Fast room temperature cation exchange synthesis of Mn-doped

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Functional Inorganic Materials and Devices

Fast room temperature cation exchange synthesis of Mn-doped CsPbCl3 nanocrystals driven by dynamic halogen exchange Daqin Chen, Su Zhou, Gaoliang Fang, Xiao Chen, and Jiasong Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13316 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Applied Materials & Interfaces

Fast room temperature cation exchange synthesis of Mn-doped CsPbCl3 nanocrystals driven by dynamic halogen exchange Daqin Chen 1,2,*, Su Zhou 2, Gaoliang Fang 2, Xiao Chen 2, Jiasong Zhong 2

1College

of Physics and Energy, Fujian Normal University, Fuzhou, Fujian, 350117, P. R. China

2College

of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, Zhejiang,

310018, P. R. China

Abstract

Currently, it is of a great challenge to achieve cation exchange in CsPbX3 (X=Cl, Br, I)

perovskite nanocrystals on account of rigid Pb2+ octahedral coordination protected by six halogen anions (PbX64-). Herein, we demonstrate that dynamic halogen exchange can effectively open up PbX64octahedrons and enable fast Mn-to-Pb cation exchange at room temperature in a few seconds. Importantly, Cl concentration rather than Mn one is demonstrated to be dominant factor for cation exchange, where different Mn2+/Cl- salts can be adopted as Mn/Cl sources and Cl-to-Cl or Cl-to-Br anion exchange is the necessary prerequisite. Such facile synthesizing method can lead to the feasibility of tuning emissive colors for the Mn-doped CsPb(Cl/Br)3 NCs by controlling both cation and anion exchange and open a new way to replace Pb2+ in CsPbX3 NCs by other nontoxic metal elements.

Keywords: perovskite; CsPbCl3; Mn4+; cation exchange; optical materials

1

ACS Paragon Plus Environment

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Introduction Lead halide perovskites APbX3 (A=CH3NH3, NH2CH=NH3, Cs; X=Cl, Br, I) have drawn extensive attention as active media for their promising applications in optoelectronic fields, such as solar cells, displays and lasers.1-10 Recently, perovskite colloidal nanocrystals (NCs), especially perovskite quantum dots (PQDs), have been successfully prepared and exhibit superior optical properties including high photoluminescence quantum yield (PLQY, up to 90%), narrow full width at half maximum (FWHM, down to 12 nm), wide emissive color gamut (up to 140% NTSC color standard), negligible self-absorption and Fӧrster resonant energy transfer, weak PL blinking effect as well as excellent defect tolerance.11-17 Currently, one of hot issues is to reduce the usage of toxic lead in PQDs. In fact, great endeavors have been devoted towards fabricating Pb-free PQDs such as CsSnX3, Cs3Sb2X9 and A3Bi2X9, but they generally suffered from poor stability and low PLQY.

18-22

As an alternative, partial cation replacement of Pb2+ by other divalent metal ions has been

explored considering that Pb2+ ions play an important role in remaining long-term stability as well as high PLQY of PQDs. Among them, Mn2+ doping has been demonstrated to potentially impart extra magnetic and optical properties and even further improve stability of PQDs.23-29 Benefited from their ionic crystal feature, halogen anion exchange has been well developed as a facile strategy to tune PL of APbX3 PQDs covering over the whole visible spectral region.30-35 Unfortunately, cation exchange, which is widely adopted in traditional semiconductor QDs, is quite difficult to realize the substitution of Pb2+ by other metal ions, probably owing to the rigid octahedron structure of Pb2+ protected by six halogen anions (PbX64-). Most recently, a novel halide exchange driven cation exchange (HEDCE) method was reported to enable the replacement of Pb by Mn via opening up PbX64- accompanied by Cl-to-Br anion exchange.36 However, the harsh requirements of both space and time conditions made only MnCl2 molecules instead of mixture of Mn and Cl ions capable of incorporating into the specific CsPbBr3 PQDs, leading to long exchange reaction time. In addition, no ionic exchange between MnCl2 and CsPbCl3 was found so far.36 Different to the case previously reported, herein we demonstrate for the first time that fast cation exchange of Pb by Mn can be achieved at room temperature (RT) in a few seconds, and Mn2+ ions rather than MnCl2 molecules can diffuse into PQD lattice with assistance of dynamic halogen exchange. Importantly, strong Mn2+ red luminescence is immediately observed when mixing CsPbCl3 solution and MnCl2 solution. As far as we know, this is the first report demonstrating Mn2+ doping into CsPbCl3 at room temperature via a 2


