Synthesis and luminescence properties of Mn-doped Cs₂KBiCl₆ double perovskite phosphors

Abstract

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.

Introduction

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 A₂M^IM^IIIX₆ (A = Cs⁺; M^I = Cu⁺, Ag⁺, Na⁺, K⁺; M^III = 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⁺ [3], Cs₂NaBiCl₆:Mn²⁺ [10, 11], Cs₂AgBiX₆:Mn²⁺ (X = Cl, Br) [12], Cs₂AgInCl₆:Mn²⁺ [13], A₂BAlF₆:Mn⁴⁺ (A = Rb, Cs; B = K, Rb) [14] and Cs₂Na_1−xAg_xBiCl₆:Mn²⁺ [15] 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.

Experimental details

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) [18], 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.

Fig. 1

XRD patterns of Cs₂KBiCl₆:Mn²⁺ (Mn/Bi = 0, 0.2, 0.4, 0.6, 0.8 and 1) powders

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 2p_3/2 and Mn 2p_1/2, 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 [19]. The EPR spectrum also suggests that Mn²⁺ ions have been successfully doped into Cs₂KBiCl₆ matrix.

Fig. 2

EPR spectrum of Cs₂KBiCl₆:Mn.²⁺ (Mn/Bi = 0.4)

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.

Fig. 3

PL intensity of Cs₂KBiCl₆:Mn²⁺ (Mn/Bi = 0.2, 0.4, 0.6, 0.8 and 1). Inset shows the images of pristine Cs₂KBiCl₆ (left) under visible irradiation and Cs₂KBiCl₆:Mn²⁺ (Mn/Bi = 0.4, right) under 365 nm excitation

Figure 4 shows the TRPL decay profiles of Cs₂KBiCl₆ and Cs₂KBiCl₆:Mn²⁺ (Mn/Bi = 0.4). The lifetime (τ_av) are calculated from the fitted curves using double-exponential function of Eq. (1).

$$\tau_{{{\text{av}}}} = {{\mathop \sum \limits_{i} A_{i} \tau_{i}^{2} } \mathord{\left/ {\vphantom {{\mathop \sum \limits_{i} A_{i} \tau_{i}^{2} } {\mathop \sum \limits_{i} A_{i} \tau_{i} }}} \right. \kern-0pt} {\mathop \sum \limits_{i} A_{i} \tau_{i} }}$$

(1)

Here A_i and τ_i 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.

Fig. 4

TRPL decay curves of a Cs₂KBiCl₆ and b Cs₂KBiCl₆:Mn.²⁺ (Mn/Bi = 0.4)

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.

Conclusions

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.