Article Cite This: J. Phys. Chem. C 2019, 123, 14239−14245
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Efficient Trap-Mediated Mn2+ Dopant Emission in Two Dimensional Single-Layered Perovskite (CH3CH2NH3)2PbBr4 Binbin Luo,*,† Yan Guo,† Xianli Li,† Yonghong Xiao,† Xiaochun Huang,† and Jin Z. Zhang‡ †
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou, Guangdong 515063, China ‡ Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States
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S Supporting Information *
ABSTRACT: In this work, Mn2+ has been efficiently and homogeneously doped into two-dimensional (2D) distorted single-layered EA2PbBr4 (EA: ethylammonium) via a reprecipitation method. Both the doped and undoped 2D layered lead halide perovskites (LHPs) were characterized using a combination of X-ray, electron microscopy, and spectroscopy techniques. The Mn 2+ -doped EA 2 PbBr 4 (EA2PbBr4:Mn2+) shows a 78% photoluminescence (PL) quantum yield (QY) with complete quenching of self-trapped exciton emission because of efficient exciton trapping by defects created by dopants and small activation energy (∼9.8 meV) between the defect states and Mn2+ d states. Compared to the long lifetime (∼1.5 ms) of Mn2+ emission in CsPbCl3, the lifetime in 2D EA2PbBr4 is found to be ∼0.75 ms, resulting from the heavy atom effect. Additionally, the PL QY of Mn2+ emission can be further increased by codoping Zn2+ or Cd2+, which is attributed to a high density of trap states created by codoping, facilitating exciton to Mn2+ energy transfer. These results reveal the key role of trap states in the energy transfer of Mn2+-doped 2D LHPs.
1. INTRODUCTION Lead halide perovskites (LHPs) have attracted enormous attention in the past few years owing to their broad versatility of chemical composition and tunability of physical and chemical properties.1−8 By designing the structure of materials and implanting different impurities such as transition metal ions, unique physical and chemical properties can be realized and used for various optoelectronic applications.4,9−15 Among the transition metal ions, Mn2+ is one of the extensively investigated dopants in II−VI semiconductor nanocrystals (NCs) due to its characteristic emission band with the ms scale lifetime and paramagnetism.15 LHPs have a widely tunable band gap spanning the whole visible range, suggesting that energy transfer from the LHP host to Mn2+ can be easily achieved by adjusting the energy levels of the host. Two previous studies reported in 2016 have successfully synthesized Mn2+-doped CsPbCl3 NCs with a characteristic Mn2+ emission band around 600 nm.16,17 Although these studies have triggered intensive investigations of Mn2+-doped LHPs for improving structural stability,18,19 O2 sensor,20 perovskite solar cells, 2 1 and light-emitting diodes (LEDs),22,23 some anomalous behaviors, such as the enhancement of both band edge and dopant emission upon low-level doping of Mn2+ and the mechanism of energy transfer, are still not well understood. Through cryogenic and time-resolved spectroscopic studies, recent studies suggest that the trap states mediate the energy transfer to sensitize Mn2+ dopants at room temperature in the 3D CsPbCl3 host, while direct transfer of © 2019 American Chemical Society
energy from the band edge states to dopants occurs at lower temperatures ( 2σ(I)]a R indices (all data)a largest diff. peak and hole, e Å−3
298 K 619.02 Cu Kα (λ = 1.54184 Å) monoclinic P21/c 11.7549(2) 8.2374(1) 8.1699(1) 108.855(2) 748.64(2) 2 2.7459 g/cm3 95.9% 1.056 0.0301 R1 = 0.0457, wR2 = 0.1314 R1 = 0.0472, wR2 = 0.1334 2.226/−2.561
a R 1 = ∑||F o | − |F c ||/∑|F o |; wR 2 = {∑[w(F o 2 − F c 2 ) 2 ]/ ∑[w(Fo2)2]}1/2.
