Bismuth selenide is a semiconductor and a thermoelectric material. While stoichiometric bismuth selenide should be a semiconductor with a gap of 0.3 eV, naturally occurring selenium vacancies act as electron donors, so BiSe is intrinsically n-type.
Bismuth selenide has a topologically insulating ground-state. Topologically protected Dirac cone surface states have been observed in Bismuth selenide and its insulating derivatives leading to intrinsic topological insulators, which later became the subject of world-wide scientific research.
Bismuth selenide is a van der Waals material consisting of covalently bound five-atom layers (quintuple layers) which are held together by van der Waals interactions and spin-orbit coupling effects. Although the (0001) surface is chemically inert (mostly due to the inert-pair effect of Bi), there are metallic surface states, protected by the non-trivial topology of the bulk. For this reason, the BiSe surface is an interesting candidate for van der Waals epitaxy and subject of scientific research. For instance, different phases of antimony layers can be grown on BiSe, by means of which topological pn-junctions can be realised. More intriguingly, Sb layers undergo topological phase transitions when attached to the BiSe surface and thus inherit the non-trivial topological properties of the BiSe substrate.