Article pubs.acs.org/cm
Photoluminescence Temperature Dependence, Dynamics, and Quantum Efficiencies in Mn2+-Doped CsPbCl3 Perovskite Nanocrystals with Varied Dopant Concentration Xi Yuan,† Sihang Ji,† Michael C. De Siena,‡ Liling Fei,† Zhao Zhao,†,∥ Yunjun Wang,§ Haibo Li,† Jialong Zhao,*,†,∥ and Daniel R. Gamelin*,‡ †
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China ‡ Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States § Suzhou Xingshuo Nanotech Co., Ltd. (Mesolight), Suzhou 215123, China ∥ College of Physics, Jilin Normal University, Siping 136000, China S Supporting Information *
ABSTRACT: A series of Mn2+-doped CsPbCl3 nanocrystals (NCs) was synthesized using reaction temperature and precursor concentration to tune Mn2+ concentrations up to 14%, and then studied using variable-temperature photoluminescence (PL) spectroscopy. All doped NCs show Mn2+ 4T1g → 6A1g d−d luminescence within the optical gap coexisting with excitonic luminescence at the NC absorption edge. Room-temperature Mn2+ PL quantum yields increase with increased doping, reaching ∼60% at ∼3 ± 1% Mn2+ before decreasing at higher concentrations. The low-doping regime is characterized by singleexponential PL decay with a concentration-independent lifetime of 1.8 ms, reflecting efficient luminescence of isolated Mn2+. At elevated doping, the decay is shorter, multiexponential, and concentration-dependent, reflecting the introduction of Mn2+−Mn2+ dimers and energy migration to traps. A large, anomalous decrease in Mn2+ PL intensity is observed with decreasing temperature, stemming from the strongly temperature-dependent exciton lifetime and slow exciton-to-Mn2+ energy transfer, which combine to give a strongly temperature-dependent branching ratio for Mn2+ sensitization.
C
based on densely packed NCs,23,27−30 or to tune back-energytransfer equilibria that lead to dual exciton/d−d emission.31 Mn2+-doped NCs usually have long excited-state lifetimes and enhanced photochemical stabilities compared to the corresponding undoped NCs.21,29 The above properties make Mn2+-doped NCs attractive for light-emitting diodes (LEDs), luminescent solar concentrators, and related photonic technologies. The luminescence QY is an important factor in determining the performance of NC-based devices. Defects and traps in NCs can act as nonradiative recombination centers to reduce the PL QY. Luminescence decay curves can provide insights into the PL QYs, because nonradiative recombination from the emissive excited state shortens the luminescence lifetime, and inhomogeneities or other features may result in multiexponential decay.23,32−34 Elongated Mn2+ PL decay and a change from multiexponential to single exponential have been observed upon
esium lead halide (CsPbX3, X = Cl, Br, I) semiconductor nanocrystals (NCs) have attracted considerable attention because of their efficient tunable photoluminescence (PL) across the whole visible range, high PL quantum yields (QY) of ∼90%, broad absorption, narrow luminescence bandwidths, and reduced emission blinking, which make these materials potentially useful for various optoelectronic applications.1−11 Recently, Mn2+ doping has been explored as an approach to modify the spectroscopic properties of CsPbX3 NCs.12−20 Mn2+ doping has been thoroughly explored in various chalcogenide and oxide semiconductor NCs, where it is frequently found to modify the optical and magnetic