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Doping Mn2+ in Lead Halide Perovskite Nanocrystals: Successes and Challenges Amit K. Guria, Sumit K. Dutta, Samrat Das Adhikari, and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India ABSTRACT: Mn2+ ions doped in high-energy absorbing semiconductor host nanocrystals take away the exciton energy and result in spin-polarized d−d emission. For the last three decades this has been widely studied on group II−VI semiconductors. Recently, the doping has been extended to CsPbX3 perovskite nanocrystals. Although the optical transition follows a similar principle, in which the exciton energy is transferred to dopant Mn d-state, doping in perovskite also revealed several new fundamental aspects of doping and dopant-induced new optical properties. Here, anions which mostly tune the band gap controlled the fate of the appearance of Mn emission. Also, the doping process was observed to be different than traditional growth doping. Hence, in perovskite host nanocrystals, while some aspects of Mn doping are found to be in agreement with previous findings, some new facts also surfaced. Combining all these facts, this Perspective focuses on the journey of Mn doping from group II−VI semiconductors to lead halide perovskite nanostructures and provides an outline for plausible future studies.
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ight-emitting doped semiconductor nanocrystals, a new era of current research, offer a unique pathway for designing solid-state lighting and light-harvesting materials with minimized self-absorption.1−5 Enormous effort has been expended on understanding the doping process and designing highly efficient doped nanocrystals.6−13 However, the pros and cons of doping result in a debate surrounding semiconductor nanocrystals regarding whether or not to dope.3,14,15 Because doping introduces the possibility of creating a charge and size imbalance center in the host lattice, it brings the concern that the illuminated tiny nanocrystals will lose their original crystal structure and emissions.1,12,16 However, for optically active dopants, the host emissions might be hijacked to a new color window.1,5−12 Both chemists and physicists take advantage of doping to monitor this optical switching of colors. The most advanced study on this aspect is Mn doping in high band gap semiconductor hosts where the excitation energy is transferred to a Mn d-state, resulting in short-range tunable yellow-orange d−d emission.5,6,8 More than two decades passed, but search for new hosts, understanding more about the doping mechanism, and finding new insights hidden behind the optical excitation and de-excitation process are still going on. After coming a long way, currently Mn doping is now performed in recently surfaced light-emitting perovskite nanocrystals.17−20 The same story repeated here, the high-energy host emission is switched to Mn yellow-orange emission; however, this also revealed a different doping path and several new findings. The strategic synthesis of growth doping where adsorption remains the key for doping did not work well for these hosts.21 Mn2+ ions, occupying the substitution position, are preferably incorporated during © 2017 American Chemical Society
Mn2+ ions, occupying the substitution position, are preferably incorporated during perovskite nanocrystal formation. perovskite nanocrystal formation. The synthetic process and purification pathways remain hectic, but the delightful colors in lead halide perovskites with halide ion switching provide new physical insights and attract additional attention.22−35 Keeping these in mind, in this Perspective we discuss the recently developed Mn-doped lead halide perovskite nanocrystals and their synthesis and optical properties. CsPbX3 and Mn-Doped CsPbX3: The Illumination. In comparison to group II−VI semiconductors, lead halide perovskite nanocrystals have higher absorption coefficient and have narrow emission of relatively longer excited-state lifetime.36−42 The loss of energy due to interference of surface or trap states is more suppressed than that of traditional quantum dots.43−48 These take over the advantages for promoting the transfer of exciton energy to Mn d-states and result in the Mn d−d emission. When the halide ions (X−) change from Cl− to Br− to I−, the emission color of CsPbX3 perovskites is tuned from blue to red.49,50 For Mn d−d transition, CsPbCl3 has the appropriate band gap for the exciton energy transfer.17 Typical illuminated colors from chloride, bromide, and iodide perovskite nanocrystals in the reaction flask are shown in panels a, b, Received: February 28, 2017 Accepted: April 7, 2017 Published: April 7, 2017 1014
DOI: 10.1021/acsenergylett.7b00177 ACS Energy Lett. 2017, 2, 1014−1021
Perspective
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
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Figure 1. (a−d) Digital images of CsPbCl3, CsPbBr3, CsPbI3, and Mn:CsPbCl3 nanocrystals in illuminated reaction flask. (e) Band positions of CsPbX3 and Mn d-states. (f) Atomic model showing a typical Mn:CsPbCl3 crystal where Mn is placed in the position of Pb.