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cation exchange method. To understand the exact reaction mechanism, different soluble Mn2+ salts such as Mn(Ac)2, C15H21MnO6 and Mn(OA)2 and Cl-contained salts such as NH4Cl, ZnCl2, GdCl3, and SnCl4 were selected to provide Mn2+ and Cl- ions for ionic exchange with CsPbCl3 PQDs, and it is found that the cation exchange of Pb by Mn is highly dependent on the Cl- concentration in solution rather than Mn2+. This study may open a new way for further work to achieve the replacement of Pb2+ in CsPbX3 NCs with other nontoxic elements.

Experimental Section Materials. Cs2CO3 (Aladdin, 99.9%), octadecene (ODE, Aladdin,90.0%), oleic acid (OA, Aldrich, 90.0%), oleylamine (OAm, Aladdin, 80-90%), PbCl2 (Macklin, 99.9%), PbBr2 (Macklin, 99.0%), MnCl2 (Aladdin, 99.0%), MnCl2 (Aladdin,99.0%), MnAc2 (Macklin,99.9%), C10H14MnO4 (Macklin,97.0%), NH4Cl (Macklin,99.0%), ZnCl2 (Macklin,98.0%), GdCl3 (Macklin, 99.0%), SnCl4 (Macklin, 99.0%), hexane (Aladdin, 99.0%), toluene (Aladdin,99%), N,N-Dimethylformamide (DMF, Macklin ,99.5%). All chemicals were directly used without further purification.

Preparation of Cs-oleate. Cs2CO3 (0.407 g, 1.25 mmol) was loaded into a 50 mL 3-neck flask along with ODE (18 mL) and OA (1.74 mL), dried for 1 h at 120 °C, and then heated under N2 to 150 °C until all Cs2CO3 was reacted. Notably, since Cs-oleate can precipitate out of ODE at room temperature, it has to be preheated to 100 °C before injection. Synthesis of CsPbCl3 or CsPbBr3 NCs. The hot-injection method was used to synthesize CsPbCl3 (CsPbBr3) NCs. PbCl2 or PbBr2 (0.2 mmol 0.0556 g, 0.0734 g), OA (2 mL), OAm (2 mL) and ODE (5 mL) were added into 25 mL three-neck flask then heated to 120 °C under N2 for about 1 h. After PbCl2 or PbBr2 was completely dissolved, reaction temperature was elevated to 170°C and then as-prepared 1mL Cs-oleate was injected into solution quickly. Finally, the reaction product was quickly cooled by an ice-water bath. Preparation of precursor solution for cation exchange. In the present work, two kinds of precursor solutions were prepared. (1) DMF was selected as solvent: MnX2 (X=Cl- or Br-) or Mn2+ salt (Mn(Ac)2, C10H14MnO4) and ACln (A=NH4+, Zn2+, Gd3+ or Sn4+, and n=1, 2, 3, 4 respectively), were dissolved in 0.5 mL DMF at room temperature for further experiment. (2) ODE was selected as solvent: Mn2+ salt (Mn(Ac)2, C10H14MnO4) and ACln (A= Zn2+ or Gd3+) were added in 25 mL three-neck flask containing ODE (5 mL) and OA (3 mL) and then heated to 150°C under N2 for about 1h until complete dissolution. Then the product was cooled to room temperature for further experiment. Cation exchange reaction. The ionic exchange reaction was performed at room temperature (25°C, 3



HangZhou). CsPbCl3 or CsPbBr3 NCs were dispersed in 5 mL toluene and then the as-prepared precursor solution was introduced. After vigorous stirring for different durations, the products were extracted from the crude solution by centrifuging at 10000 rpm for 5 min to discard the supernatant containing unreacted precursor and by-products. Finally, the precipitations were re-dispersed in hexane forming stable colloidal solutions for further characterization.

