Multicolor and white light upconversion luminescence in α-NiMoO4:Yb3+/Ln3+ (Ln = Tm, Ho, Tm/Ho) nanoparticles

In this study, α-NiMoO4:Yb3+ (15 mol%)/Ln3+ (Ln = Tm, Ho, Tm/Ho) phosphors with different doping concentrations of Tm3+ and Ho3+ (0.25, 0.50, 0.75, and 1.00 mol%) were successfully synthesized using a microwave hydrothermal method. Nanorods and attachment of nanoparticles (pebble and sphere) in nanorods were observed in host and doped samples, respectively. According to high-resolution transmittance electron microscopy (HRTEM) images, the interplanar spacings of the samples of 0.22 nm (040), 0.27 nm (−222), 0.35 nm (−112), and 0.61 (110) were well matched with the planes of X-ray diffraction (XRD) patterns. X-ray photoelectron spectroscopy (XPS) and TEM elemental mapping confirmed the incorporation of dopant ions and the uniform distribution of the elements in the samples. By changing the dopant species and concentration, multicolor (blue, green, and red) and white emissions could be achieved in the α-NiMoO4:Yb3+/Ln3+ (Ln = Tm, Ho, Tm/Ho) phosphors under 980 nm laser diode (LD) excitation. The generation of various colors and white light was further confirmed by the CIE color coordinates. The photon process and energy transfer mechanism from Yb3+ to Tm3+ and Ho3+ are also described. Finally, MTT (3-[4,5-dimethylthiazole-2-yl]−2,5-diphenyltetrazolium bromide) assay revealed the non-cytotoxic nature of the samples at low concentrations (below 0.125 mg/mL) in bovine skeletal muscle cells. These findings open the potential application of α-NiMoO4:Yb3+/Ln3+ (Ln = Tm, Ho, Tm/Ho) phosphors for multicolor bio-imaging, color displays, and white light sources.