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Harnessing Defect-Tolerance at the Nanoscale: Highly Luminescent Lead Halide Perovskite Nanocrystals in Mesoporous Silica Matrixes

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.


Mesoporous silica (meso-SiO 2 ) matrices
All mesoporous silica materials used as template matrixes were purchased from commercial vendors, and were used as received. Their properties and identification information are given in Table S1. .

Template-assisted synthesis of lead halide perovskite nanocrystals (NCs)
Typical template-assisted synthesis was conducted as outlined in Figure S1. The templates were dried at 150° C for 12 h under vacuum, reaching a final pressure of 5·10 -5 mbar, before use. Then, 2.0 mg of mesoporous silica were impregnated with 10 µL of 0.3 M solution of methylammonium lead iodide (MAPbI 3 , 186 mg mL -1 ) or with similarly concentrated CsPbI 3 , formamidinium lead bromide (FAPbBr 3 ) or MAPbBr 3 in MFA. For the synthesis of CsPbBr 3 NCs, a lower concentration (0.1 M) solution in MFA was used due to the lower solubility of CsPbBr 3 . After impregnation, the excess solution was removed by damping with filter paper. The as-obtained powder was sandwiched between two glass slides and heated up to 120°C (for MA and FA salts) or 150°C (for Cs salts) in a vacuum oven for 40 minutes. Afterwards, the powder was allowed to cool to 80 °C under vacuum and then to room temperature in air. Larger-scale syntheses (for >50 mg of silica powder) were performed in an analogous way except for in one step: excess solution was removed by vacuum on a glass filter (porosity class 2) instead of by damping with filter paper.

Preparation of dispersions and films of mesoporous silica microparticles impregnated with lead halide perovskite NCs
Glass substrates were washed with acetone, isopropanol, and deionized water in an ultrasonic bath, then treated with reduced air plasma (100W for 5 minutes) and dipped into a 5% OTMS solution in toluene for 10 minutes, as described elsewhere. 3 These hydrophobized substrates were then carefully rinsed with toluene and dried at 70 °C for 2 minutes.
To prepare films, a mixture of 2 mg of silica 4nm-SiO 2 impregnated with CsPbBr 3 NCs, 2 mg AOT and 2 mg polystyrene was dispersed in 200 µL toluene and sonicated. This mixture was drop-cast onto freshly hydrophobized substrates and allowed to dry naturally.

Estimation of the MAPbI 3 solubility in organic solvents
MAPbI 3 solubility was measured near the reaction temperature, 100 °C, in order to check the possible effect of perovskite solubility on the growth of the NCs (such a solubility evaluation at 120 °C is complicated due to iodide oxidation to iodine on a similar time scale as that of the experiment). In order to avoid potential problems with impurities or stoichiometry uncertainty, we have used only single crystals which have been prepared as reported elsewhere. 5 Small single crystals were added to 0.2 mL of the solvent at 100 °C until no more could be dissolved. Then 50 µL of the obtained saturated solution was drop-cast on a glass substrate, dried, and weighed.

Characterization techniques Scanning Transmission Electron Microscopy (STEM)
STEM investigations were performed on the aberration-corrected HD-2700CS (Hitachi; cold-field emitter), operated at an acceleration potential of 200 kV. A probe corrector (CEOS) is incorporated in the microscope column between the condenser lens and the probe-forming objective lens providing excellent high-resolution capability (minimum beam diameter ~0.1 nm). Images were recorded using a high-angle annular dark field (HAADF) detector leading to atomic number (Z) contrast. Furthermore, a secondary electron (SE) detector was installed inside the column of the HD-2700CS microscope above the sample allowing studies of sample morphology. The images (1024 x 1024 pixels) were recorded with frame times of 20 s. Energy-dispersive X-ray spectra (EDXS) of selected areas were recorded with an EDAX detector (Gemini system, EDAX).

Powder X-ray diffraction (XRD)
Powder XRD was performed using a STOE STADI P diffractometer, operating in transmission mode. A germanium monochromator, Cu Kα 1 irradiation and a silicon strip detector (Dectris Mythen) were used.

Photoluminescence (PL) spectroscopy
A Fluoromax-4 Horiba spectrofluorimeter equipped with a PMT detector was used to acquire the steady state PL spectra of the silica microparticles impregnated with perovskite NCs. Samples were sandwiched between 2 cover slips. All spectra were corrected for the detector sensitivity.

Photoluminescence quantum yield (PL QY) measurements
The absolute value of the PL QY was measured by methods similar to those described elsewhere. 6 Use of an integrating sphere with a short-pass filter allowed us to simultaneously measure absorbance corrected to reflectance and scattering losses. As the excitation source, a CW Laser Diode Module was used at a wavelength of 405 nm with a power of 0.3 W modulated at 30 Hz. For spatial averaging, an integrating sphere (IS200-4, Thorlabs) was applied. The absorbed and emitted light was measured using short-pass and long-pass filters (FES450 and FEL450, Thorlabs). This light was measured by a broadband (0.1-20 µm) UM9B-BL-DA pyroelectric photodetector (Gentec-EO). The modulated signal from the detector was recovered by a lock-in amplifier (SR 850, Stanford Research). The ratio between the emitted and absorbed light gives an energy yield. This value was then transformed into the PL QY taking into account the difference in photon energies for the laser and PL band (the PL band was used as the average energy). The PL QY was corrected by transmission of applied edge-pass filters.        Figure S7. Stability of the PL decay rate for CsPbBr 3 NCs synthesized in 7nm-SiO 2 . Between the collection of the two traces, the NCs were exposed to pulsed irradiation for 30 minutes, which corresponds to 10 10 laser shots with an intensity of 2 µJ/cm 2 . Figure S8. TEM image of 4nm-SiO 2 microparticles (as received).