SnSe

SnSe, or tin(II) selenide, is a layered IV–VI semiconductor that has attracted attention for its thermoelectric properties. At room temperature it forms an orthorhombic crystal structure and can be prepared as bulk crystals, thin films, or exfoliated nanosheets.

The room-temperature structure is orthorhombic Pnma, consisting of puckered SnSe layers arranged in a low-dimensional, layered geometry. With increasing temperature, SnSe undergoes a phase transition to a higher-symmetry phase near 800 K (to Cmcm). The material has a narrow band gap around 0.8–0.9 eV and tends to be p-type due to tin vacancies and intrinsic defects.

SnSe exhibits exceptionally low lattice thermal conductivity because of its layered structure and strong lattice anharmonicity, while maintaining relatively high Seebeck coefficients. This combination leads to high thermoelectric figure of merit (ZT) at elevated temperatures, especially along certain crystallographic directions.

Thermoelectric performance: Early studies reported ZT values up to about 2.6 in single-crystal SnSe at around 923 K. In polycrystalline and nanostructured samples, ZT values around 1–2 have been observed with appropriate doping and processing. Researchers have explored alloying and nanostructuring to optimize power factors and reduce thermal conductivity further.

Synthesis and processing: SnSe can be prepared by direct reaction of tin and selenium or by chemical vapor transport, Bridgman growth, or zone-melting techniques to produce high-quality bulk crystals. Thin films and nanoscale forms are fabricated by chemical vapor deposition, molecular beam epitaxy, or solution-based routes, and may be mechanically exfoliated to few-layer sheets.

Applications: The material is considered a strong candidate for thermoelectric energy conversion in the mid-to-high temperature range and is studied for waste-heat recovery and solid-state cooling. Ongoing research includes phase stability, doping strategies, and integration into devices.