Synthesis and characterization of bismuth doped strontium oxide powder and thin films
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Date
2019-01
Authors
Abdelrehman, Mogahid Hassan Mohammed
Journal Title
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Publisher
University of the Free State
Abstract
The main aim of this project was to investigate the synthesis and characterization of bismuth-doped strontium oxide powder and thin films. Firstly the luminescent properties and stability under electron beam irradiation of the SrO:Bi3+ phosphor powder were investigated and secondly the luminescent properties of SrO:Bi3+ thin films prepared by different techniques were studied. The luminescence from Bi3+ ions can be useful in obtaining blue to red emitting phosphors by using different hosts, when excited by ultraviolet (UV) light due to efficient conversion to longer wavelengths. The energy levels of Bi3+ ions are host dependent. Bi3+ is a low cost activator, which provide strong absorption of UV light and can be efficiently converted to longer wavelengths. These emissions are related to the 3P1 – 1S0 or 1P1 – 1S0 transitions of Bi3+ ions, which are strongly dependent on the host. The alkali-earth oxide phosphors offer a potential low-cost alternative to lanthanide-based blue phosphors. Bi3+ doped strontium oxide (SrO:Bi) phosphor powders were synthesized by the sol-gel combustion method using metal nitrates as precursors and citric acid as fuel.
A wide range of temperatures (800 - 1200 °C) and concentrations of Bi3+ (0.05 - 0.7 mol%) were used to determine the optimum sample annealing temperature and Bi3+ concentration. The optimum doping concentration, for a fixed annealing temperature of 1200 °C (2 h), was found to be 0.2 mol% and a further increase in the Bi3+concentration resulted in concentration quenching. Samples of this concentration were annealed at various temperatures and the optimum annealing temperature was found to be 1100 °C (2 h). The X-ray diffraction patterns (XRD) corresponded with the well-known face-centered cubic structure of SrO after high-temperature annealing that ranged between 1100 °C up to 1200 °C. Below 1100 °C strontium hydroxide peaks were also present. Williamson-Hall plots showed that the crystallite size was in the range of ~180 nm. Diffuse reflectance measurements of the pure host material showed it was strongly reflecting (~100%) down to a wavelength of about 230 nm, but when doped with Bi3+ an absorption band at 275 nm was observed that increased with increasing Bi3+ concentration. Scanning electron microscopy (SEM) revealed a cubic morphology and the grain size increased with annealing temperature.
Photoluminescence (PL) measurements indicated that the phosphor exhibited efficient blue emission around 445 nm under UV excitation, which also occurred for electron irradiation, but slightly shifted about 5 nm to a longer wavelength. PL results showed that the emission intensity did increase with an increase in the annealing temperature up to 1100 °C. The increased intensities were attributed to two factors. The first one is due to a combination of the decrease of the Sr(OH)2, and the second one segregation/diffusion of the Bi3+ ions from the bulk to populate the surfaces of the particles with a consequent loss in Bi3+ due to volatile species as a result of the increased annealing temperature. The intensity increased up to 1100 °C due to a decrease in the hydroxyl concentration and thereafter at higher temperatures resulted in a Bi3+ deficiency from the sample’s surface and therefore leading to a decrease in the dopant concentration. Auger electron spectroscopy (AES) was employed to analyze the surface chemical composition of the powder after pumping to a vacuum pressure of 2.6 × 10−8 Torr and back-filling the vacuum system with O2 to a pressure of 1.0 × 10−7 Torr. The presence of all major elements of SrO, namely Sr and O were confirmed, but Bi3+ was not observed due to its low concentration. Cl and C were also detected as contaminations on the surface. X-ray photoelectron spectroscopy (XPS) results for the Sr1-xO:Bix=0.002 sample also indicated the presence of the major components Sr and O of this material and some contaminations on the surface. By simultaneous monitoring of the cathodoluminescence (CL) and AES peak-to-peak heights over time for 22 h, the CL degradation of the phosphor was investigated. The slight decrease of the CL intensity (less than 20%) was due to the removal of C from the surface due to the electron stimulated surface chemical reactions (ESSCRs) which took place during electron bombardment. During the ESSCR process, the electron beam dissociates the O2 and other background species such as H2O to atomic species which subsequently react with C to form volatile compounds (COX, CH4, etc.). The CL intensity reduced slightly more and at a higher rate in the O2 back-filled environment than in vacuum during the degradation studies, due to the reaction of O2 with the adventitious C at a higher rate to form volatile compounds on the surface of the irradiated sample. The SrO was found to be stable under electron irradiation. XPS results indicated surface contaminated elements were completely removed after degradation.