Characterizations. XRD patterns were recorded to identify crystal phase structure for the as-prepared Mn-doped CsPbCl3 NCs using a powder diffractometer (MiniFlex600 RIGAKU) with CuKα radiation (λ = 0.154 nm) operating at 40 kV. TEM observation was carried out on a JEOL JEM-2010 transmission electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope at 200 kV accelerating voltage. TEM specimen was prepared by directly drying a drop of a dilute toluene solution of NCs on the surface of a copper grid. The actual compositions of Mn-doped products were determined by X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCA Lab Mark II spectrometer equipped with two ultra-high-vacuum 6 chambers. All the binding energies were referenced to the C1s peak of the surface adventitious carbon at 284.6 eV. Absorption, PLE, PL, and Mn2+ decay curves for the Mn-doped CsPbCl3 NCs were recorded on an Edinburgh Instruments (EI) FS5 spectra fluorometer equipped with continuous (150 W) and pulsed xenon lamps. Time resolved spectra for exciton emission of NCs were detected on a fluorescent lifetime spectrometer (Edinburgh Instruments, LifeSpec-II) based on a time correlated single photon counting technique under the excitation of 375 nm picosecond laser. Photoluminescence quantum yield (PLQY), defined as the ratio of emitted photons to absorbed ones, was determined by a spectra fluorometer (FS5) equipped with the xenon lamp as the excitation source and a 15 cm integrating sphere.

Results and discussion Abs

400 n

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λ = P LE e m 6 1 0 n m = LE λ e m

P

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30 s

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Figure 1 (a) Absorption, PL and PLE spectra of Mn-doped CsPbCl3 NCs. (b) PL spectra of Mn-doped CsPbCl3 NCs prepared 4


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with different cation exchange times. Insets are the corresponding luminescent photographs under the irradiation of UV lamp.

We firstly synthesized CsPbX3 NCs via a hot-injection method and observed Mn2+ emission immediately when mixed CsPbBr3 and MnCl2 precursors at room temperature (RT) (Figure S1). Similar result has been previously reported by Y. Cui et al, however, long reaction time was required to complete cation exchange in their experiment and no Mn2+ luminescence was observed when using CsPbCl3 as the exchange host (Table S1). 36 Herein, when DMF solution of MnCl2 is introduced into CsPbCl3 toluene solution, intense red luminescence is detected in a few seconds, indicating the occurrence of fast RT cation exchange of Pb by Mn and the formation of Mn-doped CsPbCl3 product. Photoluminescence (PL) spectrum shows a typical dual-color emissions centers at about 400 nm and 610 nm, which are assigned to exciton recombination of CsPbCl3 PQDs and ligand-field transition from 4T1 to 6A1 of Mn2+, 37,38 respectively (Figure 1a). Evidently, PL excitation (PLE) spectrum of Mn2+ emission resembles the exciton absorption and excitation of PQDs (Figure 1a), verifying that Mn2+ ions are successfully incorporated into CsPbCl3 host, and the observed Mn2+ luminescence is realized via exciton-to-Mn energy transfer.28 As revealed in Figure S2, the influence of Mn-to-Pb molar feeding ratio (MnCl2: CsPbCl3) was investigated. Mn2+ emission relative to exciton one gradually enhances with elevation of feeding ratio and reaches saturation when the feeding ratio is 1:1. Ionic exchange reaction time dependent PL spectra were further recorded, as shown in Figure 1b. Importantly, intense Mn2+ emission is observed after reaction for 30 s and its intensity keeps almost unchanged when further extending time to 60 min, confirming that fast cation exchange between Pb and Mn indeed occurs. Mn2+ time-resolved curves for the products prepared by different exchange time (30s~60min) show typical decay lifetimes in millisecond scale and exhibit no obvious change with increase of reaction time (Figure S3), confirming the completion of cation exchange in a short time. Especially, the PLQY of CsPbCl3 precursor is only 1.2%, and can be significantly enhanced up to 18.3% after exchange reaction for 30s due to the incorporation of Mn2+ into crystalline lattice. (a)

