Rare earth-based Cs₂NaRECl₆ (RE = Tb, Eu) halide double perovskite nanocrystals with multicolor emissions for anticounterfeiting and LED applications

Abstract

Metal halide double perovskites are a promising alternative to lead-based perovskites. Especially, rare earth (RE)-based halide double perovskites have attracted much attention due to their unique optical properties. As RE³⁺ ions could be served not only as the component of the matrix but also as the luminescence centers, the chemical composition of RE-based halide double perovskites can be simplified and tunable luminescence could be achieved. In this work, Cs₂NaRECl₆ (RE = Tb, Eu) halide double perovskite nanocrystals were synthesized by the hot injection method. Cs₂NaRECl₆ nanocrystals exhibit not only the intrinsic host emission but also the characteristic emissions of RE³⁺ ions. Furthermore, the excitation-wavelength-dependent multicolor emissions of Cs₂NaRECl₆ nanocrystals make them highly potential for anticounterfeiting applications. Multicolor LED devices based on various components of Cs₂NaRECl₆ could be obtained, providing promising candidates for optoelectronic applications.

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Introduction

Metal halide perovskite CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) have attracted considerable attention due to their attractive optoelectronic features such as their narrow and tunable emission,¹ high photoluminescence quantum yields (PLQYs),² direct band gaps,³ and high charge carrier mobilities.^4,5 However, the undesirable stability of CsPbX₃ NCs and the inherent toxicity of lead ions have seriously hindered their practical applications.⁶ Therefore, it is significant to design and synthesize lead-free halide perovskites with excellent optoelectronic properties and outstanding stability. The double perovskite Cs₂B^IB^IIIX₆, which consists of one monovalent cation (B⁺) and one trivalent cation (B³⁺), has great freedom in the composition of B-site metal ions.⁷ In addition, the three-dimensional structure of Cs₂B^IB^IIIX₆ is similar to that of CsPbX₃, making it an acceptable substitute for lead-based perovskites.⁷ The energy band structures and optoelectronic properties of the double perovskites could be effectively regulated by adjusting the element composition, which has attracted extensive attention.^8–14

Trivalent rare earth ions (RE³⁺) with unique electronic configurations could produce intense photoluminescence (PL) covering ultraviolet (UV) and visible to near-infrared (NIR) range, which is the important component of various luminescent materials.^15–18 The octahedral coordination environment and the valence state of B³⁺ cations in double perovskites provide the feasibility for the introduction of RE³⁺ ions.^19–22 For example, Yb³⁺ and Er³⁺ ions are doped in the double perovskites to produce NIR emissions.²² Similarly, Cs₂AgInCl₆ NCs doped with Yb³⁺ ions exhibit a broadband host emission at 607 nm and NIR emission of Yb³⁺ ions at 996 nm. Er³⁺ ions doped in Cs₂AgInCl₆ NCs undergo the energy transition of ⁴I_13/2 → ⁴I_15/2 and emit the NIR emission at 1537 nm under 300 nm excitation.²³ For RE³⁺ ion-doped halide double perovskites, the PL properties are greatly affected by the ratio of dopants. Generally, exploring the optimal ratio is laborious and time-consuming.²⁴ For RE-based halide double perovskites, RE³⁺ ions can be served not only as the component of the matrix but also as the luminescence centers by selecting the appropriate RE³⁺ ions. Therefore, the chemical composition can be simplified and interference from other dopants can be minimized. At present, RE-based halide double perovskites mainly include single crystals^25–28 and NCs.^29,30 The colloidal synthesis of RE-based halide double perovskite NCs could potentially provide good uniformity and tunable morphology,³¹ which has been inadequately investigated. Therefore, it is of great significance to develop RE-based halide double perovskite NCs and investigate their PL performances.

Herein, Cs₂NaRECl₆ (RE = Tb, Eu) halide double perovskite NCs were fabricated through a hot injection method. Cs₂NaRECl₆ (RE = Tb, Eu) NCs could exhibit not only intrinsic host emission but also the characteristic emissions of Tb³⁺ or Eu³⁺ ions under UV light excitation. Especially, Cs₂NaRECl₆ (RE = Tb, Eu) NCs could exhibit multicolor emissions under different light excitation, showing great potential in anticounterfeiting. The proof-of-concept of anticounterfeiting pattern, figure, and QR code made of Cs₂NaRECl₆ (RE = Tb, Tb_0.5Eu_0.5, Eu) NCs were further performed. Furthermore, based on various components of Cs₂NaRECl₆ (RE = Tb, Eu) NCs, multicolor LED devices were fabricated, suggesting the potential in the optoelectronic field.

