New Phenomena, Mechanism, And Application Exploration Based On Luminescence Of Ianthanide Ions

Inorganic luminescent materials have received much attention for their wide applications from science to engineering, such as, lighting, sensor and so on. Recently, with the development of purification process of lanthanide ions, luminescent materials based on lanthanide doped particles have attracted considerable attention for the advantages of sharp bandwidth emission, long lifetimes, and good photostability. e. g. white light emitting diodes (LEDs) with lanthanide doped phosphors, bioimaging labeled by upconversion (UC) from lanthanide doped nanoparticles, and improving the efficiency of solar cells by the modulated spectrum of sunlight. However, much effort was focused on improving the luminescence efficiency of lanthanide ions, ignoring the complexity of lanthanide luminescence for their rich 4f levels. Therefore, exploring the new luminescence phenomena and mechanism based on lanthanide ions would be an efficient way for the development and applications of lanthanide luminescence.I. Enhancing UC luminescence from lanthanide ions under the excitation of two-wavelength, simultaneously.As 4f electrons of lanthanide ions are shielded by the electrons of 5s25p6 and those 4f-4f transitions are partially forbidden, therefore, the absorption cross-section and absorption coefficient of lanthanide ions are small. Besides, nonlinear absorption of UC process also determines the low UC efficiency from lanthanide ions. Considering the property of sunlight with continuous spectrum and the ladder-like arranged energy levels of lanthanide ions, we hope to enhance the UC luminescence of lanthanide ion under the excitation of two-wavelength simultaneously, where the energies are matched well with the ground state absorption (GSA) and excited state absorption (ESA) of lanthanide ions, respectively. We first chose hexagonal NaYF4 as the matrix, and Ho3+ as the lanthanide ion, then excited the samples with 1150nm and 975 nm, simultaneously, whose energies matched well with GSA and ESA of Ho3+ The UC luminescence intensity with the excitation of two-wavelength, simultaneously, has been enhanced by a factor of 14.04, compared to the sum intensities from the excitation by two single-wavelength. The enhanced UC luminescence has also been discovered with Er3+ doped in hexagonal NaYF4, under the excitation of 1520 nm and 790 nm, where the green UC luminescence has been enhanced by the factor of 3.05. It might be an efficient method for broadening the responded area of solar cells in near-infrared area with the GSA, ESA, and photo-phonon coupling of lanthanide ions.II. Modulating the luminescence of lanthanide ions and exploring the mechanism under the excitation of magnetic field.For the paramagnetic property of lanthanide ions and the anisotropy of crystal field, the interaction between lanthanide ions along different orientation could be various and the local site symmetry around lanthanide ions might be modulated with the application of magnetic field, then influence the luminescence from lanthanide ions. We first probed the luminescence from Eu3+ with the hexagonal NaYF4 as the matrix, for the luminescence of Eu3+ is hypersensitive to the local site symmetry, and observed the splitting and shifting of luminescence bands, accompanying with the increase of the number of luminescence peaks with increasing magnetic field intensity. The mechanism of those phenomena could be ascribed to Zeeman effect induced by magnetic field. Besides, the luminescence intensity of Eu3+ was suppressed with increasing the magnetic field intensity, which is ascribed to the interplay of the change in site symmetry and the enhanced cross-relaxation rate with the application of magnetic field.Then, we chose hexagonal NaGdF4 as the matrix for the seven unpaired electrons of Gd3+, which would generate stronger interaction with magnetic field. With the application of pulse magnetic field, the magnetic-optical hysteresis behavior has been observed, where the luminescence intensity from Eu3+ do not overlap with each other in the process of increase and decrease magnetic field, forming a loop. The microscopic mechanism of magnetic-optical hysteresis behavior was tentatively ascribed to the magnetic response of paramagnetic dopant ion.Finally, by modulating the energy transfer between lanthanide ions with the application of magnetic field, we detected the enhanced UC luminescence from Yb3+-Er3+ and Yb3+-Ho3+ for the first time, where the matrix is hexagonal NaYF4. Compared the luminescence behaviors from single-doped and co-doped samples, we observed the decreased UC luminescence intensity from single-coped samples while enhanced luminescence from co-doped samples in the rise of magnetic field. The different UC luminescence behaviors from single-doped and co-doped samples indicate the enhanced energy transfer from Yb3+ to Ln3+ with the increase of magnetic field, and enhanced the UC luminescence from co-doped samples.The luminescence behaviors of lanthanide ions with the application of magnetic field can be used as the self-calibrated sensor for magnetic field. Besides, modulating UC luminescence by magnetic filed might be a new way for enhancing UC luminescence from lanthanide ions.III. Polarized UC luminescence and potential applications of lanthanide doped single nanoparticle.Recently, considerable attention has focused on the luminescence behaviors of ensemble nanoparticles, ignoring the luminescence and microscopic mechanism from lanthanide doped single nanoparticle. Exploring the luminescence and mechanism from single nanoparticle would be a new method to find the new luminescence phenomena based on lanthanide ions. We chose Er3+ as the luminescence center for its wide application in bioimaging, then, scanned the single particle with AFM and detected the polarized UC luminescence in a home-built system. The polarized behaviors of lanthanide ions from various crystal planes are different, where the UC luminescence intensity from lanthanide ions in (0001) almost unchanged while that from (1010) became obvious polarized with changing the excitation polarization angle. Those phenomena are connected with the site symmetry of lanthanide ions in different crystal planes. The site symmetry of lanthanide ions in (0001) is centrosymmetric, cancelled the polarization behaviors. Considering the application in bioimaging of lanthanide doped nanoparticle with ultra-low concentration, we probed the UC luminescence from few-particle aggregates and observed the evident change of UC luminescence intensity with changing the excitation polarization angle, indicating the significant influence of polarized UC luminescence from single particle for the applications of ultra-low concentration nanoparticles.IV. Discovering new phosphors by the single particle diagnosis approach.Considering the special luminescence behaviors of lanthanide doped single particle, we disconver a new phosphor by a single particle diagnosis approach. The principle of the approach is straightforward:to treat an individual luminescent particle in a complex mixture as a tiny single crystal, distinguish it by its characteristic luminescence color, morphology and size, and finally to determine its structure and property using microscale characterization techniques such as super-resolution single-crystal X-ray diffraction, single particle luminescence imaging and spectroscopy. Then we disconver the new single particle Mg(Si, A1)4(O, N)6:Eu2+ with the blue-green luminescence and the tetrahedral shape in MgO-Si3N4-AlN-Al2O3 system by single particle diagnosis approach. The particle is finally refined in hexagonal crystal structure with a= 3.03130 A, b= 3.03130 A, c= 41.61600 A, a= β= 90°, γ= 120°, V= 331.168182 A. Then we prepared the pure-phase phosphor Mg(Si, Al)4(O, N)6:Eu2+ by gas-pressure sintering process and achieved the crystal structure by XRD Rietveld analysis.