Insights of Doping and the Photoluminescence Properties of Mn

Apr 16, 2019 - Keeping all these developments in mind, this perspective focused on the insights of doping and the photoluminescence properties of Mn2+...
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Perspective Cite This: J. Phys. Chem. Lett. 2019, 10, 2250−2257

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Insights of Doping and the Photoluminescence Properties of MnDoped Perovskite Nanocrystals Samrat Das Adhikari, Amit K. Guria,* and Narayan Pradhan*

J. Phys. Chem. Lett. Downloaded from pubs.acs.org by UNIV PARIS-SUD on 04/28/19. For personal use only.

School of Materials Science and Technical Research Center, Indian Association for the Cultivation of Science, Kolkata, India 700032 ABSTRACT: Doping Mn2+ in semiconductor nanocrystals is widely known for its longlifetime Mn d−d orange emission. While this had been extensively studied for chalcogenide nanostructures, recently this was also extended to perovskite nanocrystals. Being that CsPbCl3 has a wide bandgap, the exciton energy transfer was found to be more efficient, but the dopant-induced photoluminescence was also obtained for layered perovskites and quantum-confined CsPbBr3 nanocrystals. In recent years significant advances have been achieved in understanding the physical insights of doping following various approaches and optimizing the conditions for obtaining intense dopant emission. In addition, several new properties associated with these doped nanocrystals were also reported, and by modulating the compositions, the host bandgap and the dopant emission positions were also tuned. Keeping all of these developments in mind, this Perspective focuses on the insights of doping and the photoluminescence properties of Mn2+-doped perovskite nanocrystals. In addition, it also proposes possible future prospects of both synthesis and optical properties of these nanomaterials. cally in Figure 1a. The first and common approach of using MnCl2 with certain concentrations along with Pb and Cs precursors was the most optimized one, which led to the doped nanocrystals following in situ or simultaneous doping along with the formation of host nanocrystals (Figure 1a, scheme I).12,18 The key feature in this approach was the halide ion-rich reaction system, which mostly came from MnCl2. Moreover, excess MnCl2 in bulk solution was required to compete with Pb(II) even when just a few percent was inserted into the crystal lattice.26 To reduce the Mn(II) precursor concentration, different chloride ion sources were used, which also helped with efficient doping in the host CsPbCl3 nanocrystals (Figure 1a, scheme II). Further, using alkylammonium ions and varying their concentration, the doping efficiency was enhanced, and this also helped in tuning the dimensions of the host nanocrystals. Figure 1b presents a digital image of the reaction flask under illumination in which Mn-doped CsPbCl3 nanocrystals were obtained.14 While this reaction was carried out at higher temperature, Mn could also be inserted inside of the CsPbCl3 nanocrystals using HCl at room temperature.25 Further, interesting observations were obtained upon Mn doping, achieved via postsynthetic cation exchange with the incorporation of other metal/nonmetal chlorides (Figure 1a, scheme III).27 These chloride salts helped in inserting Mn2+ inside of the host perovskites. In addition to all of these methods, a mixed halide approach was also extended for Mn2+doping.17,28,29 Following anion exchange with CsPbBr3 in the presence of MnCl2, Mn-doped mixed halides were obtained. This process slowly replaced the Br−

