Bismuth selenide
Bismuth selenide (Bi2Se3) is a gray powder that is a compound of bismuth and selenium also known as bismuth(III) selenide. It is a semiconductor and a thermoelectric material. While perfect stoichiometric bismuth selenide should be a semiconductor (with a gap of 0.3 eV) naturally occurring selenium vacancies act as electron donors and it often acts as a semimetal. Topologically protected surface states have been observed in Bismuth selenide which is the subject of ongoing scientific research.