Light emitting diodes (LEDs) are widely used as backlight illumination sources in liquid crystal displays (LCDs). Narrow-band-emitting green phosphors have become a research focus due to their potential to extend the color gamut of LCDs, yielding high-definition/high-resolution displays with excellent picture quality. Current commercial LED backlight technologies make use of β-SiAlON∶Eu²⁺ narrow-band green phosphors [emission wavelength of λ_em=540 nm, full width at half maximum (FWHM) of 54 nm]and K₂SiF₆∶Mn⁴⁺ narrow-band red phosphors (λ_em =631 nm, FWHM of 3 nm) in conjunction with a 460-nm InGaN blue light chip. The peak emission level at 540 nm, and FWHM emission linewidth of around 54 nm of β-SiAlON∶Eu²⁺ green phosphors limit their application in wide color gamut displays. Therefore, there is a need to develop high-performance green phosphors with an emission peak at a wavelength of around 525 nm and with a narrower emission linewidth to address the shortcomings of existing commercial green phosphors.

Potassium carbonate (K₂CO₃, mass fraction of 99.99%), sodium carbonate (Na₂CO₃, mass fraction of 99.99%), zinc oxide (ZnO, mass fraction of 99.99%), silicon dioxide (SiO₂, mass fraction of 99.99%), boric acid (H₃BO₃, mass fraction of 99.99%), and manganese dioxide (MnO₂, mass fraction of 99.99%) from Aladdin^{companyare chosen as raw materials. A series of powder samples, K_2-xNa_xZnSiO₄∶Mn2+(0≤x≤2), are synthesized by the conventional, high-temperature solid phase method. The physical structure of the materials is analyzed by an XRD-6100 powder diffractometer. Structural refinement of the XRD data of the samples is carried out by GSAS software. The morphology, particle size, and chemical composition of samples are analyzed with a JSM-7610FPlus field emission scanning electron microscope (SEM) and X-MaxN energy dispersive X-ray spectroscopy (EDS). The excitation spectra, emission spectra, and variable temperature spectra of samples are tested by an FLS920 steady-state/transient fluorescence spectrometer from Edinburgh Instruments Ltd., UK. Thermogravimetric tests are performed on the samples in an air atmosphere by a simultaneous thermogravimetric analyzer, i.e., STA449F3 thermogravimetric analyzer. The quantum efficiency of the samples is analyzed with a Hamamatsu C11347 absolute quantum efficiency tester.}

The crystal phase transition from the orthogonal phase K₂ZnSiO₄ to the monoclinic phase Na₂ZnSiO₄ is gradually achieved after K⁺ in the matrix is replaced with Na⁺. With the increase in the Na⁺ doping concentration, the main diffraction peaks of the K_2-xNa_xZn_0.94SiO₄∶Mn²⁺ (0≤x≤2) samples are continuously shifted to larger angles, which indicates that Na⁺ (with a smaller ionic radius) has been successfully doped into the K₂ZnSiO₄ material. As the Na⁺ doping concentration increases, the physical phase of the samples gradually transitions from K₂ZnSiO₄ to Na₂ZnSiO₄. This observation proves that Na⁺ gradually replaces the lattice position of K⁺ in the original material K₂ZnSiO₄ and forms a new phase (Fig. 1). The replacement of K⁺ with Na⁺ results in an increase in the average bond length of the central atom Zn－O, which leads to weakened crystal field strength around Mn²⁺ and a reduction in the degree of splitting [Eq. (1)]. As a result, it brings about higher energy emission wherein the wavelength emitted by Mn²⁺ becomes shorter. This is evidenced by the central wavelength of the emission spectrum from the sample blue-shifted from 578 nm to 517 nm, and the luminescence intensity of the sample is effectively increased (Fig. 4). The green phosphor K_2-xNa_xZn_0.94SiO₄∶0.06Mn²⁺ (x=2) is also subjected to variable temperature and thermogravimetric tests. At temperature of 150 ℃, the luminous intensity of the sample is 43% of that at room temperature. At temperature of 250 ℃, the residual mass ratio and the mass loss of the K_2-xNa_xZn_0.94SiO₄∶0.06Mn²⁺ (x=2) phosphor is 96.94% and of 3.06%, indicating that this phosphor has good thermal stability at operating temperatures typical of backlight LED devices (Fig. 7). Thus, the results demonstrate that a narrow-band, green phosphor with a narrower emission linewidth and shorter peak emission wavelength than the commercial β-SiAlON∶Eu²⁺ green phosphor is successfully prepared, achieving color tuning from deep yellow to green (Fig. 8).

This work demonstrates the synthesis of color-tunable K_2-xNa_xZn_0.94SiO₄∶0.06Mn²⁺ (0≤x≤2) phosphors with the high-temperature solid-phase method. The effect of the replacement of K⁺ with Na⁺ on the photoluminescence performance of K_2-xNa_xZn_0.94SiO₄∶0.06Mn²⁺ (0≤x≤2) phosphors is investigated. Under the excitation of blue light at 427 nm and 448 nm, the luminescence of samples gradually increases, and the main emission peak is blue-shifted as x increases. Ultimately, a narrow-band green phosphor with the main emission peak at 517 nm, the quantum yield of 29.4%, and an FWHM emission linewidth of 32 nm is obtained, which is narrower than that of the commercial green phosphor β-SiAlON∶Eu²⁺. This work presents a method and pathway to the development of new and novel narrow-band green light emitting phosphors for the next generation of backlight display technologies.