Essentially trap-free CsPbBr3 colloidal nanocrystals by post-synthetic thiocyanate surface treatment

Transcription

1 Supporting Information for: Essentially trap-free CsPbBr3 colloidal nanocrystals by post-synthetic thiocyanate surface treatment Brent A. Koscher 1,3,4, Joseph K. Swabeck 1,3,4, Noah D. Bronstein 1,3,4, A. Paul Alivisatos 1,2,3,4 1 Department of Chemistry, University of California, Berkeley, CA 94720, USA. 2 Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA. 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 4 Kavli Energy NanoScience Institute, Berkeley, CA 94720, USA. Materials: Cs2CO3 (99.9% Aldrich), octadecene (ODE, 90%, Aldrich), oleic acid (OA, 90%, Aldrich), oleylamine (OLA, 70%, Aldrich), PbCl2 (99.999%, Aldrich), PbBr2 (99.999%, Aldrich), PbI2 (99%, Aldrich), hexanes (>99%, Aldrich, Anhydrous), Toluene (99.8%, Aldrich, Anhydrous), Ethyl Acetate (99.8%, Aldrich, Anhydrous), Methyl ethyl ketone (99%, Aldrich), Ammonium Thiocyanate (99.99%, Aldrich), Sodium Thiocyanate (99.99%, Aldrich), Ammonium Bromide (99.99%, Aldrich), Lead Nitrate (99.0%, Aldrich), Lead Thiocyanate (99.5%, Aldrich), Guanidium Thiocyanate (99.0%, Aldrich), Didecyldimethylammonium Bromide (98%, Aldrich) Synthetic Methods: Preparation of Cesium Oleate Stock Solution: A stock solution of cesium oleate was prepared following the reported procedure by Protesescu et al. (Ref. 3a of the main text). Briefly, 2.5 mmol Cs2CO3 and 2.5 ml OA was loaded into a three neck flask with 40 ml ODE, dried/degassed for 1 hour at 120 C under vacuum, and then heated under dry Argon gas to 150 C until all Cs2CO3 reacted with OA. Synthesis of CsPbBr3 Nanocrystals: ODE (5 ml) and PbBr2 (0.188 mmol) were loaded into a 3-neck flask and dried/degassed under vacuum at 120 C for 1 hour, then heated under dry Argon gas to 140 C, and dried OA (0. 5 ml) and OLA (0.65 ml) were injected. Following complete dissolution of the PbBr2 salt, the temperature was adjusted between C before injection of hot cesium oleate solution (0.5 ml, M) to obtain different size CsPbBr3 nanoparticles. The reaction mixture is cooled after 5-10 s using an ice-water bath. Isolation and purification of CsPbBr3 Nanocrystals: Three isolation methods work with high efficacy for CsPbBr3 samples. In the first method, the crude solution was first centrifuged at 4000 RPM or lower for 3 min to remove aggregates, excessively large nanocrystals, and a small quantity of brown-colored by-products, keeping the supernatant. The supernatant was then centrifuged at RPM for up to 10 min at low temperatures (0-10 C), re-dispersing the isolated particles in anhydrous hexanes for storage. While this method results samples with good optical performance, the samples have excess ligand and ODE. In the second method, the crude solution was first centrifuged at 4000 RPM or lower for 3 min to remove unwanted by-products, keeping the supernatant. By adding antisolvent, namely ethyl acetate or methyl-ethyl ketone, followed by centrifugation at 8000 RPM help with isolation. While this method very effectively isolates clean samples, even samples of small particles, the optical performance of the samples are hindered by the anti-solvent addition. For the third method, around 5 ml of hexanes are added to the crude reaction solution and centrifuged at 4000 RPM to remove unwanted by-products, namely very-large crystals and agglomerated precursors. By recognizing that ODE is not a great solvent for the CsPbX3 colloids and by evaporating some of the added hexanes in a step-wise fashion, different sizes can be easily isolated from the reaction solution by centrifugation. This method produces clean samples with high optical efficiency, with the S1