and c of Figure 1, repsectively. Figure 1d presents the digital image of Mn-doped CsPbCl3 (Mn:CsPbCl3). Relative energy levels of host bands and dopant state are provided in Figure 1e.51,52 An atomic model showing a Mn2+ ion in a substituted position of Pb2+ is provided in Figure 1f. Doping Strategy. The widely reported and most trusted doping strategy in covalent solid nanocrystals is growth doping in which dopants are allowed to adsorb onto host nanocrystals during the growth.6,21,53 Another doping protocol that is widely accepted is diffusion doping, in which added dopant ions substituted host ions by ion exchange and reside in the crystal lattice, which is performed mostly via thermal annealing.54−56 However, recently developed strategies for doping in perovskites suggest that the dopant precursor is required to be introduced from the beginning of the reaction,17−20 but it does not follow the conventional nucleation doping.7 The nucleation doping path consists of nucleation of dopant clusters on which the host is allowed to grow, and during the annealing dopant ions are diffused from clusters. However, in the doped perovskite case, the only strategy reported is simultaneous formation; hence, Mn precursors are introduced along with Pb precursor at the beginning.17−20 It was noted that achieving Mn doping in inorganic CsPbX3 or hybrid perovskite (CH3NH3)PbX3 nanostructures was more favored when X = Cl. Also, among various other manganese(II) salts, MnCl2 was proved to be the superior precursor for doping in CsPbCl3.17 The synthetic strategy of doping using various Mn salts is shown schematically in Figure 2. As Mn substituted the Pb in the crystal lattice, it was evident that for successful doping, Mn−X bond strength in the Mn-precursor should be comparable to Pb−X bond strength in CsPbX3. For example, Mn:CsPbCl3−xBrx nanocrystals could be directly synthesized using MnBr2 and PbCl2 because the weaker Mn−Br bond would be required to be broken.17 On the other hand, doping was observed to be difficult for CsPbBr3 and CsPbI3 directly irrespective of using MnBr2 and MnI2 as dopant precursor. Rather, Br and I were incorporated via anion exchange on Mn:CsPbCl3 (discussed later) or the synthesis was carried out in mixed halides.17,18 However, the most striking observation noticed here was the incorporation of a small fraction of the used concentration of Mn precursor in all cases irrespective of
Figure 2. Schematic presentation showing the synthetic reaction of Mn-doped CsPbX3 nanocrystals and the comparison of reactivity of MnCl2, MnBr2, and MnI2.
the shape of the resultant nanocrystals. This has been reported by almost all recent reports on Mn:CsPbCl3 (Table 1). Parobek et al. used Mn:Pb in a 3:2 mol ratio to get 0.2% doping in orthorhombic CsPbCl3 nanocubes.18 In contrast, Liu et al. suggested a much lower Mn:Pb precursor ratio was required for doping in cube-shaped cubic CsPbCl3 nanostructures.17 Nag and co-workers noticed that only a fraction of the used Mn concentration was doped and concluded that insertion of Mn in CsPbCl3 remained difficult.19 Liu et al. noticed that with higher doping efficiency, the crystallinity of the host decreases.20 Nevertheless, they also used a very high Mn:Pb ratio (10:1) of precursor for getting 27% and 46% of Mn doping at 170 and 210 °C, respectively. This result obviously reveals that higher reaction temperature increases the doping efficiency for a particular molar ratio of Mn:Pb precursor. Comparison of all these reaction conditions is provided in Table 1. Dopant Emission. The exciton energy transfer to Mn d-states typically depends on the host band gap and relative positions of Mn 4T1 and 6A1 states.5,6,8 As shown in Figure 1e, CsPbCl3 is ideal for accomplishing the Mn emission. Br incorporation to some extent, though, would red tune the absorption, but it still would have the possibility of energy transfer. Figure 3a presents absorption, photoluminescence (PL), and PL excitation (PLE) spectra of undoped and doped cube-shaped CsPbCl3 nanocrystals.17 The presence of dopants is clearly reflected from the new emission at longer wavelength. The PLE spectrum for the 1015
DOI: 10.1021/acsenergylett.7b00177 ACS Energy Lett. 2017, 2, 1014−1021
ACS Energy Letters
Perspective
Table 1. Reaction Conditions for Synthesis of Different Mn-Doped Lead Halide Perovskite Nanocrystals doped nanocrystals
mode of synthesis
Mn:CH3NH3PbCl3 Mn:CH3NH3PbClxBr3−x Mn:CsPbCl3 cube Mn:CsPbClxBr3−x Mn:CsPbBr3 Mn:CsPbI3 Mn:CsPbCl3 Mn:CsPbClxBr3−x Mn:CsPbCl3 Mn:CsPbBr3 Mn:CsPbCl3 Mn:CsPbCl3 Mn:CsPbCl3
one step one step one step one step via anion exchange via anion exchange one step one step one step via anion exchange one step one step one step
Pb:Mn 2:1 2:1 5:2 5:2
2:3 2:3 10:1 1:2.5 1:10 1:10
% doping
shape/phase
9.6 9.6 − − 0.2 0.2 2 − 3 27 46
cube cube cube/cubic cube/cubic cube cube cube/orthorhombic cube/orthorhombic platelet/cubic(tetragonal) platelet/cubic(tetragonal) cube/tetragonal cube/tetragonal cube/tetragonal
dopant PL QY(%)
27
