Intrinsic Formamidinium Tin Iodide Nanocrystals by Suppressing the Sn(IV) Impurities

Structural and optical control through anion and cation exchange processes for Sn-halide perovskite nanostructures

Metal halide perovskite nanostructures, characterized by their ionic nature, present a compelling avenue for the tunability of dimensions and band gaps through facile compositional transformations involving both cationic and anionic exchange reactions. While post-synthetic ion-exchange processes have been extensively explored in Pb-halide perovskite nanocrystals, the inherent instability of Sn2+ has limited the exploration of such processes in Sn-halide perovskite nanostructures. In this study, we present a straightforward cation exchange process wherein 2D [R-NH3]2SnX4 Ruddlesden-Popper (RP) nanostructures with n = 1 transition to 3D ASnX3 nanocrystals at room temperature with the addition of A-cation oleate. In addition, anion exchange processes have been demonstrated for both 2D [R-NH3]2SnX4 RP nanostructures and 3D nanocrystals, showcasing transitions between iodide and bromide counterparts. Furthermore, we have fabricated a thin film of 2D [R-NH3]2SnX4 RP nanostructures for cation exchange, wherein A-cation diffusion through a liquid-solid interface facilitates the transformation into a 3D ASnX3 crystal. This investigation underscores the versatility of ion exchange processes in engineering the composition of Sn-halide perovskite nanostructures and, consequently, modulating their optical properties.

Stable Inorganic Colloidal Tin and Tin-Lead Perovskite Nanocrystals with Ultralong Carrier Lifetime via Sn(IV) Control

Inorganic tin (Sn) perovskite nanocrystals offer a promising solution to the potential toxicity concerns associated with their established lead (Pb)-based counterparts. Yet, achieving their superior stability and optoelectronic properties remains an ongoing challenge. Here, we report a synthesis of high-symmetry α-phase CsSnI3 nanocrystals with an ultralong 278 ns carrier lifetime, exceeding previous benchmarks by 2 orders of magnitude through meticulous Sn(IV) control. The nanocrystals demonstrate excellent colloidal stability, uniform monodispersity, and a distinct exciton peak. Central to these outcomes is our designed solid-liquid antioxidation suspension of tri-n-octylphosphine (TOP) and zerovalent tin (Sn(0)) that fully addresses the unique coexisting oxygen-driven and solvent-driven Sn oxidation mechanisms in Sn perovskite nanocrystal synthesis. We uncover the largely undervalued function of TOP in mitigating oxygen-driven Sn oxidation and introduce Sn(0) powder to generate a synergistic antioxidation function with TOP, significantly reducing Sn(IV)-induced defects and distortions and contributing to enhanced optoelectronic properties. Strikingly, this approach also profoundly impacts inorganic Sn-Pb perovskite nanocrystals, boosting lifetimes by 2 orders of magnitude and increasing photoluminescence quantum yield over 100-fold to 35%. Our findings illuminate the potential of Sn-based nanocrystals for optoelectronic applications.

Colloidal Aziridinium Lead Bromide Quantum Dots

The compositional engineering of lead-halide perovskite nanocrystals (NCs) via the A-site cation represents a lever to fine-tune their structural and electronic properties. However, the presently available chemical space remains minimal since, thus far, only three A-site cations have been reported to favor the formation of stable lead-halide perovskite NCs, i.e., Cs+, formamidinium (FA), and methylammonium (MA). Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a facile colloidal synthesis of AZPbBr3 NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots (QDs) in the regime of strong quantum confinement. NMR and Raman spectroscopies confirm the stabilization of the AZ cations in the locally distorted cubic structure. AZPbBr3 QDs exhibit bright photoluminescence with quantum efficiencies of up to 80%. Stabilized with cationic and zwitterionic capping ligands, single AZPbBr3 QDs exhibit stable single-photon emission, which is another essential attribute of QDs. In particular, didodecyldimethylammonium bromide and 2-octyldodecyl-phosphoethanolamine ligands afford AZPbBr3 QDs with high spectral stability at both room and cryogenic temperatures, reduced blinking with a characteristic ON fraction larger than 85%, and high single-photon purity (g(2)(0) = 0.1), all comparable to the best-reported values for MAPbBr3 and FAPbBr3 QDs of the same size.