Efficient doping and energy transfer from ZnO to Eu3+ ions in Eu(3+)-doped ZnO nanocrystals

Toward RGB LEDs based on rare earth-doped ZnO

By using ZnO thin films doped with Ce, Tb or Eu, deposited via radiofrequency magnetron sputtering, we have developed monochromatic (blue, green and red, respectively) light emitting devices (LEDs). The rare earth ions introduced with doping rates lower than 2% exhibit narrow and intense emission peaks due to electronic transitions in relaxation processes induced after electrical excitation. This study proves zinc oxide to be a good host for these elements, its high conductivity and optical transparency in the visible range being as well exploited as top transparent electrode. After structural characterization of the different doped layers, a device structure with intense electroluminescence is presented, modeled, and electrically and optically characterized. The different emission spectra obtained are compared in a chromatic diagram, providing a reference for future works with similar devices. The results hereby presented demonstrate three operating monochromatic LEDs, as well as a combination of the three species into another one, with a simply-designed structure compatible with current Si technology and demonstrating an integrated red-green-blue emission.

Excitation-induced tunable luminescence of luminomagnetic Dy and Ce co-doped ZnO nanoparticles

Here, 1 mol% Dy,Ce co-doped ZnO nanoparticles were synthesized via a simple, cost-effective combustion method which could produce large-scale products. The structure and phase purity of synthesized nanoparticles were shown by X-ray diffraction, selected-area-electron-diffraction patterns and Raman spectroscopy to be a hexagonal wurtzite structure with no secondary peaks. Spherical morphology was shown by field emission scanning electron microscopy and high-resolution transmission electron microscopy. The presence of dopants with elemental composition was authenticated by energy dispersive spectroscopy and elemental mapping. Enhanced reflectance in the visible region for Ce-doped ZnO samples was noted by diffuse reflectance spectroscopy. An increase in the bandgap for doped samples was indicated by the Kubelka-Munk function. Significant visible luminescence was observed, which varied with different excitations. The room-temperature weak ferromagnetic behaviour of 2 mol% Ce-doped ZnO and distinct paramagnetic behaviour of 1 mol% Dy,Ce co-doped ZnO was detected using vibrating sample magnetometry.

White Light Emission from Thin-Film Samples of ZnO Nanocrystals, Eu3+ and Tb3+ Ions Embedded in an SiO2 Matrix

In this work, a method was developed to determine the concentration of Eu³⁺ and Tb³⁺ ions in a thin-film sample of SiO₂, co-doped with ZnO-nanocrystals (ZnO-nc), to produce a sample of any desired colour in the International Commission on Illumination (CIE) colour space. Using this method, a white light emitting sample was fabricated. The thin-film sample combines red, green and blue emissions from the Eu³⁺ ions, Tb³⁺ ions and ZnO-nc, respectively, to create white light or light of any desired colour. The emissions at 614 nm and 545 nm from Eu³⁺ and Tb³⁺ ions, respectively, is due to the energy transfer from the excited ZnO-nc to the rare-earth (RE) ions. In this way, only a single excitation wavelength is needed to excite the ZnO-nc, Eu³⁺ and Tb³⁺ ions in the sample to produce emission of a desired colour from the sample. We developed an empirical 4th-degree polynomial equation to determine the concentrations of Eu³⁺ and Tb³⁺ ions to produce light of any desired colour in the CIE colour space. Based on the above empirical equation, the concentration of Eu³⁺ and Tb³⁺ ions for a white light emitting sample was found to be 0.012 and 0.024 molar fractions, respectively. The white light emission from the sample was confirmed by fabricating the sample using the low-cost sol–gel process. The stimulated emission spectra and the experimental emission spectra of the white light sample fit very well. The results presented in this work are important to develop energy efficient solid state lighting devices.

Study of energy transfer mechanism from ZnO nanocrystals to Eu(3+) ions

In this work, we investigate the efficient energy transfer occurring between ZnO nanocrystals (ZnO-nc) and europium (Eu(3+)) ions embedded in a SiO2 matrix prepared using the sol-gel technique. We show that a strong red emission was observed at 614 nm when the ZnO-nc were excited using a continuous optical excitation at 325 nm. This emission is due to the radiative (5)D0 → (7)F2 de-excitation of the Eu(3+) ions and has been conclusively shown to be due to the energy transfer from the excited ZnO-nc to the Eu(3+) ions. The photoluminescence excitation spectra are also examined in this work to confirm the energy transfer from ZnO-nc to the Eu(3+) ions. Furthermore, we study various de-excitation processes from the excited ZnO-nc and their contribution to the energy transfer to Eu(3+) ions. We also report the optimum fabrication process for maximum red emission at 614 nm from the samples where we show a strong dependence on the annealing temperature and the Eu(3+) concentration in the sample. The maximum red emission is observed with 12 mol% Eu(3+) annealed at 450 °C. This work provides a better understanding of the energy transfer mechanism from ZnO-nc to Eu(3+) ions and is important for applications in photonics, especially for light emitting devices.