EA2PbBr4 consists of corner-shared [PbBr6]4− separated with bilayers of EA+ as spacer cations, as shown in Figure 1a. Compared to 3D lead bromide perovskites, the lead bromide framework in EA2PbBr4 is highly twisted (Figure 1b), resulting from the dual interaction including hydrogen bonding (average length of hydrogen bonds: ∼2.6 Å) and electrostatic interaction between ammonium and [PbBr6]4− octahedra. The Pb−Br−Pb bond angles strongly deviate from the planar geometry with an average angle of ca. 150.56°. Different from the outer layers of EA4Pb3Br10,28 the [PbBr6]4− units maintain 14240
DOI: 10.1021/acs.jpcc.9b02649 J. Phys. Chem. C 2019, 123, 14239−14245
Article
The Journal of Physical Chemistry C
is attributed to the STE emission due to the highly distorted structure. The white light emission has been widely observed in other distorted single-layered LHPs.12,27,28,30,33 3.2. Mn2+ Doping of EA2PbBr4. Previous studies have suggested that the high PL QY of CsPbCl3:Mn2+ NCs originates from the good alignment between the host band gap and d−d transition of Mn2+.34,35 With the substitution of Cl− with Br− (band gap decreasing), the intensity and PL QY of Mn2+ dopant emission dropped progressively,16,35 implying a mismatch in energy levels between the host band gap and excited states of the Mn2+ dopant. Therefore, the band gap alignment plays an important role in determining the efficiency of energy transfer from the host to the Mn2+ dopants. 2D EA2PbBr4 exhibits a similar band gap as CsPbCl3 according to its absorption spectrum, suggesting that 2D EA2PbBr4 may function as a potential host material for the Mn2+ dopant. What’s more, the intrinsic pathway of energy transfer of Mn2+ can be determined by examining the quench effect of Mn doping on the white light emission of EA2PbBr4 because of the comparable lifetime between exciton trapped by lattices and defects. The SEM image (Figure S3) was taken to present more morphological details of Mn2+-doped EA2PbBr4. Micron-sized crystals with the plicate surface were observed, implying the layered structure of EA2PbBr4:40%Mn2+ remains upon Mn2+ doping. With increasing nominal dopant levels of Mn2+, the PXRD (Figure 3a) of EA2PbBr4:Mn2+ shows a small
Figure 1. (a) Crystal structure of EA2PbBr4. (b) Bond length and angle of the lead bromide framework in EA2PbBr4. (c) PXRD patterns of as-prepared products. (d) SEM image of EA2PbBr4.
a quasi-ideal geometry of octahedron with an average length of ca. 2.99 Å, which is close to the bond length of the 3D structure (ca. 2.97 Å). The antisolvent precipitation method was modified to prepare EA2PbBr4 powders.29 The precursor molar ratios have a great effect on the products, as shown in Figures S1 and S2. Although no diffraction peaks of impurity are observed in the sample prepared with the stoichiometric ratio (nEABr/nPbBr2 = 2:1), the absorption band of three-layered EA4Pb3Br10 can be found. Therefore, the molar ratio of nEABr/nPbBr2 = 3:1 was used for the synthesis of the EA2PbBr4 powder. Powder X-ray diffraction (PXRD) patterns of the samples are shown in Figure 1c. The diffraction peaks agree well with the simulated patterns of EA2PbBr4 and no other diffraction peaks are observed, showing the high purity of the as-prepared sample. The wrinkled feature at the edge of crystals denoted by the red-dashed circle can be clearly observed from the SEM image (Figure 1d), further indicating the 2D morphology of EA2PbBr4. For layered LHPs, their optical properties are strongly dependent on their crystal structure, especially in terms of the number of layers. EA2PbBr4 shows sharp excitonic absorption (Figure 2) similar to that of single-layered LHPs peaked at 396 nm (∼3.13 eV) with different spacer cations.30−32 Upon UV excitation at 360 nm, a narrow peak at 415 nm and a broad PL band spanning the entire visible range can be observed, which
Figure 3. (a) PXRD patterns of EA2PbBr4:Mn2+ with different dopant concentrations. The dashed line indicates the shift of diffraction the peak. (b) EPR spectrum of EA2PbBr4:Mn2+ at room temperature. (c) EDS mapping of EA2PbBr4:20%Mn2+.
