RGB tri-luminescence in organic–inorganic zirconium halide perovskites

Abstract

Materials with two or more fluorescence features under different excitation sources have great potential in optical applications, but luminous materials with three emission characteristics have been largely undeveloped. Here, we report a novel zero-dimensional (0D) organic–inorganic hybrid ((C₂H₅)₄N)₂ZrCl₆ perovskite with multiple emissions. The zirconium-based perovskite exhibits a red emission around 620 nm, a green emission at 527 nm, and a blue emission around 500 nm. The red and green emissions come from self-trapped excitons (STEs) and the d–d transitions of Zr(IV), respectively, which are caused by distortion of the [ZrCl₆]²⁻ octahedra. The blue emission is caused by thermally activated delayed fluorescence (TADF), which is similar to that of Cs₂ZrCl₆. The absolute photoluminescence quantum yield (PLQY) of the red and blue double emission is up to 83% and the PLQY of the green emission is 27%. With different combinations of ((C₂H₅)₄N)₂ZrCl₆ samples, we achieve a variety of applications, including a two-color luminescent anti-counterfeiting device, a white light-emitting diode (WLED) with a color rendering index (CRI) of 95 and information encryption with different excitations. We also synthesize other hybrid zirconium perovskites with tri-luminescence through a similar method. Our work provides a potential set of excitation-dependent luminescent materials and is expected to expand the basic research and practical applications of multi-luminescence materials.

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Introduction

Materials with multi-excitonic emission within a single structure have attracted much attention due to their properties in optoelectronic applications,¹ such as imaging,² multi-excitation anti-counterfeiting³ and white light emitting diodes (WLED).^4–10 For instance, multi-excitonic emission, originating from multiple types of excitons within one phosphor, achieves broadband emission for WLEDs with a high colour rendering index (CRI). Broadband emission through multi-excitonic emission within a single structure can be obtained in two main ways. One approach is to achieve multi-luminescence through structure, which has been applied in various lead-free halides.^11–14 By applying (TBA)₂Cu₂I₄ as a single phosphor, a WLED with a CRI of 89 and low correlated color temperature (CCT) of 4516 K was fabricated.¹¹ The other way is by adding a luminescent dopant to the host.^15,16 In particular, Bi³⁺, Sb³⁺, and Te⁴⁺ can be doped into A₂BX₆ perovskite structures, providing additional luminescence energy levels.^3,17–22 Among such materials, Cs₂ZrCl₆:1.5%Sb has been used in a stable single-component WLED with a record half-lifetime of 2003 h.¹⁹ Co-doped Cs₂HfCl₆:Bi³⁺/Te⁴⁺ microcrystals show warm-white emission and were applied to a WLED with a CRI of 75.4 and CIE colour coordinate of (0.370, 0.393).²³

As a typical group of A₂BX₆ perovskites, Zr-based halide zero-dimensional (0D) perovskites have been researched as both hosts and luminescence centers for lighting applications.^{17–21,24–30} Emission from Zr-based perovskites mainly arises via three pathways. The first is through thermally activated delayed fluorescence (TADF). TADF was observed in Cs₂ZrCl₆, in which the photoluminescence and lifetime exhibited a temperature dependence.²⁵ Typically, Cs₂ZrCl₆ exhibits a broad blue emission centered at 450 nm with a large Stokes shift of up to 200 nm. The large Stokes shift and broadband emission suggest that the photoluminescence (PL) may originate from host self-trapped excitons (STEs) caused by the distortion of the [ZrCl₆]²⁻ octahedron. Thus, the emission is also called STE emission.^25,28–31 As the blue fluorescence exhibit thermal activation delayed nature, we call the broad blue emission through the first pathway TADF to distinguish it. The second pathway is through extrinsic STE emission, also exhibiting a broadband emission, which has been reported in research on Hf-based 0D perovskites.^32–34 Recently, another emission mode was attributed to the spin-allowed d–d transitions of Zr⁴⁺ in the green emission region.³⁵ The d–d transition has only been reported for 0D organic–inorganic hybrid Zr-based perovskites, which is possibly due to the distortion between the [ZrCl₆]²⁻ octahedra, similar to [MnX₆]⁴⁻ complexes.³⁶ Thus, organic–inorganic hybrid Zr-based perovskites with distorted structures are potential multiple-emission materials. Recently, organic–inorganic Zr-based perovskites have been explored as potential materials for X-ray imaging and anticounterfeiting with different dopants.³⁷ However, the multiple emissions in zirconium perovskites are aroused by other dopant ions, such as Sb³⁺,²¹ Bi³⁺ and Te⁴⁺.^17,33 Controllable synthesis and the multi-emission nature of zirconium perovskites still require further research.

