Multiple Energy Transfer Channels in Rare Earth Doped Multi‐Exciton Emissive Perovskites

Abstract

Revealing the energy transfer (ET) process from excitons to rare earth ions in halide perovskites has great guiding value for designing optoelectronic materials. Here, the multiple ET channels in multi‐exciton emissive Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ are explored to comprehend the ET processes. Förster–Dexter ET theory reveals that the sensitizer concentration rather than the overlap integral of the spectra plays the leading function in the comparison of the ET efficiency among multiple ET channels from the host self‐trapped excitons (STEs) and dopant triplet STEs to Nd³⁺ ions. Besides, Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ enables varied color delivery and has great potential as anti‐counterfeiting material. Under X‐ray irradiation, Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ presents a high light yield of ≈13300 photons MeV⁻¹ and promising X‐ray imaging ability. This work provides new insight for investigating the ET efficiency among multiple ET processes and presents great potentiality of multi‐exciton emissive perovskites in the fields of anti‐counterfeiting and X‐ray imaging.

Förster–Dexter energy transfer (ET) theory reveals that the sensitizer concentration rather than the overlap integral of the spectra plays the leading function in the comparison of the ET efficiency among multiple ET channels from the host self‐trapped excitons (STEs) and dopant triplet STEs to Nd³⁺ ions in Cs₂ZrCl₆:Sb³⁺,Nd³⁺. Besides, Cs₂ZrCl₆:Sb³⁺,Nd³⁺ presents great potentiality in the fields of anti‐counterfeiting and X‐ray imaging.

1. Introduction

Self‐trapped excitons (STEs) widely exist in alkali metal halide crystals, silica, and simple organic molecular crystals, and form in the excited‐state, where electron–phonon coupling is strong enough for excited electrons and holes to severely deform the lattice to create a potential well and trap the excitons in it.^{[ 1} , ² , ³ , ⁴ , ^{5 ]} Particularly in past few years, STEs play a leading function in improving the optical property of lead‐free perovskites, which usually display low photoluminescence quantum yields (PLQYs) due to the indirect bandgap or the parity‐forbidden transition in direct bandgap perovskites.^{[ 6} , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ^{13 ]} STEs in lead‐free perovskites could be divided into intrinsic STEs and extrinsic STEs. The former exists in lead‐free perovskite host and the latter usually could be induced via introducing ns² dopants, such as Sb³⁺, Bi³⁺, Te⁴⁺, etc.^{[ 14} , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ^{22 ]}

Apart from improving the luminescent performance of lead‐free perovskites, STEs could sensitize other emission centers (rare earth ions, RE³⁺; manganese ions, Mn²⁺, etc.) via the energy transfer (ET) channel.^{[ 23} , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ^{28 ]} A part of these luminescent ions, such as Sm³⁺, Er³⁺, Tm³⁺, Nd³⁺, and Ho³⁺, always exhibit weak light absorption caused by the electric‐dipole‐forbidden of 4f → 4f transitions, which limits their downshifting luminescent performance. However, building the ET channels from the STEs to these ions could overcome this drawback because the STEs always possess broad and intense UV or blue light absorption. As a consequence, the ET channel has been studied deeply and employed to tune the optical performance of lead‐free perovskites in the visible and near infrared region (NIR).^{[ 29} , ³⁰ , ^{31 ]} Meanwhile, previous reports focused on the ET channel from single STEs to single or multiple emission centers. As far as we know, there is a lack of sufficient research on the ET channels from multiple STEs to single emission center and the efficiency of different ET processes in rare earth doped multi‐exciton emissive perovskites. It is challenging and important to explore and understand these multiple ET processes for designing optoelectronic materials.

Compared with those common lead‐free perovskites with two dopant extrinsic STEs, such as Cs₂SnCl₆:Sb³⁺ and Cs₂NaInCl₆:Sb³⁺/Bi³⁺, 0D lead‐free perovskite Cs₂ZrCl₆:Sb³⁺ with a host intrinsic STEs and two dopant extrinsic STEs was selected as multi‐exciton emissive material for the more abundant STEs.^{[ 32} , ³³ , ³⁴ , ^{35 ]} According to the previous reports, the three kinds of STEs in Cs₂ZrCl₆:Sb³⁺ could be assigned to the host intrinsic STEs in the [ZrCl₆]²⁻ octahedra, the singlet and triplet dopant extrinsic STEs in the [SbCl₆]³⁻ octahedra, respectively. Among common emission centers (RE³⁺, Mn²⁺, etc.), Nd³⁺ ions as an intense NIR emitter could be sensitized by the STEs in perovskite and have no visible emission that would overlap and disturb the broad STEs emissions.^{[ 36 ]} Consequently, it is feasible for choosing Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ perovskite to investigate the different ET processes between multiple STEs and other emission centers.

