ISSN 1998-0124 CN 11-5974/O4

2020, 13(1): 282–291 https://doi.org/10.1007/s12274-019-2611-5

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Synthesis and transformation of zero-dimensional Cs3BiX6 (X = Cl, Br) perovskite-analogue nanocrystals Hanjun Yang1, Tong Cai1, Exian Liu2, Katie Hills-Kimball1, Jianbo Gao2, and Ou Chen1 ()

1 Department of Chemistry, Brown University, 324 Brook Street, Providence, Rhode Island 02912, USA 2 Ultrafast Photophysics of Quantum Devices Laboratory, Department of Physics and Astronomy, Clemson University, South Carolina

29634, USA © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 29 October 2019 / Revised: 15 December 2019 / Accepted: 16 December 2019

ABSTRACT The unique structure of zero-dimensional (0D) perovskite-analogues has attracted a great amount of research interest in recent years. To date, the current compositional library of 0D perovskites is largely limited to the lead-based Cs4PbX6 (X = Cl, Br, and I) systems. In this work, we report a new synthesis of lead-free 0D Cs3BiX6 (X = Cl, Br) perovskite-analogue nanocrystals (NCs) with a uniform cubic shape. We observe a broad photoluminescence peak centered at 390 nm for the 0D Cs3BiCl6 NCs at low temperatures. This feature originates from a self-trapped exciton mechanism. In situ thermal stability studies show that Cs3BiX6 NCs remain stable upon heating up to 200 °C without crystal structural degradation. Moreover, we demonstrate that the Cs3BiX6 NCs can transform into other bismuth-based perovskite-analogues via facile anion exchange or metal ion insertion reactions. Our study presented here offers the opportunity for further understanding of the structure-property relationship of 0D perovskite-analogue materials, leading toward their future optoelectronic applications.

KEYWORDS perovskite nanocrystals, zero-dimensional perovskite-analogue, bismuth-based perovskite, post-synthetic reactions, anion exchange

1 Introduction Lead-halide perovskite (APbX3, A = methylammonium (MA), formamidinium (FA), Cs, and X = Cl, Br, I) nanocrystals (NCs) have attracted a great amount of research attention due to their unique optoelectronic properties, leading to potential applications including light emitters, displays, X-ray imaging and solar energy harvesting [1–16]. The unique electronic properties of lead-halide perovskites have been largely regulated by the geometric connectivity of their [PbX6]4− octahedral con-stituents [17, 18], which can be defined as their ‘dimensionality’ [19]. In this regard, the most studied type of lead-halide perovskites (i.e., APbX3) can be classified as a three-dimensional (3D) perovskite structure since the octahedral units connect and extend in all three crystallographic directions (Scheme 1(a)). Perovskite-analogues are the materials that possess similar metal-halide octahedral networks. They often exhibit crystal structures with lower dimensionalities. Examples of low dimensional perovskite-analogues include the two-dimensional (2D) structure with octahedral units connected in only lateral dimensions, or the zero-dimensional (0D) structure with isolated octahedra. Decreasing the dimensionality from 3D to 0D leads to a localization of the material’s electronic structure. Especially, complete removal of inter-octahedral interaction in 0D perovskite-analogues leads to molecule-like optoelectronic properties such as large exciton binding energies and low carrier mobilities [20–22]. In addition to the unique electronic structure, the structural flexibility of 0D perovskite-analogues

through post-synthetic transformation reactions, such as anion exchanges or metal ion insertions, makes 0D perovskite- analogues good starting materials for the fabrication of other perovskite-related materials, including the ones that are difficult to access through other means [23–26]. Although promising, to date, most studies on 0D perovskite-analogues are limited to the Cs4PbX6 (X = Cl, Br, I) systems [27, 28].

