High-efficiency tunable self-trapped exciton emission in one-dimensional β-Cs₃Cu₂Br₅via Ag alloying for optoelectronic applications

Abstract

Low-dimensional lead-free metal halides with efficient self-trapped exciton (STE) emission have attracted tremendous attention in lighting applications. Despite great efforts in manipulating the STE emission properties, it is still a great challenge to achieve controllable STE emission tuning. Herein, we successfully synthesized novel one-dimensional (1D) β-Cs₃Cu₂Br₅ with tunable emission color modulation from blue-greenish to yellow by alloying with different Ag contents. In particular, the sample with a ratio of 12.4% exhibits broadband emission peaking at 508 nm with a high quantum yield of ca. 100%. Temperature-dependent single crystal X-ray diffraction, photoluminescence, and femtosecond transient absorption (TA) measurements further reveal that the Ag-induced lattice distortion of β-Cs₃Cu₂Br₅ contributed to its controllable STE emission properties. Moreover, stable Ag-alloyed crystals have shown potential application prospects in encryption, radiation thermometry and solid-state light emitting devices. This lattice distortion tuning strategy provides a new vision for the controllable STE emission of low-dimensional metal halides.

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Introduction

Low-dimensional lead-free metal halides are attractive substitutes for lead halide perovskites due to their facile solution-processed fabrication, low toxicity and superior emission properties.^1–7 These excellent emitters have been widely explored in applications such as light-emitting diodes (LEDs),^8–11 photodetectors,^12–15 anti-counterfeiting^16–18 and thermometry.^19,20 Different from the narrow emission of three-dimensional perovskites, the soft lattice feature and strong quantum confinement effect in low-dimensional metal halides endow them with broadband emission originating from the unique excited-state species, self-trapped excitons (STEs).^21,22 The chemical and structural diversity of low-dimensional metal halides provides an ideal platform for the development of various STE emission materials.

Since an STE refers to an exciton being trapped in a potential well due to electron–phonon coupling and generating lattice distortion, STE emission is strongly related to the lattice structure.^23–26 So far, external high pressure compression has been used to change the material inert structure to modulate the STE emission.^27–33 For example, for the double perovskite Cs₂Na_0.4Ag_0.6InCl₆,³¹ the structural shrinkage of octahedra upon applying high pressure strengthened the metal–halogen bond energy and resulted in tunable emission from warm-white to deep-blue. The photoluminescence (PL) efficiency of 2D vacancy-ordered perovskite Cs₃Bi₂Cl₉ nanocrystals (NCs) could be significantly improved by pressure compression.²⁷ The initially non-emissive Cs₃Bi₂Cl₉ NCs could emit bright yellow-greenish light at high pressure. Furthermore, pressure-induced lattice compression in zero-dimensional [(C₆H₅P)₂]₄SbCl₅ also decreased the distance between organic cations and metal ions and accelerated the charge transfer, which finally promoted the peak position and emission efficiency.³³ Although external pressure modulation could cause structural evolution to improve the optical properties of low-dimensional metal halides, tuning the internal intrinsic lattice distortion to modulate the emission properties at ambient pressure is mostly essential for practical solid-state lighting applications.

Element substitution is a strategy to modulate the STE properties of metal halides by adjusting the internal intrinsic structure.^34–37 For example, the incorporation of Cu⁺ in Rb₂AgBr₃ can improve the lattice strength, significantly enhancing the emission efficiency to 98.8%.³⁸ However, most of these substitutions only either modulate the host emission or generate new emissions, and the emission colors of the doped systems are independent of the dopant concentration.^39–41 In addition, the modulation of STE thermal stability is also a key aspect for achieving high-performance solid-state light emitting devices.^42–44 Thermally stable STE emission has been reported to be achieved in ion doped 0D metal halides, such as Rb₃InCl₆:xSb³⁺ and (BTPP)₂MnCl₄:2.0%:Sb. In this work, we synthesized a novel 1D β-Cs₃Cu₂Br₅ single crystal stabilized by Ag alloying, which exhibits controllable STE emission from blue-greenish to yellow by modulating the lattice distortion at room temperature and pressure. All the Ag-alloyed β-Cs₃Cu₂Br₅ samples exhibit high photoluminescence quantum yields (PLQYs) of 78–100%. The underlying tunable emission mechanism is revealed by a combination of temperature-dependent PL spectroscopy and structural analysis based on single crystal X-ray diffraction (SCXRD). The results demonstrate that Ag alloying generates lattice distortion and then improves the host STE emission. To the best of our knowledge, this is the first example of achieving controllable STE emission by tuning lattice distortion via ion alloying in low-dimensional metal halides under ambient conditions. The alloyed samples with high efficiency PL and superior stability also showed potential applications in information encryption, radiation thermometry and the solid-state lighting field.