Herein, we fabricated Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ microcrystals (MCs) with three STEs emissions and Nd³⁺ NIR emission through room temperature precipitation method. The efficiency of multiple ET processes from the host STEs and dopant STEs to Nd³⁺ ions was investigated by the time‐resolved photoluminescence (PL) decays. Förster–Dexter ET theory demonstrates the sensitizer concentration rather than overlap integral of the PL spectrum of sensitizer and the absorption spectrum of activator plays the dominant role in the comparison of the ET efficiency among multiple ET channels from the different STEs to Nd³⁺ ions. Furthermore, Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ MCs display the great potential as anti‐counterfeiting material and X‐ray scintillator.

2. Results

Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ MCs were synthesized through a coprecipitation method in a mixed solution of hydrochloric acid and methanol at room temperature. The actual dopant concentrations in Sb³⁺ doped and Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ MCs were ascertained by inductively coupled plasma optical emission spectrometry (ICP‐OES), which provided a much lower actual Nd³⁺ concentrations than its feeding concentrations due to the quite different solubility of RE halides from other metal halides in mixture solution (Tables S1 and S2, Supporting Information). Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ MCs possess vacancy‐ordered double perovskite structure (space group Fm $\overline{3}$ m), in which isolated [ZrCl₆]²⁻ octahedra are surrounded by Cs⁺ cations (Figure 1a).

Figure 1.

a) Schematic of the cubic crystal structure of Cs₂ZrCl₆:Sb³⁺,Nd³⁺ MCs. b) Powder XRD patterns (left panel) and magnified XRD peaks (right panel) of Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺ MCs. c) XPS spectrum of Cs₂ZrCl₆:0.5%Sb³⁺,80%Nd³⁺. d) SEM image and elemental mappings of Cs₂ZrCl₆:0.5%Sb³⁺,80%Nd³⁺ MCs. e) UV–vis absorption spectra of undoped, Sb³⁺ doped, and Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ MCs. f) Normalized PL (dashed lines) and PLE (solid lines) spectra of undoped (top) and 0.5% Sb³⁺ doped Cs₂ZrCl₆ MCs (bottom).

After doping Nd³⁺ and Sb³⁺ ions, the formed [SbCl₆]³⁻ octahedra and [NdCl₆]³⁻ octahedra can partially replace [ZrCl₆]²⁻ octahedra. In Figure 1b and Figure S1 (Supporting Information), the powder X‐ray diffraction (XRD) patterns of Cs₂ZrCl₆:Sb³⁺ and Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺ MCs agree well with the standard pattern of cubic Cs₂ZrCl₆ (PDF#74‐0505), indicating all the prepared MCs are pure phase without impurity. Meantime, with the increasing concentrations of Sb³⁺ or Nd³⁺ ions, the characteristic diffraction peak (220) shifts gradually to lower angle, implying the expansion of lattice due to the larger ion radius of Sb³⁺ ions (0.76 Å, CN = 6) and Nd³⁺ ions (0.98 Å, CN = 6) than that of Zr⁴⁺ ions (0.72 Å, CN = 6).^{[ 37 ]} X‐ray photoelectron spectroscopy (XPS) identifies the presence of Cs⁺, Zr⁴⁺, Cl⁻, Sb³⁺, and Nd³⁺ in Cs₂ZrCl₆:Sb³⁺,Nd³⁺ MCs, indicating the successful doping of Nd³⁺ and Sb³⁺ ions (Figure 1c). The SEM image and the elemental mappings reveals homogeneous distribution of Cs⁺, Zr⁴⁺, Cl⁻, Sb³⁺, and Nd³⁺ in Cs₂ZrCl₆:Sb³⁺,Nd³⁺ MCs (Figure 1d).

