Templated Self-Assembly of One-Dimensional CsPbX3 Perovskite

Nanocrystal Superlattices

Aizhao Pan,a,b Matthew Jurow,b,c Yanrui Zhao,a Fen Qiu,b Ya Liu,c,d Juan Yang,c

Jeffrey J. Urban,b Ling He,*a Yi Liu*b,c

aDepartment of Chemistry, School of Science, Xi’an Jiaotong University, Xianning West Road, 28,

Xi’an, 710049, China.bThe Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720,

United States.cMaterials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,

United States.dInternational Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in

Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China

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Electronic Supplementary Material (ESI) for Nanoscale.This journal is © The Royal Society of Chemistry 2017

Experimental Section

General Materials: All reagents were purchased from Sigma and Aldrich and used as received

without purification. All reagents were at least 99% pure (Reagent grade) except oleic acid and

oleylamine, which were 90% pure (Analytical Reagent grade).

Characterization Methods: Ultraviolet and visible absorption (UV-vis) spectra of colloidal solutions were collected using a Cary 5000 UV-Vis-NIR spectrophotometer. Photoluminescence spectra (PL) and quantum yield (PLQY) measurements were recorded using an integrating sphere, recorded on an Edinburgh Instruments FLS920 spectrophotometer. Powder X-ray Diffraction (PXRD) data were acquired using a Bruker AXS D8 Discover GADDS X-Ray Diffractometer equipped with a Vantec-500 area detector and was operated at 35 kV and 40 mA at a wavelength of Co K (1.79 Å). SEM images were acquired on a Zeiss Gemini Ultra-55 Analytical Field Emission Scanning Electron Microscope. TEM and high-resolution TEM (HR-TEM) data were acquired on a FEI Tecnai G220S-TWIN electron microscope operating at 200 kV with a Gatan SC200 CCD camera.

Synthesis of CsPbBr3 Nanocubes: CsPbBr3 nanocubes or nanoplates were prepared under ambient conditions following a modified procedure.1, 2 octadecene (ODE, 5 mL) and PbBr2

(0.188 mmol, 0.069 g) were loaded into a 25 mL 3-neck flask and dried under vacuum for 1 h at

120 ºC. Equimolar organic acid and amine pairs were pre-mixed with the following composition for

nanocubes and nanoplatelets, respectively: oleic acid (OA)/oleylamine (OAm) (0.50 mL/0.50 mL),

hexanoic acid/oleylamine (0.20 mL/0.50 mL). The acid/amine mixture was dried under vacuum and

injected at 120 ºC under N2. After complete dissolution was achieved, the temperature was raised

to 160 ºC, and the CsOAc/acid mixture (0.4 mL, 0.125 M in ODE) was quickly injected. After

mixing for 5 seconds, the reaction mixture was cooled by immersion in an ice-water bath. The crude

solution was directly transferred to centrifuge tubes for separation and purification. After

centrifuging at 13,000 rpm for 10 min, the precipitate was collected and re-dispersed in either pure

hexane or hexane/acetone mixture (hexane:acetone=0.9:0.1 by volume). This procedure was

repeated five times, after which the solid precipitate was collected and redispersed in toluene or

hexane for further characterization.

Synthesis of PbSO4 clusters. The OAm-ligated PbSO4 aggregates were synthesized following

a modified reaction procedure between lead halide PbX2 (X=Cl, Br or I) and tetrabutylammonium

hydrogen sulfate (TBAHS, 98%) at room temperature.3 A solution of PbCl2 was prepared by mixing

PbCl2 (139 mg, 0.5 mmol) in OAm (8 mL) at room temperature. The mixture was heated at 150 ℃

under N2 for 1 h. A stock solution of TBAHS was prepared by dissolving TBAHS (424 mg, 1.25

mmol) in acetone (10 mL). The PbCl2 stock solution (0.4 mL) was mixed with the TBAHS stock

solution (0.2 mL), OA (0.4 mL), and either chloroform or THF (10 mL) in a vial. The resulting

mixture was stirred at room temperature for 10 min to give a clear colorless solution. The PbSO4

clusters were obtained as a white precipitate by subsequent addition of 30 mL of ethanol into the

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above solution, followed by centrifugation (8000 rpm, 5 min). Crude material was re-dispersed in

chloroform or THF (2.5 mL) to form a clear colorless solution with a concentration of ca. 0.01M.

Self-assembly of NC superlattices. The linear assembly of colloidal NCs was realized by

modifying a previously reported procedure.3 To a solution of PbSO4 clusters (~0.025 mmol in

CHCl3 or THF) was added a solution of NCs in hexane (2 mL). The resulting mixture was incubated

at room temperature. To assemble superlattice chains, after the addition of a small amount of OAm

(~0.5 mL), the mixture was left unstirred and the assembly was completed in 30 min. The assembled

NC chains were isolated by centrifugation and re-dispersed in hexane for further characterization.