Herein, we apply organic cations to synthesize 0D organic–inorganic hybrid ((C₂H₅)₄N)₂ZrCl₆ perovskite samples. The ((C₂H₅)₄N)₂ZrCl₆ samples exhibit tri-luminescence: blue emission around 500 nm and red emission at 620 nm through STE emission and green emission at 527 nm through the d–d transitions of Zr(IV). The ((C₂H₅)₄N)₂ZrCl₆ crystallizes in the monoclinic phase with distorted [ZrCl₆]²⁻ octahedra, leading to STE emission and d–d transitions in the samples. Compared with the inorganic Cs₂ZrCl₆, the organic–inorganic hybrid ((C₂H₅)₄N)₂ZrCl₆ exhibits multiple emissions, which enables different luminescence applications without introducing other rare-earth materials. By controlling the reaction conditions, the emission behaviour of ((C₂H₅)₄N)₂ZrCl₆ can be adjusted. Various applications, such as anti-counterfeiting, WLEDs and information encryption, are achieved through different combinations of the ((C₂H₅)₄N)₂ZrCl₆ samples in this work.

Results and discussion

The ((C₂H₅)₄N)₂ZrCl₆ samples exhibit excitation-dependent multiple emissions, as shown in Fig. 1. The absorption spectrum of ((C₂H₅)₄N)₂ZrCl₆ single crystals exhibits a sharp peak at 261 nm, with shoulders around 310 nm and 380 nm (Fig. 1a), suggesting the existence of multiple excited states. The absorption spectra of ((C₂H₅)₄N)₂ZrCl₆ powder and single-crystal samples show similar peak positions, but the relative intensity of each peak is different. Compared with the overall excitation in Fig. S1† and Fig. 1c, the overall excitation spectra and absorption spectra both exhibit a strong peak at 261 nm. The excitation at 261 nm is mainly linked to the blue emission at 500 nm, meaning that the absorption at 261 nm can effectively excite the blue emission part. Thus, the absorption at 261 nm is related to the blue emission part. Similarly, the second shoulder around 310 nm is related to both the green and red emission parts. The third shoulder around 380 nm is related to the red emission part.

Fig. 1 Optical properties of ((C₂H₅)₄N)₂ZrCl₆ samples. (a) Absorption spectra of ((C₂H₅)₄N)₂ZrCl₆ single crystal, blue powder, red powder and green powder. (b) Photographs, (c) photoluminescence (PL) emission spectra and photoluminescence excitation (PLE) spectra and (d) time-resolved photoluminescence spectra of ((C₂H₅)₄N)₂ZrCl₆ powder samples. Samples only exhibiting obvious blue emission are defined as blue powder; samples exhibiting white and orange-red emission are defined as red powder; samples exhibiting blue and green emission are defined as green powder.

The emission behaviour of the ((C₂H₅)₄N)₂ZrCl₆ powders can be adjusted through the reaction conditions. Here, we define the main three groups of powder samples in Fig. 1b and Fig. S2:†: a sample with blue emission under a 254 nm lamp and almost no emission under a 365 nm lamp is defined as blue powder; a sample with white emission under a 254 nm lamp and orange-red emission under a 365 nm lamp is defined as red powder; a sample with blue emission under a 254 nm lamp and green emission under a 365 nm lamp is defined as green powder. As shown in Fig. 1c, the blue powder exhibits an emission peak around 500 nm with a wide full width at half maximum (FWHM) of 131 nm under irradiation at 254 nm. This broad emission band with a large Stokes shift of 239 nm has a strong excitation band at around 260 nm, which is similar to previous results for Cs₂ZrCl₆.^25,29 For the blue emission from blue powder, the PL decay spectrum exhibits a PL decay lifetime of 3.9 μs at 300 K (Fig. 1d and Table S1†). With the temperature decreasing to 273 K and 77 K, the average PL decay lifetime of the ((C₂H₅)₄N)₂ZrCl₆ blue powder increases to 4.9 μs and 146.7 μs, respectively (Fig. S3 and Table S1†). The changes in lifetime involve reverse intersystem crossing (RISC) from long-lived triplet excited states back to short-lived singlet excited states, validating a TADF nature according to previous research on the blue emission of [ZrCl₆]²⁻ perovskites.^25,29,30

