Novel lead-free double perovskite phosphors of Mn-doped Cs2KBiCl6 (Cs2KBiCl6:Mn2+) have been facilely synthesized by using typical hydrothermal method. X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and photoluminescence measurements confirm that the synthesized Cs2KBiCl6:Mn2+ phosphors behave double perovskite structure, good morphology, excellent stability, and superior optical properties. An optimal doping concentration of Mn/Bi = 0.4 is achieved in Cs2KBiCl6:Mn2+ phosphors, showing maximum photoluminescence quantum yield of 87.2%, lifetime of 0.98 ms, and orange-red fluorescence with the emission peak at 595 nm under UV light excitation. The probable luminescence mechanism could be ascribed to excitation energy transferring from Cs2KBiCl6 to Mn, and accordingly contributing to the 4T1–6A1 transition of the Mn d electron. Superb optical properties provide much room for in-depth fluorescence researches and potential applications of Cs2KBiCl6:Mn2+ phosphors.
Newly emerged double perovskite phosphors are highly spotlighted due to their potential applications to stylish displays and lighting. The lead-free metal halide double perovskite of A2MIMIIIX6 (A = Cs+; MI = Cu+, Ag+, Na+, K+; MIII = Bi3+, Sb3+, In3+; X = Cl, Br, I) possesses specific 3D electronic structure, good stability, and environment-friendly sustainability, receiving increasing attentions in luminescence researches [1,2,3]. Cs2AgBiX6 (X = Cl, Br) is the first reported fluorescence material with double perovskite structure in 2016 [4,5,6]. Sb- or Mn-doped Cs2NaInCl6 are also good representatives of double perovskite phosphors [3, 7,8,9]. Cs2KBiCl6 shows similar double perovskite structure and could be considered as a promising phosphor. At present, to the best of our knowledge, there are rare reports on Cs2KBiCl6 and its composites. However, it is predictably deduced that pristine Cs2KBiCl6 just like Cs2NaInCl6 usually produces negligible or undetectable fluorescence under UV light excitation due to the parity-forbidden nature [3, 8, 9].
It is well established that the Sb3+ or Mn2+ doping could break such an intractable parity-forbidden transition mechanism, and thus considerably improving the optical performance. For instance, Cs2NaInCl6:Sb3+ [7,8,9], Cs2NaInCl6:Mn2+ , Cs2NaBiCl6:Mn2+ [10, 11], Cs2AgBiX6:Mn2+ (X = Cl, Br) , Cs2AgInCl6:Mn2+ , A2BAlF6:Mn4+ (A = Rb, Cs; B = K, Rb)  and Cs2Na1−xAgxBiCl6:Mn2+  are well-known double perovskite phosphors. In this study, we explore a novel double perovskite phosphor of Cs2KBiCl6 with Mn doping (Cs2KBiCl6:Mn2+). Efficient excitation energy transferring from the host matrix of Cs2KBiCl6 to the dopant of Mn is occurred, contributing to the 4T1–6A1 transition of the Mn d electron, and yielding orange-red fluorescence with the emission peak at 595 nm and maximum photoluminescence quantum yield (PLQY) of 87.2%. Superb optical properties provide much room for in-depth fluorescence researches and potential applications of Cs2KBiCl6:Mn2+ phosphors.
Chemical materials of CsCl (99.99%, Alfa Aesar), KCl (99.99%, Alfa Aesar), BiCl3 (99.99%, Alfa Aesar), MnCl2·4H2O (99%), HCl (37%), and (CH3)2CHOH (99.9%) were commercially purchased and used as raw materials without further purification. Firstly, CsCl, BiCl3, and MnCl2·4H2O were directly dissolved into HCl solution, preparing 1 mol/L CsCl solution, 0.5 mol/L BiCl3 solution, and 1 mol/L MnCl2 solution, respectively. Then, Cs2KBiCl6:Mn2+ powders were synthesized by using a typical hydrothermal method. 0.03727 g KCl powders, 1 mL CsCl solution, 1 mL BiCl3 solution, and x mL MnCl2 solution (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, corresponding to Mn/Bi = 0, 0.2, 0.4, 0.6, 0.8, 1, respectively, in the final products of Cs2KBiCl6:Mn2+) were sequentially added into a stainless-steel autoclave equipped with a Teflon liner. Adding extra HCl solution till the total volume reaches 5 mL. After reaction at 180 °C for 2 h, the precipitates were washed with (CH3)2CHOH and dried at 60 °C for 6 h. The final products of Cs2KBiCl6:Mn2+ powders were obtained.
