Luminescence Control And Applications Based On The Energy Transfer Of Rare Earth Ions

Inorganic phosphor is an important class of functional material. In recent years, researches on luminescence materials have been concentrated on energy and biological applications rather than energy-efficient lighting solid materials driven by intensified evolution of energy crisis, shortage of natural resources, and environmental pollution etc., and the upcoming nanometer era. Rare earth ion as a typical isolated luminescence center is closely related to the development of high-tech materials. Therefore, research on the optical properties of rare earth ions and their related materials has become a wordwide important subject. In this context, we have carried out a series of targeted and systematic researches which focus on the application requirements and the problems to be solved. Simultaneously, some new concepts have been proposed and the related exploratory investigations have been adopted. The following shows the three reseach topics in this thesis:I. Quantum cutting and spectral modification based on energy transfer of rare earth ionsThe research of quantum cutting and spectral modification based on energy transfer of rare earth ions, which focuses on the application of solar cell photoelectric conversion efficiency improvement, usually refers to the RE3+(RE3+=Tb3+, Tm3+, Pr3+) sensitizer and Yb3+luminescence center. However, such RE3+can only absorb the ultraviolet (UV)/blue solar light in a featured narrow wavelength range weakly due to their inherent drawback orignated from4f-4f transition characteristics. This results in a disadvantage in pratical application. To solve the problem, we have proposed that so-called broadband spectral modification by using rare earth ions with f-d transition nature to replace the existed senziters. We have successfully demonstrated the broadband absorption in Eu-+doped borate glass. The valence state of Eu has been analysed by absorption spectra and electron spin resonance (ESR) spectra. The energy transfer between Eu2+and Yb3+has been confirmed by the steady state and transient state fluorescent spectra at room temperature, while the fluorescent quantum efficiencies of single ions’luminescence has been calculated based on steady state fluorescent spectra at low temperature. Subsequently, we inferred that the quantum cutting process with total quantum efficiency above100%has occurred through cooperative down-conversion between Eu2+and Yb3+.Considering that the high-energy visible region is the major component of the solar energy, we expect to redshift the absorption band of Eu2+from UV region observed in the broate glass matrix, to the blue region or even cover the whole UV/blue region. Based on the ligand field theory, the aluminosilicate glass has been chosen as the host material. We have adjusted the absorption band of Eu2+to250-480nm wavelength region. Besides, the absorbing energy could be converted to near～infrared (NIR) emission through the cooperative down-conversion energy transfer between Eu2+and Yb3+.To realize the efficient quantum cutting, the energy absorbed by Eu2+in broad wavelength region must be released in NIR region efficiently. To achieve this purpose, Er3+has been chosen as the luminescence center due to its abundant appropriate energy levels to emit NIR photons. Eu2+-Er3+doped phosophors has been prepared. The energy transfer mechanism in Ca8Mg(SiO4)4Cl2sphor has been analyzed by excitation spectra, emission spectra, decay curves, time-resovled emission spectra and power dependent emission intensity function. It has been confirmed that the absorbed energy in250-500nm wavelength region was transferred to the NIR emission through the energy transfer between Eu2+-Er3+and the cross relaxation processes occurred among different Er3+ions.Different from Eu2+, which could be obtained in rigid reduction condition, Ce3+is easy to be doped just by using trivalent rare earth salts and Ce3+also exhibits broadband/strong absorption feature of f-d transition. Thus, Ce3+-Er3+co-doped YAG phosphor has been chosen as an example to further illustrate the multi-photon related NIR quantum cutting process followed by broadband aborption. It has been observed that the absorption band of Ce3+in YAG covered the high-energy visible region, and met the requirement for multi-photon emitting of Er3+. Apparent NIR emission enhancement of Er3+has been observed after the sensitization of Ce3+under excitation at467nm. The energy transfer dynamic process has been analyzed by time-resolved emission spectra, and the energy transfer efficiency has been calculated by the transient fluorescence spectra. Furthmore, the optimized NIR emission has been obtained by adjusting the Er3+doping concentration. To confirm the multi-photon transition related NIR emission of Er3+, we have compared the excitation spectrum of YAG:Er3+1531nm emission and the absorption spectrum of YAG:Er3+. In order to explore the application possibility of NIR quantum cutting in solar cells, the frequently researched Pr3+-Yb3+couple doped transparent fluoride oxide glass has been chosen to demonstrate the spectral modification effect combining with the c-Si solar cell. We have found that external quatum effieciency (EQE) values of the solar cells decreased in some featured wavelength related to the absorption of the doping rare earth ions. The enhancement effect of NIR quantum cutting to the solar cell EQE fell far-short of the negative effect coming from the absorption of Pr3+and Yb3+. In regards to the encountered problem, series of possible solutions have been proposed.