functionalities of the NCs in interesting ways.21−25 Such Mn2+-doped NCs often display orange or red emission coming from the Mn2+ 4T1g → 6A1g d−d transition, with high PL QYs (over 50%) and large emission bandwidths (about 258−409 meV).13−16,18−24,26 This emission is typically sensitized by energy transfer from the photoexcited host lattice.14,15 Consequently, the effective Stokes shift between the absorption and luminescence can be controlled by the bandgap of the host NCs. The Stokes shift can be tailored to avoid undesired reabsorption losses in optoelectronic devices © 2017 American Chemical Society
Received: August 4, 2017 Revised: August 23, 2017 Published: August 24, 2017 8003
DOI: 10.1021/acs.chemmater.7b03311 Chem. Mater. 2017, 29, 8003−8011
Article
Chemistry of Materials growth of CsPbCl3 shells around in Mn2+:CsPbCl3 NCs, which separate the Mn2+ ions from the NC surfaces.17 The Mn2+ concentration also has a significant effect on the luminescence properties of Mn2+-doped NCs.13,16 Mn2+ concentrations influence energy-transfer rates from the host to Mn2+ ions, and also affect whether recombination occurs exclusively from isolated Mn2+ ions or if it may come from Mn2+−Mn2+ pairs.13,16,35,36 The PL QY of Mn2+ emission in Mn2+:CsPbCl3 NCs has been described in several studies, but no consensus has been reached about its dependence on Mn2+ concentration. For example, the QY was increased to a maximum value of 27% by increasing doping levels to a B-site Mn2+ concentration of 9.6%.14 NC crystallinity was found to deteriorate at higher Mn2+ loading, and PL QYs also decreased.16 Other Mn2+:CsPbCl3 NCs showed a peak Mn2+ PL QY of 54% at 27% Mn2+,13 but Mn2+:CsPbCl3 nanoplatelets showed a maximum PL QY of 20% at only 0.8% Mn2+ concentration.12 Further systematic investigation to clarify the dependence of Mn2+:CsPbCl3 NC PL on the dopant concentration is warranted. Temperature is an important variable in PL, and variable-temperature experiments can elucidate fundamental features of a material’s excited-state dynamics. To date, there has been limited investigation of the effects of temperature on the emission of Mn2+:CsPbCl3 NCs. This has been mostly limited to heating experiments above 300 K,17−20 and two reports of the luminescence of Mn2+:CsPbCl3 NCs at cryogenic temperatures (77 K),13,20 but to our knowledge there has been no reported analysis of Mn2+:CsPbCl3 NC PL as a function of temperature. Here, Mn2+:CsPbCl3 NCs with different Mn2+ concentrations were studied by temperature-dependent steady-state and timeresolved PL spectroscopy. Mn2+:CsPbCl3 NCs with different dopant concentrations were synthesized via a one-pot injection method. The effects of reaction temperature and Mn/Pb molar feed ratio on the Mn2+-doping level were investigated. The PL of these NCs was then investigated as a function of temperature and Mn2+ concentration. The results reveal anomalous temperature dependence in both intensity and lifetime of the Mn2+ PL that are not observed in comparable Mn2+-doped II−VI NCs. On the basis of these results, several key distinctions between Mn2+:CsPbCl3 NCs and Mn2+-doped II−VI NCs are identified and discussed.
Figure 1. Representative TEM images (a−d) and XRD patterns (e) of Mn2+:CsPbCl3 NCs prepared at 170 (a), 190 (b), 210 (c), and 230 (d) °C using a Mn/Pb molar feed ratio of 2/1. The XRD pattern of cubic CsPbCl3 are shown at the bottom as reference.
