Light-Induced Self-Assembly of Cubic CsPbBr3 Perovskite

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Light-Induced Self-Assembly of Cubic CsPbBr3 Perovskite Nanocrystals into Nanowires Jiakai Liu, Kepeng Song, Yongwoo Shin, Xin Liu, Jie Chen, Ke Xin Yao, Jun Pan, Chen Yang, Jun Yin, Liang-Jin Xu, Haoze Yang, Ahmed M. El-Zohry, Bin Xin, Somak Mitra, Mohamed Nejib Hedhili, Iman S Roqan, Omar F. Mohammed, Yu Han, and Osman M. Bakr Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00680 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Light-Induced Self-Assembly of Cubic CsPbBr3 Perovskite Nanocrystals into Nanowires Jiakai Liu†,∥, Kepeng Song‡,∥, Yongwoo Shin⊥,∥, Xin Liu∇, Jie Chen†, Ke Xin Yao‡, Jun Pan†, Chen Yang#, Jun Yin#, Liang-Jin Xu†, Haoze Yang#, Ahmed M. El-Zohry#, Bin Xin#, Somak Mitra#, Mohamed Nejib Hedhili§, Iman S. Roqan#, Omar F. Mohammed*,#, Yu Han*,†,‡ and Osman M. Bakr*,†

†KAUST Catalysis Center (KCC), Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ‡KAUST Advanced Membranes and Porous Materials Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ⊥Advanced Materials Lab, Samsung Research America, Burlington, MA 10803, U.S.A ∇School of Chemistry, Dalian University of Technology, Dalian, 116024, China #Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia §Imaging and Characterization Core Lab, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia


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2 ABSTRACT: The rapid development of halide perovskite synthesis offers the opportunity to fabricate high-quality perovskite nanocrystals (NCs), whose structural uniformity can lead to assembled supra-structures with improved device performance and novel collective properties. Light is known to significantly affect the structure and properties of halide perovskites and plays a crucial role in the growth and assembly of their crystals. Nevertheless, the light-induced growth mechanisms of perovskite NCs are not yet clearly understood. In this work, we performed a systematic study of the visible-light-induced template-free synthesis of CsPbBr3 nanowires (NWs) generated through self-assembly of cubic (in phase and close to cubic morphology) NCs. Using atomic-resolution electron microscopy, we visualized the cubic-toorthorhombic phase transition in NCs and the interface between coalesced NCs. Remarkably, the images of the interface revealed the coexistence of CsBr and PbBr2 surface terminations in halide perovskites. Our results shed light on the mechanism underlying the observed anisotropic assembly of halide perovskites and elucidate the vital role of light illumination during this process. More importantly, as an elegant and promising green-chemistry approach, light-induced self-assembly represents a rational method for designing perovskites.


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3 I. INTRODUCTION Light-induced synthesis has been of continuing interest as a promising route to realizing new types of materials structures and morphologies.1-2 Light can be used to stimulate the synthesis or assembly of light-responsive materials, such as silver,3 gold,4 and Cd-based nanoparticles,5 offering an alternative route to conventional synthetic methods. Similarly to other light-sensitive materials, halide perovskites tend to actively respond to light through changes in their structure and properties. For instance, substantial effects of light on the phase transition,6-7 structural dynamics,8 ion migration and conductivity,9-10 and nucleation and growth11-12 of halide perovskites have been reported. The rapid development of halide perovskite synthesis has led to the fabrication of high-quality, uniform perovskite nanocrystals (NCs), which benefited device performance and opened gateways to the assembly of novel supra-structures.13-27 However, fabricating halide perovskite nanostructures using light remains largely unexplored. The only notable studies were performed using laser or UV-light irradiation, which induced the self-assembly of CsPbBr3 nanoplatelets into stacked lamellar structures, presumably based on strong van der Waals attraction among the nanoplatelets.28-29 However, halide perovskites are extremely susceptible to undergo aggressive degradation upon exposure to laser or UV-light environments.30 To the best of our knowledge, the assembly of smaller perovskite NCs (the most prevalent type of perovskite nanocrystals) that exhibit weaker inter-particle interactions driven by visible light has not been reported, but is important to understanding the growth kinetics, assembly mechanisms, and surface ligand chemistry of highly ionic perovskite NCs (given that virtually all such syntheses are carried out in the visible light, without much head to its effect on the reaction).31-34 Here, we report a conceptually new method for assembling CsPbBr3 NCs using visible light that does not require the particles to have a special morphology or to be laboriously functionalized with photo-responsive ligands switches. Under visible light, defect-free