X-ray diffraction (XRD, Bruker D8 Advance), X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi), and scanning electron microscope (SEM, JEOL JEM-2100F) equipped with energy-dispersive X-ray spectroscopy (EDS) were used for analyzing the crystal phase structure, element distribution mapping, chemical state, and surface morphology. Brooke 300 200 electron paramagnetic resonance (EPR) spectrometer with an X-band microwave frequency of 10 GHz was used for investigating the EPR characteristics. PerkinElmer Lambda 365 UV–vis Spectrophotometer, and Edinburgh Instruments FS5 spectrofluorometer equipped with an integrating sphere were used for measuring UV–vis absorption spectra, PL spectra, PLQYs, and time-resolved PL (TRPL) decay profiles. The PL spectra were measured under 365 nm excitation, while the TRPL profiles were obtained using excitation wavelength of 430 nm and emission wavelength of 590 nm.
Results and discussion
Figure 1 shows the XRD patterns of Cs2KBiCl6:Mn2+ (Mn/Bi = 0, 0.2, 0.4, 0.6, 0.8 and 1). It is observed that the Cs2KBiCl6:Mn2+ remains similar crystal structure as Cs2KBiCl6, although the diffraction peaks around 2θ = 14° slightly shift to high angles after doping Mn. The ionic radius of Mn2+ (0.67 Å) is smaller than those of K+ (1.38 Å) and Bi3+ (1.03 Å) [3, 16,17,18]. The Mn2+ substitution results in lattice shrinkage, which accounts for the high-angle shifting of diffraction peaks. The radius of Mn2+ is largely deviated from Cs+ (1.67 Å) , it is boldly speculated that the Mn2+ occupies Bi3+ site or K+ site and forms a double perovskite architecture. The diffraction peak at 2θ = 14.4° shifts to higher angle in Mn/Bi = 0.4 sample as compared with the Mn/Bi = 0.2 sample. It indicates that more Mn2+ substitution results in more excitation energy transferring from Cs2KBiCl6 matrix to the doped Mn2+ and enhances PLQYs. There is no observable high-angle shifting with further increasing Mn2+ concentrations (Mn/Bi = 0.6, 0.8 and 1), suggesting that excessive Mn2+ doping attenuates PL intensity as a result of concentration-induced quenching effect.
The SEM images of Cs2KBiCl6 and Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) are shown in Fig. S1 in Supporting Information. It is observed that both show micron size and similar appearance with certain flake shape. More flaky Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) maybe caused by the hygroscopicity and deliquesce of MnCl2·4H2O. The EDS mappings (Fig. S2 in Supporting Information) show the elements distribution of Cs, K, Bi, Cl, and Mn of Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4). It is obvious that the components are uniformly distributed and the Mn2+ is fully doped into the host matrix of Cs2KBiCl6.
The XPS core level spectra of Cs2KBiCl6 and Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) are shown in Fig. S3 in Supporting Information. The element signals of Cs, K, Bi, Cl and Mn are distinctly detected. The binding energies of Mn 2p are positioned at 641.4 eV and 654.3 eV, corresponding to Mn 2p3/2 and Mn 2p1/2, respectively. As compared with pristine Cs2KBiCl6, the signals of K 2p species in Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) are considerably weakened, may arising from the substitution of Mn2+ and/or the alteration of surface states such as carbon absorbing.
The EPR spectrum of Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) is explored and shown in Fig. 2. A sixfold hyperfine coupling pattern is observed, showing a hyperfine constant A of 90.9 G and a characteristic g-factor of 2.047 ± 0.001. Such specific characteristics are derived from the isotropic hyperfine coupling of Mn2+ electron spin state and nuclear spin . The EPR spectrum also suggests that Mn2+ ions have been successfully doped into Cs2KBiCl6 matrix.
The UV–vis absorption spectra of Cs2KBiCl6:Mn2+ (Mn/Bi = 0, 0.2, 0.4, 0.6, 0.8 and 1) are depicted in Fig. S4 in Supporting Information. The pristine Cs2KBiCl6 shows an absorption peak at about 390 nm (Fig. S4a), owing to the intrinsic self-trapped exciton (STE) absorbance of double perovskite materials. There is no visible emission under UV excitation for pristine Cs2KBiCl6. With Mn2+ doping, some absorption peaks in the visible region (particularly 400–600 nm) are observed (Figs. S4b–f). These newly appeared absorption peaks are originated from the 4T1–6A1 transition absorption of Mn2+. It indicates that strong lattice vibration reduces the STE emission of Cs2KBiCl6, and accordingly transferring the excitation energies to Mn2+ as well as producing robust (stronger than STE) d–d transition emission of Mn2+. In addition, the visible emission peaks position of Cs2KBiCl6:Mn2+ (Mn/Bi = 0.2, 0.4, 0.6, 0.8 and 1) does not change with the variation of Mn concentration (Figs. S4b–f), which also reflects that the fluorescence shows invariant emission peak for Cs2KBiCl6:Mn2+ phosphors.
