Achieving Intrinsic Dual-Band Excitonic Luminescence from a Single Three-Dimensional Perovskite Nanoparticle Through Ni²⁺-Mediated Halide Anion Exchange

Introduction

In recent years, three-dimensional (3D) all-inorganic cesium lead halide (CsPbX₃, X = Cl, Br, or I) perovskite nanoparticles (NPs) have gained prominence in multidisciplinary research areas attributable to their unique optoelectronic and photovoltaic properties.^1–8 Thereinto, highly dynamic crystal lattices of perovskites and 3D interconnections for their corner-sharing [PbX₆]⁴⁻ octahedra readily result in rapid halide anion-exchange reactions between different halide perovskite NPs.^9–14 As a consequence, the composition-dependent bandgaps of CsPbX₃ NPs can be arbitrarily adjusted from the visible to infrared spectral regions by mixing various halide precursors in appropriate ratios.^15–21 However, such fast anion-exchange kinetics has also proven to be a bottleneck for creating a single 3D perovskite NP consisting of two or more different perovskite components of 3D CsPbX₃ particularly by using the conventional synthetic strategies such as the hot-injection method, ligand-assisted reprecipitation approach, spin-coating method, etc.²²

Generally speaking, 3D CsPbX₃ NPs in contact with other halide components tend to undergo fast anion exchange and then produce mixed-halide perovskite NPs with labile monochromatic emission,^23–25 which impede further practical applications because of the subsequent anomalous charging hysteresis and light-induced phase segregation.^26–30 Conventional attempts to overcome the detrimental halide anion exchange are capping the 3D CsPbX₃ NPs with either an inorganic layer or organic shell/cage such as SiO₂,^31–33 polymers,³⁴ and metal–organic frameworks,^35,36 but these approaches can inevitably affect the luminescent efficiencies and charge carriers transport for perovskites due to the dielectric characteristic of external shells.^37,38

Considering the above-mentioned issues and inspired by the completely isolated [PbBr₆]⁴⁻ octahedra inside the crystal lattice of zero-dimensional (0D) Cs₄PbBr₆, which are different from the corner-sharing [PbX₆]⁴⁻ octahedra in 3D CsPbX₃ NPs and may restrict the rapid halide anion exchange in perovskites ( Supporting Information Figure S1), herein, we first report a Ni²⁺-mediated halide anion-exchange strategy to achieve the highly desirable 3D CsPbX₃ NPs with ultra-stable intrinsic dual-band excitonic luminescence. By combining the experimental results such as femtosecond transient absorption (fs-TA) with first-principles calculations based on density functional theory (DFT), we successfully visualize the dynamic process of internal anion exchange for nanoscale perovskite materials, and reveal the underlying mechanism as illustrated in Scheme 1, where the Ni²⁺-substituted [PbBr₆]⁴⁻ octahedra, namely isolated [NiBr₆]⁴⁻ octahedra in the 0D Ni²⁺-doped Cs₄PbBr₆ perovskites serving as meta-stable intermediates, can lead to a lower anion diffusion barrier in corresponding regions than those within the 0D Cs₄PbBr₆ and residual 3D CsPbBr₃ regions. Therefore, after adding supplementary halide anions in the synthetic procedure, the anion diffusion preferentially occurs in the proximity of the substitutional Ni_Pb centers, in favor of the precise control of halide anion-exchange reactions, thereby delivering intrinsic dual-band excitonic luminescence from a single 3D CsPbX₃ NP with two different perovskite individuals of CsPbCl₃ and/or CsPbBr₃. In addition, defect-free 3D CsPbCl₃ NPs with ultra-narrow bright blue emission and excellent stability, which make them potential blue-emissive perovskite phosphors for highly efficient optoelectronic devices, can be further fabricated by adding more Cl⁻ ions.