Results and discussion

The copper bromide single crystals are synthesized by gradually reducing the temperature of their hydrobromic acid solution. When the precursor solutions (CsBr/CuBr = 4 : 1 in 1 mL HBr and 0.2 mL H₃PO₂) are slowly cooled down, the products exhibit weak blue emission and can be identified as a pure 0D Cs₃Cu₂Br₅ phase (Fig. S1†), with [Cu₂Br₅]³⁻ dimers isolated by Cs⁺ cations in the Pnma space group of an orthorhombic crystal system.^19,45 However, when the precursor solutions are rapidly cooled down, needle-like crystals with intense cyan emission are produced (Fig. 1a). The PL spectra of the two emissive crystals are shown in Fig. S2.† The peak position of needle-like crystals shifts to 493 nm compared to 0D Cs₃Cu₂Br₅ (460 nm), and the full-width at half maximum (FWHM) becomes broader from 80 to 86 nm. The needle-like crystals were unstable and transformed into blue emissive 0D Cs₃Cu₂Br₅ crystals after being exposed to air conditions within 1 min (Fig. S3†). We inferred that the blue-emissive crystals were thermodynamically stable products and the cyan-emissive cystals were kinetically favored products as a metastable state.^46,47

Fig. 1 (a) The synthesis of cyan emissive and Ag-alloyed copper bromide single crystals, and the photograph of the samples with different Ag contents upon UV light irradiation. (b) PXRD patterns of Ag alloyed samples. (c) The resolved crystal structure of Cs₃(Cu/Ag)₂Br₅. (d) SEM elemental mapping of Cs, Cu, Ag and Br of Cs₃Cu₂Br₅:12.40%Ag⁺ single crystals. Scale bars are 500 μm. XPS analysis of Ag (e) and Cu (f), respectively.

