Effect of Crystal Phase Transition for Green Light Emitting K_2-xNa_xZn_0.94SiO₄: 0.06Mn²⁺ Phosphor on Photoluminescence Properties

Objective 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 beta-SiAlON:Eu2+ narrow-band green phosphors [emission wavelength of lambda em= 540 nm, full width at half maximum (FWHM) of 54 nm]and K2SiF6:Mn4+ narrow-band red phosphors (lambda(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 beta-SiAlON:Eu(2+)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. Methods 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 company are chosen as raw materials. A series of powder samples, K2-xNaxZnSiO4: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. Results and Discussions 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 K2-xNaxZn0.94SiO4: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 K2-xNaxZn0.94SiO4:0.06Mn(2+) (x=2) is also subjected to variable temperature and thermogravimetric tests. At temperature of 150 degrees C, the luminous intensity of the sample is 43% of that at room temperature. At temperature of 250 degrees C, the residual mass ratio and the mass loss of the K2-xNaxZn0. 94SiO4:0.06Mn(2+) (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 beta-SiAlON:Eu2+ green phosphor is successfully prepared, achieving color tuning from deep yellow to green (Fig. 8). Conclusions This work demonstrates the synthesis of color-tunable K2-xNaxZn0. 94SiO4:0.06Mn(2+) (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 K2-xNaxZn0. 94SiO4:0.06Mn(2+) (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 beta-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.