Photoluminescent research and mechanism

The scientific research on phosphors has a long history going back more than 100 years. A prototype of the ZnS-type phosphors, an important class of phosphors for television tubes, was first prepared by Théodore Sidot, a young French chemist, in 1866 rather accidentally. It seems that this marked the beginning of scientific research and synthesis of phosphors

From the late 19th century to the early 20th century, Lenard et al. in Germany performed active and extensive research on phosphors, and achieved impressive

results. They prepared various kinds of phosphors based on alkaline earth chalcogenides (sulfides and selenides) and zinc sulfide, and investigated the luminescence properties

They established the principle that phosphors of these compounds are synthesized by introducing metallic impurities into the materials by firing. The metallic impurities, called luminescence activators, form luminescence centers in the host. Lenard and coworkers tested not only heavy metal ions but various rare-earth ions as potential activators. Alkaline chalcogenide phosphors developed by this research group are called Lenard Phosphors, and their achievements are summarized in their book

Pohl et al. in Germany investigated Tl+ activated alkali halide phosphors in detail inthe late 1920s and 1930s. They grew single-crystal phosphors and performed extensive spectroscopic studies. They introduced the configurational coordinate model of luminescence centers in cooperation with F. Seitz in the U.S. and established the basis of present day luminescence physics

Leverenz et al. at Radio Corporation of America also investigated many practical phosphors with the purpose of obtaining materials with desirable characteristics to be used in television tubes. Detailed studies were performed on ZnS-type phosphors. Their achievements are compiled in Leverenz’s book

Since the end of World War II, research on phosphors and solid-state luminescence has evolved dramatically. This has been supported by progress in solid-state physics, especially semiconductor and lattice defect physics; advances in the understanding of the optical spectroscopy of solids, especially that of transition metals ions and rare- earth ions, have also helped in these developments

Research on phosphors and their applications requires the use of a number of fields in science and technology. Synthesis and preparation of inorganic phosphors are based on physical and inorganic chemistry. Luminescence mechanisms are interpreted and elucidated on the basis of solid-state physics. The major and important applications of phosphors are in light sources, display devices, and detector systems. Research and development of these applications belong to the fields of illuminating engineering, electronics, and image engineering. Therefore, research and technology in phosphors require a unique combination of interdisciplinary methods and techniques, and form a fusion of the above mentioned fields

In fluorescent materials electrons have energies in the impurity state band for a very short time (10-9 to 10-6 seconds). Then, they emit light as their energy changes to energy in the valence band. As a result, fluorescent materials will only glow while light of sufficient energy shine on them. In phosphorescent objects, the electrons remain in the impurity band for a while. After this time delay the electrons emit light as their energy changes. Thus, phosphorescent substances have the ability to store

up light and release it gradually. The notion of a metastable state explains this. If the molecules of the substance can get from the ground state to a metastable state, and if the metastable state can slowly decay back to the ground state via photon emission, then we have phosphorescence. Typically, the metastable state is a triplet state, and the ground state is a singlet one. Ground state molecules absorb photons and go to excited singlet states. Most of them immediately hop right back to the ground state, emitting a photon, but non-radiative processes take a few to a less energetic triplet state. Once these molecules get to the lowest triplet state, they are stuck there, at least for a while. Some low probability process accomplishes the triplet-singlet conversion, and the molecules slowly leak out light

The electrons remain with energy in the impurity band because the physical situation “forbids” a direct transition from impurity to valence band. To change energy to that of the valence band the electrons must first gain back the thermal energy that they lost when they made the transition to the impurity band. Because the energy is small, it can generally be provided by the ambient energy in the air