However, we can obtain stable cyan-emissive crystals with high crystallinity (Fig. S4†) by adding a small amount of AgBr (e.g., 0.027 mmol) into the precursor solution. The actual ratio of Ag in crystals was also determined by inductively coupled plasma-mass spectrometry (ICP-MS) and is listed in Table S1.† For clarity, the sample is denoted as Cs₃Cu₂Br₅:xAg⁺ (x is the molar ratio of Ag/Cu in crystals). Interestingly, the emission color is found to be Ag-content dependent varying from blue-greenish to yellow (Fig. 1a). To acquire the structural information, powder X-ray diffraction (PXRD) is performed. They have very similar diffraction patterns, while the diffraction peaks exhibit a 0.2° shift toward small angles with increasing Ag alloying content up to Cs₃Cu₂Br₅:32.35%Ag⁺ (Fig. 1b). This suggests that they will have the same host crystal structures, but different lattice expansions due to the involvement of Ag⁺. The impurity of CsAgBr₂ will appear at Ag feeding ratios higher than 1.07 (Fig. S5†). To determine the underlying crystal structure, SCXRD measurement was performed on Cs₃Cu₂Br₅:32.35%Ag⁺ samples. The resolved crystal structure is shown in Fig. 1c, in which Ag substitutes the position of Cu and they have a disordered distribution in the lattice (the detailed parameters are summarized in Table S2†). The final emissive products are determined to be 1D β-Cs₃Cu₂Br₅ with the involvement of Ag, which belongs to an orthorhombic system with the Cmcm space group. A novel 1D phase structure of β-Cs₃Cu₂Br₅ is generated with the Cu–Br chains consisting of edge-sharing [CuBr₄]³⁻ tetrahedra surrounded by Cs⁺ cations, which is totally different from 0D Cs₃Cu₂Br₅ with isolated [Cu₂Br₅]³⁻ dimers. The experimental PXRD patterns of β-Cs₃(Cu/Ag)₂Br₅ in Fig. 1b excellently matched the calculated ones from single crystal structure refinement, confirming the novel-synthesized 1D phase structure and the reliability of the refinement results. Moreover, we also resolved the crystal structure of the β-Cs₃Cu₂Br₅:6.52%Ag⁺ sample, which has the smallest lattice expansion and the closest structure to pure β-Cs₃Cu₂Br₅. As shown in Fig. S6,† the diffraction peaks of the unstable cyan-emissive crystals are a mixture of the simulated peaks based on the crystal structures of 0D Cs₃Cu₂Br₅ and Cs₃Cu₂Br₅:6.52%Ag⁺. Typically, the diffraction peak at 31.42° belonging to the (131)-oriented plane is in line with the simulated peaks of β-Cs₃Cu₂Br₅:6.52%Ag⁺. This further confirms that the initially cyan emissive crystals without Ag involvement are pure β-Cs₃Cu₂Br₅. As discussed above, 0D Cs₃Cu₂Br₅ is suggested to be a thermodynamically stable phase and 1D β-Cs₃Cu₂Br₅ is suggested to be a kinetically favored phase. We further provided the formation energy results through density functional theory (DFT) calculations. As shown in Fig. S7,† the calculated formation energy of 0D Cs₃Cu₂Br₅ is −2.519 eV, which is much lower than that of 1D β-Cs₃Cu₂Br₅ (−1.697 eV). This suggests that 0D Cs₃Cu₂Br₅ is a thermodynamically stable product, while 1D β-Cs₃Cu₂Br₅ is a metastable state with a kinetically favored phase. But the incorporation of Ag⁺ could significantly reduce the formation energy of 1D β-Cs₃Cu₂Br₅ to −2.588 eV, which indicates that the involvement of Ag⁺ can further stabilize the metastable-state 1D β-Cs₃Cu₂Br₅ to be thermodynamically favorable. The energy-dispersive spectroscopy (EDS) elemental mapping of β-Cs₃Cu₂Br₅:12.40%Ag⁺ further demonstrates the uniform distribution of Cs, Ag, Cu and Br in β-Cs₃(Cu/Ag)₂Br₅, respectively (Fig. 1d). The ratio of Cs : (Cu + Ag) : Br is about 3 : 2 : 4.5, which is consistent with the stoichiometric ratio of single crystal refinement results (Fig. S8†). Furthermore, X-ray photoelectron spectroscopy (XPS) was further performed to uncover the electronic structures of β-Cs₃(Cu/Ag)₂Br₅ with different chemical states. As shown in Fig. 1e, two typical Ag⁺ 3d spectrum peaks at 373.9 eV (3d_3/2) and 367.9 eV (3d_5/2) were observed, respectively. The peaks at 951.5 eV and 931.6 eV are assigned to Cu⁺ 2p_1/2 and 2p_3/2 (Fig. 1f), respectively. All these results support that the Ag and Cu ions are presented in the form of +1 valence chemical states.

