Te⁴⁺-doped zero-dimensional Cs₂ZnCl₄ single crystals for broadband yellow light emission

Abstract

As an all-inorganic zero-dimensional (0D) lead-free metal halide, Cs₂ZnCl₄ is a potentially excellent matrix for preparing chemically stable and environmentally friendly photoluminescent materials through an ion-doping strategy. Therefore, it is necessary to study the emission properties of every kind of doping ion in the tetrahedral coordination of Cs₂ZnCl₄. Here, for the first time, we successfully synthesized Te⁴⁺-doped Cs₂ZnCl₄ single crystals via a facile hydrothermal method. X-ray crystallography clearly reveals the strong distortion of the tetrahedral units in Cs₂ZnCl₄ caused by doping with Te⁴⁺. Broadband yellow-light emission covering the range from 450 nm to 700 nm with a large Stokes shift is observed in Cs₂ZnCl₄:Te⁴⁺ single crystals, and this is attributed to self-trapped excitons (STEs) caused by the lattice vibration of the distorted structure. It is worth mentioning that this broadband yellow phosphor can be excited by a wide range of blue light, implicitly giving it an excellent ability to adapt to multiple models of blue LED chips, thus serving as a suitable yellow phosphor that can be applied to fabricate WLEDs without the need to worry about the lack of red light.

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Introduction

Luminescent all-inorganic metal halides with flexible crystallography/electronic structures have been widely studied as optoelectronic materials in recent years.^1,2 Among them, lead halide perovskites have drawn most attention due to their high photoluminescence quantum yields (PLQYs) and color tunability, which are beneficial for their application in light-emitting diodes (LEDs).³ However, the toxicity of lead and the poor stability of lead halide perovskites have limited their practical applications in solid-state lighting devices.⁴ To avoid these two nonnegligible shortcomings of lead-based perovskites, a series of lead-free metal halides have been developed at present.⁵ Because of their similar electronic configurations to Pb²⁺, the ions Ge²⁺, Sn²⁺, Sb³⁺ and Bi³⁺ are considered to be several of the most promising candidates to take the place of Pb²⁺ in the B-site of ABX₃.^6–9 Nevertheless, most lead-free halides belong to the perovskite-type structure that consists of BX₆ octahedral units, and only a few examples adopt non-perovskite structures. It is generally believed that low-dimensional metal halides are more conducive to lattice deformation; therefore it is easier to produce high-efficiency light emission.¹⁰ At present, 1D and 0D metal halide materials with various color luminescence and high PLQYs have been realized in Sn-, Cu-, In- and Sb-based single crystals.^11–14

Among many lead-free metal halides, Cs₂ZnCl₄ did not attract too much attention in the field of optoelectronics in the early years. Under X-ray irradiation, Cs₂ZnCl₄ exhibits an ultraviolet light emission with a maximum at 310 nm and two well-defined shoulders at 260 and 410 nm.¹⁵ Benefiting from rapid Auger-free luminescence in the ultraviolet spectral region, this compound is usually used as a scintillator in high-energy X-ray detection and has the advantages of high timing resolution and detection efficiency.¹⁶ As an all-inorganic 0D metal halide featuring easy synthesis, good chemical stability and a wide band gap, Cs₂ZnCl₄ is a potentially excellent host to study the emission properties of every kind of doping ion in tetrahedral coordination.

A large number of reports have confirmed that doping is an effective mean to induce new emission centers and boost the photoluminescent properties of metal halides.¹⁷ Among the positive examples, several doping studies with Cs₂ZnCl₄ as the host show inspiring results as efficient emitters in recent studies. Among them, Cu⁺-doped Cs₂ZnCl₄ was synthesized and found to generate an intense sky-blue emission with a high photoluminescence quantum yield.¹⁸ Meanwhile, Mn²⁺- and Sn²⁺-doped Cs₂ZnCl₄ produce green and red light-emission, respectively.^19,20 Remarkably, it was found that the incorporation of Sb³⁺ into the Cs₂ZnCl₄ matrix led to a high-efficiency broadband near-infrared emission.²¹ However, an intense broadband yellow light emission based on all-inorganic 0D Cs₂ZnCl₄ through a doping strategy in tetrahedral coordination, which is more desirable for display and solid-state lighting,²² has remained unexplored up to the present. Usually, one of the implementation methods for a white LED is to utilize blue InGaN LEDs coated with a yellow-emitting YAG:Ce³⁺ phosphor, but this kind of white light source often looks too “cold” to the naked eye due to the narrow range emission of yellow phosphor.^23,24 Therefore, it is significant to develop chemically stable and environmentally friendly 0D metal halides with high-efficiency broadband yellow PL emission.