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4 nanowires (NWs) were spontaneously formed from individual cubic CsPbBr3 NCs through assembly and fusion. Counterintuitively, the initial cubic-phase CsPbBr3 NCs do not assemble by isotropic coalescence but undergo anisotropic linear growth via oriented attachment, and more interestingly, this process coincides with a cubic-to-orthorhombic phase transformation. Because these two phases (i.e. cubic and orthorhombic) present significantly different optoelectronic and stability character (when integrated in devices); and it remains controversial whether the crystal structure of CsPbBr3 NCs is cubic or orthorhombic, the exploration of the underlying perovskite crystal structure is crucial to comprehending the NCs’ structure-property relationships.13, 35-36 Moreover, we systematically investigated the morphological evolution and phase transition of CsPbBr3 NCs as a function of illumination time and photon energy. The results show that the self-assembly of CsPbBr3 NCs is initiated by the light-induced selective desorption of surface ligands, namely, oleic acid (OA) and oleylamine (OAm). This desorption leads to reduced steric repulsion between adjacent NCs. The remaining carboxylate-rich ligand-shell creates an anisotropic environment to facilitate the formation of NWs. The resultant NWs display a photoluminescence lifetime of ~125 ns – about two orders of magnitude longer than that of the initial precursor NCs (~2 ns). Our study represents the first example of the use of visible light to direct the assembly and subsequent growth of perovskite NCs, providing important insight into the mechanisms underlying the observed light-induced assembly behavior. II. RESULTS AND DISCUSSION


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Figure 1. (a) TEM images of CsPbBr3 NCs. (b) SEM image of CsPbBr3 NWs. (c) XRD patterns of NCs and NWs. (d, g) Atomically resolved high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) image of CsPbBr3 NCs with cubic phase along [001]c zone axis (d) and CsPbBr3 NWs with orthorhombic phase along [1-10]o zone axis (g). Inserted: The low magnification of the HAADF-STEM image of the CsPbBr3 NW. (e, h) The corresponding experimental fast Fourier transform (FFT) patterns of NCs (e) and NWs (h). (f, i) Simulated selected area electron diffraction patterns of cubic CsPbBr3 structures along [001]c (f) and orthorhombic CsPbBr3 structures along [1-10]o direction. Purple, gray and red spheres represent Cs, Pb and Br atoms, respectively.

In our experiment, CsPbBr3 NCs were prepared following a previously reported method,13 The NCs possess cubic morphologies with an average diameter of 7 nm (Figure 1a). Highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging (Figure 1d) and fast Fourier transform (FFT) (Figure 1e)37 analysis indicate that the assynthesized CsPbBr3 NCs are of a cubic phase, including the rare particles that exhibited a somewhat more rectangular morphology (Figure S2). This assignment is supported by the


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6 correspondence between the experimental FFT and the simulated electron diffraction (ED) patterns (Figure 1f) of the cubic CsPbBr3 structure (ICSD 29073; Pm3̅m (221); a = 0.5874 nm) along the [001]c zone axis. As the contrast of atomic columns in the HAADF-STEM image scales approximately with the square of the corresponding atomic number Z (∼Z1.7), one can identify different atomic columns in CsPbBr3 NCs along the [001]c direction in the order of contrast: Pb-Br > Cs > Br. The observed atom positions perfectly match the structural model of the cubic CsPbBr3 phase projected along the [001]c direction (Figure 1d, inset), further confirming this assignment. A solar simulator generating an irradiance of 1.7 suns (170 mW/cm2) was used as the light source to drive the assembly of CsPbBr3 NCs. Under irradiation, nearly defect-free singlecrystalline NWs (Figures 1b and S1) spontaneously assembled from individual NCs and gradually settled at the bottom of the containing vial. Powder X-ray diffraction (PXRD) data reveal that during the self-assembly process, the initial cubic CsPbBr3 NCs underwent a phase transformation from cubic phase to the thermodynamically more stable orthorhombic phase, as reflected by the splitting of diffraction peaks associated with the decreased symmetry (see Figure 1c). The PXRD pattern of the resulting NWs can be well indexed according to the orthorhombic phase of CsPbBr3 (ICSD 97851, Pbnm (62), a = 0.8207 nm, b = 0.8255 nm, c = 1.1759 nm). A representative HAADF-STEM image of a CsPbBr3 nanowire is displayed in Figure 1g. The observed atomic positions match well with the structure model of orthorhombic CsPbBr3 phase projected along [1-10]o direction. The axis of the nanowire is along [110]o, as determined form the corresponding FFT patterns (Figure 1h). This indicates that the nanowire has a [110]o-oriented growth direction. Electron diffraction pattern of orthorhombic CsPbBr3 along the [1-10]o zone axis was simulated and presented in Figure 1i for comparison with the FFT pattern to consolidate the phase determination.