shift toward the high degree, attributed to the small radius of Mn2+ with respect to Pb2+. EPR spectra (Figure 3b) of EA2PbBr4:Mn2+ show the hyperfine splitting without significant interference, which rules out the existence of Mn2+ with a different coordination environment and suggests the weak Mn−Mn exchange interaction.36 Because the strength of the Mn−Mn exchange interaction highly depends on the distance of Mn2+ ions,37 the weak Mn−Mn interaction observed in 2D (EA)2PbBr4 is ascribed to the large interlayer distance (∼11 Å) and homogenous doping of Mn2+. The splitting energy with ∼93 G is close to that in Mn2+-doped CsPbCl3 NCs and bulk
Figure 2. Absorption and PL spectrum (λex = 360 nm) of EA2PbBr4. Inset: The photograph of as-prepared EA2PbBr4 under UV light. 14241
DOI: 10.1021/acs.jpcc.9b02649 J. Phys. Chem. C 2019, 123, 14239−14245
Article
The Journal of Physical Chemistry C
Figure 4. (a) Absorption and (b) normalized PL spectra (λex = 360 nm) of EA2PbBr4:Mn2+ with different nominal Mn2+ concentrations. (c) PLE spectra (λem = 616 nm) of EA2PbBr4:Mn2+ with different doping levels. PL decay curves of EA2PbBr4:Mn2+ monitored at different wavelengths (d) 415 nm and (e) 616 nm. (f) Temperature-dependent PL spectra of EA2PbBr4:40%Mn2+. Inset: Boltzmann analysis of the Mn2+ PL intensity as a function of T.
time of exciton to Mn2+ indicates that Mn2+ doping should not have a significant impact on the STE emission. The anomalous behavior and high PL QY of Mn2+ emission suggest that an intermediated state has been introduced simultaneously upon Mn2+ doping that are more competitive than the formation of STEs for exciton trapping. Usually, doping impurities creates high density of defect states, which trap excitons around several tens of picoseconds time scales.47 Therefore, these intermediated states are very likely trap states. The PL excitation (PLE) spectra (Figure 4c) of the EA2PbBr4:Mn2+ samples collected at 616 nm show features that resemble the absorption band, implying that the Mn2+ dopant emission arises from the absorption of the host. Note that a weak and broad PLE peak at 435 nm is clearly observed and the intensity presents a positive correlation with the concentration of Mn2+, attributed to the absorption of the trap states created by Mn2+ doping. This result indicates that the energy of Mn emission can derive from the absorption of trap states. The much lower intensity suggests that the energy of Mn2+ emission is mainly derived from the EA2PbBr4 band edge absorption. To better understand the energy transfer process, timeresolved PL was conducted as shown in Figure 4d. The PL decay curves of the emission with the maximum intensity show a double-exponential fit with average lifetimes ranging from 4.0 to 1.4 ns when the doping level of Mn2+ was increased from 0 to 60 at. %. The great decrease in the average lifetime implies the highly efficient trapping of excitons by defects, which transfer the energy to Mn2+ sites subsequently. As for the Mn2+ emission, all the PL lifetimes of EA2PbBr4:Mn2+ samples can be fitted with a single exponential (Figure 4e). The lifetimes are calculated to be ∼0.747 ms, caused by the spin-forbidden transition from 4T1 to 6A1.31 Interestingly, varying the Mn2+ doping concentration does not seem to have a significant influence on the lifetime of the Mn2+ dopant emission. The constant lifetime of Mn2+ emission in our case further indicates the negligible Mn2+−Mn2+ dipole−dipole interaction in the EA2PbBr4 host. Compared