D

oping Mn2+ in semiconducting materials has been extensively studied for decades.1−5 This has a very unique identity, having long-lifetime orange emission originating from its d states.3,6,7 Excitation of the host and emission from the dopant remain the most striking feature of these materials, and the emission energy is less concerned with the bandgap of hosts.8,9 From the most widely studied ZnS host to other chalcogenide nanocrystals, insights on doping of Mn2+ in the crystal lattice and the obtained new photoluminescence (PL) properties are largely understood.10,11 In recent developments, the doping has also been extended to perovskite nanocrystals. Among these, the high-bandgap perovskite, CsPbCl3, became an ideal host for efficiently transferring its exciton energy and has been widely studied for the last couple of years.12−14 The high-temperature synthesis method reported by Kovalenko and co-workers15 was the backbone, and the same was extended to Mn-doping originally by Klimov’s research group.12 This breakthrough of doping in perovskites was also summarized earlier,7 but with time, there have significant investigations of both the doping and the dopantinduced optical properties. Several new methods have been developed,14,16−18 the impact of different additives on doping has been established, temperature-dependent new observations of the photoluminescence (PL) have been obtained, and the doping has been extended to layered perovskites as well as to CsPbBr3 nanocrystals.14,18−25 Keeping all of these developments in mind, this Perspective focuses on the challenges tackled and remaining in the synthesis, the obtained dopantinduced PL properties and its tunability, and extended doping in other hosts, including layered perovskites. Insights of Mn2+ doping in CsPbCl3 nanocrystals were first analyzed, and several chemical processes involved in insertion of the dopant inside of the lattice are summarized schemati© XXXX American Chemical Society

Received: January 21, 2019 Accepted: April 16, 2019 Published: April 16, 2019 2250

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Figure 1. (a) Schematic presentation of various developed synthesis strategies. (I) Represents typical high-temperature synthesis using chloride salts of Pb and Mn. It also presents the increase of reaction temperature as well as Mn concentration, which enhanced the doping efficiency. (II) Depicts the scheme of synthesis of Mn-doped perovskites using ammonium salts at high temperature and HCl at room temperature. (III) Shows the metal chloride-induced Mn doping via postsynthesis of hosts. (IV) Presents the Mn-doped mixed halide perovskites that started with CsPbBr3 to mixed halides. (b,c) Digital images of reaction flasks showing yellow−orange and orange−red emissions under illumination (365 nm). The image in (b) was adopted from the ref 14, with copyrights obtained from Wiley-VCH.

ions in the original host nanocrystals and enhanced the Cl concentration in the same host nanocrystals (Figure 1a, scheme IV).16,30 Summarizing several such reports on Mndoped perovskites, details of the reactions, obtained PL quantum yields (PLQYs), etc. are provided in Table 1. While most of the efficient Mn-doped CsPbCl3 nanocrystals emit in yellow−orange window (∼580−590 nm), by tuning the compositions, the emission was tuned from the yellow− orange to orange−red region (∼625 nm). This was achieved mostly by increasing the Mn content in the host lattice.19,31 A digital image of the reaction flask having low-energy-emitting Mn-doped CsPbCl3 nanocrystals under illumination is shown in Figure 1c, and this red shifting is discussed more later. Following the above schemes, the PL obtained from respective doped nanocrystals is presented in Figure 2. Varying the oleylammonium chloride concentrations, the changes of host exciton and Mn dopant emission spectra are shown in Figure 2a. The normalized spectra at the host emission confirmed that the third material (oleylammonium chloride) indeed enhanced the intensity of dopant emission.14 This salt acted as a dual source; first this provided excess chloride ions, and second, this also supplied adequate ammonium ions, which helped for better surface passivation.24,32 In addition, this salt also restricted the phase change of the nanocrystals and retained the cubic phase even with hours of prolonged annealing. Gamelin and co-workers18 had shown that higher reaction temperature also facilitate efficient doping. With the increase of reaction temperature, they reported enhancement of the intensity of Mn d−d emission. Figure 2b presents the host emission normalized PL spectra of Mn-doped CsPbCl3 with variations of reaction temperature, and the 230 °C reaction indeed showed the most intense dopant emission. Hence, for efficient doping, reaction temperature also matters. Beyond these reports, several other strategies were also developed, and one such process was cation exchange. Gao et

