Photoluminescence Lifetimes and Thermal Degradation of Mn2+

Sep 21, 2018 - different reaction temperatures to control the NC size from. 5.3 to 17.4 nm and then ..... ORCID. Xi Yuan: 0000-0001-8731-216X. Jialong...
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Cite This: J. Phys. Chem. C 2018, 122, 23217−23223

Photoluminescence Lifetimes and Thermal Degradation of Mn2+Doped CsPbCl3 Perovskite Nanocrystals Sihang Ji,† Xi Yuan,*,† Ji Li,† Jie Hua,† Yunjun Wang,‡ Ruosheng Zeng,§ Haibo Li,† and Jialong Zhao*,† †

J. Phys. Chem. C 2018.122:23217-23223. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/12/18. For personal use only.

Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China ‡ Suzhou Xingshuo Nanotech Co., Ltd. (Mesolight), Suzhou 215123, China § School of Life and Environmental Sciences, Guilin University of Electronic Technology, Guilin 541004, China S Supporting Information *

ABSTRACT: The Mn2+-doped CsPbCl3 nanocrystals (NCs) with a low Mn2+ doping concentration were synthesized using different reaction temperatures to control the NC size from 5.3 to 17.4 nm and then were studied by means of steady-state and time-resolved photoluminescence (PL) spectroscopy at various temperatures. The Mn2+ emissions with different quantum yields in the doped NCs in hexane exhibited nearly size-independent and single-exponential decay lifetimes of 1.8 ms at room temperature. The PL lifetimes in all Mn2+ in CsPbCl3-doped NCs had similar temperature dependence from 80 to 300 K, whereas they were size-dependent at elevated temperatures, reflecting thermal degradation of doped NCs. The degradation mechanisms of Mn2+ PL were attributed to the amount of surface defects as nonradiative recombination centers generated in size-unchanged and grown Mn2+:CsPbCl3 NCs. The study provides the detailed understanding of the thermal degradation mechanisms in doped perovskite NCs for optoelectronic applications.

1. INTRODUCTION All-inorganic halide perovskite nanocrystals (NCs) have been paid much attention because of their size-tunable or composition-tunable photoluminescence (PL) wavelength, high PL quantum yield (QY), and narrow emission linewidth, which exhibit the practical applications in solar cells, photodetectors, and light-emitting diodes.1−3 Highly luminescent manganese ion (Mn2+)-doped CsPbCl3 perovskite NCs with a bright orange Mn2+ emission band around 600 nm were successfully synthesized in 2016.4−20 It is well known that the broad orange-red emissions of Mn2+ doped in traditional II−VI semiconductor NCs are attributed to the Mn2+ 4T1 → 6A1 d−d transition through energy transfer from excitons in the host to dopants, which can be used for white light-emitting devices.21−30 The most commonly studied Mn2+ emissions are usually observed at around 2.12 eV (585 nm) from Mn2+doped ZnS and ZnSe NCs, whereas they are at 2.06 eV (600 nm) for Mn2+ in CdS and CdSe NCs.21,26−30 The tunable Mn2+ emissions from yellow to red were observed in Mn2+doped NCs by varying not only the concentrations and positions of dopants but also the sizes and structures of NCs.31−37 At the same time, the various PL lifetimes of Mn2+ emissions were reported from microseconds to milliseconds because the spin-forbidden Mn2+ d−d transitions in doped II− © 2018 American Chemical Society

