Tin selenide

Tin selenide, also known as stannous selenide, is an inorganic compound with the formula (SnSe), where Tin has a +2 oxidation state. Tin(II) selenide is a narrow band-gap (IV-VI) semiconductor and has received considerable interest for applications including low-cost photovoltaics and memory-switching devices. Tin(II) selenide is a typical layered metal chalcogenide; that is, it includes a Group 16 anion (Se2−) and an electropositive element (Sn2+), and it is arranged in a layered structure. Tin selenide, also known as stannous selenide, is an inorganic compound with the formula (SnSe), where Tin has a +2 oxidation state. Tin(II) selenide is a narrow band-gap (IV-VI) semiconductor and has received considerable interest for applications including low-cost photovoltaics and memory-switching devices. Tin(II) selenide is a typical layered metal chalcogenide; that is, it includes a Group 16 anion (Se2−) and an electropositive element (Sn2+), and it is arranged in a layered structure. Tin(II) selenide exhibits low thermal conductivity as well as reasonable electrical conductivity, creating the possibility of it being used in thermoelectric materials. In 2014, a team at Northwestern University has established the world record performance for thermoelectric material efficiency. Tin(II) selenide (SnSe) has stiff bonds and distorted lattice, crystallizing in the orthorhombic GeSe (germanium selenide) structure. To be isomorphous, two substances must have the same chemical formulation, and they must contain atoms with corresponding chemical properties and with similar atomic radii. Tin(II) selenide exists in a doubled layered structure that derives from a distorted rock-salt structure. Within these double layers, each tin atom is covalently bonded to three neighboring selenide (Se) atoms, and each selenide atom is covalently bonded to three neighboring tin atoms. The double layers are then held together primarily by van der Waals forces. Tin(II) selenide’s layered structure bestows both anharmonic and anisotropic bonding to the compound. 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 a CsCl structure. 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. SnSe nanocrystals have also been synthesized by a gas-phase laser photolysis reaction that used Sn(CH3)4 and Se(CH3)2 as precursors. 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.