Cooperative Transitions Of Yb³⁺ Clusters

Cooperative transition process represents a special type of electronic transitionsoccurring in the spectral regions where the single ions do not have absorption oremission. The first cooperative luminescence (CL) was demonstrated with a pair ofexcited Yb3+ions in1970. Since then, the CL from Yb3+dimers (pairs) has beenstudied extensively both in theory and experiment. For the CL of dimers, D. L. Dexterhas firstly given the calculation method of probability with first-order perturbationtheory. Subsequently, researchers gradually improved the cooperative transitiontheory and have reported its potential applications in three dimensional display,optical bistability, and structure probe. For the CL of three ions (trimers) Dexterconcluded that―From the form of the first-order wave functions of equation it is clearthat they cannot, at least under these approximations. There is no way to couple tripleexcitations to the ground-state wave function with a perturbation depending only onsums of two-electron interactions, and thus the position of one of the three atoms isimmaterial. This leads to a net zero matrix element through term-by-term cancellation.Second-order corrections to the wave functions can indeed allow such transitions,which would presumably be down by a factor possibly no smaller than q2.‖Beforeour works, no one tried to calculate the probability of cooperative transition for morethan two identical ions. Cooperative transition process referring to three or more ionshad been considered not exist before our observations. Cooperative luminescenceusually describes a process that emits one photon by simultaneous depopulation of apair of ions. Now, the―inherent concept‖was revised as we have found thecooperative process of three ytterbium ions. Our works make a important progress inthe field of cooperative transition and broaden the study range of CL and cooperativeenergy transfer. Cluster in the thesis refers to the structure unit of CL consisted ofthree or more ions and we call it rare earth luminescence cluster. The thesis focuseson the cooperative transition processes of Yb3+clusters and its potential applications. This thesis is consisted of three parts.1. The cooperative emission intensity of Yb3+-dimers was significantly enhanced viaoptimization the synthesis method. On this basis, an emission peaking at343nm wasobserved at room temperature. The intensity of the UV emission showed a cubicdependence on the NIR pump power, whereas the luminescence lifetime is nearly onethird of that of single Yb3+ions. All these results indicated that the UV emission isoriginated from the CL of three Yb3+ions. The structure unit of CL is calledYb3+-trimers. We measured the emission spectra of Yb3+ions, Yb3+-dimers, andYb3+-trimers at15K and obtained emission spectra with fine structures usinghigh-resolution spectroscopy method. The emission has a clear spectral structure, inwhich most emission peaks are consistent with the self-convoluted spectra from singleYb3+ions. Blue shifts were observed with certain peaks.2. We for the first time observed the upconversion emission (315nm) of Gd3+ionsin CaF2:Yb3+, Gd3+under excitation of980nm laser. Spectroscopy analysis confirmedthat the population of energy levels for Gd3+ions are came from the cooperativeenergy transfer of Yb3+clusters. The energy gap of Gd3+ions between the groundstate8S7/2and the first excited state6P7/2is~31,796cm1, meaning that Gd3+ionscannot be sensitized either by one Yb3+ions or by Yb3+pairs. We also firstly observedthe upconversion emission, peaking at282nm, from6IJlevel of Gd3+ions inCaF2:Yb3+, Gd3+under excitation of980nm laser. The pump-power dependences ofthe UC emission of Gd3+and n value was4.15±0.1for315nm emission peak,indicating that the UV emission most likely came from the sensitization ofsimultaneous de-excitation of four Yb3+ions (Yb3+-tetramers).3. We studied on the down conversion process of Yb3+dopant CaF2. Excitationspectra indicated intense absorption from250nm to310nm. Under excitation of UVlight, Yb3+ions could emit several emission peaks around980nm, which areattributed to the transitions of Stark splitting between the excited state and the groundstate. The selective excitation spectra of these Yb3+emission peaks indicated that theUV absorption is derived from Yb-O charge transfer band. We measured the quantum yield (~82%) for the down conversion by using our self-designed devise. We foundthat our sample can markedly increase the UV response of the solar cells. We firstlyobserved the CL of Yb3+-dimers under UV excitation, which means that our samplescould efficiently transfer energy from UV to NIR region.Our results indicated the existence of cooperation of three ions and cooperativesensitization of four ions, which broadened the study range of CL and cooperativeenergy transfer processes. Cluster sensitization is a new mechanism, which is aneffective complement of UC.