In parallel with tuning the crystal structural dimensionality, expanding the compositional space, especially searching for lead-free perovskite systems driven by environmental and toxicity concerns, has been a fast-moving research direction ever since the burst of lead-based perovskite studies in 2009 [29–33]. Because of the similar electron configuration of Bi3+ to Pb2+, bismuth-based perovskite-analogues have recently attracted increasing attention [34–38]. Especially, the layered 2D bismuth- halide materials (also known as the vacancy-ordered perovskite structure, A3Bi2X9 (X = Cl, Br)) (Scheme 1(b)) have shown promising potentials in photovoltaics, lighting, and other optoelectronic applications [39–45]. For example, MA3Bi2Br9 and Cs3Bi2Br9 NCs have been investigated as highly emissive lead-free phosphors [46, 47]. Dimer crystal structure (MA)3Bi2I9 thin-film has been developed as a photovoltaic absorber due to its relatively small band gap (around 2.1 eV) [48], and the prototype device has already shown a promising solar power conversion efficiency of up to 3.2% [48–51]. Despite of rapid progress in the development of 2D bismuth-halide materials, studies on their 0D counterparts, i.e., Cs3BiX6 (Scheme 1(c)) have lagged behind. In this regard, Cs3BiCl6 and Cs3BiBr6 bulk

Address correspondence to ouchen@brown.edu

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materials have been synthesized and utilized as X-ray scintillation materials and photodetectors, respectively [52, 53]. Recently, Gamelin and coworkers demonstrated successful syntheses of 0D Cs3BiX6 (X = Cl, Br, I) NCs through either a direct hot-injection method using trimethylsilyl halide (TMS-X) as an active halide precursor or through a post-synthetic anion exchange route [54]. However, no emission properties or structural stability studies have been presented so far.

Herein, we report a facile synthesis of uniform Cs3BiX6 (X = Cl, Br) NCs via a hot-injection method using benzoyl halides as halide precursors. While 0D Cs3BiBr6 NCs were non-emissive, a broad photoluminescence (PL) peak centered at 390 nm as a result of a self-trapped exciton (STE) mechanism was observed for Cs3BiCl6 NCs. In situ thermal stability studies showed that the 0D perovskite-analogue structure of the thin-films assembled from either Cs3BiCl6 or Cs3BiBr6 NCs remained stable upon heating to ~ 200 °C, demonstrating their potential to be used in future device applications at elevated temperatures. Interestingly, the 0D Cs3BiX6 NCs showed high chemical reactivity and structural flexibility in post-synthetic transformation reactions. A halide anion exchange reaction allowed a continuous con-version from Cs3BiCl6 NCs to Cs3BiBr6 NCs, and a transformation to 0D dimer Cs3Bi2I9 NCs. A metal ion insertion reaction using BiX3 precursors can efficiently transform Bi-poor Cs3BiX6 NCs to Bi-rich Cs3Bi2X9 NCs without disturbing NC integrity. Our work provides insights on the optical properties, structural stabilities, and chemical/structural transformations of 0D Cs3BiX6 (X = Cl, Br) NCs, which may facilitate further understanding of dimensionality-property relationships in lead-free perovskite- analogue materials.

2 Result and discussion

2.1 Synthesis of Cs3BiX6 (X = Cl, Br) NCs

0D Cs3BiX6 (X = Cl, Br) NCs were synthesized using benzoyl halides as the halide precursors through a well-developed hot-injection method (see Electronic Supplementary Material (ESM) for details) [55]. In a typical reaction, a mixture of cesium carbonate and bismuth acetate in 1-octadecene, oleic acid, and oleylamine was degassed at 90 °C for 1 h to remove water and oxygen. A halide precursor, i.e., benzoyl chloride or benzoyl bromide was then injected into the reaction solution at 160 °C, and the reaction was immediately terminated by cooling the solution in a water bath. After purification with ethyl acetate, the obtained Cs3BiCl6 and Cs3BrBr6 NCs were redispersed and stored in toluene.

