Disorder and Halide Distributions in Cesium Lead Halide Nanocrystals as Seen by Colloidal 133Cs Nuclear Magnetic Resonance Spectroscopy

Colloidal nuclear magnetic resonance (cNMR) spectroscopy on inorganic cesium lead halide nanocrystals (CsPbX3 NCs) is found to serve for noninvasive characterization and quantification of disorder within these structurally soft and labile particles. In particular, we show that 133Cs cNMR is highly responsive to size variations from 3 to 11 nm or to altering the capping ligands on the surfaces of CsPbX3 NCs. Distinct 133Cs signals are attributed to the surface and core NC regions. Increased heterogeneous broadening of 133Cs signals, observed for smaller NCs as well as for long-chain zwitterionic capping ligands (phosphocholines, phosphoethanol(propanol)amine, and sulfobetaines), can be attributed to more significant surface disorder and multifaceted surfaces (truncated cubes). On the contrary, capping with dimethyldidodecylammonium bromide (DDAB) successfully reduces signal broadening owing to better surface passivation and sharper (001)-bound cuboid shape. DFT calculations on various sizes of NCs corroborate the notion that the surface disorder propagates over several octahedral layers. 133Cs NMR is a sensitive probe for studying halide gradients in mixed Br/Cl NCs, indicating bromide-rich surfaces and chloride-rich cores. On the contrary, mixed Br/I NCs exhibit homogeneous halide distributions.

n-octane at 120 °C in air.The TOPO solution was synthesized analogously without addition of metal halides.TOP-Cl2 0.5 M in toluene.TOP (6 mL, 13 mmol) with approximatively 130 mmol Cl2 gas, which was produced by slowly adding HCl (15 mL, 170 mmol) in aqueous solution to the excess of KMnO4 under argon flow.The insertion of a water-filter flask guaranteed the complete removal of traces of HCl.No vacuum grease but Teflon® joints and only equipment made of glass were used in order to avoid chlorine glass leaks.TOPCl2 is solid at 0 °C and liquid at room temperature.The final stock solution was obtained by adding toluene (18.7 mL).TOP-Br2 0.5 M in toluene.TOP (6 mL, 13 mmol) was mixed with toluene (18.7 mL) and Br2 (0.6 mL, 11.5 mmol) under inert atmosphere.Oleylammonium iodide (OLAI).OLA (62.5 mL, 0.19 mol) and HI (21.5 mL, 0.19 mol) were mixed in EtOH (500 mL) and purified by recrystallization from Et2O and EtOH.Ligand stock solution (0.1 mg/μL) in mesitylene.The respective ligand (PEA or PPA) was dissolved in mesitylene up to the desired concentration.
Lecithin-capped CsPbX3 (X = Br, Cl) NCs. 3 For CsPbBr3 NCs, CsOA in ODE (4 mL, 1.6 mmol), Pb(OA)2 (5 mL, 2.5 mmol) and lecithin (0.324 g, ca.0.45 mmol) were dissolved in ODE (10 mL) and heated under vacuum to 100 ⁰C, where upon the atmosphere was changed to argon and TOP-Br2 in toluene (5 mL, 2.5 mmol, 5 mmol of Br) was injected.The reaction was cooled immediately by an ice bath.For CsPbCl3 NCs, CsOA (4 mL, 1.6 mmol), Pb(OA)2 (5 mL, 2.5 mmol) and lecithin (0.641 g, ca.0.90 mmol) were dissolved in ODE (5 mL) and heated under vacuum to 150 ⁰C, whereupon the atmosphere was changed to argon and TOP-Cl2 in toluene (5 mL, 2.5 mmol, 5 mmol of Cl) was injected.The reaction was cooled immediately by an ice bath.Isolation and purification.The crude solution was precipitated by the addition of 2 volumetric equivalents of acetone, followed by the centrifugation at 29500g (g is the earth gravity) for 10 minutes.The precipitated fraction was dispersed in 10 mL of toluene and then washed three more times.Each time the solution was mixed with two volumetric equivalents of acetone and centrifuged at 29500 g for 1 minute, and subsequently dispersed in the progressively smaller amounts of the solvent (5 mL for the second cycle, 2.5 mL for the third cycle).After the last precipitation, NCs were dispersed in 2 mL of toluene and centrifuged at 29500 g for 1 minute to remove any non-dispersed residue.