In this work, we realized broadband yellow light emission by incorporating Te⁴⁺ ions which have a relatively high cationic valence state into a zero-dimensional all-inorganic and stable metal halide Cs₂ZnCl₄ single crystal via a facile hydrothermal method. Under blue light excitation, the resultant material Cs₂ZnCl₄:Te⁴⁺ shows bright broadband yellow emission peaking at 570 nm and spanning 450 nm to 700 nm, which should be attributed to the self-trapped exciton (STE) emission of the distorted [TeCl₄] and [ZnCl₄]²⁻ units being formed by doping Te⁴⁺ into the Cs₂ZnCl₄ host. Moreover, by combining the Cs₂ZnCl₄:Te⁴⁺ powder with a blue InGaN LED (450 nm) chip, a tentative white LED has been fabricated showing the CIE chromaticity coordinates (0.3422, 0.3543) with a correlated color temperature (CCT) of 5116 K. These corresponding parameters indicate that the described yellow phosphor of Cs₂ZnCl₄:Te⁴⁺ entirely satisfies the color properties required by WLEDs.

Experimental

Chemicals

The raw materials tellurium dioxide (TeO₂, 99.9%), cesium chloride (CsCl, 99.9%), zinc chloride (ZnCl₂, 99.9%), and ethanol (C₂H₅OH, 99.8%) were purchased from the Aladdin Chemical Reagent Factory. Hydrochloric acid (HCl, 37 wt% in water) was purchased from the Zhongxing Chemical Reagent Factory. All the chemical reagents used for synthesis were commercially purchased and used without further purification.

Synthesis of Cs₂ZnCl₄ single crystals

The Cs₂ZnCl₄ single crystals were synthesized through a hydrothermal method. Here is an example to illustrate the experimental details. First, 2 mmol CsCl (99.9%), 1 mmol ZnCl₂ (99.9%), and 4 mL of hydrochloric acid (37 wt%) were successively loaded into a 15 mL Teflon autoclave container. Then, the autoclave was put into a drying oven, followed by heating up to the target temperatures (160 °C). After keeping the reaction at the preset temperature for 24 h, the solution was slowly cooled down to room temperature (RT) overnight. Finally, the products in the form of lamellar crystals were obtained after immediate filtration and washing with ethanol three times.

Growth of Cs₂ZnCl₄:Te⁴⁺ single crystals

The procedure was identical to that described above for Cs₂ZnCl₄, except for the addition of TeO₂.

Characterization

The crystal structure and phase composition were checked using powder X-ray diffraction and single-crystal X-ray diffraction. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance X-ray diffractometer with Cu K_α radiation (λ = 1.5406 Å). The data were collected in a step-scanning mode with a step size of 0.02° and 5 s counting time per step. The single-crystal X-ray diffraction data were collected using a SMART APE II DUO X-ray four-circle single-crystal diffractometer (Bruker) equipped with a CCD-detector, a graphite monochromator, and a Cu Kα radiation source.

UV-vis diffuse reflectance spectra (DRS) were measured on a UV-visible diffuse reflectance spectrometer (SolidSpec-3600) consisting of an integration sphere using BaSO₄ as a reference. The photoluminescence excitation and emission spectra were collected using a FluoroMAX-4-TCSPC fluorescence spectrophotometer, equipped with a 450 W xenon arc lamp and a photomultiplier tube (measurement range 200–900 nm).

