Abstract
A series of ZnO, ZnO-SG:Dy3+, ZnO-SG:Sm3+, and ZnO-SG:Dy3+/xSm3+ (where SG = BO33-, PO42-, and SO42-) (where x = 1, 1.5, 2.0, 2.5 mol %) nanophosphors were successfully synthesized via combustion method using zinc nitrate hexahydrate as a zinc source and urea used as a fuel. The concentrations for single doping were kept fixed at 1 mol% for each dopant, and for the Dy3+/Sm3+ co-doped samples the Dy3+ concentration was kept at 1 mol% for the entire study while the Sm3+ concentration was varied as 1.0, 1.5, 2.0, 2.5 mol%. The prepared nanophosphors have been characterized by means of X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Ultraviolet-Visible spectroscopy (UV-Vis) and photoluminescence (PL) spectroscopy, for their crystal, morphological, optical, and luminescent characterization. The XRD results revealed a formation of hexagonal wurtzite crystal structure for undoped ZnO, and singly (Dy3+, Sm3+) doped, and co-doped (Dy3+/xSm3+) ZnO and ZnO-SO4 phosphors while a complete change of structure was observed for the incorporation of BO33- and PO42- ions into the ZnO matrix. The incorporation of borate changed the crystal structure to a metaborate cubic crystal structure, and the phosphate caused the structure change to a monoclinic pyrophosphate. The optical properties studied by the UV-Vis measurements revealed that the incorporation of anionic group systems (BO33-, PO42-, SO42-) into ZnO can reduce the band gap of the ZnO material. In the case of singly doped Dy3+ materials, the PL emission exciting by the 348 nm showed the characteristic Dy3+ emissions such as blue, yellow and red emission at 484 nm, 575 nm, and 664 nm respectively for borate and phosphate samples while a low intense broad emission was observed for the ZnO and sulphate materials. Similarly, for the Sm3+ singly doped materials, the dominating and characteristic emissions of Sm3+ ions were observed for the Zn4B6O13 and Zn2P2O7 materials when excited by 403 nm wavelength. In the co-doped samples, the PL emission spectra excited by 348 nm wavelength (Dy3+ excitation band) revealed the intense Dy3+ emissions as well as the Sm3+ emission at low intensity for Zn4B6O13 and Zn2P2O7 materials. The lifetime decay measurement confirmed the energy transfer as there was a decrease in the lifetime of Dy3+ ions when the concentration of Sm3+ ions were increased. From all the anionic group substitution, the best colour purity was observed to be from the Zn2P2O7:Dy3+ doped materials with colour purity of 99% in the yellow region and Zn4B6O13:Sm3+ a colour purity of 98% in the orange region of the CIE coordinates. On the other hand, when exciting the Dy3+ ion (484 nm), enhanced yellow emissions were observed in all cases, which is mostly due to the efficient energy transfer from Dy3+ ion to the Sm3+ ion for all Zn4B6O13 and Zn2P2O7 phosphor materials. This energy transfer mechanism from Dy3+ to Sm3+ ions in the co-doped ZnO phosphor materials played an important role in tuning warm to cold white light produced under UV excitation. These phosphor materials could be a potential candidate for tunable light emission in different white light emitting diode (w-LED) applications.