Presence of Metal Chloride for Minimizing the Halide Deficiency and

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Presence of Metal Chloride for Minimizing the Halide Deficiency and Maximizing the Doping Efficiency in Mn(II)-Doped CsPbCl3 Nanocrystals Samrat Das Adhikari, Rakesh Kumar Behera, Suman Bera, and Narayan Pradhan* School of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032 India

J. Phys. Chem. Lett. Downloaded from pubs.acs.org by UNIV OF ADELAIDE on 03/19/19. For personal use only.

S Supporting Information *

ABSTRACT: Pretreatment using metal chlorides during the formation of halide deficient free perovskite nanocrystals is reported. Among several metal chlorides, Cu(II)Cl2 was observed to be ideal for the synthesis of highly emitting CsPbCl3 nanocrystals at high reaction temperature. Because high temperature remained more favorable for the dopant insertion, doping of Mn(II) was carried out under this haliderich system, and nearly 68% photoluminescence quantum yield was recorded. Analysis could not provide strong evidence of insertion of Cu(II) inside the nanocrystals; rather, it was established that Cu(II)Cl2 in the system helped to stabilize the reaction even at and above 260 °C and provided an adequate chloride source for obtaining the highly emitting host as well as doped nanocrystals. Details of the physical process involved for this metal ion-induced uplifting of the reaction temperature and the consequent impacts on the nanocrystal formation are studied in detail and reported in this Letter.

A

temperature without forming any cross products or salting out of any salt. Keeping these issues in mind, herein, high-temperature (>250 °C) reactions were designed considering series of metal chlorides, typically used for room-temperature passivation for both undoped CsPbCl3 and Mn-doped CsPbCl3 nanocrystals. The optimized one focused on Cu(II)Cl2, which provided an ideal environment for the reaction and led to minimized halide-deficient nanocrystals with 63% absolute PLQY of the excitonic emission. After harvesting, these nanocrystals also remained almost inactive to further metal halide sensing. Because the reaction was carried out at high reaction temperature with chloride rich environment, doping of Mn(II) was successful with minimized amount of Mn(II)Cl2 and led to a record ∼68% PLQY for the Mn d−d emission. This approach also remained ideal for Mn(II) insertion as this prevented the Mn−Mn interaction-induced PL quenching of the doped nanocrystals. Details of the assistance of the of secondary metal halide, the uplifting of the reaction temperature, role of excess halides at high temperature during formation of nanocrystals, the impact of doping and preventing excess Mn insertion in the nanocrystals are studied in detail and reported in this Letter. Series of metal chlorides were selected for carrying out the high-temperature perovskite synthesis reaction. The major challenge in designing such reactions remained with the solubility of these metal salts and sustainability of the reaction

mong the leading energy materials, light-emitting cesium lead halide perovskite nanocrystals remain in the forefront of current research.1−8 These are typically prepared in a high-temperature colloidal synthetic approach and dispersed in organic solvents.9−11 The exciting feature of these materials is their unprecedented high photoluminescence quantum yield (PLQY), and these were extensively studied and optimized during their synthesis as well as postsynthesis modifications.9,12−22 Moreover, while most of the studies were related to intensifying and stabilizing the host exciton emission, doping Mn2+ in high-energy emitting CsPbCl3 has recently emerged as a new class of materials having its own emission in a different window.23−26 It is already established that high temperature favors efficient doping,27,28 and hence, establishing the reaction where formation of host and simultaneous doping, preferably at high reaction temperature, could result in the desired nanocrystals with minimized defects. Recent literature reports reveal that postsynthesis treatment of different metal halide salts enhance the emission intensity of perovskite nanocrystals.14,15,18,29−31 For CsPbCl3, this has shown multifold enhancements and even achieved near unity PLQY.32 These exciting results indeed solved the major issue of obtaining intense blue emission from the high band gap perovskite nanocrystals; however, for doping and observing the efficient host-to-dopant energy transfer, the ideal case would be high-temperature dopant insertion. This could be achieved by introducing secondary metal halides at high reaction temperature, which could assist the supply of adequate amount of halide ions during the formation of the desired nanocrystals. In addition, this system could also help efficient doping of Mn(II) ions. However, such reaction protocols are not straightforward and need special architect designs for sustainability at high © XXXX American Chemical Society

