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

Abstract

We provide the first demonstration of pure red emission in the visible light region via three-photon excitation in monodisperse Na 3 ZrF 7 :Er nanoparticles (NPs) by using a laser operating in the telecommunication band. NPs of ∼22 nm in diameter are synthesized at 260°C by the thermal decomposition method. The experimental results reveal that the Na 3 ZrF 7 :Er NPs exhibit pure red emission in the visible region under 1480 nm laser excitation, and the emission intensity is significantly influenced by the Er 3þ ion concentration. The decay times of the 4 S 3∕2 → 4 F 15∕2 and 4 F 9∕2 → 4 F 15∕2 transitions of the Er 3þ ions at 540 and 655 nm, respectively, are reduced by increasing the Er 3þ ion concentration in the Na 3 ZrF 7 :Er NPs. The suppressed emission intensity result from the defect-related quenching effect: when trivalent Er 3þ ions replac tetravalent Zr 4þ ions, extra Na þ ions and F − vacancies are formed to re-balance the charge in the Na 3 ZrF 7 matrix. The emission color of the Na 3 ZrF 7 :Er NPs is related to the cross relaxation between Er 3þ 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.