Luminescence properties of ZnO and ZnO: Eu³⁺ nanostructures and thin films
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Date
2019-10
Authors
Hasabeldaim, Emad Hasabeldaim Hadi
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Eu3+ doped ZnO thin films and ZnO nanorods were successfully prepared by using different techniques. Successful incorporation of Eu3+ ions in the ZnO matrix and preferred orientation along the c-axis for the films and the nanorods were achieved. The structure, morphology, luminescence and stability of the samples under electron beam irradiation were investigated. Firstly: Low Eu3+ concentration (0.4, 0.6, 0.8, and 1 mol%) doped ZnO thin films were successfully prepared using the spin coating technique. The preferred orientation of the films was reduced with increasing Eu3+ content. The average particle sizes and the optical band gap of the films decreased with increasing Eu3+ concentration. The films were excited at 325 nm and 464 nm using a xenon lamp. Upon excitation at 325 nm, the films exhibited band to band emission at ~378 nm and a broad deep level emission due to defects, with a small peak associated with characteristic Eu3+ emission at 614 nm that protruded from the broad band deep level emission. Upon excitation at 464 nm the characteristic Eu3+ emission features were observed and their intensity increased with increasing Eu3+ content until 0.6 mol% of Eu3+ and was then quenched. Multipole-multipole interaction, defects created due to the differences in ionic radii and charge states of Eu3+ and Zn2+ were found to contribute to luminescence quenching. Judd-Ofelt intensity parameters and asymmetry ratio analysis revealed the dependence of the Eu3+ emission intensity on the local environment around the Eu3+ ions in the host. Secondly: ZnO thin films doped with higher Eu3+ concentration up to 4 mol% were also successfully prepared using a sol-gel spin coating technique. X-ray photoelectron spectroscopy (XPS) confirmed the presence of Zn atoms in their doubly ionized state (Zn2+), while Eu atoms were found to be present in their divalent (Eu2+) and trivalent (Eu3+) states. Excitation spectra showed a broad band near 288 nm which was attributed to the charge transfer between O to Eu3+. For the excitation at 464 nm, the doped samples exhibited only the characteristic emissions of Eu3+ which were attributed to the 5D0-7FJ (J = 0, 1, 2, 3, 4) transitions, respectively. The Eu3+ emission intensity increased with increasing Eu3+ concentration up to 3 mol% and was then quenched. Cathodoluminescence (CL) spectra showed only the Eu3+ characteristic emission similar to PL excited at 464 nm. Judd-Ofelt intensity indicated strong covalence of Eu-O bond and higher asymmetry in the vicinity of the Eu3+ ions. The optimum sample (3 mol%) was degraded in vacuum under electron beam irradiation for 160 C/cm2 (about 22 h). The CL intensity showed a slight decrease at the initial electron dose at ~ 30 C/cm2 and then stabilized at further electron dosages. XPS analysis confirmed the formation of defects as a result of electron beam irradiation. Slight changes of the surface morphology and roughness were observed from the degraded area. Thirdly: Eu3+ (3 mol%) doped ZnO thin films were deposited by pulsed laser deposition (PLD) at different oxygen partial pressures (vacuum, 5.9 x 10-2 Torr, 8 x 10-2 Torr and 10 x 10-2 Torr). The 002 X-ray diffraction (XRD) peak of the thin film initially increased with an increase in the oxygen partial pressure, but then slightly decreased. The film thickness, roughness and emission intensity also followed the same trend. The films' morphology improved as a function of increasing oxygen pressure. When excited by a He-Cd laser at 325 nm, the film deposited in vacuum exhibited an intense UV emission at ~ 379 nm, broad-weak deep level emission in the region from 450 nm to 700 nm, as well as a small peak associated with the characteristic emission of the 4f – 4f transitions of Eu3+ at 616 nm standing out from the deep levels emission for the films deposited in oxygen partial pressure. When the Eu3+ ions were selectively excited at 464 nm, only the characteristic emission of the 4f – 4f transitions of Eu3+ were observed at 536 nm, 578 nm, 595 nm, 616 nm, 656 nm and 707 nm corresponding to the 5D1 – 7F0, 5D0 – 7FJ (J = 0, 1, 2, 3 and 4) transitions. When excited at 288 nm, the film deposited in vacuum only exhibited a broad peak centred at 585 nm which was due to the ZnO deep defect levels. The O to Eu3+ charge transfer band near 288 nm was observed for the films deposited in oxygen, and its intensity increased with increasing oxygen pressure. The samples prepared in oxygen exhibited characteristic emission