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7 As shown in Figure S3, experiments under both dark and light conditions were further carried out to provide solid evidence for the effects of light on NWs growth. Because the strong illumination produced an elevated temperature (38 ˚C), the possibility that the heat generated by the illumination was responsible for the self-assembly process was also considered. By comparing samples heated to 40 ˚C without illumination, this possibility was excluded (Figure S3c). Elevated temperature could not trigger the self-assembly of NCs into NWs or nanobelts, and higher temperatures resulted in the aggregation and decomposition of CsPbBr3 NCs rather than oriented growth. To explore the effective waveband of the broad-spectrum white light, we cut the entire light spectrum at a wavelength of 550 nm, close to the band-edge of CsPbBr3 NCs, which is approximately 2.4 eV. When the NC solution was irradiated with light of wavelengths longer than 550 nm, self-assembly did not occur (Figures S4 and S5). In the absence of an exciting light during the reaction, the emission spectra – measured by taking reaction aliquots – remained constant over time, further indicating that no self-assembly occurred in the CsPbBr3 NC solution (Figure S5). This finding indicates that the self-assembly of perovskite NCs can only be realized with light that has sufficient energy to generate electrons and holes in CsPbBr3 NCs.


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Figure 2. (a-d) Shape evolution of 1D CsPbBr3 NWs with illumination time under a light intensity of 1.7 suns. (a) NCs formed after 2 hours, (b) nanorods formed after 5 hours, (c) NWs formed after 20 hours, (d) NWs formed after 24 hours. (e) HRTEM images showing the oriented attachment of CsPbBr3 intermediate NCs (at 5 hours of illumination) coalescing along the [110]o crystallographic direction and the corresponding FFT patterns (inset). (f) HAADF-STEM image of nanorods acquired along the [1-10]o zone axis, showing an interface between two nanorods that are attached through (001)o surfaces. (g) Magnification of the selected area in (f) clearly showing the continuous CsBr layer and PbBr2 layer interfaces between CsPbBr3 intermediates. (h) Illustration of shape evolution process driven by light from CsPbBr3 NCs to nanorods and further to NWs. (i) Schematic diagram of half-unit cell fusion process on CsBr- and PbBr2-terminated (001)o surface.

To elucidate the formation process of NWs, we monitored the evolution of the growth reaction at different time points by HRTEM. The shape evolution from the original NCs to the final NWs is schematically illustrated in Figure 2a. At the initial stage (0 ~ 2 hours), the NCs spontaneously aligned and fused to form NW prototypes (Figures 2a and S6). Figure 2b illustrates the early formation stage of nanorods, which were composed of aggregated CsPbBr3 NCs. The structures show features of oriented-attachment-dominant fusion and coalescence. At this stage (2 ~ 5 hours), nanorods began to appear. Over time, the NCs progressively


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9 disappeared. At a later stage (5 ~ 20 hours), the nanorods were continuously grafted into NWs (Figures 2c and S7). The next step was the attachment of adjacent NWs, followed by their evolution into nanobelts and even longer and thicker NWs, as shown in Figures 2d and S8, via homoepitaxial growth.38 The final products were a mixture of nanobelts and NWs with diameters of hundreds of nm and typical chain lengths in the range of 10 – 20 μm. Extremely long but flexible chains with lengths exceeding 50 μm formed, corresponding to approximately 7000 close-packed NCs. The 1D NWs growth is a surprising outcome given the seeming lack of directional interactions in perovskite nanocrystals. Perhaps counterintuitively, CsPbBr3 NCs with a highly symmetric cubic lattice could grow into 1D chains, growing anisotropically rather than isotropically. In general, highly symmetric cubic-phase crystal structures lack inherent anisotropy and typically yield nanocubes or large crystals with no-extreme anisotropy. However, the initial self-organization of NCs shown in Figures 2a and S6 indicates the formation of linear chains. The observation of the straight arrays suggests that understanding the self-assembly process requires understanding the intermolecular forces between the crystals.39-40 Indeed, a driving force must be imparted and manipulated among the NCs. The driving force for the spontaneous alignment and fusion of CsPbBr3 NCs is believed to be dipole-dipole interactions, which have been widely investigated in gold nanorods and wurtzite and zinc-blende structures.39,

