Pure red visible emission via three-photon excitation of colloidal Na3ZrF7:Er nanoparticles using a telecom-band laser

We provide the first demonstration of pure red emission in the visible light region via three-photon excitation in monodisperse Na3ZrF7:Er nanoparticles (NPs) by using a laser operating in the telecommunication band. NPs of similar to 22 nm in diameter are synthesized at 260 degrees C by the thermal decomposition method. The experimental results reveal that the Na3ZrF7: Er NPs exhibit pure red emission in the visible region under 1480 nm laser excitation, and the emission intensity is significantly influenced by the Er3+ ion concentration. The decay times of the S-4(3/2) -> F-4(15/2) and F-4(9/2) -> F-4(15/2) transitions of the Er3+ ions at 540 and 655 nm, respectively, are reduced by increasing the Er3+ ion concentration in the Na3ZrF7: Er NPs. The suppressed emission intensity result from the defect-related quenching effect: when trivalent Er3+ ions replac tetravalent Zr4+ ions, extra Na+ ions and F- vacancies are formed to re-balance the charge in the Na3ZrF7 matrix. The emission color of the Na3ZrF7: Er NPs is related to the cross relaxation between Er3+ ions. These results provide an important step toward more effective biological imaging and photodynamic therapy by minimizing the scattering of the excitation light and increasing the penetration depth.