Admirable stability achieved by ns² ions Co-doping for all-inorganic metal halides towards optical anti-counterfeiting

Abstract

Optical materials play a momentous role in anti-counterfeiting field, such as authentication, currency and security. The development of tunable optical properties and optical responses to a range of external stimuli is quite imperative for the growing demand of optical anti-counterfeiting technology. Metal halide perovskites have attracted much attention of researchers due to their excellent optical properties. In addition, co-doping methods have been gradually applied to the research of metal halide perovskites, by which more abundant luminescence phenomena can be introduced into the host perovskite. Herein, the ns² ions of bismuth (Bi³⁺) and antimony (Sb³⁺) ions co-doped zero-dimensional Cs₂SnCl₆ metal halide with an excitation-wavelength-dependent emission phenomenon is synthesized as an efficient multimodal luminescent material, the luminescence of which is tunable and covers a wide region of color. What's more, a dynamic dual-emission phenomenon is captured when the excitation wavelength changes from 320 nm to 420 nm for Cs₂SnCl₆:Bi_0.08Sb_0.12 crystals. Moreover, the Bi³⁺ and Sb³⁺ doped metal halide material shows great enhancement in solvent resistance and thermal stability compared to the pristine Cs₂SnCl₆. The admirable stability and distinguishable photoluminescence (PL) phenomenon of this all-inorganic metal halide has great potential to be applied in optical anti-counterfeiting technology. Furthermore, the co-doping method can accelerate the discovery of new luminescence phenomena in original metal halide perovskites.

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Introduction

Nowadays, counterfeit products have penetrated into plenty of industries, such as digital, food, clothing and so on, which is a widespread and disturbing phenomenon in our daily life.¹ Optical materials offer the advantages of visibility, high throughput, and facile design and are widely applied in the anti-counterfeiting field.^2–6 However, in traditional fluorescent anti-counterfeiting techniques, monochromatic fluorescent phosphors are generally used.^7,8 This is still unsafe because monochromatic fluorescence can be easily achieved by other fluorescent materials. Therefore, the development of polychromatic fluorescence anti-counterfeiting technology is urgent to meet our practical demands. To solve this problem, organic and rare earth materials with abundant fluorescence are applied into anti-counterfeiting field to achieve excitation-wavelength-dependent fluorescent colors.^4,9–11 But the poor stability of organic material and the lack of rare earth material might limit their further development. Therefore, single component and tunable fluorescent material with high stability are in great demand in the practical application.¹²

Due to excellent optical properties, metal halide perovskite has attracted much attention of researchers.^13–15 Among them, lead halide perovskite owns particularly outstanding performance. However, the toxicity and instability of lead perovskite restrict its development steps.^16,17 As well as looking for lead replacements, the researchers also altered the optical properties by reducing the dimension of perovskite.^18–21 For the fact that lower dimensionality seems more probable to lead octahedral distortion and STEs formation, attempts on lower dimensional metal halides is increasing rapidly in this era of intense scientific research, especially for 0 dimensional metal halides.^22–25

