Kailash C. Mishra

A scattered wave model of electronic structure of Eu2+ in BaMgAl10O17 and associated excitation processes

Abstract The excitation spectrum of a commercial blue phosphor, BaMgAl10O17:Eu2+ (BAM) has been investigated assuming a distribution of the activator ions at three different sites: Beevers–Ross, anti-Beevers–Ross and mid-oxygen sites in the intermediate plane. The energy levels and wavefunctions of the Eu2+ ions located at these sites have been calculated using the self-consistent field Xα scattered wave cluster molecular orbital approach (SCF-Xα-SW-MO). The molecular orbital eigenvalues are then utilized to develop an energy level diagram and the associated excitation spectrum. A comparison of the calculated excitation peaks with those from experiment provides support for distribution of Eu2+ ions at multiple sites in BAM.

Model of work function of tungsten cathodes with barium oxide coating

Using a full-potential band structure approach, we have investigated the work function of barium oxide coated tungsten cathodes in low pressure discharge lamps. The main objective of this work is to understand why the work function for such cathodes is lower than that of the uncoated tungsten. The model studied in this work is based on a well known supposition that the source of thermionic electrons is the barium atoms released from the barium oxide coating due to a chemical reaction with the underlying metallic tungsten. For the unrelaxed seven-layer model of (100) surface of barium on barium oxide, the work function is calculated to be 2.22 eV, which is lower than that of BaO, Ba, and W metals separately. For a fully relaxed nine-layer surface, it becomes 1.36 eV. Although this value of the work function is lower than those estimated for the fluorescent cathodes by electrical measurements, which averages contributions from surfaces in all possible random orientations, this model provides a satisfactory ...

Investigation of Fluorescence Yield of Phosphors in Vacuum Ultraviolet Region

A theoretical model describing the dependence of quantum efficiency of a phosphor on the intensity of exciting radiation is developed. This model takes into account three dominant processes contributing to the loss of radiation when the host is excited by vacuum ultraviolet radiation above its bandgap energy, and fluorescence occurs through transfer of radiation from the host to the activator ions. The three processes are depletion of the ground state, nonradiative energy transfer in the excited state, and surface recombination of electron-hole pairs. It has been shown that the surface recombination effect can be treated adequately by assuming that the excited state lifetime of the host material is modified by a loss factor associated with this effect.

Investigations of Nuclear Quadrupole Interaction in BaMgAl10O17:Eu2+

Local environments of divalent europium ions in β-lattice of barium magnesium aluminate, BaMgAl10O17 (BAM) have been investigated using a recently developed ab initio band structure method. This method is a variant of the full potential LMTO method. The reliability of this method for calculating electric field gradient (EFG) was tested in the case of aluminum oxide in corundum structure. The calculated EFGs at both cation and anion sites compare satisfactorily with those from other ab initio methods and from recent measurements. The nuclear quadrupole coupling constants for Eu2+ calculated by this method at various possible sites in BAM, when compared to recently measured 151Eu Mössbauer spectroscopy results, suggest three locations for Eu2+: a Beevers–Ross site, a mid-oxygen site and an anti-Beevers–Ross site. Energetically, the anti-Beevers–Ross site appears to be more stable than the other two sites.

Investigation of Luminescence from Dy3 + in AlN

The preparation of powder samples of aluminum nitride activated by dysprosium, AlN/Dy 3+ , was achieved for the first time using a low-temperature, solution-based approach. Aqueous solutions of aluminum and dysprosium nitrates was first converted to hydroxides and then to ammonium-metal hexafluorides (Al 1-x Dy x (NH 4 ) 3 F 6 . Finally, this complex ammonium salt was converted to finely divided powders of Al 1-x Dy x N via a pyrolysis reaction in a high-purity ammonia flow inside a tubular quartz furnace at 900°C. The phase purity and crystallinity of the material were determined using X-ray diffraction and energy dispersive spectroscopy. The intraconfigurational f-f transitions of Dy 3+ were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) measurements. The excited states of Dy 3+ in this large-gap nitride were identified from a careful analysis of PL and PLE results. It was also observed from the PLE measurements that the Dy 3+ ions in this host are excited through energy transfer from both the excited state of the host lattice and defects. Measurements using site-selective spectroscopy suggest multiple sites for Dy 3+ ions in AlN. A theoretical model describing the excitation process and saturation of excited states of Dy 3+ at higher intensity of the exciting radiation is also proposed.