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Experimental Methods

Chemicals and materials

Cesium carbonate (Cs₂CO₃, 99.9%), lead bromide (PbBr₂, 99.999%), lead chloride (PbCl₂, 99.999%), nickel bromide (NiBr₂, 99.9%), oleic acid (OA, technical grade 90%), and 1-octadecene (ODE, technical grade 90%) were purchased from Sigma-Aldrich (Shanghai, China). Oleylamine (OAm, 80–90%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Acetone, cyclohexane, hydrochloric acid, hydrobromic acid, ethyl acetate, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used without purification unless otherwise noted.

General procedure for the synthesis of 0D Cs₄PbBr₆:Ni NPs

The 0D Cs₄PbBr₆:Ni NPs were synthesized by using our previously reported method with some modifications. Taking the 0D Cs₄PbBr₆:Ni NPs with a nominal NiBr₂/(NiBr₂+PbBr₂) mole ratio of 80 mol % as an example, a mixture of 0.150 mmol NiBr₂ and 0.038 mmol PbBr₂ was first added into a 50 mL two-neck round-bottom flask containing 5 mL ODE, 0.5 mL OA, and 0.5 mL OAm, degassed under N₂ flow at room temperature for 10 min, and then heated to 120 °C under N₂ flow with constant stirring for 60 min. After complete solubilization of NiBr₂ and PbBr₂, the temperature was raised to 150 °C and the Cs-oleate precursor solution (0.4 mL, 0.125 M in ODE) was quickly injected. After reacting for 5 min, the reaction mixture was rapidly cooled down to 25 °C in an ice-water bath to obtain monodisperse Cs₄PbBr₆:Ni NPs. All the as-synthesized Cs₄PbBr₆:Ni NPs were collected by centrifugation at 12,000 rpm for 5 min and stored in a glovebox under argon protection. For the other Cs₄PbBr₆:Ni NPs with the molar ratios of Ni indicated in Supporting Information, the synthetic procedures were identical to those of the Cs₄PbBr₆:Ni NPs (80 mol %), except for changing the NiBr₂/(NiBr₂+PbBr₂) molar ratio (10–90 mol %) in the mixed ODE OA, and OAm solution containing 0.188 mol of the PbBr₂ and NiBr₂ precursors.

General procedure for the halide anion exchange in 0D Cs₄PbBr₆:Ni NPs

In a typical procedure, 31.5 μL hydrochloric acid (or 20 μL hydrobromic acid) as the supplementary anion source was first injected into a mixture solvent of 0.5 mL OA, 0.5 mL OAm, and 5 mL ODE, which was then diluted to an expected concentration and degassed under N₂ flow at room temperature for 10 min. Meanwhile, the pre-synthesized 0D Cs₄PbBr₆:Ni NPs under argon protection were dispersed in cyclohexane and stored in a reaction vessel for subsequent reaction. The diluted anion source was injected into the reaction vessel including Cs₄PbBr₆:Ni NPs dropwise with stirring under a N₂ atmosphere. After reacting for 60 min, all the NPs were collected by centrifugation at 12,000 rpm for 5 min. Subsequently, the resultant precipitates were dispersed again into cyclohexane and stored in a refrigerator at 4 ^oC, accompanied by slow anion diffusion.

Computational methods

All the first-principles calculations are performed under the framework of the DFT, where the codes are implemented in the Vienna ab initio simulation package with the projector augmented plane-wave method.³⁹ The generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE) is selected to treat the exchange-correlation potential.⁴⁰ The cut-off energy for the plane wave is set to 450 eV. The energy criterion is set to 10⁻⁵ eV in an iterative solution of the Kohn–Sham equation. The Brillouin zone was sampled with allowed spacing between k points in 0.2 Å⁻¹, with a Γ-centered Monkhorst–Pack k-point grid. All the structures are relaxed until the residual forces on the atoms have decreased to less than 0.01 eVÅ⁻¹. The formation energy of Cs₄PbBr₆:Ni is defined by