Received: March 1, 2019 Accepted: March 18, 2019

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Figure 1. Schematic presentation of reactions at high temperature in the presence of different metal chlorides along with Pb(II) and Mn(II) precursor. A digital image of the flask showing turbidity is from the control reaction, and more images are provided in Supporting Information. The digital image of the flask showing an orange, clear solution is from the Cu(II)Cl2 reaction system. In the presence of Mn(II), this Cu(II)Cl2containing flask resulted in Mn-doped CsPbCl3, which is illuminated under irradiation at room temperature. Excitation wavelength is 365 nm.

Figure 2. (a) Absorbance and (b) corresponding PL spectrum of CsPbCl3 nanocrystals obtained from the reaction carried out at 260 °C in the presence of CuCl2. (c) Plot of reaction temperature versus PLQY of the obtained nanocrystals. (d) PL spectra of Mn-doped CsPbCl3 obtained from reactions with different Cs precursor injection temperatures and normalized at the exciton emission position. (e) Representative absorbance and PLE spectra of Mn-doped CsPbCl3 nanocrystals. (f) Plot of PLQY versus reaction temperature of the obtained doped nanocrystals.

of the flask of the reaction having MnCl2 in the reaction medium. In a typical synthesis of the host CsPbCl3 nanocrystals, Cu(II)Cl2 and PbCl2 along with trioctylphosphene, octadecene, oleic acid, and oleylamine were taken in the reaction flask and Cs precursor was injected at 260 °C. For obtaining the reaction system free from excess Cs or Pb, equimolar amounts of both precursors were introduced; however, CuCl2 was taken in excess for supplying sufficient chloride ions during nanocrystal formation. Similarly, for doping Mn, MnCl2 was introduced along with PbCl2 at the beginning. However, minimum MnCl2 was used in comparison to other literature reports23,28,33,34 (discussed below). Figure 2a presents the UV−visible spectrum and Figure 2b shows the corresponding PL spectrum of CsPbCl3 nanocrystals obtained at 260 °C reaction. The absolute PLQY of the assynthesized nanocrystals measured in film remained 63%.

without having any cross products above 250 °C. In the modified reaction approach for synthesizing CsPbCl3 reported by Kovalenko and co-workers, it was observed that with ZnCl2, SbCl3, CdCl2, SnCl4, and CuCl, the reaction medium turned turbid beyond 200 °C (Figures 1 and S1); but with Cu(II)Cl2, the reaction medium was more ideal even at 280 °C (Figure 1). The turbidity of the reaction might be due to salting out of salts, the inability to form a solubilized compound, or formation of cross products. From these observations, CuCl2 was determined to be more ideal, and hence, this was adopted as a model system for synthesizing the possible halide deficiency free nanocrystals at higher reaction temperature. The reaction temperature was chosen to be 260 °C for obtaining crystalline nanocrystals and triggering efficient Mn(II) doping with minimum Mn(II) salt addition. The scheme in Figure 1 also presents the illuminated digital image 1531

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Figure 3. (a) HAADF-STEM and (b−d) TEM images at different resolutions of Mn-doped CsPbCl3 obtained from the reaction carried out at 260 °C. The Cs-to-Pb ratio was 1:1, and the Pb-to-Mn ratio was 1:0.2.