Powder X-ray diffraction (XRD) unambiguously confirmed the crystal structure of the as-synthesized Cs3BiX6 NCs. Three sets of characteristic dual peaks were observed and located in the 2θ ranges of 12.8°–15.6°, 20.8°–22.9° and 28.3°–31.2° for both Cs3BiCl6 and Cs3BiBr6 NCs. These fingerprint Bragg diffraction peaks can be assigned to the (111) and (311

＿

); (511)

and (113＿

); and (621＿

) and (622＿

) crystal planes of the 0D Cs3BiX6 monoclinic phase (space group: C2/c) (Fig. 1(a)). The rest of the XRD diffraction peaks matched well with the same phase (Fig. 1(a)). Rietveld fittings of the XRD patterns gave the lattice parameters of a = 27.03 Å, b = 8.204 Å, c = 13.18 Å, β = 99.46o and a = 28.31 Å, b = 8.617 Å, c = 13.71 Å, β = 99.48o for the Cs3BiCl6 and Cs3BiBr6 NCs, respectively (Fig. S1 and Tables S1–S3 in the ESM).

Transmission electron microscopy (TEM) images of Cs3BiCl6 and Cs3BiBr6 NCs showed a cubic shape with average edge- lengths of 9.8 ± 1.3 nm and 10.9 ± 1.5 nm, respectively (Figs. 1(b) and 1(d)), in good agreement with the crystal sizes derived from XRD measurements (~ 10 nm) (Table S1 in the ESM). High- resolution TEM images of the samples showed clear lattice fringes with d-spacings of 2.9 and 4.1 Å, which were associated with the (622

＿

) and (113＿

) planes of 0D Cs3BiCl6 and Cs3BiBr6 crystal phases, respectively (Figs. 1(c) and 1(e)). In addition, we found that the size of 0D Cs3BiCl6 NCs can be tuned from 9.8 ± 1.3 to 6.9 ± 0.9 nm by decreasing the reaction temperature from 160 to 140 °C (Fig. S2 in the ESM). Interestingly, we have successfully assembled smaller Cs3BiCl6 NCs (7.1 ± 1.0 nm) into highly ordered NC superlattices with a simple cubic structure (lattice constant of 8.1 nm, Figs. 1(f) and 1(g)). The corresponding wide-angle electron diffraction (ED) pattern showed a preferred particle orientation alignment with the [103] crystallographic direction aligning parallel to the direction of the incident electron beam (Fig. 1(h)).

Absorption spectra exhibited well-defined electronic transition features located in the ultra-violet (UV) spectral region (Fig. 2(a)). Additionally, the absorption profiles showed a high similarity to those of Bi3+ ([Xe]4f145d106s2) ions doped metal halide salts with a similar octahedral coordination environment (e.g., NaCl or KBr), consistent with the assigned electronically isolated 0D perovskite-analogue structure (Scheme 1(c)) [56]. Specifically, three sets of absorption bands were observed, labeled as A, B, and C with increasing electronic transition energy (Fig. 2(a)). The first absorption band (A band) at 332 nm for Cs3BiCl6 and 380 nm for Cs3BiBr6 can be assigned to the spin forbidden 1S0–3P1 transition of the Bi3+ ion [56–58]. The weak, shoulder-like band (B band) was fitted at 238 nm for Cs3BiCl6, and 284 nm for Cs3BiBr6, which was from the vibration-assisted transition of 1S0–3P2 of the Bi3+ ion [58, 59]. The doubly-split peaks (C band) at a higher energy range (209 and 223 nm for Cs3BiCl6; 236 and 261 nm for Cs3BiBr6) can be assigned to the 1S0 – 1P1 transition of the Bi3+ ion, where the splitting was caused by a dynamic Jahn-Teller effect [59]. The absorption spectra and the band assignments indicated that electronic transitions of the 0D perovskite-analogue Cs3BiX6 NCs were solely determined by their isolated [BiX6]3− octahedral sub-unit [60]. As a result, the absorption profile was independent to the size of the NCs, indicating no strong quantum confinement effect is present in our samples (Fig. S2 in the ESM). This conclusion was further supported by nearly unchanged absorption profiles after

 Scheme 1 Schematic illustration of the crystal structures of 3D CsPbX3 (X = Cl, Br, I) (a), 2D Cs3Bi2X9 (X = Cl, Br) (b), and 0D Cs3BiX6 (X = Cl, Br) (c). Insets: local structures of metal-halide octahedron and its closest neighboring octahedra.

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replacing the Cs+ (1.81 Å) cation of the Cs3BiX6 NCs with a smaller Rb+ (1.66 Å) cation (Fig. S3 in the ESM) [61].

