Color tunable green–yellow–orange–red Er3+/Eu3+-codoped PbGeO3:PbF2:CdF2 glass phosphor for application in white-LED technology
Introduction
Much interest has recently been paid to the development of alternative lighting sources to replace conventional incandescent and fluorescent illuminants, and exhibiting much lower energy consumption, highly reliable, and incorporating environmental-friendly production technology [1], [2]. LED-based lighting systems is regarded nowadays as the next generation solid-state illumination technology [3] since the realization of white-light emitting diodes [4], [5], [6], [7]. The LED-based sources incorporate valuable properties including low power consumption, high electrical energy to light conversion efficiency, long-life, low-cost, and easy maintenance. Moreover, they incorporate environmental advantages because their production do not require emission of greenhouse gases (CO2), and provokes no mercury pollution [8], [9]. There exist essentially two methods to produce white-light sources employing LEDs [1]. One takes advantage of LEDs producing the three fundamental (red, green, and blue), or a combination of complementary colors light. The other utilizes the combination of emission from either blue or ultraviolet LED excited down-converted light from a phosphor. In particular, white LEDs produced by means of a combination involving a blue or ultraviolet LED and a yellow emitting phosphor [3], is the most utilized approach nowadays, due to the low cost, easy fabrication process, and high brightness emission. However, this technique suffers from low color rendering index owing to the fact that they are based upon two colors mixing, and the generated white-light changes either with excitation power or temperature [10]. Another deficiency of the approach is the absence of red components in their emission spectra which prevent the generation of light in the red spectral region and the combination of yellow phosphor and blue-LED produces rather high correlated color temperatures (CCT>4500 K). In order to overcome these drawbacks, a white LED fabricated via UV-blue excited red, green, and blue emitting phosphors is required. In the last few years, there have been a large number of reports dedicated to new phosphors for application in white-LED technology based upon UV-blue excited down-converted fluorescence emission in rare-earth(RE3+) doped solid-state materials, including oxides, sulfides, oxysulfides, nitrides, and oxynitrides, amongst many [11], [12], [13], [14], [15].
In the past few years, several attempts have been made to mix RE3+ multi-doped phosphors to produce either multicolor or white-light emission [16], [17], [18], [19], [20], [21]. The possibility of color tunability incorporates versatility/functionality to the multicolor phosphor for the pursued application in the so called “smart light” technology. Emission color tunability in several phosphor materials induced via excitation power [22], [23], [24], temperature [25], [26], and adjustment of the active ions concentration combination has been demonstrate by many [16], [21], [27], [28]. In this work, we report on the generation of color tunable wide gamut light covering the greenish, yellow-green, yellow, orange, and reddish chromaticity region using europium/erbium co-doped phosphors excited by UV/blue light emitting diodes. Results indicate that the color-tunable fluorolead germanate erbium/europium codoped glass phosphor herein reported is a promising novel contender for application in LED-based illumination technology.
Section snippets
Experimental
The glass samples were prepared with reagent grade PbF2 and CdF2 (P.A. Aldrich) and glassy PbGeO3. Starting reagents were mixed in an agate mortar using n-heptane as homogenizing medium. After melting in an open Pt–Au crucible at 800 °C for 30 min in air, liquids were quenched at room temperature in graphite molds. Some 30 min annealing treatments at temperatures around the glass transition were performed. Rare-earth ions were introduced in the form of oxides (in several different concentrations).
Results and discussion
Fig. 3 presents typical emission spectra of radiation emanating from the Er3+/Eu3+-codoped phosphors excited at 385 nm, and with a fixed erbium concentration of 1.0 mol% (a), and varying europium content: 0.0→1.50 mol% (in 0.25 mol% increments), and fixed europium content of 0.5 mol% (b) and changing erbium concentration (b): 0.0→1.50 mol%(in 0.25 mol% increments). They were excited via a cw LED operated at 385 nm, with a fixed power of ∼2 mW. The Er3+/Eu3+co-doped samples exhibited four distinct
Conclusion
Color tunable wide gamut light covering the greenish, yellow-green, yellow, orange, and reddish color chromaticity region in europium/erbium co-doped PbGeO3:PbF2:CdF2 glass phosphor was demonstrated. The color tunability was actually obtained via proper combination of active ions concentrations. The phosphor samples were synthesized, and their light emission properties examined under UV/blue light-emitting-diode excitation. Luminescence emission around 525, 550, 590, 610, and 660 nm was obtained
Acknowledgments
The financial support for this research by CNPq(Conselho Nacional de Desenvolvimento Científico e Tecnológico) and FACEPE(Fundação de Amparo a Ciência e Tecnologia do Estado de Pernambuco) Grant APQ 0504-1.05/08, Instituto Nacional de Ciência e Tecnologia(Infos-Instituto Nacional de Fotônica, Opto-Eletrônica e Spintrônica) is gratefully acknowledged. Wellington S. Souza was supported by a studentship from CAPES(Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior), and Renata O.
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