TEM grid, similar structures were observed. Additional such images are presented in Figure S3. Powder X-ray diffraction (XRD) patterns for both undoped and doped nanocrystals are shown in Figure 4, and the patterns confirmed both retained

However, decrease of reaction temperature reduced the PLQY. Figure 2c shows the plot of PLQY versus the reaction temperature, and the data corroborated that temperature indeed played an important role in obtaining high-quality nanocrystals. This as-synthesized nanocrystal PLQY was observed to be comparable to several leading reports on the synthesis of CsPbCl3.11,35,36 For Mn doping, Gamelin and co-workers had reported that higher reaction temperature resulted in higher possibility of dopant ion insertion; but in those cases, significant amount of MnCl2 was used (150% with respect to Pb), and the reaction temperature was 230 °C.28 On the other hand, herein, maximum 20% MnCl2 (with respect to Pb) was used and efficient doping (∼1% retained in the crystal) was observed at 260 °C reaction. Figure 2d presents the PL spectra of the Mndoped CsPbCl3 nanocrystals obtained at different reaction temperatures with Pb to Mn intake ratio 1:0.2 in the reaction flask. The best PLQY at 260 °C was observed 68% which retained ∼2% Mn (with respect to Pb) in the nanocrystal (EDS shown in Figure S2). Figure 2e presents the typical absorption and the PL excitation spectra for the nanocrystals obtained in 260 °C, suggesting the dopant emission originated from host excitation. Figure 2f shows the PLQY of the dopant emission for the doped nanocrystals obtained from reactions carried out at different reaction temperatures. These results confirmed that like host exciton emission, the dopant emission intensity also successively increased with rise in reaction temperature. Hence, like chalcogenides, reaction temperature remained the important parameter for obtaining highly emissive perovskite nanocrystals. It is worth noting that the reaction also sustained and the medium did not turn turbid even at 280 °C, but the Mn d−d emission PLQY did not change further. This might be due to the optimum value for the d−d transition emission. However, beyond 280 °C, the solution turned turbid; hence, 260 °C was the best optimized temperature. Because CuCl2-assisted reaction mixtures remained transparent without having turbidity, this facilitated the instant nucleations upon injection of Cs precursor. This indeed helped in obtaining highly monodisperse nanocrystals which were reflected from the formation of superstructures on the electron microscopic grid. Figure 3a shows a High-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) image, and panels b−d of Figure 3 present transmission electron microscopy (TEM) images of cubeshaped Mn-doped CsPbCl3 nanocrystals at different resolutions. In all these images, the cube-shaped nanocrystals were seen arranged like superstructures, and this could be possible because of their narrow size distributions. Everywhere on the

Figure 4. Powder X-ray diffraction patterns of undoped CsPbCl3 with variation of Cu(II)Cl2 composition with Pb and doped CsPbCl3 nanocrystals with variation of Mn composition with respect to Pb taken in the reaction system. Ratios of Pb to Mn provided in the figure are per the loading amount taken in the reaction flask.

the cubic phase. For the control reactions without Mn, while samples were measured with variation of Pb to Cu taken in the reaction system, no change in the positions of the peaks was observed (top two patterns in Figures 4 and S4). This suggested that Cu(II) might not have incorporated in the crystal lattice of the nanostructure. On the other hand, while the Mn percentage was varied, marginal shifting was observed only with excess Mn loading. Otherwise, for lesser amounts of Mn, no change in any of the peaks of the XRD patterns was noticed. Upon Mn doping, the concentration of Mn in the host crystal strongly influences the photoluminescence properties (position, lifetime, etc.).13,27,28,33,34,37−39 It is already established that more Mn in the lattice quenches the dopant PLQY 1532

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Figure 5. (a) PL spectra of Mn-doped CsPbCl3 with emission center at 585 nm and (b) PL spectra of the doped nanocrystals with emission center at 610 nm. These were prepared by varying Mn loading amount in the reaction flask; however, reactions in both cases were carried out at 260 °C. (c and d) PL excited-state decay lifetimes for the two sets of nanocrystals having emission centered at 585 and 610 nm, respectively.