Results and discussion

The hydrothermal reactions of cesium chloride, zinc chloride and hydrochloric acid at 160 °C afforded colorless single crystals of Cs₂ZnCl₄. X-ray crystallography reveals that Cs₂ZnCl₄ has a 0D structure and adopts an orthogonal Pnma space group. Crystallographically, every Zn^II center is surrounded by four chlorine atoms in a highly symmetrical elongated tetrahedral geometry, thus forming [ZnCl₄]²⁻ tetrahedra, as shown in Fig. 1a. Each unit cell contains two complete [ZnCl₄]²⁻ tetrahedra and two angular tetrahedra and each [ZnCl₄]²⁻ tetrahedron consists of a longer Zn–Cl bond (2.262 Å) and three identical shorter Zn–Cl bonds (2.258 Å). Overall, the crystal structure of Cs₂ZnCl₄ is composed of ordered but disconnected [ZnCl₄]²⁻ tetrahedra with charge-balancing Cs⁺ cations filling the vacant positions and further enhancing the structural stability. The crystal structure of Cs₂ZnCl₄:Te⁴⁺ obtained through incorporating Te⁴⁺ into Cs₂ZnCl₄ has also been characterized in detail by single-crystal X-ray crystallography and shows a similar structure to that of the undoped crystal. The small increase in cell volume (Table S1, ESI†) conforms with the fact that Te⁴⁺ (r = 0.97 Å) is bigger than Zn²⁺ (r = 0.60 Å). The obvious results shown in Fig. 1b demonstrate that the [ZnCl₄]²⁻ tetrahedron becomes more distorted compared with its predecessor after doping with Te⁴⁺ ions, because the distance between Zn^II and the surrounding chlorine atoms ranges from 2.259 Å to 2.273 Å, which is distinctly different from the Zn–Cl bonds in the matrix lattice. In addition, the adjacent bond angles of Cl–Zn–Cl of the [ZnCl₄]²⁻ tetrahedra also clearly changed after doping with Te⁴⁺. As can be seen in Fig. S1 (ESI†), the four Cl–Zn–Cl bond angles parallel to the page in a [ZnCl₄]²⁻ tetrahedron include two 106.488° and two 109.884° in pristine Cs₂ZnCl₄. After doping with Te⁴⁺, the angles become different to each other with a range from 105.925° to 115.684°. These significant structural differences between the host and Cs₂ZnCl₄:Te⁴⁺ also provide evidence that Te⁴⁺ ions have substituted for a small part of the Zn²⁺ sites. As a common phenomenon in metal halide materials, the distorted coordination units and the resultant close contact of Cl⋯Cl (3.62–3.83 Å) between nearest neighbours in Cs₂ZnCl₄:Te⁴⁺ will probably contribute to the formation of Cl₂⁻ species upon excitation, affording self-trapped holes and self-trapped electrons that originate from short-range electron–phonon coupling in the strongly deformable lattice.^25–27 The powder X-ray diffraction (PXRD) patterns of x% Te-doped Cs₂ZnCl₄ (x = 0–12) samples, which correspond well with the standard PXRD pattern of Cs₂ZnCl₄ (JCPDS card 70-1965), are shown in Fig. 1c and no impurity phase has been detected. This uniformity of the patterns also confirms that the Cs₂ZnCl₄:Te⁴⁺ single crystals retain the isomorphic structure of pristine Cs₂ZnCl₄.

Fig. 1 (a) The crystal structure diagram of Cs₂ZnCl₄. (b) The lengths of the Zn–Cl bonds in a [ZnCl₄]²⁻ tetrahedron before and after doping. (c) A comparison of the PXRD patterns of Cs₂ZnCl₄ samples doped with different amounts of Te⁴⁺.

The elemental composition of Te⁴⁺-doped Cs₂ZnCl₄ has also been strictly qualitatively and quantitatively characterized. The energy dispersive spectroscopy (EDS) elemental mapping of the Cs₂ZnCl₄:Te⁴⁺ crystal shows even distributions of Cs, Zn, Cl, and Te elements (Fig. S2 and S3, ESI†), indicating that Te⁴⁺ ions have been uniformly doped in the lattice of Cs₂ZnCl₄ with high phase purity. On the other hand, characterization with an inductively coupled plasma optical emission spectrometer (ICP-OES) reveals that the actual Te⁴⁺ concentrations in Cs₂ZnCl₄:Te⁴⁺ single crystals are lower than the experimentally expected values (Fig. S4 and Table S2, ESI†). When the molar ratio of the feeding amount based on Te atoms in the experiment is 1–12%, the actual percentage of atoms entering the lattice of Cs₂ZnCl₄ is only 0.21–4.02%. This may be due to the larger radius and higher valence state of the Te⁴⁺ ion which means it does not easily replace the lattice site of Zn²⁺. Additionally, it should be noted that the experimentally expected values will be used to express the doping contents for convenience of description in the full text. In order to further determine the valence state of Te⁴⁺ doping in Cs₂ZnCl₄, X-ray photoelectron spectroscopy (XPS) measurement is employed and gives high-resolution spectra of the 2p orbital of Zn²⁺, the 3d orbital of Cs⁺, the 2p orbital of Cl⁻ and the 3d orbital of Te⁴⁺, which are shown in Fig. S5 (ESI†). The two peaks observed at 586.6 and 576.3 eV can be assigned to Te 3d_3/2 and Te 3d_5/2,²⁸ proving the presence of Te in the as-synthesized samples. This result also demonstrates the oxidative stability of tellurium because of its unchanged valence state compared with the precursor. Evidence for the high yield and phase purity of Cs₂ZnCl₄:Te⁴⁺ single crystals was also provided by Fourier-transform infrared spectroscopy (FTIR) and Raman spectra (Fig. S6, ESI†).