Raman spectroscopic characterizations showed red-shifts to lower energy (28 cm−1 for Cl-containing samples, and 30 cm−1 for Br-containing samples) for terminal Bi-X stretching modes of the [BiX6]3− octahedra in the 0D Cs3BiX6 samples as compared to their 2D Cs3Bi2X9 counterparts (Fig. 2(b)) [62]. These redshifts

suggested weaker Bi-X bonds in 0D Cs3BiX6 NCs [63], in line with the slightly longer Bi-X bond lengths than those in the 2D Cs3Bi2X9 NCs determined by crystal structure characterizations (2.70 Å in Cs3BiCl6 and 2.56 Å in Cs3Bi2Cl9 for the terminal Bi-Cl bond; 2.82 Å in Cs3BiBr6 and 2.71 Å in Cs3Bi2Br9 for the terminal Bi-Br bond, Tables S1–S3 in the ESM). Moreover, to confirm the chemical composition and valence states, X-ray

 Figure 1 (a) XRD patterns of Cs3BiCl6 and Cs3BiBr6 NCs. The bars represent standard XRD diffractions. (b) and (c) Low-resolution and high-resolution TEM images of Cs3BiCl6 NCs. (d) and (e) Low-resolution and high-resolution TEM images of Cs3BiBr6 NCs. (f) A low-resolution TEM image of self-assembled Cs3BiCl6 NC superlattices. (g) The corresponding fast Fourier transform (FFT) pattern. (h) ED pattern of the assembled NC superlatticeshowing NC orientational alignment.

 Figure 2 (a) Absorption spectra of Cs3BiCl6 and Cs3BiBr6 NCs. The fitted peaks of the absorption spectra are marked in shaded areas. (b) Raman spectraof Cs3BiX6 and Cs3Bi2X9 NCs (X = Cl, Br). (c)–(f) XPS of Cs3BiX6 and Cs3Bi2X9 NCs (X = Cl, Br) for Cs 3d (c), Bi 4f (d), Br 3d (e), and Cl 2p (f).

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photoelectron spectroscopy (XPS) measurements were carried out and revealed the existence of Cs+, Bi3+, and X– ions in the Cs3BiX6 NCs (Figs. 2(c)–2(f)). A shift to higher binding energy (~ 0.4 eV) was observed when comparing the Bi 4f peak of the Cs3BiCl6 NCs to that of the Cs3BiBr6 NCs. This shift in binding energy was likely due to the chemical shift caused by the higher electronegativity of Cl− ions than Br− ions [64]. In addition, Fourier-transform infrared (FT-IR) spectroscopic measurements revealed the co-existence of both oleic acid and oleylamine ligands on the NC surfaces (Fig. S4 in the ESM) [65–68]. Taken together, we demonstrated the successful direct syntheses of high-quality Cs3BiX6 (X = Cl, Br) NCs with a 0D perovskite- analogue structure through a hot-injection method. However, our efforts failed in attempting to directly synthesize 0D Cs3BiI6 NCs under similar synthetic conditions. In contrast, only Cs3Bi2I9 NCs can be obtained (Fig. S5 in the ESM) featuring face-sharing [Bi2X9]3− unit, also known as “0D dimer” structure (space group: P63/mmc) [69].

Further, we found that controlling the reactivity of the bismuth precursor is crucial to obtain 0D Cs3BiX6 (X = Cl, Br) NCs. The syntheses with relatively high oleylamine concentrations (> 10% volume fraction in ODE) and low reaction temperatures (< 180 °C) resulted in a decreased reactivity of the Bi precursor and Cs3BiX6 NCs with a Bi-poor 0D crystal phase were preferentially obtained. Whereas when lowering the oleylamine concentration and/or increasing the reaction temperature, the bismuth precursor became more reactive, thus Bi-rich 2D Cs3Bi2X9 phase became the dominant product (Figs. S5 and S6 in the ESM, and see ESM for details).