Figure 6. (a) PL spectra obtained from as-synthesized CsPbCl3 nanocrystals dispersed in hexane and after NiCl2 treatment at room temperature. (b) PL spectra of doped CsPbCl3 nanocrystals before and after treatment of NiCl2. These nanocrystals were prepared from CuCl2-assisted reaction at 260 °C. (c) PL spectra with and without NiCl2 treatment with doped CsPbCl3 nanocrystals obtained from literature method.

and also red shifts the optical emission.28 The same was also observed here in the CuCl2-assisted reactions. The highest PLQY was observed for the emission at 585 nm and with 20% Mn precursor intake (with respect to Pb), and with more Mn precursor intake, the PLQY was drastically reduced. Figure 5a presents a typical yellow-orange emitting PL spectrum, and Figure 5b shows the same with red-shifted emission at 610 nm with decrease of PLQY from 68% to only 16%. The excitedstate decay lifetimes for these two emissions are presented in Figure 5c and 5d for 585 and 610 nm respectively, and this showed with red shifting, the decay lifetime also reduced. Further, while the excitonic emissions for both cases were compared, the red tuned dopant emission showed significant quenching and this suggested that there might be additional tarp or defect states induced with more Mn incorporation. This not only quenched the dopant emission but also reduced

the intensity of the host excitonic emission. It is established by Son and co-workers that upon optimum doping, both emissions observed in room-temperature steady-state measurement were originated from same nanocrystals, and hence, quenching of host exciton should not be exclusively related with increase of dopant emission.33 Also, for obtaining intense emission or facilitating efficient host-to-dopant energy transfer, excess Mn content is not required; rather, a high-quality host is required with minimum dopant. Moreover, these CuCl2assisted reactions which provide adequate chloride ions minimized the MnCl2 insertion which indeed prevented the possibility of more Mn incorporation and so also the PLQY. This also possesses advantage over our previously reported oleylammonium chloride-assisted Mn(II) doping in CsPbCl3 where the PLQY was comparatively lower (∼30%) and the particle dimensions were simultaneously tuned with Mn 1533

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cooled from 260 °C, and the XRD pattern obtained from the precipitated mixture is presented in Figure S11. The pattern still overlapped, similar to the layered perovskites; however, it was much less intense. It might be possible that small amounts of layered perovskites were formed even with rapid cooling or were retained in small dimension at 260 °C. More likely, it could be expected that Cu ions remained as a soluble complex with TOP in the reaction mixture as without TOP; at 260 °C, no such nanocrystal emissions were formed (Figure S12). Moreover, it was not possible for us to analyze the complex or any intermediate species formed at 260 °C because in situ analysis at 260 °C was practically difficult. Hence, we can only assume with these supporting experiments that CuCl2, which remained the prime source to supply excess Cl during hightemperature formation of CsPbCl3 or Mn-doped CsPbCl3, remained in dispersive form. Ideally, it can be stated here that these Cu(II) ions acted as a sacrificial reagent for promoting the reaction for obtaining high-quality nanocrystals. Its redox reaction to form Cu(I) in the presence of TOP/amine might help release excess halide ions to the reaction system. Importantly, while this contribution was under preparation, Rogach and co-workers46 reported thermally stable Cu-doped mixed halide perovskites following a different synthetic approach. The insertion of Cu(II) in the lattice was signified even from the XRD peaks which were shifted right with increase of Cu content. However, we did not observe the same in our case. Another report that appeared recently suggested Cu adsorption on the surface of the nanocrystals enhanced the PLQY of the nanocrystals.47 However, our synthetic protocol is independent from these reports, and the experimental findings also do not overlap or provide strong evidence supporting Cu(II) ions retained in the host crystal structure. In conclusion, chloride-rich high-temperature reactions for the formation of minimized halide deficient doped and undoped CsPbCl3 nanocrystals are reported. Cu(II)Cl2 was observed as an ideal metal salt under the adopted synthetic protocol for providing an appropriate platform to carry out the reaction. This could result in CsPbCl3 nanocrystals with efficient blue emission, and upon doping Mn(II), ∼68% absolute PLQY for the Mn d−d emission was recorded in the perovskite system. Moreover, as metal chlorides supplied excess chloride ions, minimum Mn(II) precursor was required for obtaining the desired doped nanocrystals. These nanocrystals did not show any further PL intensity enhancement after postsynthesis metal chloride addition. No strong evidence of involvement of copper inside the crystal lattice was observed, but the presence of CuCl2 indeed helped in uplifting the reaction temperature, which led to efficient doping with efficient emission. Hence, these results regarding supplying excess halides at high reaction temperature during nucleation, growth, and doping and the impacts on the PL properties contribute new insights in understanding the crystal growth of perovskite nanocrystals in solution.