The Te⁴⁺-doped Cs₂ZnCl₄ single crystal exhibits transparency and colourlessness under natural light and emits bright yellow light when it is exposed to the irradiation of a 4 W and 365 nm ultraviolet lamp (Fig. 2a), whereas the undoped crystals are transparent and show no luminescence under UV light excitation (Fig. S7, ESI†). The excellent transmittance of the crystals verifies their high crystallinity. To investigate the optical properties of undoped and Te⁴⁺-doped Cs₂ZnCl₄ single crystals, steady-state UV-Vis diffuse reflectance spectra with different doping contents of Te⁴⁺ were first carried out and are presented in Fig. 2b. The Cs₂ZnCl₄ host shows a plateau of high reflection in the wavelength range of 500–800 nm and then starts to decrease gradually from 300 to 500 nm. However, according to relevant reports of metal halide luminescent materials, it is natural to consider that the differences between the Cs₂ZnCl₄:Te⁴⁺ single crystals and the pristine host in the region of 360–500 nm originate from the characteristic of free excitons in the low-dimensional and deformable structure of metal halides with Te⁴⁺ ions introduced into the lattice.^29,30 Along with the increase in the concentration of Te⁴⁺ in the Cs₂ZnCl₄ host, the corresponding band edge absorption intensities are also significantly enhanced and the absorption edges gradually shift to a low energy band. Referring to the relevant literature, the band gap (E_g) of a Cs₂ZnCl₄ matrix is defined to be 4.60 eV.²¹ According to the Kubelka–Munk equation, as the doping content of Te⁴⁺ increases, the band gaps of 1, 2, 4, 6, 8, 10 and 12 mol% Te⁴⁺-doped Cs₂ZnCl₄ are obtained as 4.01, 3.35, 2.79, 2.70, 2.67, 2.63 and 2.32 eV, respectively. There is no doubt that the reduction in these E_g values should be attributed to the doping by Te.

Fig. 2 (a) Optical microscopy images of a Cs₂ZnCl₄:Te⁴⁺ single crystal under natural light (left) and UV light (right). (b) The diffuse reflectance spectra of 0, 1, 2, 4, 6, 8, 10, and 12 mol% Te⁴⁺-doped Cs₂ZnCl₄. (c) The PL spectra of Cs₂ZnCl₄:xTe⁴⁺ (x = 1, 2, 4, 6, 8, 10, and 12.0 mol%) single crystals excited at 417 nm. (d) The PLE spectra of Cs₂ZnCl₄:xTe⁴⁺ monitored at 570 nm.

Upon 417 nm excitation, Te⁴⁺-doped Cs₂ZnCl₄ single crystals exhibit a broadband yellow-light emission spanning the visible light region from 450 nm to 700 nm with a Stokes shift of about 140 nm. The PL intensities of different feeding ratios of Te⁴⁺-doped Cs₂ZnCl₄ are significantly transformed with the trend rising first and then falling because of the concentration quenching effect, and reaching a maximum with a half-maximum full width (FWHM) of up to 118 nm when the concentration of Te⁴⁺ is 8 mol% (Fig. 2c). These results indicate the successful conquering of the problem of self-absorption in the near-UV region. It is very clear that the emission spectrum of Te⁴⁺-doped Cs₂ZnCl₄ single crystals consists of a shoulder band at 470 nm and a main broad band at 570 nm. So that it is reasonable to attribute the high-energy shoulder to the free excitons and the low-energy broadband emission to self-trapped excitons because of the similar two-band emission profiles to those metal halides.^31,32 The excitation spectra of different Te⁴⁺ feed ratios are shown in Fig. 2d, which show a certain correlation to the absorption spectra. Because of the similar electronic configurations between Te⁴⁺ and Sb³⁺, its energy band is composed of ground state ¹S₀, singlet excited state ¹P₁ and triplet excited state ³P_n (n = 0, 1, 2).^33,34 We can see in Fig. 2d that when the doping concentration increases, the optimal excitation wavelength changes from 417 nm (A-band) at 1 mol% to 451 nm (B-band) at 12 mol% Te⁴⁺-doped Cs₂ZnCl₄. According to the transition selection law and considering the contribution of lattice vibration to the transition process, the A-band is due to the ¹S₀ → ³P₁ transition of Te⁴⁺ through spin–orbit coupling which is a double peak caused by the dynamic Jahn–Teller effect, and the B band is due to the ¹S₀ → ³P₂ transition of Te⁴⁺.²⁸ For the above-mentioned reasons, this broad excitation band also indicates that the broadband yellow phosphor can be excited by a wide range of blue light, making it promising for applications in the field of solid-state lighting.