To gain a deep understanding of the controllable emission mechanism of Ag-alloyed β-Cs₃Cu₂Br₅, steady-state spectroscopy and time-resolved PL spectroscopy were performed. As shown in Fig. 2a, the β-Cs₃Cu₂Br₅ crystals exhibit broad cyan emission with a peak at 493 nm, a full width at half maximum (FWHM) of 86 nm and a large Stokes shift of 153 nm. Broadband emission with large Stokes shifts is characteristic of STE emission in low-dimensional metal halides.^24,25,48 The PL spectrum of β-Cs₃Cu₂Br₅ could be well fitted with two Gaussian-shaped peaks, a high energy (HE) peak located at 490 nm and a low energy (LE) peak centered at 512 nm. This suggests that the emission may originate from two STE states. We subsequently recorded the time-resolved PL spectra of β-Cs₃Cu₂Br₅ at room and low temperatures, respectively. Fig. 2b shows that the PL decay profile at 77 K can be well fitted by using double-exponential functions, yielding two lifetimes, τ₁ = 0.53 µs and τ₂ = 627.18 µs, respectively. The PL decay profile gives a lifetime of only 139.37 μs (Fig. S9†) at room temperature, suggesting that the two STE states are in thermal equilibrium.⁴⁹ More importantly, we found that 1D Ag-alloyed β-Cs₃Cu₂Br₅ not only possessed high air and thermal stability under ambient conditions (Fig. S10†), but also exhibited remarkably controllable emission properties depending on the Ag concentration. Fig. 2c displays the tunable bright emission with the light color changing from blue-greenish to yellow with the increase of the Ag ratio from 0.11 to 0.85 under UV irradiation. The corresponding Commission Internationale de l'Eclairage (CIE) chromaticity coordinates are shown in Fig. S11.† Meanwhile, the PL peaks show a continuous red-shift from 502 nm to 545 nm. The FWHM also became broader from 108 to 155 nm (Fig. S12†), suggesting that Ag plays a vital role in tuning the emission of Ag-alloyed β-Cs₃Cu₂Br₅. Fig. 2d shows that the highest PLQY could be up to 100% when the Ag/Cu ratio is 12.40%. The PLQY of pure 1D β-Cs₃Cu₂Br₅ is determined to be 68.2% (Fig. S13†), which is supposed to be lower than the true value due to the rapid phase transformation during the test process. Moreover, similar to the pristine crystal, all PL spectra of Ag-alloyed crystals could also be well fitted by two Gaussian shaped emission peaks (Fig. S14†), a HE one denoted as peak 1 and a LE one denoted as peak 2, and both peak 1 and peak 2 show a continuous red shift with the Ag ratio increasing from 6.52% to 32.35%. This suggests that the incorporation of Ag ions can directly modulate the emission of the inherent host STE state rather than introducing a new emissive STE state.^23,50 The time-resolved PL curves of the samples with different Ag contents are presented in Fig. 2e and Table S3,† where all the samples show a double-exponential decay with a short component of 2–33.4 μs and a long component of 12.6–149.2 μs. This further excellently confirms the existence of the two STEs. Specifically, with the increase of the Ag ratio from 0.11 to 0.85, the short component contribution of 33.4 µs is correspondingly enhanced from 8.56% to 61.61% and the relative intensity of peak 2 originating from STE2 is also enhanced. This suggests that the short components of 2–33.4 μs are associated with the STE2 state, while the long components of 12.6–149.2 μs correspond to the STE1 state.

Fig. 2 (a) PL spectra of β-Cs₃Cu₂Br₅ single crystals. (b) Time-resolved PL decay curve of β-Cs₃Cu₂Br₅ single crystals at 77 K for clarity. PL spectra (c), PLQYs (d) and time-resolved PL decay curves (e) of Ag-alloyed β-Cs₃Cu₂Br₅ single crystals.

We further performed femtosecond transient absorption (fs TA) spectroscopy, which can provide a direct observation of STEs with characteristic broadband photo-induced absorption (PIA) signals on the red side of the exciton absorption.⁴⁸Fig. 3a shows the pseudo-colored TA plot of β-Cs₂Cu₂Br₅:6.52%, 12.40% and 32.35%Ag⁺ samples with excitation at 340 nm. Immediately after the pump pulse, we observed two broad PIA signals at around 400 and 620 nm. The former has a slow generation process, reaching a maximum after 50 ps. While the latter has a fast formation time within the instrumental response resolution (∼100 fs). Over the next 5 ps, a rapid decay of the PIA signal at 620 nm was observed, accompanied by an increase in the PIA band centered at about 525 nm. This suggests a transformation between two excited-state species. Both the PIA bands at 400 and 525 nm do not decay to zero in the probed time window. As discussed above, the broad emission of the Ag-alloyed samples is ascribed to the recombination of spin-triplet STE1 and STE2 with a microsecond long lifetime.^22,45 The observation of two long-lived PIA bands at around 400 and 525 nm agrees well with the assignment and further confirms the presence of STE1 and STE2. Moreover, we also found that the relative intensity of the low-energy PIA bands was enhanced with increasing Ag content (Fig. S15†), which is consistent with the change of the ratio of the fitted peak 2 in the steady-state PL. Hence, we believe that the PIA band at 400 nm is associated with STE1 and the PIA band at 525 nm is related to STE2.

Fig. 3 (a) Pseudo-colored fs TA spectra of β-Cs₂Cu₂Br₅:6.52%, 12.40% and 32.35%Ag⁺, respectively. (b) TA kinetics of β-Cs₂Cu₂Br₅:6.52%Ag⁺ probed at 400 and 617 nm. (c) Tetrahedral parameters Δd and σ² of Ag-alloyed β-Cs₃Cu₂Br₅. (d) Schematic illustration of [Cu₂Br₆]⁴⁻ units in 1D β-Cs₂Cu₂Br₅ and the evolution of the Br–Br–Br tilt angle of Ag-alloyed β-Cs₃Cu₂Br₅. Temperature-dependent PL spectra (e) and time-resolved PL decay curves (f) of β-Cs₂Cu₂Br₅:12.40%Ag⁺.