2 ability to isolate very tight size distribution samples. In all of the methods, the resulting samples are long term stable and can be stored in a glovebox or in air (depending on timeframe to use the samples). Surface Treatments The surface treatment reactions were conducted on both freshly synthesized and aged samples, with results being very consistent for treatments inside or outside of a glovebox. For the thiocyanate salt treatments, the thiocyanate salt was dried and degassed, if needed, to ensure the samples were not exposed to extra unnecessary water, and were stored in nitrogen dryboxes. This step could be necessary considering that many thiocyanate salts are deliquescent. The dry salts were added directly to the nanocrystal sample in excess. The amount of thiocyanate available in the solution at any given time is controlled by the limited solubility of the ionic salts into the non-polar solvents. The heterogeneous mixture was stirred at room temperature, with most changes occurring in 20 minutes and very little occurring afterwards. The resulting solution following the treatment is a turbid mixture. The remaining salt was removed from solution by either centrifugation followed by decanting or by using a PTFE syringe filter. The isolated samples were then appropriate to use for further studies. Characterization: Absorbance: Steady state absorbance spectra were collected using a Shimadzu UV-3600 double beam spectrometer using the slowest scanning rate, with one second integration and 2nm slit width. Steady-state Fluorescence: Steady state fluorescence spectra were measured using a Horiba Jobin-Yvon FluoroLog 2 spectrofluorometer (wavelength calibration to the 397 nm Raman peak under 350nm excitation to within 1 nm accuracy). Source light is a xenon short arc lamp. Typically integration times of 0.2 s/nm with 1 nm slit widths were used. Quantum Yield Measurements: For absolute measurements a home built integrating sphere spectrofluorometer was used. A Fianium SC450 supercontinuum pulsed laser is used as a white light source, where two sets of monochromators select the appropriate excitation wavelength (between 470 and 2500 nm). A small part of the excitation beam is reflected to a ThorLabs S120VC calibrated silicon photodiode to continuously measure the power and the majority is focused into a Spectralon integrating sphere from LabSphere. The beam hits a sample within a cylindrical quartz cuvette. Light leaving the exit port of the sphere, fitted with a baffle to prevent direct reflections, is focused onto a detection monochromator and the spectrum is detected with a Princeton Instruments PIXIS 400B thermoelectrically cooled silicon CCD. The sensitivity as a function of wavelength is calibrated with a NIST-traceable radiometric calibration lamp. For relative measurements, those requiring excitation below 470 nm, photoluminescence quantum yields are estimated according to standard procedure using the FluoroLog. A more complete detailed account of our home-built integrating sphere setup can be found in [1]. Powder X-ray Diffraction (XRD): Patterns of nanocrystal samples were obtained using a Bruker D-8 GADDS Diffractometer equipped with a Co Kα. Transmission electron micrographs (TEM): TEM images were obtained with a FEI Tecnai T20 equipped with a Gatan SC200 CCD camera and LaB6 filament operated at 200 kv. Photoluminescence Lifetimes: Photoluminescence lifetimes were measured using a PicoQuant Fluotime 300, equipped with a PMA 175 detector and a LDH-P-C-405 diode laser with an excitation wavelength of nm. The laser diode is capable of reaching a repetition rate of 80 MHz, however repetition rates were adjusted as appropriate to observe the full decay. S2

3 X-ray Photoelectron Spectroscopy (XPS): XPS were taken on a PHI 5400 XPS system. Samples were deposited onto p-type silicon wafers. Before deposition the wafers were etched in a dilute HF solution and thoroughly oxidized using a Piranha solution. Additional Supporting Figures: S1: Typical PL spectra for a sample of cleaned freshly synthesized CsPbBr3 nanocrystals, isolated using the third method described earlier in the procedures, using the slow removal of hexanes. The sample was excited at 420 nm. For this sample the peak position is at 509 nm, with a 16 nm (77 mev) linewidth, very comparable to single particle literature on the material (Ref. 9a, 9b from main text). S2: Example of the spectra used to calculate absolute photoluminescence quantum yield using the home-built integrating sphere setup. Shown are example spectra under 470 nm laser light excitation, a blank and a sample scan. Typically 5-10 scans are used for each excitation wavelength for both the sample and the blank. The change in the laser line (~470 nm) reflects the absorption of the sample, and the change in the emission (~510 nm) reflects the emission of the sample. The comparison of the two results in the quantum yield. S3

4 S3: TEM images used for sizing and shape analysis, both before and after treatment. Included in the micrograph is the average size and distribution, along with the distribution and squareness of the particles used in the study. The squareness of the particles is defined as the ratio of the shorter side to the longer size, a square would represent a value of 1. S4: Small angle X-Ray scattering (SAXS) intensity map of aged nanocrystal samples both before and after treatment with thiocyanate. The periodic spacing of the dried samples before treatment was nm and after treatment the spacing was nm. SAXS data was taken under vacuum at room temperature in a Bruker NanoStar equipped with a Cu target. S4

5 S5: Representative HR-TEM of the CsPbBr3 used in the sample. Shown is the lattice spacing of 0.58nm consistent with either a Pm3m or Pmna CsPbBr3 crystal structure. Shown is a scale bar of 2 nm. S6: Plots of the wavelength-dependent photoluminescent quantum yields (PLQYs) for the fresh and aged samples both treated and untreated. The particular treatment we are highlighting in the figure is the treatment with ammonium thiocyanate. Shown in each graph is the absorption of the sample (red), and quantum yield including the changes from scan to scan (green) and the quantum yield including both the changes from scan to scan and comparison to the traceable blackbody source for absolute uncertainty (blue). S5