The optical properties of as‐prepared Zr‐based MCs were investigated. In Figure 1e, the absorption spectra reveal the additional overlapping double peaks ranging from 300 to 425 nm, corresponding to the ¹S₀ → ^{1, 3}P₁ transitions of Sb³⁺ ions. Besides, a narrow absorption peak at 584 nm only observed in Nd³⁺/Sb³⁺ co‐doped Cs₂ZrCl₆ MCs could be ascribed to ⁴I_9/2 → ⁴G_5/2 transition of Nd³⁺ ions.^{[ 38 ]} Then, the PL and PL excitation (PLE) spectra of undoped and Sb³⁺ doped Cs₂ZrCl₆ MCs were carried out (Figure 1f and Figure S2, Supporting Information). Two broad PL peaks at 490 and 630 nm in the PL spectra of Cs₂ZrCl₆:Sb³⁺ excited by 320 nm correspond to the singlet STEs (¹STEs) and the triplet STEs (³STEs) in the [SbCl₆]³⁻ octahedra, respectively. As the increasing of Sb³⁺ ions concentration, the two dopant STEs emissions become stronger and ³STEs emission become more dominant than ¹STEs emission at the meantime, which may be ascribed to the enhanced ET process from ¹STEs to ³STEs.^{[ 35 ]} Besides, under the excitation of 256 nm, the PL spectra of Cs₂ZrCl₆:Sb³⁺ exhibit the weak emission peaked at 450 nm that also exists in the PL spectrum of undoped MCs, corresponding to the host STEs emission in the [ZrCl₆]²⁻ octahedra. The decreased PL intensity of host STEs emission in Cs₂ZrCl₆:Sb³⁺ as the increase of Sb³⁺ doping concentration could be ascribed to the increasing defect density caused by Sb³⁺ ions rather than the ET process from host STEs to dopant STEs, because the emission processes of them are mutually independent, which could be largely attributed to the hindered charge carriers transport between the neighboring octahedra in Cs₂ZrCl₆:Sb³⁺ perovskite.^{[ 34 ]} The PLE spectrum of undoped Cs₂ZrCl₆ monitored at 450 nm exhibit a single PLE peak at 256 nm that corresponds to the Zr ─Cl charge transfer transition in the [ZrCl₆]²⁻ octahedra. Besides, for the emissions at 490 and 630 nm, the PLE spectra of Cs₂ZrCl₆:0.5%Sb³⁺ show the different patterns. The former exhibits two PLE peaks at 256 and 320 nm, corresponding to the Zr─Cl charge transfer transition and ¹S₀ → ¹P₁ transition of Sb³⁺ ions, respectively. Here, the existence of PLE peak at 256 nm in Cs₂ZrCl₆:0.5%Sb³⁺ could be ascribed to the distribution of host STEs at 490 nm. The latter consists of a doublet (peak fitting 1, 277 nm; peak fitting 3, 340 nm) band and a weaker band (peak fitting 2, 310 nm), corresponding to the ¹S₀ → ³P₁ and ¹S₀ → ¹P₁ transitions of Sb³⁺ ions, respectively (Figure S2d, Supporting Information). Interestingly, the doublet band shows a greater splitting than that of Sb³⁺‐doped Cs₂NaInCl₆ in previous reports (≈20 nm for Sb³⁺‐doped Cs₂NaInCl₆; ≈63 nm for Sb³⁺‐doped Cs₂ZrCl₆), implying a dynamic Jahn–Teller distortion of the excited‐state (³P₁) with a greater extent for crystal environment that is more prone to octahedral distortion.^{[ 39} , ⁴⁰ , ^{41 ]} Moreover, the PLE peak attributed to ¹S₀ → ¹P₁ transition exists in the PLE spectrum of Cs₂ZrCl₆:0.5%Sb³⁺ monitored at 630 nm, implying the ET from the singlet excited‐state (¹P₁) to the triplet excited‐state (³P₁).