Chloride ions were the key for promoting the insertion of Mn2+ in perovskite nanocrystals for room-temperature reactions. al.33 reported the synthesis of luminescent CsPb1−xMnxCl3 nanocrystals through partial and reversible cation exchange reaction between Pb and Mn in both CsMnCl3 and CsPbCl3 nanocrystals. In an interesting approach, Chen et al. showed room-temperature anion exchange-induced Mn insertions in CsPbCl3 nanocrystals.27 The authors performed this by using different metal chloride salts, and it was established that chloride ions were the key for promoting the insertion of Mn2+ in perovskite nanocrystals for room-temperature reactions. This was confirmed by using Mn-acetate (instead of MnCl2) as the Mn source, but this could not help in obtaining the Mn d− d emission even after stirring for 1 h at room temperature along with the host nanocrystals.27 As shown in Figure 2c, using NH4Cl, ZnCl2, GdCl3, and SnCl4 indeed prompted the cation exchange of Pb with Mn and generated the orange Mn d−d emission.27 The higher the valence of the metal in the chloride salt, the better the doping efficiency, and this was related to the chloride contents in the reaction system to proportionate Mn insertion via ion exchange. Hence, excess chloride ions mostly facilitated Mn2+ doping in CsPbCl3 nanocrystals. While all of the above hosts were CsPbCl3, doping of Mn2+ was also extended to mixed halide perovskites. This was performed starting with either CsPbBr3 by chloride addition16 or Mn-doped CsPbCl3 by bromide insertion.17,28 The reaction could be started with the doped nanocrystals, or the dopant Mn2+ ions might be inserted during the halide ion exchange process. An increase of chloride contents in the mixed halide perovskites helped in widening the bandgap, and therefore, this 2251

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Table 1. Synthesis Information of Selected Mn-Doped Perovskite Nanocrystals Sl no.

materials

Mn precursor

03 04 05 06 07 08 09 10 11 12 13 14 15 16

Mn:CsPbCl3 Mn:CsPbCl3 Mn:CsPb(Cl/Br)3 Mn:CsPbCl3 MAPb1−xMnxBr3−(2x+1)Cl2x+1 Mn:CsPbCl3 Mn:CsPbCl3 CsPbxMn1−xClyBr3−y Mn:(C4H9NH3)2PbBr4 CsPb0.88Mn0.12Cl3 CsPb0.835Mn0.165Cl3 Mn:CsPbCl3 Mn:CsPbCl3 Mn:CsPbCl3 Mn:CsPbCl3 Mn:CsPbCl3 platelets Mn:CsPbCl3

MnCl2 MnCl2 MnCl2 Mn acetate MnCl2 MnCl2 MnCl2 MnCl2 MnBr2 MnCl2 MnCl2 MnCl2 MnCl2 MnCl2 MnCl2 MnCl2 MnCl2

17 18 19 20 21 22 23 24 25 26

Mn:CsPbCl3 Mn:Cs2AgInCl6 2D Mn:CsPbCl3 CsPb0.77Mn0.23Cl3 CH3NH3PbxMn1−xCl3 Mn:CsPbCl3 Mn:CsPbCl3 Mn:CsPb(ClyBr1−y)3 Mn:CsPbBr3 platelets Mn:CsPbCl3

MnCl2 Mn acetate MnCl2 MnCl2 MnCl2 MnCl2 MnCl2 MnCl2 MnBr2 MnCl2

27 28 29 30

Mn:CsPbCl3 Mn:(butyl)2PbBr4 Mn:CsPbCl3 Mn:(butyl)2PbBr4

MnCl2 MnBr2 MnCl2 MnBr2

01 02

Pb:Mn precursor ratio 1:0.4 1:1.5 1:0.005 1:9 1:1 1:2 1:5 1:0.05 1:4 1:6 1:1.5 1:1 1:0.05 1:0.4 1:0.05 1:10 1:2.5 1:2.5 1:10 1:5 1:10 1:1.5 − 2:1 1:1 1:0.75 1:2.5 1:2 1:2 1:5 1:0.25 1:0.5 1:1 1:1 1:1 1:0.2 1:1.08