Experimental section

Materials

Europium acetate hydrate (Eu(CH₃CO₂)₃·xH₂O, 99.99%), terbium acetate hydrate (Tb(CH₃CO₂)₃·xH₂O, 99.99%) were purchased from Sigma-Aldrich. 1-Octadecene (ODE, 90%), oleic acid (OA, 90%), oleylamine (OAm, 90%), chlorotrimethylsilane (TMSCl, 99%), sodium acetate (CH₃COONa, 99.99%), and cesium carbonate (Cs₂CO₃, 99.99%) were purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Cyclohexane (99.5%), and ethyl acetate (99.5%) were purchased from Xilong Scientific Co., Ltd (Guangdong, China). All chemicals were obtained from commercial suppliers and used without further purification. The LED chips were purchased from Shenzhen Looking Long Technology Co., Ltd.

Synthesis of Cs₂NaRECl₆ (RE = Tb, Eu) NCs

Eu(CH₃CO₂)₃·xH₂O (0.5 mmol) or Tb(CH₃CO₂)₃·xH₂O (0.5 mmol), Cs₂CO₃ (0.115 g), CH₃COONa (0.025 g), ODE (10 mL), OAm (0.8 mL), and OA (3.2 mL) were added into a 100 mL three-neck flask, dried under vacuum for 1 h at 120 °C, and then heated at 180 °C under N₂. Subsequently, TMSCl (0.6 mL) was swiftly injected, and after 5 min, the solution was cooled in an ice bath.

Synthesis of Cs₂NaTb_0.5Eu_0.5Cl₆ NCs

For Cs₂NaTb_0.5Eu_0.5Cl₆ NCs, the same synthesis process as the preparation of Cs₂NaRECl₆ NCs was adopted, except that 0.5 mmol Eu(CH₃CO₂)₃·xH₂O or Tb(CH₃CO₂)₃·xH₂O was replaced with 0.25 mmol Eu(CH₃CO₂)₃·xH₂O and 0.25 mmol Tb(CH₃CO₂)₃·xH₂O.

Purification of Cs₂NaRECl₆ (RE = Tb, Eu) NCs

The crude solution was centrifuged at 9500 rpm for 8 min, and the supernatant was discarded. The precipitation was then dispersed in 4 mL of cyclohexane and centrifuged at 9500 rpm for 8 min. Then, the precipitation was dispersed in 4 mL of cyclohexane for subsequent characterizations. To obtain the solids of Cs₂NaRECl₆, the dispersive solution was mixed with 6 mL of ethyl acetate, and centrifuged at 10 000 rpm for 10 min to precipitate Cs₂NaRECl₆ NCs.

Characterizations

The phase formation was analyzed by using an X-ray diffractometer (D8 Advance, Bruker AXS, Germany) equipped with a ceramic X-ray tube Cu Kα radiation source (λ = 0.15405 nm) at a scanning rate of 4° min⁻¹ in the 2θ range from 10° to 60°. X-Ray photoelectron spectroscopy (XPS) was conducted using a Thermo Fisher Scientific-ESCALAB 250Xi. UV-visible (UV-vis) absorption spectra were recorded using a Shimadzu 3600i plus UV-vis spectrophotometer. The PLQYs were gained directly by the absolute PLQY measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). The composition of the samples was studied using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi) equipped with an EDX spectrometer. Transmission electron microscopy (TEM), high angle annular dark field scanning TEM (HAADFSTEM), and elemental mapping measurements were carried out using an FEI Tecnai G2S-Twin instrument with a field emission gun operating at 200 kV. The PL spectra were recorded on an Edinburgh Instruments FLS 920 fluorescence spectrometer equipped with a 450 W xenon lamp as the excitation source at room temperature. The temperature-dependent PL spectra were also measured using a fluorescence spectrometer equipped with a xenon lamp as the excitation source (Edinburgh Instruments FLS 920) and a temperature controller (INSTEC temperature controller, American Instec). The fluorescent decay curves were obtained using a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) taking a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The PL decay curves were fitted with an exponential decay function, as expressed: . The average PL lifetime could be calculated by using . A_i represents constant, τ_i is the decay time for the exponential component, I₀ and I(t) are PL intensities at time 0 and t, respectively.