VI semiconductor NCs were influenced by a spin−orbit coupling at Mn2+ and the anion.38−43 Therefore, tuning the energies of Mn2+ emissions in CsPbCl3 NCs is promising to achieve novel functionalities and superior advantages for white light-emitting diodes by changing the NC composition and size and the dopant concentration. Recently, the emitting colors of Mn2+-doped CsPbCl3 NCs have been controlled for fabrication of the warm white lightemitting diodes. The wavelength of exciton emission in CsPbCl3 was tuned to 450 nm through anion exchange of Mn:CsPbCl3 NCs with PbBr2 to obtain Mn:CsPbClxBr3−x NCs.4,19 Meantime, the emission wavelength of Mn2+ in CsPbCl3 NCs was shifted from 595 to 620 nm by changing the Mn2+ doping concentration from low to high, whereas the PL dynamics was significantly reduced from a single-exponential decay lifetime of 1.6 ms to multiexponential decay one of about 0.4 ms at room temperature,5−11 which is attributed to the formation of Mn2+−Mn2+ dimers or defects at higher Mn2+ doping levels.15,18 However, the NC size dependence of the PL energy and lifetime for Mn2+-doped in CsPbCl3 NCs have not Received: August 26, 2018 Revised: September 21, 2018 Published: September 21, 2018 23217

DOI: 10.1021/acs.jpcc.8b08295 J. Phys. Chem. C 2018, 122, 23217−23223

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The Journal of Physical Chemistry C

Figure 1. Absorption (a), PL (b), PLE spectra (c), Mn2+ PL decay curves (d), and PL lifetimes and QYs (e) of Mn2+:CsPbCl3 NCs with varied sizes. The color in (c) represents the intensity of the emission bands. All NCs are dispersed in hexane solutions.

ODE (6 mL) in the 50 mL three-necked flask. The reaction mixture was degassed at 120 °C for 30 min and then heated up to 190 °C under argon flow. A Cs−oleate precursor (0.25 ml) (made by dissolving Cs2CO3 (0.26 g) in OA (1 mL) and ODE (7 mL) at 150 °C) was quickly injected into the reaction solution, and after 15 s the solution was immediately cooled down to room temperature by immersing in a cold water bath. The Mn2+:CsPbCl3 NCs with different particle sizes could be obtained by varying the reaction temperature, surface ligand content, and reaction solution concentration. Keeping the other conditions constant, when the reactants at 130 °C are injected into the precursor, small NCs of about 5.3 nm were synthesized. Large NCs with about 17.4 nm were synthesized at 210 °C by adding appropriate ligands and reaction solvents. For purification, the Mn2+:CsPbCl3 NCs were repeatedly precipitated by acetone/hexane, centrifuged, decanted, and suspended in hexane. The final precipitates were dispersed in hexane. The Mn2+ doping concentrations of Mn:CsPCl3 NCs with various sizes were measured to be 3.0, 3.1, 2.9, 2.8, and 2.7%, respectively, for NC397, 400, 403, 406, and 409 by inductively coupled plasma mass spectrometry (ICP−MS). 2.4. Characterization. Absorption spectra were recorded on a UV−visible spectrophotometer (Shimadzu UV-2700). PL and 2D PL excitation (PLE) spectra, PL QYs, and PL decay curves were recorded by using a spectrometer (HORIBA Jobin Yvon Fluorolog-3) with a QY accessory and a time-correlated single-photon counting lifetime spectroscopy system. The morphologies of the NCs were acquired by using a JEOL JEM2100 transmission electron microscope. X-ray powder diffraction (XRD) patterns were measured by a Rigaku D/ Max-2500 diffractometer using Cu Kα radiation. A PerkinElmer NexION 350-X ICP−MS instrument was used for elemental analysis. Janis VPF-500 vacuum liquid nitrogen cryostat with a temperature controller was used to mount the Mn2+:CsPbCl3 NC film samples (on silicon substrates) for the temperature-dependent PL measurements.

been reported yet. Further improving the stability of CsPbX3 (X = Cl, Br, I) NCs by doping Mn2+ in the host has been demonstrated recently.14 Therefore, it is convenient to reveal the thermal stability of Mn2+ in Mn:CsPbCl3 NCs by monitoring the change in their single-exponential decay PL lifetimes, which is very important for the practical applications in solid-state lighting. In this work, the size-varied Mn2+:CsPbCl3 NCs with low doping concentration were synthesized using different reaction temperatures and then studied by variable-temperature, steadystate, and time-resolved PL spectroscopy. It is surprisingly found that the Mn2+ emissions with different QYs in all the doped NCs in hexane almost exhibit the same singleexponential decay time of 1.8 ms at room temperature and the same temperature dependence of lifetimes from 80 to 300 K. The thermal degradation mechanisms of Mn2+ emissions in doped NCs with varied sizes were studied in vacuum at temperatures ranging from 300 to 400 K by measuring the change in the PL intensity, lifetime, NC size, and structure.