with the dimension of CsPbxMn1−xCl3 NCs reduced with increasing the temperature in a previous report.13 The insets of Figure 1b,d exhibit high-resolution TEM images of the Mn2+:CsPbCl3 NCs prepared at 190 and 230 °C. Clear lattice fringes are observed, which suggest that the as-obtained NCs are highly crystalline. XRD data for these Mn2+:CsPbCl3 NCs indicate that they retain the structures of the parent cubic CsPbCl3 NCs (Figure 1e). The diffraction peaks are shifted to higher angles as the Mn2+ concentration increases, consistent with contraction of the lattice caused by substituting Pb2+ ions with smaller Mn2+ ions. The four types of Mn2+:CsPbCl3 NC samples all exhibit strong diffraction peaks at (100) and (200), revealing that they have (100) preferred orientation. Figure 2 shows absorption, PL, and PL excitation (PLE) spectra, as well as time-resolved Mn2+ PL decay curves, of this series of Mn2+:CsPbCl3 NCs. The NCs with low dopant concentration show their first absorption band at about 398 nm. This absorption band shifts to 384 nm when the concentration of Mn2+ is 7.0% (Figure 2a), attributable to the effect of Mn2+ alloying with the host CsPbCl3, similar to that observed in Mn2+:CdSe NCs.37 Quantum confinement contributes little to the blue shift, because the diameters of these NCs are about 8.9− 10.5 nm, which are larger than the Bohr exciton diameter (∼5 nm) for CsPbCl3.12 PL spectra of these Mn2+:CsPbCl3 NCs all show two peaks, one at ∼405 nm and the other at ∼600 nm (Figure 2c). The energies of these peaks remain constant when the excitation wavelength is changed from 300 to 385 nm (Figure 2b for 190 °C sample), indicating negligible heterogeneity.38 The blue−violet PL bands can be attributed to band-edge emission of the
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RESULTS AND ANALYSIS Synthesis and General Characterization. Mn2+-doped CsPbCl3 NCs were synthesized by methods adapted from the literature.13−15 A series of samples was prepared using a particular Mn/Pb molar feed ratio of 2/1 and different reaction temperatures of 170, 190, 210, and 230 °C. The Mn2+ concentrations (relative to Pb2+) in the doped NCs across this series were determined by inductively coupled plasma mass spectrometry (ICP−MS) to be 2.0%, 2.4%, 6.5%, and 7.0%. This result reveals that Mn2+ incorporation is enhanced by increasing the reaction temperature. The actual concentrations of Mn2+ present in the final NCs are much smaller than the Mn2+ precursor concentrations, consistent with previous reports.12−15 Figure 1 shows transmission electron microscope (TEM) images and X-ray diffraction (XRD) data collected for this series of Mn2+:CsPbCl3 NCs. The NCs prepared at different temperatures all show a similar monodispersed and cubic morphology, and their average cube lengths were estimated to be 10.5, 10.2, 10.5, and 8.9 nm for 170, 190, 210, and 230 °C, respectively. The size of the Mn2+-doped NCs synthesized at 230 °C is a little smaller than that of the others, which is consistent 8004
DOI: 10.1021/acs.chemmater.7b03311 Chem. Mater. 2017, 29, 8003−8011
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Chemistry of Materials
Table 1. Analytical Mn2+ Concentrations (Relative to Pb2+ Ions) in Various Mn2+:CsPbCl3 NCsa, Determined by ICP− MS sample
0.5/1
1/1
2/1
3/1
5/1
170 °C 190 °C 210 °C 230 °C
1.2% 1.2% 1.5% 2.2%
1.9% 2.0% 2.5% 4.1%
2.0% 2.4% 6.5% 7.0%
3.7% 5.4% 7.2% 8.3%
4.1% 7.1% 14.6% 14.3%
a
The NCs were prepared with different Mn/Pb molar feed ratios (from 0.5/1 to 5/1) and at various reaction temperatures (from 170 to 230 °C).
