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This talk reviews work on the optical properties of Eu-doped GaN at the Semiconductor Spectroscopy laboratory of the University of Strathclyde. The principal experimental technique used has been lamp-based Photoluminescence/Excitation (PL/E) spectroscopy on samples produced mainly by high-energy ion implantation and annealing, either at low or high pressures of nitrogen, as described by Lorenz et al. . These have been supplemented by samples doped in-situ either by Molecular Beam Epitaxy or Metallorganic Vapour Phase Epitaxy. Magneto-optic experiments on GaN:Eu were carried out in collaboration with the University of Bath.
We report site-selective studies of the Zeeman splittings that are observed for magnetic fields up to 6.6T for different Eu incorporation sites in GaN. Utilizing resonant excitation with visible light, we are able to distinguish the site and find for one center (Eu1) a splitting into five components as expected for C3v symmetry. The corresponding g-values are 1.66 and 1.90. The two lines of another center Eu2 each split into two levels corresponding to g-values of 1.9 and 2.84. Most surprisingly a third center, for which only one line is clearly identified, a g-value of 6.16 is found which is larger than can be explained for a 7F2 purely ionic Eu state.
The co-doping of hexagonal GaN with Er and O is investigated by means of density functional calculations. Predominantly Er-O defect-pairs characterized by a binding energy around 0.5 eV are formed. Different geometric configurations with various orientations (i.e. axial and basal pairs with C3v or C1h symmetry) are expected with similar formation energies. Independent of the particular configuration, the presence of oxygen does not deeply affect the atomic structure and the electronic charge distribution around the Er centers. The relatively high binding energy suggests that Er-O pairs should survive thermal treatment. An investigation of the binding energy per bond indicates that on the other hand Er-Ox complexes (x=2,3,4) are not likely to be formed (differently from Er-O co-doped Si). Rather, as long as the oxygen fluence does not overtake the Er fluence, different oxygen ions will be bound to different Er-centers.
A series of Ca1-xSrxS:Eu2+y mol% phosphors were synthesized with solid state reactions and with various Ca/Sr ratio and Eu2+ doping concentrations. The influences of the lattice composition and the Eu2+ doping level on photoluminescent properties were analyzed. With doping concentrations between 0.1 to 3 mol%, concentration quenching takes place leading to the decrease of luminance; the emission maxima are also red-shifted. Further, this work reports enhanced photosynthetic activities of intact spinach leaves due to spectral modification of simulated solar irradiation by one synthesized phosphor (Ca0.4Sr0.6S:Eu1 mol%). The CO2 assimilation rates of intact spinach leaves were monitored with an effective homemade photosynthesis measurement system with controlled light conditions. The phosphor could efficiently convert the photosynthetically less active green part of the solar spectrum into the red, with a broad-band red emission centered at 650 nm and a halfband-width of 68 nm, giving an excellent match with the absorption spectrum of spinach chloroplasts. By careful referencing the photon flux, we found an enhanced photosynthetic activities by about 30 % due to the emission of the phosphor.
X-ray-excited luminescence of GaN doped with Eu ions as a luminescent center was observed in the wavelength range from 350 nm to 650 nm. Three peaks at 375 nm, 550 nm and 622 nm were found. To survey the mechanism of the photoluminescence due to non-resonance excitation, photoluminescence X-ray excitation spectra are also measured. The mechanism of the luminescence occurrence was briefly discussed based on the model developed by Emura et al.
SiO2 nanoparticles (NPs) were coated with Eu3+-doped calcium phosphate (CP) and Mn2+-doped ZnO to give Zn2SiO4 via a modified Pechini sol-gel process. Annealing at high temperatures resulted in NPs with an amorphous core and a crystalline luminescent shell. It was shown that this procedure can be applied to silica cores with diameters below 300 nm. By transmission electron microscopy, elemental analysis and from X-ray diffraction patterns it was determined that shell composition and structure are influenced by the annealing temperature and pH of the coating solution. Measurements of photoluminescence intensities displayed their dependency on the concentration of dopant in the resulting core/shell NPs.