PL QY (%)

synthesis highlights 180 °C synthesis 200 °C synthesis 200 °C synthesis HCl added, shell growth room-temperature synthesis 190 °C synthesis 190 °C synthesis room-temperature synthesis solid-state grinding 170 °C synthesis 170 °C synthesis postsynthesis KCl treatment 170 °C synthesis oleylammonium chloride added, 180 °C synthesis 185 °C synthesis room temperature 170 °C synthesis 150 °C synthesis 190 °C synthesis 190 °C synthesis 210 °C synthesis 210 °C synthesis 200 °C synthesis 105 °C synthesis solvothermal treatment 160 °C synthesis room-temperature synthesis 190 °C synthesis 150 °C synthesis ion exchange layer perovskite to 3D perovskite layered perovskite to doped platelets

microwave-assisted synthesis 120 °C synthesis CuCl2-assisted 260 °C synthesis 100 °C synthesis

27 58 16 40 51 10 ± 2 62 40 37 48 34 54 60 27 27 20.3 54 5 22 36 43 17 30 16 ± 4 21 26.1 15.2 48 51.9 59.3 27 33.5 35 36 65 61 68 26

research group/ re Klimov12 Son13 Meijerink25 Im40 Brovelli41 Gamelin18 Li42 Kundu21 Xia30 Chen28 Chen43 Liu44 Pradhan14 Samanta31 Nag45 Zhang26

He46 Manna47 Zheng48 Jung49 Luo50 Han51 Zhao20 Xia52 Son23 Pradhan53

Yang54 Pradhan22 Pradhan57 Nag58

Figure 2. (a) Successive PL spectra of the Mn-doped CsPbCl3 obtained with variation of oleylammonium chloride salts in the reaction mixture. (b) PL spectra of Mn-doped CsPbCl3 synthesized at different reaction temperatures. The inset shows a plot of the PLQY versus reaction temperature. (c) PL spectra of Mn-doped CsPbCl3 obtained in a postsynthesis ion exchange process. (d) PL spectra of mixed halide perovskites obtained with successive change of the halide contents. (e) PL spectra of Br−-treated Mn-doped CsPbCl3 nanocrystals showing that an optimum amount of Br− helps in obtaining intense dopant emission. (a) Reprinted with permission from ref 14. Copyright Wiley-VCH. (b−e) Obtained with permission from refs 18, 27, 28, and 17, respectively.

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nanocrystals with an intense dopant emission at ∼610 nm are also reported thereafter.50 To understanding the Mn distribution in the host crystals, the authors collected room-temperature X-band EPR spectra of the doped nanocrystals (Figure 3b). EPR spectra with a hyperfine coupling constant of A = 86 G were observed for 1 and 2% Mn-doped nanocrystals, which was expected for Mn2+ and confirmed that Mn remained in the octahedral environment. However, with an increase of Mn content, only one peak remained, and this was expected because of strong Mn−Mn coupling. From EPR data and Mn emission decay behavior, it was concluded that the exchange interaction between Mn2+ ions was responsible for the red shift. Soon after the development of 3D perovskites, emissive 2D layered perovskites were also reported,36,37 and Mn2+ doping was also extended to these microstructures. The doping was successfully carried out following both solution-phase and solid-state synthesis. Among the layered L2PbX4 (X = Cl, Br, and I) perovskites, L2PbBr4 (atomic model shown in Figure 4a) having shorter n-butylammonium ions (as L) was found as the ideal host for being able to transfer the exciton energy to the Mn d states.21,22 Hence, efficient Mn d−d emission was observed for only L2 PbBr 4 , though doping could be successfully achieved for L2PbCl4 and L2PbI4.22 A typical optical microscopic image of these Mn:L2PbBr4 microcrystals obtained under illumination is shown in Figure 4b. The PL spectra with varying Pb to Mn precursor ratio is presented in Figure 4c, which followed similar to the doping in 3D perovskites, wherein with enhanced Mn2+ concentration in the host nanocrystals the intensity of Mn d−d emission was observed to be improved along with the quenching of the subsequent host emission. While these microstructures were obtained in the solution phase using presynthesized butylammonium bromide, similar doping observations were also reported for solid-state synthesis. Kundu and co-workers, in the first study of its kind, used the solid-state approach by grinding L2PbBr4 and MnBr2 to obtain the doped layered perovskites21 and the PL spectra of a typical case presented in Figure 4d, which showed intensity tuning of the host and dopant emissions with a variable percentage of Mn. Son and co-workers reported Mn:CsPbBr3 nanocrystals by injection of Cs precursor to the layered perovskites (L2[Pb1−xMnx]Br4; L: ligand).23 A schematic presentation of the synthesis of these nanostructures is shown in Figure 5a. These layered perovskites having one monolayer and Mn2+

