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Browsing Physics by Subject "Absorption"

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    ItemOpen Access
    Investigation of the luminescent properties of metal quinolates (Mqx) for use in OLED devices
    (University of the Free State, 2014-01) Duvenhage, Mart-Mari; Swart, H. C.; Ntwaeaborwa, O. M.
    Since Tang and VanSlyke developed the first organic light emitting diode (OLED) in the late 80’s using tris-(8-hydroxyquinoline) aluminium (Alq3) as both the emissive and electron transporting layer, a lot of research has been done on Alq3 and other metal quinolates (Mqx). The optical, morphological and electrical properties of these Mqx have been studied extensively. Alq3 has, however, a disadvantage as it tends to degrade when stored under atmospheric conditions. These degraded products are non-luminescent and lead to poor device performance. A good understanding of what happens during the degradation process and ways of eliminating this process are needed. In this study different Mqx compounds were synthesized and their degradation behavior was studied to see what effect it has on their luminescent properties. One way to tune the emissive colour of Alq3 is to introduce electron-withdrawing or electron-donating groups (EWG and EDG) onto the hydroxyquinoline ligands. These groups have an effect on the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. In this study Alq3 powders were synthesized with an EDG (-CH3) substituted at position 5 and 7 ((5,7-dimethyl-8-hydroxyquinoline) aluminium) (5,7Me-Alq3) and EWG (-Cl) at position 5 ((5-chloro-8-hydroxyquinoline) aluminium) (5Cl-Alq3). A broad absorption band at ~ 380 nm was observed for un-substituted Alq3. The bands of the substituted samples were red shifted. The un-substituted Alq3 showed a high intensity emission peak at 500 nm. The 5Cl-Alq3 and 5,7Me-Alq3 samples showed a red shift of 33 and 56 nm respectively. Optical absorption and cyclic voltammetry measurements were done on the samples. The optical band gap was determined from these measurements. The band gap did not vary with more than 0.2 eV from the theoretical value of Alq3. The photon degradation of the samples was also investigated and the 5,7Me-Alq3 sample showed the least degradation to the UV irradiation over the 24 h of continuous irradiation. By encapsulating the Alq3 molecule with glass (SiO2) or a polymer-like polymethyl methacrylate (PMMA), the oxygen and moisture responsible for degradation have a lesser effect on the degradation of the Alq3 molecule. The as prepared SiO2-Alq3 sample’s emission was blue shifted by 10 nm from that of Alq3. The sample was subjected to UV irradiation and after 24 hours, no luminescence intensity was detected. According to literature the SiO2 will decompose into Si and O species under UV irradiation. These O species have reacted with the Alq3 to form non-luminescent products. The Alq3:PMMA samples showed a maximum emission at 515 nm. There was a decrease in luminescence intensity when the sample was irradiated with UV photons. This was due to the decomposition of PMMA into elemental species and the O again reacted with the Alq3 molecule to form non-luminescent products. However, the intensity stabilized after 100 h of irradiation. X-ray photoelectron spectroscopy (XPS) and infra red (IR) measurements were done on the as-prepared and degraded Alq3 samples. It revealed that the Al-O and Al-N bonds stayed intact, but C-O and C=O bonds formed during degradation, indicating that the phenoxide ring ruptures during degradation. It is known that the luminescent centre of the molecule is located on the quinoline rings and the rupturing of one of these rings will destroy this centre, leading to a decrease in luminescence intensity. When the Al3+ ion was replaced with a Zn2+ ion to form Znq2, it showed higher emission intensity and, compared to Alq3, did not degrade as fast. This might be due to the fact that Znq2 only has two quinoline rings. The effect of solvent molecules, in the solid state crystal lattice, on the photoluminescence properties of synthesized mer-[In(qn)3].H2O. 0.5 CH3OH was studied. Single crystals were obtained through a recrystallization process and single crystal x-ray diffraction (XRD) was performed to obtain the unit cell structure. The main absorption peaks were assigned to ligand centered electronic transitions, while the solid state photoluminescence excitation peak at 440 nm was assigned to the 0-0 vibronic state of In(qn)3. Broad emission at 510 nm was observed and was ascribed to the relaxation of an excited electron from the S1-S0 level. A powder sample was annealed at 130 °C for two hours. A decrease in intensity was observed and could possibly be assigned to a loss of solvent species. To study the photon degradation, the sample was irradiated with an UV lamp for ~ 15 hours. The emission data was collected and the change in photoluminescence intensity with time was monitored. High resolution XPS scans of the O-1s peak revealed that after annealing, the binding energy shifted to lower energies indicating a possible loss of the H2O and CH3OH present in the crystal. The O-1s peak of the degraded sample indicated the possible formation of C=O (~ 532.5 eV), C-O-H and O=C-O-H (~ 530.5 eV) on the phenoxide ring. Commercial Alq3 is normally used in the fabrication of OLEDs. In this study Alq3 was synthesized using a co-precipitation method and it was purified using temperature gradient sublimation. The Alq3 was then used to fabricate a simple two layer OLED with a device structure: ITO/NPB/Alq3/Cs2CO3:Al. The electroluminescence (EL) spectrum of the device consisted of a broad band with a maximum at ~520 nm and was similar to the photoluminescence (PL) spectrum observed from the synthesized Alq3 powder. The luminance (L)–current density (J)–voltage (V) characteristics of the device showed a turn on voltage of ~ 2 V, which was lower than the current density of the device fabricated using the commercial Alq3. The external quantum efficiency (ηEQE) and the power conversion efficiency (ηP) of the device were 1% and 2 lm/W, respectively.