Results and discussion

RE-based Cs₂NaRECl₆ (RE = Tb, Eu) double perovskite NCs were prepared by the hot injection method with post-injection of halides. Cs₂NaRECl₆ NCs possess cubic crystal structure and belong to the Fm3̄m space group. The crystal structure of Cs₂NaRECl₆ is composed of alternating arrangement of [NaCl₆]⁵⁻ and [RECl₆]³⁻ octahedra, with Cs⁺ ions located in the cavity of the octahedra (Fig. 1a). As shown in Fig. 1b, the X-ray diffraction (XRD) pattern of Cs₂NaTbCl₆ NCs is consistent with the standard card ICSD-96062 without other impurities. The XRD pattern of Cs₂NaEuCl₆ NCs is similar to that of Cs₂NaTbCl₆, and the diffraction peaks of each crystal plane slightly shift to the lower angle compared with those of Cs₂NaTbCl₆, which is attributed to the fact that the lattice constant of Cs₂NaEuCl₆ (a_Eu = 10.8095 Å) is larger than that of Cs₂NaTbCl₆ (a_Tb = 10.7636 Å). The transmission electron microscopy (TEM) and the high-resolution TEM (HR-TEM) images of Cs₂NaRECl₆ double perovskite NCs are shown in Fig. 1c–f. The size distribution histograms of Cs₂NaRECl₆ NCs are shown in Fig. S1 (ESI†). It could be seen that Cs₂NaRECl₆ NCs show regular cubic morphology with a size of 14–18 nm. The lattice fringes with the lattice spacing of 2.69 Å for Cs₂NaTbCl₆ NCs and 2.73 Å for Cs₂NaEuCl₆ NCs, corresponding to the (400) crystal plane, are consistent with XRD results. Cs₂NaRECl₆ NCs were further characterized using X-ray photoelectron spectroscopy (XPS) to detect the valence state of RE ions (Fig. S2, ESI†). In Fig. S2b (ESI†), the binding energies of 1242.05 eV and 1276.41 eV correspond to Tb³⁺ 3d_5/2 and Tb³⁺ 3d_3/2, respectively, proving the valence state of terbium in Cs₂NaTbCl₆ is trivalent. In Fig. S2d (ESI†), the peaks of 1135.29 eV and 1164.70 eV are assigned to Eu³⁺ 3d_5/2 and Eu³⁺ 3d_3/2, respectively. The peaks at 1125.20 eV and 1154.80 eV are attributed to Eu²⁺ 3d_5/2 and Eu²⁺ 3d_3/2, respectively. These results reveal that Eu³⁺ and Eu²⁺ exist in Cs₂NaEuCl₆. The existence of Eu²⁺ may result from the reduction of partial Eu³⁺ to Eu²⁺ by OAm or the trace impurity originating from raw reagents. Scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDS) element mappings of Cs₂NaRECl₆ are shown in Fig. S3 and S4 (ESI†). The result exhibits the homogeneous distribution of Cs, Na, RE (RE = Tb, Eu), and Cl elements in Cs₂NaRECl₆ NCs. The energy dispersive X-ray (EDX) images also confirm the composition of Cs, Na, Tb (or Eu), and Cl elements in Cs₂NaRECl₆ (RE = Tb, Eu) NCs (Fig. S5, ESI†).

Fig. 1 (a) Crystal structure and (b) XRD patterns of Cs₂NaRECl₆ (RE = Tb, Eu) NCs. TEM and HR-TEM images of (c) and (d) Cs₂NaTbCl₆ and (e) and (f) Cs₂NaEuCl₆ NCs.