Cs₂SnX₆ (X = Cl, Br, I) nanocrystals have been investigated for its tunable light emission and photocatalytic activity.^26–31 What's more, mixed halide Cs₂SnCl_6−xBr_x single crystals have been successfully applied into narrowband photodetection.³² Doping methods have also been applied to the research of Cs₂SnCl₆ crystals/nanocrystals. Tang'group reported bismuth-doped Cs₂SnCl₆ crystals as blue emissive phosphors.³³ Later, Xia'group reported antimony-doped Cs₂SnCl₆ nanocrystals which realized broad-band emission.³⁴ Liu'group realized Bi/Sb doped Cs₂SnCl₆ separately and tune visible emission of Cs₂SnCl₆:Bi and Cs₂SnCl₆:Sb phosphors by changing post-annealing temperature.³⁵ This development opens up new opportunities for these materials as candidates for solid state lighting. On the other hand, the method of multi-ion co-doping has been gradually applied to the research of metal halide perovskite, by which more abundant luminescence phenomenon can be introduced into the host perovskite.³⁶ Xu's group synthesized Mn²⁺ and Sb³⁺ co-doped Cs₂NaInCl₆ nanocrystals, displaying adjustable and continuous luminescence.³⁷ Chen's group achieved dual-band-tunable white-light emission in Cs₂SnCl₆ by co-doping Bi³⁺ and Te⁴⁺.³⁸ The Bi³⁺ and Sb³⁺ codoped Cs₂SnCl₆ system has also been reported.³⁹ However, the dynamic dual emission phenomenon of the co-doping system and its related application prospects remain to be explored. Herein, we synthesized a series of lead-free zero-dimensional Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16, 0.2) metal halides crystals by doping Bi³⁺ and Sb³⁺ simultaneously, from which there is an interesting excitation-wavelength-dependent phenomenon. The PL intensity will change not only by the dopant content, but also by the different excitation wavelength. The impurity ions doped metal halide material shows an enhancement in PLQY about 40% compared to the pristine Cs₂SnCl₆ where there is no PL phenomenon being observed under the UV light. The UV-vis spectrum, X-ray photoelectron spectroscopy (XPS) and Raman spectrum were applied to investigate the effect of Bi and Sb co-doping on properties of Cs₂SnCl₆ metal halide crystals. The variable excitation wavelength PL spectrum and the time-resolved photoluminescence (TRPL) spectrum are used to study the dual-emission phenomenon. With excellent stability against air, high temperature and organic solvents, the Cs₂SnCl₆:Bi_0.2−xSb_x metal halide material has great potential to become anti-counterfeiting material.

Results and discussion

The zero-dimensional Cs₂SnCl₆ metal halide crystals are almost transparent under ambient, which are crystallized in the space group cubic Fm3̄m. The tin cation(IV) is coordinated with six chloride anions, constructing a [SnCl₆]²⁻ octahedron. Each [SnCl₆]²⁻ octahedron is separated by Cs⁺, which owns large radius, forming a typical zero-dimensional structure (Fig. 1a). The powder X-ray diffraction (PXRD) in Fig. 1b shows the Cs₂SnCl₆:Bi_0.2−xSb_x crystals can well correspond to pure Cs₂SnCl₆ phase, indicating that the Bi³⁺ and Sb³⁺ co-doping has little influence in the host crystal structure.

Fig. 1 (a) The diagram of Bi³⁺/Sb³⁺ co-doping in zero-dimensional Cs₂SnCl₆ metal halides (Sn cyan, Cs dark yellow, Cl green, Bi light blue, Sb light orange). Full range (b) and selected range (c) of the PXRD pattern of measured data for Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0.04, 0.08, 0.12, 0.16, 0.2) sample. (d) The SEM image and EDS mapping of Cs₂SnCl₆:Bi_0.08Sb_0.12.

In Fig. 1c, The PXRD peaks of impurity doping sample exhibit varying degrees shift compared to the pristine sample, which suggest that the Bi³⁺ and Sb³⁺ cations are incorporated into the host crystal lattice successfully. For the fact that the radius of Bi³⁺ (1.03 Å) and Sb³⁺ (0.90 Å) are bigger than the radius of Sn⁴⁺ (0.71 Å), the peak of PXRD shifts to a lower angle when impurity doping due to the lattice expansion.^40–42 The field emission scanning electron microscope (FESEM) image (Fig. 1d) shows that the crystals possess excellent crystal morphology, which shows the high crystallinity. The energy dispersive X-ray spectrometry (EDS) mapping was performed to verify the composition of the Cs₂SnCl₆:Bi_0.08Sb_0.12 sample by collecting Cs, Sn, Cl, Bi, and Sb signatures. As shown in Fig. 1d, these five elements are evenly distributed over grains and perfectly covered each other, demonstrating the uniform composition of the crystals.⁴³ In order to understand the actual doping amount of impurity ions, we carry out ICP-MS test on the codoped crystals. The result of ICP-MS is showed in Table S1.† From the Table S1,† we can find that the Bi³⁺ and Sb³⁺ in precursor were only partially incorporated into the products. But for the sake of expression, we still use the original formula.