The red powder exhibits a wide emission under a 254 nm lamp. The wide emission originates from a blue emission around 500 nm and a red emission around 620 nm (Fig. S4†). With decreasing excitation energy, the blue emission gradually disappears and the sample exhibits a single emission peak at 620 nm with a FWHM of 124 nm (Fig. 1c). Interestingly, the blue emission of the blue powder can only be effectively excited in the range of 220∼280 nm, while the blue emission of the red powder can be excited at lower energy. The blue emission state in the red powder sample is related to the red emission state. For the blue emission band, the PL decay spectrum exhibits an average lifetime of 5.5 μs (τ₁ = 4.6 μs, 96%; τ₂ = 28.1 μs, 4%) for excitation under 254 nm light (Fig. 1d and Table S1†) owing to the TADF state. For the red powder, the FWHM of the red emission band decreased from 124 nm to 120 nm with the temperature decreasing from 300 K to 273 K (Fig. S5†). The PL position shifts slightly from 620 nm at 300 K to 619 nm at 273 K. This phenomenon is similar to the STE emission of ((CH₃)₄N)₂ZrCl₆ in previous research.³⁵ In addition, the PL decay spectrum exhibits a lifetime of 4.9 μs to 8.7 μs with excitation from 254 nm to 350 nm (Fig. S6 and Table S1†). The extension of the lifetime is attributed to STE emission showing an excitation-energy-dependent property.³⁸ Based on these results, the red emission is attributed to STEs.

As displayed in Fig. 1c, the green powder exhibits a broad blue emission under irradiation by a 254 nm lamp and a narrow green emission under a 365 nm lamp (Fig. S7†). The narrow green emission peak centred at 527 nm with a FWHM of 47 nm (Fig. 1c) has not been reported for zirconium halide complexes yet. For the blue emission, the PL decay spectrum exhibits an average lifetime of 4.7 μs for excitation under 254 nm light (Fig. 1d and Table S1†) owing to a TADF state. For the green emission band, the PL decay spectrum exhibits a shorter lifetime τ₁ = 1.6 μs (71%) and a longer lifetime τ₂ = 19.7 μs (29%) for excitation under 310 nm light. The shorter lifetime is owing to non-radiative recombination and the longer lifetime is ascribed to the d–d transitions of Zr(IV).³⁵ The blue and green emission bands of the green powder both exhibit strong excitation around 250 nm and 300 nm. The similar excitation spectra may originate from the overlap in the blue and green emission. Thus, a three-dimensional excitation–emission matrix (EEM) fluorescence spectrum is necessary.

For better understanding of the multi-emissive behaviour of the ((C₂H₅)₄N)₂ZrCl₆ powders, the EEM fluorescence spectra of the red and green powders are displayed in Fig. 2. Different from the narrower excitation range (220–280 nm) of the blue emission of the blue powder, the blue emission of the red powder can be excited at lower energy, with a wavelength of 340 nm (Fig. 2a and c). The red sample exhibits double-peak emission around 500 nm and 620 nm with excitation from 250 to 340 nm. With the excitation energy decreasing below that at 340 nm, the red sample exhibits a single broad emission peak around 620 nm. The emission peak position did not shift with the change of the excitation wavelength, which means that the red emission is not generated by a defect state. The emission at 620 nm, with a large Stokes shift of 270 nm and broad FWHM of 124 nm, is attributed to STE emission due to lattice distortions.^35,39 As shown in Fig. 2a and c, the blue emission of the red sample dominates the luminescence under 254 nm light. The lifetime of the blue emission of the red powder is similar to that of the blue and green powder (Table S1†). Under excitation at 310 nm and 320 nm, the red sample exhibits a double peak at 500 nm and 620 nm. Both the shorter lifetime τ₁ and the longer lifetime τ₂ decrease, and the ratio of the longer τ₂ increases. Under excitation at 340 nm, the blue luminescence part is relatively weak compared with the red part. Both τ₁ and τ₂ exhibit a decreasing trend. Under 350 nm light, the red powder only exhibits a single emission peak at 620 nm. The decrease in τ₁ and τ₂ for the blue emission accompanied with the appearance of the red emission suggests energy transfer between the two states. The absolute photoluminescence quantum yield (PLQY) of the double-peak emission is up to 83% at 270 nm and for the single red emission it is up to 14% at 340 nm (Fig. 2c).