also influenced the Mn d−d emission. A typical example of a reaction started with Mn-doped CsPbCl3 is presented in Figure 2d, where with Br− addition the Mn d−d emission was slowly diminished and the exciton emission was red-shifted. This was because of the enhanced Br− content in the mixed halide perovskite. The exchange rate was also observed to be tuned with tuning of the Pb to Mn precursor content in the reaction. In another report, Xu and Meijerink even stated that addition of a certain amount of Br− indeed helped in enhancing the energy transfer efficiency from the host to dopant emissive states.17 The successive PL spectra with composition variation in the Br−-treated Mn-doped CsPbCl3 nanocrystals in such a system are presented in Figure 2e. This has been established by comparing the relative PL intensity of the nanocrystals obtained during successive Br− addition.

This red tuning was because of the lattice contraction-induced change in the crystal field strength, which in turn influenced the energy gap between emissive Mn d states. Even though the size, shape, and bandgap of the hosts are suitable for transferring their exciton energy to Mn to obtain the Mn d−d emission, they did not favor the tunability of the emission energy; however, with Mn2+ environment variations, the emission was observed to be tuned. This tunability covered only a short window, and the intensity of the emission was also reduced with red shifting.19,31,34 This has been extensively studied in chalcogenide hosts.35 This red tuning was because of the lattice contraction-induced change in the crystal field strength, which in turn influenced the energy gap between emissive Mn d states. Importantly, Samanta and co-workers reported that similar observations could be obtained in Mndoped CsPbCl3 perovskite nanocrystals.31 In the first of its kind in perovskite systems, they could tune the emission position from ∼585 to ∼625 nm. This was achieved by increasing the Mn content in the reaction system. Figure 3a presents PL spectra of host CsPbCl3 and with variation of Mn insertion up to 15.5% and with a rise of Mn in the composition; the Mn d−d emission intensity was enhanced with suppression of high-energy host emission. It is also important to mention here that Mn-doped CH3NH3PbCl3

Figure 3. (a) PL spectra of CsPbCl3 nanocrystals with various Mn contents. With an increase of Mn concentration, the dopant emission is redshifted. (b) Corresponding electron paramagnetic resonance (EPR) spectra of Mn-doped CsPbCl3 nanocrystals with Mn variation. Panels of this figure are reprinted with permission from ref 31, and copyrights were obtained from the Royal Society of Chemistry. 2253

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Figure 4. (a) Atomic model of L2PbBr4, where L is n-butylammonium ion. (b) Optical microscopic image of Mn-doped L2PbBr4 under UV irradiation (λ = 340−380 nm). (c) PL spectra obtained with variation of Pb to Mn precursors during loading in the reaction system. The excitation wavelength was 350 nm. These microcrystals were obtained in solution-phase synthesis. (d) PL spectra of Mn-doped L2PbBr4 with different Mn content obtained following solid-state synthesis. (a−c) Reprinted with permission from ref 22; (d) reprinted with permission from ref 21.