where $E[\text{total}]$ is the total energy of Cs₄PbBr₆:Ni perovskite structure, $E[\mathrm{Cs}]$, $E[\mathrm{Pb}]$, $E[\mathrm{Br}]$, and $E[\mathrm{Ni}]$ are the energies of isolated atoms forming the structure, and $n_{\mathrm{Cs}}$, $n_{\mathrm{Pb}}$, $n_{\mathrm{Br}}$, and $n_{\mathrm{Ni}}$ represent the atom number of Cs, Pb, Br, and Ni in Cs₄PbBr₆:Ni perovskite, respectively. For the electronic band structure calculations, due to the underestimation of GGA in the framework of PBE functionals and the large nuclear charge of Pb, which also underestimates the bandgap to a certain degree,⁴¹ the spin–orbit coupling effect will significantly affect the bandgap calculations in lead halide perovskites. Additionally, considering the energy level splitting caused by the magnetic element Ni in the Ni²⁺-doped Cs₄PbBr₆ structure, spin-polarized calculations have been employed including the majority spin and minority spin conditions. Here we applied a higher level computational method, Heyd–Scuseria–Ernzerhof (HSE06) hybrid functionals,⁴² to predict the bandgap of the bulk structures, where the predicted gap is in good agreement with the experimental data.

Results and Discussion

In our design, 0D Ni²⁺-doped Cs₄PbBr₆ NPs containing a trace amount of 3D CsPbBr₃ perovskite components (termed as Cs₄PbBr₆:Ni) were first synthesized in a Pb²⁺-poor and Br⁻-rich reaction environment by using a modified hot-injection procedure we previously developed, which then served as a meta-stable intermediate to prepare ultra-stable 3D CsPbX₃ (X = Cl and Br) NPs with intrinsic dual-band excitonic luminescence after adding supplemental halide anions in the synthetic procedure (Scheme 1). For comparison, pure 0D Cs₄PbBr₆ perovskite NPs were also prepared as a reference by using a similar synthetic procedure without adding NiBr₂ precursor into the reaction environment. 0D Cs₄PbBr₆ perovskite was chosen as the host material for substitutional Ni²⁺-doping because of its unique crystal lattice featuring isolated [PbBr₆]⁴⁻ octahedra separated by Cs cations, which is thought to be capable of restricting the fast halide anion exchange as commonly observed in 3D CsPbX₃ NPs with corner-sharing [PbBr₆]⁴⁻ octahedra. What is more, due to the much smaller ionic radius of Ni²⁺ (∼0.69 Å) than that of Pb²⁺ (∼1.19 Å), the substitutional Ni_Pb centers (that is, [NiBr₆]⁴⁻ octahedra) in the as-synthesized Cs₄PbBr₆:Ni NPs should be unstable (or in a meta-stable state) relative to the other perovskite components in the same perovskite NP such as 0D Cs₄PbBr₆ and 3D CsPbBr₃, which can be well evidenced by our first-principles calculations based on DFT ( Supporting Information Table S1) showing that the formation energy (E_f, which is defined as the energy difference between the integrated nanocrystal and all of its isolated atoms, usually used as a reliable indicator for material stability) for the Cs₄PbBr₆:Ni NPs is much higher than that of their pure counterparts. In view of this, the meta-stable Cs₄PbBr₆:Ni NPs should have a lower halide anion diffusion barrier particularly in the proximity of the substitutional Ni_Pb centers than those within the 0D Cs₄PbBr₆ and/or 3D CsPbBr₃ regions. As a result, after providing supplementary halide anions in the synthetic procedure, the halide anion-exchange process preferentially occurs in close proximity to the substitutional Ni_Pb centers rather than in the 0D Cs₄PbBr₆ and 3D CsPbBr₃ regions, which thereby makes it possible to produce highly desirable intrinsic dual-band excitonic luminescence in a single perovskite NP (Scheme 1).

