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Doping Mn in Lead Halide Perovskite Nanocrystals: Successes and Challenges Amit K. Guria, Sumit K Dutta, Samrat Das Adhikari, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00177 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017
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ACS Energy Letters
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
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Abstract: Mn2+ ions doped in high energy absorbing semiconductor host nanocrystals take away the exciton energy and result the spin polarized d-d emission. For last three decades this has been widely studied on group II-VI semiconductors. Recently, the doping has been extended to CsPbX3 perovskite nanocrystals. Though the optical transition followed similar principle where the exciton energy was transferred to dopant Mn d-state, but the doping in perovskite also unfolded several new fundamental aspects on doping and the dopant induced new optical properties. Here, anions which mostly tune the band gap controlled the fate of appearance of Mn emission. Also, the doping process was observed different than the traditional growth doping. Hence, in perovskite host nanocrystals while some aspects on Mn doping are retained to its previous findings; but some new facts were also surfaced. Combining all these facts, this perspective focused the journey of Mn doping from group II-VI semiconductors to lead halide perovskite nanostructures and provided some outlines for plausible future studies. TOC:
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ACS Energy Letters
Light emitting doped semiconductor nanocrystals, a new era of current research, offer the unique pathway for designing solid state lighting and light harvesting materials having minimized selfabsorption.1-5 Enormous efforts have been put forwarded for understanding the doping process and designing highly efficient doped nanocrystals.6-13 However, the pro and cons of doping puts the question on the semiconductor nanocrystals whether to dope or not to dope.3,14,15 As doping has the possibility of creating a charge and size imbalance center in the host lattice, it brings anxiety for the illuminated tiny nanocrystals to 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 Chemists and physicists, both take advantages of doping to monitor this optical switching of colors. The most advance study on this aspect is the Mn doping in high bandgap semiconductor hosts where the excitation energy is transferred to Mn d-state resulting 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 walking a long way, currently Mn doping is now performed in recently surfaced light emitting perovskite nanocrystals.17-20 The same story repeated here as the high energy emission is switched to Mn yellow-orange emission; but this also unfolded a different doping path and several new findings. The strategic synthesis of growth doping where adsorption remains the key for doping did not worked well for these hosts.21 Mn2+ ion, being occupied the substitution position, are preferably incorporated during the perovskite nanocrystals formation. The synthetic process and purification pathways though remain hectic, but the delightful colors in lead halide perovskites with halide ion switching provide new physical insights and attract additional attentions.22-35
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Keeping these in mind, in this perspective we discuss on the recently developed Mn doped lead halide perovskite nanocrystals; their synthesis and optical properties.
Figure 1. (a-d) Digital images of CsPbCl3, CsPbBr3, CsPbI3 and Mn:CsPbCl3 nanocrystals in illuminated reaction flask. (e) Band positions of CsPbX3 halides and Mn d-states. (f) Atomic model showing a typical Mn:CsPbCl3 crystal where Mn is placed in the position of Pb.
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 traditional quantum dots.43-48 These take over advantages for promoting the transfer of exciton energy to Mn d-states and results the Mn d-d emission. Changing halide ions (X-) from Cl- to Br- to I-, the emission color of CsPbX3 perovskites 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 Figure 4 ACS Paragon Plus Environment
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1a-1c. 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 Atomic model showing a Mn2+ ion in a substituted position of Pb2+ is provided in Figure 1f.
Figure 2. Schematic presentation showing the synthetic reaction of Mn doped CsPbX3 nanocrystals, and the comparison of reactivity of MnCl2, MnBr2 and MnI2.
Doping strategy The widely reported and most trusted doping strategy in covalent solid nanocrystals is the growth doping where dopants are allowed to adsorb onto host nanocrystals during the growth.6,21,53 Other doping protocol which is widely accepted is the diffusion doping, where added dopant ions substituted host ions by ion exchange and reside in the crystal lattice, performed mostly via thermal annealing.54-56 However, the recent 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 Nucleation doping path consists of nucleation of dopant clusters on which host is allowed to grow and with annealing dopant ions are diffused from clusters. However, in doped perovskite case the only strategy
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reported is the simultaneous formation and 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 nanostructure 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 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 as weaker Mn-Br bond would require to be broken.17 On the other hand, doping was observed 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 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 3:2 mole ratio to get 0.2% doping in orthorhombic CsPbCl3 nanocubes.18 In contrast, Liu et al. suggested 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 were doped and concluded that insertion of Mn in CsPbCl3 remained difficult.19 Liu et al. noticed that with higher doping efficiency, the crystallinity of host decreases.20 Nevertheless, they also used a very high Mn to Pb ratio (10:1) of precursor for getting 27% and 46% of Mn doping at 170 oC and 210 oC respectively. This result obviously reveals that higher reaction temperature increases the doping 6 ACS Paragon Plus Environment
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efficiency for a particular molar ratio of Mn to Pb precursor. Comparison of all these reaction conditions is provided in Table 1. Table 1. Reaction conditions for synthesis of different Mn doped lead halide perovskite nanocrystals.
Doped nanocrystals
Mode of synthesis
Pb:Mn
Mn:CH3NH3PbCl3
one step
2:1
cube
17
Mn:CH3NH3PbClxBr3-x
one step
2:1
cube
17
Mn:CsPbCl3 cube
one step
5:2
9.6
cube / cubic
Mn:CsPbClxBr3-x
one step
5:2
9.6
cube / cubic
Mn:CsPbBr3
via anion exchange
--
cube