The dynamics at 400 nm exhibits two generation processes with time constants of 247 fs and 7.15 ps and a long-lived decay component (τ > 1 ns), as shown in Fig. 3b. Upon photoexcitation, free excitons were first formed and then trapped by a deformable lattice, producing spin-singlet STEs. Subsequently, spin-triplet STEs were formed via an intersystem crossing (ISC) process. Therefore, we ascribe the first ultrafast component of 247 fs to the trapping of free excitons and assign the second component of 7.15 ps to the ISC process from spin-singlet STE1 to spin-triplet STE1. The short-lived PIA signal at 620 nm is attributed to spin-singlet STE2. Its fast decay represents the ISC process from spin-singlet STE2 to spin-triplet STE2. Lifetime analysis at 617 nm reveals that this process occurs with a time constant of 2.49 ps. The long-lived components of 400 and 617 nm are associated with the decay of the resulting spin-triplet STE states.

To verify the influence of Ag alloying on the crystal structures, we resolved the crystal structures of the samples with different Ag contents. We first focus on the distortion of single [CuBr₄]³⁻ units, and the detailed individual angle and bond length are summarized in Table S4.† The tetrahedral parameters Δd (tetrahedral elongation) and σ² (bond angle variance) were calculated to evaluate the structural deviations of [CuBr₄]³⁻ tetrahedra induced by Ag ions.⁵¹ As shown in Fig. 3c, as the Ag ratio increases from 6.52% to 32.35%, the bond angle variance (σ²) shows no obvious change, while a decrease in bond distance deviation is observed. Since the Ag ratio of 12.40% corresponds to the highest PLQY, this suggests that the strong Jahn–Teller effect of [CuBr₄]³⁻ units may be beneficial for improving the emission efficiency in this system. Subsequently, we also analyzed the distortion between adjacent [Cu₂Br₆]⁴⁻ units. Fig. 3d shows the evolution of the [Cu₂Br₆]⁴⁻ unit tilt angle in different Ag-alloyed samples; the corner-sharing Br–Br–Br angle decreases from 168.652 to 166.580° as x increases from 6.52% to 32.35% and the smaller tilt angle indicates strong lattice distortion in the crystals.⁵² As a result of the highly structural distortion in higher Ag alloyed samples, the strength of strong electron–phonon coupling would increase, leading to a change in the ratio of the two intrinsic STEs in β-Cs₃Cu₂Br₅ and a red shift of the emission peak. In addition, the trapped excitons could be easily annihilated by the phonon-assisted nonradiative recombination process due to the strong electron–phonon coupling, and this was the reason for the lower PLQY in highly Ag-alloyed samples. The above results confirm that Ag ions can directly induce lattice distortion and thus control the STE emission properties.

We also recorded the temperature-dependent and time-resolved PL spectra of Ag-alloyed β-Cs₃Cu₂Br₅ to investigate the effect of temperature on the STE emission. As shown in Fig. 3e and S16,† as the temperature decreases from 300 to 80 K, all of the PL spectra of different concentration Ag-alloyed β-Cs₃Cu₂Br₅ become narrower and the PL peaks become gradually blue-shifted. Except for β-Cs₃Cu₂Br₅:32.35%Ag⁺, the temperature-dependent PL intensity of Ag-alloyed samples exhibits high thermal stability in the range of 80–340 K (Fig. S17†). Furthermore, the Huang–Rhys factor (S) was calculated by fitting the temperature-dependent FWHM of β-Cs₃Cu₂Br₅:x%Ag⁺, and the S value increased from 28.15 to 61.50 as the Ag ratio increased from 6.52 to 32.35% (Fig. S18†). This indicates the enhancement of the strength of electron–phonon coupling, proving that strong lattice distortion would enhance the strength of electron–phonon coupling.²⁵ The samples with different Ag ratios exhibit the same PL peak at 502 nm at 80 K (Fig. S19†). The PL tail at 550–700 nm increases with increasing Ag content (Fig. S20†). It is notable that 6.52% and 12.40% Ag⁺-alloyed crystals exhibit different emission colors at room temperature, but have identical PL spectra at 80 K. We measured their cell volumes at 100 K and found that the cell volume of β-Cs₃Cu₂Br₅:6.52%Ag⁺ (1304.995 ų) is close to that of β-Cs₃Cu₂Br₅:12.40%Ag⁺ (1309.705 ų). The coincidence of the PL profiles of these two samples at 80 K may originate from their similar structures. Hence, we infer that β-Cs₃Cu₂Br₅ has a very soft lattice and the lattice distortion can also be modulated by temperature, leading to tunable emission colors. The temperature-dependent SCXRD analysis results of β-Cs₃Cu₂Br₅:32.35%Ag⁺ with detailed crystal lattice parameters at different temperatures are provided in Table S5,† which demonstrates that there is lattice shrinkage but no phase transition with decreasing temperature. We also calculated the tetrahedral parameters of [CuBr₄]³⁻ at different temperatures (Fig. S21†), and the results indicate that the temperature could largely suppress the lattice distortion of β-Cs₃Cu₂Br₅:32.35%Ag⁺, leading to the enhancement of the emission intensity and the blue shift of the emission peak. These results also imply the intrinsic feature of the soft crystal lattice of β-Cs₃Cu₂Br₅, which is the reason why Ag alloying can induce significant changes in crystal structures.