(b)

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Mn:CsPbCl3 70

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Figure 2 (a) TEM and (b,c) HRTEM images of Mn-doped CsPbCl3 NCs prepared via cation exchange. Insets of (a, c) are the corresponding SAED and FFT patterns. (d) XRD patterns of Mn-doped CsPbCl3 NCs prepared after different cation exchange times. The diffraction bars of cubic CsPbCl3 phase (JCPDS No. 75-0411) is also given in (d). High-resoluiton XPS spectra of (e) Cs 3d, (f) Pb 4f, (g) Cl 2p and (h) Mn 2p for the CsPbCl3 and Mn-doped CsPbCl3 samples.

Microstructure characterizations of Mn-doped CsPbCl3 NCs are shown in Figure 2. Transmission electron microscope (TEM) observation demonstrates that the as-prepared CsPbCl3 precursor shows cubic morphology with sizes of 8~15 nm (Figure S4). After cation exchange of Pb by Mn, their shape and size are not significantly altered (Figure 2a). Selected area electron diffraction (SAED) pattern indicates that the NCs possess pure cubic crystal structure. High resolution TEM (HRTEM) images and the corresponding fast Fourier transformation (FFT) pattern (Figure 2b, 2c) confirm their crystalline nature with high-crystallinity. Evidently, the (100) and (110) crystalline planes of CsPbCl3 with the corresponding inter-planar spacings of 5.60 Å and 3.96 Å are clearly marked in Figure 2c. X-ray diffraction (XRD) (Figure 1d) patterns further confirm that the precursor NCs possess the crystalline structure of cubic CsPbCl3 (JCPDS No. 75-0411), and cation exchange will not induce any impure phase. Elemental signals of Cs, Pb, Cl and Mn are easily detected in the energy dispersive X-ray (EDX) spectrum (Figure S5) and the mapping data show homogeneous distribution of these elements (Figure S6), verifying the incorporation of Mn into CsPbCl3 NCs. Additionally, Cs, Pb and Cl elements can be clearly observed in the X-ray photoelectron spectroscopy (XPS) for the CsPbCl3 product (Figure 2e-2g) and extra Mn signal is indeed discerned after cation exchange (Figure 2h). Notably, slight red-shift for the Cs, Pb and Cl signals towards high binding energy after cation exchange is probably ascribed to the alteration of crystalline field environments after the substitution of Pb by Mn. Generally, Mn2+ has a broadband d-d emission ranging from green to red highly depending on crystal field environment, which can be discerned with the assistance of Tanabe-Sugano diagram. To achieve red emission of Mn2+, it is necessary to incorporate Mn2+ dopants into a host with strong crystal-field. Therefore, it is difficult to observe bright Mn2+ red luminescence if Mn2+ ions are only attached on the surface of PQDs. In fact, similar Mn2+ red emissive behaviors, including emission position, bandwidth, decay lifetime and 6


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exciton-to-Mn energy transfer, have been well demonstrated in Mn2+-doped CsPbCl3 host previously prepared by traditional hot-injection method 24, 26, 29, where Mn2+ dopants are believed to enter into CsPbCl3 crystal lattice. As a consequence, it is expected that the successful incorporation of Mn2+ dopants into CsPbCl3 crystal lattice is realized via cation exchange .

