Tin selenide, also known as stannous selenide, is an inorganic compound with the formula SnSe. Tin(II) selenide is a typical layered metal chalcogenide as it includes a group 16 anion (Se2−) and an electropositive element (Sn2+), and is arranged in a layered structure. Tin(II) selenide is a narrow band-gap (IV-VI) semiconductor structurally analogous to black phosphorus. It has received considerable interest for applications including low-cost photovoltaics, and memory-switching devices. Because of its low thermal conductivity as well as reasonable electrical conductivity, tin selenide is one of the most efficient thermoelectric materials. Structure Tin(II) selenide (SnSe) crystallizes in the orthorhombic structure that derives from a distorted rock-salt structure. It is isomorphous to germanium selenide (GeSe). The unit cell encompasses two inverted layers. Each tin atom is covalently bonded to three neighboring selenium atoms, and each selenium atom is covalently bonded to three neighboring tin atoms. The layers are held together primarily by van der Waals forces. At temperatures above 800 K its structure changes to rock-salt structure. At pressures above 58 GPa, SnSe acts as a superconductor; this change of conductivity is likely due to a change in the structure to that of CsCl. In recent years, it has become evident that new polymorphs of SnSe exists based upon the cubic and orthorhombic crystal systems, known as π-SnSe (space group: P213, No. 198) and γ-SnSe (space group: Pnma, No. 62) Synthesis Tin(II) selenide can be formed by reacting the elements tin and selenium above 350 °C. Problems with the composition are encountered during synthesis. Two phases exist—the hexagonal SnSe2 phase and the orthorhombic SnSe phase. Specific nanostructures can be synthesized, but few 2D nanostructures have been prepared. Both square SnSe nanostructures and single-layer SnSe nanostructures have been prepared. Historically, phase-controlled synthesis of 2D tin selenide nanostructures is quite difficult. Sheet-like nanocrystalline SnSe with an orthorhombic phase has been prepared with good purity and crystallization via a reaction between a selenium alkaline aqueous solution and tin(II) complex at room temperature under atmospheric pressure. A few-atom-thick SnSe nanowires can be grown inside narrow (~1 nm diameter) single-wall carbon nanotubes by heating the nanotubes with SnSe powder in vacuum at 960 °C. Contrary to the bulk SnSe, they have the cubic crystal structure. Chemistry Tin(II) selenide adopts a layered orthorhombic crystal structure at room temperature, which can be derived from a three-dimensional distortion of the NaCl structure. There are two-atom-thick SnSe slabs (along the