Unlike the relation between the blue and red emissions of the red powder sample, the blue and green emissions of the green powder are relatively independent. Although the PLE spectra of the blue and green emissions are similar (Fig. 1c), the EEM spectra in Fig. 2b and d imply that the two emission groups belong to two independent pathways. The blue emission part for the green powder can only be excited with energy higher than that at 280 nm, similar to the blue powder. Besides the blue emission, a new emission at 527 nm appears with excitation from 265 nm to 340 nm (Fig. S8†). The absolute PLQY of the blue emission is up to 57% at 250 nm and for the green emission it is up to 27% at 295 nm (Fig. 2f). The green emission under a 365 nm lamp is stimulated by the broad emission of the source (Fig. S7†). The broad peak enables us to observe the green emission at 527 nm, which has not yet been discovered in previous research on Zr-based halide 0D perovskites. As the driving voltage increases from 3 V to 12 V, the PL peak positions for both the green and red powders show no obvious shift. The green powder exhibits luminescence at 527 nm, while the red powder exhibits a double peak around 500 nm and 620 nm (Fig. S9 and S10†). The PL intensity of both samples shows a linear dependence on the normalized excitation power. This phenomenon may exclude the possibility that the additional luminescence peak originates from permanent defects.^12,40,41 In addition, the red emission observed for other hybrid Zr-based perovskites has been identified as STE emission in previous research.³⁵

Fig. 2 Luminescence properties of red and green ((C₂H₅)₄N)₂ZrCl₆ powder samples. PL emission spectra under different excitations for (a) red and (b) green ((C₂H₅)₄N)₂ZrCl₆ powders. Three-dimensional excitation-emission matrix (EEM) fluorescence spectra for (c) red and (d) green ((C₂H₅)₄N)₂ZrCl₆ powders. PLQY under various excitation wavelengths for (e) red and (f) green powders.

((C₂H₅)₄N)₂ZrCl₆ single crystals are grown via the evaporation method. It is worth noting that the absorption spectra (Fig. 1a) show that both the powder and the single crystals show multiple excited states with different relative intensities. By slowly evaporating the precursor solution, a single crystal is obtained, which is 4 mm × 3 mm × 1 mm in size, and transparent under room lighting (Fig. 4a). Scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS) mappings show that the distribution shapes of the Cl, Zr and N elements are consistent with the single-crystal profile, indicating uniformity of the final product (Fig. S11 and S12†). Single-crystal X-ray diffraction (SCXRD) measurements showed that the ((C₂H₅)₄N)₂ZrCl₆ single crystal adopted the monoclinic space group C2/c (no. 15). The crystal structure consists of two tetraethylammonium cations and a zirconium hexachloride(II) anion complex (Fig. 3a). In the ((C₂H₅)₄N)₂ZrCl₆ crystals, each Zr⁴⁺ is coordinated with six Cl⁻ ions to form [ZrCl₆]²⁻ octahedra, and each adjacent octahedron exhibits a different spatial orientation (Fig. S13†). The Zr–Cl1, Zr–Cl2 and Zr–Cl3 lengths are 2.457, 2.463 and 2.474 Å, respectively. The Cl1–Zr–Cl2, Cl1–Zr–Cl3 and Cl2–Zr–Cl3 angles in the octahedron are 90.55°, 90.39° and 89.50°, respectively (Fig. 3b). It is obvious that the [ZrCl₆]²⁻ octahedron is slightly deformed. Furthermore, the torsion between two neighbouring [ZrCl₆]²⁻ octahedra, evaluated through the longest Cl–Zr–Cl bond, is 14.49°, as displayed in Fig. 3c. In 0D systems, the STE property mainly depends on the distortion in a single octahedron.^38,39,42 In a centrosymmetric octahedron, the d–d transition is forbidden by the Laporte selection rule, making the luminescence very weak.⁴³ In previous research on Mn(II) complexes in the octahedral crystal field, the distortion between adjacent octahedra breaks the symmetry, leading to observable luminescence.^36,44,45 Thus, the distortion of [ZrCl₆]²⁻ octahedra in the structure also enables the d–d transitions of Zr(IV). Therefore, structural distortion of [ZrCl₆]²⁻ octahedra leads to the multiple emission of the ((C₂H₅)₄N)₂ZrCl₆ samples.

