Enhancement Of Yb³⁺ Clusters Cooperative Luminescence Through Surface Modification

Cooperative quantum transitions include cooperative luminescence,cooperative sensitization,cooperative absorption and cooperative energy receptivity.Cooperative luminescence refers to the process of de-excitation of more than two same ions in ex-cited states at the same time to produce a high-energy photon.The energy of the high-energy photon is the sum of the energy of all ions down transition.In 1961,Varsanyi and Dieke discovered cooperative absorption of Pr³⁺,which was the first time that a cooperative transition was observed experimentally.D.L.Dexter theoretically studied the cooperative transitions of two ions by first-order perturbation theory.He proposed that the contribution of the first-order perturbation function to the cooperative transi-tion of three ions is 0,hence only under the first-order perturbation theory,the coop-erative transition of three ions cannot occur,while the second-order modification of the coupled wave function can allow it to occur,the probability of the transition is so small that it is almost negligible.In 2013,our group reported the cooperative luminescence of three Yb³⁺ions(Yb³⁺-trimers)at the International Conference on Luminescence Dynamics,which is the first public report on the cooperative luminescence of Yb³⁺-trimers.Since then,we have conducted in-depth studies on the mechanism of cooperative luminescence and cooperative sensitization of Yb³⁺trimers.Our study includes the cooperative sensiti-zation of three Yb³⁺ions to an Cu²⁺ion and an Pb²⁺ion,four Yb³⁺ions to an Gd³⁺ion,and the cooperative susceptibilities of three Yb³⁺ions to Ce³⁺ions,covering almost all mechanisms of cooperative quantum transitions.This research is a major break-through in the field of cooperative quantum transitions and has far-reaching implica-tions.The transition rate of Yb³⁺-trimers is calculated theoretically.Compared with that of two Yb³⁺ions(Yb³⁺-dimers),the transition rate in calcium fluoride matrix is about1/20000,which is basically consistent with our experimental data.The small transi-tion rate reflects weak luminescence intensity in experiment,which also brings us many difficulties in practical measurement.Therefore,in order to reduce the difficul-ties in the follow-up study of cooperative luminescence of Yb³⁺-trimers,this paper focuses on how to improve the luminous intensity of cooperative luminescence of Yb³⁺-trimers under the same conditions,including the following contents:1.The surface modification of CaF₂:Yb³⁺material by zirconia was realized by high temperature solid state method.Under the excitation of 980 nm laser,the cooper-ative luminescence intensity of Yb³⁺-dimers increases first and then decreases with the increasing concentration of zirconia,while the cooperative luminescence intensity of Yb³⁺-trimers increases continuously.When the zirconia concentration is more than3 mol%,an obvious impurity peak appears at 361 nm.The impurity peak is derived from the transition of Tm³⁺ions from ~1D₂ to ~3H₆.We believe that the impurity Tm³⁺ion comes from Yb F₃ with a purity of 99.999%.2、The surface of CaF₂:Yb³⁺samples were coated with Zr O₂.Under the excita-tion of 980 nm near infrared light,the cooperative luminescence of Yb³⁺-dimers and Yb³⁺-trimers was significantly enhanced.The cooperative luminescence intensity of Yb³⁺ions increases first and then decreases with the increasing of the coating thick-ness.The XRD results show that this phenomenon is caused by the blocking effect of the coating on the excitation light and cooperative luminescence.We propose for the first time the nano-shell effect of bulk materials,and use this effect to explain the above experimental phenomenon.The experimental study of luminescence kinetics shows that the fluorescence lifetime of the three luminescent centers of the sample is significantly improved after coating,which fully proves the existence of this effect.The low temperature emission spectra of the samples before and after coating were measured by high resolution spectroscopy.It is found that the fine structure of the emission spectra of the samples before and after coating has almost no change.3、The surface modification of CaF₂:Yb³⁺material by barium chloride was realized by high temperature solid phase method.Under the excitation of 980 nm near infrared light,the cooperative luminescence intensity of Yb³⁺-dimers and Yb³⁺-trimers firstly increased and then decreased.When the concentration is 1 mol%,the cooperative lu-minescence intensity reaches the maximum.In order to explore the causes of en-hanced cooperative luminescence,surface modification of CaF₂:Yb³⁺materials were performed by Ba F₂ and Ca Cl₂respectively.The experimental results show that the enhancement of cooperative luminescence is due to the charge conservation of the whole system with the addition of Cl^–ions.Kinetic analysis showed that the fluores-cence lifetime of the samples decreased with the increase of Ba Cl₂ concentration.This is due to Ba²⁺ions occupy the lattice of Ca²⁺ions,or directly replace the Yb³⁺ions occupying the lattice of Ca²⁺ions,leading to the structural destruction of Yb³⁺ion clusters,thus resulting in the fluorescence quenching of structural destruction.