Figure 1a displays a representative transmission electron microscopy (TEM) image for the as-synthesized 0D Cs₄PbBr₆:Ni NPs, from which one can see all the 0D Cs₄PbBr₆:Ni NPs are approximately in a hexagonal shape with a mean nanocrystal size of 15.7 ± 0.8 nm analogous to that of their pure counterparts (Figure 1a,b and Supporting Information Figure S2). A high-resolution TEM image reveals the high crystallinity of the Cs₄PbBr₆:Ni NPs (Figure 1c), as well evidenced by their clear lattice fringes with a measured d-spacing of 0.324 nm that is in agreement with the lattice spacing of (131) plane of rhombohedral-phase Cs₄PbBr₆ crystal structure (ICSD No. 162158) ( Supporting Information Figure S3). High-angle annular dark-field scanning TEM (HAADF-STEM) images along with the corresponding two-dimensional energy-dispersive X-ray spectroscopy (EDS) element mapping of some randomly selected Cs₄PbBr₆:Ni NPs (Figure 1d) shows that the Ni²⁺ ions are homogeneously distributed among the whole NPs, indicative of the successful doping of Ni²⁺ ion in the lattice of 0D Cs₄PbBr₆:Ni NPs. This result can be further corroborated by the presence of the typical 2p state of divalent Ni²⁺ ion plus the distinct X-ray photoelectron spectroscopy (XPS) peak shifting toward lower binding energies for Cs 3d, Pb 4f, and Br 3d states in the as-synthesized Cs₄PbBr₆:Ni NPs when compared with the pure reference (Figure 1e). The actual Ni²⁺-doping content in these 0D Cs₄PbBr₆:Ni NPs can be roughly estimated by using inductively coupled plasma atomic emission spectroscopy ( Supporting Information Table S2), which was found to deviate substantially from the nominal amount of NiBr₂ precursor added to the reaction environment, similar to the cases of Mn-doped or Sn-doped diverse perovskite nanocrystals we previously reported.⁴³

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Benefiting from the successful doping of Ni²⁺ ion, these as-synthesized 0D Cs₄PbBr₆:Ni NPs were found to feature dual-band emissions in the bluish-violet (∼437 nm) and bluish-green (∼495 nm) spectral regions upon UV excitation at 365 nm, thereby giving rise to an overall cerulean blue color output with an absolute photoluminescence quantum yield (PLQY) of about 14.1% (Figure 2a, upper). This observation is completely different from the case of the reference Cs₄PbBr₆ NPs without any fluorescence in the visible spectra region (Figure 2a, lower). In particular, dissimilar time-resolved PL (TRPL) decays and fs-TA dynamics were observed for these two emissions in the 0D Cs₄PbBr₆:Ni NPs (Figure 2b,c and Supporting Information Table S3), which strongly supports the presence of two types of newly emerged emission centers in the as-synthesized Cs₄PbBr₆:Ni NPs. Based on our first-principles calculations as well as a set of control experiments ( Supporting Information Figures S4–S7), we are confident that the bluish-violet emission centered at ∼437 nm is primarily associated with the substitutional Ni_Pb center after Ni²⁺ doping in the lattice of 0D Cs₄PbBr₆ NPs,^44–46 whereas the bluish-green emission at ∼495 nm is originated from the sub-nanometer CsPbBr₃ impurities embedded within 0D Cs₄PbBr₆ host matrix as previously reported.^47–49 That is to say, the as-synthesized 0D Cs₄PbBr₆:Ni NPs we prepared are composed of three kinds of constituents including substitutional Ni_Pb center, sub-nanometer 3D CsPbBr₃ perovskite residue, and 0D Cs₄PbBr₆ host matrix (Figure 2d). As a consequence, apart from the intrinsic band edge absorption of 0D Cs₄PbBr₆ perovskite at ∼313 nm, we detected two band edge absorptions peaking at ∼435 and ∼490 nm, respectively, in the absorption spectrum for the as-synthesized Cs₄PbBr₆:Ni NPs (Figure 2a). More interestingly, the bluish-violet emission at ∼437 nm can rapidly degrade to a non-luminescent state after exposure to elevated temperature or under ambient air conditions, in stark contrast to the retarded degradation of the bluish-green emission of ∼495 nm under otherwise identical experimental conditions (Figure 2e,f), which indeed reveals the substitutional Ni_Pb centers in the as-synthesized Cs₄PbBr₆:Ni NPs are unstable compared with the other two components as we anticipated.