The fluorescence characteristics of Cs2KBiCl6:Mn2+ phosphors are shown in Fig. 3. Considerable PL with emission peak at 595 nm is observed under 365 nm excitation. Moreover, the PL intensity is gradually increased with increasing Mn concentration and reaches maximum when Mn/Bi = 0.4 in Cs2KBiCl6:Mn2+. Further increasing Mn concentration (Mn/Bi = 0.6, 0.8 and 1) attenuates PL intensity. Excessive Mn doping would increase non-radiative center and cause concentration-induced quenching. The measured PLQYs of Cs2KBiCl6:Mn2+ (Mn/Bi = 0.2, 0.4, 0.6, 0.8 and 1) are shown in Fig. S5 in Supporting Information. The sample with Mn/Bi = 0.4 behaves the highest value of 87.2%, indicating superb optical properties and providing much room for fluorescence researches and potential applications. The insets in Fig. 3 show the Cs2KBiCl6 image under visible irradiation, since there is no observable fluorescence under UV light excitation. While the Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) image appears distinct orange-red under 365 nm excitation. The probable emission mechanism in Cs2KBiCl6:Mn2+ could be ascribed to the excitation energy transferring from the Cs2KBiCl6 matrix to the dopant of Mn2+, which is similar to the previously reported Mn-doped double perovskite phosphors [3, 11, 19]. The 4T1–6A1 transition from Mn d electron contributes to the orange-red emission of Cs2KBiCl6:Mn2+ under UV light excitation.
Figure 4 shows the TRPL decay profiles of Cs2KBiCl6 and Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4). The lifetime (τav) are calculated from the fitted curves using double-exponential function of Eq. (1).
Here Ai and τi are the weight coefficient and radiation time, respectively. The fitted results indicate that pristine Cs2KBiCl6 shows short lifetime of only 0.54 μs, while Cs2KBiCl6:Mn2+ behaves typical millisecond (ms) or sub-ms lifetime of 0.98 ms. The ms lifetime also suggests low concentration of trap states on surface or inside of the Cs2KBiCl6:Mn2+, contributing to larger carrier diffusion length and higher PL intensity. Moreover, it is rationally deduced that the STE emission could be nearly excluded in Cs2KBiCl6:Mn2+ phosphors, and efficient energy transferring from Cs2KBiCl6 to Mn2+ is occurred, since the lifetime of STE emission is usually within the μs range.
The stability of Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) is investigated via investigating the XRD patterns of fresh Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) and placing for 6 months in air conditions (Fig. S6 in Supporting Information). The XRD patterns do not show observable difference, suggesting that the crystal phase structure is not changed. It reflects good stability and advances potential applications of Cs2KBiCl6:Mn2+ phosphors.
Cs2KBiCl6:Mn2+ phosphors have been facilely synthesized by using a typical hydrothermal method. XRD, SEM, XPS and EPR measurements are taken. The experimental results show that synthesized Cs2KBiCl6:Mn2+ phosphors behave double perovskite structure, good morphology, and superior stability. The Mn is effectively doped into the Cs2KBiCl6 matrix, while the optimal doping concentration is Mn/Bi = 0.4. The PL measurements reveal that Cs2KBiCl6:Mn2+ (Mn/Bi = 0.4) shows maximum PLQY of 87.2%, lifetime of 0.98 ms, and distinct orange-red fluorescence with the emission peak at 595 nm under 365 nm excitation, owing to excitation energy transferring from Cs2KBiCl6 to Mn, and accordingly contributing to the 4T1–6A1 transition of the Mn d electron.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Q. Zhong and S. Yu acknowledge the funding support from Special Construction Fund of Faculty of Engineering (No. 46201503).
The research was supported by the Special Construction Fund of Faculty of Engineering (Grant No. 46201503).
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Zhong, Q., Yu, S., Zheng, Q. et al. Synthesis and luminescence properties of Mn-doped Cs2KBiCl6 double perovskite phosphors. Discover Nano 18, 42 (2023). https://doi.org/10.1186/s11671-023-03820-w
- Double perovskite phosphor
- Mn doping
- Orange-red emission