samples, demonstrating the successful replacement of octahedrally coordinated Pb2+ by Mn2+ ions.17,38 EDS mapping (Figure 3c) also indicates the uniform distribution of Mn2+ in EA2PbBr4. Interestingly, the doping efficiency (Table S1) of Mn2+ in 2D EA2PbBr4 is much higher than that of CsPbCl3 NCs, in which only a small amount of the added Mn2+ can be incorporated into the crystal lattice.14 It is believed that the 2D single-layered feature of EA2PbBr4 plays a key role in promoting the substitution process because all the Pb2+ sites can be exposed and exchanged with Mn2+ in the synthesis. The fast substitution is also evidenced by the easy synthesis of Mn2+-doped EA2PbBr4 from simply grinding EABr, PbBr2, and MnBr2 precursors together. As shown in Video S1 provided in the Supporting Information, an intense orange light is immediately observed after placed under UV light, which cannot be realized in 3D CsPbCl3 and MAPbCl3. In the absorption spectra, a sharp absorption edge with an onset at 430 nm (Figure 4a) is observed for all Mn2+ doped samples, attributed to the first excitonic absorption of the host. Correspondingly, a broad (fwhm = 81 nm) and characteristic Mn2+ dopant emission (Figure 4b) at 616 nm can be found after Mn2+ doping, resulting from the Mn2+ d−d emission corresponding to the spin-forbidden 4T1 → 6A1 transition.35,39−42 Unexpectedly, with increasing the input concentration of the Mn2+ dopant from 1 to 60 at. %, the STE emission is completely quenched. As a result, the total PL QY of EA2PbBr4:Mn2+ samples reach up to 78% with 40 at. % Mn2+ nominal doping level, which is higher than that of most reported host systems.23,31,39,43−45 Some representative photographs of EA2PbBr4:Mn2+ samples under room light and UV light are shown in Figure S4, respectively. An intense orange light is observed when the samples are placed under UV light (λex = 365 nm). It has been demonstrated that the formation of STEs from free excitons occurs on ps time scales in 2D LHPs,27 while the transfer time of exciton to Mn2+ is around hundreds of ps.43,46 Compared to the much faster formation rate of STEs, the slower transfer 14242
DOI: 10.1021/acs.jpcc.9b02649 J. Phys. Chem. C 2019, 123, 14239−14245
Article
The Journal of Physical Chemistry C to the PL lifetime (∼1.5 ms) of Mn2+ dopants in the 3D CsPbCl3 system,23 the lifetime of EA2PbBr4:Mn2+ is much shorter, usually attributed to the Mn2+−Mn2+ interaction upon a high doping level.31,35,48 However, this may not be the reason in our case because no significant decrease of the lifetime and redshift of Mn2+ emission were observed with higher Mn2+ doping concentrations.45 The reason for the reduction of the Mn2+ PL lifetime from ∼1.5 ms of the CsPbCl3 system to 0.75 ms of the EA2PbBr4 host could be the heavy atom effect that increases the rate of intersystem crossing, as reflected by the fact that the lifetime of Mn2+ increases from 0.767 to 2.896 ms once Br is progressively replaced by Cl in our case (Figure S5). A recent study demonstrated that the heavy atom effect can enhance the intersystem crossing from the perovskite host to the organic ligand to enhance the room-temperature phosphorescence.49 In addition, the central wavelength of the Mn2+ emission band can be slightly tuned from 616 nm (orange light) to 630 nm (pink light) by progressively substituting Br with Cl (Figure S5a), ascribed to the difference of strength of the ligand filed. The temperature-dependent PL spectrum (Figure 4f) of EA2PbBr4:40%Mn2+ sample enables us to gain deeper insight into the energy transfer mechanism of EA2PbBr4:Mn2+. The PL spectrum at 77 K exhibits a sharp emission peak at 395 nm and a broad emission band with a maximum at 635 nm, arising from the band edge emission of single-layered EA2PbBr4 and Mn2+ d−d transition, respectively. The red shift of Mn2+ dopant emission stems from the enhancement of the ligand field strength because of the contraction of octahedra.44,50 The progressive decrease of Mn2+ emission confirms the thermally activated excitation mechanism of dopants. An activation energy ΔEc is calculated to be 9.8 meV based on the Boltzmann analysis shown in the inset of Figure 4f, which is much smaller than that (∼314 meV) in CsPbCl3 NCs.24 Such a small potential barrier enables the efficient transfer of excitons to Mn2+ and high PL QY at room temperature. 