6 S7: Information on some of the other treatments tried and some from the literature. The first two treatments were the native ligands, oleic acid and oleylamine. Treating with oleic acid resulted in a decrease in PLQY and the presence of a long lifetime component following treatment, whereas not much happens to the sample optically when treating with oleylamine. There is a small improvement optically when treating with didodecyldimethylammonium bromide (DDAB), shown in the change in the photoluminescent lifetime, whereas there is a larger improvement when using ammonium bromide, although not to same extent as sodium or ammonium thiocyanate. The final graphs are the results of a treatment with guanidinium thiocyanate and lead thiocyanate, each resulting in very little, if any, change in the optical performance of the samples. S6

7 S8: Photoluminescence lifetimes of a sample treated with lead nitrate, suggested in the literature as a potential treatment [2] to improve the optical performance of CsPbI3 nanocrystals. The analogous treatment of CsPbBr3 nanocrystals results in samples that perform worse optically than untreated samples. S9: Photoluminescent lifetimes of a sequential washing series using anhydrous ethyl acetate. In sequential washes, a long lifetime component becomes pronounced in the overall lifetime, accompanied by a rapidly decreasing optical quantum yield. While the first (red) and second (green) wash seem similar in the long lifetime component, the sample performed very poorly optically (estimated to have ~1000x lower quantum yield than the unwashed sample) and the lifetime is strongly convoluted with the instrument response (IRF, black). A third wash was attempted, however the sample was unmeasurable optically. S7

8 Treatments that Improve Optical Performance Ammonium Thiocyanate Sodium Thiocyanate Ammonium Bromide (somewhat) Didodecyldimethylammonium Bromide (somewhat) Treatments that do Nothing or Harms Optical Performance Guanidinium Thiocyanate Lead (II) Thiocyanate Potassium Selenocyanate Potassium Bromide/Sodium Bromide Potassium Cyanide/Sodium Cyanide Ammonium Acetate/ Ammonium Formate Cesium Carbonate Ammonium Citrate Tetrabutylammonium Bromide Tetraoctylammonium Bromide Oleylamine Oleic Acid Lead Nitrate Lead Acetate S10: Table that summarizes the changes in the optical performance of other treatments not included in the more detailed section for brevity. In the table the two columns divide the treatments into those which improve the optical performance of CsPbBr3 samples and those which either do nothing or harm the optical performance. S11: X-Ray photoelectron spectra of the sulfur region (Sulfur 2p3/2) for both an untreated and treated aged sample of nanocrystals. The two XPS spectra are offset by an additional 25 counts per second (a.u.) for clarity. No significant difference is observed for the region as a result of the treatment. S8

9 S12: Molecular orbital diagram describing the orbitals which lead to the optical transition in CsPbBr3 nanocrystals, in which the conduction band is made of Pb 6p and Br 4p orbitals and the valence band is made of Pb 6s and Br 4p orbitals. Also shown is the end result of a lead-rich surface, shallow electron levels, and two potential consequences of the thiocyanate treatment (i.e. the passivation of surface lead raising the levels out of the bandgap or removal of the troublesome lead altogether. S13: Single-exponential fitting of the photoluminescent decays for an untreated (left) and treated (right) aged sample from the Picoquant FluoTime 300. Including with each decay is the reduced chi-squared goodness of fit for a single exponential fit. S9

10 S14: Test of the optical stability of a dilute colloidal sample of treated and untreated particles under continuous illumination with ~100mW of 365 nm UV light. The untreated sample initially presents an 81% quantum yield and after continuous illumination that value decreases to ~10% along with a red-shift of the luminescence peak. The treated sample decreases from 99% quantum yield to 88% in the same timeframe with similar illumination conditions. S15: The pre-irradiation and post-irradiation treated and untreated colloidal samples used for the UV-light stability test. The treated sample presented a 99% quantum yield before and 88% after irradiation, whereas the untreated sample presented an 81% quantum yield before and 10% quantum yield following irradiation. S10

11 S16: Time-resolved photoluminescence lifetimes of the untreated and treated samples both before and after continuous irradiation with UV-light. Lifetimes were collected with 407.1nm pulsed excitation with a 1 MHz repetition rate using a PicoQuant Fluotime 300. Supporting Information References: [1] Bronstein, N. D. (2015) Material and optical design rules for high performance luminescent solar concentrators (Order No ). Retrieved from Dissertations & University of California; ProQuest Dissertations & Theses A&I; ProQuest Dissertations & Theses Global. ProQuest document ID: [2] Swarnkar, A., Marshall, A. R., Sanehira, E. M., Chernomordik, B. D., Moore, D. T., Christians, J. A., Chakrabarti, T., Luther, J. M., Science, 2016, 354, S11