2.2 Emission property of Cs3BiCl6 NCs

Low dimensional perovskite-analogues such as 2D (C6H5C2H4NH3)2PbCl4 and 0D Cs4SnBr6 often exhibit an efficient PL through a STE mechanism [18, 27, 69–74]. Unlike band edge emission, STE-induced PL typically possesses a large Stokes shift and a broad peak width, making it a potential white light source [75, 76]. To examine the emission properties

and mechanisms of our Cs3BiX6 perovskite-analogue NCs, we conducted PL measurements at both low (liquid nitrogen temperature) and room temperatures. For Cs3BiCl6 NCs, while only very weak emission was observed at room temperature, a broad and slightly asymmetric PL peak at 391 nm with drastically enhanced intensity was observed when lowering the temperature to 80 K (Fig. 3(a)). The peak position and width of the Cs3BiCl6 NC PL were similar to those of Cs3BiCl6 single crystals [52]. Overall, a large Stokes shift (59 nm, 0.56 eV) and a broad PL profile (full-width-at-half-maximum (FWHM): 60 nm, 0.47 eV) suggested that the STE-induced emission mechanism was responsible for the observed PL in Cs3BiCl6 NCs [77]. The PL excitation (PLE) spectral onset of Cs3BiCl6 NCs monitored at the PL peak position (i.e., at the wavelength of 390 nm) matched well with the absorption profile at room temperature, thus confirming the PL peak originated from the Cs3BiCl6 NCs. PL spectra measured at different PLE wavelengths (Fig. 3(b)) further confirmed that the excitation of Cs3BiCl6 NCs is responsible for the PL peak. It should be noted that a weaker peak at 440 nm emerged when 344–360 nm excitation was used, which is caused by the exceptional strong spin-orbit coupling of Bi3+ [78–80]. Further, temperature-dependent PL measurements of the Cs3BiCl6 NCs film showed an increase in intensity and a decrease in PL peak width without changing the peak position upon lowering the temperature (Fig. 3(c)), further supporting the STE-based emission mechanism (Fig. 3(d)) [72]. In contrast, no emission has been detected for 0D Cs3BiBr6 NCs at either room temperature or liquid nitrogen temperature.

2.3 Thermal stability of Cs3BiX6 (X = Cl, Br) NCs

Thermal stabilities of Cs3BiX6 NCs were tested by in situ monitoring of the XRD pattern evolution. Upon heating in ambient conditions, the crystal phase of both Cs3BiCl6 and Cs3BiBr6 NCs remained stable up to ~ 200 °C (blue-shaded area in Figs. 4(a) and 4(b)). The widths of the XRD peaks remained nearly the same as those at room temperature, indicating that no significant NC fusion occurred (Figs. 4(a) and 4(b)).

Figure 3 (a) Absorption (ABS, purple), PL (red), and PLE (blue) spectra of Cs3BiCl6 NCs. PL and PLE spectra were measured at 80 K. Absorption spectrumwas measured at room temperature. (b) PL spectra of Cs3BiCl6 NCs measured at different excitation wavelengths. (c) PL spectra of Cs3BiCl6 NCs measured at different temperatures. Inset: PL peak intensity and the corresponding FWHM as a function of temperature. (d) Schematic illustration of the self-trappedexciton emission mechanism.

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Additionally, all the XRD peaks shifted to lower angles due to the thermal expansion of the crystal lattice. When the temperature reached above 200 °C (yellow-shaded area in Figs. 4(a) and 4(b)), new diffraction peaks started to emerge, which can be assigned to the 2D Cs3Bi2X9 crystal phase (space group: P3

＿

m1), indicating a decomposition of the Cs3BiX6 NCs. For example, the (102) and (110) peaks (at ~ 23.1°) and the (204) and (220) peaks (at ~ 47.2°) of the 2D Cs3Bi2Cl9 structure emerged when the temperature reached 220 °C (Figs. 4(c) and 4(d)). For the Cs3BiBr6 case, pronounced peaks at 27.4° and 31.4° emerged when heated to 225 °C, along with a shoulder in the 2θ range of 45.1°– 45.5°, which were associated with the (201), (202) peaks, and the (204) and (220) peaks of the 2D Cs3Bi2Br9 structure, respectively (Figs. 4(e) and 4(f)). Moreover, a new peak at ~ 29.0° was also observed when heating the Br-containing NCs to elevated temperatures (> 225 °C), which can be assigned to the (110) diffraction of body-centered cubic CsBr (Figs. 4(b) and 4(e)). These observations confirmed the decomposition of 0D Cs3BiX6 NCs through the following chemical reaction