Fig. 3f shows the temperature-dependent time-resolved PL spectra of β-Cs₃Cu₂Br₅:12.40%Ag⁺. The decay curves of β-Cs₃Cu₂Br₅:12.40%Ag⁺ could be fitted by using bi-exponential functions at different temperatures. The lifetimes are provided in Table S6.† The long component of the lifetimes shows a dramatic decrease from 331.3 µs to 26.55 µs with increasing temperature from 100 to 340 K. Moreover, the two component contribution is strongly related to temperature. At 80 K, the fast component with a lifetime of 8.73 µs only contributes 1.85%, while the ratio of the fast component gradually increases to 31.6% as the temperature increases to 400 K. Similar phenomena also occur in other Ag-alloyed β-Cs₃Cu₂Br₅ crystals (Table S7†). This further suggests that temperature can regulate the ratio of STE1 and STE2.

Based on the tunable emission colors and lifetimes at different temperatures of Ag-alloyed β-Cs₃Cu₂Br₅, we explored its potential application in encryption, radiation thermometry and solid-state lighting devices. As shown in Fig. 4a, Ag-alloyed β-Cs₃Cu₂Br₅ crystals exhibit an obvious emission color change after being heated at 125 °C. β-Cs₃Cu₂Br₅:17.32%Ag⁺ exhibits an apparent tunable color change from cyan to yellow with the change of temperature (Fig. 4b), and the CIE coordinates of (0.17, 0.43) convert to (0.35, 0.43) when the temperature increases from 80 to 400 K. This motivates us to develop a Morse code-based encryption method by utilizing the temperature-induced tunable emission of Ag-alloyed β-Cs₃Cu₂Br₅, where we define yellow emission as the dash and the other color emission as the dot. Fig. 4c displays the encryption information corresponding to “2023”. Under normal conditions, the wrong information “3894” is shown. It can be decrypted to “2023” after heating. Subsequently, the true information “2023” can be encrypted again with emission color recovery after cooling. As shown in Fig. S22,† the Morse code-based encryption strategy could also be applied to the encryption of complex words. The pattern displays the wrong information “bwljes” at room temperature, while the true information “copper” could only be decrypted after heating. This simple and reversible method provides a new idea for information encryption. In addition, the lifetime of Ag alloyed β-Cs₃Cu₂Br₅ dramatically decreases with increasing temperature, which is available for remote thermometry. The relationship between temperature and PL lifetimes could be well fitted by using the Boltzmann-sigmoid model, , where τ₀ is the intrinsic radiative lifetime, T and T_B represent the temperature and quenching temperature, and ΔT is the half-width of the temperature sensitive range, and the temperature sensing range is calculated to be T_B + 2ΔT.⁵³ The temperature sensing range is found to be dependent on the content of Ag ions, where β-Cs₃Cu₂Br₅:6.52%Ag⁺ has an ultra-broad sensing range of 119–351 K, whereas the 32.35% Ag⁺-alloyed sample exhibits a narrower sensing range of 102–246 K. The sensing area is shown in Fig. 4d and Fig. S23† with a color-filled area. The alterable sensing range of Ag-alloyed β-Cs₃Cu₂Br₅ indicates that the phonon-assisted thermal effect could be regulated by Ag-induced lattice distortion,⁵³ which provides the first example for adjusting the temperature sensing range within a single-phase metal halide. As shown in Fig. S24,† the PL lifetime of β-Cs₃Cu₂Br₅:32.35%Ag⁺ shows a linear relationship with temperature in the range of 80 to 260 K, wherein the rate of change of lifetime is 1.15 µs K⁻¹, indicating that Ag-0.85 is a promising candidate for thermometry applications. The thermometry performance is described by the specific sensitivity, . The maximum value reaches 2.8% K⁻¹ for β-Cs₃Cu₂Br₅:32.35%Ag⁺ (Fig. 4e). This value is comparable to that of other lifetime-based thermometry (Table S8†),^54–57 which implies that Ag-alloyed β-Cs₃Cu₂Br₅ is a promising material for radiation thermometry. Furthermore, the highly efficient Ag-alloyed β-Cs₃Cu₂Br₅ crystals are desirable candidates for fabricating phosphor-converting devices. As shown in Fig. 4f, Ag-alloyed β-Cs₃Cu₂Br₅ crystal powder with different ratios was coated on a commercial 340 nm UV light emitting diode (LED) chip. The devices could display different emission colors ranging from blue-greenish to yellow (Fig. S25†), and the performances of these devices are summarized in Table S9.† In addition, the emission spectra remain unchanged at different voltages (Fig. S26†), indicating the desirable stability of the device. To achieve a white light emission device, pure 0D Cs₃Cu₂Br₅ (blue emission) and β-Cs₃Cu₂Br₅:32.35%Ag⁺ (yellow emission) crystals were mixed in a ratio of 2 : 1 and coated on a 275 nm LED chip. The as-fabricated device exhibits cold, pure and white light at different voltages (Fig. S27†), which is due to the PLQY difference between the two samples.