2. EXPERIMENTAL SECTION 2.1. Materials. MnCl2 (≥99%), PbCl2 (99.99%), Cs2CO3 (99.99%), and trioctylphosphine (TOP, 90%) were purchased from Aladdin. Oleylamine (OLA, 70%) and oleic acid (OA, 90%) were purchased from Aldrich. 1-Octadecene (ODE, 90%) was purchased from Alfa Aesar. Hexane, toluene, acetone, and ethanol were purchased from Beijing Chemical Works. All chemicals were directly used without further purification. 2.2. Synthesis of the Cs−Oleate Precursor. Cs2CO3 (0.26 g), ODE (7 mL), and OA (1 mL) were loaded into the three-necked flask and then degassed at 120 °C for 30 min. After that, the mixed solution was heated up to 150 °C under the protection of argon and held at least 15 min until the solution became clear and transparent. 2.3. Synthesis of Mn:CsPbCl3 NCs. The Mn2+:CsPbCl3 NCs with varied particle sizes were synthesized using a one-pot injection synthesis method, which was modified from previously reported approaches. In a typical procedure, PbCl2 (0.2 mmol, 0.054 g) and MnCl2 (0.4 mmol, 0.05 g) were mixed with OLA (1.5 mL), OA (1.5 mL), TOP (1 mL), and

3. RESULTS AND DISCUSSION Following the methods adapted from the literature,4,5,15 Mn2+:CsPbCl3 NCs with five selected sizes were synthesized by changing the reaction temperature. The detailed synthesis 23218