In Fig. 2, there are the pictures of pristine Cs₂SnCl₆ and Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16, 0.2) crystals irradiated by a UV lamp at 365 nm and 395 nm. The PL of Cs₂SnCl₆:Bi_0.2−xSb_x changes between blue, purple and red. Fig. 3 shows the PLE and PL spectrum of different co-doping Bi³⁺/Sb³⁺ content of Cs₂SnCl₆ at the excitation wavelength of 365 nm and 395 nm. The pristine Cs₂SnCl₆ sample shows negligible emission peak when excited, which is centered at 454 nm. The PL and PLE spectra of pristine Cs₂SnCl₆ are showed in Fig. S1.†Fortunately, for the Cs₂SnCl₆:Bi_0.2 sample, there is a strong peak centered at 454 nm. What's more, as the Sb³⁺ content increased in Cs₂SnCl₆:Bi_0.2−xSb_x, the intensity of peak centered at 666 nm enhanced and peak centered at 454 nm attenuated gradually. When the Sb³⁺ reached the maximum ratio of Cs₂SnCl₆:Sb_0.2, the peak of 454 nm reached to the minina. In the meanwhile, the emission peak of 666 nm reached the maximum intensity. The PL spectrum of different co-doping Bi³⁺/Sb³⁺ content of Cs₂SnCl₆ exhibited similar trend but different PL intensity when excited by the excitation light of 395 nm. The Commission Internationale de l'Eclairage (CIE) coordinate at (0.1419, 0.0728), (0.1503, 0.0847), (0.1586, 0.0895), (0.1717, 0.0966), (0.1939, 0.1139), (0.5757, 0.3792) when excited at 365 nm and (0.1416, 0.0729), (0.1580, 0.0843), (0.2758, 0.1798), (0.3434, 0.2200), (0.4107, 0.2613), (0.6093, 0.3848) when excited at 395 nm for Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16, 0.2), which are marked in the CIE chromaticity diagram (Fig. S2†). The marked points in Fig. S2b† are relatively evenly distributed compared to Fig. S2a,† which is consistent with the PL phenomenon. Inspired by the interesting PL phenomenon, Cs₂SnCl₆:Bi_0.08Sb_0.12, which has the distinct difference of PL phenomenon (Fig. 2), was characterized by variable excitation wavelength PL spectrum.

Fig. 2 Photograph of pristine Cs₂SnCl₆ and Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0.04, 0.08, 0.12, 0.16, 0.2) crystals irradiated by a UV lamp at 365 nm and 395 nm.

Fig. 3 (a–f) PLE and PL spectrum of Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16, 0.2). Note: Ex1 = 365 nm, Ex2 = 395 nm.

Fig. 4a shows the different patterns composed by Cs₂SnCl₆:Bi_0.08Sb_0.12 powder at natural light and irradiated by a UV lamp at 365 nm and 395 nm. When the Cs₂SnCl₆:Bi_0.08Sb_0.12 crystals are ground into a powder, it can be processed into any shape. The pattern shows blue and bright red patterns when irradiated by 365 and 395 nm UV light, respectively. What's more, Fig. S3† shows the school emblem printed by the anti-counterfeiting ink, which is composed of Cs₂SnCl₆:Bi_0.08Sb_0.12 powder, dispersant and solvents. The photos were taken in natural light and irradiated by a UV lamp at 365 nm and 395 nm. The pattern shows blue and bright red patterns when irradiated by 365 and 395 nm UV light, respectively. This demonstrated the Cs₂SnCl₆:Bi_0.08Sb_0.12 crystals are excellent anti-counterfeiting material, which can be applied into anti-counterfeiting technology. In Fig. 4b, there is an excitation-wavelength-dependent dual-emission phenomenon for the Cs₂SnCl₆:Bi_0.08Sb_0.12 sample when the excitation wavelength varied from 320 to 420 nm. From 320 to 350 nm, there is only one peak centered at 454 nm, the intensity of which enhanced gradually as the excitation wavelength increased. At the same time, the peak centered at 666 nm appeared when the excitation wavelength increased to 350 nm. When the Ex = 360 nm, the intensity of the emission peak at 454 nm reached its maximum. With the excitation wavelength increasing from 360 to 390 nm, the intensity of peak at 454 nm attenuated and the peak at 666 nm enhanced inch by inch. When the excitation wavelength varied between 400 to 420 nm, both of the two peaks attenuated simultaneously. The CIE chromaticity coordinates of Cs₂SnCl₆:Bi_0.08Sb_0.12 at different excited wavelength from 320 nm to 420 nm showed the obvious excitation-dependent photoluminescence phenomenon (Fig. 4c). The photoluminescence of the co-doped material covers a wide color gamut, which is hard to be copied. The optical material is not easy to replicate because of its unique photoluminescence properties. This intriguing optical property enable us to apply this material into optical anti-counterfeiting applications.