Ⅱ. Up-conversion enhancement design and application based on energy transfer of rare earth ionsUp-conversion based on energy transfer of rare earth ions has extensive application prospects. Recently, a multitude of representative works of up-conversion have sprung up in solar cell-oriented application. The commen ground of these works is the growing trend of broadband and efficient up-conversion. Herein, we have proposed a new approach to realize broadband and efficient up-conversion, which refered to the multi-color excitation based on the ground state absorption, excited state absorption and phonon-coupled absorption of rare earth ions. The final application enviorment of the up-converter for solar cells refers to solar irradiation, which is composed of numerous continuous distributed single colors irradiation. We suggested to excitation with multi-color to simulate solar irradiation. In experiment, two-color excitation has been adopted to demonstate the above concept, and apparent up-conversion emission enhancement has been obtained successfully. In detail, Ho3-doped LaF3phosphor has been chosen as the up-conversion material. Comparative up-conversion phenoma under970nm,1150nm single color excitation and two-color excitation have been investigated. Order of magnitude increasements of the visible up-conversion emission intensities have been observed by comparing the two-color excitation case to the single color excitation case. Furhermore, intensity-power relation and fluorescence transient evolution of Ho3+visible emission have been analyzed to examine the up-conversion meachanims and the reason for emission enhancement.On the other hand, up-conversion of rare earth ions in biological fluorescence imaging application has received unprecedented attention, especially for the NIR-to-NIR luminescence of rare earth ions. A novel approach has been developed for the realization of efficient NIR-to-NIR up-conversion and down-shifting emission in nanophosphors. The efficient dual-modal NIR-to-NIR emission is realized in a β-NaGdF4:Nd3+@NaGdF4:Tm3+-Yb3+core-shell nanocrystal by careful control of the identity and concentration of the doped rare earth ion species and by manipulation of the spatial distributions of these RE ions. This approach facilitates simultaneous high-efficiency up-conversion luminescence by energy transfer from Yb3+to Tm3+and down-shifting emission from Nd3+in the core-shell structured nanocrystal, both of which enable the adoption of NIR excitation sources owing to the f-f absorption bands of Yb3+and Nd3+ions, respectively. Moreover, the optical properties of steady-and transient-state emissions were optimized by the effective suppression of the non-radiative energy loss between RE dopants, which was achieved through the selective doping of targeted RE ions in different regions of such composite-like nanoparticles. The photoluminescence results reveal that the emission efficiency increases at least two-fold when comparing the materials synthesized in this study with those synthesized through traditional approaches. Hence, these core-shell structured nanocrystals with novel excitation and emission behaviors enable us to obtain tissue fluorescence imaging by detecting the up-converted and down-shifted photoluminescence from Tm3+and Nd3+ions, respectively. As demonstrated in this work, this could make these materials a promising alternative to existing NIR luminescent nanomaterials for bioimaging. The reported approach thus provides a new route for the realization of high-yield emission from RE ions doped nanocrystals. which could prove to be useful for the design of optical materials containing other optically active centers.Ⅲ. New up-conversion phenomenon and mechanism exploration based on energy transfer of rare earth ionsUp-conversion luminescence in rare earth ions doped nanoparticles has attracted considerable research attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, and biological imaging etc. However, there have been no reports on RE ions doped nanoparticles to investigate their polarized energy transfer up-conversion, especially for single particle. Herein, the polarized energy transfer up-conversion from RE ions doped fluoride microrods were demonstrated in a single particle spectroscopy mode for the first time. Unique luminescent phenomena, e.g.. sharp energy level split and singlet to triplet transitions at room temperature, multiple discrete luminescence intensity periodic variation with polarization direction, were observed upon excitation with980nm linearly polarized laser. Furthermore, nanorods with the controllable size and symmetry were fabricated for analysis of the mechanism of polarization anisotropy. The comparative experiments suggest that intra-ions transition properties and crystal local symmetry dominate the polarization anisotropy, which was also confirmed by density functional theory (DFT) calculations. Taking advantages of the RE ions based up-conversion, potential application in polarized microscopic multi-information transportation is suggested for the polarization anisotropy from RE ions doped fluoride single nanorod or nanorod array.