G.K. Liu

Hydrothermal synthesis and photoluminescence properties of red phosphor BaSiF6:Mn4+ for LED applications

An efficient red phosphor, BaSiF6:Mn4+ free of manganese oxide impurities, has been synthesized by using a one-step hydrothermal method at 120 °C for 12 h. The synthesis and chemical reactions have been investigated in detail and it includes: (a) influence of starting materials, concentrations of KMnO4 and HF, reaction temperature and time on products; (b) redox reaction of KMnO4 by HF; (c) chemical reaction mechanism for red phosphor BaSiF6:Mn4+. Mn4+ ions, acting as activators in the phosphor, originated from KMnO4 reduced by HF. The composition and crystal structure were determined with XRD, EDS and XPS. The morphology and thermal stability were investigated by SEM and TG-DSC, respectively. The maximum absorption band (at 460 nm) of the phosphor overlaps well with the blue emission of commercial InGaN LED. The room temperature emission spectrum of the phosphor shows a typical emission (centered at 632 nm) of Mn4+ in complex fluorides. The characteristic broad excitation band and sharp emission peaks of this red phosphor are desirable for potential applications as a red phosphor or for improving the color rendering index of conventional white LEDs.

Confinement on energy transfer between luminescent centers in nanocrystals

The luminescence dynamics of optical centers in nanocrystals depends critically on the phonon density of states (PDOS), which is quite distinct from that of bulk materials. It is shown that energy transfer (ET) in nanocrystals is confined by discrete PDOS as well as direct size restriction. Temperature-, concentration-, and size-dependence of the fluorescence decay from the 4S3/2 state of Er3+ in Y2O2S nanocrystals have been investigated using laser spectroscopic experiments and computational simulations. A set of microscopic rate equations that govern the evolution of the excitation probability Pi(t) are solved iteratively using a Monte Carlo method. The simulations of ET based on a theoretical model with five parameters are in good agreement with the experimental results. It is shown that phonon-assisted ET processes in Er3+:Y2O2S nanocrystals contribute partly to the fluorescence decay at 295 K, and is negligible at 5 K. For applications, the nanoconfinement effects on ET may significantly reduce the e...

Optimized photoluminescence of red phosphor K2TiF6:Mn4+ synthesized at room temperature and its formation mechanism

A red phosphor K2TiF6:Mn4+ (KTFM) has been synthesized by etching Ti(OC4H9)4 in HF solution with KMnO4 and KF at room temperature for 30 min. The formation mechanism of red phosphor KTFM has been discussed based on detailed experimental results. We studied the influences of synthetic procedure and KMnO4 concentration on the powder color and intensity of phosphor luminescence. The actual doping concentration of Mn4+ in the K2TiF6 (KTF) host lattice of the phosphor has been investigated by measuring the concentration of filtrate through ICP-AES analysis. The results showed that about 32.4 mol% of Mn elements was doped into KTF crystals at optimal Mn4+ concentration (in precursor solution). The presence of HF was found to be essential to doping Mn4+ into KTF due to the weakly acidic and complexing properties of HF. The red luminescence of Mn4+ in KTF with a crystal structure matching standard card JCPDs (#28-0825), was first observed in the sample prepared from HF solution concentrations lower than 5 wt%. The dependence of the intensity of the luminescence on HF concentration might be due to the varying of Mn4+ concentrations in KTF crystals. Higher HF concentration was associated with lower yield, because KTF is soluble in HF at high concentrations. Encapsulation of the red phosphor KTFM with YAG:Ce on a GaN layer produces “warm” white LEDs with color rendering of 86 at 3251 K.

Confinement of electron–phonon interaction on luminescence dynamics in nanophosphors of Er3+:Y2O2S

Observation of an anomalous thermalization effect induced by optical excitation of Er 3+ in nanocrystals of Y 2 O 2 S was recently reported (Liu et al. Nano Lett. 2 (2002) 535). Due to the absence of low-energy phonon modes in nanocrystals, the nonradiative relaxation between crystal field levels of Er 3+ is significantly diminished, whereas this confinement effect on high-energy phonon relaxation and thermalization is negligible. It is also shown that absorption line broadening for Er 3+ ions in the surface layer of the nanoparticles enables coincident excitation of Er 3+ ions at defect and intrinsic sites. As a result of the combined excitation and relaxation processes, Er 3+ population accumulates in the upper crystal field levels of the 4 I 15/2 ground state; and the intensity of hot bands originating from these levels increases abruptly as temperature decreases below 7 K. This anomalous thermalization effect is interpreted satisfactorily based on calculations of temperature-dependent multiphonon relaxation rates in nanocrystals of confined phonon modes.

X-ray excited optical luminescence (XEOL) detection of x-ray absorption fine structure (XAFS)

The x-ray excited optical luminescence (XEOL) from a variety of rare-earth ions was used as a detection mode for the collection of L-edge x-ray absorption fine-structure (XAFS) data. In order to understand the source of the observed optical signal, advantage is taken of the known luminescent response of f ions in a variety of transparent host materials. Whereas some samples exhibit an optical response that is indistinguishable from the transmission XAFS data, other samples show marked differences between the data obtained with the two different detection schemes. The unexpected optical luminescence of a Gd2O3 sample is traced to a Eu impurity. An optical spectrum of 0.4% Tb in Gd2O2S, excited by x-ray photons at the Gd edge, is used to demonstrate that the optical signal may arise from an ion different from the absorbing ion. The implications of this energy transfer are discussed in terms of the suitability of XEOL as a detection scheme for XAFS spectroscopy.

