Light conversion

Divalent europium-activated BaECl was the first rare-earth-activated x-ray phosphor (24). The advantage of BaECLEu " over the conventional CaWO material is in the higher x-ray absorption and better x-ray-to-visible light conversion. The problem with BaECl for x-ray appHcation is in the lower density (4.56 g/cm vs 6 g/cm for CaWO and plate-like morphology. 

But not all materials emit the same amount of light when heated to the same temperature there is a spectral distribution of electromagnetic waves. For example, a piece of glass and a piece of iron when heated in the same furnace look different the glass is nearly colourless yet feels hotter to the skin because it emits more infrared light conversely, the iron glows because it emits visible as well as infrared light. 

When a candle burns, it emits heat and light. Conversely, heat is required to melt ice. In this chapter, the relationship between the amount of heat released or taken by a reaction will be discussed. 

Finally, a promising trend in the study of the photoelectrochemical behavior of objects, whose nature is close to that of semiconductors, is related to photobiology, in particular to processes of light conversion in natural photosynthesizing objects. Certain elementary stages of photosynthesis, particularly photoelectrochemical ones, can, apparently, be simulated in some cases within the framework of the concepts of photoelectrochemistry of semiconductors. 

A few other applications of dithiolenes make use of their redox properties. Kumar et a/.219 proposed the use of dithiolenes as photosensitizers. Umezawa et al.22<> coated a Pt cathode with (Et4N)Ni(mnt)2 and saw a modest degree (1.4 x 10 4 %) of light conversion upon irradiation. On the opposite side, Bradley et al.721 used dithiolenes to stabilize n-type Si anodes against photoanodic decomposition. 

Chl-coated semiconductor (n-type) electrodes and metal electrodes can act as efficient photoanodes and photocathoes, respectively, for visible light conversion. The former system functions as a dye-sensitized semiconductor electrode, while the latter is presumably driven by the photoconductive properties of a Chi solid layer and/or charge separation involving the Chl-metal contact barrier. 

Fig. 18. Light-conversion photochemical molecular device, consisting of two components, a light collector (or antenna, lightabsorbing groups A) and a light emitter E, and performing a three-step process absorption (A), energy transfer (ET), and emission (E).

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