Investigation of photoluminescence properties and energy transfer in Sm3+ and Eu3+ co-doped Na2Ca(SO4)2 nanophosphors prepared by combustion technique

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A series of Na2Ca(SO4)2 nano phosphors doped singly with Sm3+(1 mol%) and Eu3+ (1 mol%) ions and co-doped with Sm3+(1 mol%) and Eu3+(x)(where x = 0.5, 1.0 and 1.5 mol%) ions were synthesized using a chemical combustion method. Structural, morphological and optical properties of prepared nanophosphors were studied using X-ray diffraction (XRD), Field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), Diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) characterization techniques. XRD results confirmed that prepared nanophosphors are crystalized in a monoclinic phase with space groups of C2/c. Surface morphological studies indicate that the addition of dopants into the Na2Ca(SO4)2 phosphors could have influenced the particle sizes and shapes and their agglomeration. Optical band gap energy values were calculated from the Kubelka-Munk theory using the reflection spectra data. All of the Na2Ca(SO4)2 nanophosphors series were excited with different ultraviolet (UV) excitation such as 403 nm and 394 nm wavelength to investigate the PL features of the Sm3+ and Eu3+ ions, respectively. When Sm3+ ions excited were under 404 nm wavelength, it is observed that in addition to Sm3+ ions, Eu3+ ion emission was found in the PL spectra. This was due to the energy transfer from Sm3+ ions to the Eu3+ ions in the Na2Ca(SO4)2: Sm3+, Eu3+ (1.5 mol%) nanophosphors. Using the PL data, Commission Internationale de l'Elcairage (CIE) color coordinates were obtained. Using the CIE color diagram these coordinates were found to be lying in the orange and red color region. For increasing Eu3+ ion concentration, a color shift from orange to red was obtained and found to be tunable in the pure red color emission for an increasing Eu3+ ion concentration in the Na2Ca(SO4)2 nanophosphors. From all the results, it is noticed that Sm3+ and Eu3+co-doped Na2Ca(SO4)2 materials are potential candidates for efficient orange and red emitting luminescent phosphors for application in white light emitting diodes (W-LEDs).