3.3. Enhancing Mn2+ Emission through Codoping. The results shown above suggest that trap states play a key role in energy transfer from the host to the dopant. Therefore, a relatively high density of trap states seems to facilitate energy transfer and thereby enhance Mn2+ emission. Usually, dopant emission is not observed in transition metal ions with the d10 filling configuration. Therefore, Cd2+ is codoped with 5 at. % Mn2+ into EA2PbBr4 spontaneously to further understand the sensitization mechanism. As shown in Figure 5a, the Mn2+ and Cd2+ codoped samples with a homogeneous distribution can be obtained. The ICP-AES results (Table S2) show that varying the doping level of Cd2+ in the range of 0−15% has a limited impact on the content of Mn2+. As expected, a significant enhancement of Mn2+ emission (Figure 5b) was observed upon codoping Cd2+, while further increasing the doping level reduces the intensity. This behavior is also observed for the Zn2+ and Mn2+ codoped samples (Figure S6). The PL QY rises from 41.1 to 62.9% once increasing the doping level of Cd2+ from 0 to 10 at. %, demonstrating the trap-mediated energy transfer mechanism. As for optimized 40 at. % Mn2+ doped EA2PbBr4, the PL QY can be slightly improved when codoping Cd2+ with a low doping level in the range of 1−5 at. % (Figure 5c) because a high density of defects have been created by Mn2+ dopants. However, further increasing the concentration of Cd2+ diminishes the QY of Mn2+ dopant emission, which is ascribed to the deterioration
Figure 5. (a) EDS mapping and (b) PL spectra of Mn2+ and Cd2+ codoped EA2PbBr4 samples, the PL intensity located at 415 nm are normalized. The doping level of Mn2+ is kept at 5 at. % and the content of Cd2+ are varied from 0 to 15 at. %. (c) PL QY of EA2PbBr4:40%Mn2+ with different doping levels of Cd2+.
of the crystal quality of the host and also the competition effect of Cd2+ in the doping reaction.
4. CONCLUSIONS In summary, 2D single-layered EA2PbBr4 with broad STE emission has been prepared via a reprecipitation method. By doping Mn2+ into 2D single-layered EA2PbBr4, the STE emission is completely quenched because of the efficient exciton trapping by the induced shallow defects, resulting in 78% PL QY of Mn2+ dopant emission. The activation energy between trap states and Mn2+ d states is calculated to be ∼9.8 meV, which is much smaller than that in the 3D CsPbCl3 host. Moreover, codoping Zn2+/Cd2+ into EA2PbBr4:Mn2+ also enhance the Mn2+ emission, further supporting the trapmediated energy transfer mechanism. These results indicate the importance of trap states in host to dopant energy transfer in Mn2+ doped 2D LHPs.
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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.9b02649. XRD patterns, absorption and PL spectra, SEM images, and ICP-AES analysis data for the doped EA2PbBr4 samples (PDF) Crystallographic data for 180504g (CIF) Observation of an intense orange light when the samples are placed under UV light (AVI) Datablock for 180504g (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Binbin Luo: 0000-0001-9652-7998 Xiaochun Huang: 0000-0001-5495-7914 14243
DOI: 10.1021/acs.jpcc.9b02649 J. Phys. Chem. C 2019, 123, 14239−14245
Article
The Journal of Physical Chemistry C
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Jin Z. Zhang: 0000-0003-3437-912X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by National Natural Science Foundation of China (NSFC: 51702205, 21571122) and STU Scientific Research Foundation for Talents (NTF17001). We thank Mingyang Li (Sun Yat-Sen University) for the TRPL measurements and Tongtong Xuan (Sun Yat-Sen University) for the PL QY characterization.
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DOI: 10.1021/acs.jpcc.9b02649 J. Phys. Chem. C 2019, 123, 14239−14245
Article
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DOI: 10.1021/acs.jpcc.9b02649 J. Phys. Chem. C 2019, 123, 14239−14245
