Selenium is a nonmetal discovered in 1817, with a density of about 4.81 g/cm³ and a melting point around 221 °C, combining photoconductivity with semiconductor behavior, and finding uses in optoelectronics, glass, and nutrition. Electrostatic copying and laser printing are selenium's most classic uses: the amorphous selenium or selenium-alloy photoreceptor on the drum conducts under light and insulates in the dark, transferring toner to paper through an electrostatic latent image, making it the core component of copiers and laser printers. In the glass industry, selenium acts as both decolorizer and colorant: a trace cancels the green tint from iron impurities to make glass clear, while a slightly larger amount makes red glass for signal lights and art glass. In nutrition, selenium is an essential trace element; sodium selenite and related compounds are used for food fortification, animal feed additives, and dietary supplements, and selenium supplementation was once promoted in low-selenium regions of China to prevent Keshan disease. Selenium also goes into rectifiers, photovoltaic cells, and alloying additives.
Sodium selenite Na2SeO3 is the most common selenium-supplement compound, used for food fortification, animal feed, and dietary supplements, and is also a laboratory selenium source. Cadmium selenide CdSe is an important semiconductor material used in red pigments, photovoltaic cells, and quantum dots; modern quantum-dot displays heavily employ CdSe quantum dots. Zinc selenide ZnSe is transparent to infrared light and is used to make infrared lenses, windows, and laser optics. Lead selenide PbSe is a mid-infrared detector material used in thermal imaging and gas analysis. Selenium sulfide SeS2 is used as an anti-dandruff shampoo active and as an antifungal agent. Selenium dioxide SeO2 serves as an oxidant in organic synthesis and as an additive in manganese electrolysis. Sodium selenosulfate also sees limited use in photographic fixing and chemical analysis. Amorphous selenium thin film is the direct-conversion photoconductor layer of flat-panel digital X-ray detectors, used in medical imaging and security screening.