Fig. 4 (a) Photograph of Cs₂SnCl₆:Bi_0.08Sb_0.12 at natural light and irradiated by a UV lamp at 365 nm and 395 nm. (b) PL spectrum of variable excitation wavelength for Cs₂SnCl₆:Bi_0.08Sb_0.12 sample. (c) The CIE chromaticity coordinates of Cs₂SnCl₆:Bi_0.08Sb_0.12 at different excited wavelength from 320 nm to 420 nm.

Excellent PL and chemical stability is fundamental to optical anti-counterfeiting materials. Herein, the thermogravimetric analysis (TGA) was applied for pristine Cs₂SnCl₆ and Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16, 0.2). From the Fig. 5a we can find that the pristine Cs₂SnCl₆ crystals would dehydrate in the beginning. Then, it became to decompose at 300 °C. With the temperature going on constantly, the crystals began to decompose into SnCl₄ (Fig. S4†).⁴⁴ Encouragingly, there is no obvious decomposition observed until 450 °C in the thermogravimetric analysis (TGA) of the Cs₂SnCl₆:Bi_0.2−xSb_x crystals, which indicated the dopants enhanced the thermal stability of the pristine Cs₂SnCl₆ crystals. When the Cs₂SnCl₆:Bi_0.08Sb_0.12 crystals were soaked in DMF, DMSO, CH₃OH, CH₃CH₂OH and CH₃CN for a week, there is no distinct difference between the crystals soaked in different solvent and the pristine one in PL intensity (Fig. 5b and c). In addition, the PXRD for crystals soaked in different solvents shows similar pattern, which demonstrate the high chemical stability for the crystals (Fig. 5d). The excellent thermal and PL stability demonstrate it has great potential to be applied into the optical anti-counterfeiting application.

Fig. 5 (a) The TGA curves of pristine Cs₂SnCl₆ and Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16, 0.2) metal halides crystals. PL spectrum excited by 365 nm (b) and 395 nm (c) of pristine Cs₂SnCl₆:Bi_0.08Sb_0.12 and Cs₂SnCl₆:Bi_0.08Sb_0.12 after being soked in different solvent for a week. (d) PXRD pattern of pristine Cs₂SnCl₆:Bi_0.08Sb_0.12 and Cs₂SnCl₆:Bi_0.08Sb_0.12 after being soked in different solvent for a week. (e) UV-visible absorption spectrum (f) Raman spectrum of Cs₂SnCl₆, Cs₂SnCl₆:Bi_0.2, Cs₂SnCl₆:Bi_0.08Sb_0.12 and Cs₂SnCl₆:Sb_0.2 sample. (g) TRPL spectra of Cs₂SnCl₆:Bi_0.2−xSb_x sample at 454 nm (x = 0, 0.04, 0.08, 0.12, 0.16). (h) TRPL spectra of Cs₂SnCl₆:Bi_0.2−xSb_x sample at 666 nm (x = 0.04, 0.08, 0.12, 0.16, 0.2).

To further reveal the reasons for the differences in crystal properties after doping, we also did some other characterization. The UV-vis spectrum curves are shown in Fig. 5e. New absorption peaks appear at 340 nm and 313 nm in Cs₂SnCl₆:Bi_0.2 and Cs₂SnCl₆:Sb_0.2, which can be ascribed to the impurity doping of Bi³⁺ and Sb³⁺, respectively.

