Study On The Spectroscopy Of Up-conversion Materials Under NIR Excitation And Its Enhancement Mechanism

Rare-earth （RE） doped fluoride up-conversion nanoparticles （UCNPs） have beenwidely investigated, due to rare-earth ions’ excellent up-conversion （UC）luminescence emissions, thus they have found potential applications in a number offields, such as up-conversion lasers, vivo biology imaging, photodynamic therapy,3Ddisplay, solar cells and so on. However, the main limitation for UCNPs is their lowUC efficiency, which make how to enhance UC efficiency and UC intensity an urgentissue to be settled. This work conducted a spectroscopic study on the up-conversionemission spectra of Nd³⁺ions doped in NaYF₄, CaF₂, CaF₂@CaF₂nanoparticles andits enhancement mechanisms, under a near infrared （NIR） laser excitation at808nm.NaYF₄, CaF₂, CaF₂@CaF₂nanoparticles were first prepared and characterizedwith Nd³⁺ion doping concentration0.5%to5%. With the experimental system, theup-conversion emission spectra were obtained in a range from UV to NIR. The UCemission peaked at242nm，562nm and726nm, first demonstrated in this work, havepotential applications in UC lasers, solar cell and bioimaging, respectively. Bycomparing and analyzing up-conversion emission spectra under differentexperimental conditions (i.e. Nd³⁺ion doping concentration, host materials), CaF₂:3%Nd³⁺@CaF₂core/shell nanoparticle was found with highest enhancementup-conversion luminescence （³times of NaYF₄）, and CaF₂²times of NaYF₄,which indicate that coating an homogeneous layer outside the core nanoparticles is aneffective way to enhance the up-conversion efficiency, and CaF₂is a better hostmaterial for Nd³⁺’s UC emission.Then, this work performed theoretical analysis on the mechanisms of Nd³⁺ionup-conversion luminescence. By studying the dependence of up-conversion emissionintensity on the laser excitation power, the up-conversion processes in UV and visibleto NIR range were found to be a two-photon and three-photon process, respectively.Besides, the energy level diagram and the proposed up-conversion energy transfermechanisms were built according to Nd³⁺ion’s stark energy levels. Moreover, for theup-conversion emissions based on the⁴G_9/2level i.e.562nm and726nm, theirintensities were found to be proportional to the square of excitation laser power bysolving the up-conversion rate equation and steady equation, which indicates atwo-photon process and that energy transfer is the dominate mechanism.Finally, this work conducted a time-resolved spectroscopic study on the up-conversion fluorescent lifetime, which is approximately200μs⁵00μs. The decayprofiles were found not just an exponential one but with an increase in the front.Besides, a shorter lifetime determines a higher intensity, which is subject to thenon-radiative energy transfer theory.This work provides a reliable reference and experimental and theoreticalfoundation for further study and potential applications of RE-doped UCNPs.