Multiphoton Near-Infrared Quantum Cutting In Rare Earth Ions Doped Luminescent Materials

Since the first demonstration of near-infrared (NIR) quantum cutting (QC) in Tb3+/Yb3+codoping, NIR-QC has been widely studied as efficient downconversion of an ultraviolet(UV)-blue photon to two~1000nm photon. Especially promoted by the promising applicationof NIR-QC downconverter to increase c-Si solar cells efficiency, the energy transfer (ET)mechanisms involving in various donors (Ds), such as rare earth ions RE3+(RE=Tb, Pr, Tm,Er, Nd, Ce, Ho, Dy) and RE2+(RE=Eu, Yb), transition metal ions (Bi3+, Mn2+) and ionicgroups (VO43-, MoO42-, WO42-), etc., and Yb3+acceptors codoping are considered to becooperative or resonant NIR-QC, which own quantum efficiency (QE) greater than unity.However, many of the aforementioned novel NIR-QC of Ds/Yb3+, especially the broadbandNIR-QC, are only determined by photo-excitation and emission spectra, and decay curves ofDsas a function of Yb3+contents, which might be not rigorous and convinced proofs.Moreover, in some cases, these spectroscopic phenomena are just resulted from one-stepresonant ET of1Dsâ†'1Yb3+through charge transfer state (CTS) or assisted by multiphonon,but not the1Dsâ†'2Yb3+two-photon NIR-QC. Accordingly, following the systematic andin-depth study on the NIR emission of Ds/Yb3+codoping, the same fluorescence phenomenaare always explained using different ET mechanisms, such as i) the NIR emission and ETmechanisms of Ce3+/Yb3+are respectively declared to be1Ce3+â†'2Yb3+cooperative NIR-QCand1Ce3+â†'1Yb3+downshifting via Ce4+-Yb2+CTS, ii) the NIR-QC of Pr3+/Yb3+is mainlycontributed from the two-step resonant ET but not from the previously reported1Pr3+â†'2Yb3+cooperative ET, iii) the cooperative QC for~1000nm emission of Tm3+/Yb3+does notoccur in a host lattice with high phonon energy but does by phonon-assisted one Yb3+~1000nm photon emitting and another Tm3+~1800nm photon emitting, etc. Therfore, for thefurther research, it is necessary to rigorously resolve the relevant controversies, and also tostudy the NIR emission and ET mechanisms of single RE3+donor without Yb3+acceptor.In1957, Dexter theoretically treated the possibility to obtain QE greater than unity, thatis: i) for an activator with three energy levels excited by a vacuum ultraviolet (VUV) photon,if the energy difference between adjacent levels corresponds to visible photon, the radiative transition probabilities would be sizable from the excited state to intermediate state, and fromthe intermediate state to ground state, sequentially emitting two visible photons; ii) for dualions codoped system excited by a VUV photon, the energy of sensitizer can be equallytransferred to two activators, thereby emitting two visible photons. The sequential visiblephoton emission was first realized in VUV-excited YF3:Pr3+, and then VUV-excited Gd3+/Eu3+codoping is demonstrated for efficient visible QC. In principle, a high-energy photonabsorbed can be cut into two low-energy photons re-emitting. Accordingly, NIR-QC was firstdemonstrated in RE3+/Yb3+(RE=Tb, Tm, Pr) codoping by cooperative ET, and, furthermore,the novel NIR-QC via resonant ET was achieved in Pr3+/Yb3+and Er3+/Yb3+codoping, whichgreatly extend and promote the development of Dexter QC theory. Because the resonant ETrate is1000times over the cooperative ET rate, to design and obtain the novel resonantNIR-QC of RE3+/Yb3+becomes a research hotspot. However, the energy of UV photon ismore than2or3times over that of Yb3+~1000nm NIR photon, so, is it possible to obtainthree-photon NIR-QC in RE3+/Yb3+codoping? What are the corresponding ET mechanisms?On the other hand, from Dexter QC theory, it can be predicted that the two-photon sequentialNIR emission must occur in RE3+-doping. So, what are the ET mechanisms of a sequentialNIR-QC? Moreover, the energy of UV-blue photon is several times over that of certain NIRphotons, so, does three-photon or multiphoton sequential NIR luminescence exist or not?How are going the related ET mechanisms? If there exists the three-photon or multiphotonNIR-QC, how to optimize the fluorescence processes?This dissertation is to extend and promote the development of Dexter QC theory, and toclarify and resolve the existing controversies. By using various host lattices activated by RE3+ions, and the static-dynamic fluorescence testing technologies (especially the time-resolvedemission spectrum), the above-mentioned scientific problems are well explored and studied.The main research results of this dissertation are:(1) On the basis of the cooperative ET mechanism of1Ce3+â†'2Yb3+and the one-stepET mechanism of1Ce3+â†'1Yb3+, decay curves of Ce3+:5dâ†'4f emission as a function ofYb3+contents can be feasibly fit by Monte-Carlo simulation. The corresponding resultsdemonstrate that the Yb3+~1000nm emission is just attributed to the1Ce3+â†'1Yb3+ luminescence downshifting but not the1Ce3+â†'2Yb3+NIR-QC.