After Nd³⁺ ions are introduced into Cs₂ZrCl₆:0.5%Sb³⁺ MCs, sharp NIR emission appears resulting from the ET from STEs to Nd³⁺ ions. The PL spectra of Cs₂ZrCl₆:0.5%Sb³⁺,(20–80%)Nd³⁺ MCs excited by 256 and 320 nm are shown in Figure 2a,b,d,e. As the increase of Nd³⁺ concentration, both the host STEs and dopant ³STEs emissions decline to a great extent along with the rising of Nd³⁺ NIR emission, implying the possibility of ET processes from the host STEs and dopant ³STEs to Nd³⁺ ions. Interestingly, compared with the PL intensity the host STEs and ³STEs, that of ¹STEs exhibits slight decline with the Nd³⁺ ions concentration increasing, implying that ¹STEs may contribute negligible carriers to Nd³⁺ ions and indicating that the Nd³⁺ concentration could tune the relative intensity of ¹STE and ³STE emissions (Figure S3, Supporting Information). The ET channel could be further investigated by the PLE spectra. In Figure 2c, the PLE spectrum of Cs₂ZrCl₆:0.5%Sb³⁺,80%Nd³⁺ monitored at 1075 nm exhibits a broad band (275–400 nm) and a narrow peak at 256 nm. The former shares similar excitation pattern with that of ³STEs emission measured at 630 nm while the latter originates from the Zr─Cl charge transfer transition and could also be observed in the PLE spectrum monitored at 450 nm, implying that Nd³⁺ share the same excited‐state with the host STEs and dopant ³STEs, and Nd³⁺ emission is sensitized by the ET from the host STEs and dopant ³STEs to Nd³⁺ ions rather than direct excitation of Nd³⁺ ions. Besides, compared with that monitored at 490 nm, the PLE spectrum monitored at 1075 nm does not show a prominent PLE peak at 320 nm (¹S₀ → ¹P₁), indicating the negligible ET contribution from ¹STEs.

Figure 2.

PL spectra of Cs₂ZrCl₆:0.5%Sb³⁺,(0–80%)Nd³⁺ MCs in the visible region excited by a) 256 nm and b) 320 nm. c) Normalized PLE spectra of Cs₂ZrCl₆:0.5%Sb³⁺,80% Nd³⁺ MCs monitored at different wavelength. PL spectra of Cs₂ZrCl₆:0.5%Sb³⁺,(0–80%)Nd³⁺ MCs in NIR region excited by d) 256 nm and e) 320 nm. PL spectra versus temperature of Cs₂ZrCl₆:0.5%Sb³⁺,80% Nd³⁺ MCs in visible region excited by f) 256 nm and g) 320 nm. PL spectra versus temperature of Cs₂ZrCl₆:0.5%Sb³⁺,80% Nd³⁺ MCs in NIR region excited by h) 256 nm and i) 320 nm.

The PL spectra recorded at 100–340 K could also prove and understand the ET process in Cs₂ZrCl₆:0.5%Sb³⁺,80% Nd³⁺ MCs. Excited by 256 nm, the host STEs emission become stronger (100–220 K) and then weaker (220–340 K) with the increasing temperature (Figure 2f). This abnormal behavior may be attributed to the competition between thermal‐induced PL quenching and thermally activated delayed fluorescence, the latter involves the reverse intersystem crossing between the triplet and singlet excited‐states.^{[ 42} , ^{43 ]} Meantime, under the excitation at 256 nm, the PL intensity of Nd³⁺ NIR emission first increases (100–280 K) and decreases subsequently (280–340 K) with the increasing temperature (Figure 2h). This phenomenon could be ascribed to the competition between thermal‐induced PL quenching and carrier transition to Nd³⁺ ions, implying the existence of ET process from the host STEs to Nd³⁺ ions.^{[ 44 ]} Besides, excited by 320 nm, the PL intensity of ¹STEs emission gradually decreases with the increase of temperature from 100 to 340 K, however, the ³STEs emission undergoes an abnormal increase from 160 to 190 K, which could be ascribed to the ET from the excited‐state ¹P₁ to ³P₁ upon increasing the temperature (Figure 2g; Figure S4, Supporting Information). In addition to this abnormal increase phenomenon, its PL intensity decreases gradually in the range of 100–160 K and 190–340 K. Furthermore, the Nd³⁺ emission excited by 320 nm first rises (from 100 to 220 K) and declines subsequently (from 220 to 340 K), and the similar phenomenon implies the ET channel from ³STEs to Nd³⁺ ions (Figure 2i). In combination with the results mentioned above, it could be demonstrated the existence of two ET channels from the host STEs and dopant ³STEs to Nd³⁺ ions.