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Thanks to the unstable nature of these substitutional Ni_Pb centers, we did find that these as-synthesized 0D Cs₄PbBr₆:Ni NPs worked very well as meta-stable intermediates to deliver intrinsic dual-band excitonic luminescence from a single 3D CsPbX₃ NP (Figure 3a). As compared in Figure 3b, after intentionally adding some supplementary Br⁻ ions into the cyclohexane solution of the Cs₄PbBr₆:Ni NPs even at low temperature (4 °C), the bluish-violet emission centered at ∼437 nm in the as-synthesized Cs₄PbBr₆:Ni NPs was observed to gradually fade away and then was transformed into a brand-new blue emission peaking at ∼463 nm with increased reaction time, which is completely different from the slightly red-shifted bluish-green emission from the residual 3D CsPbBr₃ component in terms of line position and intensity, leading to a gradual change in the overall color output from cerulean blue to peacock blue (Figure 3b, upper). Of particular note, accompanied by the disappearance of the absorption peaks of the substitutional Ni_Pb center (∼432 nm) and Cs₄PbBr₆ perovskite (∼313 nm), an additional absorption peak around 455 nm came into being in these Cs₄PbBr₆:Ni NPs after being subjected to the post-treatment of supplementary Br⁻ ions (Figure 3c), which strongly supports that a brand-new component other than the pre-existing substitutional Ni_Pb center and residual CsPbBr₃ is produced accounting for the newly emerged blue emission at ∼463 nm.

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By combining the above spectral analyses with TRPL decays of the brand-new component and residual CsPbBr₃ showing almost identical trends in the rapid decay stage ( Supporting Information Figure S8), we argue that such a brand-new component should be a type of micro 3D CsPbBr₃ perovskite (hereafter referred to as micro CsPbBr₃) with the distinct blue-shifted PL emission compared with the residual CsPbBr₃ component (Figure 3b), which is thought to be due to the enhanced excitonic localization level in the micro CsPbBr₃ as previously reported.^50,51 To verify this hypothesis, we then carried out a series of time-dependent research on the underlying generation mechanism for the micro CsPbBr₃ component. As shown in the representative TEM images of NPs after adding some supplementary Br⁻ ions (Figure 3d), two weeks later, the morphology of NPs exhibited a remarkable transformation from the initial rhombohedral phase to the resultant cubic phase, as further evidenced by the evolution of characteristic powder X-ray diffraction (XRD) peaks (Figure 3e) from a rhombohedral-phase Cs₄PbBr₆ crystal structure (ICSD No. 162158) to a cubic-phase CsPbBr₃ crystal structure (ICSD No. 29073), demonstrating the brand-new component can be naturally attributable to cubic CsPbBr₃ crystal, namely the micro CsPbBr₃ perovskite.

To verify the need of substitutional Ni_Pb centers designed to precisely control the halide anion-exchange reaction in 0D Cs₄PbBr₆:Ni NPs, we further tracked the locations of Ni²⁺ ions before and after the reaction. As shown in Figure 3f and Supporting Information Figure S9, the HAADF-STEM images coupled with their corresponding EDS elemental mapping display that a majority of Ni²⁺ ions escaped from the Cs₄PbBr₆:Ni NPs by free Pb²⁺ replaced in cyclohexane solution (Figure 3a) and then were dispersed outside the resultant cubic-phase CsPbBr₃ NPs with the reaction proceeding. This result is further supported by our high-resolution XPS results (Figure 3g), where the absence of typical XPS peaks from Ni 2p orbital in resultant CsPbBr₃ NPs (Cs₄PbBr₆:Ni + Br⁻) as that in reference Cs₄PbBr₆ is obviously different from the case in the as-synthesized 0D Cs₄PbBr₆:Ni NPs exhibiting typical Ni 2p peaks. Additionally, the escaped Ni agglomerated into NiBr₂ compounds, which can be found in the final solution environment by TEM characterization ( Supporting Information Figure S10). These compositional analysis results also reveal that the brand-new micro CsPbBr₃ component is Ni-free product, excluding the influence of Ni element on its PL property. In a word, by utilizing 0D Cs₄PbBr₆:Ni NPs as meta-stable intermediates for post-treatment of supplementary Br⁻ ions, we can achieve the unreported single 3D CsPbBr₃ NP consisting of the micro 3D CsPbBr₃ perovskite (blue emission centered at ∼463 nm) and residual 3D CsPbBr₃ component (green emission centered at ∼505 nm), showing intrinsic dual-band excitonic luminescence upon excitation at 365 nm at room temperature.

