Yb³⁺/Eu³⁺/Ho³⁺ Tridoped Cs₂Ag_0.3Na_0.7InCl₆ Double Perovskite with Excitation-Wavelength-Dependent Triple Emission for Anti-Counterfeiting Application

Abstract

Developing a secure anti-counterfeiting technology with more dimensional encryptions is urgently demanded. The lead-free double perovskite (DP) family represented by A₂B_IB_IIIX₆ hold great potential for applications in advanced fluorescence anti-counterfeiting owing to of large-bandgap engineering via B_I/B_III site transmutation or exotic dopants. Herein, Ln³⁺ (Ln³⁺ = Eu³⁺, Ho³⁺, and Yb³⁺)-doped Cs₂Ag_0.3Na_0.7InCl₆ DP microcrystals (MCs) were firstly successfully synthesized by a hydrothermal method. By selective excitation of different luminescence centrals through precise control of excitation wavelength, we demonstrate dynamic color tuning in the Ln³⁺-doped Cs₂Ag_0.3Na_0.7InCl₆ DPMCs. Specifically, under various excitations of UV 300, 394, and NIR 980 nm, the as-synthesized DPMCs display triple emissions of warm yellow, red, and green, respectively. The warm yellow light stems from the self-trapped exciton (STE) downconversion (DC) luminescence of the DP matrix, while the red and green lights can be attributed to the strong Eu^{3+ 5}D₀→⁷F_J (J = 1, 2, 3, 4) DC luminescence and Ho³⁺ (⁵F₄→⁵I₈) upconversion (UC) luminescence sensitized by Yb³⁺. Thus, the as-synthesized Ln³⁺-doped Cs₂Ag_0.3Na_0.7InCl₆ DPMCs, which possess tunable combined DC/UC luminescence, show great potential to be an anti-counterfeiting material with a high security level.

1. Introduction

The security of information and data has attracted great attention, and is closely related to anti-counterfeiting technology. Fluorescence anti-counterfeiting technology has been highlighted by a facile decryption process, simple device requirement, and huge information-loading capacity [1,2]. The traditional single-mode fluorescent anti-counterfeiting, which is usually achieved by downconversion (DC) luminescence, shows inadequate security for high-level data and information due to its simple technical barriers. Thus, developing reliable luminescent materials with multiple colors and tunable luminescence by simultaneously combining DC and upconversion (UC) luminescence has become an important but challenging topic for increasing the security of anti-counterfeiting technology [1].

Lead halide perovskite is expected to be the newcomer material for anti-counterfeiting because of its excellent optical properties including tunable bandgap, low-cost solution synthesis, and defect tolerance [3,4]. Unfortunately, these materials suffer from the two drawbacks of the toxicity of Pb and their poor stability. The lead-free double perovskite (DP) family represented by A₂B_IB_IIIX₆ has thus garnered particular interest in recent years due to its environmentally friendly nature and long-term operation, and especially enabling of large-bandgap engineering via B_I/B_III site transmutation or exotic dopants [1,5]. The doping of metal ions with s- and d-electrons in DPs, which can provide multiple emission centers, is considered one of the most common methods for achieving multicolor emission by DC luminescence. Typically, the emission color of Sb³⁺ and/or Mn²⁺ doped Cs₂NaInCl₆ DPs can be tuned from yellow to red or blue [6,7]. However, the simultaneously achieved DC and UC luminescence in perovskite materials, which is highly important for increasing the security level, is still rarely obtained [8,9,10]. To achieve the goal, lanthanide ions (Ln³⁺) are considered an ideal candidate due to their unique 4f electronic configurations and rich UC energy-level structures. Moreover, doping rare earth ions in DPs can be easily achieved due to their similar ionic radius and valence state, as well as the preferred coordination number (CN) of 6 (same as trivalent metal cations in DPs) [11,12,13,14], which have achieved their prospective applications in light-emitting diodes (LEDs) and near-infrared bioimaging, etc. [14,15,16,17,18]. Thus, lanthanide ions doped DPs can be expected to be one of the most promising anti-counterfeiting materials for achieving multicolor emission with both DC and UC luminescence, which has rarely been studied until now.