To further understand the ET process and investigate the ET efficiency, the time‐resolved PL decays of Cs₂ZrCl₆:0.5%Sb³⁺,(0–80%)Nd³⁺ MCs were carried out. As exhibited in Figure 3a, after being fitted by the mono‐exponential function, the PL decay curves of Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺ monitored at host STEs emission (λ_ex = 256 nm) could provide the lifetimes of 14.82, 13.74, 13.55, 13.05, and 12.19 µs, corresponding to the Nd³⁺ concentrations of 0%, 20%, 40%, 60%, and 80%, respectively. These lifetimes match well with the previous reports and could be attributed to the host STEs recombination.^{[ 34 ]} Similarly, as shown in Figure 3b and Table S3 (Supporting Information), the PL decay curves monitored at 630 nm provide a series of lifetimes after fitted by mono‐exponential function, which can be assigned to the dopant ³STEs recombination. It could be observed that with the increasing Nd³⁺ ions concentration, the lifetimes of both host STEs and dopant ³STEs decrease due to the ET from them to Nd³⁺ ions. To further compare these ET processes, the ET efficiency η _t was adopted and can be obtained by the following equation:

$${\eta }_{\mathrm{t}}=1-\frac{{\tau }_{\mathrm{x}}}{{\tau }_{\mathrm{s}}}$$ (1)

where τ_x and τ_s are the lifetimes of STEs in the presence or absence of Nd³⁺ ions.^{[ 25 ]} As shown in Figure 3c, with Nd³⁺ concentration increasing, the ET efficiency η _t increases gradually and host STEs have the higher value of η _t than that of dopant ³STEs at all Nd³⁺ feeding concentrations (20%, 40%, 60%, and 80%).

Figure 3.

The decay curves of Cs₂ZrCl₆:0.5%Sb³⁺, (0–80%) Nd3⁺ MCs monitored at a) 450 nm and b) 630 nm. c) The ET efficiency (η _t) variation of Cs₂ZrCl₆:0.5%Sb³⁺,(20–80%)Nd³⁺ MCs (monitored at 450 and 630 nm) as a function of Nd³⁺ feeding ratio. d) Schematic diagram of the proposed multiple ET model.

Furthermore, according to Förster–Dexter theory, the ET probability P _SA can be estimated by the following expressions^{[ 45} , ^{46 ]}:

$$P_{\mathrm{SA}}=\frac{1}{{\tau }_{\mathrm{s}}}{\left(\frac{R_0}{\mathrm{r}}\right)}^6$$ (2)

$$P_{\mathrm{SA}}=\frac{1}{{\tau }_{\mathrm{s}}}\left(\frac{{\eta }_{\mathrm{t}}}{1-{\eta }_{\mathrm{t}}}\right)$$ (3)

where R ₀ is the critical distance and r is the distance between sensitizer and activator. To our knowledge, the dopants are uniformly distributed in the lattice and the three cations (Zr⁴⁺, Sb³⁺, and Nd³⁺) share the same lattice sites in the unit cell, hence, the distance r between sensitizer and activator can be estimated according to Blasse's equation ^{[ 47 ]}:

$$r=2{\left(\frac{3V}{4\pi Nc}\right)}^{1/3}$$ (4)

where c is the total concentration of sensitizer and activator, N is the number of host cations that might be occupied by the activator and sensitizer in the unit cell, and V is the volume of the unit cell. Then, Equations (2) and (4) were substituted into Equation (3) to obtain Equation (5) as follows:

$$\frac{{\eta }_{\mathrm{t}}}{1-{\eta }_{\mathrm{t}}}=\frac{{\pi }^2{N}^2{R}_0^6{c}^2}{36V^2}$$ (5)

Then, the related parameters of two ET processes in Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺ were substituted into Equation (5) to obtain Equation (6).