(2) Under excitation of blue light, Pr3+-doping and Dy3+-doping can yield the efficientsequential two-photon NIR emissions at~1000nm, respectively, that is: i) with1G4acting asan intermediate level, the energy in excited Pr3+:3Pjstates can be sequentially split into twoNIR photons emitting by the first-step transition of3P1,0â†'1G4(~915nm) and the second-stepof1G4â†'3H4(~990nm), where internal QE is calculated to be104%; ii) with6H7/2,6F9/2or6H5/2serving as an intermediate level, the energy in excited Dy3+:4F9/2state can besequentially split into two NIR photons emitting by the first-step transition of4F9/2â†'6F9/2,6H7/2(~834nm) or4F9/2â†'6H5/2(~1000nm) and the second-step of6F9/2,6H7/2(6H5/2)â†'6H15/2(~1000nm), where internal QE is calculated to be111%and107%accordingto Judd-Ofelt theory and the measured luminescence decay time, respectively. On the otherhand, in GdVO4:Dy3+, the broadband UV-excited [VO4]3-groups can strongly sensitize Dy3+ions, thereby achieving the efficient NIR downconversion in250-500nm.(3) As Ho3+:5F4,5S2excited, the energy can decay via sequential radiative transitions of5F4,5S2â†'5I6~1015nm and5I6â†'5I8~1190nm with5I6acting as an intermediate level. Besides,through the strong sensitization of broadband UV-excited [VO4]3-to Ho3+in YVO4:Ho3+, notonly the full spectrum downconversion can be realized in250-560nm, but the NIR emissionintensities increase by a factor of~10. On the other hand, under excitation of an UV photon~287nm, Ho3+-doped β-NaYF4can yield three NIR photons emitting at850,1015and1180nm, respectively. In term of excitation and emission spectra, decay curves, and time-resolvedvisible and NIR emission spectra, the mechanisms of sequential three-photon NIR-QC and ETof Ho3+are reported, that is: with5F4,5S2and5I6acting as intermediate levels, the energy of3D3excited state can be sequentially decayed to5I8ground state by three-step radiativetransitions, where internal QE is estimated to be124%. Furthermore, as Yb3+introduced intoβ-NaYF4:Ho3+, the radiative transition of3D3â†'3K8,5F2~850nm and that of5F4,5S2â†'5I6~1015nm would become the resonant ET paths to increase the emission intensity of Yb3+~1000nm and that of Ho3+~1180nm by several times, which downconverts the absorbed UVenergy of Ho3+to NIR emission completely, and optimizes the three-photon NIR emission ofHo3+greatly. According to the experimental and theoretical calculation, the10mol.%Yb3+ codoped β-NaYF4:1%Ho3+is demonstrated to have the optimal internal QE of~224%.(4) In term of static fluorescence spectra, dynamic luminescence decay curves, andtime-resolved NIR emission spectra, it can be found that, under excitation of a blue photon~470nm, the energy of Tm3+:1G4excited state is sequentially de-excited by three photonsemitting at1185,1466and1800nm with3H4and3F4acting as intermediate levels, whereinternal QE is evaluated to be160%. Moreover, by using integrating sphere, the practical QEof β-NaYF4:1%Tm3+phosphors was roughly measured to be32%, rather smaller than thetheoretical value~160%. On the other hand, in Gd2O2S:Tm3+, as Tm3+:1D2state excited by anUV photon~364nm, the efficient cross-relaxations of Tm3+:1D2â†'3H4+Tm3+:3H6â†'3F2,3andTm3+:3H4â†'3F4+Tm3+:3H6â†'3F4would sequentially occur, quadruply populating Tm3+:3F4state to yield four NIR photons~1789nm. By changing Tm3+contents, the absorbed UVenergy of Tm3+can be efficiently downconverted and concentrated on~1789nm emission,which greatly benefits the practical application of four-photon NIR-QC materials activated byTm3+. On the basis of the four-photon NIR-QC processes and the measured decay curves, theinternal QE of Gd2O2S:10%Tm3+is calculated to be maximum~275.4%. Furthermore, bycomparing the excitation spectra with the diffuse reflection spectra of Gd2O2S:Tm3+, the totalQE of Gd2O2S:10%Tm3+is rationally optimized to be~239.4%.