The f–f transitions in Ln³⁺ usually exhibit a very low absorption coefficient due to the forbidden f–f transition [19], which was generally overcome by the codoping of sensitizers in phosphors such as Yb³⁺. Yb³⁺ has a large absorption coefficient near 980 nm and the energy can be effectively transferred to activators such as Ho³⁺, Er³⁺ for achieving UC luminescence [8,17,20,21], which is largely attributed to the feature of the single-transition channel of ²F_7/2→²F_5/2 in Yb³⁺ ion [22]. To achieve multicolor emission with both DC and UC luminescence, herein lead-free Yb³⁺/Eu³⁺/Ho³⁺ tridoped DPs are developed as high-security anti-counterfeiting material that exhibits trimodal luminescence of warm yellow, red, and green under light excitations at 300, 394 and 980 nm, respectively. The warm yellow emission observed in Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ can be attributed to the self-trapped exciton (STE) emission of the DP matrix, while the red and green emissions originate from the strong transitions of ⁵D₀→⁷F_J (J = 1, 2, 3, 4) of Eu³⁺ and ⁵F₄→⁵I₈ of Ho³⁺, individually. In addition, the effect of the doping concentration of Ln³⁺ on the optical properties and the corresponding photon relaxation dynamics of as-prepared Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ are also systemically investigated.

2. Materials and Methods

Materials: Cesium chloride (CsCl, Macklin, 99.99%, Shanghai, China), silver chloride (AgCl, Macklin, 99.5%, Shanghai, China), sodium chloride (NaCl, Macklin, 99.5%, Shanghai, China), indium chloride (InCl₃, Macklin, 99.99%, Shanghai, China), Europium(III) chloride hexahydrate (EuCl₃·6H₂O, Macklin, 99.9%, Shanghai, China), holmium chloride hexahydrate (HoCl₃·6H₂O, Macklin, 98%, Shanghai, China) and Ytterbium(III) chloride hexahydrate (YbCl₃·6H₂O, Macklin, 98%, Shanghai, China) were used without further purification.

Synthesis of Cs₂Ag_0.3Na_0.7InCl₆:100%Yb³⁺, 50%Eu³⁺, 50%Ho³⁺ Microcrystals (MCs): As shown in Figure S1 (see supplementary), firstly, CsCl (2 mmol), AgCl (0.3 mmol), NaCl (0.7 mmol), InCl₃ (1 mmol), EuCl₃·6H₂O (0.5 mmol), HoCl₃·6H₂O (0.5 mmol), YbCl₃·6H₂O (1 mmol) and HCl (12 mL, 10 M) were added in a 25 mL Teflon autoclave. Then, the mixture solution was magnetically stirred vigorously for 30 min at room temperature. The sealed stainless-steel reactor was heated to 180 °C and kept for 12 h. The reactor was cooled to room temperature at a speed of 5 °C h⁻¹. The as-synthesized MCs were washed with isopropanol three times and then placed on the filter paper for natural drying. The dried samples were ground with a mortar for further characterization.

Characterization: The structure, morphology, and composition of the MCs were characterized by field-emission scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan), X-ray diffraction (XRD, D8 Advance, Bruker, Karlsruhe, Germany), and transmission electron microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan). The UV-vis absorption spectra of the phosphors were recorded on a UV-visible spectrophotometer (Hitachi UV-3900, Tokyo, Japan). The steady-state PL, PLE spectra, and PL QYs were recorded using a spectrometer (Fluromax-4P, Horiba Jobin Yvon, Paris, France) equipped with a QY accessory. The PL decay curves were recorded on a spectrometer(FLS1000, Edinburgh Instruments, Livingston, Britain).

3. Results and Discussion

Figure 1a presents the typical SEM image of Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ MCs, and a truncated octahedral morphology with grain sizes of ~2–5 μm can be determined. The diffraction rings in the selected area electron diffraction (SAED) pattern (Figure 1b) can be assigned to (222), (331), (521), and (800) crystal planes of the sample, which have high accordance with the XRD result. The HRTEM image exhibits a legible lattice fringe for the (331) plane with an interplanar distance of 0.248 nm, indicating the high crystallinity of the sample (Figure 1c). The EDX spectrum (Figure S2) indicates the present of Cs, Ag, Na, In, Eu, Ho, Yb and Cl in the Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ MCs, and the uniform distribution of above elements are further confirmed by the EDX mapping images, as shown in Figure 1d–l, which indicates that the Ln³⁺ are successfully doped into Cs₂Ag_0.3Na_0.7InCl₆ matrix without obvious aggregation.