We prepare Nd-Bi codoped zeolites by a method consisting of a simple ion-exchange process and subsequent high-temperature annealing. The emission covers the range of 970∼1450 nm, corresponding to the electronic transitions of Nd3+ ions and Bi-related active centers (BiRAC), respectively. The introduction of Bi distinctly broadens the excitation band of Nd3+ in the visible region, and the lifetime of Nd3+ reaches as long as 354 μs. In the zeolite matrix, Bi ions exist as BiRAC and Bi oxide agglomerates. The former one act as a sensitizer of Nd3+ ions, and the latter one act as a blockage to avoid the quenching effect of coordinated water, which enable Nd3+ ions to show efficient near-infrared (NIR) emission even the zeolites contain large amount of coordinated water. The excellent optical and structural properties make these NIR emitting nanoparticles promising in application as laser materials and biological probes.
Near-infrared-to-visible upconversion materials have many promising applications, including use in luminescent solar concentrators, in next-generation displays, and as biological labels. NaYF4 nano-particles doped with Yb and Er exhibit efficient upconversion and are easily deployed in these applications. It is known that a rough metal surface may increase the yield of fluorescence of a nearby fluorophore, by local field enhancement due to plasmonic resonances, and by modification of the radiative rate(s) of the fluorophore. Thus, properly chosen metallic nanostructures can potentially increase the upconversion efficiency of lanthanide-doped nanoparticles, yet the optimal design of these nanostructures is still an active area of research. In our experiments, we use a spectroscopic imaging system to study the upconversion efficiency of NaYF4: Er3+/ Yb3+ through spatially-resolved upconversion spectra, using a custom-built scanning confocal microscope system with infra-red excitation, and wide-field fluorescence imaging. We present spectrally-resolved upconversion images of NaYF4:Yb3+/Er3+ nanoparticles on plasmonic substrates, including silver nanowires and patterned substrates of gold and silver, which show localized regions (∼ 1μm) of relatively stronger intensity and modified upconversion spectra, and compare these to wide-field fluorescence images of samples with and without plasmonic substrates.
We developed ultra-violet field-emission devices using rare-earth nitrides of Al1-xGdxN grown by a reactive radio-frequency magnetron sputtering technique. The Al1-xGdxN phosphor film excited by high-energy electrons shows a resolution limited, narrow intra-orbital luminescence from Gd3+ ions at 318 nm. The devise characteristics depend on injected current and acceleration voltage, which were analyzed by considering multiple excitation process of injected high-energy electrons.
In this work, the damage formation subsequent to Eu implantation at 300 keV has been investigated by coupling the TEM, XRD and RBS/C techniques. It has been found that GaN exhibits a specific damage buildup in three main steps: (i) clustering of point defects and formation of a network of stacking faults defects in the bulk, (ii) propagation of the planar defect network towards the surface and (iii) breakdown of the surface layer. This occurs through different strain saturation regimes. Around 5x1014 Eu/cm2, the strain along the implantation direction saturates to 0.6%. At higher fluence, whereas the peak at 0.6% is maintained, there is an increase of the strain throughout the implanted layer which probably continues to extend. A second saturation occurs when the stacking fault network reaches the layer surface.
Spherical monodisperse core/shell-type nanoparticles, comprising an amorphous SiO2 core coated with a luminescent phosphor layer were synthesized by the modified Pechini processes. The sol-gel method allows covering the 50 – 500 nm core particles with different inorganic phosphor layers of about 10 nm thickness, doped with rare-earth or transition metal ions which determine the luminescent properties. Particles comprising a Zn2SiO4 shell, doped with Mn2+ ions, are not only fluorescent under UV irradiation (260 nm), but store the activation energy by trapping electrons/holes at lattice defects. This energy is released as phosphorescence in the time scale of seconds and minutes, or as photostimulated luminescence under the excitation of red light (650 nm). Traps related to these processes are different, and their concentration is affected by the preparation conditions of the particles.
Nano-colloids and nano-crystals doped with ions of rare-earth elements have recently attracted a lot of attention of scientific community. This attention is due to unique physical, chemical and optical properties attributed to nanometer size of the particles. They have great potential of being used in applications spanning from new types of lasers, especially blue and UV lasers, phosphorous display monitors, optical communications, and fluorescence imaging. In this paper we investigate the infrared-to-visible upconversion luminescence in bulk crystals doped with ytterbium and holmium co-doped and ytterbium and thulium co-doped NaYF4 upconversion phosphors. The phosphors were prepared by using simple co-precipitation synthetic method. The initially prepared phosphor has very weak upconversion fluorescence. The fluorescence significantly increased after the phosphor was annealed at a temperature of 600 0C. Nanocolloids of this phosphor were obtained using methanol as solvents and they were utilized as laser filling medium in photonic crystal fibers. Under 980 nm laser excitation very strong upconversion signals were obtained for ytterbium and holmium co-doped phosphor at 541 nm, 646 nm and 751 nm, and 376 nm, 476 nm, 646 nm, 696 nm and 803 nm for ytterbium and thulium co-doped phosphor. The particle sizes of the nanocolloids were analyzed using Atomic Force Microscope. The reported nanocolloids are good candidates for fluorescent biosensing applications and also as a new laser filling medium in fiber lasers.