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    ItemOpen Access
    Luminescence investigations of CaS:Eu2+ powder and pulsed laser deposited thin films for application in light emitting diodes
    (University of the Free State, 2015-06) Nyenge, Raphael Lavu; Ntwaeaborwa, O. M.; Swart, H. C.
    The main objective of this thesis was to investigate the luminescent properties of commercial CaS:Eu2+ powder and pulsed laser deposited thin films for application in light emitting diodes. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and photoluminescence (PL) spectroscopy data suggest that the CaS:Eu2+ phosphors contain secondary phases that were possibly formed during the preparation or due to unintended contamination. An intense red PL broad band with a maximum at 650 nm was observed when the powder was excited at 484 nm using a monochromatized xenon lamp. When the powder was excited using a 325 nm He-Cd laser an additional PL emission peak was observed at 384 nm. The origin of this emission is discussed. Auger electron spectroscopy and Cathodoluminescence (CL) spectroscopy were used to monitor the changes in the surface chemical composition and CL intensity when the phosphor was irradiated with a 2 keV electron beam in vacuum. Possible mechanism for the degradation of CL intensity is presented. Thermal quenching in CaS:Eu2+ occurred at a relatively low temperature of 304 K. The kinetic parameters, namely activation energy and order of kinetics of γ-irradiated CaS:Eu2+ were determined using initial rise and peak shape methods, respectively. An Edinburgh Instruments FS920 fluorescence spectrometer equipped with a Xe lamp as the excitation source was used to collect emission and excitation spectra at low temperature. The samples were exposed to γ-radiation ranging from 10 to 50 Gy for thermoluminescence studies, from a 60Co source. The thermoluminescence data were obtained using a Harshaw thermoluminescence Reader (Harshaw 3500 TLD Reader). The possible mechanism leading to the decay of luminescence is explored. Pulsed laser deposited thin films of CaS:Eu2+ phosphor were grown on Si (100) or Si (111) substrates using the Q-switched Nd: YAG laser. For the purpose of this work, the deposition parameters which were varied during the film deposition are: laser wavelength, working atmosphere, number of laser pulses, deposition pressure, and substrate temperature. The film thickness, crystalline structure, surface morphology, and the photoluminescent properties of the thin films were found to be a function of the laser wavelength. The results from XRD showed that the as-deposited CaS:Eu2+ thin films were amorphous, except for the (200) diffraction peak observed from the films deposited at the wavelengths of 266 and 355 nm. The Rutherford backscattering (RBS) results indicate that film thickness depends on the laser wavelength used during deposition. Atomic force microscopy and scanning electron microscopy results show that the roughness of the samples is determined by the laser wavelength. The interaction of laser with matter is discussed, and the best wavelength for ablating this material is proposed. With RBS, it was possible to look at the variation of composition with depth as well as to determine the thickness of the thin films. Compositional analysis carried out using the energy dispersive X-ray spectroscopy showed that the films contained oxygen as an impurity. The films prepared in an oxygen atmosphere were amorphous while those prepared in a vacuum and argon atmosphere showed a degree of crystallinity. The roughness of the films has a strong influence on the PL intensity. The PL intensity was better for films in the argon atmosphere; showing bigger surface structures with respect to the other films. The emission detected at around 650 nm for all the films was attributed to 6 1 7 4 f 5d 4 f transitions of the Eu2+ ion. An emission at around 618 nm was observed, and was attributed to 2 7 0 5 D  F transitions in Eu3+, suggesting that Eu2+ was unintentionally oxidized to Eu3+. Results from time-of-flight secondary ion mass spectroscopy study show that all the films contain oxygen although the film prepared in oxygen contain more oxygen. The PL intensity of the CaS:Eu2+ films was found to depend on the pulse rate, with PL intensity increasing as the number of pulses is increased. XRD studies showed that there was an improvement in crystallinity of CaS:Eu2+ thin films upon post-deposition annealing, and subsequently an improvement on the PL intensity . PL intensity also improved significantly at a substrate temperature of 650oC. The best PL intensity as a function of deposition pressure was obtained at an argon pressure of 80 mTorr.