attributed to enhancement of Mn2+ ionic activity at these elevated temperatures. The PL spectra of the Mn2+:CsPbCl3 NCs prepared at various reaction temperatures (170 to 230 °C) with different Mn/Pb molar feed ratios (0.5/1 to 5/1) are summarized in Figure 3a−d. The excitonic PL intensity gradually decreases with increasing Mn2+ concentration, while the Mn2+ 4T1g → 6A1g intensity increases (Figure 3a−d). These results show that energy transfer from the CsPbCl3 host NCs to the Mn2+ dopants becomes more efficient with increasing Mn2+ concentration. Additionally, a small red shift of the Mn2+ emission is observed at high Mn2+ concentrations, consistent with formation of Mn2+−Mn2+ pairs.33−36 For example, the peak of the Mn2+ emission in the NCs prepared at 210 °C with a Mn/Pb feed ratio of 5/1 is redshifted to 632 nm. These trends are less pronounced in the NCs prepared at 170 and 190 °C, but are more evident in those prepared at 210 and 230 °C because of the enhanced Mn2+doping efficiencies at elevated temperatures. Notably, nearly complete suppression of excitonic emission is demonstrated in the Mn2+:CsPbCl3 NCs prepared at 230 °C with a Mn/Pb feed ratio of 5/1, which has not been achieved in previously reported Mn2+-doped CsPbX3 NCs.12−16,18,19 Figure 3e−h plots PL decay curves of the corresponding Mn2+:CsPbCl3 NCs. All of the Mn2+:CsPbCl3 NCs synthesized at 170 °C exhibit single-exponential PL decay curves. Singleexponential decay curves are also observed for the NCs prepared at higher temperatures when lower Mn/Pb feed ratios are used. This result is similar to the observation of single-exponential PL decay in Mn2+:CdS/ZnS and Mn2+:ZnSe NCs at low Mn2+ concentrations.40,41 Single-exponential PL decay is a reflection of a homogeneous chemical environment for the ensemble of Mn2+ ions within the various NCs. As the reaction temperature or Mn/ Pb precursor ratio is increased, faster multiexponential decay is observed, suggesting that this homogeneity is destroyed. The fast components in the multiexponential decay can be attributed to Mn 2+ −Mn 2+ pairs, surface-exposed Mn 2+ ions, or trap states.22,23,35,36,42 Upon closer inspection, it is seen that the Mn2+:CsPbCl3 NCs exhibit single-exponential PL decay when their peak PL wavelength is 605 nm or shorter, but show multiexponential decay when their PL is red-shifted to longer wavelengths. The relationship between the PL peak wavelength and the decay dynamics supports the interpretation of both observations with respect to the formation of Mn2+−Mn2+ pairs in Mn2+:CsPbCl3 NCs at high doping levels.35,36 Figure 4 summarizes the average PL lifetimes and QYs of Mn2+ emission from the NCs shown in Figure 3, estimated from the data using eq 1.32
Figure 2. (a) UV−vis absorption spectra of Mn2+:CsPbCl3 NCs synthesized at 170, 190, 210, and 230 °C prepared with a Mn/Pb feed ratio of 2/1. (b) Two-dimensional PL excitation spectrum of Mn2+:CsPbCl3 NCs prepared at 190 °C. The colored contours represent the PL intensity. PL spectra (c) and Mn2+ emission decay curves (d) of the Mn2+:CsPbCl3 NCs synthesized at different temperatures. The excitation wavelength is 330 nm. The inset in part c exhibits the total PL QYs of the doped NCs measured in solution plotted as a function of reaction temperature.
