results of Bhargava et al on Mn doped ZnS nanophosphor Khosravi et al reported synthesis of manganese doped ZnS nanoparticles by aqueous method. Aqueous solution of zinc chloride