Fig. 3 Crystal structure of ((C₂H₅)₄N)₂ZrCl₆. (a) Detailed view of the crystal of ((C₂H₅)₄N)₂ZrCl₆. (b) Detailed view of a [ZrCl₆]²⁻ octahedron. (c) Detailed view of adjacent [ZrCl₆]²⁻ octahedra of the ((C₂H₅)₄N)₂ZrCl₆ sample.

As displayed in Fig. 4a & b, ((C₂H₅)₄N)₂ZrCl₆ single crystals exhibit blue luminescence under a 254 nm lamp, green luminescence under a 310 nm UV chip and weak red luminescence under 375 nm irradiation. The blue emission exhibits a peak around 500 nm and the weak red emission exhibits a peak around 620 nm. The ((C₂H₅)₄N)₂ZrCl₆ single crystal exhibits yellow luminescence under a 365 nm lamp (Fig. S14†). In addition, ((C₂H₅)₄N)₂ZrCl₆ single crystals exhibit weak broadband emission at lower excitation energy (Fig. S15†). We infer that energy transfer between the two states occurs in the single crystals. Unlike the green powder samples, the green luminescence of the single crystal exhibits split peaks centred at 498, 527, 550, and 576 nm (Fig. 4d), which are derived from the d–d transitions of Zr(IV).^35,46 Emission through the d–d transitions of Zr(IV) has only been reported for 0D organic–inorganic hybrid Zr-based perovskites, and is due to the distortion of the [ZrCl₆]²⁻ octahedra, similar to [MnX₆]⁴⁻ complexes.³⁶ Although the green powder exhibits a single emission peak under a 365 nm lamp, its peak shape fits well with the split peaks of the ((C₂H₅)₄N)₂ZrCl₆ single crystal (Fig. 4d). We posit that the narrow green emission of the green powder is attributed to the d–d transitions of Zr(IV), which will be elucidated later. In addition, the XRD patterns of both the single crystal and powder samples match well with the simulated structure, as displayed in Fig. S16.† Briefly, there are three emission pathways for((C₂H₅)₄N)₂ZrCl₆: TADF emission around 500 nm, STE emission around 620 nm and d–d transitions of Zr(IV) due to lattice distortion, as shown in Fig. 4e. Energy transfer occurs between the two STE emission states.

Fig. 4 Luminescence and structural properties of ((C₂H₅)₄N)₂ZrCl₆. Photographic images under (a) room lighting and (b) UV irradiation. (c) PL emission spectra of the ((C₂H₅)₄N)₂ZrCl₆ single crystal under different excitations. (d) PL spectra of the single crystal and green powder under excitation at 310 nm. (e) Energy transfer diagram of FE, TADF, STE and d–d transition states.

It is worth noting that all the ((C₂H₅)₄N)₂ZrCl₆ samples, including different powder samples and single crystals, exhibit multiple excited states, but their relative intensities are different (Fig. 1a). As shown in Fig. S17,† the blue powder exhibits octahedral-shaped granules with sizes of ∼30 μm while the red and green powders exhibit smaller-sized granules. For the narrow green emission at 527 nm, we attribute it to the small-sized zirconium perovskite granules, which has not been reported in previous research on organic–inorganic hybrid zirconium perovskite single crystals.³⁷ To confirm this hypothesis, we applied a top-down method to grind the ((C₂H₅)₄N)₂ZrCl₆ single crystal to a powder with different luminescent properties. As shown in Fig. S18,† ((C₂H₅)₄N)₂ZrCl₆ powder only exhibits blue emission under a 254 nm lamp after being ground once, while the ((C₂H₅)₄N)₂ZrCl₆ powder ground twice to a smaller size exhibits blue emission under a 254 nm lamp and green emission under a 365 nm lamp. The green emission peak is located at 527 nm (Fig. S19†). We further apply ball milling to ((C₂H₅)₄N)₂ZrCl₆ single crystals to fully pulverize the samples. As shown in Fig. S20,† the ((C₂H₅)₄N)₂ZrCl₆ powders exhibit uniform blue and green emission under 254 nm and 365 nm lamps, respectively. The blue and green emissions are from the same areas (Fig. S21†). Therefore, we assume that the narrow green emission of the green powder is attributed to the d–d transitions of Zr(IV) and is obtained by controlling the grain size of ((C₂H₅)₄N)₂ZrCl₆ powder.