The PL excitation (PLE) and PL spectra of Cs₂NaRECl₆ (RE = Tb, Eu) NCs are shown in Fig. 2. Under the excitation of 360 nm, multiple emission peaks of Cs₂NaTbCl₆ NCs appear, including the host emission peak centered at 430 nm and the emission peaks of Tb³⁺ ions. Specifically, the PL intensity of Tb³⁺ ions at 548 nm is higher than that of the host emission in Cs₂NaTbCl₆ NCs. In addition, the emission peak of Tb³⁺ ions at 490 nm partially overlaps with the host emission of Cs₂NaTbCl₆ NCs (Fig. 2a). As shown in Fig. 2b, the PLE spectrum of Cs₂NaTbCl₆ NCs monitored at 548 nm consists of several excitation peaks, with the narrow main peak at 279 nm. The main excitation peak is assigned to the 4f⁸ → 4f⁷5d¹ transition of Tb³⁺. This transition could also be seen in the absorption spectrum (Fig. S6, ESI†). The PL spectrum exhibits the characteristic emissions of Tb³⁺ ions under the excitation of 279 nm at 490, 548, 584, and 621 nm, which are assigned to the ⁵D₄ → ⁷F_J (J = 6–3) transitions in the f-orbitals of Tb³⁺ ions, respectively. The host emission of Cs₂NaTbCl₆ NCs is annihilated. Cs₂NaTbCl₆ NCs exhibit PLQYs of 13.9% and 15.3% under 360 and 279 nm excitation, respectively. In addition, the time-resolved PL decay curves of Cs₂NaTbCl₆ NCs are displayed in Fig. S7 (ESI†). The PL decay curve of the host emission (430 nm) could be fitted by a multiexponential function with an average decay lifetime of 4.76 ns, and the decay pathway may be attributed to carrier trap, charge recombination, and trap-state-associated decay.³² Furthermore, the PL lifetime of Tb³⁺ ions emission was found to be 2.78 ms, which is attributed to the spin-forbidden f–f transitions in Tb³⁺ ions.³³

Fig. 2 PLE and PL spectra of Cs₂NaTbCl₆ NCs: (a) λ_em = 430 nm, λ_ex = 360 nm, (b) λ_em = 548 nm, λ_ex = 279 nm. PLE and PL spectra of Cs₂NaEuCl₆ NCs: (c) λ_em = 430 nm, λ_ex = 360 nm, and (d) λ_em = 593 nm, λ_ex = 330 nm.

The PL spectra of Cs₂NaEuCl₆ NCs also consist of multiple emission peaks, including the host emission of Cs₂NaEuCl₆ and the characteristic emissions of Eu³⁺ ions. The PLE spectrum of Cs₂NaEuCl₆ NCs monitored at 430 nm consists of a single-peak curve located at 360 nm. It could be seen that the PL intensity of the host emission is higher than that of the Eu³⁺ ion under 360 nm excitation (Fig. 2c). For the emissions of Eu³⁺ ions, several narrow emission peaks located at 593, 616, 656 and 699 nm can be assigned to characteristic Eu³⁺ electronic transitions associated with ⁵D₀ → ⁷F_J (J = 1–4) transitions, respectively. The PLE spectrum of Cs₂NaEuCl₆ NCs (λ_em = 593 nm) consists of a broad band (330 nm) and two sharp peaks (362 nm and 381 nm), which originate from the charge transfer band within [EuCl₆]³⁻ octahedra and f–f transitions of Eu³⁺, respectively (Fig. 2d). The charge transfer band could be seen in the absorption spectrum (Fig. S6, ESI†).

The PLQYs of Cs₂NaEuCl₆ NCs are 10.4% and 5.3% under 360 and 330 nm excitation, respectively. The PL lifetime of the host emission of Cs₂NaEuCl₆ NCs is 3.56 ns, and the decay pathway is similar to that of Cs₂NaTbCl₆ NCs. For the emission of Eu³⁺ ions, PL lifetime was measured to be 2.84 ms, which is assigned to the spin-prohibited f–f transitions in Eu³⁺ ions (Fig. S8, ESI†). Based on the above PL results, the PL mechanism may be that Cs₂NaRECl₆ NCs emit a broad host emission centered at 430 nm under the excitation of 360 nm, and transfer part of the energy to RE³⁺ ions, resulting in the characteristic emissions of RE³⁺ ions (Fig. S9, ESI†).³⁴

