White emitting GeO2PbOBi2O3SrF2 glass and nano-glass ceramics co-doped with Eu3+, Tb3+ and Tm3+ ions
Graphical abstract
GeO2PbOBi2O3SrF2 glass triply co-doped with Eu3+, Tb3+ and Tm3+ (point E)
Introduction
Solid-state lighting technology in the form of phosphors is highly efficient, capable of converting energy much more effectively than the conventional lighting sources. In addition to energy saving, it is also more environmentally friendly and, therefore, has been considered as a future-generation lighting technology [1]. One of the common approaches to produce white phosphors nowadays is a method that blends three primary colours, viz. red, green and blue (RGB). This method requires three appropriate types of lanthanide(III) (Ln3+) ions as emitters [2], [3], [4], [5], [6], [7]. Among them Eu3+, Tb3+ and Tm3+ ions could be essential ingredients of the phosphors applied in general lighting. However, deficiency of red emission is the main reason for their poor color rendering index (CRI) and high correlated color temperature (CCT). Thus, as a result they produce a colder and bluer white light in comparison to conventional incandescent bulbs [8], [9]. In this paper a parent glass and its devitrified (nano-glass-ceramic) counterpart of the same composition triply co-doped with Eu3+, Tb3+, and Tm3+ ions are demonstrated as white phosphors.
In the case of Ln3+ ions optical absorption transitions of the type f-f are strongly forbidden by the parity selection rule. By ‘stealing’ intensity from allowed transitions (mixing of orbital functions with e.g. the 4f-5d transitions) the selection rule is somewhat relaxed and optical absorption transitions of the ions are present but they are relatively weak [10]. It means that very low values of the molar absorption coefficient is the characteristic feature for Ln3+. Thus, the reverse process, i.e. emission, is also forbidden and in consequence is of low intensity. This results in long-lived excited states, up to milliseconds. A way to circumvent the low absorption coefficient and subsequently low emission intensity is to apply any type of energy transfer effect. This effect is a point of interest in this study.
Materials called glass-ceramics are obtained by controlled devitrification (crystallization) of a glass. The appropriate glasses are subjected to carefully programmed thermal treatments which cause the nucleation and then growth of crystalline phases. The process is designed to convert the initial vitreous phase partly into a polycrystalline material. The glass-ceramics always contain a residual glassy phase and one or more embedded crystalline phases. The crystallinity varies between 0.5 and 99.5%, most frequently between 30 and 70% [11].
The glass-ceramics is generally opaque although translucent and even transparent nano-glass ceramics have been produce in certain cases (the crystallinity only being revealed by X-ray diffraction). The transparent nano-glass ceramics turn out not only the same advantages as glasses, but first of all exhibit improved optical properties [12], [13]. Particularly the nanophase glass-ceramics are characterized by higher thermal stability and desirable mechanical stability than their parent glasses, frequently possessing upper use temperature of higher than 800 °C [14].
Oxyfluoride glass-ceramics doped with Ln3+ ions arouse a great interest owing to their attractive optical properties and potential applications in luminescent materials [15], [16]. These materials are obtained by controlled crystallization of the vitreous precursor doped with Ln3+ [17], [18], [19], [20], [21], [22]. The fluoride nanostructured phases in the glass-ceramics offer crystalline environment of lanthanide sites with low phonon energy that prevents non-radiative multiphonon relaxation [23]. So the glass-ceramics combines physicochemical properties of oxide host with profitable optical properties of fluoride nanocrystals doped with Ln3+ ions [24], [25], [26], [27], [28], [29], [30], [31]. For instance, oxyfluoride transparent nano-glass-ceramics possess luminescence characteristics that is significantly different from those of the parent glasses. Namely, such system combines advantage of lanthanide crystals as narrow spectral lines and longer lifetimes of luminescent levels. The ease and low cost of fabrication is the additional advantage in the preparation of nano-glass-ceramic white phosphors. Nothing but nanocrystals doped with appropriate Ln3+ ions can be also applied as white emitting materials [32].
In general GeO2PbOBi2O3 glasses present a broad transmission window (400 nm–4.5 μm) and high refractive index (n = 2.0) which are consequences of the small field strengths and relatively large masses of the metal components in such oxide as PbO and Bi2O3 [33]. Moreover, presence of PbO and Bi2O3 in an oxide glass leads to decrease of their phonon energy [34].
The aim of the study is to prepare a white phosphor and to provide more information about the energy transfer effects that occur between co-doping Eu3+, Tb3+ and Tm3+ ions in vitreous and nano-glass ceramic matrices of the type GeO2PbOBi2O3SrF2. The paper presents also comparison of spectral properties of the parent glass and its nano-glass ceramic analog.
Section snippets
Sample preparation
Glasses with the nominal composition 79GeO219PbO1Bi2O31Ln2O3 were already presented by A.M. Kłonkowski et al. in ref. 33. In order to reduce more the phonon energy of the studied glasses in comparison to the GeO2PbOBi2O3 ones we used SrF2 in the glass compositions at the expense of GeO2 and PbO. Then, to stabilize the glass we added more Bi2O3 (3 mol. %) as a non-classical network former in the presence of a conventional glass former such as GeO2. Thus, in this study we proposed glass matrix
DSC/TG curves
Results of DSC and TG measurements are presented in Fig. 1. The DSC curve (a) shows two endothermic peaks and one small exothermic signal. The endothermic peak at 543 °C indicates the glass transition temperature (Tg) and the next endothermic signal at 1119 °C is related to melting point (Tm). Then the exothermic peak was assigned to crystallization point (Tc) at 758 °C. The TG curve (b) shows very small loss of mass (ca. 0.2%) during heating up to 1200 °C. Owing to the low evaporation of the
Conclusions
The Tb3+ → Eu3+ energy transfer was observed in the materials doubly co-doped with Eu3+ and Tb3+ and the triply co-doped with Eu3+, Tb3+ and Tm3+ ions. Only in the GPBOSF:3Eu12Tb5Tm glass was seen also the reverse transfer from Eu3+ to Tb3+. Spectral analysis of luminescence of the triply co-doped glass and its devitrified counterpart suggests also presence of the Tm3+ → Tb3+ energy transfer.
Intensity domination of the 5D0 → 7F2 band above 5D0 → 7F1 one in the Eu3+ emission spectrum of the
Acknowledgement
We would like to express our gratitude to Marcin Łapiński, PhD (Nanotechnology Centre A, Gdańsk University of Technology, Gdańsk, Poland) for his helpful technical assistance.
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