DOI: 10.1021/acs.jpcc.8b08295 J. Phys. Chem. C 2018, 122, 23217−23223

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The Journal of Physical Chemistry C procedure is provided in the Experimental Section. Figure 1 shows the absorption, PL, PLE spectra, Mn2+ PL decay curves, PL lifetimes, and PL QYs of differently sized Mn2+:CsPbCl3 NCs. As seen in Figure 1a,b, the absorption spectra show wellresolved peaks from exciton states in Mn2+:CsPbCl3 NCs synthesized at various reaction temperatures. The sharp lowest exciton absorption bands of these NCs shift to the high-energy side from 402 to 386 nm with the decreasing NC size, indicating the quantum size effect. The blue shift of the exciton absorption band is only 130 meV. It is known that the size of the as-prepared NCs is much larger than their exciton Bohr radius (2.5 nm) of CsPbCl3, so that the quantum size effect is not significant.44 The double emission bands peaked at around 400 and 598 nm are clearly observed from each Mn2+:CsPbCl3 NC in Figure 1b, which are considered to originate from the radiative recombination of excitons near the band edge and the isolated Mn2+ ions, respectively.4,5 The wavelengths of exciton luminescence bands are 397, 400, 403, 406, and 409 nm, thus the NC samples are labeled as NC397, 400, 403, 406, and 409, respectively. In addition, it is noted that the full width at the half-maximum (fwhm) of exciton emission in Mn2+:CsPbCl3 NCs slightly increases with the decreasing NC size. However, it is surprisingly observed that the Mn2+ luminescence band maintains a constant peak wavelength at 598 nm for all the differently sized Mn2+:CsPbCl3 NCs.5 In addition, no clear change in the fwhms of NCs is also observed. On the other hand, the PL wavelengths of excitons and Mn2+ remain unchanged when the PLE wavelength is varied from 280 to 385 nm as seen in Figure 1c, indicating relatively uniform size distribution.45 It is interesting that all the Mn2+:CsPbCl3 NCs with different sizes exhibit almost the same Mn2+ PL decay curves as seen in Figure 1d. This means that the NC size has no effect on their PL lifetimes, which is quite different from those of Mn2+-doped ZnS and ZnSe NCs in the previous reports.38−43 The PL decay curves were well fitted by a singleexponential function I(t) = I0 × exp(−t/τ), where I0 and τ are the initial intensity and time constant, respectively. The PL lifetimes of Mn2+ in CsPbCl3 NCs in hexane with different sizes are obtained to be about 1.8 ms, which are the longest ones in the reported Mn2+ in CsPbCl3 NCs.4−11,15,18 The Mn2+-doped CsPbCl3 NCs had a single-exponential decay with a lifetime of up to 1.8 ms at the doping concentration of about 3% and a multiexponential decay with a reduced lifetime with the increasing doping concentration, originating from the radiative recombination of isolated Mn2+ ions and Mn2+−Mn2+ dimers.15 Therefore, the emission bands at 598 nm in all Mn2+doped CsPbCl3 NC samples can be attributed to isolated Mn2+ ions. The PL QYs of Mn2+ in the doped NCs with different sizes are plotted in Figure 1e. The maximum PL QY is over 50% for the Mn2+-doped NCs with an exciton emission of 400 nm, whereas it is only 20% for the doped NCs with an exciton emission of 397 nm, which may be related to the amount of defects/traps in the doped NCs with small size. The corresponding PL QYs of exciton emissions in doped NCs with varied sizes are also summarized in Table S1. The experimental result indicates that the PL lifetimes of Mn2+ in the doped CsPbCl3 NCs are nearly the same despite a significant difference among PL QYs of the doped NCs with varied sizes. The transmission electron microscopy (TEM) images and XRD patterns of Mn2+:CsPbCl3 NCs are shown in Figure 2. As seen in Figure 2a−e, the Mn2+:CsPbCl3 NCs exhibit a significant increase in size for NC397, 400, 403, 406, and

Figure 2. TEM images and XRD patterns of Mn2+:CsPbCl3 NCs for NC397, 400, 403, 406, and 409. The inset in (c) shows a HRTEM image.

409 with increasing reaction temperature from 130 to 210 °C and a relatively narrow size distribution. These monodispersed, doped NCs have a cubic morphology. The lattice spacing in the high resolution TEM (HRTEM) image of a NC for NC403 is clearly seen in the inset of Figure 2h, suggesting the NCs with good crystallinity. The corresponding histograms of Mn2+:CsPbCl3 NCs are shown in Figure 2f−j, respectively. The NC size distribution spectra are fitted with a Gaussian line shape. The average sizes of Mn2+:CsPbCl3 NCs for NC397, 400, 403, 406, and 409 are determined to be 5.31 ± 0.55, 7.23 ± 0.45, 8.74 ± 0.60, 13.35 ± 1.16, and 17.42 ± 1.58 nm, respectively. Further, Mn2+:CsPbCl3 NCs with different sizes have a cubic structure (#75-0411) as seen in Figure 2k. It can be found that there are two relatively obvious diffraction peaks at 15.8 and 31.9°, which correspond to (100) and (200) directions, respectively, which are well consistent with our previous report.15 The diffraction peaks of Mn2+:CsPbCl3 NCs with different sizes do not show a clear shift after introduction of Mn2+ dopants despite the remarkable broadening of diffraction peaks with the reduction of the NC size.4 To study the size effects of Mn2+ luminescence properties in the Mn2+:CsPbCl3 NCs, the variable-temperature luminescence spectra and decay curves of Mn2+, and band-edge emissions in Mn2+:CsPbCl3 NC samples were measured. Figure 3 shows the representative variable-temperature PL spectra and decay curves of Mn2+ in CsPbCl3 NC films deposited on silicon substrates for NC403 and the integrated PL intensities and lifetimes for all the doped NCs. The variable-temperature PL spectra and decay curves of band-edge emissions in Mn2+:CsPbCl3 NC films and the integrated PL intensities, peak energies, and fwhms for all Mn2+:CsPbCl3 NCs are shown in Figures S1 and S2 in the Supporting Information. As seen in Figure 3a,b, the Mn2+ PL is quite weak at 80 K. The PL intensity for NC397 gradually increases with increasing temperature to 260 K and then decreases rapidly. The maximum luminescence intensity at 260 K is about 13 times the initial intensity at 80 K. The enhancement of Mn2+ 23219