CsPbCl3 host NCs. The full widths at half-maximum (fwhm) are very narrow (∼91 meV), and small Stokes shifts (76−152 meV) are found, consistent with exciton recombination. The red PL band shows a larger fwhm (289−362 meV) and exhibits a long PL lifetime of 0.8−1.8 ms (Figure 2d). These properties are similar to those observed in Mn2+-doped II−VI semiconductor NCs, and are consistent with the red PL coming from the spinand parity-forbidden Mn2+ 4T1g → 6A1g d−d transition.16,39 The ratio of Mn2+ to exciton PL intensities increases with increasing reaction temperature between 170 and 230 °C, consistent with the increase in Mn2+ concentration observed analytically. The total PL QY increases markedly across this series, reaching a maximum value of 62% at the dopant concentration of 2.4%, and then decreases with further increases in Mn2+ concentration (Figure 2c, inset). Furthermore, single-exponential PL decay is observed in the Mn2+:CsPbCl3 NCs with low Mn2+ concentrations (Figure 2d). The PL decay dynamics are discussed more in the following section. The effect of reaction time on the PL properties of Mn2+:CsPbCl3 NCs is described in the Supporting Information. The NCs with reaction time longer than 1 h exhibit poorly resolved excitonic absorption features, and, correspondingly, almost no exciton PL, suggesting deterioration of crystallinity. For exploration of the synthetic parameter space more broadly, a series of Mn2+:CsPbCl3 NC samples was then prepared using various Mn/Pb molar feed ratios (0.5/1 to 5/1) across the same set of reaction temperatures (170−230 °C). The analytical Mn2+ concentrations in these NCs are summarized in Table 1. The Mn2+-doping level could be increased both by increasing the reaction temperature and by increasing the Mn/ Pb feed ratio. The Mn2+ concentration changes little (from 1.2% to 4.1%) across the precursor series at 170 °C but varies from 1.5% to 14.6% at 210 °C across the same series. The Mn/Pb feed ratio thus more effectively controls the doping concentration at higher reaction temperatures (210 and 230 °C), an observation
τav = 8005
∑i Ai τi ∑i Ai
(1) DOI: 10.1021/acs.chemmater.7b03311 Chem. Mater. 2017, 29, 8003−8011
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Chemistry of Materials
Figure 3. Normalized PL spectra (a−d) and decay curves (e−h) of Mn2+ ions in the Mn2+:CsPbCl3 NC colloids synthesized with various Mn/Pb molar feed ratios at different temperatures. The excitation wavelength is 330 nm.
Moreover, from Figure 3, each one also displays nearly monoexponential PL decay. The maximum QY is thus achieved at the highest Mn2+ concentration for which monoexponential decay is still observed, which for these NCs is in the range 3 ± 1%. This is a surprisingly high concentration when compared to analogous results for Mn2+-doped II−VI NCs, where, for example, Mn2+ concentrations well below 1% have been found to yield higher Mn2+ PL QYs.28,41 We note that the importance of careful Mn2+:CsPbCl3 NC purification to remove unincorporated Mn2+ has been stressed in the recent literature,15 and care was taken here to eliminate adventitious Mn2+ by using repeated purification cycles in conjunction with analytical Mn 2+ concentration measurements (see Experimental Methods). The Mn2+ emission observed here originates from energy transfer from photoexcited CsPbCl 3 NCs to the Mn 2+ dopants.12−16 The observed Mn2+ PL QY is thus dependent not only on the internal quantum efficiency of Mn2+ radiative decay (ΦMn) but also on the efficiency of energy transfer (ΦET) from host to dopant. 43 The energy-transfer time in Mn2+:CsPbCl3 NCs has been estimated to be nanoseconds,14,15 which is much slower than that in analogous Mn2+doped II−VI NCs (about tens of picoseconds),33,34 but it is much faster than the decay time of the Mn2+ emission (milliseconds). The Mn2+ PL decay thus reflects only the Mn2+ d−d transition, not the energy-transfer process. The PL lifetime has little dependence on Mn2+ concentration below ∼3.7%. The increasing Mn2+ PL QY with increasing Mn2+ concentration in this range is mainly due to a growing probability of energy transfer from the CsPbCl3 exciton to a Mn2+ when more Mn2+ is present, i.e., increased ΦET. When Mn2+ doping exceeds ∼4%, the PL QY drops, and the Mn2+ lifetime decreases with increasing Mn2+ concentration, accompanied by the PL red shift and attributed to Mn2+−Mn2+ pair formation, which at this concentration has a statistical prevalence of ∼16%.44 Similar PL red shift and quenching have been demonstrated in Mn2+doped II−VI semiconductor NCs at high Mn2+ concentrations.35,36 The crystallinity of Mn2+:CsPbCl3 NCs is also decreased at high doping levels,13,16 suggesting an increase in the density of defect/trap states, and some of the decreased
Figure 4. (a−d) Average PL lifetimes (●) and colloidal-solution QYs (■) of Mn2+ ion d−d emission in the Mn2+:CsPbCl3 NC colloids synthesized with various Mn/Pb molar feed ratios at different temperatures. The ★ indicate the samples with the highest PL QY for each synthesis temperature. In order of increasing temperature, the Mn2+ concentrations of these star samples are 3.7%, 2.4%, 2.5%, and 2.2%.