$$\left(\frac{{\eta }_{t1}}{1-{\eta }_{t1}}\right)/\left(\frac{{\eta }_{t2}}{1-{\eta }_{t2}}\right)=\frac{R_{01}^6}{R_{02}^6}\times {\left(\frac{c_{Nd}+c_{Zr}}{c_{Nd}+c_{Sb}}\right)}^2$$ (6)

where η _t1 and η _t2 refer to efficiency of ET channels from [ZrCl₆]²⁻ octahedra to Nd³⁺ ions and from [SbCl₆]³⁻ octahedra to Nd³⁺ ions, respectively. R ₀₁ and R ₀₂ are the critical distance of ET channels from [ZrCl₆]²⁻ octahedra to Nd³⁺ ions and from [SbCl₆]³⁻ octahedra to Nd³⁺ ions, respectively. C _Nd, C _Zr, and C _Sb correspond to the concentrations of Nd³⁺, Zr⁴⁺ and Sb³⁺. As a result, the value of η _t/(1‐η _t) is influenced by two factors: the critical distance R ₀ and the total concentration of sensitizer and activator. The critical distance R ₀ can be estimated by the following expression^{[ 46 ]}:

$$R_0^6=\frac{3h^4{c}_{light}^4{Q}_A}{4\pi n^4}\underset{0}{\overset{\infty }{\int }}\frac{f_S\left(E\right)F_A\left(E\right)}{E^4}dE$$ (7)

where h is the Planck constant, c _light is the speed of light, f _S is the normalized PL spectrum of sensitizer, F _A is the normalized absorption spectrum of activator, n is the refractive index of the host medium, Q _A is the oscillator strength of the absorption transition of the activator, and E is the average energy of the overlapping transition. For the two ET channels in Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺, the critical distance R ₀ is proportional to the overlap integral of the normalized PL spectrum of sensitizer and the normalized absorption spectrum of activator. Considering the absorption peak of Nd³⁺ at 584 nm mentioned above, the PL spectrum of ³STEs possesses a larger overlap integral than that of the host STEs (Figure S5a, Supporting Information). As a result, the ET channel from the host STEs (ET1) shows a smaller R ₀ (R ₀₁ < R ₀₂) and a larger total concentration of sensitizer and activator. From the experimental results above, the ET1 possesses a larger ET efficiency η _t, implying a larger value of η _t/(1 − η _t) (Figure S5b, Supporting Information). This result indicates that conmpared with ET2, the advantage of the sensitizer (Zr⁴⁺) concentration is enormously sufficient to overcome the disadvantage of the critical distance for ET1 to display larger η _t and η _t/(1 − η _t). Therefore, the sensitizer concentration plays the leading function in the comparison of the two ET efficiency. The possible PL mechanism is proposed in Figure 3d, for Cs₂ZrCl₆:Sb³⁺,Nd³⁺ MCs, after the excitation of 320 nm, the electrons in the ground‐state (¹S₀) are excited to the ¹P₁ and ³P₁ excited‐states in [SbCl₆]³⁻ octahedra along with the ET from ¹P₁ to ³P₁, and then trapped to the singlet and triplet self‐trapped states, giving out cyan and orange broad emissions, corresponding to the recombination of ¹STEs and ³STEs, respectively. At the same time, an ET process with a lower efficiency from triplet self‐trapped state to excited level of Nd³⁺ occurs. Besides, upon excitation at 256 nm, electrons are excited from the ground‐state to excited‐state in [ZrCl₆]²⁻ octahedra, leading to blue emission resulting from the host STEs recombination. Meantime, partial energy is transferred from host self‐trapped state to excited level of Nd³⁺ with a higher efficiency. Finally, electrons located in the excited level of Nd³⁺ are relaxed to the ground‐state, resulting in the intense Nd³⁺ emission.

The PLQYs of Cs₂ZrCl₆, Cs₂ZrCl₆:0.5%Sb³⁺, and Cs₂ZrCl₆:0.5%Sb³⁺,(20–80%)Nd³⁺ MCs under the excitation of 256 and 320 nm are displayed in Table S4 (Supporting Information). Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺ display decreasing PLQYs with the Nd³⁺ concentration increasing, which could be attributed to the increasing defect density caused by Nd³⁺ dopant. Furthermore, the stability of Cs₂ZrCl₆:0.5%Sb³⁺,80%Nd³⁺ MCs was investigated. As displayed in Figure S6a (Supporting Information), the thermogravimetric (TG) curve measured in nitrogen indicates that the perovskite MCs display no obvious weight loss (below 5%) up to 400 °C, indicating the remarkable structure stability of Cs₂ZrCl₆:0.5%Sb³⁺,80%Nd³⁺ MCs. In Figure S6b (Supporting Information), after being stored for 100 days, the MCs exhibit the essentially unchanged XRD pattern. Meantime, the PL emissions of the MCs could be remained at ≈90% of the initial intensities, manifesting the great air stability of Cs₂ZrCl₆:0.5%Sb³⁺,80%Nd³⁺ MCs (Figure S6c–f, Supporting Information).

