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We report tunable luminescence in oxygen-deficient CaO-Al2O3-GeO2 glasses. The glass samples were prepared by adding metal Al instead of corresponding oxide (Al2O3). Efficient blue and red emissions were observed when excited by 300 and 370 nm ultraviolet light, respectively. By adjusting the content of metal Al, we could control the quantities of the defects which results in tunable luminescence from blue to white to red. Furthermore the resultant oxygen-deficient glasses have shown bright white luminescence when the excitation wavelength tuned to 335nm. Our method opens a new route for the white light illumination.
©2007 Optical Society of America
1. Introduction
Defects bring to mind imperfection and blemish. Usually, micro-defects such as point defects and dislocations could affect the properties of materials. Whereas, for some materials defects play an important role for the realization of various functions [1]. Extensive investigations have shown that control of various defects could improve performance and applications over the “ideal” material: (i) Light induced Si- and Ge-related defects in GeO2/SiO2 glass play a key role in fiber grating, which has been widely used in Dense Wavelength Division Multiplexing (DWDM) optical communication system [2,3]; (ii) Lasing has been achieved by using F-centers [4]; (iii) External electromagnetic fields induced defects-related photostimulated luminescence and long-lasting phosphorescence glasses have potential applications for X-ray sensors, optical memory and optical displays [5–9]. These defects are induced by external fields or controlling atmosphere. Apparently, various novel functions can be found if the species, quantities and spatial distribution of defects can be controlled in an active manner.
CaO-Al2O3-GeO2 glasses show good mechanical strength and chemical durability [10,11]. They are transparent from visible to infrared wavelength region, and have applications as infrared window and sensor. However, there have been few investigations on the defect formation in these glasses. Here, we report a simple method to control oxygen-deficient defects in CaO-Al2O3-GeO2 glasses by adding metal Al instead of corresponding oxide (Al2O3). We observed tunable luminescence from blue to white to red from the glass samples when excited by ultraviolet (UV) light. The glasses may be useful as colorless and transparent fluorescent materials, such as for lamps, displays, and white LED lighting [12,13].
2. Experiment
The glass composition was 40CaO-30Al2O3-30GeO2 (mol%). In order to prepare oxygen-deficient glass samples, a part of Al2O3 was substituted by Al. The glass samples without Al, using Al instead of 1, 2, 3, 5, 10mol% Al2O3 are referred to samples A, B, C, D, E and F, respectively. The raw materials were reagent-grade CaCO3, Al2O3, GeO2 and Al. Approximately 30 g batches were mixed and then melted in alumina crucibles with alumina caps at 1550°C for 1h under the ambient atmosphere. The melts were poured onto a stainless plate and pressed with another stainless plate to obtain glass plates. All of the obtained glasses were transparent. The glass samples were cut, polished and subjected to optical measurement. The absorption spectra of the samples were measured by a spectrophotometer (Hitachi-4100). The photoluminescence and excitation spectra were measured with a fluorescence spectrometer (Hitachi-4500), and the luminescence photographs of glass samples were excited by a Xe lamp in the spectrophotometer. Electron spin resonance (ESR) spectra were obtained using a JEOL JES-FA200 ESR spectrometer (300 K, 9.063 GHz, X-band). All of the measurements were carried out at room temperature.
3. Results and discussion
Fig. 1. Absorption spectra of CaO-Al2O3-SiO2 glasses. a, b, c, d, e, and f are absorption spectra of glass samples A, B, C, D, E, and F, respectively.
Figure 1 shows the absorption spectra of CaO-Al2O3-SiO2 glass samples. When Al was added instead of part Al2O3 in the glass, it was observed that the absorption edge shifts to the shorter wavelength at first and then to longer wavelength with increasing Al content. When using Al instead of part Al2O3 as glass composition, oxygen content decreases in the network, and Al enters the glass network through combing with NBO for Al have strong reducibility, the process can be proposed as follows:
NBO+Al→Al-BO
It has been reported that [14], the shift of the absorption edge to shorter wavelength is related to the formation of bridging oxygen (BO), which binds electrons more tightly than NBO. With the addition of Al, a part of NBO converted into BO, leading to the absorption edge shift to shorter wavelength. However when a large number of Al added, lots of NBOs may be produced as follows, resulting in the red-shift of the absorption edge.
