A novel halide hybrid white light emitter with remarkable structural and emission stability

Addressing the issues of self-absorption effect among different components in traditional white light-emitting diodes (WLEDs) and the high cost associated with rare earth elements, we propose the potential utility of zero-dimensional organic–inorganic metal halide hybrids (0D OIMHs) in white WLEDs fabrication. In this study, we synthesized a novel 0D OIMH, (C5N2H14)ZnCl4, characterized by broadband white light emission. The (C5N2H14)ZnCl4 crystal features a short ultraviolet cutoff situated at 190 nm and emits cold white light with a Color Rendering Index (CRI) of 73 under an excitation wavelength of 260 nm. Furthermore, (C5N2H14)ZnCl4 demonstrates exceptional structural and emission stability in an air atmosphere over a span of three months.


Intorduction
White light-emitting diodes (WLEDs), as successors to incandescent and fluorescent lamps, have ushered in a revolution in the field of solid-state lighting [1,2] .Conventionally, WLEDs are produced by integrating blue light-emitting diodes (LED)chips with a blend of yellow, green, and red phosphors that contain rare-earth metals [3,4] .However, such a blend of phosphors inevitably gives rise to several issues.Firstly, the emitters exhibit varied lifetimes, leading to changes in the color of the white light over time [5,6] .Secondly, the overlapping absorption and emission energies of different components result in a self-absorption effect, which is detrimental to practical applications [7,8] .Lastly, the use of expensive rare-earth metals poses a challenge to large-scale industrial production.To tackle these problems, one effective approach is to synthesize single-phase broadband emitters that do not contain rare-earth metals.Such emitters can emit photoluminescence (PL) across the entire visible spectrum, offering a promising solution for the aforementioned issues.
Zero-dimensional organic-inorganic metal halide hybrids (0D OIMHs) are emerging as highly promising materials for solid-state lighting phosphors, owing to their low cost, simple synthesis process, and wide emission spectrum, which is expected to span the entire visible region [9][10][11][12] .These attributes make 0D OIMHs potential candidates for achieving high color-rendering index white light emission.In this work, we have successfully synthesized a novel white light 0D OIMH, denoted as (C5N2H14)ZnCl4.This compound exhibits cool white light emission with a color rendering index as high as 73.We have performed comprehensive analyses to characterize the structure of (C5N2H14)ZnCl4 and to investigate its luminescence properties.

Materials and Methods
We grew single crystals of (C5N2H14)ZnCl4 using the aqueous solution method, with C6N2H14, ZnCl2 and HCl as starting materials.Specifically, we dissolved approximately 0.342 g (3 mmol) C6N2H14 and 0.409g ZnCl2 in 4 mL of HCl solution.This well-mixed solution was then heated at 110 °C for 24 hours in an oil bath with continuous stirring.Subsequently, the solution was allowed to cool slowly to room temperature.After an additional 24 hours, we successfully obtained the target compound.
We performed single-crystal X-ray diffraction measurements using a Rigaku AFC10 diffractometer equipped with a graphite-monochromated Kα (λ = 0.71073 Å) radiation.We employed the Crystal Clear software for data extraction and integration, and the XPREP program for face-indexed absorption corrections.The structures were initially solved using direct methods with SHELXS-97, and further refined by full-matrix least-squares refinement on F2 using SHELXL-97, which is part of the Win GX software suite.We verified the structures using the ADDSYM algorithm from the program PLATON and found no higher symmetries [13][14][15] .