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With respect to halide perovskite systems, this dipolar

potential field has been simulated for the CsPbI3 system and could explain the tendency to form linear structures.45 However, there could also be concomitant phase changes that are driving or magnifying this tendency. To explore the driving force behind the self-assembly process, HRTEM was utilized to inspect the phase changes during self-assembly. The FFT pattern in Figure 2e, indicates that the intermediate NCs (under illumination for 5 hours) generated before the formation of NWs belong to the orthorhombic phase, suggesting that the phase transition


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10 of CsPbBr3 NCs may occur at an earlier stage of the assembly process. Because of the distortion of Pb-Br6 octahedra in the orthorhombic structure, the symmetry of the structure breaks, and a dipole moment induced field is generated accordingly. After two polarized orthorhombic NCs coalescence, the lowest dipole field gradient is achieved when the third orthorhombic phase NC approaches along the linear alignment rather than through side attachment. The linear selforganization of these NCs guarantees the NCs to grow in one dimension.45 The self-assembly process consists of two main steps: the merging of NCs into nanorods and NWs (stage I) and the extension of nanorods or NWs into nanobelts (stage II). In stage I, the intermediates were mainly formed by the oriented attachment of CsPbBr3 NCs along the [110]o axis of the orthorhombic phase (Figure 2e), consistent with the growth direction of the final NWs. Structural analysis showed that the (110)o surfaces were terminated by either CsBr or PbBr2, as illustrated in Figure S9. The detailed growth mechanisms were investigated by first-principles density functional theory (DFT), through which we systematically studied the surface energies of CsPbBr3 NCs with all possible surface terminations. According to our computations, CsBr- and PbBr2-terminated (110)o surfaces exhibited similar surface energies, (i.e., 0.32 J/m2 and 0.4 J/m2 respectively); this finding suggests that the two type of (110)o surface terminations potentially coexisted, providing the possibility for assembly. Intuitively, the cubic phase has highly symmetric Pb-Br6 octahedral sites, so this phase has symmetric surfaces. However, the orthorhombic phase has rotated octahedral Pb-Br6 sites which break the symmetry on surfaces. Therefore, (110)o surface and (001)o surface are different terminations on the orthorhombic phase (Figure S10). In this sense, compared with the (110)o facets, the (001)o facets possess a smaller surface energy and maintain the atomic densest stacking mode. This finding was further verified by DFT calculations, as shown in Table S2, and the energies of the (001)o plane for CsBr- and PbBr2-terminated surfaces were determined to be 0.07 J/m2 and 0.11 J/m2, respectively; moreover, the (001)o surface energy was determined to be


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11 distinctly lower than that of the (110)o surface (0.32 J/m2 for CsBr termination and 0.40 J/m2 for PbBr2 termination). The growth of NWs along the [110]o direction leads to maximal exposure of low energy (001)o surfaces, and is therefore thermodynamically favorable. In stage II, the nanorods evolved into nanobelts and NWs, as shown in Figure 2f. Either nanorods or nanobelts and NWs prefer to assemble along the [110]o direction. Despite the assembly along the [110]o direction is predominant, lateral assembly of nanorods into nanobelts along [001]o is also observed at the later stage. This shape evolution from NCs to NWs is schematically illustrated in Figure 2h. According to our observations, CsPbBr3 NCs grow through an oriented-attachment mechanism rather than through Ostwald ripening.33 Inevitably, both CsBr- and PbBr2terminated surfaces must coexist, which is a necessary condition for oriented-attachment growth. However, to the best of our knowledge, direct evidence of coexisting surface terminations has not yet been reported. Hence, we employed HAADF-STEM to gain atomicscale insight into the interfacial structure. Because the initial NC surfaces are usually covered with large amounts of ligands, intermediate nanorods were chosen for the observation of the interfacial structure. The atomic structure of the interfaces between assembled CsPbBr3 nanorods was clearly obtained in Figure 2f and 2g, unequivocally demonstrating that both CsBr and PbBr2 surfaces terminations coexisted. Thus, it can be concluded that two half-unit cells with different terminations (PbBr2 or CsBr termination) coalesced to become a new slab; the corresponding projected structural model is shown in Figure 2i.