For the red emission from the STE states, we find that the emission behaviour can be adjusted by humidity or water. After being stored at a relative humidity (RH) of 100% for one day, some of the green powder turns into red powder, forming yellow luminescence (Fig. S22†). By applying the powder on a 310 nm UV chip, blue, green and red luminescence are achieved simultaneously (Fig. S23†). This phenomenon is more obvious in another 0D organic–inorganic hybrid (C₁₃H₃₀N)₂ZrCl₆ perovskite sample. (C₁₃H₃₀N)₂ZrCl₆ exhibits similar multiple emissions around 510 nm, 660 nm and 527 nm (Fig. S24†). The red emission part attributed to STE gradually increases with increasing numbers of washes with water, as shown in Fig. S25.† The CIE chromaticity coordinate of the (C₁₃H₃₀N)₂ZrCl₆ sample changes from (0.38, 0.47) to (0.44, 0.43) after being washed twice.

Both the blue and green powders exhibit high stability in high-temperature and dry storage environments. As shown in Fig. S26,† after being stored in an oven at 80 °C for 60 days, the PLQY of the blue powder decreased slightly from 90% to 89% under excitation at 250 nm. The PLQY of the green powder remains at 27% under 300 nm irradiation. Compared with the blue and green powders, the red powder is less stable. After being stored in an oven for 60 days, the PLQY of the red powder decreases from 83% to 53% at 270 nm, from 22% to 12% at 315 nm and from 14% to 7% at 340 nm. Therefore, we think the red emission is related to humidity or water. However, the changes in structure need further study in future research.

The organic–inorganic hybrid ((C₂H₅)₄N)₂ZrCl₆ perovskite powders exhibit multiple emissions and the emission tendency can be adjusted by the reaction conditions. Utilizing the unique luminescence properties, we explore various applications with the ((C₂H₅)₄N)₂ZrCl₆ powders. First, the ((C₂H₅)₄N)₂ZrCl₆ powders with multiple luminescence wavelengths under different excitation energies in one matrix, such as the red and green powder samples, are suitable for anti-counterfeiting security. Traditional anti-counterfeiting fluorescent materials exhibit monochromatic emission under different excitations, which make them easily counterfeited.⁴⁷ However, simply mixing phosphors with different emissions may lead to inhomogeneous dispersion of the materials.⁴⁸ Therefore, it is necessary to develop a single-matrix material exhibiting multiple emissions, which would be difficult to replicate and have a higher level of anti-counterfeiting security.^21,27,49Fig. 5a shows that the ((C₂H₅)₄N)₂ZrCl₆ green powder emits blue and green luminescence under different UV lamps, exhibiting great potential for application in the anti-counterfeiting field. Compared with previous 0D Zr-based perovskite anti-counterfeiting applications,^3,21,27 the ((C₂H₅)₄N)₂ZrCl₆ powders do not require the introduction of other rare-earth materials.

Fig. 5 Applications of ((C₂H₅)₄N)₂ZrCl₆ samples. (a) Photographs of an anti-counterfeiting pattern using the ((C₂H₅)₄N)₂ZrCl₆ powder sample under different lights. (b) Emission spectra and (c) CIE chromaticity diagram of green, red and white LEDs through green, red and mixed samples on a UV chip (310 nm). The insets are the photographs of the corresponding LEDs. (d) Photographs of the green and blue powders under different irradiations. (e) Photographs of pixelated patterns of the green and blue powders under different irradiation conditions.