(a)

NH4Cl (b)

ZnCl2 increase of Cl content (c) GdCl3 (d)

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SnCl4 400

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Figure 3 PL spectra of Mn-doped CsPbCl3 products synthesized by cation exchange between CsPbCl3 and Mn(Ac)2 with the assistance of different Cl-contained salts: (a) NH4Cl, (b) ZnCl2, (c) GdCl3 (Cl content gradually increases along the direction of arrow) and (d) SnCl4. Evolutional PL spectra of Mn-doped perovskite NCs synthesized by different ionic exchange procedures to validate the successful Mn-to-Pb cation exchange prerequisites: (e) between CsPbCl3 and MnBr2, (f) between CsPbBr3 and MnCl2, (g) pure CsPbBr3, (h) bewteen CsPbBr3 and MnBr2, (i) among CsPbBr3, Mn(Ac)2 and PbBr2, (j) bewteen CsPbBr3/MnBr2 and GdCl3, (k) via step by step additoin of CsPbBr3, MnBr2 and GdCl3 and (l) via step by step addition of CsPbBr3, Mn(Ac)2 and GdCl3.

To understand the exact mechanism of fast RT cation exchange, Mn(Ac)2 is selected as the Mn source and no Mn emission is detected after reaction for 60 min, indicating that Cl source is necessary to realize the incorporation of Mn into CsPbCl3 host. As demonstrated in Figure 3a-3d, the addition of different Cl sources, such as NH4Cl, ZnCl2, GdCl3 and SnCl4, indeed prompts cation exchange of Pb by Mn and produces both sharp exciton violet emission of CsPbCl3 and broad Mn2+ red luminescence. Similarly, PLE spectra by monitoring 400 nm and 610 nm emissions show the same exciton absorption (Figure S7), verifying the successful incorporation of Mn2+ into CsPbCl3 NCs and the occurrence of exciton-to-Mn energy transfer. Furthermore, the influence of Mn and Cl adding contents on cation exchange is investigated. When Mn content is fixed, increasing Cl concentration in solution during cation exchange will induce gradual 7



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enhancement of Mn2+ luminescence relative to exciton one (Figure 3c), indicating that more Mn2+ ions enter into CsPbCl3 host by replacing Pb2+ with the assistance of Cl- ions. XRD patterns of Mn2+-doped CsPbCl3 samples (Figure S8) show the gradual shift of diffraction peaks towards high-angle direction with increase of Cl-to-Mn feeding ratios, which is believed to attribute to the substitution of Pb2+ with larger ionic radius by Mn2+ with smaller ionic radius. The exact atomic ratios of Cs/Pb/Mn/Cl in Mn2+-doped CsPbCl3 products were evaluated from XPS data and were tabulated in Table S2. Evidently, with elevation of Cl-to-Mn ratios, the Mn/Pb ratio gradually increases. All these results confirm that the average perovskite crystal structure undergoes Pb2+ substitution by Mn2+. To further confirm this, PL decay curves by monitoring Mn2+ 610 nm emission for these corresponding samples were recorded, as shown in Figure S9. Evidently, Mn2+ decay lifetime monotonously decreases with elevation of Cl adding content due to increase of Mn content in CsPbCl3 host and the resulted Mn concentration quenching effect. As a comparison, when Cl content is fixed, no obvious enhancement of Mn emission is found with increase of Mn content (Figure S10). All these results prove that the replacement of Pb2+ by Mn2+ is highly dependent on Cl- concentration instead of Mn2+. Furthermore, we extended the experimental study to other related systems. Firstly, manganese acetylacetonate (C10H14MnO4) was selected as Mn2+ source and mixed with GdCl3 and CsPbCl3 to induce cation exchange. Interestingly, similar dual-color exciton and Mn2+ emissions and exciton-to-Mn energy transfer can be observed (Figure S11). Secondly, other solution system instead of DMF was adopted to produce fast RT cation exchange. Precursors of Mn2+ and Cl- sources are dissolved in octadecene (ODE) and oleic acid (OA), then dropped this mixture into CsPbCl3 solution, the identical result can be obtained (Figure S12). Notably, when the Mn(Ac)2 ODE/OA solution is added into CsPbCl3 toluene solution without Clsource, a weak Mn2+ broadband emission can be observable, indicating that cation exchange can actually occur without the assistance of Cl-to-Cl anion exchange. When manganese acetate salts are dissolved in OA/OED solution, acetate ligands can be replaced by oleate ligands and form manganese-oleate complexes.39 It is believed that this kind of complexes may enhance the activity of Mn2+ and make it possible that Mn2+ ions incorporate into CsPbCl3 lattice to substitute Pb2+ ones. Indeed, the addition of Cl- ions can prompt cation exchange and Mn2+ emission is gradually enhanced with increase of Cl content. Thirdly, MnBr2 is used to provide Mn source and mixed with CsPbCl3. No Mn emission is detected although red-shift of exciton emission is achieved due to Br-to-Cl anion exchange (Figure 3e). In contrast, mixing MnCl2 with CsPbBr3 can lead to both blue-shift of exciton emission owing to Cl-to-Br anion exchange and intense Mn luminescence ascribing to Mn-to-Pb cation exchange (Figure 3f). This is reasonable since the replacement 8