Powder XRD characterization is performed to determine the crystallinity and phase purity of the as-prepared Ln³⁺-doped/undoped Cs₂Ag_0.3Na_0.7InCl₆ DPs. As presented in Figure 2, all samples exhibit obvious diffraction peaks, which correspond to the cube phases of Cs₂Ag_0.3Na_0.7InCl₆ (JCPDS No. 74-0484), meaning the phase structure of the matrix can be retained after doping. It is worth noting that all of the diffraction peaks are sharp and without any additional peaks after doping, indicating their excellent crystallinity and high purity [19]. The split XRD peaks observed between 40° and 55° can be confirmed as the overlapped two peaks corresponding to pure Cs₂Ag_0.3Na_0.7InCl₆ host and the tridoped Cs₂Ag_0.3Na_0.7InCl₆ MCs, meaning the presence of a mixed phase in the as-prepared tridoped Cs₂Ag_0.3Na_0.7InCl₆ sample, which should be further optimized by precisely controlling the reaction parameters. Ln³⁺ of Yb³⁺ (0.86 Å), Eu³⁺ (0.95 Å), and Ho³⁺ (0.9 Å) can replace In³⁺ (0.8 Å), Na⁺ (1.02 Å) or Ag⁺ (1.15 Å) theoretically. The replacement of Na⁺ and Ag⁺ by Yb³⁺/Eu³⁺/Ho³⁺ will induce the shift of the diffraction peaks to a lower angle, which is inconsistent with our situation [23]. Thus, the In³⁺ in Cs₂Ag_0.3Na_0.7InCl₆ will be preferentially replaced by Yb³⁺/Eu³⁺/Ho³⁺ attributable to the similarity of ionic radii and ionic charges, which can be confirmed by the undetectable shift of the diffraction peak in our case.

Figure 3 shows the absorption spectra of Ho³⁺/Yb³⁺ co-doped, Eu³⁺/Yb³⁺ co-doped, and Yb³⁺/Eu³⁺/Ho³⁺ tridoped Cs₂Ag_0.3Na_0.7InCl₆ DPs in the range of 350–1350 nm. The transition of the Eu³⁺, Ho³⁺, and Yb³⁺ from the ground states to the excited states is identified by the characteristic lines in the absorption spectra. It can be observed that the absorption peak at 960 nm (marked as 9) is the ²F_7/2→²F_5/2 transition of Yb³⁺ (Figure 3a). The characteristic absorption bands centered at 417, 452, 483, 538, 642, and 1150 nm corresponding to the transitions of Ho³⁺ from the ground state of ⁵I₈ to the excited states of ⁵G₅, ⁵G₆, ⁵F₃, ⁵F₄, ⁵F₅ and ⁵I₆, respectively (Figure 3b). Meanwhile, the absorption bands centered at 390 and 472 nm (marked as 1 and 2) can be ascribed to the transition of Eu³⁺ from the ground state of ⁷F₀ to the excited states of ⁵D₂ and ⁵D₁ (Figure 3c). The absorption bands of Ho³⁺/Eu³⁺/Yb³⁺ can be observed in Yb³⁺/Eu³⁺/Ho³⁺ tridoped sample, suggesting the successful doping of Yb³⁺/Eu³⁺/Ho³⁺ ions into the Cs₂Ag_0.3Na_0.7InCl₆ host.

