the elemental or free ion quantum properties of the luminescent center and the host

material has an effect on photon energy of the transition elements [40].

 2.6 ZnS:ReF3 as ACTFEL Phosphor

 The electroluminescence studies of zinc sulphide doped with various rare earths

were initiated in the late 1960s and several colors were observed from the ZnS:ReF3 thin

films [41,42]. Now, rare-earth activated ZnS phosphors have attracted attention as

suitable candidates for full color ac thin film electroluminescent displays [43, 44].

Though a lot of work has been done of understanding visible emission, infrared emission

has been ignored. In this research, particular attention will be given to the infrared

emission of phosphors because they have wide applications in industry, such as

developing night vision equipment like helmet mounted cameras, goggles for friend/foe

identification, transmitting signals for fiber optical communication and handheld remote

controls, to name just a few.

2.6.1 ZnS

 As we have mentioned before, ZnS is by far the most widely studied and also the

most commonly used host material for commercial ACTFEL displays due to its unique

physical nature and excellent optical properties.

 Physically, ZnS lattice sites are tetrahedrally coordinated. Since Laporte-forbidden

transitions can be relaxed when the atom or ion is placed in a site without a center of

symmetry, the tetrahedral nature of ZnS makes the brightest red and green ACTFEL

phosphors efficient.

Optically, ZnS has a bandgap of 3.8 eV. It is therefore transparent throughout the visible

range and optical absorption in the phosphor layer is minimal. In addition, high dielectric

breakdown strength (1 MeV/cm) allows efficient excitation of the luminescent centers.