Ultra-stable CsPbBr3 Perovskite Nanosheets for X-Ray Imaging Screen

Highlights A gram-scale CsPbBr3 perovskite nanosheet colloid was synthesized by a green method at room temperature. The perovskite nanosheet colloid shows uncompromised photoluminescence quantum yield upon storage for over 8 months. A self-assembly crack-free thin film of the colloidal nanosheets demonstrated an efficient X-ray imaging screen. Electronic supplementary material The online version of this article (10.1007/s40820-019-0283-z) contains supplementary material, which is available to authorized users.


S1.2 Fabrication and Purification of CsPbBr3 Nanocrystals (NCs)
The fabrication of CsPbBr3 NCs was conducted by using a modified method reported by G. Konstantatos et al. [ S1]. Specifically, 384 mg of CsAc,12 mL of 1-PrOH and 24 mL of Hex were loaded into a 100 mL vial and stirred in air at ambient temperature firstly. Then, 3.67 g PbBr2 was added into a 20 mL vial with 5.4 mL of 1-PrOH, 5.4 mL OcAc and 5.4 mL OcAm and heated at 90 °C in air under vigorous stirring. After complete dissolution of the PbBr2 salt, the hot solution was quickly injected into the prepared CsAc solution under vigorous stirring at ambient condition. The color of the mixture turned greenish immediately, and 2 min later the reaction completed. Finally, the mixture was centrifuged at 4,000 rpm, and the wet pellet of the NCs was dispersed in 36 mL toluene for further characterization.

S1.3 Photoluminescence and Absorption
The as-prepared nanocrystals were diluted for 100 times in n-hexane for steady-state measurements of photoluminescence and absorption, respectively. A Cary5000 UV-Vis spectrometer (Agilent Technologies) was used for absorption measurements in the range from 450 to 700 nm. A FluoroMax-4 spectrofluorometer (Horiba Scientific; a slit width of 2 nm and a scan rate of 500 nm min -1 ) was used to record the photoluminescence spectra. The excitation wavelength used for theCsPbBr3 nanocrystals were set at 400 nm for thin film and 365 nm for colloid, respectively. The PLQY data were collected on in an integrated sphere coupling on a steady state PL spectrometer (Edinburg FS5), under an excitation at 365 nm. Three trials on thinfilm samples were conducted and results were averaged to present the final PL QY values.

S1.4 Transmission Electron Microscopic Measurement
Transmission electron microscopic (TEM) images were acquired using a Tecnai transmission electron microscope with an acceleration voltage of 120 keV. HRTEM images were acquired using the same instrument. Low-dose HRTEM images were acquired by a Gatan K2 direct-detection camera operated in electron-counting mode (camera counting frame rate of 400 fps (frames per second) at 4 k × 4 k resolution) with a final image output rate of 40 fps at 4 k × 4 k resolution. An exposure of 3 s therefore results in an image stack of 120 individual image frames. These frames can be summed to improve the signal-to-noise ratio (SNR).

S1.5 X-ray Diffraction Measurements
Powder X-ray diffraction (XRD) was performed using a Bruker AXS D8 diffractometer with Cu-Ka radiation ( = 1.5406 Å). The samples were prepared via the drop casting of the nanocrystal suspension onto a clean glass slide, followed by drying at room temperature.

S1.6 Streak Camera Measurement
Time-resolved photoluminescence color map was collected using a high-resolution streak camera (Hamamatsu C10910) where the pump beam is generated with the second harmonic (410 nm) of a Spectra-Physics MaiTai eHP and Inspire HF-100 OPO. APE Pulse Select pulse picker is used to select the repetition rate of the pulse beam to 4 MHz. Measurements were performed at room temperature with the excitation fluence of 1 pJ cm -2 .

S1.7 Radioluminescence and X-ray Imaging
Radioluminescence were obtained by an Edinburgh FS5 fluorescence spectrophotometer (Edinburgh Instruments Ltd., UK) equipped with a miniature Xray source (AMPEK, Inc.). The photographs of X-ray-excited luminescence were acquired with a digital camera (Canon 600D with EF-S 35mm f/2.8 IS STM lens).

S2 Ligands Density Calculation
In order to figure out the ligands density on the CsPbBr3 nanosheets, the TGA measurement was performed (Fig. 3c). The ligand is found to contribute about 25% to the total weight. The nature of ligand is found to be octylamine according to the boiling points of octylamine.
The TGA result expresses a mass relation between a CsPbBr3 nanosheet and the ligands on its surface. Hence, we get the mass of ligands on a CsPbBr3 nanosheet as shown in Eq. S2: Then, the total number of ligands on a CsPbBr3 nanosheet can be obtained according Eq. S3: where is the molar mass of octylamine, is the Avogadro constant.

Fig. S2
Photographs showing CsPbBr3 NC colloids of varied stability which is induced by size difference. Both 70-nm and 28-nm samples turned yellow, which is less active in PL. In contrast, 3-nm-thick nanosheets (21 nm in diagonal length) remains green.