Fig. 4 (a) Thermochromic properties of x%Ag:β-Cs₃Cu₂Br₅. (b) CIE coordinates of 17.32%Ag:β-Cs₃Cu₂Br₅ at different temperatures. (c) Images of Ag-alloyed β-Cs₃Cu₂Br₅ based encryption and decryption patterns. (d) Temperature dependence of the PL lifetime of 32.35%Ag:β-Cs₃Cu₂Br₅. (e) Specific sensitivity as a function of temperature in the temperature range from 80 to 400 K. (f) Electroluminescence spectra and photograph of a solid-state lighting device based on Ag-alloyed β-Cs₃Cu₂Br₅ samples and a 340 nm UV LED chip.

Conclusions

In summary, we synthesized a novel 1D cyan-emissive copper bromide β-Cs₃Cu₂Br₅ crystal, which can be further stabilized by Ag ions with controllable STE emission properties. It has highly efficient tunable emission from blue-greenish to yellow with the highest PLQY of up to 100% for β-Cs₃Cu₂Br₅:12.4%Ag⁺. Temperature-dependent PL, TRPL and fs TA measurements further revealed the mechanism of tunable emission, in which Ag-induced lattice distortion regulated the strength of electron–phonon coupling, leading to its highly efficient tunable STE emission behavior. As a proof-of-concept experiment, the synthesized Ag alloyed samples have great potential applications in information encryption, thermometry and the solid-state lighting field. Our work provides a new insight into the STE emission mechanism of lattice distortion modulation and presents a new direction toward the development of high-performance and environmentally-friendly luminescent materials.

Author contributions

Xiaojing Liu and Ruiling Zhang designed the experiment, revised the manuscript and obtained the research grants; Xin Xu conceived the idea, conducted the material synthesis and characterization studies, and wrote the manuscript; Zhongyi Wang, Qingkun Kong and Siping Liu led the discussion and conducted the device performance measurements. Keli Han obtained the research grants. All authors contributed to the general discussion.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Taishan Scholar Project of Shandong Province of China, the Scientific Research Starting Foundation of Outstanding Young Scholars of Shandong University, the National Natural Science Foundation of China (grant no. 22103050), and the Young Scholars Future Program of Shandong University.

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Footnote

† Electronic supplementary information (ESI) available: Experimental, supplementary Fig. S1–S27, and Tables S1–S9. See DOI: https://doi.org/10.1039/d4qi00970c

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