A First-Principles Investigation of the Electronic Structure of Trivalent Rare Earth Ions in Gallium Nitride

Using first-principles electronic structure methods, we have investigated the electronic structure and associated properties of Pr3+, Nd3+, Eu3+, Tb3+, Dy3+ and Er3+ substituting for Ga3+ in GaN. It is found that these rare earth ions (RE3+) are stable at the Ga site in tetrahedral coordination with nitrogen atoms. The nitrogen ligands move away from the RE3+ site to allow for the large ionic radii of the RE3+ ions. The equilibrium bond lengths of RE–N are found to vary between 2.15 to 2.30Å in good agreement with structural data available for Eu3+, Tb3+ and Er3+. Using the calculated energy bands, excitation energies for the 4f→5d transition have also been predicted. Most of these energies are larger than the band gap of GaN and cannot be exploited in transferring host excitation energies to the RE3+ ions.

An Investigation of Self-Absorption and Corresponding Spectral Shift in Phosphors

Many light emitting diode (LED) phosphors have overlapping emission and absorption bands, such as Ce 3+ -doped garnets and Eu 2+ -doped silicates, nitrides, and oxynitrides. These overlapping bands inevitably lead to self-absorption effects through either nonradiative energy transfer mediated via electron-electron correlation process or actual emission and radiative energy transfer which involves emission and absorption of a photon due to spectral overlap. The first process has been often discussed in the literature. The second process has been investigated in this work through a semi-quantitative formalism. Spectral shift, changes in emission peak shape, photoluminescent conversion efficiency, and phosphor characterization are discussed in light of self-absorption. The simulated results are compared to experimental data and shown to correlate well with observed data.

Study of GaN : Eu3 + Thin Films Deposited by Metallorganic Vapor-Phase Epitaxy

Using metallorganic vapor-phase epitaxy, thin films of gallium nitride activated by Eu 3+ (GaN:Eu 3+ ) have been deposited on sapphire substrates at atmospheric pressure. Luminescence from Eu 3+ ions in GaN has been investigated using photoluminescence (PL) and PL excitation spectroscopy. Experimental results show that Eu 3+ ions are excited via energy transfer from the host. Analyses of the observed emission and excitation spectra indicate occupancy of multiple sites in the nitride lattice. Using a pulsed laser source, variation of emission intensity with increasing excitation intensity has also been examined. The possibility of emission saturation at high excitation intensity is discussed from the perspective of application in light-emitting diode sources.

A Study of Luminescence from Tm3 + , Tb3 + , and Eu3 + in AlN Powder

Nitride alloys of Ga, In, and Al activated by rare-earth ions (Re 3+ ) are being considered for application in nitride-based solid-state light sources. The potential applications involve using such materials as the active layer in a heterostructure design or as a fluorescent material for converting the emission from a light-emitting diode to white light. In this paper, we report luminescence from Tm 3+ , Tb 3+ , Eu 3+ , and Tb 3+ -Eu 3+ couple in A1N powder samples synthesized for this purpose. Using the photoluminescence and photoluminescence excitation responses of Re 3+ -doped A1N samples, the multiplet structures of these Re 3+ ions in A1N with tetrahedral coordination have been determined. The excitation energy transfer processes from the host and defects to Re 3+ and from Tb 3+ to Eu 3+ have been observed. These processes are critical for developing nitride-based luminescent materials for white-light emission.

Theoretical Investigation of Intercalated Water Molecules and Hydroxyl Groups in BAM ( BaMgAl10O17 : Eu2 + ) Phosphor and Associated Degradation Processes

Intercalation of water molecules into the p-alumina lattice of barium magnesium aluminate (BaMgAl 10 O 17 ) has been recently reported to be responsible for degradation of this phosphor. The associated degradation process is characterized by a green shift of the emission spectrum accompanied by oxidation of the activator ions. However, the structure and location of intercalated water molecules, and the degradation mechanism are not understood. In this report, both the location and geometry of water molecules are investigated using an atomistic simulation method with empirical interatomic potentials and a quantum mechanical method of structure optimization using full periodicity of the lattice. The oxidation of activator ions is modeled based on dissociation of water molecules to hydroxyl groups in this lattice. It is shown that the dissociation to hydroxyl groups is energetically favorable, and that the protons released in the process could be responsible for oxidation of europium ions.