Spectroscopic investigations of Dy3+ doped borogermanate glasses for laser and wLED applications
Introduction
White light-emitting diodes (wLEDs) have a promising future in general illumination and display backlights due to advantages such as eco-friendly nature, spectral tunability, color stability, high reliability, high capability in transforming electrical to optical power and long lifetime over conventional lighting sources [1,2]. There are two common approaches in the production of commercial wLEDs. First one is by coating an InGaN blue LED with a yellow-emitting phosphor. In spite of its extensive usage and high luminous efficacy, low color rendering index (CRI<80) and high correlated color temperature (CCT>6000 K) were reported as its drawbacks [3]. The other approach is encapsulation of two or three different types of phosphors combined with blue pumping LED. Although higher CRI values have been achieved, this method is limited by the difficult preparation and poor emitting-color stability [2]. In both cases matrix material is applied to fix the phosphor powder on the LED chip. This material may reduce the life cycle of LED under high temperature and long-term UV irradiation [4]. Therefore, besides epoxy resin free manufacturing process, rare earth (RE) ion doped glasses might be a good option to replace phosphor powders as they present improved properties such as uncomplicated production technique, relatively easy shaping, low fabrication cost, homogenous light emission, high thermal and mechanical stabilities [5].
Although borate glasses exhibit many desirable properties, including low melting point, thermal stability, and formation of various structural units they suffer from high phonon energy (∼1300 cm−1) resulting in non-radiative losses and limits their performance in photonic applications. The lower phonon energy of germanate glasses enhance the radiative transition probability and improve the luminescence quantum efficiency. Borogermanate glass, composed of these aforementioned glass systems, was selected as the host matrix for the present study regarding to its high density and RE doping capacity as well as lower melting point and phonon energy than silicate and borosilicate systems [6]. RE ion doped glasses represent a highly diversified application area in photonic devices involving lasers, wLEDs, display devices, optical waveguides, optical fibers, storage devices and temperature sensors [7,8]. Among various RE ions, trivalent dysprosium (4f9) is one of the most significant one in the generation of white light emission which is maintained by the suitable combination of its intense yellow and blue emissions. The luminescence spectrum of dysprosium ion contains two intense emission bands in the wavelength of 470–500 nm (blue, magnetic dipole, 4F9/2 → 6H15/2) and 570–600 nm (yellow, electric dipole, 4F9/2 → 6H13/2) and a feeble 640–690 nm emission band (red, 4F9/2 → 6H11/2). 4F9/2 → 6H13/2 transition is hypersensitive and it can be affected by surrounding environment causing alterations in yellow to blue (Y/B) ratio. Therefore, by appropriate values of Y/B, Dy3+ ions can generate white light emission. Also, Y/B ratio indicates the local symmetry around Dy3+ ions and the covalent nature of RE-ligand (Dy-O) bond. Dy3+ doped lead tellurofluoroborate [9], borotellurite [10], borophosphate [11], borosilicate [12,13] and bismuth borate [14] glasses were investigated for wLED and laser applications. However according to our knowledge this is the first study dealing with the white light emission in Dy3+ doped borogermanate glass system. Besides the presence of gadolinium oxide in the glass composition enhances the density of glass and provides effective energy transfer to the activator ions [15,16].
The aim of the present study is to prepare Dy2O3 doped B2O3-GeO2-Gd2O3 glasses and to reveal the optical, luminescence and radiative properties by systematic spectroscopic investigations including absorption, photoluminescence, Judd-Ofelt, decay time and CIE chromaticity analysis for potential applications in solid state lasers and wLEDs.
Section snippets
Experimental methods
Dy3+ doped gadolinium borogermanate glasses in 30B2O3-40GeO2-(30-x)Gd2O3-xDy2O3 (x = 0.25, 0.5, 1 denoted as BGGD0.25, BGGD0.50 and BGGD1.00, respectively) composition were prepared from high purity raw materials such as boric acid (H3BO3, Alfa Aesar), germanium dioxide (GeO2, Sigma Aldrich), gadolinium oxide (Gd2O3, Sigma Aldrich), dysprosium oxide (Dy2O3, Sigma Aldrich) by melt-quench method. About 15 g batches were thoroughly mixed and melted in an electrical furnace to a temperature of
Results and discussion
The XRD pattern of BGGD1.00 are presented in Fig. 1. The non-crystalline character of prepared glass was established by XRD analysis. The absence of specific crystalline peaks confirms the glassy state of BGGD1.00.
Conclusion
The absorption, luminescence and decay time measurements were used to investigate the optical and luminescence properties of BGGDx glasses. The negative value of bonding parameters suggests the ionic nature of bonding between Dy3+ ions and ligands. The optical band gaps are reduced by the elevated content of Dy3+ due to the structural changes in glass matrix. Higher degree of covalency for Dy-ligand bond and higher asymmetrical ligand environment for Dy3+ ions are confirmed by the calculated Ω2
Declaration of interests
None.
Acknowledgments
The authors would like to thank TÜBİTAK-The Scientific and Technological Research Council of Turkey [grant number 114M477] for financial support. Authors would also like to thank to ŞİŞECAM Science and Technology Center for absorption measurements.
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