To further understand the PL properties, the temperature-dependent PL measurements of Cs₂NaRECl₆ (RE = Tb, Eu) NCs were conducted. Fig. 3a shows the PL spectra of Cs₂NaTbCl₆ NCs in the temperature range of 100–300 K under 360 nm excitation. The PL intensity of Cs₂NaTbCl₆ NCs decreases with the increase in temperature, which is caused by the thermal quenching effect. The PL intensity of Cs₂NaTbCl₆ NCs does not diminish drastically during the initial temperature increase and could maintain comparatively high PL intensity at relatively high temperatures, indicating that the synthesized Cs₂NaTbCl₆ NCs have good stability. Fig. 3b visualizes the change in PL intensity of Cs₂NaTbCl₆ NCs throughout the heating process. Cs₂NaTbCl₆ NCs show intense host emission until 240 K, and then the PL intensity appears to decrease substantially in the temperature range of 240–300 K. The narrow-band emissions of Tb³⁺ ions maintain distinctly throughout the heating process and undergo a substantial emission decay as the temperature increases (Fig. 3c). In addition, the peak position of host emission changes slightly throughout the heating process, while the peak positions of Tb³⁺ ion emissions do not change (Fig. S10, ESI†). Under the excitation of 279 nm, the PL spectrum of Cs₂NaTbCl₆ NCs mainly exhibits Tb³⁺ ion emissions in the temperature range of 100–300 K (Fig. 3d). Tb³⁺ ion emissions of Cs₂NaTbCl₆ NCs presents narrow linear shape, indicating that the full width at half maximum (FWHM) does not change distinctly and Cs₂NaTbCl₆ NCs can maintain good structural stability as the temperature increase. Similarly, as the temperature increases from 100 to 300 K, the change in PL intensity of different emissions of Cs₂NaTbCl₆ NCs under 279 nm excitation was statistically analyzed (Fig. 3e). The PL intensity of Tb³⁺ emissions decreases slowly in the range of 100–260 K, and could maintain 55.7% of the initial intensity at 300 K (Fig. 3f). Moreover, the position of Tb³⁺ ion emission centered at 548 nm remains constant (Fig. S11, ESI†).

Fig. 3 (a) Temperature-dependent PL spectra, (b) pseudo-color mapping, and (c) normalized PL intensity changes of Cs₂NaTbCl₆ NCs under 360 nm excitation from 100–300 K (blue: host emission, green: Tb³⁺ ions emission). (d) Temperature-dependent PL spectra, (e) pseudo-color mapping, and (f) normalized PL intensity changes of Cs₂NaTbCl₆ NCs under 279 nm excitation from 100–300 K (blue: host emission, green: Tb³⁺ ions emission).

Subsequently, the temperature-dependent PL spectra of Cs₂NaEuCl₆ NCs were recorded. Under 360 nm excitation, the PL spectrum of Cs₂NaEuCl₆ NCs is dominated by the host emission with relatively weak Eu³⁺ ion emissions. Fig. 4a presents the PL variation of Cs₂NaEuCl₆ NCs in the temperature range of 100–300 K. As the temperature increases from 100 to 240 K, the host emission of Cs₂NaEuCl₆ NCs can maintain high PL intensity and the emission peak does not change significantly, indicating the good stability of the Cs₂NaEuCl₆ matrix (Fig. S12, ESI†). The PL intensity of the host emission appears to be significantly decreased and the peak position red-shifts in the range of 240–300 K. Compared with the host emission, the PL intensity of Eu³⁺ emissions decreases continuously during the heating process with a larger fluorescence loss (Fig. 4b). The PL intensity of the host emission reduces to 41% of the initial intensity, while the PL intensity of Eu³⁺ ions exhibits only 17.6% of the initial intensity after the same heating process (Fig. 4c). As exhibited in Fig. 4d, the temperature-dependent PL spectra (100–300 K) of Cs₂NaEuCl₆ NCs under 330 nm excitation are mainly composed of intense Eu³⁺ ions emissions and relative weak host emission. With the increasing temperature, the PL intensity of both the host emission and Eu³⁺ ions emission of Cs₂NaEuCl₆ NCs show a stepwise decrease, and the decrease rate of the host emission is smaller than that of Eu³⁺ ions emission (Fig. 4e). When the temperature increases to 300 K, the PL intensity of Eu³⁺ ions emission greatly weakens to 14.2% of the initial intensity (Fig. 4f), and the peak position of Eu³⁺ ions emission is always located at 593 nm (Fig. S13, ESI†).

