Properties Associated with Phosphors

The following are specific aspects of phosphors we need to discuss before we can delve further into energy dissipation processes. 

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A general feature of doped semiconductive NC materials is described in a review paper authored by Bhargava, one of the founders of this area (8). It is generally accepted that solid particles are richer in crystallographical defects when their diameter becomes smaller. Quantum size effects associated with nanoparticles are very sensitive to the defects. In most cases, defects influence negatively the luminescent properties of phosphors. Most of these drawbacks of NC phosphors are attributed to the larger specific surface area, since defects tend to concentrate themselves in a near-surface region. 

Phospholipids, which are common in mammalian membranes, contain a phosphoric acid ester. A simple example is phosphatidic acid which replaces the fatty acids associated with soaps. The molecules of these acids contain two hydrocarbon hydrophobic chains as indicated in the lipid-water systems of Fig. 1.17. The similarity between the structure and behaviour of lipids and soaps has led to a resurgence of interest in the properties of soap molecules. 

Some substances exist in more than one liquid form liquid crystals or anisotropic liquids, forming various types of mesomorphic phases (nematic, smectic, cholesteric). They share properties normally associated with both liquids and crystals. Sodium soaps have numerous mesomorphic states. Amorphous (metastable) phases of a substance may also display polymorphism. However, often the term polymorph is explicitly restricted to the solid phase and polymorphic transformations are restricted to transformations involving phases with different crystal structures which are part of a single component system such solid polymorphs form an identical liquid on melting. For example, phosphorous has many solid allotropes, which all revert to the same P4 form when melted to the liquid state. In contrast, for isomers the difference between them persists in the liquid phase. Polymorphs (allotropes) can be combined with other substances to give the same compounds isomers give different reaction products. 

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