Here, τi and Ai are the time components and weights of the multiple exponential functions used for analyzing the PL decay curves, respectively. As seen in Figure 4a,b, τav of the 170 (and 190) °C NCs has a value of 1.8 ms that is independent of Mn/Pb ratio until this ratio reaches its highest value 5/1, at which point τav decreases to 1.7 ms (1.3 ms). In contrast, the Mn2+:CsPbCl3 NCs prepared at higher temperatures (210 and 230 °C) show τav values that decrease with increasing Mn2+ concentration for all Mn/Pb feed ratios (Figure 4c,d). As the Mn/Pb feed ratio increases, the QY of Mn2+ emission for the NCs prepared at 170−210 °C increases and then decreases, while that for the NCs prepared at 230 °C only decreases. The maximum QYs of Mn2+ emission are 33%, 56%, 41%, and 27% for the NCs prepared at 170, 190, 210, and 230 °C, respectively, and are marked with ★ in Figure 4. From Table 1, these maximum-QY samples all have a similar analytical Mn2+ concentration of between 2.2% and 3.7%. 8006
DOI: 10.1021/acs.chemmater.7b03311 Chem. Mater. 2017, 29, 8003−8011
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Chemistry of Materials lifetime and QY is likely attributable to enhanced energy migration to traps at these elevated Mn2+ concentrations. Such concentration quenching is common in dopant-activated phosphors.45 Variable-Temperature Photoluminescence. To investigate these NCs in more detail, variable-temperature PL measurements were performed. Although some variable-temperature spectral data for Mn 2+:CsPbCl 3 NCs have been reported,13,17−20 to our knowledge variable-temperature PL lifetime data have not been reported, and analyses of such variable-temperature spectra or lifetime data have not been reported. Figure 5 presents steady-state and time-resolved PL
Figure 6. Temperature-dependent integrated PL intensities normalized at 80 K (a) and decay times (b) of Mn2+ PL in Mn2+:CsPbCl3 NC films deposited on silicon substrates with 2.0% (■), 2.4% (●), 14.6% (▲), and 7.0% (▼) Mn2+. The samples were prepared using Mn/Pb feed ratios of 2/1, 2/1, 5/1, and 2/1, synthesized at 170, 190, 210, and 230 °C, respectively. Figure 5. Temperature-dependent PL spectra (a) and decay curves (b) of Mn2+ PL in Mn2+:CsPbCl3 NC films deposited on silicon substrates (Mn/Pb feed ratio of 2/1 synthesized at 190 °C). Similar behavior is observed for other samples (see the Supporting Information). The excitation wavelength is 330 nm.