ASC18-capped CsPbBr3
NCs. 1 ASC18 (0.216 g) was added into a 25 mL three-neck flask along with CsOA in ODE (0.4 mL, 0.16 mmol), Pb(OA)2 (0.5 mL, 0.25 mmol, warm) and 5 mL ODE.The reaction vessel was purged 3 times and heated under inert gas to 130 ⁰C, followed by the injection of TOP-Br2 (0.5 mL, 0.5 mmol).The reaction was immediate, and the resulting crude solution was cooled to room temperature using a water-ice bath.Isolation (washing step 1): The crude solution was centrifuged at 29464 g for 10 minutes.The supernatant was isolated and mixed with 12 mL of ethyl acetate and the mixture was centrifuged at 29464 g for 10 minutes.The precipitate was redispersed in 3 mL of toluene.Purification (washing steps 2-4): the colloid can be further purified by up to 3 rounds of precipitation and redispersion, each comprising sequential addition of 6 mL ethyl acetate, centrifugation at 29500 g for 1 minute and subsequent redispersion in 3 mL of toluene.The final NC dispersion can be centrifuged at 29500 g for 1 minute again to remove larger NCs.DDAB-capped CsPbBr3 NCs. 4 55 mg of PbBr2 and 5 mL of ODE were added into a three-neck flask and heated to 180 °C under vacuum.At 120 ⁰C the atmosphere was changed to argon and 0.5 mL of dried OA along with 0.5 mL of dried OLA were injected.At 180 ⁰C, 0.8 mL of 0.125 M CsOA was injected into the reaction flask.After 15 s, the reaction was quenched with ice-bath.The crude solution was centrifuged at 12000 g for 3 minutes.Subsequently, the supernatant was discarded, while the precipitate was redissolved in 0.3 mL of hexane and centrifuged at 12000g for 3 minutes.After centrifugation, the precipitate was discarded and 600 µL of toluene along with 150 µL of 0.1M DDAB/PbBr2 were added to the supernatant.Solution was centrifuged at 12000 g for 1 hour.Subsequently, 1.8 mL of ethyl acetate was added as antisolvent, and the solution was centrifuged at 12000 g for 3 minutes.The precipitate was redispersed in 600 µL of toluene, followed by the addition of 3 mL of ethyl acetate and centrifugation for 5 minutes at 4400 g.The supernatant was discarded, and the precipitate was redispersed in 600 µL toluene-d8.
PEA-and PPA-capped CsPb(Br/Cl)3 and CsPb(Br/I)3 NCs. 2 For each cesium lead halide composition, Table S1 specifies precursors and their quantities, as well as reaction conditions.In general, lead precursor (PbBr2 for CsPb(Br/Cl)3 and CsPb(Br/I)3, Pb(DOPA)2 for CsPbCl3) and zinc halide precursor(s) (ZnCl2, ZnBr2 and/or ZnI2, used as convenient halide sources, e.g.PbCl2 does not dissolve in n-octane with TOPO) were placed in one reaction vial and diluted with n-hexane.The reaction mixture was vigorously stirred at room temperature, followed by the swift injection of the cesium precursor (CsDOPA or CsOA), which initiated the reaction.In general, CsDOPA was used for CsPb(Cl/Br)3 NCs, whereas iodide containing NCs were synthesized with CsOA.CsPbBr3 NCs were possible to synthesize with both precursors.The reaction was quenched with the addition of the 0.1 mg/L stock solution of ligand (C8C12 PEA, for CsPb(Cl/Br)3 or C8C12 PPA, for CsPb(Br/I)3) after the particles reached the desired size.The reaction mixture was stirred for one additional minute after ligand addition.For purification of the NCs, one to two equivalents of washing solution (EtOAc:ACN,2:1 v:v, dried over molecular sieves and filtered through 0.2 μL PTFE filter) were added to the crude reaction mixture to cause NCs precipitation, followed by centrifugation.The supernatant was discarded, and precipitate was redispersed in a minimal amount of nhexane.The NCs were washed three times.CsBr NCs.CsBr NC synthesis was adapted from the literature. 5ZnBr2 (90 mg, 0.04 mmol) was dissolved in 2 mL ODE, 1 mL OA and 1 mL OLA at 75 °C.0.5 mL of CsOA (0.4 M) was added.After 5 minutes, the reaction was stopped by cooling with a water bath to room temperature.The CsBr NCs were collected by centrifugation and redispersed in toluene-d8.

