Tm3+; Yb3+:Zn2TiO4 near infrared to blue upconversion phosphors for anti-counterfeit applications

Abstract

Tm³⁺; Yb³⁺:Zn₂TiO₄ samples have been synthesized using a solid state reaction route. The phase, lattice parameters, crystallite size has been examined using X-ray Diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). An intense peak of Yb³⁺ codoped samples is observed near ∼957 nm due to the ²F_7/2 → ²F_5/2 transition in diffuse reflectance spectra (DRS), which confirms the presence of Yb³⁺ ion in the prepared compound. The optical band gap of Yb³⁺ codoped samples has been calculated using Kubelka–Munk function. The Raman spectra corresponds to incorporation of Tm³⁺/Yb³⁺ at the octahedral and tetrahedral site of the spinel host. The emission spectra recorded by using 370 nm excitation wavelength shows intense blue colour band corresponding to the ¹G₄ → ³H₆ transition of Tm³⁺ ion. The upconversion (UC) emission spectra recorded by using 980 nm laser excitation source shows emission bands due to the ¹G₄ → ³H₆, ¹G₄ → ³F₄ and ³H₄ → ³H₆ transitions of Tm³⁺ ion in the host matrix lying in the blue, red and NIR regions respectively. There is effective enhancement of about ∼35 times in the blue UC emission intensity with incorporation of Yb³⁺ at 3% doping concentration in the prepared sample. The anti-counterfeit application of the optimized upconverting phosphor has been successfully demonstrated.