Figure 5. (a) Synthesis scheme for introducing Cs(I) to layered perovskite L2PbBr4 to obtain Mn:CsPbBr3 nanocrystals. (b) PL spectra of Mndoped CsPbBr3 nanoplatelets and (c) successive PL spectra with different HBr addition to obtain CsPbBr3 nanocubes. (d) Successive PL spectra of different monolayers of Mn:CsPbBr3 nanoplatelets. (b,c) Reprinted with permission from ref 23; (d) reprinted with permission from ref 38.

In another report, Nag et al. studied room-temperature ion exchange-induced Mn−doping in CsPbBr3 nanocrystals.38 The authors performed the ion exchange in different monolayers of platelets by adding MnBr2 and obtained the Mn d−d emissions. Figure 5d presents the PL spectra of three different size platelets; the thinner the platelets, the better the doping efficiency. This might be explained again using a similar reason for quantum confinement as stated by Son and co-workers.23 Even the host was changed from chalcogenides to perovskites, but the Mn d−d emission position, excited-state decay lifetime, and Mn concentration-dependent emission variations remained similar. However, the most striking observation in the dopant emission properties reported was the low-temperature PL. Gamelin and co-workers18 reported that upon going from room temperature to liquid N2 temperature the host exciton emission was observed to be enhanced similar to chalcogenide nanocrystals, but interestingly, the Mn d−d emission was found to be quenched. This observation was opposite from that of the low-temperature dopant emission in chalcogenides.18,20 However, returning back to room temperature again, both emissions returned to their original position. Digital images of the 3D Mn-doped perovskite nanocrystals captured at room temperature, liquid nitrogen temperature, and in between during heating are presented in Figure 6. The yellow−orange color obtained at room temperature was reversibly regained when heated from liquid nitrogen temperature. The plausible explanation provided for this reverse observation in comparison to doped chalcogenide was the faster recombination rate of the exciton. As a result, energy transfer from the host to Mn d states in perovskites became insufficient, and this diminished the

also showed the Mn d−d emission. However, with injection of Cs-oleate at high temperature (200 °C), these were transformed to Mn-doped CsPbBr3 nanocrystals as the final product. However, these doped nanocrystals contained a mixture of nanocubes and thin nanoplatelets, and each species was separated via centrifugation, and both showed intense Mn d−d emission. The quantum-confined CsPbBr3 platelets having a high bandgap performed efficient energy transfer to Mn d states, leading intense Mn dopant emission. Figure 5b shows the PL spectra of these nanoplatelets, showing dominant Mn d−d emission. However, this doping was observed to be facilitated by HBr concentration. Unlike CsPbCl3, where bulk crystals could also emit Mn d−d emission, CsPbBr3, having a certain size within the quantum confinement, was only able to transfer the host exciton energy to Mn d states. Hence, doped platelets and smaller size cubes could be the ideal host for obtaining this emission. This was confirmed by synthesizing different size cubes of CsPbBr3 by varying the amount of HBr, and the smaller size cubes obtained with more HBr showed intense Mn d−d emission. The observation is reflected from the PL spectra (Figure 5c) of the cubes obtained with HBr concentration variations in the reactions.

The quantum-confined CsPbBr3 platelets having a high bandgap performed efficient energy transfer to Mn d states, leading intense Mn dopant emission.

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to dope the perovskite nanocrystals still remains elusive and needs further exploration.

Overall, how to dope the perovskite nanocrystals still remains elusive and needs further exploration.

Figure 6. Digital images of hexane-dispersed Mn-doped CsPbCl3 nanocrystals at room temperature (RT), liquid nitrogen (LN2) temperature, and during heating up from LN2 to RT. Images with dual colors signify the temperature differences in two parts of the container. The excitation wavelength is 365 nm.

