Tunable multicolour emission from dysprosium-doped mixed rare-earths oxyorthosilicate nanophosphors for application in ultraviolet-pumped multicolour and white light emitting diodes
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Date
2015-02
Authors
Ogugua, Simon Nnalue
Journal Title
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Publisher
University of the Free State
Abstract
Phosphors have many uses today in applications such as electronic information displays, solid state lighting, solar cells, advertising and theft prevention. By using urea-assisted solution combustion method, we prepared tunable multicolour and white light emitting dysprosium (Dy3+) doped rare earth oxyorthosilicate (R2SiO5) (R = La, Y, Gd) powder phosphors. We prepared five sets of powder samples, namely LaYSiO5:Dy3+, LaGdSiO5:Dy3+, GdYSiO5:Dy3+, La2-xGdxSiO5:Dy3+ (x = 0, 0.5, 1.0, 1.5 and 2.0) and LaGdSiO5:Dy3+ x mol% (x = 0.05, 0.1, 0.25, 0.75, 1.0, 1.5, 2.0, 3.0 and 5.0). The structure and the stretching modes of vibration of the phosphors were analyzed using X-ray diffractometer (XRD) and Fourier transform infrared (FT-IR) spectrometer respectively while the morphologies and the elemental composition of the phosphors were analyzed using, respectively, field emission scanning electron spectroscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). In addition, X-ray photoelectron spectroscopy (XPS) was also used to analyze the elemental composition, chemical and electronic states of the phosphors while the distribution of atomic and molecular ionic species on the surface region of the samples was studied using the time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The elemental composition analysis indicated that there was a correlation among the EDS, XPS and TOF-SIMS data. The TOF-SIMS overlay images suggested that the dopant ions were evenly distributed and co-localized with major ionic species on the surface. The crystallite sizes calculated from the X-ray diffraction peaks using Williamson-Hall equation were in the range of 8.0 to 21.0 nm. The band gaps of the phosphors determined from the diffuse reflectance data using Tauc plot were found to vary from 5.0 to 4.45 eV. The photoluminescence spectra recorded when the samples were excited using the 325 nm He-Cd laser consisted of broad band and line emission peaks which we assigned respectively to self-trapped excitons (STE) in SiO2 and 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+. The peak intensities of the emission bands were shown to depend on the molar ratios of La to Gd, La to Y and Gd to Y on the mixed rare-earths oxyorthosilicate hosts. The colour purity of the bands estimated using CIE coordinates confirmed that our samples were emitting tunable multicolour and white light. These results suggest that our material can be used as single host phosphors in energy efficient UV-pumped multicolour and white light emitting diodes (LED). The structure, particle morphology, surface chemical composition and electronic states, photoluminescent properties and possible applications of these materials in UV-pumped LEDs were investigated.
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Keywords
Combustion synthesis, Phosphors, X-ray diffraction, Photoluminescence, Microscopy, Spectroscopy, Dissertation (M. Nanoscientiae (Physics)) - University of Free State, 2015