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Figure 3. (a) Illustration of light-induced ligand removal mechanism. (b) FTIR spectra of NCs and NWs. (c) High-resolution N 1s core-level XPS spectra of NCs and NWs.

The factor dominating the self-assembly process is the interaction between the ligands and NC surfaces. To determine the underlying mechanism of NW formation under visible-light irradiation, we investigated the ligand interactions among NCs via experimental characterizations and theoretical calculations. It is known that ligands are intrinsically mobile and susceptible to detachment from NC surfaces.46-47 Illumination can generate excitons (electron-hole pairs), which play an important role in the selective destabilization of surface stabilizers.48-49 After photoexcitation, dissociated excitons, which are charged, diffuse to the perovskite NC surfaces, and are easily captured by surface ligands (i.e., OAm and OA). The proposed light-induced ligand removal mechanism in CsPbBr3 NCs is illustrated in Figure 3a. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to elucidate ligand detachment and variation during the assembly process; the results are shown in Figure 3b,c. In the FTIR spectra, the absorption features at 1535 cm-1 and 1710 cm-1 are ascribed to the oleate anions (-COO-) and C=O stretching vibrations,16 respectively. In NW samples, the former feature vanishes while the latter feature appears miniscule, indicating that the deprotonated -COO- disappears and that small amounts of OA are still


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13 present. Unlike the NCs, the NWs exhibit relatively weak peaks at 3310 cm-1 (corresponding to -NH2) and 1635 cm-1 (corresponding to -NH3+), which indicates the desorption of OAm (NH2) and protonated amine groups (-NH3+) during the self-assembly process. The N 1s core level in the XPS survey also confirms that protonated amine groups (-NH3+, 401.8 eV) are reduced to oleylamine (-NH2, 399.9 eV) due to light-induced reduction. Therefore, light illumination efficiently detached the surface ligands, and then NCs ultimately fused via baresurface contact. However, based on the relative peak values in Figure 3b, it could be surmised that despite the desorption of both ligands, OAm was not as severely affected by the lightinduced desorption as OA. The different types of ligands attached to the surface of NCs often determines the anisotropic interactions that dominate their shape evolution as they grow. To further understand the NWs’ formation mechanism, we investigated the effect of OAm and OA surface ligands on the shape evolution of perovskite NCs. We found that a small amount of OAm benefits the self-assembly of NWs (Figure S11a), while an excess OA leads to large 3D crystals (Figure S11b). Such phenomena indicate that OAm tends to promote the formation of anisotropic structures.50 D. Roo et al. reported that perovskite NCs surfaces are dynamically stabilized with oleylammonium bromide and oleylammonium oleate (OA-OAm).46 Specifically, when the system contains excess OAm, the OA adhered to the CsPbBr3 NCs can bind to the surrounding OAm to form an ion pair. Consequently, the formed OA-OAm pairs can be easily removed under visible-light irradiation. Hence, the excess OAm is consumed by OA, which reduces the steric repulsion and eventually benefits assembly. Indeed, we confirmed this mechanism by changing the reaction solvent from octadecene (ODE) to OAm, resulting in the formation of NWs (Figure S12), which implies that OAm plays an essential role in the anisotropic growth of the structure along a certain direction.51 In general, ligands selectively adhered to particular atoms on specific NC facets to reduce their surface energy. Consequently, the growth direction


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14 can be ascribed to the existence of OAm: OAm facilitates the growth of orthorhombic NCs intermediates along the [110]o direction while preventing their growth along the [001]o direction. Ultimately, OAm leads to the formation of 1D NWs. It is clear that both OA and OAm bind to the NCs surfaces; however, the molecular form in which the ligands bind to the surfaces (either in the neutral form or in the protonated or deprotonated form) is unclear. In addition, the ligands distribution and coverage on perovskite NCs surfaces are extremely difficult to characterize. Therefore, DFT calculations were used to investigate the ligand binding energy on CsPbBr3 NCs surfaces. We considered all possible configurations, including the binding of electron-donating groups to surface cations, the formation of interfacial hydrogen bonds, and a combination of the two (Figure S13). The results clearly show that the binding energy of OAm on surface Pb-ions (-0.86 eV) is much stronger than that of OA (-0.75 eV). Although the binding energies of these two surface-ligands are quite similar for the CsBr-terminated surfaces, however, the relatively smaller binding energy of CsBr-terminated surfaces usually drives the ligands on these surfaces to detach sooner. Therefore, the later desorbed OAm ligands play a paramount role in effecting the NC's shape and growth direction. In other words, the OA ligand detaches relatively more easily from the surface than the OAm ligand does, yielding anisotropic growth. Illumination ultimately drives the ligands to detach from the NCs, mainly because the binding energies of the surface ligands are not sufficiently large to avoid desorption. After the removal of the surface ligands, the surfaces of CsPbBr3 NCs are exposed. Consequently, CsPbBr3 NCs can fuse and eventually evolve into 1D NWs or nanobelts.