Ca3Y2(SiO4)3 powder activated with Eu was prepared with ceramic method. Its spectroscopic properties in VUV-UV-Vis region together with structure and morphology were analyzed. Luminescence measurements indicated that Ca3Y2(SiO4)3 prepared in a reducing atmosphere contained both Eu2+ and Eu3+ ions. The excitation could be tuned to generate a complex luminescence consisting of a broad band related to a blue-greenish 5d → 4f emission of Eu2+ and 4f → 4f red luminescence of Eu3+. Their superposition covered the whole visible part of spectrum and appeared almost white among others upon excitation around 390 nm.
Inorganic erbium-doped glasses are widely used in telecommunications due to the sharp intra-atomic 4I13/2 → 4I15/2 transition in the 4f orbital of erbium resulting in an emission at ∼ 1.5 μm, which is the low loss window of silica optical fibres. The limited erbium concentration of about 1020 ions.cm−3 in inorganic erbium-doped glasses and the low absorption coefficient of the Er3+ ions, imply that relatively long lengths of fibre are required. The organic erbium complexes present higher absorption cross sections due to the photosensitization of erbium by organic conjugated ligands and broader emission bands than those of the free Er3+ ion. Such properties open the possibility to develop compact, low power and broadband infrared emitting devices. We present the study of a highly doped organic thin film obtained from organic erbium complexes deposited by a vacuum sublimation technique. This deposition method allows the realization of an erbium-doped thin film without the help of an organic polymer matrix, which is a potential source of vibrationnal luminescence quenching. The ligands used in the present study are fluorinated in order to limit the vibrationnal quenching of the ligand itself, and to increase the volatility of the complexes. In this paper, we report the synthesis, the sublimation process and the characterization of the thin films.
This article provides the key technical niche that alternative rare earth metals and oxides can offer as an alternative to the ones that are under ‘Potential China Export Embargo’. The potential areas of national interest that affected by limited rare earth and the implications of such short supplies on the US businesses are discussed. The paper discusses the technology areas where US based academic and industries have an opportunity in developing the alternate rare earth materials (REM) through innovations in recycling existing rare earth (RE) metals/oxides and develop alternate solutions. Some examples are provided on how the nanotechnology research in the alternative material technologies in the rare earth metals and oxide materials significantly affect the industry trend of rare earth dependence.
We investigated the electroluminescence (EL) properties of Eu-doped GaN-based light-emitting diodes (LEDs) grown by organometallic vapor phase epitaxy (OMVPE). The thickness of the active layer was varied to increase the light output power. With increasing the active layer thickness, the light output power monotonically increased. The maximum light output power of 50 μW was obtained for an active layer thickness of 900 nm with an injected current of 20 mA, which is the highest value ever reported. The corresponding external quantum efficiency was 0.12%. The applied voltage for the LED operation also increased with the active layer thickness due to an increase in the resistance of the LED. Therefore, in terms of power efficiency, the optimized active layer thickness was around 600 nm. These results indicate that the optimization of the LED structure would effectively improve the luminescence properties.
In-situ doped Eu ions in GaN grown by Organometallic Vapor-phase Epitaxy (OMVPE) at different pressures were investigated under different excitation methods and through the use of the following experimental techniques: (1) resonant site-selective laser irradiation (2) electron beam excitation, and (3) a dual excitation using a combination of electron beam and laser irradiation. With these means, we have examined the difference in the excitation pathways that result from resonant laser and electron hole (e-h) pair excitation of Eu ions for two different distinct incorporation sites, which are responsible for most of the luminescence. We have obtained clear evidence that e-h pairs do not have the ability to excite all of the ions and that there is excitation trapping by defects involved in the Eu excitation.