of Eu3+ with an increase in intensity for increasing oxygen partial pressure. No CL was observed for the sample prepared in vacuum, whereas only the characteristic emission of Eu3+ was detected for the films obtained in oxygen partial pressure. Current-voltage measurements of the p-type Si/ZnO:Eu3+ junctions showed a diodelike behaviour with turn on voltage of about 10 V. Fourthly: For Eu3+ doped ZnO (ZnO:Eu3+) thin films deposited by PLD, the oxygen working atmosphere, deposition time and target-substrate distance were optimized to achieve the best luminescence and morphology properties. The surface and luminescence stability of the film under electron beam irradiation was also studied. The CL intensity of the Eu3+ dominant peak at 616 nm increased slightly during the initial stage of electron irradiation, after which it stabilized. XPS high resolution spectra of the O 1s peak confirmed the creation of new defects during electron beam irradiation. Atomic force microscopy images revealed that the particle sizes increased slightly during irradiation (degradation). Colour rendering and purity of the CL spectra were slightly changed during degradation. Fifthly: Preferentially c-axis oriented ZnO nanorods were grown on a ZnO seed layer spin coated on a crystalline silicon substrate. A low temperature aqueous chemical growth method using the chemical bath deposition (CBD) technique was used to grow the ZnO nanorods. The samples were annealed at 700 °C in a reducing atmosphere (H2/Ar) with a relative ratio of 5%:95% for different times (20, 30 and 50 min). XRD analysis revealed that the crystallite sizes increased with increasing annealing time up to 30 min and then decreased for longer annealing time. Scanning electron microscope images showed a successful growth of the vertically-aligned ZnO nanorods which were approximately 500 nm in length and 50 nm in diameter. The diameter of the nanorods increased with increasing annealing time up to 30 min and then decreased when the annealing time was increased further. PL measurements confirmed that the un-annealed sample exhibited two distinct emissions, namely the band to band emission around 378 nm and a broad orange emission centred at 600 nm which was due to the oxygen related defects. The annealed samples exhibited only a broad green emission centred at 500 nm and their intensities increased with annealing time. The highest intensity was recorded for the sample annealed for 30 min and the intensity decreased for further annealing time. The deconvoluted PL peak of the green emission indicated that three different kinds of defects were responsible for the emission at 500 nm. The decay measurements indicated that the green emission (500 nm) had an average lifetime of 11.58 µs. The quantum yield of the sample annealed for 30 min was measured using an integrating sphere at a wavelength of 325 nm and it was found to be 43%. The surface state, chemical and luminescence stability of the sample with higher luminescence intensity were investigated under electron beam irradiation. Auger electron spectroscopy, XPS and secondary electron microscopy (SEM) were used. The CL intensity was monitored concurrently with the Auger peak-to-peak heights using the same electron beam. The degradation experiment was performed in vacuum and in an oxygen ambient. According to the AES spectra, all the principal elements (zinc and oxygen) were detected as well as carbon, which was removed at the initial stage of electron beam irradiation. No chemical change was observed during electron beam irradiation. In vacuum, the CL intensity decreased to almost half of its initial intensity after 100 C/cm2 electron dose and then stabilized. In the oxygen atmosphere, the CL intensity also decreased initially up to a dose of ~10 C/cm2 and thereafter recovered to about 90% of its original intensity and stabilized after a dose of ~100 C/cm2. No difference in the chemical state of Zn was observed with XPS for the original and degradaded samples. Only a small change in the defect contribution part of the O peak was observed. SEM images for the original and degraded samples showed that the electron beam irradiation induced surface changes in the morphology of the ZnO nanorods. These results suggest that the ZnO nanorods and ZnO:Eu3+ thin films are promising green and red materials for optoelectronic devices such as light emitting diodes (LEDs) and flat panel displays (FPDs) among others.
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Keywords
Thesis (Ph.D. (Physics))--University of the Free State, 2019, Luminescence, Zink oxide, Europium ions, Thin films, Nanorods, Green emission, Electron beam irradiation, Light emitting diodes, Flat panel displays