Two groups of samples are rationally designed to study the luminescence mechanism of as-prepared Yb³⁺/Eu³⁺/Ho³⁺ tridoped Cs₂Ag_0.3Na_0.7InCl₆ DPs. For the first series, the molar concentration ratios (all relative to In³⁺) of Yb³⁺ and Eu³⁺ are both fixed at 100%, while that of Ho³⁺ is varied from 20% to 100% (Figure 4a,c,e). In the second series, the concentrations of Eu³⁺ and Ho³⁺ are varied while keeping the total ratio of 100% unchanged (Figure 4b,d,f). As shown in Figure 4a,b, a broad emission band located in the 320–720 nm range with a PL quantum yield (QY) of 28.1% is observed under the excitation of UV 300 nm, which derived from the STEs of the Cs₂Ag_0.3Na_0.7InCl₆ matrix with an average decay lifetime of 5.304 μs, is similar to that found in other lead-free double perovskites (Figure S4) [12,24], while the narrow emission bands at around 490, 542, 650, 590, 615, and 698 nm can be attributed to the transitions of Ho^{3+ 5}F₃ →⁵I₈, ⁵F₄→⁵I₈, ⁵F₅→⁵I₈ and Eu^{3+ 5}D₀ →⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₄, respectively. The STE emission decreases sharply with increasing Eu³⁺ concentration (Figure 4a,b), suggesting the presence of an energy transfer from the host to the energy levels of the Eu³⁺ [25,26], leading to the competition PL between Eu³⁺ and STE. The emission band peaked at 650 nm can be attributed to ⁵F₅→⁵I₈ of Ho³⁺ rather than the Eu³⁺, which otherwise would have other characteristic transitions of Eu³⁺ in the PL spectra. Furthermore, a sharp concave peak can be seen at 540 nm in Figure 4a,b, and the concave peak is more obvious in the samples with heavier Ho³⁺ doping, which seems due to the ⁵I₈→⁵F₄(⁵S₂) resonance absorption of Ho³⁺. Under the excitation of UV 394 nm (Figure 4c,d), the Eu³⁺ ions can be directly excited to generate Eu³⁺ characteristic red emissions, which exhibit strong Eu^{3+ 5}D₀→⁷F_J (J = 1,2) emissions peaked at 590 and 615 nm, and weak transitions of ⁵D₀→⁷F₃ (650 nm) and ⁵D₀→⁷F₄ (698 nm) with a PL QY of 4.1%. The peak at 590 nm can be attributed to the magnetic-dipole transition, while the peaks at 615, 650, and 695 nm should correspond to the electric-dipole transitions, which will be allowed when the Eu³⁺ ions occupy the crystallographic site lack inversion symmetry [19,27]. In our case, the ⁵D₀→⁷F₂ electric-dipole transition displays the strongest emission at 615 nm, which means the Eu³⁺ ions mainly occupy the crystallographic site which lacks inversion symmetry. The decay curve by monitoring the emission at 613 nm can be fitted by a signal exponential decay function with a lifetime of 108 μs (Figure S4), which suggests that the 4f electrons of the Eu³⁺ ions are well screened from surrounding defect sites, via the closed 5s²5p⁶ outer shell electrons of Eu³⁺ [25,26]. The PL intensity decreases with increasing Ho³⁺ concentration, indicating the suppressor effect of Ho³⁺ on the Eu³⁺ PL. Under the excitation of near-infrared laser (NIR) 980 nm (Figure 4e,f), the green UC luminescence from the Ho³⁺ (⁵F₄→⁵I₈) sensitized by Yb³⁺ dominated the emission spectra, the Yb³⁺ ions in the ground state can be excited to the ²F_5/2 level by absorbing 980 nm photons. Energy on the excited ²F_5/2 level Yb³⁺ ions can be transferred to the Ho³⁺ ions, result in the strong green emission for the DPs co-doped with Yb³⁺ and Ho³⁺. Thus, the phenomenon of strong green emission is observed in the UC emission. Besides the green emission of Ho³⁺, the other typical emission peaked at 652 nm were observed, which origins from the transition ⁵F₅→⁵I₈ of Ho³⁺ and can be suppressed by the doping of Eu³⁺.

Figure 5a shows PLE spectra (λ_em = 613 nm) of the samples with a fixed content of Eu³⁺ and increasing content of Ho³⁺. As shown, besides a broad band, many sharp peaks presented in the PLE spectra, confirming that the characteristic luminescence of Eu³⁺ emission peaked at 613 nm can be obtained by direct excitation of the Eu³⁺ in Cs₂Ag_0.3Na_0.7InCl₆ host, especially by the ⁷F₀→⁵D₃ transition at 394 nm. The PLE intensity gradually decreases with increasing Ho³⁺ concentration, which indicates the suppressed Eu³⁺ emission by the doping of Ho³⁺. In addition, the presence of the broad PLE band in the 240 to 330 nm range is due to the overlapped emission of host STEs and Ho³⁺ emission at 613 nm was monitored (Figure 4a) [20]. This speculation can be further confirmed by the PLE spectra detected at 570 nm as shown in Figure 5b, in which a similar PLE band range from 240 to 330 nm is presented. Figure 4b shows the PLE spectra of this series of samples detected at 570 nm. As shown, except the decreased PLE intensity with increasing Ho³⁺ concentration, no detectable spectra shift can be observed, which indicates that Ho³⁺ doping weakens but does not change the origination of STEs emission of the Cs₂Ag_0.3Na_0.7InCl₆ host [1].