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of Cl (r=181 pm) by Br (r=196 pm)40 with large ionic radius will probably block the incorporation of Mn into PbX64- octahedron. Finally, using CsPbBr3 as ionic exchange host and MnBr2 as Mn and Br sources, exciton emission is not changed and no Mn emission is detected (Figure 3g, 3h). The addition of other Mn and Br sources such Mn(Ac)2 and PbBr2 will not change the situation (Figure 3i). To exclude the possibility that the lack of Mn emission is due to energy mismatch between exciton emission and Mn2+ absorption, the products after cation exchange between CsPbBr3 and MnBr2 for 60 min were obtained by centrifugation and re-dispersed in solution, and then Cl- ions (provided by GdCl3) are further introduced to induce anion exchange. As revealed in Figure 3j, Cl-to-Br anion exchange surely enable blue-shift of exciton emission from green to blue but Mn2+ emission is still undetectable, confirming that Mn-to-Pb cation exchange cannot occur when using CsPbBr3 as the host and MnBr2 as the ion exchange source. As a supplement, we designed a step-by-step adding experiment where MnBr2 was firstly introduced into CsPbBr3 solution and then GdCl3 was subsequently added. Similarly, we only observe blue-shift of exciton emission without emergence of Mn2+ luminescence (Figure 3k). However, when Mn(Ac)2 was adopted as Mn source and GdCl3 was subsequently added to induce Cl-to-Br anion exchange, Mn2+ emission surprisingly occurs and gradually intensifies, accompanied with blue-shift of exciton emission (Figure 3l), confirming that Mn-to-Pb cation exchange in CsPbBr3 host is actually achievable with the usage of appropriate Mn source and the assistance of Cl-to-Br anion exchange. As demonstrated in Figure S13, S14, using the combination of Mn(Ac)2 and ZnCl2 as well as C10H14MnO4 and GdCl3, we can indeed observe Mn broadband red luminescence in CsPbBr3 host. Herein, we proposed a possible mechanism of fast room temperature cation exchange driven by dynamic halogen exchange to understand the observed experimental phenomena, as schematically illustrated in Figure 4a. When only Mn2+ ions are introduced into CsPbCl3 solution, they are hard to enter into CsPbCl3 crystal lattice owing to rigid octahedron structure of Pb2+ protected by six Cl- anions (PbCl64-). The addition of Cl- ions will lead to dynamic Cl-to-Cl anion exchange, being beneficial to temporarily open PbCl64octahedrons and induce cation exchange of Pb by Mn (Figure 4a, left). Massive Cl- ions can break more PbX64- octahedron structure, and the probability of cation exchange will be significantly increased. As above-mentioned, Mn-to-Pb cation exchange quickly occurs when mixing CsPbCl3 with MnCl2 or CsPbCl3 with other Mn salts (Mn(Ac)2, C15H21MnO6, Mn(OA)2) and Cl-contained salts but does not work when mixing CsPbBr3 with MnBr2 or CsPbCl3 with MnBr2. In the most successful cation exchange cases, the bond energy of Mn-Cl (338 kJ/mol) is close to that of Pb-Cl (301 kJ/mol),29,41 enabling the substitution of Pb by isovalent 9



