Abstract
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.
Highlights
In the last decade metal halide perovskites (MHPs) have emerged as a new class of promising semiconductors owing to their peculiar physical properties that make them suitable for different optoelectronic applications. (Fu et al, 2019; Shamsi et al, 2019; Arya et al, 2020; Chouhan et al, 2020; Roy et al, 2020)
One of the most remarkable features of MHPs is the possibility to tune their fundamental bandgap by varying their chemical composition over a broad range of energies, from blue to near infrared, Cs2SnX6 Perovskite Nanocrystals producing a wide gamut of emitted colors. (Mittal et al, 2016; Huang et al, 2017a; Chouhan et al, 2020)
While OLA leads to the formation of 3D NCs as expected, we observe that shorter chain amines cause a modification of the NC morphology leading to the growth of 2D nanoplatelets (NPs) and we investigated the correlation between the ligand molecule length and the changes in the shape, crystal structure and optical properties of the samples
Summary
In the last decade metal halide perovskites (MHPs) have emerged as a new class of promising semiconductors owing to their peculiar physical properties that make them suitable for different optoelectronic applications. (Fu et al, 2019; Shamsi et al, 2019; Arya et al, 2020; Chouhan et al, 2020; Roy et al, 2020). When the size of a crystalline solid is reduced to the nanoscale it starts displaying new properties due to quantum confinement effects, which are not present at macroscopic level To this regard, MHP NCs offer new appealing features in comparison to their bulk form: they present an extremely high photoluminescence quantum yield, reaching peak values of almost 100%, and very narrow emission linewidths. (Swarnkar et al, 2015; Zhang et al, 2015; Gonzalez-Carrero et al, 2016; Shamsi et al, 2019) Another important advantage of perovskite NCs is their easy compositional tunability: being colloidally dispersed, their chemical composition can be modified through facile post-synthetic anion and cation exchange processes. While OLA leads to the formation of 3D NCs as expected, we observe that shorter chain amines cause a modification of the NC morphology leading to the growth of 2D nanoplatelets (NPs) and we investigated the correlation between the ligand molecule length and the changes in the shape, crystal structure and optical properties of the samples
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