Multi‐exciton emissive Cs₂ZrCl₆:0.5%Sb³⁺,Nd³⁺ MCs are not only suitable for exploring the mutiple ET processes, but also served as anti‐counterfeiting materials for dynamic color delivery in the visible region by tuning the excitation wavelength. Considering that excessive Nd³⁺ ions would quench the host STEs and dopant ³STEs emissions severely, Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ with the lowest Nd³⁺ concentration was selected among Cs₂ZrCl₆:0.5%Sb³⁺,(20‐80%)Nd³⁺ MCs. In Figure S7a–c (Supporting Information), it could be observed that Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs render dynamic broad emissions from blue to orange with the excitation wavelength increasing from 250 to 380 nm, making it well suited for anti‐counterfeiting and information encryption. Undoped Cs₂ZrCl₆ and Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ powder were selectively filled into a pixelated pattern (5 × 20 dot matrix; Figure 4a). These compounds show the same color under sun light (white) or 254 nm UV light (blue), and the encrypted message could not be decrypted at this moment. Then, the excitation light source was replaced by 365 nm UV light, the “CIAC” orange pattern composed of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ powder could be observed to complete the decryption of encrypted information. Moreover, as shown in Figure 4b, by filling Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ powder into designed patterned grooves, delicate quick response (QR) code could be clearly presented. As‐prepared QR code image could be recognized directly by a commercial mobile phone application (WeChat), suggesting its prospects as high‐resolution PL imaging agent.

Figure 4.

a) Images of security patterns made of pure Cs₂ZrCl₆ and Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺. The decryption patterns emitting orange light upon excitation with 365 nm UV lamp were produced using Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺, and the remaining area was filled with pure Cs₂ZrCl₆ to form encrypted patterns. b) Patterned QR code of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ under visible light (top), and 254 nm UV light (bottom). Schematic diagram of as‐prepared QR code image is recognized directly by a commercial mobile phone application (WeChat) (right). c) RL spectra of Cs₂ZrCl₆:0.5%Sb³⁺,20% Nd³⁺, and LuAG:0.5%Ce³⁺ at dose rate of 669 µGy s⁻¹. d) RL integral intensity of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺,^, and LuAG:0.5%Ce³⁺ as a function of X‐ray dose rate. e) RL stability of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ under cyclical of X‐ray illumination at dose rate of 63 µGy s⁻¹. f) Schematic of the X‐ray imaging system. g) The photographs of a metallic key and a metallic allen wrench under the visible light (top) and X‐ray (bottom) with X‐ray dose of 669 µGy s⁻¹, which are all partially wrapped in plastic casing. h) MTF of the Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ thin film scintillation screen measured by the slanted‐edge method (inset).