3 6 3 2 92Cs BiX Cs Bi X 3CsX + (1)

2.4 Halide anion exchange reactions

Halide anion exchange reactions have been well-developed in perovskite materials as an effective means to fine-tune the halide composition, and hence the band gap of perovskite NCs post-synthetically [3, 81]. To test anion exchange reactions in 0D Bi-based perovskite-analogue NCs, we used TMS-X as the exchange precursors and the results are shown in Fig. 5 [82]. When starting from Cs3BiCl6 NCs and adding TMS-Br precursor, both XRD and UV–vis absorption measurements showed a

gradual transition to Cs3BiBr6 with miscible anion-alloyed intermediate Cs3Bi(Cl1–xBrx)6 NCs (Figs. 5(a), 5(c), 5(e), and 5(g)). During the anion exchange process, all the XRD peaks gradually shifted to lower angles with increasing the amount of added TMS-Br, indicating a continuous lattice expansion (Figs. 5(e) and 5(g)). The final XRD pattern of the Cs3BiBr6 NCs was nearly identical to that of the directly synthesized sample (Fig. 1(a)), confirming the retainment of the 0D crystal phase. In addition, the invariant XRD peak width during the reaction revealed the conservation of the NC dimensions (Figs. 5(e) and 5(g)). The UV–vis absorption peak continuously shifted from 332 to 380 nm during the process of anion exchange (Fig. 5(c)). This gradual shift was consistent with the XRD results, confirming the miscibility between the Cs3BiCl6 and Cs3BiBr6 crystal phases. However, the UV–vis absorption peak width evolution showed an initial broadening followed by a narrowing process (Fig. S7 in the ESM). This trend is different from the previous observations for the anion exchange reactions of 3D CsPbX3 perovskite NCs, which showed less distortion in the absorption spectra profile [3, 81]. This difference can be explained by different origins of the electronic transitions caused by different dimensionalities (3D vs. 0D structures). In a 3D perovskite phase such as CsPbX3, the electronic bands are composed of electronically coupled [PbX6]4− networks throughout the lattice in all three geometric directions, resulting in a single energy state within a given NC. Whereas, the electronic structure of a 0D Cs3Bi(Cl1−xBrx)6 crystal is determined by the individual [Bi(Cl1−xBrx)6]3− octahedron units due to their isolation within the lattice. In this case, the absorption spectrum of the Cs3Bi(Cl1−xBrx)6 NCs is composed of the sum of individual octahedron units [Bi(Cl1−xBrx)6]3− following a normal distribution for a given overall anionic ratio, thus resulting in

Figure 4 (a) and (b) Temperature-dependent XRD patterns for Cs3BiCl6 (a) and Cs3BiBr6 (b) NCs. Blue-shaded regions indicate the temperature range where no NC decompositions were observed. Yellow-shaded regions indicate the temperature range where the NC decomposition occurred. (c)–(f) Zoomed-in areas of the dashed rectangles in (a) and (b) for the clear identifications of crystal structure changes for Cl- ((c) and (d)) and Br-containing ((e) and (f))samples. The standard diffraction patterns for Cs3Bi2X9 (top bars) and Cs3BiX6 (bottom bars) (X = Cl, Br) are shown in colored bars, while the standard diffraction patterns of CsCl or CsBr are shown in grey bars. Note that at elevated temperature all the XRD peaks shifted to lower angles as compared to thestandard peak positions, which is due to the thermal expansion of the crystal lattice at high temperatures.

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the observed absorption peak broadening (Fig. 5(c) and Fig. S7 in the ESM). TEM images taken during the anion exchange reaction from Cs3BiCl6 to Cs3BiBr6 confirmed the preservation of structural integrity of the NCs with a slight size increase (Fig. S8 in the ESM).