Taken together, these observations are not consistent with the common process of thermal PL quenching. In this scenario, the decreasing PL lifetimes observed here with increasing temperature should be accompanied by decreasing PL intensities. This temperature dependence is also not consistent with that of the intrinsic dual emission reported previously for Mn2+-doped II− VI NCs31,46 and hypothesized for Mn2+:CsPbX3 (X = Cl, Br, I) NCs.14 In this scenario, decreasing the temperature should increase both the intensity and the lifetime of the Mn2+ PL. Clearly, the unique temperature dependence of the Mn2+ PL in Mn2+:CsPbCl3 NCs has a distinct origin that is different from those observed previously in Mn2+-doped II−VI and related NCs. Further analysis of the Mn2+:CsPbCl3 NC PL temperature dependence can thus reveal new insights into the properties of this interesting class of materials. To fully understand this unique behavior, it is necessary to consider the temperature dependence of the full PL spectrum. Complete PL spectra including both Mn2+ and excitonic emission for the sample from Figure 5, collected at various temperatures, are provided as Supporting Information. These spectra are summarized in Figure 7, which plots the integrated excitonic and Mn2+ PL intensities versus temperature. Similar spectra have been reported previously but were not analyzed.13,20 These data show a substantial decrease in NC excitonic intensity with increasing temperature, concomitant with the increase in Mn2+ PL intensity shown in Figure 6a. The inverse correlation of these two intensities suggests interpretation of the Mn2+ PL intensity trends in Figure 6a in terms of a temperature-dependent energy transfer from the photoexcited CsPbCl3 NCs to the Mn2+ dopants, i.e., a temperature dependence of ΦET. Control measurements on undoped CsPbCl3 NCs show a similar large decrease in excitonic PL intensity with increasing temperature over the same temperature range, accompanied by an increase in the excitonic PL decay time from ∼5 ns at 100 K to ∼35 ns at room temperature (see the Supporting Information). This unusual temperature dependence has been noted in other CsPbX3 NCs and associated with exciton fission.4,6 We hypothesize that the anomalous temperature dependence of
data for Mn2+:CsPbCl3 NCs (190 °C, Mn/Pb feed ratio = 2/1) collected as a function of temperature from 80 to 300 K. As the temperature is raised, the PL decay becomes gradually faster, accelerating by approximately a factor of 2 over this range. Normally, this result might be considered indicative of thermal PL quenching, but here the Mn2+ emission intensity increases by more than a factor of 6 over the same temperature range. A blue shift of the Mn2+ PL maximum is also observed with increasing temperature, consistent with both the decrease in crystal-field strength caused by thermal expansion of the host lattice, similar to Mn2+:ZnS and Mn2+:ZnSe NCs,29 and the thermal activation of vibronic hot bands. Figure 6 summarizes the Mn2+ PL intensities and lifetimes as functions of temperature for Mn2+:CsPbCl3 NCs of various Mn2+ concentrations (see the Supporting Information for primary data). Similar trends are observed for the other Mn 2+ concentrations, except at the most elevated concentrations. The Mn2+ emission intensity in the NCs prepared at 170 and 190 °C increases significantly with increasing temperature, whereas the increase is smaller for the NCs synthesized at 210 and 230 °C that have more Mn2+. This intensity increase is >500% for the two low-Mn2+ samples, but drops to 140% and 28% for the two high-Mn2+ samples. It is noted that these measurements were performed on NC films deposited on silicon substrates, and the room-temperature PL QYs are thus smaller than those measured for the NCs in solution (e.g., Figure 4). The temperature dependence of the PL maxima for the two high-concentration NCs is also more complex, suggesting contributions from species other than isolated Mn2+ ions (see the Supporting Information). Notably, the Mn2+ PL lifetimes show nearly identical changes with temperature for all Mn2+ concentrations, despite the relatively large variations in PL QYs with temperature for the same samples. 8007
DOI: 10.1021/acs.chemmater.7b03311 Chem. Mater. 2017, 29, 8003−8011
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Chemistry of Materials