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Figure 4. (a) Absorption (solid line) and emission (dash line) spectra of CsPbBr3 NCs (blue) and NWs (red). (b,c) Lifetime measurements on CsPbBr3 NCs and the obtained NWs via light-induced self-assembly. 2D-color transient emission map of CsPbBr3 NCs (b) and NWs (c). The inset shows the corresponding time-resolved PL decays curves.

The optical properties of the CsPbBr3 NCs and NWs were characterized by UV-Vis absorption and photoluminescence (PL) spectroscopies. The 1D NWs showed a clear spectral red shift (0.14 eV) in their absorption and PL spectra compared with the original NCs (see Figure 4a). To further probe the charge carrier dynamics in CsPbBr3 NWs, PL lifetimes were measured using a streak-camera system (400-nm excitation at room temperature). The initial NCs show PL carrier lifetime of ~2 ns (Figure 4b), whereas the assembled NWs display PL lifetimes of ~125 ns (Figure 4c). These two orders of magnitude longer carrier lifetime of NWs compared with NCs could be attributed to a long carrier diffusion length in the highly crystalline of CsPbBr3 NWs and, potentially, appreciable photon recycling within the NWs as previously reported.52-53


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16 III. CONCLUSION In summary, we demonstrated that light can induce the self-assembly of CsPbBr3 NCs into 1D NWs and nanobelts; such light-induced synthesis is an alternative method for synthesizing CsPbBr3 NWs and potentially other perovskite supra-structures. Light-induced synthesis serves as an efficient green-chemistry approach as well as a rational method for designing perovskites. The light-induced self-assembly mechanism of the perovskite NCs was clearly elucidated by monitoring the shape evolution and verifying the atomic structure at different stages via (S)TEM and DFT simulation. We found that the contribution of illumination mainly lies in the selective destabilization of surface ligands of NCs. During the desorption process, the ligand OA detaches much more easily from NC surfaces than does the ligand OAm, giving rise to anisotropic growth and leading to the formation of NWs. The self-assembly of the CsPbBr3 NCs occurs via anisotropic fusion along the [110]o crystallographic direction of the orthorhombic NCs. Furthermore, for the first time, the coexistence of both CsBr or PbBr2 surface terminations were observed by HAADF-STEM, which was systematically investigated via DFT simulation. Interestingly, the obtained low-defect and high uniformity of NWs, exhibit long PL lifetimes in comparison to the NCs. Overall, we conclude that light is crucial to the self-assembly process of CsPbBr3 NCs, and, more generally, to the reaction trajectory of perovskite materials. The use of light to induce the self-organization of perovskite NCs in solution provides a new degree of flexibility in the construction of colloid-based perovskite structures and devices.

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17 Supporting Information

Details of the chemicals, experimental section, nanocrystals and nanowires synthesis, materials’ characterizations (XRD, XPS, TEM, absorption and emission spectra, FTIR), and computations methods are contained in Supporting Information.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID Omar F. Mohammed: 0000-0001-8500-1130 Yu Han: 0000-0003-1462-1118 Osman M. Bakr: 0000-0002-3428-1002 Author Contributions ∥Jiakai

Liu, Kepeng Song and Yongwoo Shin contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by King Abdullah University of Science and Technology (KAUST).


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18 Xin Liu was supported by the National Science Foundation of China (NSFC) (21771029 and 21573034). J. Liu would like to acknowledge Dr. Jiangtao Jia from KAUST Advanced Membranes and Porous Materials Center for his valuable suggestions. The supercomputer time was supported by the Supercomputing Core Laboratory at KAUST.

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