Fig. 4 (a) Temperature-dependent PL spectra, (b) pseudo-color mapping, and (c) normalized PL intensity changes of Cs₂NaEuCl₆ NCs under 360 nm excitation from 100–300 K (blue: host emission, red: Eu³⁺ ions emission). (d) Temperature-dependent PL spectra, (e) pseudo-color mapping, and (f) normalized PL intensity changes of Cs₂NaEuCl₆ NCs under 330 nm excitation from 100–300 K (blue: host emission, red: Eu³⁺ ions emission).

The PL of RE-based Cs₂NaRECl₆ (RE = Tb, Eu) NCs consists of both host emission and RE³⁺ ion emission, and there is an apparent difference in the PLE wavelength of the two PL compositions. Consequently, the PL spectra of Cs₂NaRECl₆ NCs were recorded under successive changes of excitation wavelength, as shown in Fig. 5. Under the excitation in the range of 260–300 nm, Cs₂NaTbCl₆ NCs exhibit characteristic green emissions of Tb³⁺ ions accompanied by changes in PL intensity (Fig. 5a). In the excitation range of 300–370 nm, the host PL intensity of Cs₂NaTbCl₆ NCs increases continuously. Subsequently, the PL color of Cs₂NaTbCl₆ NCs changes with the excitation wavelength (Fig. 5b). As shown in Fig. 5c, under 280 nm excitation, Cs₂NaTbCl₆ NCs show green emission with Commission Internationale de L'Eclairage (CIE) color coordinates of (0.301, 0.577), while under 370 nm excitation, Cs₂NaTbCl₆ NCs exhibit blue emission with CIE color coordinates of (0.167, 0.144). Analogously, Cs₂NaEuCl₆ NCs exhibit various emission colors under different excitation lights. Fig. 5d displays the PL spectra of Cs₂NaEuCl₆ NCs under the excitation from 300 to 370 nm. Under the short-wavelength light excitation, the PL spectra of Cs₂NaEuCl₆ NCs mainly exhibit characteristic emissions of Eu³⁺ ions. The host emission of Cs₂NaEuCl₆ NCs enhances with the increasing wavelength of the excitation light, while the PL intensity of Eu³⁺ ion emissions becomes weaker (Fig. 5e). Under 300 nm excitation, Cs₂NaEuCl₆ NCs show red luminescence with the CIE color coordinate of (0.610, 0.351). Under 370 nm light excitation, the PL of Cs₂NaEuCl₆ NCs turns blue with the CIE color coordinates of (0.195, 0.184). It is indicated that the PL color of Cs₂NaEuCl₆ NCs changes gradually from red to blue as the excitation wavelength varies from 300 to 370 nm (Fig. 5f). The above results demonstrate that Cs₂NaRECl₆ (RE = Tb, Eu) NCs can display multicolor emissions under different excitation light, which provides the possibility for anticounterfeiting application.

Fig. 5 (a) PL spectra, (b) pseudo-color mapping, and (c) CIE color coordinate of Cs₂NaTbCl₆ NCs with varying excitation wavelengths from 260 to 370 nm. (d) PL spectra, (e) pseudo-color mapping, and (f) CIE color coordinate of Cs₂NaEuCl₆ NCs with varying excitation wavelengths from 300 to 370 nm.

Based on the excitation-wavelength-dependent multicolor emission features of Cs₂NaRECl₆ (RE = Tb, Eu) NCs, the anticounterfeiting patterns were fabricated in combination with Cs₂NaTb_0.5Eu_0.5Cl₆ NCs to explore the anticounterfeiting performance of RE-based halide double perovskites. As presented in Fig. 6a, the leaf patterns filled with Cs₂NaEuCl₆, Cs₂NaTb_0.5Eu_0.5Cl₆, and Cs₂NaTbCl₆ NCs excited by 302 nm emit red, magenta, and green colors, and in contrast, under 365 nm light irradiation, the patterns show pink, bluish violet, and blue colors, respectively. To take advantage of the characteristics of Cs₂NaRECl₆, we also designed an anticounterfeiting pattern of digital alignment using Cs₂NaEuCl₆ NCs, Cs₂NaTb_0.5Eu_0.5Cl₆ NCs and Cs₂NaTbCl₆ NCs (Fig. 6b). We used Cs₂NaEuCl₆ NCs to fill the first number “8” and the vertical line on the top right of the second number “8”. The rest of the second number “8” is padded by Cs₂NaTb_0.5Eu_0.5Cl₆ NCs. The last number pattern is filled by Cs₂NaTbCl₆ NCs. The pattern exhibits two red numbers “8” and one green number “8” under 302 nm excitation. However, the middle number “8” of the digital alignment then turns to a number “6” and one pink vertical bar irradiated by 365 nm light. Finally, by filling Cs₂NaTbCl₆ NCs into the grooves of the QR code (Fig. 6c), the QR code can be identified to the information of “chemistry” by phone applications such as WeChat. It is suggested that Cs₂NaRECl₆ (RE = Tb, Eu) NCs have great potential for anticounterfeiting applications.