Besides anti-counterfeiting, the ((C₂H₅)₄N)₂ZrCl₆ powders can further be applied to UV-pumped WLEDs. The LEDs are fabricated by coating the ((C₂H₅)₄N)₂ZrCl₆ powders on a UV chip (310 nm). Fig. 5b shows the spectra of the LEDs through different ((C₂H₅)₄N)₂ZrCl₆ powders. The CIE chromaticity coordinates of the green LED and red LED are (0.26, 0.65) and (0.44, 0.39), respectively, at a driving voltage of 3 V. The CRI of the red LED reaches as high as 89 with a CCT of 2747 K. From the spectra, it is noted that the emission of the green LED can cover the absence of emission of the red LED in the range of 500 nm to 600 nm via excitation at 310 nm. Therefore, by mixing the green and red ((C₂H₅)₄N)₂ZrCl₆ powders on a UV chip, a WLED is produced that can achieve a high CRI of up to 95 and low CCT of 3441 K with a CIE chromaticity coordinate of (0.42, 0.42) in the 1931 CIE colour space.

Finally, the different excitation-dependent emission performance of the blue and green ((C₂H₅)₄N)₂ZrCl₆ powders provides an opportunity for information encryption applications. The blue and green powders exhibit similar luminescence under a 254 nm lamp but different luminescence under a 365 nm lamp (Fig. 5d and e and S27†). Thus, the blue and green powders are chosen to be selectively deposited into a pixelated hole pattern (ϕ = 2 mm, d = 5 mm) in a PMMA plate (Fig. S28†). As shown in Fig. 5d and S28,† the blue and green powders both exhibit a pale yellow colour under room lighting and do not exhibit luminescence under lower energy excitation with a wavelength of 375 nm, which makes them hard to distinguish. The pattern composed of the blue and green powders showed uniformly blue luminescence under 254 nm excitation, expressing signals in the pixel holes (Fig. 5g). However, on excitation under a 365 nm UV lamp, cryptographic information emerged as green emission due to the excitation-dependent emission of the green powder (Video S1†). Different luminescent behaviours in each pixel hole can be decoded as binary data of “0” or “1” for digital information.

Besides light conversion applications, ((C₂H₅)₄N)₂ZrCl₆ can also be applied to X-ray detection. The radioluminescence (RL) spectrum of the blue ((C₂H₅)₄N)₂ZrCl₆ samples is carried acquired under X-ray irradiation with a tube voltage of 100 kV and tube current of 100 mA. The emission peak is around 500 nm, which is similar to the emission peak under a 254 nm lamp (Fig. S29†).

Conclusions

In summary, we synthesized ((C₂H₅)₄N)₂ZrCl₆ samples with multiple emissions. The ((C₂H₅)₄N)₂ZrCl₆ crystallized in the monoclinic phase with distorted [ZrCl₆]²⁻ octahedra, leading to multi-mode luminescence. The blue, red and green luminescence are attributed to STE emission and the d–d transitions of Zr(IV). The three emission modes exist simultaneously in both the single crystal and powder samples. The relative intensity of the three luminescence colours can be adjusted in experiments to form three major types of powder sample. The blue powder sample mainly exhibits emission at 500 nm with an excitation energy higher than that at 280 nm. The red powder sample exhibits blue emission at 500 nm and a red emission peak at 620 nm. Energy transfer occurs between the two STE emissions, which is observed for the red powder sample. The green sample exhibits blue emission at 500 nm and a narrow green emission peak at 527 nm with a FWHM of 47 nm. Various optical applications are achieved through different combinations of the three kinds of powders. An anti-counterfeiting application with two chromatic emissions is achieved using the green ((C₂H₅)₄N)₂ZrCl₆ powder under 254 nm and 365 nm UV lamps. A WLED with a CIE chromaticity coordinate of (0.42, 0.42), CCT of 3441 K and CRI of up to 95 is obtained by applying green and red ((C₂H₅)₄N)₂ZrCl₆ powders on a 310 nm UV chip. The application of information encryption is achieved by depositing blue and green powders in a pixelated hole pattern in a PMMA plate. Besides the ((C₂H₅)₄N)₂ZrCl₆ samples, (C₁₃H₃₀N)₂ZrCl₆ with a different organic cation also exhibits multiple emissions. We believe the unique tri-luminescent ((C₂H₅)₄N)₂ZrCl₆ developed in this study could pave the way towards multi-emitting lead-free perovskites and is expected to expand the application of multi-luminescence materials in lighting, anti-counterfeiting and encryption fields.