Contrary to the smooth transition from Cs3BiCl6 to Cs3BiBr6 NCs, the anion exchange in Cs3BiBr6 using TMS-I resulted in the formation of 0D dimer Cs3Bi2I9 (Figs. 5(a), 5(b), 5(d), and 5(f)). The first absorption peak of Cs3BiBr6 NCs did not red-shift upon adding the TMS-I precursor. Instead, a set of new absorption peaks ranging from 450 to 485 nm emerged (Fig. 5(b)). The new absorption profile was consistent with that of the previously reported 0D dimer Cs3Bi2I9 NCs [83, 84]. The distinctive change in the crystal structure is clearly shown in the XRD spectra with the appearance of a new set of diffraction peaks that can be assigned to the 0D dimer Cs3Bi2I9 crystal structure (space group: P63/mmc) (Figs. 5(d) and 5(f)). For example, the characteristic (202) and (204) peaks of the Cs3Bi2I9 structure can be clearly observed at 25.8° and 29.7°, respectively. In addition to the formation of Cs3Bi2I9, the newly emerged diffraction peaks at 27.6°, 39.5°, and 48.9° can all be associated with CsI (Figs. 5(d) and 5(f)). Therefore, the XRD results suggest the following anion exchange reaction mechanism

3 6 3 2 92Cs BiBr 12TMS I Cs Bi I 3CsI 12TMS Br+ - + + - (2)

The as-formed Cs3Bi2I9 phase showed significantly narrowed diffraction peaks, indicating the loss of nano-sized domains as a result of the dramatic change in crystal structure and material composition during the exchange reaction. While other reports have shown a great structural versatility of 0D perovskite-

analogues, the failure of fabricating Cs3BiI6 through an anion exchange reaction suggests that there is a thermodynamic instability of the Cs3BiI6 NCs. We notice that a successful synthesis of Cs3BiI6 NCs through a similar anion exchange reaction has been recently reported [54]. The difference is likely due to the use of larger starting Cs3BiBr6 NCs (~ 15.5 nm), imparting additional phase stability of the obtained large Cs3BiI6 NCs because of the presence of less amount of surface atoms [85].

2.5 Metal ion insertion reactions

The metal-deficient nature of 0D perovskite-analogue NCs offers them a strong tendency to undergo metal ion insertion reactions [23, 24, 86]. In particular, BiX3 can be inserted into the Bi-poor 0D Cs3BiX6 NCs, forming Bi-rich 2D Cs3Bi2X9 NCs (Fig. 6(a)). To test this hypothesis, we have conducted the ion insertion reaction by soaking BiX3 solid salt (with the same halide anion to avoid any anion exchange reactions) into the toluene solution of colloidal Cs3BiX6 NCs. A small amount of oleic acid and oleylamine was also added into the reaction solution to facilitate the dissolution of BiX3 solids (see ESM for reaction details). Upon mixing, while the original absorption feature gradually disappeared, new absorption peaks emerged at 370 and 430 nm for the Cl and Br cases, respectively (Figs. 6(f) and 6(g)). The metal ion insertion reactions were completed in ~ 5 min, shown by no further variances of the absorption features (Figs. 6(f) and 6(g)). The final absorption profiles were nearly identical to those of the directly synthesized Cs3Bi2X9 NCs (Fig. S5 in the ESM) [87]. The XRD measurements unambiguously confirmed the corresponding layered 2D Cs3Bi2X9 structures, in good accordance with the absorption measurement results (Figs. 6(h) and 6(i)). TEM images showed

Figure 5 Anion exchange reactions of Cs3BiX6 (X = Cl, Br) NCs. (a) Schematic illustration showing the Cs3BiCl6, Cs3BiBr6 and Cs3Bi2I9 unit cells. (from bottom to top respectively). (b) and (c) Absorption spectra evolution during the anion exchange for Cl-to-Br (c), and Br-to-I (b). Dashed lines are drawn as a guide to highlight subtle changes. (d) and (e) XRD pattern evolution during anion exchange for Cl-to-Br (e) and Br-to-I (d). The standard XRD diffraction patterns for Cs3BiCl6, Cs3BiBr6 and Cs3Bi2I9 are shown in purple, cyan and red bars, respectively. The standard XRD diffraction patterns for CsIare shown in blue bars. (f) and (g) Zoomed-in area of dashed rectangles in (d) and (e) respectively, signifying the representative area of phasetransformation.