DOI: 10.1021/acs.jpcc.8b08295 J. Phys. Chem. C 2018, 122, 23217−23223

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temperature 300 K. It is surprisingly noted that the PL lifetimes of Mn2+ for other NCs almost have similar temperature dependence from 80 to 320 K. It is well known that the emissions of Mn2+ are considered to originate from the Mn2+ 4T1 → 6A1 d−d transition through energy transfer of excitons in hosts to dopants. Because of the spin-forbidden Mn d−d transition, the lifetime of Mn2+ emission is very long, which is in the range of micro- and milliseconds.38−43 Meanwhile, the ultrafast energy transfer occurring within the picosecond−nanosecond range will have no effect on the Mn2+ PL decay dynamics.13,46,47 Therefore, the Mn2+ PL lifetime in general can be decided by the coordination environment and the strength of the ligand field around Mn2+ such as Mn2+ doping concentration and the size of doped NCs. The experimental result may mean that all Mn2+ ions in CsPbCl3 NCs with different sizes have a uniform environment because the crystallization in ionic perovskite NCs is excellent under low concentration doping conditions. On the other hand, the small red shift of band-edge emissions for the doped NCs is 93 meV when the NC size is varied from 5.3 to 17.4 nm because the sizes of CsPbCl3 NCs are larger than the exciton Bohr radius of 2.5 nm, indicating a weak quantum confinement effect.44 In addition, no significant change in the lattice constant of CsPbCl3 NCs is demonstrated in the XRD experiment. Therefore, the weak size dependence of Mn2+ PL energies and lifetimes indicates that the Mn2+ ions with nearly the same crystal field environment are homogeneously distributed in CsPbCl3 NCs. The thermal degradation of Mn2+ emissions in CsPbCl3 NCs above 320 K can be better understood by monitoring the change in their unique PL spectra and lifetimes at room temperature. The PL spectra and decay curves of Mn2+:CsPbCl3 doped NC films for NC400 and NC409 without/with heat treatment at 320, 340, 360, 380, and 400 K for 20 min are shown in Figure 4. The normalized PL spectra of the exciton and Mn2+ emissions in Mn2+:CsPbCl3 NC films for NC400 and 409 under heat treatment at various temperatures are shown in Figure S4 in the Supporting

Figure 3. Variable-temperature PL spectra (a), intensities (b), decay curves (c), and lifetimes (d) of Mn2+ in the NC403 film or Mn2+doped NC with different NC sizes. The excitation wavelength is 330 nm.

