Sol-gel processes for protection and synthesis of luminescent materials

Sol-gel chemistry has a very broad application area such as thin films and coatings, monoliths, powders, grains and spheres, fibers, composites, porous gels and membranes. From thin film to powder, different kind of materials with excellent control of stoichiometry, density and microstructure can be synthesized using simple equipment without the need for vacuum at relatively low temperatures. Using sol-gel, material properties can be modified by changing only one parameter during the preparation. Rare earth doped alkaline earth binary and ternary sulfides have a very special place in luminescent materials because of their relatively low synthesis temperature and broad emission spectra upon doping with europium and cerium. Particularly to obtain red or orange emission CaS:Eu2+ and SrS:Eu2+ are considered suitable candidates. Ca1−xSrxS:Eu2+ phosphors with a strong absorption in the blue region are currently also used in white-light emitting diodes. In addition to wavelength converters in LEDs, alkaline earth sulfide phosphors are employed in different areas such as display applications, electroluminescent devices and optical information storage. Nevertheless, the lack of stability with respect to water and other atmospheric components hinders their usage as phosphor hosts. A number of encapsulation techniques have been utilized to improve the stability of sulfide phosphors and sol-gel is the one of the most attractive techniques. This work focuses on thin films, protection coatings, powders and luminescent materials prepared using sol-gel chemistry. TiO2 and Al2O3 powders, thin films and protection layers were synthesized via water free sol-gel. In addition to the optical and structural properties of the products, the effect of the coating on the stability of sulfide particles was investigated. Al2O3 synthesized via sol-gel was also compared with Al2O3 synthesized via ALD (atomic layer deposition) as protection layer. Lastly, undoped and persistent luminescent Eu and Nd-doped CaAl2O4 powders were synthesized via water free sol-gel technique. Effect of the calcinations temperature and the doping concentration on the structure and photoluminescence properties was investigated.