Apparently, the multiple STEs emissions of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs show large Stokes shifts, which is generally favourable for Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs as the scintillator since self‐absorption caused by small Stokes shift could result in efficiency loss for scintillators.^{[ 11} , ^{48 ]} In order to investigate the scintillation performance of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺, its absorption coefficient and X‐ray attenuation efficiency are compared with those of common scintillator Lu₃Al₅O₁₂:0.5%Ce³⁺ (LuAG:0.5%Ce³⁺). In Figure S8a (Supporting Information), Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs show lower absorption coefficient than LuAG:0.5%Ce³⁺ from 15 to 20 keV. Then, the samples with thickness of 0.8 mm (Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺) and 1.0 mm (LuAG:0.5%Ce³⁺) were fabricated based on identical X‐ray attenuation efficiency (99%, at 17.5 keV) to unify the absorbed X‐ray energy of these two kinds of scintillators (Figure S8b, Supporting Information). X‐ray radioluminescence (RL) spectra of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs are presented in Figure 4c and Figure S8c (Supporting Information), host STEs emission and dopant ³STEs emission could be observed, which reveals the same luminescene mechanism as that excited by UV light. As the X‐ray dose rate increasing, the RL intensity of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ increases and presents linear response curve, from which its X‐ray light yield (LY) could be obtained indirectly by means of the commercial scintillator LuAG:0.5%Ce³⁺ (the LY of 25000 photons MeV⁻¹) as a reference (Figure 4d). Under the same X‐ray absorption cross sections, the response of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs is 0.532 times of LuAG:0.5%Ce³⁺ scintillator, thus the LY of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs is calculated as ≈13 300 photons MeV⁻¹, which is close to that of Cs₂ZrCl₆:2.1%Sb³⁺ scintillator (18 000 ± 700 photons MeV⁻¹) that reported recently.^{[ 15 ]} The lowest detection limit could determine the minimum dose rate required for detection and could be estimated when the signal‐to‐noise ratio (SNR) is 3. In Figure S8d (Supporting Information), the lowest detection limit of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs is estimated to be 1.424 μGy s⁻¹, which is far lower than the typical medical X‐ray diagnostic requirement.^{[ 49 ]} Moreover, the RL intensity of the MCs exhibits negligible fluctuations under X‐ray cyclical irradiation for 1 h, implying the great radiation stability of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs (Figure 4e). To investigate the X‐ray imaging ability of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ MCs, transparent and flexible scintillation film was fabricated by embedding the MCs powder into polydimethylsiloxane (PDMS) (5 cm × 5 cm × 0.1 cm; Figure S8e, Supporting Information). Subsequently, the object and flexible Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺@PDMS thin film were put in self‐built X‐ray imaging system (Figure 4f). A metallic key and a metallic allen wrench partially wrapped in plastic casing were selected as the testing objects to verify X‐ray detection ability of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺@PDMS thin film. The X‐ray contrast images with distinct inside structure are successfully disclosed due to the difference X‐ray transmittance between metal and plastic (Figure 4g). The modulation transfer functions (MTF) of X‐ray images was calculated using slanted‐edge images to further evaluate image quality (Figure 4h).^{[ 50 ]} The spatial resolution of Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺@PDMS thin film is acquired as 5.77 lp mm⁻¹ at MTF = 0.2. Compared with X‐ray imaging of recently reported lead‐free perovskite scintillator Cs₂ZnBr₄:25%Mn@PDMS thin film (5.06 lp mm⁻¹) and Cs₂Ag_0.6Na_0.4In_0.85Bi_0.15Cl₆ (4.4 lp mm⁻¹), Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺@PDMS thin film also exhibits great X‐ray imaging ability.^{[ 11} , ^{51 ]} The above‐mentioned advantages of as‐designed Cs₂ZrCl₆:0.5%Sb³⁺,20%Nd³⁺ exhibit its potentiality as a candidate of X‐ray scintillator.

3. Conclusion

In summary, Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ MCs with Nd³⁺ characteristic emission, host STEs emission and dopant singlet/triplet STEs emissions were prepared by a room temperature precipitation method. The multiple ET channels from the host STEs and dopant ³STEs to Nd³⁺ ions were explored and revealed. According to the Förster–Dexter ET theory, benefit from the high ion concentration of sensitizer rather than the small spectral overlap, the ET from the host STEs to Nd³⁺ ions exhibit the larger ET efficiency than that from the dopant ³STEs to Nd³⁺ ions. Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ enables dynamic color delivery under selective excitation and exhibits great potential as multi‐mode anti‐counterfeiting material to encrypt multilevel optical codes. Furthermore, Sb³⁺/Nd³⁺ co‐doped Cs₂ZrCl₆ presents excellent X‐ray scintillation performance as well as promising X‐ray imaging ability. This work not only provides a new study perspective to understand multiple ET processes and the related influencing factors in rare earth doped multi‐exciton emissive perovskites, but also exhibits their great potentiality as anti‐counterfeiting materials and next generation X‐ray scintillators.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Li H., Han K., Li Z., Yue H., Fu X., Wang X., Xia Z., Song S., Feng J., Zhang H., Multiple Energy Transfer Channels in Rare Earth Doped Multi‐Exciton Emissive Perovskites. Adv. Sci. 2024, 11, 2307354. 10.1002/advs.202307354

Data Availability Statement

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