Motivated by the successful construction of a single 3D CsPbBr₃ NP with intrinsic dual-band excitonic luminescence, we reasoned that the highly desirable single 3D CsPbX₃ NP containing both 3D CsPbCl₃ and 3D CsPbBr₃ components with their respective spectral characteristics could be realized by using the Ni²⁺-mediated halide anion-exchange strategy to precisely control the anion-exchange reaction in the proximity of substitutional Ni_Pb centers within the meta-stable 0D Cs₄PbBr₆:Ni NPs (Figure 4a), breaking through the limitation of rapid anion exchange between different components of 3D CsPbX₃ perovskite. Similar to the generation process of single 3D CsPbBr₃ NP with intrinsic dual-band excitonic luminescence, as depicted in Figure 4b, after adding supplementary Cl⁻ ions into the cyclohexane solution of as-synthesized Cs₄PbBr₆:Ni NPs kept at 4 °C, the bluish-violet emission centered at ∼437 nm is gradually transformed into an emerging deep-purple emission peaking at ∼395 nm after reacting for two weeks, which is different from the constant bluish-green emission peaking at ∼495 nm for the residual 3D CsPbBr₃ component and results in a change in the overall color output from cerulean blue to indigo blue (Figure 4b, inset). Correspondingly, the UV–vis absorption spectra (dotted lines) reveal that an additional absorption peak centered at ∼390 nm appears in these Cs₄PbBr₆:Ni NPs after undergoing the post-treatment of supplementary Cl⁻ ions, while the absorption peaks of the substitutional Ni_Pb center (∼432 nm) and Cs₄PbBr₆ perovskite (∼313 nm) fade away, supporting that another brand-new component with purple emission peaking at ∼395 nm is fabricated.

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To shed more light on the origin of the above purple-emission component, we then performed TEM and XRD to visualize the transformation in morphology and phase. The morphological change for the as-synthesized Cs₄PbBr₆:Ni NPs after adding supplementary Cl⁻ ions from rhombohedra to cubes analogous to the case after adding supplementary Br⁻ ions was recorded in the time-dependent TEM images (Figure 4c), as further supported by XRD patterns on relevant NPs (Figure 4d), where the typical diffraction peaks ascribed to rhombohedral-phase Cs₄PbBr₆ (ICSD No. 162158) crystal evolved into the peaks in good agreement with cubic-phase CsPbCl₃ (ICSD No. 29072) crystal after 2 weeks. Notably, as compared in the HAADF-STEM images and EDS analyses ( Supporting Information Figures S11 and S12), the anion-exchange process in Cs₄PbBr₆:Ni NPs with the post-treatment of supplementary Cl⁻ ions is also accompanied by the above-mentioned Ni²⁺ ions escaping from NPs. Taking together all the above observations and analyses, we reasonably conclude that the purple-emission component should be a type of micro 3D CsPbCl₃ perovskite possessing cubic-phase structure (termed as micro CsPbCl₃) and the expected 3D CsPbX₃ NPs with coexisting micro CsPbCl₃ and residual CsPbBr₃ perovskite components can be successfully prepared through the Ni²⁺-mediated halide anion-exchange. Furthermore, we find that the PL emission peaks related to 3D CsPbCl₃ and 3D CsPbBr₃ components in the as-synthesized single 3D CsPbX₃ NP with satisfactory crystalline stability under ambient conditions after 16 days show no clear evidence of shifting in the corresponding PL emission spectra ( Supporting Information Figure S13), thereby demonstrating that our strategy contributes to restricting the rapid halide anion exchange in perovskites over long time scales.

