Green-Emitting Lead-Free Cs4SnBr6 Zero-Dimensional Perovskite Nanocrystals with Improved Air Stability

We report the synthesis and characterization of nanocrystals of a novel fully inorganic lead-free zero-dimensional perovskite, Cs4SnBr6. Samples are made of crystals with an average size of ∼20 nm with green emission centered around 530 nm. Interestingly, both colloidal suspensions and thin films show an enhanced air stability with respect to that of any other previous tin-based nanocrystalline system, with emission persisting for tens of hours under laboratory air.

Synthesis of Cs 4 SnBr 6 Nanocrystals. Nanocrystals were synthesized using a hot-injection method. In a typical procedure, the Cs-oleate precursor was synthesized separately. First, a mixture of 0.0656 g of Cs 2 CO 3 and 0.2 mL of ODE, 0.2 mL of OLA and 0.2 mL of OA was stirred and degassed at 130 °C under vacuum for 1 hour to generate Cs-oleate precursor. Second, 0.3481 g of SnBr 2 are dissolved in 1,2 mL of TOP and quickly added to the mixture in a 10-mL three-neck flask, followed by mild degassing and nitrogen purging. The reaction was quenched after 1 minute. To the as-synthesized nanocrystals was added 1-butanol followed by centrifugation at 4000 rpm for 3 min and dispersion in 2 mL of hexane.

X-ray diffraction
The crystal structures of the samples were characterized by room temperature Cu-radiation Powder X-ray diffraction (XRD) on a Bruker D8 diffractometer. Scans were performed in the 10-40° range, with a step size 0.04° and a counting time of 3 s per step.

Transmission electron microscopy (TEM)
S5 TEM images were collected on a JEOL JEM-1200 EX II microscope operating at 100 kV (tungsten filament gun) and equipped with the TEM CCD camera Olympus Mega View G2 with 1376 x 1032 pixel format. Samples were prepared by drop-casting the solution on coated copper grids.

Steady State PL and Time-Resolved Photoluminescence Measurements
Steady state and time resolved photoluminescence measurement where carried out on Horiba a Fluorolog-3, with a PMT as detector. The excitation source for the TCSPC is a Horiba nanoLED-370 with an excitation wavelength of 369nm, a pulse duration of 1.3ns and a repetition rate of 1 MHz.

Time-Resolved Photoluminescence Measurements
For TRPC measurements the samples have been loaded into air-tight resonant cavity (low intensity measurements) and open cell (high intensities) holders in a N 2 filled glovebox. The excitation source for the TCSPC is a Horiba nanoLED-39 370 with an excitation wavelength of 369 nm, a pulse duration of 1.3 ns and a repetition rate of 100 KHz.

EDX Analysis
Elemental analyses of the powders were performed by Energy Dispersive X-ray Analysis (EDX) by a X-max 50 mm 2 probe (Oxford Instrument) connected to a EVO MA10 scanning electron microscope (SEM). The powders were dispersed on graphite bi-adhesive supports fixed on Al stubs S6 inside a glove box under Ar atmosphere. The stubs were inserted in a home -made sample holder that was sealed in the glove box and in which the low vacuum was made by a rotary pump. In this way, the samples were transferred in the SEM chamber avoiding the exposition to air. Subsequently, the sample holder was open and the measurements performed under ultra-high vacuum at a working distance of 8.5 mm and with an electron generation voltage of 20 kV.

Computation Details
Periodic density functional calculations (DFT) were performed using the Dmol 3 program in the Materials Studio package. 1 All calculations were carried out using Perdew-Burke-Ernzerhof exchange-correlation functions in the framework of general gradient approximation. 2 The double numerical basis set including d-polarization functions (DND) was utilized to describe the valence electrons, with the core electrons described by the effective core potential. The convergence criteria applied for geometry optimizations were 1.0 × 10 −5 A, 2.0 × 10 −3 Å, and 5.0 × 10 −3 Å for energy change, maximum force, and maximum displacement, respectively. A double numerical basis sets, plus polarization functional version 3.5 and an orbital cutoff of 5.1 Å were used. The threshold for self-consistent-field density convergence was set to1.0 × 10 −6 eV; while converging (8 × 8 × 6) Monkhorst-Pack grids were employed to perform integration in the first Brillouin zone for Cs 4 SnBr 6.
Optimization of lattice constants and total energy calculations were performed with plane wave cutoff energy of 600 eV.