Fig. S1. TEM micrographs illustrating the assembly process over time. (a) TEM images of pure

PbSO4-pods in hexane. (b), (c) and (g) TEM images of PbSO4-pods mixed with CsPbBr3 NCs after

0, 5 min and 30 min incubation, respectively. (d) TEM and (e) STEM images of typical single

PbSO4-pod. (f) High magnification TEM image of (g) about typical single assembly array of PbSO4-

pods templated with CsPbBr3 NCs after 30 min incubation.

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Fig. S2. UV-Vis absorption spectra of the 1D self-assembled superlattices of CsPbBr3 NCs induced by the C-clusters made from PbCl2. Incubation times are listed in legend.

Fig. S3. (a) TEM image of CsPbBr3 NC self-assembled superlattices induced by C-PbSO4 clusters made from PbBr2. Insets are optical photographs under UV light. (b) Optical absorption and (c) PL emission spectra of the 1D self-assembled superlattices induced by C-PbSO4 clusters made from PbCl2, PbBr2 and PbI2 over 1 month.

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Fig. S4. (a) Photograph of CsPbBr3 NC solution, CsPbBr3 NC+Na2SO4 solution and CsPbBr3 NC + tetrabutylammonium hydrogen sulfate solution, all taken after mixing for 30 min under UV light. (b) Photographs of a hexane solution of CsPbBr3 NCs (left) and a mixture of NCs with CHCl3 (right) under UV light, showing gradual blue shift due to chloride substitution. (c) UV-Vis absorption and (d) PL emission spectra of the samples before and after adding CHCl3 and stirring for 24 h.

Fig. S5. TEM micrograph showing the morphology of the self-assembled structure from incubation with C-PbSO4 clusters after 1 month. Insets: optical photograph under UV light and the corresponding HR-TEM image

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Fig. S6. (a) Powder X-ray diffraction (PXRD) pattern of the NCs and the assemblies from T-clusters for 1 month, (b) PL emission spectra of CsPbBr3 NCs and the self-assembled superlattices with T-PbSO4-clusters over time. Insets in (b) are optical photographs under UV light over time. (c) The PL stability test of the pure NCs and the superlattice assemblies from T-clusters stored in ambient environment (room temperature, relative humidity: 40-60%). (d) Plot of PLQYs at different temperatures for the NCs and the superlattice assemblies from T-clusters in both colloidal solutions and thin films. (e-j): Photographs of glass slides coated with the thin films of NCs and the superlattice assemblies from T-clusters before (e and f) and after exposure to air for 5 days (g and h) or after soaking in water (i and j).

Discussion of the stability of the superlattice assembliesWe have carried out further studies of materials’ stability to ambient conditions,

water, and temperature before and after self-assembly. The PL spectra, relative PLQY, SEM, TEM and optical images of the solution and film samples were summarized in Fig. S6.

The stability of the self-assembled superlattices to ambient conditions was evaluated by storing at room temperature and RH≈40-60%. Neither the PL peak position nor the PXRD pattern of the superlattice samples displayed any notable changes after 1 month of incubation (Figure S6a and S6b). As shown in Fig. S6c, the PL intensity of the CsPbBr3 assemblies slowly decreased (by less than 20%) after 5 days exposure to ambient conditions. Slightly faster decomposition was observed for the free CsPbBr3 NCs.

Further degradation was observed when exposure times were extended to 30 days. Nevertheless, the final PL intensity of the CsPbBr3 superlattices remained significantly higher than the free CsPbBr3 NC samples. In addition, the SEM and TEM studies of

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the CsPbBr3 assemblies after one-month exposure revealed that the majority retained their 1D pea-pod morphology, with only small portions of 1D shells decorated with external NCs (Figure 2e and 2f in main text).

Figure S6d shows the PL intensity changes at different temperatures. Similar degradation tendencies were observed for the free CsPbBr3 NCs and the superlattice assemblies in both solution and thin films as the temperature was increased. The corresponding PL property of films was also investigated by spin-casting a film of both free NCs and the assembled superlattices. As shown in Figure S6g and S6h, both films still exhibit strong photoluminescence under UV light illumination at ambient conditions for 5 days. Films of free CsPbBr3 NCs were completely quenched after being immersed in water for 30 s, while the superlattice assemblies remained weakly emissive under the same conditions (Figure S6i and S6j).

Overall, the CsPbBr3 superlattice assemblies display comparable or slightly improved stability under various conditions such as ambient environment, water and heat. Given that the pea-pod shaped superlattice assembly structure is open-shell structured and can only provide partial encapsulation of the nanocrystals, such comparable stability is not unexpected.

Fig. S7. TEM images showing the formation of pure PbSO4-pods in hexane in the absence of

CsPbBr3 NCs after 30 min incubation.

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Fig. S8. SEM images of self-assembled superlattices outside of the T-PbSO4 clusters after 30 min sonication.

Fig. S9. TEM images of the self-assembled superlattices of (a) CsPbCl1.5/Br1.5 NCs and (b) CsPbI3 NCs. Inset in (a): the magnified TEM image

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