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

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.

Metal Work Function Effects on the Performance of UV–Visible–NIR Cs2SnI6 Photodetectors for Flexible Broadband Application

“…[45,47] The higher α value in wavelength from the 400-450 nm region might be due to defect states. [40,46,48] The estimated optical bandgap was 1.36 eV (Figure S7b, Supporting Information), similar to the values previously reported in the literature. [6] The band diagram of metals/CSI under the equilibrium condition is shown in Figure 2b,c.…”

Ultrafast glimpses of the excitation energy-dependent exciton dynamics and charge carrier mobility in Cs2SnI6 nanocrystals

“…1D), which is due to the existence of a huge density of electronic states (emanating from defects) lying below the conduction band in this material. 22 Unlike the CsSnX 3 counterparts, where the first perceptible peak marks the excitonic transition from valence band maxima (VBM) to conduction band minima (CBM), analogous transition in this centrosymmetric crystal structure is known to be optically inactive. 23 This is an outcome of the quantum selection constraint due to the existence of inversion symmetry, which renders this transition dipole forbidden.…”

Physically Detachable and Operationally Stable Cs2SnI6 Photodetector Arrays Integrated with µ‐LEDs for Broadband Flexible Optical Systems

“…The higher PDPs at 455nm is possibly due to a higher absorption coefficient α value (Figure S9a, Supporting Information), which is attributed to the presence of intermediate bands/defect states within in optical bandgap of CSI. [46][47][48][49][50] However, further investigation is required to understand defects states and their correlation with the broadband photodetection performance. The obtained PDPs properties of CSI PD-arrays are comparable with lead-based PD-arrays and previously reported CSI PDs (Tables S8 and S9, Supporting Information).…”