Chemical and Structural Stability of CsPbX3 Nanorods during Postsynthetic Anion-Exchange: Implications for Optoelectronic Functionality

We examine halide anion-exchange reactions on CsPbX3 nanorods (NRs), and we identify reaction conditions that provide complete anion exchange while retaining both the highly quantum-confined 1-D morphology and metastable crystal lattice configurations that span a range between tetragonal structures and thermodynamically preferred orthorhombic structures. We find that the chemical stability of CsPbBr3 NRs is degraded by the presence of alkyl amines that etch CsPbBr3 and result in the formation of Cs4PbBr6 and 2-D bromoplumbates. Our study outlines strategies for maintaining metastable states of the soft lattices of perovskite nanocrystals undergoing exchange reactions, despite the thermodynamic driving force toward more stable lattice configurations during this disruptive chemical transformation. These strategies can be used to fine-tune the band gap of LHP-based nanostructures while preserving structure–property relationships that are contingent on metastable shapes and crystal configurations, aiding optoelectronic applications of these materials.

and 8 mL DHAm, 5 mL HBr was added dropwise with water bath cooling.The mixture was stirred at room temperature for 3 hours.The precipitate was then purified with acetonitrile and hexane several times, dried under vacuum overnight, and stored in glovebox for future use.Preparation of trioctylammonium bromide (TOAmHBr).4.5 mL HBr was added drop by drop into 10 mL of TOAm under vigorous stirring.The cloudy mixture was stirred for another 2 hours, filtered and rinsed with D.I. water several times.The products were dried at 80 °C overnight, stored in glovebox for future use.Preparation of benzoyl iodide (Bz-I).1.4 mL Bz-Cl was added to an Ar-filled vial containing 3 g KI.
The mixture was stirred at around 60 °C for one day covered with aluminum foil.3 mL dried ODE was added to the vial, and the supernatant was collected with a 0.2 μ m syringe filter, stored in glovebox for future use.Preparation of PbX 2 stock solutions.2.5 mL ODE and 0.2-0.4mmol PbX 2 were loaded into a 2-neck flask and dried under vacuum at 120 °C.0.7 mL OA and 0.7 mL OAm were added under Ar atmosphere and the solution was stirred until complete dissolution of the salt.For PbCl 2 , an additional 1 mL TOP was added to the flask.The resulting concentrated stock solution was cooled to room temperature and transferred to an Ar-filled vial for storage.Synthesis of tetragonal CsPbBr 3 nanorods.5 mL ODE was loaded into a round button flask and dried under vacuum at 120 °C.It was al-lowed to cool down to 80 °C, and 0.3 mL of Pb-OA stock solution was injected.To prepare the Cs feedstock, 0.3 mL Cs-OA stock solution was mixed with 0.55 mL OA, 0.25 mL OAm, and 0.27 mL ODE.Separately, 0.085 g OAmHBr was mixed with 0.7 mL toluene and 0.4 mL ODE.0.9 mL of each of the two feedstock solutions were individually injected into the flask at a rate of 3.6 mL/hr.The samples was purified by centrifugation at 10000 rcf for 20 min.The supernatant was discarded, and the precipitate was redispersed in hexane and centrifuged at 2000 rcf for another 10 min.The final supernatant was collected for future analysis.

Synthesis of
CsPbCl 3 nanorods.CsPbCl 3 nanorods in different crystal phases were synthesized in ways similar to that of tetragonal CsPbBr 3 nanorods above.0.75 g OAmHCl was used instead, and Cs feedstock solution was prepared with required amounts of OA and OAm for different crystal phases.Post-synthetic treatment on CsPbBr 3 nanorods.Cooled crude solutions of CsPbBr 3 nanorods were transferred to Ar-filled vials.Desired amount of chemicals were then injected drop by drop under vigorous stirring at room temperature, and the mixtures were let stirring for another 1~2 hours before cleaning.Post-synthetic treatment using metal halide powders.To an Ar-filled vial containing 0.5 g metal halide powder (PbI 2 , PbCl 2 , PbBr 2 , ZnBr 2 , CuBr 2 , NaBr or KBr), 2 mL cooled crude solution of CsPbBr 3 or CsPbCl 3 nanorods was injected.The mixture was vigorously stirred in a water bath kept at 8-12 °C for some time, ranging from tens of minutes to dozens of days before cleaning.Characterization.XRD data was acquired on a BRUKER D8-Focus Bragg-Brentano X-ray Powder Diffractometer equipped with Cu K-α radiation source.UV-Vis spectra were measured on an Ocean Optics Flame-S-UV-Vis Spectrometer with an Ocean Optics DH-200-Bal deuterium and halogen lamp S-3 light source.TEM images were taken on a FEI Tecnai G2 F20 ST FE-TEM operated at 200 kV equipped with Gatan CCD camera.Lattice parameter analysis.The lattice parameters of the nanorods and conventional nanocrystals were analyzed using the software package GSAS-II.The XRD patterns were fitted to the orthorhombic phases (CsPbCl 3 : ICSD#243734; CsPbBr 3 : ICSD#244752) with the Pawley refinement.The tetragonal CsPbBr 3 nanorods were fitted to the tetragonal phase (mp-1014168).

Figure S1 .
Figure S1.XRD patterns of tetragonal and orthorhombic CsPbBr 3 nanorods (a).Close-ups on the reflections at 15 degrees (b) and 30 degrees (c).The card files for the corresponding structures (mp-1014168; ICSD#244752) are provided.Note that the peaks below 15 degrees correspond to stacked nanorods that order during the preparation of the XRD plate.

Figure S3 .
Figure S3.Fitted lattice parameter results of parent and Br-exchanged nanorods as compared to bulk materials in (a) a, (b) b, and (c) c axes.

Figure S4 .
Figure S4.(a) UV-Vis spectra and (b) XRD patterns of parent tetragonal CsPbBr 3 nanorods and samples treated with PbCl 2 for 17 and 26 days.TEM images of (c) parent CsPbBr 3 nanorods and (d) samples treated for 26days.In (a), the absorption and PL spectra are displayed in dashed and solid lines, respectively.

Figure S10 .
Figure S10.XRD patterns of CsPbBr 3 nanorods treated with (a) Bz-Cl and (b) Bz-I.In (b) the peak around 31.8 o is assigned to Cs 4 Pb(I/Br) 6 .

Figure S11 .
Figure S11.Photographs of CsPbBr 3 nanorods treated with varied concentrations of TMS-I illuminated with (a) room light and (b) UV light.The NRs became white cloudy and nearly nonflorescent in a few minutes after the addition of x2 molar ratio of TMS-I (first vial on right).

Table S1 .
Fitted lattice parameters of NCs and directly synthesized NRs.

Table S3 .
Average sizes of nanorods before and after treatment with Pb-OA.*