Results and Discussion
The single crystals of C5N2H14)ZnCl4, which can reach sizes up to 3 mm, are colorless and transparent under natural light.However, when exposed to irradiation from a Hg lamp, they take on a white hue, as depicted in Figure 1b  of (C5N2H14)ZnCl4 within the bc plane are presented .Diffuse reflectance data across the UV-Vis-NIR spectrum of the compound were acquired using a SolidSpec-3700 DUV spectrophotometer, with measurements taken over a wavelength range of 200-800 nm.Room temperature (RT) transmission data confirmed that (C5N2H14)ZnCl4 has a relatively short ultraviolet absorption cutoff at 190 nm (6.52 eV).Given its lack of conspicuous absorption in the visible spectrum, the compound remains colorless and transparent under sunlight (as depicted in Figure 1b).
The excitation and emission spectra, photoluminescence quantum yield (PLQY), decay curves at room temperature, and high temperature-dependent steady-state PL spectra were captured using a FLSP9200 fluorescence spectrophotometer, as depicted in Figure 3.These spectra reveal that the compound (C5N2H14)ZnCl4 emits cool white light peaking at 482 nm at room temperature when excited at a wavelength of 260 nm.This emission is characterized by a Stokes shift of 222 nm and a full width at half maximum (FWHM) of 139 nm.Furthermore, the Commission Internationale de l'Eclairage chromaticity coordinate for (C5N2H14)ZnCl4 is determined to be (0.24, 0.31), corresponding to a correlated color temperature (CCT) of 13068 K and a color rendering index (CRI) of 73.The average lifetime of this emission is 3.13 ns under the under the excitation at 260 nm.In order to explore whether there is only one emission center, we also obtained average lifetime at 600 nm.When measured at 600 nm, the luminescence lifetime of (C5N2H14)ZnCl4 is 3.97 ns, which is almost identical to the luminescence decay curve at 482 nm, confirming the single emission mechanism.To verify the stability of (C5N2H14)ZnCl, powder samples were placed in air for three months and powder XRD and luminescence spectra were tested.The PXRD of (C5N2H14)ZnCl in air for three months are agree well with the original PXRD.Normalized emission spectra also show that the emission spectra of the (C5N2H14)ZnCl are consistent with the original spectra after three months, and the peak pattern is unchanged, indicating the excellent structure and luminescence stability of the ZnCl4 in air.
Further investigation was undertaken to elucidate the low-energy emission mechanism in (C5N2H14)ZnCl.The results demonstrated that the average lifetime remained largely unaltered across the different excitation wavelengths, suggesting the presence of just one type of luminescence center, indicative of a single emission mechanism.To rule out the impact of defects on luminescence, the luminescence spectra of both(C5N2H14)ZnCl single crystal and powder samples were obtained.The emission profiles of both sample types showed an overlap in spectra, thereby effectively excluding the possibility of a permanent defect mechanism.
To investigate the influence of organic cations on luminescence, the emission spectrum of the organic salt has been thoroughly assessed at a wavelength of 260 nm.Under excitation wavelength of 260nm, (C5N2H14)ZnCl4 and C5N2H14Cl2 have similar emission spectra.And the average lifetime of C5N2H14Cl2 are 5.10 ns corresponding with the (C5N2H14)ZnCl4.Hence, the WL emission are derived from organic cations in the structure.
(a) (b) Figure .4T The electronic band structure, along with the PDOS allocated to the constituent elements of (C5N2H14)ZnCl4, have been meticulously calculated.We employed static density functional theory to calculate the electronic structure of (C5N2H14)ZnCl4, focusing specifically on the orbitals proximate to the energy band gap.As depicted in Figure 4a, the calculated indirect band gap is 5.12 eV, somewhat lower than the experimental band gap measured at 6.25 eV. Figure 4b presents the partial densities of states (PDOS) within (C5N2H14)ZnCl4.From this, it is evident that the p orbitals of Cl-primarily contribute to the highest occupied molecular orbitals (HOMO), with [C5N2H14] 2+ and Zn 2+ orbitals making only a minor contribution.Conversely, the s orbitals of Zn2+ predominantly contribute to the lowest unoccupied molecular orbitals (LUMO), while the orbitals of [C5N2H14] 2+ and Cl -again make a less significant contribution.

Conclusions
We synthesized a Zn-based 0D OIMHs (C5N2H14)ZnCl4 with a aqueous solution method.It has great ultraviolet transmission properties, and the short ultraviolet cutoff of 190 nm.The compound achieves cold white light emission and good stability, which is very potential as a single phosphor material.This research offers valuable insights for the synthesis of single-component white light-emitting materials.

Figure. 2
Figure.2 (a) [ZnCl4] 2-tetrahedron and organic[C5N2H14] 2+ cations; (b) The crystal structures of (C5N2H14)ZnCl4 within the ab plane are presented; (c) The crystal structuresof (C5N2H14)ZnCl4 within the bc plane are presented .Diffuse reflectance data across the UV-Vis-NIR spectrum of the compound were acquired using a SolidSpec-3700 DUV spectrophotometer, with measurements taken over a wavelength range of 200-800 nm.Room temperature (RT) transmission data confirmed that (C5N2H14)ZnCl4 has a relatively short ultraviolet absorption cutoff at 190 nm (6.52 eV).Given its lack of conspicuous absorption in the visible spectrum, the compound remains colorless and transparent under sunlight (as depicted in Figure1b).The excitation and emission spectra, photoluminescence quantum yield (PLQY), decay curves at room temperature, and high temperature-dependent steady-state PL spectra were captured using a FLSP9200 fluorescence spectrophotometer, as depicted in Figure3.These spectra reveal that the compound (C5N2H14)ZnCl4 emits cool white light peaking at 482 nm at room temperature when excited at a wavelength of 260 nm.This emission is characterized by a Stokes shift of 222 nm and a full width at half maximum (FWHM) of 139 nm.Furthermore, the Commission Internationale de l'Eclairage chromaticity coordinate for (C5N2H14)ZnCl4 is determined to be (0.24, 0.31), corresponding to a correlated color temperature (CCT) of 13068 K and a color rendering index (CRI) of 73.The average lifetime of this emission is 3.13 ns under the under the excitation at 260 nm.In order to explore whether there is only one emission center, we also obtained average lifetime at 600 nm.When measured at 600 nm, the luminescence lifetime of (C5N2H14)ZnCl4 is 3.97 ns, which is almost identical to the luminescence decay curve at 482 nm, confirming the single emission mechanism.

Figure. 3
Figure.3 (a) Excitation spectra of(C5N2H14)ZnCl4; (b) Emission spectra of (C5N2H14)ZnCl4 under 260 nm excitation; (c) The CIE of (C5N2H14)ZnCl4To verify the stability of (C5N2H14)ZnCl, powder samples were placed in air for three months and powder XRD and luminescence spectra were tested.The PXRD of (C5N2H14)ZnCl in air for three months are agree well with the original PXRD.Normalized