emission from 4T1 to 6A1 has been explained by the thermally activated exciton fission and energy transfer from the host to Mn2+ ions.15 The PL quenching of Mn2+ at high temperatures is probably related to thermal degradation of the doped NCs because of defect/trap formation. Similar enhanced PL intensities with increasing temperatures are observed for other samples, which are in good agreement with our previous report.15 However, the maximum PL intensities of Mn2+ are observed at 260, 260, 280, 300, and 340 K for NC397, 400, 403, 406, and 409, respectively, as shown in Figure 3b. The size dependence of Mn2+ PL intensities below 300 K is not clearly understood, which is perhaps related to size-dependent localized charges within the band gap of doped NCs or exciton fission, resulting in long lifetime band-edge emissions as seen in Figure S2. The peak energies of Mn2+ doped in these CsPbCl3 NCs are located at 2.05−2.08 eV at 300 K as seen in Figure S3 in the Supporting Information. The emission band of Mn2+ ions for NC403 shifts to the blue about 78 meV with increasing temperature from 80 to 360 K. The observed blue shift of the Mn2+ PL with increasing temperature is attributed to the decrease in crystal-field strength caused by thermal expansion of the host lattice, which is similar to Mn2+:ZnS and Mn2+:ZnSe NCs.21−25 As seen in Figure S3, the fwhm of Mn2+ in CsPbCl3 NCs is clearly broadened for small-sized NCs perhaps because of the interaction among NCs during film fabrication. The fwhms for all NCs in solutions are quite similar as seen in Figure 1. Further study on the widening mechanism of fwhms is needed. The fwhm of Mn2+ emission for NC403 gradually broadens from 181 to 305 meV with the increase of temperature from 80 to 360 K. The fwhm broadening of Mn2+ is considered to originate from the electron−phonon coupling in the doped NCs.29,30 As seen in Figure 3c,d, the PL decays of Mn2+ ions are gradually shortened with the increase of temperature, in contrast to their enhanced PL intensities. The PL decay curves of Mn2+ ions in CsPbCl3 NCs with varied sizes were fitted to estimate the average fluorescence lifetimes at low and high temperatures with a single-exponential or three-exponential function, respectively. The PL lifetime of Mn2+ for NC403 decreases from 3.57 ms at 80 K to 1.66 ms at room

Figure 4. PL spectra and decay curves of Mn2+ in CsPbCl3 NC films deposited on silicon substrates for NC400 (a,b) and NC409 (c,d) without/with heat treatment at 320, 340, 360, 380, and 400 K for 20 min. The PL spectra and decay curves of the samples were measured at room temperature after they were cooled down. The PL intensities of band-edge emissions are normalized. 23220

DOI: 10.1021/acs.jpcc.8b08295 J. Phys. Chem. C 2018, 122, 23217−23223

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intensity and decay time are significantly reduced with increasing annealing time as seen in Figure 5c,d. The PL lifetime for NC400 is reduced to 0.85 ms after heat treatment for 1 h from the initial value of 1.54 ms. Similar changes in PL intensities and decay times for NC409 are observed as shown in Figure S7 in the Supporting Information. The TEM images of Mn2+:CsPbCl3 NCs for NC400 and 409 under heat treatment at 360 K for 20 min indicate the growth of the NC size as shown in Figure S8 in the Supporting Information, which is consistent with that of annealed CsPbBr3 NCs.48 The principal degradation mechanisms of the CH3NH3PbI3 film were related to a defect generation process that is highly localized on surfaces and interfaces.49,50 The thermal degradation of Mn2+ PL in doped CsPbCl3 NCs is discussed as follows. At the first stage, the reduction in Mn2+ PL intensity is related to generation of surface defects as nonradiative recombination centers in doped NCs at a low temperature range from room temperature to about 340 K. Simultaneously, the PL lifetimes of Mn2+ almost remain constant, indicating that the environment of the crystal field around Mn2+ is still not varied because the size of doped NCs is unchanged. At the second stage, the reduction in PL intensity is considered to originate from the formation of the amount of surface defects as nonradiative recombination centers were formed in the doped NCs because of the NC growth at temperature above 360 K. On the other hand, more Mn2+ ions that do not emit on the NC surface are doped into the grown NCs, and then Mn2+−Mn2+ dimers are formed, resulting in red-shifted Mn2+ emissions and shortened Mn2+ PL lifetimes.15,18 Therefore, the experimental results indicate that the Mn−Cl bond is thermally stable below 340 K for largely sized Mn2+ doped NCs.

