Controllable Colloidal Synthesis of Tin(II) Chalcogenide Nanocrystals and Their Solution-Processed Flexible...

A systematic colloidal synthesis approach to prepare tin(II, IV) chalcogenide nanocrystals with controllable valence and morphology is reported, and the preparation of solution-processed nanostructured thermoelectric thin films from them is then demonstrated. Triangular SnS nanoplates with a recently-reported π-cubic structure, SnSe with various shapes (nanostars and both rectangular and hexagonal nanoplates), SnTe nanorods, and previously reported Sn(IV) chalcogenides, are obtained using different combinations of solvents and ligands with an Sn⁴⁺ precursor. These unique nanostructures and the lattice defects associated with their Sn-rich composition allow the production of flexible thin films with competitive thermoelectric performance, exhibiting room temperature Seebeck coefficients of 115, 81, and 153 μV K^-1 for SnS, SnSe, and SnTe films, respectively. Interestingly, a p-type to n-type transition is observed in SnS and SnSe due to partial anion loss during post-synthesis annealing at 500 °C. A maximum figure of merit (ZT) value of 0.183 is achieved for an SnTe thin film at 500 K, exceeding ZT values from previous reports on SnTe at this temperature. Thus, a general strategy to prepare tin(II) chalcogenide nanocrystals is provided, and their potential for use in high-performance flexible thin film thermoelectric generators is demonstrated.