Mn after opening PbCl64- octahedrons by dynamic Cl-to-Cl anion exchange. On the contrary, the energy difference (~26%) is large between Mn-Br bond (314 kJ/mol) and Pb-Br one (249 kJ/mol),29,41 making Mn tend to combine with Br and stay in solution rather than enter into CsPbBr3 crystal lattice. For the mixture of CsPbCl3 and MnBr2, the occurrence of anion exchange of Cl by Br with large ionic radius will prevent the incorporation of Mn into CsPbCl3 lattice although the energy different between Mn-Br bond (314 kJ/mol) and Pb-Cl one (301 kJ/mol) 29,41 is small. Therefore, it is necessary to use other Mn compounds such as Mn(Ac)2 and C10H14MnO4 in replace of MnBr2 to provide Mn source and add Cl source to trigger Cl-to-Br anion exchange to temporarily open PbBr64- octahedrons and induce cation exchange of Pb by Mn in CsPbBr3 host (Figure 4a, right).

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Figure 4 (a) Sketches of fast room temperature Mn-to-Pb cation exchange in CsPbCl3 (left) and CsPbBr3 (right ) hosts assisted by dynamic Cl-to-Cl (left) and Cl-to-Br (right) halide exchanges. Variation of PL spectra (b) for CsPbCl3 NCs after Mn-to-Pb caiton exchange followd by graduall increase of Br-to-Cl anion exchange and (c) for CsPbBr3 NCs after simultaneous Mn-to-Pb cation exchange and Cl-to-Br anion exchange with monotonous increase of MnCl2 adding content. (d, e) Luminescent photographs for the corresponding products dispersed in solution under the irradiation of UV lamp.

Finally, we demonstrate the feasibility of tuning emissive colors by controlling both cation and anion exchange. Using CsPbCl3 NCs as host, cation exchange between CsPbCl3 and MnCl2 produces dual-color emissions and subsequently anion exchange by introducing PbBr2 solution can result in red-shift of exciton 10


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emission from violet to blue due to gradual substitution of Cl by Br and monotonous decrease of Mn orange emission owing to mismatch between perovskite bandgap and Mn emitting state (Figure 4b). Using CsPbBr3 NCs as host, increasing of MnCl2 adding content can simultaneously induce blue-shift of exciton emission from green to blue due to gradual replacement of Br by Cl and monotonous increase of Mn orange emission owing to incorporation of Mn into perovskite host and enhanced energy transfer from exciton to Mn (Figure 4c). Such ionic exchange strategy makes it easy to tune emissive colors for the as-prepared Mn-doped all-inorganic perovskite NCs (Figure 4d, Figure 4e).

Conclusions In summary, a novel dynamic halogen exchange driven cation exchange strategy was developed to incorporate Mn into CsPbCl3 and CsPbBr3 perovskite hosts. Different to previously reported case, where only MnCl2 molecules were capable of doping Mn into the specific CsPbBr3 NCs, the present method alleviated the severe reaction condition and the mixtures of different Mn and Cl salts were found to be suitable for cation exchange. Benefited from the opening of rigid PbX64- octahedrons by dynamic Cl-to-Cl or Cl-to-Br anion exchange, fast exchange of Pb by Mn was completed in a few seconds and the doping of Mn into CsPbCl3 NCs via cation exchange was achieved for the first time. As a consequence, the emissive colors of perovskite products can be easily tuned with the help of simultaneous Mn-to-Pb cation and halogen anion exchanges. It is believed that the present work can provide an effective route to endow perovskite NCs with novel optoelectronic properties via fast cation exchange and even synthesize other low-Pb perovskite materials.

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ASSOCIATED CONTENT Supporting Information Table S1, S2 and Figure S1-S14. Extra PL/PLE spectra, time-resolved luminescent spectra, TEM and EDX data. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected] (D. Q. Chen) Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the National Key Research and Development Program of China (2018YFB0406704), Zhejiang Provincial Natural Science Foundation of China (LR15E020001) and National Natural Science Foundation of China (51572065).

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Fast room temperature cation exchange synthesis of Mn-doped

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