More importantly, by intentionally adding more supplementary Cl⁻ ions into the reaction solution and increasing the halide anion-exchange time, we can transform the 3D CsPbX₃ NPs with CsPbCl₃ and CsPbBr₃ components into pure micro 3D CsPbCl₃ NPs ( Supporting Information Figure S14), which have an ultra-narrow blue emission peaking at ∼395 nm with a full width at half maximum (FWHM) of ∼10 nm and an extremely high absolute PLQY of 48.3 ± 5% (Figure 4e), superior to the hot-injection synthesized counterparts (emission peak at ∼400 nm) with a FWHM of ∼16 nm and a PLQY of 11.9 ± 3%. In addition, the TRPL spectrum of the micro 3D CsPbCl₃ NPs in Figure 4f presents a mono-exponential decay of excitons in single channel with an average carrier lifetime of 2 ns less than 4.7 ns for their counterparts, which is uncommon among CsPbCl₃ perovskites, suggesting that the intrinsic lattice defects are largely reduced in the micro 3D CsPbCl₃ NPs. As an added benefit from these reduced intrinsic defects, the poor stability for cubic-phase CsPbCl₃ is significantly improved in these micro 3D CsPbCl₃ NPs relative to their pristine counterparts. As clearly indicated in Figure 4g and Supporting Information Figure S15, the PL intensity of pristine 3D CsPbCl₃ NPs, dispersed in a cyclohexane solution and stored in a cuvette under ambient air conditions after 2 days, was observed to undergo a striking degradation to 23% of its initial intensity, whereas the room temperature blue PL intensity of micro 3D CsPbCl₃ NPs was retained about 98% under otherwise identical experimental conditions, implying the excellent air-stability of the micro 3D CsPbCl₃ NPs.

Conclusions

In summary, we have developed a Ni²⁺-mediated halide anion-exchange strategy in 0D Cs₄PbBr₆:Ni perovskites to achieve highly desirable 3D CsPbX₃ NPs with two coexisting different perovskite individuals of CsPbCl₃ and/or CsPbBr₃. Our experimental results and first-principles calculations revealed that the halide anion-exchange reaction in the 0D Cs₄PbBr₆:Ni NPs serving as meta-stable intermediates preferentially occurred in close proximity to the substitutional Ni_Pb centers rather than in the 0D Cs₄PbBr₆ and 3D CsPbBr₃ regions, thereby delivering intrinsic dual-band excitonic luminescence primarily including two types of combinations, where ∼463 and ∼505 nm emissions are from the micro 3D CsPbBr₃ perovskite and residual 3D CsPbBr₃ component, while ∼395 and ∼495 nm emissions are from the micro 3D CsPbCl₃ perovskite and residual CsPbBr₃ component, respectively. Moreover, we could prepare defect-free 3D CsPbCl₃ NPs with ultra-narrow bright blue emission (FWHM, ∼10 nm; PLQY, 48.3 ± 5%) and excellent stability by modifying the aforementioned strategy. This work would unambiguously pave a new way to produce novel and efficient perovskite nanomaterials for a wide range of technological applications encompassing photovoltaics, light-emitting diodes, and photodetectors.

Supporting Information

Supporting Information is available and includes the supplemental experimental procedures, Figures S1–S15, and Tables S1–S3.

Conflict of Interest

There is no conflict of interest to report.

Funding Information

This work is supported by the Fund of Fujian Science & Technology Innovation Laboratory for Optoelectronic Information (grant nos. 2020ZZ114 and 2022ZZ204), the Key Research Program of Frontier Science CAS (grant no. QYZDY-SSW-SLH025), the National Natural Science Foundation of China (grant nos. 21731006 and 21871256), and the Fund of Advanced Energy Science and Technology Guangdong Laboratory (grant no. DJLTN0200/DJLTN0240).