Mn4+ doped (NH4)2TiF6 and (NH4)2SiF6 micro-crystal phosphors: synthesis through ion exchange at room temperature and their photoluminescence properties

The synthesis procedures of red phosphors activated by Mn4+ that are carried out especially at room temperature, are significant to industrialization. Herein, we report a facile strategy to obtain two red micro-crystal phosphors: (NH4)2TiF6:Mn4+ (NTF:Mn) and (NH4)2SiF6:Mn4+ (NSF:Mn) through ion exchange at room temperature for a few hours. Raw materials NTF and NSF act as host lattices of red micro-crystal phosphors, and activator Mn4+ ions are from K2MnF6/HF and/or mixed KMnO4/HF solution. The reaction mechanism for the synthesis process has been comprehensively discussed in detail. The typical red emitting luminescence of Mn4+ with strong absorption in the blue region and the large size of about 200 μm of red micro-crystal phosphors NTF:Mn make it an attractive candidate material as a red compensator for resin-free white light-emitting diodes with high color rendering index. The dependence of emission intensity on concentration of HF and KMnO4, and measurement temperature has been investigated. The advantage of the proposed strategy is the ability to explore Mn4+ doped red phosphors from low-cost starting materials at room temperature for large scale industrial applications.

Intensity and bandwidth of multiphonon vibronic transitions of rare-earth ions in crystals

The theory of multiphonon vibronic coupling to electronic transitions is applied in analysing fluorescence spectra of Eu2+ in BaFCI, which consist of the 4f7(6P7/2,) → 4f7(8S7/2) and 4f65d → 4f7 transitions, and the 4f7-4f65d excitation spectrum of Ce3+ in YPO4. The 4f electrons are weakly coupled to lattice vibration modes so that only weak one- and two-phonon sidebands are observable in the 4f-4f optical transitions, whereas the electron-phonon coupling is significantly stronger for a 5d electron. Accordingly, intensive multiphonon vibronic transitions overwhelmingly dominate the 4f65d → 4f7 spectrum. It is shown that the extended Judd-Ofelt theory for weak vibronic coupling in the framework of the M-process is equivalent to the Huang-Rhys theory for the δ-process. In the analysis of experimental data, contributions from local ligand modes and lattice acoustic modes are separated, and the coupling strength is evaluated, in terms of the Huang-Rhys parameter S, for the 4f-4f and 5d-4f vibronic transitions.

Analysis of the crystal‐field spectra of the actinide tetrafluorides. II. AmF4, CmF4, Cm4+:CeF4, and Bk4+:CeF4

We report a systematic analysis of the crystal‐field spectra of four fluoride compounds containing tetravalent actinide ions. The first part of this work [J. Chem. Phys. 95, 7194 (1991)] provided interpretation of the absorption spectra of UF4, NpF4, and PuF4. To extend our analysis to heavier elements of the series, low‐temperature absorption spectra of AmF4 and CmF4, and site selective laser‐induced emission and excitation spectra of Cm4+:CeF4 and Bk4+:CeF4 were obtained. A model energy level calculation was found to be in good agreement with the experimental results. It is shown that the crystal‐field interaction in combination with spin–orbital coupling results in significant J mixing in the excited states, but ground state wave functions are still relatively pure in J character for the tetravalent actinide ions Am4+, Cm4+, and Bk4+. Trends in the parameters of the effective operator Hamiltonian are compared with those of a Hartree–Fock free‐ion model. Interpretation of the ground‐state splitting of t...

Crystal field influence on the 8S7/2 ground state splitting of Bk4+ in CeF4

The one-particle models of crystal-field theory provide a qualitative interpretation for the observed ground state splitting of four Kramers doublets of the 8S7/2 of Bk4+ doped into CeF4. A set of nine nonzero (Bqk) parameters corresponding a C2v point symmetry provide a very good correlation between the experimental data and simulated energy level schemes within a rms deviation of 13.7 cm−1. The calculated and experimental energy values have the same order-of-magnitude for the ground state and excited components. The total ground state splitting of the S-state ions of f-elements such as Bk4+ in CeF4 is larger when compared with Cm3+:LaCl3 and Gd3+:La(C2H5SO4)3⋅9H2O ions. The so-called crystal-field strength parameter, Nv, increases as a function of the increasing maximum splitting of the ground state due to a decrease in the participation of the pure 8S7/2 in the final composition of the ground state eigenvector.

Nonlinear optical dynamics and Eu3+ spectral holeburning in strontium barium niobate thin film grown by pulsed laser deposition

Optical quality SrxBa1−xNb2O6 (SBN) thin films, both undoped and Eu3+-doped, of thickness less than 0.5 μm have been successfully grown on fused quartz substrates using a pulsed laser deposition technique. Optical properties of these films were characterized in high-resolution spectroscopic experiments in time and frequency domains. For undoped SBN thin films, broadband emission in the UV region extending to the visible was observed following excitation at 355 nm. This emission is attributed to exciton luminescence of the SBN film. Nonlinear optical response in the picosecond regime and the third-order nonlinear susceptibility, χ(3), were studied using degenerate four-wave-mixing methods. In transverse alignment, χ(3) is enhanced by two orders of magnitude in comparison with its bulk counterpart. A thermal annealing process, monitored via changes in spectral properties of Eu3+, was employed to convert the as-grown amorphous film into a polycrystalline film. High-resolution spectroscopic measurements in th...