Post-synthetic treatment
Size selection. 6The fractional isolation of the supernatant proceeds through portionwise anti-solvent addition (acetone), followed by centrifugation (29500 g at 17 °C, 10 minutes).The supernatant continues in the purification cycle, until no luminescence of the supernatant is observed, while the precipitate of each cycle constitutes an isolated fraction of NCs.These fractions are redispersed in toluene-d8 (0.5 mL).
Anion exchange.CsPbBr3 and CsPbCl3 NCs were mixed in several ratios (Table S2) of pure halide NCs in toluene-d8 to a final volume of 0.5 mL.

Sensitivity Calculations
In this study, we posed the question whether solution NMR is prohibited also for CsPbX3.We continued with a rough estimate for the relative signal intensity per measurement time for 133 Cs NMR vs. 77 Se and 113 Cd NMR, considering their receptivity at natural abundance relative to 1 H together with their typical T1 relaxation times and FWHM in CsPbBr3 and CdSe, respectively, determined by ssNMR (see Table S3), assuming similar linewidth and relaxation behavior for NCs compared to their bulk analogues.Based on these calculations, 133 Cs is expected to be more than 60 times more sensitive than 77 Se and 113 Cd.Based on the 77 Se data on CdSe NCs from Thayer et.al., 7 a CsPbBr3 NC sample concentration of 2.5 mg/mL would be required.We thus concluded that such solution NMR studies are fully feasible.
The sensitivity per time compared to 133 Cs were calculated with the following formula: , where 2000 Hz and 109 s are the general FWHM and T1 relaxation time of 133 Cs in CsPbBr3 (see Table S3.)Thayer et al. used labelled 77 Se (61%) for their CdSe solution NMR study. 7The concentration was around 40 mg in 0.25 mL.Compared to a 0.5 mL sample used for our study, this corresponds to 20.1 mg or 0.26 mmol of 77 Se.This would require 34.8 mg of 133 Cs, equal to 150 mg of CsPbBr3.Including the increased sensitivity of 133 Cs calculated above, about 2.5 mg of CsPbBr3 is expected to give similar intensity compared to the literature data on CdSe NCs.Unfortunately, no number of scans is mentioned for 77 Se, hampering a direct comparison.Anion-exchanged lecithin-capped NCs

CsBr NCs
Figure S12.Powder XRD pattern of the CsBr NCs with the bulk reference pattern shown in negative. 14The broad signal below 20° results from the adhesive tape used for the measurement.

( a )
Values for bulk material due to the lack of NC values.(b) Values for NCs.

Figure S5 .
Figure S5.Absorption and PL spectra of size selected fractions of ASC18 capped CsPbBr3 NCs.

Figure S13 .
Figure S13.133Cs cNMR spectra of CsPbBr3 NCs using various concentrations with the same amount of scans.

Figure S15 .
Figure S15. 133Cs ssNMR spectra of CsPbCl3 NCs acquired with a hahnecho sequence (blue) and with 1 H-133 Cs cross polarization (red) with a contact time of 2 ms.

Figure S16 .
Figure S16.Static 133 Cs ssNMR spectrum of CsPbBr3 bulk using a solution probe.

Figure S17 .
Figure S17.133Cs cNMR chemical shifts of the core peaks for monodisperse CsPbBr3 NCs capped with various ligands versus their excitonic absorption energy.

Table S2 .
Volumes of pure halide NCs used for the mixtures for the mixed halide NCs.

Table S3 .
Calculated intensities per time for 77 Se and 113 Cd in CdSe compared to 133 Cs in CsPbBr3.

Table S6 .
PL and FWHM for size selected ASC18 capped CsPbBr3 NCs used for size-dependence shown in Figure2e.

Table S8 .
PL, FWHM and size determined by TEM of PPA capped CsPb(Br/I)3 NCs.