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that the average sizes of the NCs increased from 10.5 to 11.8 nm, and 11.7 to 13.3 nm for the Cl and Br cases, respectively (Figs. 6(b)–6(e)). Slight increases in the NC sizes suggest an insertion-based reaction mechanism rather than a NC dissolu-tion and reformation mechanism, sharing a similar mechanism as previously observed in ion insertion reactions and crystal structure transformations of Pb-based perovskite NCs [23].

In all, a complete picture of the post-synthetic reactions presented here is summarized in Scheme 2. It has been noted that the 0D Cs3BiX6 perovskite-analogue NCs are highly modifiable through facile post-synthetic transformation reactions.

Scheme 2 Schematic illustration of post-synthetic transformations starting from 0D Cs3BiX6 (X = Cl, Br) perovskite-analogue NCs.

In addition, the NCs can largely maintain their morphology and nanoscale size while drastically changing their crystal structures. Understanding reaction mechanisms at an atomic level that allows perovskite and perovskite-analogue NCs to undergo diverse structural transformations without losing their nanoscale integrity still stands as an intriguing topic for future research.

3 Conclusion In conclusion, we report a facile and selective synthesis of high-quality Cs3BiX6 (X = Cl, Br) NCs. The synthesized Cs3BiX6 NCs exhibit a 0D perovskite-analogue crystal structure with uniform cubic morphology. Both optical and XRD charac-terizations confirmed the crystal structure of the 0D Cs3BiX6 NCs with isolated [BiX6]3− octahedra within the crystal lattice. Interestingly, a broad emission peak centered at 390 nm through a STE mechanism was observed for Cs3BiCl6 NCs at low temperatures. Halide anion exchange and metal ion insertion reactions were demonstrated to show the com-positional and structural versatilities of 0D perovskite-analogue NCs, allowing Cs3BiX6 NCs to serve as starting materials towards exploration and fabrication of other Bi-based materials post-synthetically. As a member of the 0D perovskite-analogue materials family, lead-free Cs3BiX6 NCs hold the potential to be applied as optoelectronic materials with less toxicity and minimized environmental concerns in future applications.

Acknowledgements O. C. acknowledges the support from Brown University startup funds and the National Science Foundation (OIA-1538893). K.

Figure 6 (a) Schematic illustration for the metal ion insertion reaction from Cs3BiX6 to Cs3Bi2X9 (X = Cl, Br). (b) and (c) TEM images of Cs3BiBr6 NCs before (b) and after (c) the metal ion insertion reaction. (d) and (e) TEM images of Cs3BiCl6 NCs before (d) and after (e) the metal ion insertion reaction. (f) and (g) Absorption spectra evolution during insertion reaction for Cs3BiBr6 NCs (f) and Cs3BiCl6 NCs (g). Dashed lines are drawn to mark the disappearance of Cs3BiX6 peak and the emergence of Cs3Bi2X9 peak. (h) and (i) XRD patterns of Br-containing (h) and Cl-containing (i) NCs before (bottom) and after (top) metal ion insertion reactions. The standard diffraction patterns for Cs3Bi2X9 (top bars) and Cs3BiX6 (bottom bars) are shown in the colored bars. The asterisk marks an unidentified impurity peak.

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H.-K. is supported by the U.S. Department of Education GAANN research fellowship (P200A150037). TEM, XRD, XPS and Raman measurements were performed at the Electron Microscopy Facility and NanoTools Facility in the Institute for Molecular and Nanoscale Innovation (IMNI) in Brown University. Part of the photoluminescence measurements was performed at Clemson University.

Electronic Supplementary Material: Supplementary material (experimental detail, crystal structure, additional figures and additional characterization) is available in the online version of this article at https://doi.org/10.1007/s12274-019-2611-5.

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