same thermal barrier of Ea ≈ 840 cm−1 for charge separation, confirming that both data sets share the same microscopic origin. This barrier is larger than but comparable to those of ∼350−400 cm−1 reported for exciton dissociation in CsPbBr3 NCs.4 We note that eqs 2 and 3 embed various assumptions about the multiplicity of the charge-separated state, the absence of other nonradiative processes, and uniform energy-transfer rate constants, for which there is little direct evidence, so the specific numerical fitting results should not be overinterpreted. Nonetheless, the primary qualitative conclusion remains sound: the strong and anomalous temperature dependence of the Mn2+ PL intensity is ultimately attributable to the strong and anomalous temperature dependence of the exciton itself, reflecting a temperature-dependent competition between exciton recombination and energy transfer to Mn2+. Interestingly, this analysis indicates efficient relaxation from the long-lived charge-separated state to the emissive Mn2+ 4T1g state. Finally, we address the temperature dependence of the Mn2+ PL lifetimes (Figure 6b). This temperature dependence is opposite that of the Mn2+ PL intensity, and it has a qualitatively different origin. The temperature dependence of the Mn2+ PL lifetime arises from vibronic activation of the radiative d−d transition. Substitutional Mn2+ in CsPbCl3 NCs has pseudooctahedral symmetry, and its 4T1g → 6A1g transition is thus parity-forbidden in addition to spin-forbidden to first order. This electronic transition can only gain electric-dipole allowedness by coupling to symmetry-breaking vibrations of the MnCl64− unit (2T1u, T2u in Oh). One quantum of this odd-parity activating mode introduces a “false” electric-dipole origin, upon which additional Franck−Condon progressions along symmetrypreserving coordinates are built to generate the intrinsically broad bandshapes of the Mn2+ PL. At low temperatures, electricdipole intensity can only come from addition of one quantum of an odd-parity vibration in the final (ground) state, but at elevated temperatures, thermal excitations of odd-parity vibrations in the initial (excited) state activate these new pathways for electricdipole transitions, leading to a net increase in the radiative transition probability with increasing temperature, and hence a decrease in the radiative decay time as seen in Figure 6b. Fitting the temperature dependence in Figure 6b provides an estimate of 226 cm−1 for the effective energy of the activating mode (see the Supporting Information). This energy is consistent with the expected energies of MnCl64− T1u and T2u cluster vibrations, as well as with the M−L phonon energies of the CsPbCl3 lattice.48 We thus interpret the decrease in Mn2+ PL lifetime with increasing temperature as an intrinsic property of Mn2+ in the CsPbCl3 lattice reflecting thermally activated dynamic symmetry breaking in the luminescent excited state. The intrinsic nature of this effect is responsible for the strong similarity of the Mn2+ lifetime temperature dependence across all samples in Figure 6b. A similar Mn2+ lifetime temperature dependence is not observed in Mn2+-doped II−VI NCs, consistent with the lack of inversion symmetry at the cation sites in these lattices.
Figure 7. Temperature-dependent integrated PL intensities in Mn2+:CsPbCl3 NC films deposited on silicon substrates with 2.4% Mn2+, normalized at 80 K. (a) Excitonic emission. (b) Mn2+ emission. These NCs were synthesized at 190 °C using a Mn/Pb feed ratio of 2/1. The dashed curves in parts a and b show fits of the data to eqs 2 and 3, respectively, using (a) kET = 0.33 ns−1, kexc = 1.04 ns−1, and A = 1053.8 ns−1, or (b) kET = 0.33 ns−1, kexc = 4.13 ns−1, and A = 1053.8 ns−1, and a globally fitted value of Ea = 838.5 cm−1. The difference in kexc between the two observables is attributed to sample heterogeneity.
the Mn2+ PL intensity seen in Figures 6a and 7b ultimately stems from a combination of this characteristic behavior of excitons in CsPbCl3 NCs and the relatively slow energy transfer to Mn2+ in these materials. Small values of kET ∼ 0.33−2.6 ns−1 have been reported for Mn2+:CsPbCl3 NCs of various relatively high Mn2+ concentrations,15,47 consistent with the relatively long exciton lifetimes of several nanoseconds in other similar Mn2+:CsPbCl3 NCs.19 The hypothesis linking the anomalous Mn2+ PL temperature dependence to these two factors is supported by temperature-dependent exciton PL decay measurements on the Mn2+:CsPbCl3 NCs studied here, which show the exciton lifetime increasing from