• Even though halide ion-induced Mn insertion inside of the host nanocrystals at room temperature was successfully achieved, how the dopant ions make the process feasible is not yet understood. The ion exchange and driving forces involved in this process need to be studied for better understanding of the doping process. • Why does one need excess Mn precursor even to obtain a few percentage of doped ions in the host perovskite nanocrystals? The Mn precursor can supply either more chloride (for MnCl2) or more Mn(II) ions in the bulk solution. Between these two, the key factor to make the insertion favorable has yet to be investigated. This would be possible only after understanding more on the fundamental insights of the doping process in these newly emerging perovskite nanocrystals. • The magnetic properties, dopant concentration-dependent Mn−Mn coupling, presence of other optically active or magnetic dopants, or making heterostructures by integrating with other semiconductors might provide new information or open up new properties and, hence, need to be investigated. • Though different strategies for improving dopant emission were reported (Table 1), all were accompanied by host perovskite emission, and hence, there should be a search for nanocrystals with pure dopant emission. • It was recently established that surface passivation and/or the presence of halide ions can manifold enhance the host exciton emission.24,32,55,56 However, their impact on the dopant emission has not yet been investigated. Ultrafast spectroscopy might provide more information on additional states and the conditions where the maximum percentage of the host exciton energy would be transferred to the dopant states. Hence, more spectroscopic studies should be carried out.

intensity of the dopant emission. This temperature-dependent branching of the relaxation channel (direct exciton recombination and Mn2+ sensitization) indeed explained such reverse observations. In a different experiment for doped layered perovskites, Dutta et al. reported similar observation but stated that in layered perovskites with an increase of Mn concentration the decrease of PL intensity of the dopant emission was less pronounced because of their intermediate characteristics.22 In an another interesting report by Brovelli and co-workers, the Mn d−d emission was observed quenched below 200 K, but, upon further cooling beyond 70 K, the Mn d−d emission again reappeared. It was predicted that originally the excitonic energy was localized on a shallow metastable state, which was transferred to the dopant state. Below 200 K, sensitization of Mn through the shallow metastable state was quenched, and hence, the excitonic emission was enhanced and the dopant emission intensity was reduced. Upon further cooling below 70 K, the gain in dopant emission was established because the energy transfer was directly from the band edge.39 Summary and Future Prospects. In summary, developments in the physical processes of synthesis of various Mn-doped 3D and 2D layered perovskites are reported. Insights into the mechanism of dopant insertions in these perovskite nanocrystals in high/room-temperature synthesis and the influence of Mn concentration, the presence of different metal/nonmetal halide salts, the reaction temperature, and addition of other halide ions are discussed. Among the properties, the temperature-dependent change in Mn d−d emission intensity in both 3D and 2D perovskites, Mn concentration-dependent enhancement/diminishing, and red tuning of the dopant emission intensity are elaborately discussed. However, even though success has been achieved in doping several perovskite nanostructures and intense dopant emission was achieved, much more science behind doping, the physical processes involved in the doping strategy, and the new properties on the doped nanocrystals still remain to be investigated. Retaining the importance of the unique long-lifetime emission of doped perovskites, some future prospects to be investigated and new insights involved in the doping process are listed for future studies: • Even though perovskite nanocrystals synthesis is widely established, slowing down the growth process could not be achieved yet. Doping is an established concept and is mostly facilitated by adsorption, and this needs slow growth of the host. Hence, even though the simultaneous presence of all precursors including Mn led to Mn d−d emission, placement and understanding of dopant implantation in the host lattice in unclear. Hence, precise control over the synthesis process and dopant insertion need to be deeply investigated.Overall, how

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.P.). *E-mail: [email protected] (A.K.G.). 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 An IACS faculty grant is acknowledged for funding. S.D.A. acknowledges CSIR, India for a fellowship. REFERENCES

(1) Bhargava, R. N.; Gallagher, D.; Hong, X.; Nurmikko, A. Optical Properties of Manganese-Doped Nanocrystals of ZnS. Phys. Rev. Lett. 1994, 72, 416−419.

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