For the fact that Raman spectroscopy can directly reveal the phonon coupling to the electronic transition between the excited and ground states, it was further employed to shed light on the exciton–phonon interactions.⁴⁵ The laser of 532 nm was used as the excitation light to eliminate the interference from PL. In Fig. 5f, four samples (the pristine Cs₂SnCl₆, Cs₂SnCl₆:Bi_0.2, Cs₂SnCl₆:Bi_0.08Sb_0.12 and Cs₂SnCl₆:Sb_0.2) were tested. All of them exhibit three intense bands at 168, 232 and 309 cm⁻¹ that are assigned to symmetric stretching (A_1g), asymmetric stretching (E_g) and 2A_1g vibrational modes in metal halide octahedron.^46–48 When Bi³⁺ was doped, the peak at 280 cm⁻¹ which ascribed to the A_1g stretching appeared.⁴⁹ And the peak at 254 cm⁻¹ ascribed to A_1g stretching as well appeared once Sb³⁺ was doped.⁴⁹ As the content of Sb³⁺ increased, the intensity of peaks at 168 and 232 cm⁻¹ decreased, indicating that the vibrational modes of metal halide octahedron have substantially weaker coupling strength. The vibrational overlap between the excited and ground-state vibrational wavefunctions was further decreased. As a result, the nonradiative processes were suppressed, which may account for the higher PLQY for Cs₂SnCl₆:Bi_0.2 than Cs₂SnCl₆:Sb_0.2.⁵⁰

The Fig. 5g and h show the TRPL spectra of Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0, 0.04, 0.08, 0.12, 0.16) at 454 nm and Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0.04, 0.08, 0.12, 0.16, 0.2) at 666 nm, respectively. The peak centered at 454 nm has monoexponential PL decay while the peak centered at 666 nm has double-exponential decay.⁵¹ The data of lifetime for these two peaks at different content of impurity doping are given in Table S2.† From the Fig. 5g, h and Table S2,† we find that both of the lifetime of radiation recombination for peaks at 454 nm and the peak at 666 nm increased with more component content of Bi³⁺ and less component of Sb³⁺. The highest PLQY above these crystals is Cs₂SnCl₆:Bi_0.2, which is up to 40.91%. And the PLQY of Cs₂SnCl₆:Sb_0.2 is 8.76%.

In Fig. 6a and b, the temperature-dependent PL spectrum of Cs₂SnCl₆:Bi_0.2 and Cs₂SnCl₆:Sb_0.2 displays that the PL intensities increase as the temperature drops from 280 K to 140 K, while the signal of emission peak for both two materials is continuously enhanced. This phenomenon reflects the non-radiative recombination process is suppressed in the low temperature region.⁵² The temperature-dependent integrated PL intensity was shown in Fig. S5.† The exciton binding energy (E_b) can be calculated by the following Arrhenius eqn (1).

where k_B is Boltzmann constant, T temperature, I₀ PL integrated intensity at 0 K, I PL integrated intensity.⁵³ Conductive to radiative recombination, the E_b value of Cs₂SnCl₆:Bi_0.2 and Cs₂SnCl₆:Sb_0.2 is calculated as 133 and 104 meV, respectively through the fitting curves of integrated PL intensity and temperature, which is much higher than the traditional 3D perovskites such as MAPbI₃ and CsSnI₃. The high value of E_b shows excitons in Cs₂SnCl₆:Bi_0.2 and Cs₂SnCl₆:Sb_0.2 are typical Frenkel excitons.⁵⁴ The resulting highly localized excitons with stronger binding reduce the probability for carrier scattering, which further suppressed the non-radiative recombination significantly.⁵⁵

Fig. 6 Temperature-dependent PL spectrum of Cs₂SnCl₆:Bi_0.2 (a) Cs₂SnCl₆:Sb_0.2 (b), Cs₂SnCl₆:Bi_0.08Sb_0.12 at 365 nm (c) and 395 nm (d).