Information. As seen in Figure 4a,c and Figure S4, it is clearly observed that the band-edge emissions for NC400 and NC409 shifts to longer wavelength under higher heat treatment temperature over 360 K, indicating the size growth of doped NCs.47,48 The Mn2+ PL bands not only shift to longer wavelength but also have significantly shortened decays after the doped NC films were annealed at above 360 K, as seen in Figure 4b,d. The average PL lifetimes of Mn2+ in doped CsPbCl3 NCs for NC400 and 409 under heat treatment at various temperatures are shown in Figure S5 in the Supporting Information. It is clearly observed that the PL lifetimes of Mn2+ for NC400 and 409 remain unchanged at temperature below 340 K, whereas they are significantly reduced to 0.55 and 1.04 ms, respectively, from their initial value of 1.45 ms, at temperatures ranging from 340 to 400 K. The PL decay curves of Mn2+ were fitted by a single-exponential or threeexponential function to calculate the PL lifetimes of NC400 and 409 under low and high (over 340 K) heat treatment temperatures, respectively. Because a large number of defects were produced around Mn2+ ions with the increase of heat treatment temperature, thus reducing the PL lifetimes of Mn2+ and introducing multiexponential PL decay of Mn2+ ions.9−11 This also indicates that largely sized Mn2+-doped CsPbCl3 NCs have better thermal stability than the small ones. In addition, the narrowed linewidths of diffraction peaks in XRD patterns of Mn2+:CsPbCl3 NC films for NC400 and 409 under heat treatment at various temperatures indicate the growth of the NC size above 360 K as shown in Figure S6 in the Supporting Information. The PL spectra and decay curves of Mn2+ in doped CsPbCl3 NC films for NC400 under heat treatment at 340 and 360 K for various times are shown in Figure 5. It is observed that the Mn2+ PL intensity for NC400 under heat treatment at 340 K rapidly decreases with increasing annealing time, whereas its PL decay time of 1.54 ms almost remains unchanged as seen in Figure 5a,b. However, under heat treatment at 360 K, both PL

4. CONCLUSIONS In summary, the PL properties of Mn2+ doped in CsPbCl3 NCs with different sizes have been studied by variable-temperature, steady-state, and time-resolved PL spectroscopy. The Mn2+doped NCs with a low Mn2+ doping concentration were synthesized using reaction temperatures to vary the NC size from 5.3 to 17.4 nm. It was found that the Mn2+ emissions in all the doped NCs in hexane almost exhibited size-independent PL wavelengths and single-exponential decay lifetimes of 1.8 ms at room temperature. Further, the Mn2+ emissions in doped NC films almost had the same temperature dependence of PL lifetimes from 80 to 300 K, whereas they had size-dependent PL lifetimes at temperatures above 340 K because of thermal degradation. On the basis of the change in Mn2+ PL spectra and lifetimes, the degradation mechanisms of Mn2+ emissions were explained by the formation of amount of nonradiative recombination centers such as defects or traps in sizeunchanged and grown NCs under higher temperature heat treatment. This study provides detailed understanding of the thermal stability of perovskite NCs for their applications in solid-state lighting and displays. The improvement of thermal stability can be expected by passivating the NC surface to effectively suppress surface defects.

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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.jpcc.8b08295.

Figure 5. PL spectra (a,c), decay curves (b,d), intensities (e), and lifetimes (f) of Mn2+ in CsPbCl3 NC films deposited on silicon substrates for NC400 under heat treatment at 340 and 360 K for various times.

Temperature-dependent PL spectra and decay curves of Mn2+ in CsPbCl3 NC films under heat treatment at 23221

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various temperatures. XRD and TEM of Mn2+-doped CsPbCl3 NCs after heat treatment (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected](X.Y.). *E-mail: [email protected](J.Z.). ORCID

Xi Yuan: 0000-0001-8731-216X Jialong Zhao: 0000-0001-9020-1436 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 11704152, 11774134, 21461006, and 51472053), National Key Research and Development Program of China (2016YFB0401701), and Development of Science and Technology of Jilin Province (20170520114JH).

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