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Exciton-to-Dopant Energy Transfer in Mn-Doped Cesium Lead Halide Perovskite Nanocrystals David Parobek, Benjamin J Roman, Yitong Dong, Ho Jin, Elbert Lee, Matthew Sheldon, and Dong Hee Son Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02772 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016
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Nano Letters
Exciton-to-Dopant Energy Transfer in Mn-Doped Cesium Lead Halide Perovskite Nanocrystals David Parobek‡, Benjamin J. Roman‡, Yitong Dong‡, Ho Jin‡, Elbert Lee‡, Matthew Sheldon†,‡* and Dong Hee Son‡*
‡Department of Chemistry, Texas A&M University, College Station, Texas 77843 †Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77843
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ABSTRACT
We report the one-pot synthesis of colloidal Mn-doped cesium lead halide (CsPbX3) perovskite nanocrystals and efficient intraparticle energy transfer between the exciton and dopant ions resulting in intense sensitized Mn luminescence. Mn-doped CsPbCl3 and CsPb(Cl/Br)3 nanocrystals maintained the same lattice structure and crystallinity as their undoped counterparts, with nearly identical lattice parameters at ~0.2% doping concentrations and no signature of phase separation. The strong sensitized luminescence from d-d transition of Mn2+ ions upon band-edge excitation of the CsPbX3 host is indicative of sufficiently strong exchange coupling between the charge carriers of the host and dopant d electrons mediating the energy transfer, essential for obtaining unique properties of magnetically doped quantum dots. Highly homogeneous spectral characteristics of Mn luminescence from an ensemble of Mn-doped CsPbX3 nanocrystals and well-defined electron paramagnetic resonance spectra of Mn2+ in host CsPbX3 nanocrystal lattices suggest relatively uniform doping sites, likely from substitutional doping at Pb2+. These observations indicate that CsPbX3 nanocrystals, possessing many superior optical and electronic characteristics, can be utilized as a new platform for magnetically doped quantum dots expanding the range of optical, electronic and magnetic functionality.
KEYWORDS: Perovskite, Mn-doping, Exciton-to-dopant energy transfer, Sensitized phosphorescence
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Colloidal nanocrystals of semiconducting perovskites have become a subject of intense research in recent years due to their superb luminescence properties and facile chemical tunability of the bandgap, which is attractive in many applications where colloidal quantum dots have been very successful as a source for photons or charge carriers1-5. In particular, allinorganic cesium lead halide (CsPbX3) nanocrystals have shown great potential for outperforming commonly used II-VI metal chalcogenide quantum dots from their capability to harvest photons, create charge carriers, and generate photons from the recombination of charge carriers efficiently, even in the absence of a passivating shell1. The facile modification of the bandgap via post-synthesis anion-exchange is also a unique attribute of perovskite nanocrystals for chemical tuning of their properties2, 3. In II-VI semiconductor quantum dots, doping with transition metal ions has been extensively explored as a way to introduce new optical, electronic and magnetic properties, making them much more functional than their undoped counterparts6. For instance, substitutional isoelectronic doping of MX (M=Cd, Zn, X=S, Se) quantum dots with paramagnetic transition metal ions such as Mn2+ can generate intense sensitized dopant luminescence7,
8
and create a magnetically-
coupled exciton state enabling optical control of magnetism9 or production of energetic hot electrons via exciton-to-hot carrier upconversion10,
11
. These new properties of Mn-doped
quantum dots result from the exchange coupling between the charge carriers of the host semiconductor and d electrons of the dopant, which opens new pathways of energy exchange or forms new coupled electronic states between the exciton and dopant12, 13. One may anticipate new optical, electronic and magnetic properties also in CsPbX3 perovskite semiconductor nanocrystals, similar to Mn-doped II-VI quantum dots, if Mn2+ ions can be doped in CsPbX3 nanocrystals stably with sufficiently strong exchange coupling between the charge carriers.
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Here, we successfully synthesized Mn-doped CsPbCl3 and CsPb(Cl/Br)3 nanocrystals exhibiting a strong sensitized Mn luminescence, indicative of the presence of the exchange coupling between the exciton and Mn sufficiently strong to mediate efficient energy transfer between them. This suggests that one may expand the range of the host semiconductor nanocrystal materials to include CsPbX3 perovskites to obtain new properties originating from the exchange coupling between the charge carriers and magnetic dopant ions, beyond the extensively studied II-VI quantum dots. The common difficulties faced with the synthesis of Mndoped colloidal semiconductor nanocrystals, in general, arise from the fact that the incorporation of Mn2+ ions in the lattice of small nanocrystals is usually kinetically unfavorable14. For this reason, many strategies to synthesize Mn-doped quantum dots rely on the delicate kinetic control of the growth and doping process during synthesis7,
8, 15, 16
. In the case of doping CsPbX3
nanocrystals, almost instantaneous formation of CsPbX3 nanocrystals during the solution-phase synthesis, and the quaternary elemental composition of the doped nanocrystals make identification of the optimum combination of precursors and synthetic conditions more challenging than the case of ternary systems. In this study, Mn-doped CsPbX3 nanocrystals were prepared via a one-pot synthesis method using MnCl2 as the precursor, employing a modified version of the reported synthesis method1 for CsPbX3 nanocrystals as shown in Scheme 1. Further details of the synthesis are in the Supporting Information. Among the various organometallic and halide Mn precursors tested, MnCl2 resulted in the most consistent production of Mn-doped nanocrystals. Briefly, PbCl2 (0.22 mmol) and MnCl2⋅(H2O)4 (0.31 mmol) were added into a mixture of oleylamine (0.8 ml), oleic acid (0.8 ml), and 1-octadecene (5 ml). After heating the mixture to 120-200 °C, additional oleylamine and oleic acid was injected to dissolve the metal salts. Subsequently, 0.4 ml of a
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cesium-oleate solution in 1-octadecene (64 mM) was swiftly injected into the heated metal salt solution, which immediately formed Mn-doped CsPbCl3 nanocrystals in seconds. The reaction mixture was quenched with an ice bath after the completion of the reaction, and the nanocrystals were precipitated with acetone then suspended in hexanes. When PbBr2 was used as the precursor instead of PbCl2, the resulting Mn-doped perovskite nanocrystals had mixed halide composition, i.e. CsPb(Cl/Br)3, and the exciton PL redshifted from that of CsPbCl3.2, 3 The ratio of Cl/Br in CsPb(Cl/Br)3 was determined to be 4.4:1 according to the energy dispersive X-ray spectroscopy (EDS) performed on a scanning electron microscope. The control, undoped CsPbX3 nanocrystals were also synthesized under similar experimental condition. Because of the large excess amount of Mn precursors and low doping efficiency in the host nanocrystal, thorough purification of Mn-doped CsPbX3 nanocrystals is important for the reliable determination of the doping concentration. The usual precipitation/resuspension cycles, which is effective for purifying other semiconductor nanocrystals such as II-VI metal chalcogenides, could not completely remove the unreacted Mn source from the reaction product. This is likely due to the insufficient difference in the solubility of Mn-doped CsPbX3 nanocrystals and Mn2+ complexes (e.g., Mn-oleate) in the solvents used for the purification. For this reason, gel permeation chromatography (GPC) was used for further purification of the nanocrystals.17 Separation of the nanocrystals from the remaining contaminants that are trapped in the pores of the gel with (