Based on the above results, the possible PL mechanism for Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ for multimodal anti-counterfeiting application is shown in Figure 6. Under UV 300 nm excitation, the electrons of Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ can be excited from the ground states to the excited states, and excited electrons can be depopulated by the following three paths: converted to the STEs through nonradiative relaxation, energy transfers to Ho³⁺ and Eu³⁺, and in turn produces the yellow emission as shown in Figure 6a. Under excitation of UV 394 nm, the Eu³⁺ ions can also be directly excited to generate Eu³⁺ characteristic red emissions (Figure 6b). While Figure 6c shows the simplified energy diagram of Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ DPs under excitation of NIR 980 nm. As shown, the Ho³⁺ is largely excited by the Yb³⁺through the energy transfer, which presents a larger infrared light absorption cross-section than that of Ho³⁺ [25]. In addition, the ground state of the Yb³⁺ can be excited to the ²F_5/2 level by absorbing 980 nm photons, and energy on the exciting ²F_5/2 level can be transferred to the Ho³⁺. The energy difference between the ⁵I₇ and ⁵I₆ levels of Ho³⁺ and the ⁷F₆ and ⁷F₀ levels of Eu³⁺ matches well. Thus, the phenomenon of the strong green emission accompanied by weakened red emission is observed in the UC emission with co-doped Yb³⁺ and Ho³⁺, and the red light in the range of 635–670 nm tends to be extinguished with increasing Eu³⁺ concentration by the resonant energy transfer process and transferred to the ⁷F₆ level of Eu³⁺. Therefore, strong green emissions can be obtained by the appropriate doping ratio of Eu³⁺ to Ho³⁺ [15].

Figure 7 shows the photographs of as-prepared DPMCs/toluene/polystyrene composite films under excitations of UV 254 nm, UV 365 nm, and NIR 980 nm, which was directly patterned the precursor solution on the black paperboard to create a film with “HUT” and “NBUT” icons with the thickness of ~68 μm (Figure S3). Information encryption and decryption can be switched on and off using the patterned film to realize the purpose of anti-counterfeiting. The pattern is colorless under natural light (encryption), while the bright yellow (Figure 7a), red (Figure 7b), and green (Figure 7c) emissions are clearly distinguished under irradiations of UV 254 nm, UV 365 nm, and NIR 980 nm (decryption), respectively. The above results directly present the potential applications of Cs₂Ag_0.3Na_0.7InCl₆: Yb³⁺/Eu³⁺/Ho³⁺ materials in high-level anti-counterfeiting technology.

4. Conclusions

In conclusion, we reported the design and the experimental synthesis of Yb³⁺/Eu³⁺/Ho³⁺ tridoped Cs₂Ag_0.3Na_0.7InCl₆ DPMCs with trimode luminescence for application in anti-counterfeiting with high security. The successful doping of Yb³⁺/Eu³⁺/Ho³⁺ into the Cs₂Ag_0.3Na_0.7InCl₆ matrix was systematically confirmed by EDS and absorption spectra characterization. Under excitation of UV 300 nm, in addition to a broad STE emission with bright yellow, some narrow emission bands corresponding to Eu³⁺ and Ho³⁺ are also presented in the PL spectra, indicating the presence of energy transfer processes from exciton to Ho³⁺ and Eu³⁺. Under 394 nm excitation, Eu³⁺ has strong characteristic luminescence at 613 nm with red emission by directly absorbing the excitation light. Under 980 nm excitation, because the sublevels of Ho³⁺ and Yb³⁺ are more matched, Yb³⁺ absorbs two photons of 980 nm and then transfers energy to Ho³⁺, which makes Ho³⁺ relax from ⁵S₂ to ⁵F₄, then emitting the characteristic green light through the ⁵F₄→⁵I₈ transition. Finally, the as-synthesized Ln³⁺-doped Cs₂Ag_0.3Na_0.7InCl₆ DPMCs–toluene–polystyrene composite films yield tunable DC–UC combined luminescence by selective excitation of different luminescent centrals through precise control of excitation wavelength, showing great potential as an anti-counterfeiting material with high security.