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Zinc sulfide is an inorganic compound with the chemical formula ZnS. It is found in nature mainly as the black mineral sphalerite, although impurities make it appear white to the naked eye. In dense synthetic form, it can be transparent and is used as a pigment.
In the presence of a few ppm of a suitable activator, zinc sulfide exhibits intense phosphorescence that is employed in many applications from cathode ray tubes to X-ray screens and glow-in-the-dark products. When silver is used as an activator, the emission maximum peaks at a wavelength of about 450 nanometers. Copper gives a greenish glow and is also used in electroluminescent panels.
As a semiconductor material, ZnS has an electron-filled valence band and an empty conduction band. The energy gap between the two is quite large, limiting the number of electrons that can migrate from the valence to the conduction bands. This makes doping with metals necessary to improve the physicochemical properties of the material.
A team of researchers at Ritsumeikan University in Japan, led by Yoichi Kobayashi, discovered that zinc sulfide crystals doped with copper (Cu) ions show remarkable photochromic behavior. When irradiated with visible and ultraviolet light, the crystals turn from creamy white to dark gray and back again in a fraction of a minute, which is much faster than other materials that exhibit this type of photochromism. The reason for this fast reaction is that the copper ions introduce intermediate electronic energy levels into the forbidden band of ZnS.