Luminescence enhancement of phosphors by doping with silver
Abbass, Abd Ellateef
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Phosphor materials doped with noble metals have attracted considerable attention for the past fifty years due to their possible applications in lighting and solar cells with improved efficiency, biology, lasers and novel display technologies. Active research has recently been focused on the interaction between noble metal nanoparticles and rare-earth ions in different phosphor hosts, with the aim of luminescence enhancement. Much attention has been paid to silver nanoparticles due to their strong absorption of electromagnetic radiation, resulting from localized surface plasmon resonance which can enhance the incident electric field by about two orders of magnitude. Although some reports have been published in regard to phosphors doped with silver, there is still room to better understand the interaction between silver and phosphors and to boost the efficiency of such phosphors. In this work, three different types of materials, namely amorphous silica, bismuth silicate and zinc oxide were used as hosts for silver and terbium. These hosts were selected due to their good physical properties and particularly because they have the appropriate refractive index, which is one of the main parameters required to control the plasmon absorption band for plasmonic enhancement. Doped and undoped amorphous silica and bismuth silicate were successfully prepared by the sol-gel method, while zinc oxide was prepared by the combustion method. The sources of the dopants used in this work were silver nitrate and terbium nitrate. The prepared phosphor powders were investigated by many techniques in order to apply appropriate conditions for phosphor enhancement. The structure, morphology and particle size were investigated by X-ray diffraction and transmission electron microscopy. Reflectance and absorption band of localized surface plasmon resonance were measured using a ultraviolet-visible spectrophotometer. X-ray photoelectron spectroscopy was used to investigate the composition of the phosphors, while optical properties were investigated using a fluorescence spectrophotometer having a xenon lamp or by exciting samples with a helium-cadmium laser. Firstly, doped and undoped amorphous silica was synthesized by the sol-gel method. The photoluminescence properties of amorphous silica doped only with silver as a function of annealing temperature were investigated in detail. The obtained results showed that the addition of silver after annealing at low temperature (500 °C) enhanced the luminescence associated with oxygen deficiency centres of the amorphous silica host, which is attributed to the formation of silver oxide. Increasing the annealing temperature to 1000 °C introduced new optically active centres in the amorphous silica. These new emission bands were related to excess oxygen due to decomposition of the silver oxide at high temperature. The additional luminescence band changed the blue emission from pure amorphous silica to near white light from the silver doped material suggesting that the silver doped silica system may be suitable for solid state lighting applications. The stability of this phosphor under ultraviolet irradiation was also investigated. To study the effect of addition of silver on the terbium luminescence, both terbium (5 mol%) and different silver concentrations were incorporated into amorphous silica using the sol-gel method. The obtained results showed significant enhancement of the terbium emission when 1 mol% silver was added after annealing at 500 °C. In previous works, the enhancement of rare-earth ion emission in the presence of silver was assigned to two possibilities, namely plasmonic enhancement associated with silver nanoparticles or energy transfer associated with silver ions. This work shows a third possibility, namely that enhancement of the rare-earth (e.g. terbium) emission is due to energy transfer from defects of the host material to the terbium ions, where the addition of silver influences the silica host defects. Secondly, powder samples of doped and undoped zinc oxide were successfully prepared by the combustion method. The photoluminescence properties of zinc oxide doped only with silver were studied in detail. More than a two fold increase in the intensity of near band edge emission of undoped zinc oxide was observed in the presence of silver nanoparticles. A new mechanism due to interaction between silver nanoparticles and zinc oxide has been proposed as being responsible for the enhancement of near band edge emission which is different from previous reports. In other samples, zinc oxide was doped with both terbium and silver. The addition of 1 mol% silver to the 5 mol% terbium doped zinc oxide system caused significant quenching on the terbium emission intensity instead of enhancement. The quenching effect is attributed to radiative energy transfer from terbium ions to silver nanoparticles (re-absorption) and was studied by means of spectral overlap and lifetime measurements. In the previous reports, researchers focused only on enhancement as a beneficial effect and considered quenching as a deleterious effect. In this work, the obtained results showed that the absorption of energy by silver nanoparticles (acting as energy acceptors) can also be beneficial in biological and polymer applications where local heating is desired i.e. photothermal applications. Another novelty of this work is that one can use the down-converting phosphor properties (containing, for example, rare-earth ions) as effective method to indirectly couple a laser to the plasmon resonance wavelength of metal nanoparticles without the need to change the particle size or shape of the nanoparticles, which requires special synthesis methods. Thirdly, bismuth silicate was synthesized using the sol-gel method and successfully doped with only terbium or silver, or co-doped with both. A simple way to select a suitable host material, when doped with any rare-earth ion and incorporated with silver nanoparticles, to cause overlap between an excitation band of the rare-earth ions and the localized surface plasmon resonance of the metallic nanoparticles in order to study possible plasmonic enhancement is presented using Mie theory calculations. Luminescence properties of the terbium doped bismuth silicate containing silver nanoparticles were explored in detail and an enhancement of the emission from the terbium ions at 545 nm when excited at 485 nm of about two and a half times is attributed to amplification of the electric field associated with the localized surface plasmon resonance of the silver nanoparticles. A particular novelty of the present work is the use of a crystalline host instead of an amorphous host to study plasmonic enhancement as in previous studies.