Experimental

Materials

Hydrochloric acid (HCl, Shanghai Lingfeng Chemical Reagent Co., Ltd, 36.0–38.0 wt%), zirconium tetrachloride (ZrCl₄, Aladdin, 98%), tetraethylammonium chloride ((C₂H₅)₄NCl, HEOWNS, 98%), methyltributylammonium chloride (C₁₃H₃₀NCl, HEOWNS, 98%), isopropanol (IPA, Aladdin, AR), toluene (99.9%, Sinopharm Chemical Reagent Co., Ltd), methyl acetate (≥99.0%, Macklin), polystyrene (PS, Aladdin) and deionized water were used in this work. All chemicals were used as received without any further purification.

Synthesis of the ((C₂H₅)₄N)₂ZrCl₆ powder

In a typical synthesis of the ((C₂H₅)₄N)₂ZrCl₆ powder (nominal concentrations in moles), 4 mmol ZrCl₄ (2 mmol ZrCl₄ for the green powder) were dissolved in 8 mL HCl and 4 mmol (C₂H₅)₄NCl were dissolved in 8 mL HCl. Subsequently, 1 mL of the (C₂H₅)₄NCl solution was injected into 1 mL of the ZrCl₄ solution under vigorous stirring; then a white precipitate appeared. The crude mixture was centrifuged at 6000 rpm for 10 minutes and the precipitate was re-dispersed in 0.5 mL HCl, then dried in a tube in an oven at 80 °C to obtain the blue ((C₂H₅)₄N)₂ZrCl₆ powder. For the green ((C₂H₅)₄N)₂ZrCl₆ powder and the red ((C₂H₅)₄N)₂ZrCl₆ powder, the precipitates were dried on a glass substrate at 80 °C and 50 °C, respectively. The green powder can also be obtained by grinding ((C₂H₅)₄N)₂ZrCl₆ single crystals in a mortar and pestle.

Synthesis of the ((C₂H₅)₄N)₂ZrCl₆ single crystals

0.4 mmol (C₂H₅)₄NCl was dissolved in 1.5 mL DI water, then 1.6 mL ZrCl₄ solution (0.25 M) was injected into the water solution to form a clear solution. Subsequently, the clear solution was sealed, with a small hole, then placed in an oven and kept at 80 °C. Transparent crystals precipitated at the bottom of the sample vial after 1 week. Finally, the crystals were washed with IPA and dried at 80 °C.

Fabrication of LEDs

The as-synthesized green and red ((C₂H₅)₄N)₂ZrCl₆ powders were mixed with 25% polystyrene (PS) toluene ink at room temperature in air, and the ink was dropped onto a UV-LED chip (310 nm).

Characterization details

Single-crystal X-ray diffraction (XRD) data were collected using an Apex DUO with Mo-Kα radiation (λ = 0.71073 Å) at room temperature. The structures were resolved and refined using direct methods with SHELXL and Olex2. X-ray diffraction (XRD) measurements were carried out using a Rigaku Ultima III X-ray diffractometer equipped with Cu Kα radiation (λ = 1.541841 Å). Ultraviolet and visible (UV-vis) absorption spectra were measured using a Shimadzu UV-3600 Plus spectrophotometer at room temperature. PL spectra were measured using a Horiba PTI QuantaMaster 400. Time-resolved photoluminescence spectra were detected using an Edinburgh Instruments FLS980 at room temperature. The absolute fluorescence quantum yields were measured using a Horiba PTI QuantaMaster 400 steady-state fluorescence system with an integrating sphere. Scanning electron microscopy (SEM) and X-ray spectroscopy (EDS) were performed on a JEOL JSM-7800F at 15 kV. Fluorescence microscopy images were captured using an Axio Vert A1 FL-LED at room temperature.

Data availability

More experimental or data associated with this article is available on request from the authors.

Author contributions

Z. D. supervised the project. C. W. conceived the study, synthesized the perovskites and performed steady-state absorption and PL spectral measurements and time-resolved photoluminescence spectra (TCSPC) measurements. W. M. and G. L. fabricated the LED devices, and G. X., M. P. and B. X. performed characterization of the LED devices. All authors discussed the results and assisted during manuscript preparation. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22075129).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Photographs, PL spectra, UV-vis absorption spectra, XRD patterns, SEM images, EDS mapping and crystal structure of hybrid Zr-based perovskite samples. See DOI: https://doi.org/10.1039/d3sc06178g

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