Importantly, no apparent change to the color properties of the broadband yellow emission is observed in Cs₂ZnCl₄:Te⁴⁺ single crystals when adjusting the excitation from 330 nm to 450 nm, confirming its nature as a broadband monochromatic emitter. Time-resolved photoluminescence decay experiments indicate the lifetime of the emission measured at 570 nm is 53.2 ns for 8 mol% Te⁴⁺-doped Cs₂ZnCl₄ single crystals, which is close to the decay time of some low-dimensional lead halide perovskites that originated from self-trapped excitons.^10,35 It can easily be found from Fig. 3b and Table S3 (ESI†) that, upon 454 nm excitation, the decay times of the intrinsic radiation for various concentration of Te⁴⁺ monitored at 570 nm decrease slightly from 62.94 to 50.87 ns with the increase in the concentration of Te⁴⁺, which is related to the increased non-radiative transition. Given the intense distortion of the tetrahedra caused by the introduction of Te⁴⁺ into the 0D all-inorganic cesium zinc chloride, we sought to investigate the origin of the broadband yellow-light photoluminescence of Te⁴⁺-doped Cs₂ZnCl₄ single crystals. In the cases of Sb³⁺- and Ce³⁺-doped Cs₂ZnCl₄, the possibility of intrinsic defect-induced emission has been excluded.^21,36 So here we also ignore the impact of defects on luminescence. On the other hand, the light emission caused by the energy level transition of Te⁴⁺ itself always features a long lifetime of tens of microseconds or milliseconds, which is inconsistent with those lifetimes we obtained.^37–39 Thus, the lifetime of Te⁴⁺ ions is strongly dependent on the crystal field in the host lattices. Considering that the emission spectra of Cs₂ZnCl₄:Te⁴⁺ single crystals possess the characteristics of broadband emission and a large Stokes shift, it is reasonable to attribute the emission centered at 570 nm to the recombination of STEs. Actually, the pristine Cs₂ZnCl₄ single crystals give a weak narrowband emission at 491 nm upon 267 nm excitation (Fig. S8, ESI†) that has been defined as originating from their STEs.^21,36 Additionally, the luminescence is too weak to be observed by the naked eye. Based on the above mentioned factors, we ascribe the low-energy broadband yellow-light emission peak at 570 nm and the small high-energy shoulder peak at 470 nm of Cs₂ZnCl₄:Te⁴⁺ single crystals to the recombination of STEs, which are formed by the strong lattice vibration originating from the distorted [TeCl₄] and [ZnCl₄]²⁻ units in the Te⁴⁺-doped Cs₂ZnCl₄ single crystals. For Cs₂ZnCl₄, the STEs produced by the doping of Te⁴⁺ ions cause a greater Stokes shift, broaden the emission spectra and greatly enhance the intensity. And as for the characteristics of fluorescence emission, the average slightly longer lifetimes of Cs₂ZnCl₄:Te⁴⁺ with different Te⁴⁺ concentrations compared to previously reported lead halide perovskites can presumably be attributed to the populated self-trapped excitons.

Fig. 3 (a) The photoluminescence emission spectra of 8 mol% Te⁴⁺-doped Cs₂ZnCl₄ single crystals upon 330 nm, 350 nm, 365 nm, 419 nm, and 450 nm excitation. (b) The photoluminescence (PL) decay of 1, 2, 4, 6, 8, 10, and 12 mol% Te⁴⁺-doped Cs₂ZnCl₄ (excited at 454 nm, probed at 570 nm) at room temperature. (c) The EL spectrum of a WLED based on Cs₂ZnCl₄:Te⁴⁺ and a blue LED chip. (d) The CIE color coordinates of a Cs₂ZnCl₄:Te⁴⁺ sample and WLED devices constructed from Cs₂ZnCl₄:Te⁴⁺ and YAG:Ce³⁺ on blue LED chips.