According to the previous report, the photoluminescence of Cs₂SnCl₆:Bi_0.2 can be ascribed to the [Bi_Sn + V_Cl] defect complex.⁵⁶ The photoluminescence of Cs₂SnCl₆:Sb_0.2 is originted from triplet self-trapped excitons, attributed to the ³P_n–¹S₀ transitions (n = 0, 1, 2).³⁴ Therefore, we propose the energy transfer (ET) process between dopants as shown in Fig. S6.† Besides, from the Table S2,† we can find that the lifetime of Cs₂SnCl₆:Bi_0.2−xSb_x (x = 0.04, 0.08, 0.12, 0.16, 0.2) at 666 nm becomes longer as the content of Bi³⁺ increases (Fig. S7†). Combined with the temperature-dependent photoluminescence phenomenon in Fig. 6d where the intensity of peak at 454 nm enhanced and then attenuated from 300 K to 140 K, it is speculated that there is an energy transfer process between [Bi_Sn + V_Cl] and STEs formed by Sb³⁺.

In order to further confirm the existence of Bi³⁺ and Sb³⁺ ions, XPS were also applied. The full range XPS spectrum for the two samples mentioned above was showed in Fig. S8a.† The Fig. S8b–f† showed the high-resolution XPS spectrum of Cs 3d, Sn 3d, Cl 2p and Sb 3d for pristine Cs₂SnCl₆ and Cs₂SnCl₆:Bi_0.08Sb_0.12 sample. As shown in Fig. S8c,† the Sn 3d spectra of pristine Cs₂SnCl₆ sample has two signature peaks of Sn 3d_3/2 and Sn 3d_5/2 at 495.6 and 487.1 eV.⁵⁷ In addition, the Cl 2p spectrum of the pristine Cs₂SnCl₆ sample showed main peaks of Cl 2p_1/2 and Cl 2p_3/2 at 199.9 and 198.3 eV (Fig. S8d†).⁵⁸ The Cs 3d spectrum showed two main peak of Cs 3d_3/2 and Cs 3d_5/2 at 738.7 and 724.6 eV (Fig. S8b†). However, when the Bi³⁺ and Sb³⁺ were induced to the Cs₂SnCl₆ crystals, the Bi 4f spectrum showed two main peaks of Bi 4f_5/2 and 4f_7/2 at 164.2 eV and 159.2 eV, and the Sb 3d spectrum showed two main peaks of Sb 3d_3/2 and Sb 3d_5/2 at 539.5 and 531.9 eV, respectively.³⁴ A slight O 1s signal is inevitably detected due to oxygen and moisture in the air.⁵⁹ What's more, the binding energy of Sn⁴⁺ 3d_3/2 and Sn⁴⁺ 3d_5/2, Cl 2p_1/2 and Cl 2p_3/2 move to lower energy while the binding energy of Cs 3d_3/2 and Cs 3d_5/2 remain unchanged. These results indicate that some Bi³⁺ and Sb³⁺ ions have entered into the lattice of the Cs₂SnCl₆ host and most probably substituted on the Sn⁴⁺ sites. Moreover, with the addition of Bi³⁺ and Sb³⁺ ions, there is a partial decreasement in the interaction between extranuclear and core electrons in both neighboring Sn⁴⁺ and Cl⁻, accounting for the peak shift to lower energy in Cs₂SnCl₆:Bi_0.08Sb_0.12 sample.⁶⁰

Experimental

Materials

Cesium chloride (AR), stannous(II) mono-sulphate (AR), bismuth(III) chloride (AR) and antimony(III) chloride purchased from Sigma Aldrich. Hydrochloric acid was purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents and solvents were used without further purification.

Synthesis

Cs₂SnCl₆. A mixture of cesium chloride (0.063 g, 0.375 mmol) and stannous mono-sulphate (0.020 g, 0.094 mmol) were first dissolved in Teflon autoclave with 10 mL hydrochloric acid. Then the solution was heated at 90 °C for 1 h in a stainless steel Parr autoclave. The solution was then slowly cooled to 30 °C with a speed of 2 °C h⁻¹. The as-synthesized crystals were then filtered out and washed with isopropanol and dried in a furnace overnight at 30 °C.

Cs₂SnCl₆:Bi_xSb_0.2−x. A mixture of cesium chloride (0.063 g, 0.375 mmol), stannous mono-sulphate (0.020 g, 0.094 mmol) and 0.0188 mmol impurity of bismuth chloride and antimony chloride were first dissolved in Teflon autoclave with 10 mL hydrochloric acid. Then the solution was heated at 90 °C for 1 h in a stainless steel Parr autoclave. The solution was then slowly cooled to 30 °C with a speed of 2 °C h⁻¹. The as-synthesized crystals were then filtered out and washed with isopropanol and dried in a furnace overnight at 30 °C.

