Material Properties of RE- Doped Ln (Ln= Y, La) oxides and oxysulfides phosphors for red-emitting devices
| dc.contributor.advisor | Dejene, F. B. | |
| dc.contributor.advisor | Swart, H. C. | |
| dc.contributor.author | Ali, Abdub Guyo | |
| dc.date.accessioned | 2018-02-12T08:57:09Z | |
| dc.date.available | 2018-02-12T08:57:09Z | |
| dc.date.issued | 2015-11 | |
| dc.description.abstract | Structural and optical properties of Eu3+-doped Ln (Ln=Y, La) oxide and oxysulfide nano crystals synthesized by sol-combustion method were analysed as a function of host to fuel ratio. Structural characterization shows crystallite nanosized particles and the hexagonal phase as the dominant structure. The red emission of Eu3+ doped Y 20 2S, La20 2S and Y 20 3 nanocrystals appearing near 624 nm was assigned to the 5Do-7F2 transition of Eu3+. Due to insufficient quantities of thiourea at the higher Ln/S mole ratio, the bright red emission has been quenched. Fourier-transform infrared spectrometry analysis showed that there was a negligible difference in the absorbed impurities with various molar ratios. The Ln/S concentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs for Ln/S=I to 76 μs for the higher concentrations. Structural and optical properties of La20 2S:Eu3+micro crystals synthesized by sol-combustion method were analyzed as a function of La/S concentration. Structural characterization shows a crystallite size of about 178 nm and the hexagonal phase as the dominant crystalline structure. The red emission of Eu3+ doped La20 2S microcrystals appearing near 624 nm was assigned to the 5D0- 7F2 transition of Eu3+. Due to insufficient quantities of thiourea at the higher La/S mole ratio, the bright red emission has been quenched. Fourier-transform infrared spectrometry analysis showed that there was a negligible difference in the absorbed impurities with various molar ratios. The La/S concentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs for La/S= I to 7 6 μs for the higher concentrations. To investigate the effect of co-doping a series of red-emitting phosphors Y 20 3:Eu3+:Ho3+ were prepared by the solution combustion method. X-ray diffraction (XRD) patterns indicate that the Eu3+ and Ho3+ doping do not show obvious effect on the cubic Y 20 3 crystal. Their crystall ite size estimated by x-ray diffractometry and scanning electron microscopy was about 8 nm. Under UV 325 nm excitation, emission wavelengths at 626 nm was quenched at higher mole percent of Ho3+ and energy was transferred from Eu3 ~ to Ho3+. Y20 3:Eu3+: Ho3+ phosphor shows a red-emitting afterglow phenomenon, and the Eu3 ' ions are the luminescent center during the decay process. The bright red emission near 626 nm has been noticeable due to the 5D0- 7F2 transition of Eu3 ... . The intensity of the luminescence has decreased with an increase of concentration of Ho3+. In sufficient quantities of Eu3+ to Ho3+, the bright red emission near 626 nm has been predominant due to 5D0- 7F2 transition of Eu3+. The decay characteristic of Y 20 3:Eu3 +: Ho3 + phosphor is according with the double exponential equation. The as-prepared powder Y 20 2S:Eu3 + was deposited on Si ( 100) substrates by using a pulsed laser deposition technique. The thin films grown under different oxygen deposition pressure conditions have been characterized using structural and luminescent measurements. The Xray diffraction patterns showed mixed phases of cubic and hexagonal crystal structures. As the oxygen partial pressure increased, the crystallinity of the films improved. Further increase of the 0 2 pressure to 140 mtorr reduced the crystallinity of the fi lm. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that an increase in 0 2 pressure affected the morphology of the films. The average band gap of the films calculated from diffuse reflectance spectra using the Kubeika-Munk function was about 4.75 eV. The photoluminescence measurements indicated red emission of Y20 2S:Eu3 + thin films with the most intense peak appearing at 619 nm, whi ch is assigned to the 5Do-7F2 transition of Eu3 +. This most intense peak was totally quenched at higher 0 2 pressures. X-ray photoelectron (XPS) indicated that Y20 3 thin films are formed on the surfaces of the Y20 2S: Eu3 + thin films during prolonged electron bombardment. The films grown in a lower 0 2 ambient consist of smaller but more densely packet particles relative to the films grown at a higher 0 2 ambient. In order to study the effect of annealing temperature on the films, four samples were annealed at various temperatures while one was kept unannealed. X-ray diffraction measurements show that the un-annealed thin film was amorphous, while those annealed were crystalline. At lower annealing temperature of 600 °c to 700 °c cubic bixbyite Y20 3:Eu3 + was formed . As the annealing temperatures were increased to 800 °c, hexagonal phase emerged. The average crystall ite size of the fi lm was 64 nm. Photoluminescence (PL) measurement indicates intense red emission around 612 nm due to the 5 00~ 7F2 transition. Scanning electron microscopy (SEM) indicated that agglomerates of non-crystalline particles with spherical shapes were present for the un-annealed film s. After annealing at high temperature, finer morphology was revealed. Atomic fo rce microscopy (AFM) further confirmed the formation of new morphology at the higher annealing temperatures. UV-vis measurement indicated a band gap in the range of 4.6 to 4.8 eY. It was concluded that the annealing temperature played an important role in the luminescence intensity and crystallinity of these films. To investigate the effect of different species of gases Y20 2S:Eu3 + thin films have been grown on Si ( I 00) substrates by using a pulsed laser deposition technique. The thin films grown under different species of gases have been characterized using structural and luminescent measurements. The X-ray diffraction patterns showed mixed phases of cubic and hexagonal crystal structures. The crystallinity of the film deposited in vacuum is poor, but improved significantly in argon and oxygen atmosphere. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that different species of gases affected the morphology of the fi lms. The average band gap of the films calculated from diffuse reflectance spectra using the Kubeika-Munk function was about 4.69 eV. The photoluminescence measurements indicated red emission of Y20 2S:Eu3+ thin films with the most intense peak appearing at 6 12 nm, which is assigned to the 5D0- 7F2 transition of Eu3+. The intensities of this most intense peak greatly depend on the species of gas with argon having the highest peak. This phosphor has applications in the flat panel displays. | en_ZA |
| dc.description.sponsorship | African Laser Center (ALC) | en_ZA |
| dc.description.sponsorship | National Research Foundation (NRF) | en_ZA |
| dc.description.sponsorship | University of the Free State | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/7808 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Solution - combustion method | en_ZA |
| dc.subject | Morphology | en_ZA |
| dc.subject | Excitation | en_ZA |
| dc.subject | Band gap | en_ZA |
| dc.subject | Luminescence | en_ZA |
| dc.subject | Rare earth ions | en_ZA |
| dc.subject | PLD | en_ZA |
| dc.subject | Laser ablation | en_ZA |
| dc.subject | Decay time | en_ZA |
| dc.subject | Red-emitting | en_ZA |
| dc.subject | Y₂O₂:Eu³⁺ | en_ZA |
| dc.subject | La₂O₂S:Eu³⁺,La₂O₃: Eu³⁺ | en_ZA |
| dc.subject | Y₂O₂S:Eu³⁺ | en_ZA |
| dc.subject | Y₂O₂S:Eu³⁺ :Ho³⁺ | en_ZA |
| dc.subject | Thesis (Ph.D. (Physics))--University of the Free State, 2015 | en_ZA |
| dc.title | Material Properties of RE- Doped Ln (Ln= Y, La) oxides and oxysulfides phosphors for red-emitting devices | en_ZA |
| dc.type | Thesis | en_ZA |