Photographs and color coordinates of WLEDs made from YAG:Ce³⁺, the as-fabricated WLEDs driven at 30 mA current and Cs₂ZnCl₄:Te⁴⁺ single crystals are shown together in Fig. 3d. The broadband light emission was determined to have Commission International de l’Eclairage (CIE) chromaticity coordinates of (0.4578, 0.5126) for Cs₂ZnCl₄:Te⁴⁺ single crystals, indicating that it could be a potential yellow phosphor. To demonstrate the commercial value of Cs₂ZnCl₄:Te⁴⁺, a white LED was fabricated by combining Cs₂ZnCl₄:Te⁴⁺ powder with a blue InGaN LED (450 nm) chip. The emission spectrum of the WLED is shown in Fig. 3c. The white light generated shows CIE chromaticity coordinates (x = 0.3422, y = 0.3543) and a correlated color temperature (CCT) of 5116 K, which is wamer than a WLED fabricated with YAG:Ce³⁺ with CIE chromaticity coordinates (x = 0.3101, y = 0.4087) and a CCT of 8312 K. The obtained results indicate that the as-synthesized Cs₂ZnCl₄:Te⁴⁺ is a potential candidate yellow phosphor that can be applied to white WLEDs.

In addition to excellent PL properties as a broadband yellow phosphor, Cs₂ZnCl₄:Te⁴⁺ single crystals also have good humidity stability, as certified by the unchanged PXRD patterns after exposure to the ambient atmosphere for 7 months, overcoming the moisture sensitivity of lead halide perovskites (Fig. 4a). Likewise, the PL intensity of the samples also remained almost unchanged after being stored in air for 7 months. The above results confirm that the Te⁴⁺-doped Cs₂ZnCl₄ single crystals have excellent air stability. Additionally, we combined thermogravimetric and differential thermal analysis (TG-DTA) of Cs₂ZnCl₄ and Cs₂ZnCl₄:Te⁴⁺ single crystals to investigate their thermal stability. The corresponding test results show that they both have a water loss platform up to about 600 °C and then undergo a phase transition up to 800 °C (Fig. S9, ESI†). Thus, although the introduction of Te⁴⁺ ions into Cs₂ZnCl₄ single crystals brings about a change in the photoluminescence property, it still retains the inherent stability of the matrix. The temperature-dependent spectra indicate that no spectral shift occurred and the spectral shapes were perfectly preserved at various temperatures (Fig. S10, ESI†). The PL intensity retained 73% of the initial value at 343 K. Therefore, Te⁴⁺-doped 0D Cs₂ZnCl₄ metal halide single crystals display impressive stability, which lays a solid foundation for their application in actual light-emitting devices.

Fig. 4 (a) PXRD patterns of as-synthesized Cs₂ZnCl₄:Te⁴⁺ and the same sample after exposure to ambient air for 7 months. (b) The steady-state PL spectrum of 8 mol% Cs₂ZnCl₄:Te⁴⁺ and the corresponding spectrum after exposure to ambient air for 7 months.

Conclusions

In conclusion, via incorporating Te⁴⁺ into the tetrahedral coordination environment in zero-dimensional Cs₂ZnCl₄ through simple and convenient hydrothermal synthesis, structurally stable and broadband yellow light-emitting single crystals covering the range from 450 nm to 700 nm (Cs₂ZnCl₄:Te⁴⁺) have been obtained successfully. After carefully analyzing the crystallographic structures of both the pristine and Te⁴⁺-doped Cs₂ZnCl₄ single crystals, it is easy to see that the [ZnCl₄]²⁻ tetrahedra in the lattice of Cs₂ZnCl₄:Te⁴⁺ become more distorted, whether from changes in bond lengths or angles; this is beneficial for exciton trapping and STE emission. In particular, this broadband yellow phosphor can be excited by a wide range of blue light, so it is able to perfectly adapt to blue light-emitting LED chips which have been mass produced industrially, allowing the assembly of natural-colored WLEDs and thus serving as a potential alternative for commercial applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation (NSF) of China (52172152, 51572200), Key Projects of NSF of Zhejiang Province (LZ20E020003), and Wenzhou Major Scientific and Technological Innovation Project (G20170005).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystal data and structure refinement data, change of bond angles, ICP, EDS, and XPS analysis, FT-IR and Raman spectra, elemental mapping, TG-DTA data for Te⁴⁺-doped Cs₂ZnCl₄, and PL and PLE data for Cs₂ZnCl₄. See DOI: 10.1039/d1tc04990a

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