Static and dynamic structures of monoalkylammonium hexachlorostannate(IV) complexes [CnH2n+1NH3]2 SnCl6 (n = 2 to 6) studied by means of NQR and NMR

Morphological and Optical Tuning of Lead-Free Cs2SnX6 (X = I, Br) Perovskite Nanocrystals by Ligand Engineering

In order to overcome the toxicity of lead halide perovskites, in recent years the research has focused on replacing lead with more environmentally friendly metals like tin, germanium, bismuth or antimony. However, lead-free perovskites still present instability issues and low performances that do not make them competitive when compared to their lead-based counterparts. Here we report the synthesis of lead-free Cs2SnX6 (X = Br, I) nanostructures of different shapes by using various surface ligands. These compounds are a promising alternative to lead halide perovskites in which the replacement of divalent lead (Pb(II)) with tetravalent tin (Sn(IV)) causes a modification of the standard perovskite structure. We investigate the effects of different amines on the morphology and size of Cs2SnX6 (X = Br, I) nanocrystals, presenting a facile hot-infection method to directly synthesize three-dimensional (3D) nanoparticles as well as two-dimensional (2D) nanoplatelets. The amines not only modify the shape of the crystals, but also affect their optical properties: increasing the length of the amine carbon chain we observe a widening in the bandgap of the compounds and a blue-shift of their emission peak. Alongside the tuning of the chemical composition and the reduction of the crystal size, our study offers a new insight in controlling the physical properties of perovskite nanocrystals by means of the capping ligands, paving the way for future research on lead-free materials.

Ligand-mediated synthesis of colloidal Cs2SnI6 three-dimensional nanocrystals and two-dimensional nanoplatelets

Cs₂SnI₆ is a variant on tin-iodide solution-processable materials and may lead to a lead-free material for use in next-generation photovoltaic cells and other optoelectronics. So far, only a few studies have been conducted where shape and geometry control of Cs₂SnI₆ nanocrystals is demonstrated. Here we report a general approach to directly synthesize Cs₂SnI₆ of two-dimensional (2D) layered nanoplatelets as well as three-dimensional (3D) nanocrystals. The shape of Cs₂SnI₆ nanocrystals could be engineered into 3D nanoparticles and different 2D nanoplatelets with well-defined morphology by choosing different organic acid and amine ligands via a hot injection process. Moreover, the thickness of layered 2D nanoplatelets could be adjusted by changing the amount of Cs-oleate present during the synthesis. The photoluminescence emission peaks changed from 643 to 742 nm based on nanomaterial shape. Our method provides a facile and versatile route to rationally control the shape of the Cs₂SnI₆ nanocrystals, which will create opportunities for applications in lead-free optoelectronics.