corporation.27 In contrast, here the halide deficiency was minimized, and hence, the PLQY was significantly enhanced in the as-synthesized nanocrystals. For these nanocrystals treated after harvesting with other metal chloride salts in their hexane solution, no or negligible change in the photoluminescence intensity was observed. Figure 6a,b presents the PL spectra of NiCl2-treated hexane dispersed undoped and doped CsPbCl3 nanocrystals prepared in the presence of CuCl2. Both results showed that the PL of both cases was inactive to NiCl2 sensing. Similar results were also obtained with CdCl2 and SnCl4 treatment. In contrast, when Mn-doped CsPbCl3 was prepared following standard protocol23 without having CuCl2, significant enhancement of the PL intensity was observed upon NiCl2 treatment (Figure 6c). These salt additions were performed at room temperature and in hexane solution. Absorption spectra corresponding to all these PL spectra are provided in the Supporting Information (Figures S5−S7). These results confirmed that CuCl2-assisted high-temperature reaction minimized the chloride deficiency in the nanocrystals. As Cu doping has been extensively studied in chalcogenides,40,41 at first glance it was assumed that Cu(II) was replacing Pb(II) like Mn(II); however, no strong evidence was obtained for the same. X-ray photoelectron spectroscopy spectra (XPS, Figure S8) also could not support Cu(II). Besides this, Cu(II) could be reduced to Cu(I) in the presence of TOP and amine in solution.42 Cu(I) atoms favor the 4-fold coordination over 6-fold, which is essentially required for doping in the crystal;43 hence, it is less likely this would accommodate the Pb(II) sites. EDS spectra collected from different places in one sample as well as from several samples showed Cu percentage varied from 10 to more than 30% (Table S1) with respect to Pb, and this inconsistency casted doubt having Cu incorporated in the lattice. Because excess of Cu(II)Cl2 was used, these might be retained along with the nanocrystals, but in different form of the material. Because the nanocrystals were observed in superstructure forms, washing out all free materials or reactants or products by centrifugations is less likely. This might be the reason why the element Cu was always observed in EDS. However, while the ratio of Cs, Pb, and Cl was verified, the probability of retaining 1:1:3 molar compositions were mostly observed in several EDS data. Moreover, Pb(II) composition was not observed to be reduced in any case, and hence, replacement of Pb(II) with Cu(II) could not be confirmed from these data. On the other hand, if Cu(II) was retained inside the crystal, its percentage could be much lower, and these might not have impact on the PL properties of Mn-doped nanocrystals. For understanding the fate of CuCl2, the annealed reaction mixture at 260 °C before injection of Cs precursor was naturally cooled to room temperature, and the products were analyzed. The reaction mixture showed an absorption peak at 334 nm, which might be obtained from L2PbCl4 or L2Pb(Cu)Cl4 layered perovskites (Figure S9).44,45 The powder XRD patterns of the sample shown in Figure S10 confirmed the same interspacing distance of ∼4 nm, almost matching with oleylammonium ion spacer ligands in L2PbCl4 layered perovskites. However, when heated up again to 260 °C, the precipitated layered perovskites did not again dissolve and the solution retained the turbidity. Hence, layered perovskites might not be the product in the reaction mixture at 260 °C, which were converted to 3D perovskites upon Cs injection. For further understanding, the reaction mixture was quickly

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00599. Experimental procedures, EDS, additional TEM images, XPS spectra, and additional optical data (PDF) 1534

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Samrat Das Adhikari: 0000-0002-5670-5179 Narayan Pradhan: 0000-0003-4646-8488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS DST and TRC (AI/1/62/IACS/2015) at IACS are acknowledged for funding. S.D.A., R.K.B. and S.B. acknowledge CSIR, India for fellowships.

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REFERENCES

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DOI: 10.1021/acs.jpclett.9b00599 J. Phys. Chem. Lett. 2019, 10, 1530−1536