An important application for phosphors is to make thin films for devices such as plasma displays and light emitting diodes. Sr1-xO:Bix=0.002 phosphor thin films were prepared by spin coating and pulsed laser deposition (PLD). Spin coating samples were obtained by sequentially depositing 10 layers at 3000 rpm for 30 s and then annealed at various substrate temperatures. The optimum annealing temperature was found to be 900 °C (2 h). For all thin films samples, XRD showed the thin film had a strong (111) preferential orientation on the cubic phase. The results imply that the crystallite size of the sol–gel-derived films increased slightly with the increasing annealing temperatures. The morphology of the samples was determined by SEM and atomic force microscopy (AFM). The main PL emission peak position of the thin films prepared by spin coating showed a shift to shorter wavelengths at 430 nm, if compared to the main PL peak position of the powder at 445 nm. Thin films were prepared by PLD of Sr1-xO:Bix=0.002 phosphor optimized for blue luminescence. The powder was pressed into a PLD target, which was annealed at 200 °C for 2 h in air to remove all adventitious water containing species. Thin films were then successfully fabricated by PLD in vacuum or an O2 working atmosphere on Si (100) substrates. Films were deposited using different types of excimer lasers namely a KrF laser (248 nm) with energy 300 mJ/pulse and a ArF laser (193 nm) with energy 150 mJ/pulse with the constant substrate temperature at 300 °C and deposited at different substrate temperatures using a Nd:YAG laser (266 nm) with energy 33.3 mJ/pulse. The microstructures and PL of these films were found to be highly dependent on the substrate temperature. XRD of the thin films obtained with the different types of excimer lasers showed the thin films also had a strong (111) preferential orientation on the cubic phase. XRD of the films deposited using the Nd:YAG laser in O2 showed that the crystallinity increased with an increase in the substrate temperature, changing from amorphous to a cubic structure. At the highest temperature of 500 °C, the 111 and 200 SrO peaks were almost the same height, as in the powder. However, for 350 °C and 200 °C, the 200 peak was much smaller, which suggests some preferential orientation for films prepared at lower substrate temperatures. All films deposited in vacuum were amorphous. All the SEM images show a very rough thin film surface that comprises of rounded irregular particles of different sizes and shapes, which were not uniformly distributed and which do not seem to be highly dependent on the substrate temperature. AFM results showed that the surface roughness decreased as the substrate temperature increased. The optimum substrate temperatures for the maximum luminescence (both (PL) and (CL)) were 200 °C and 50 °C for deposition in O2 and vacuum, respectively. The main PL emission peak position of all the PLD thin films showed a shift to shorter wavelengths at 427 nm, when compared to the powder (445 nm). The optical properties of the powder and thin films showed different results because the Bi3+ ion is very sensitive towards its environment. Time of flight secondary ion mass spectroscopy (ToF SIMS) depth profiles for the samples deposited in O2 or vacuum at different substrate temperatures look similar, except for a slight thickness variation. The PLD fabrication technique is suggested to be the best technique to fabricate the SrO:Bi3+ phosphor thin films.
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Keywords
Phosphor, Powder, Thin films, Strontium oxide, Bismuth, Sol-gel combustion method, Pulsed laser deposition, Spin coating, Annealing, Photoluminescence, Cathodoluminescence, Dissertation (M.Sc. (Physics))--University of the Free State, 2019