Cs₂SnCl₆:Bi_0.08Sb_0.12. A mixture of cesium chloride (0.063 g, 0.375 mmol), stannous mono-sulphate (0.020 g, 0.094 mmol), bismuth chloride (0.0024 g, 0.00752 mmol) and antimony chloride (0.0026 g, 0.01128 mmol) were first dissolved in Teflon autoclave with 10 mL hydrochloric acid. Then the solution was heated at 90 °C for 1 h in a stainless steel Parr autoclave. The solution was then slowly cooled to 30 °C with a speed of 2 °C h⁻¹. The as-synthesized crystals were then filtered out and washed with isopropanol and dried in a furnace overnight at 30 °C.

Anti-counterfeiting ink. The ethylene glycol, diethylene glycol monomethyl ether, and propylene glycol methyl ether were mixed in a weight ratio of 3 : 5 : 2 as the ink solvent. Then, the Cs₂SnCl₆:Bi_0.08Sb_0.12 was put into the ink solvent, and 1 wt% of dispersant BYK110 was added according to the weight of the powder. Put the mixture of ink solvent, powder and dispersant in a ball mill tank, and use a planetary ball mill to mix and disperse at a speed of 400 rpm for 5 h. The prepared mixture is filtered through a 1 micron filter membrane to prepare an inkjet printable ink.

The piezoelectric inkjet printing process (BroadTeko DP800 inkjet Printer) was used. The school emblem and the square array were successfully printed at a resolution of 1200 × 1200.

Characterization

Powder X-ray diffraction (PXRD) patterns were tested on a Rigaku D/max-IIIA diffractometer with Cu Kα (1.54 Å) radiation at 293 K. Steady-state PL spectrums were recorded by Edinburgh Instruments FLS 980 at room temperature (25 °C) for all samples. Diffuse reflectance spectrums of the microcrystalline powders were recorded on a UV/Vis spectrophotometer (SHIMADZU, UV3600Plus), calibrated by simultaneous measurement of the substance (BaSO₄ powder), and then converted to absorbance using the KubelkaMunk theory. The thickness and roughness of the samples remained consistent during the study with different doping levels. Thermogravimetric Analysis spectrums were measured on NETZSCH TG209F3 with the heating rate of 10 K per minute in N₂ atmosphere. Raman spectrums were obtained on RENISHAW inVia Reflex. The XPS measurements were performed on Thermal Scientific Escalab 250 Xi-UPS. An Al Kα (1486.6 eV) X-ray was used as the excitation source for XPS.

Conclusions

In this work, Bi³⁺ and Sb³⁺ co-doped zero-dimensional Cs₂SnCl₆ metal halide material is synthesized by co-doping method, which exhibit tunable emission covering a wide region of color. The impurities improve the pristine Cs₂SnCl₆ in stability against air, high temperature and organic solvents. By adjusting the relative co-doping content of two ions, we can target the desired PL phenomenon. The PL emission peak centered at 454 and 666 nm in Bi³⁺ and Sb³⁺ co-doped Cs₂SnCl₆ can be ascribed to the [Bi_Sn + V_Cl] defect and STEs. The Cs₂SnCl₆:Bi_0.08Sb_0.12 with excitation-wavelength-dependent emission phenomenon is highly stable and environmental-friendly, which can highly meet our practical demands. This single component and tunable luminescent material with high stability is promising to be applied into optical anti-counterfeiting technology. Additionally, co-doping method can provide a new way for researchers to enrich the luminescence of metal halide perovskite.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by National Natural Science Foundation of China (Grant No. 22005089), Natural Science Foundation of Hubei Province (Grant No. 2020CFB146), Hubei Provincial Department of Education (Grant No. Q20201002), Hubei University, Wuhan, 430062, P. R. China, National College Students’ Innovation and Entrepreneurship Training Programs (Grant No. 202210512007).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra00351e

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