Fig. 6 (a) The photographs of leaf patterns made using Cs₂NaRECl₆ (RE = Tb, Tb_0.5Eu_0.5, and Eu) NCs. The patterns emit different PL color under 302 nm and 365 nm excitation. (b) The anticounterfeiting pattern of “888” number made of Cs₂NaRECl₆ (RE = Tb, Tb_0.5Eu_0.5, and Eu) NCs under 302 nm and 365 nm excitation. (c) Schematic diagram of the QR code made of Cs₂NaTbCl₆ for encryption and decryption.

In addition, Cs₂NaRECl₆ NCs could serve as phosphors for fabricating LEDs. An orange-red LED device was fabricated by coating Cs₂NaEuCl₆ NCs on a commercial 310 nm chip (Fig. S14a, ESI†). The electroluminescence spectrum of the orange-red LED device mainly consists of the emission peaks of Eu³⁺ ions at 593 and 616 nm with the CIE color coordinate of (0.477, 0.366) and the correlated color temperature (CCT) is 2124 K. The green LED device was prepared by coating the chip with Cs₂NaTbCl₆ NCs (Fig. S14b, ESI†). The electroluminescence spectrum consists of Tb³⁺ ion emissions with the CIE color coordinate of (0.313, 0.440) and the correlated CCT is 6047 K. A warm-white LED device with CIE color coordinate of (0.396, 0.387) and CCT of 3694 K could be fabricated by mixing Cs₂NaEuCl₆ and Cs₂NaTbCl₆ NCs (Fig. S14c, ESI†). The LED devices fabricated using RE-based Cs₂NaRECl₆ (RE = Tb, Eu) NCs exhibit bright emission under 20 mA forward current, indicating that Cs₂NaRECl₆ (RE = Tb, Eu) NCs have great potential for LED applications.

Conclusions

RE-based Cs₂NaRECl₆ (RE = Tb, Eu) halide double perovskite NCs with uniform morphology were prepared by the hot injection method. Cs₂NaRECl₆ NCs could exhibit not only intrinsic host emission but also the characteristic emissions of RE³⁺ (RE = Tb, Eu). Cs₂NaRECl₆ NCs possess the excitation-wavelength-dependent multicolor emission features. The emitting color of Cs₂NaTbCl₆ NCs gradually changes from green to blue under 280–370 nm excitation, and the emitting color of Cs₂NaEuCl₆ NCs gradually changes from red to pink under the excitation of 300–370 nm light. Based on the multicolor emissions of Cs₂NaRECl₆ NCs, the patterns (leaf and digital alignment) and QR code were designed to verify their anticounterfeiting potential. Furthermore, orange-red, green and warm-white LED devices were fabricated, showing the great potential of Cs₂NaRECl₆ NCs in optoelectronic applications.

Author contributions

Y. L., Q. X. Y., H. W. L., J. F., and H. J. Z. designed the experiments, interpreted the data, and co-wrote the manuscript. Y. L and Q. X. Y. carried out the syntheses, characterization studies, and data analyses. H. W. L. participated in the measurement and data analyses. X. Y. F., H. X. Y., and Z. Y. L. gave suggestions on writing the manuscript. J. F. and H. J. Z. discussed the results and commented on the manuscript.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by financial aid from the National Natural Science Foundation of China (22271273), and the International Partnership Program of the Chinese Academy of Sciences (121522KYSB20190022).

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Footnotes

† Electronic supplementary information (ESI) available: [DETAILS]. See DOI: https://doi.org/10.1039/d4tc01697a

‡ These authors contributed equally and should be regarded as co-first authors.

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