GeO 4+Al→Ge 2++O--Al-O -(NBO)
ESR spectra of the glass samples are shown in Fig. 2. No ESR signal was observed in sample A. When Al added, an apparent ESR signal appeared at g=1.994. This signal is due to the germanium oxygen deficient centers (GODCs) [3,15].
Fig. 3. Photographs of the emission states of glass samples when excited by UV lights, a, b, c, d, e, and f are glass samples A, B, C, D, E and F, respectively. The excitation wavelengths for blue, white, and red emissions are 300, 335 and 370nm, respectively.
Figure 3 shows photographs of the emission states of glass samples excited by UV lights. No emission was observed in sample A. For the Al added samples we observed bright blue, white, and red emissions when excited by 300, 335 and 370nm, respectively.
Fig. 4. Photoluminescence (right) and excitation (left) spectra of glass samples, a, b, c, d, e and f are referred to glass samples A, B, C, D, E, and F, respectively. The excitation wavelengths of A, B, and C are 300, 335 and 370nm, respectively.
The photoluminescence and excitation spectra of glass samples are shown in Fig. 4. No PL bands were observed in sample A. In the Al added samples, blue light emission with two broad bands centered at 375 and 460nm; white emission with two broad bands centered at 400 and 510nm; and red light emission with two broad bands centered at 460 and 630 nm were observed when excited by 300, 335 and 370 nm UV lights. We choose the sample F to study the complex behavior based on the time-resolved PL spectra (Fig. 5). When excited by 300nm UV light, the sample only shows a broad band at 460nm, the band centered at 375nm disappeared for the lifetime of the 375nm band is 11µs and the shortest delay of time-resolved PL experiment was 10µs. When excited by 335nm UV light, it shows four PL bands at 405, 510, 630 and 680 nm. And it shows three PL bands at 460, 630 and 680 nm when excited by 370nm UV light. Figure 6 summarizes the proposed mechanism for the observed luminescence.
Fig. 5. Time-resolved PL spectra of sample F, excitation wavelengths are 300, 335 and 370 nm, respectively.
It has been reported that the Ge2+ center has two PL bands at 300 and 395 nm with the corresponding excitation bands at 250 and 330nm [2,3]. Therefore, we suggest that the PL bands at 370 nm and 460 nm excited by 300nm UV light may be due to the Ge2+ centers. The excitation band at 370nm with the PL bands located at 630 and 680 nm is similar to those reported in H2-loaded vapor axial deposition (VAD) germanosilicate glasses [16–18]. It was suggested that two closely related varieties of the germyl radical (GR) are responsible for the red PLs. We proposed that H2O molecules in the raw materials or in air might react with some Al during the melting process and then the produced H2 would react with Ge2+, resulting in formation of GR [15,17].
Fig. 6. Schematic of energy level diagram of Ge-related oxygen defects and transitions involved in the photoluminescence experiment.
Nowadays, there is much interest in cheap, easy generation of white light sources for lighting and display technology, our method opens a new route for the white light illumination [12,13]. The calculated color coordinates of the white light PL spectra are (0.306, 0.258), (0.308, 0.282), (0.322, 0.305), (0.359, 0.312), and (0.336, 0.339), these fall well within the white region of the 1931 CIE diagram, and is close to the perfect white color coordinates (0.333, 0.333) [19]. It is important to measure the efficiency of the energy conversion from the viewpoint of practical application, further investigation is being carried out.
Figure 7 presents the luminescence spectra of the sample F with the excitation wavelength turning from 300 nm to 395 nm, which indicates that the luminescence could be tunable. The calculated color coordinates of the PL spectra vary from (0.198, 0.204) to (0.335, 0.340) and to (0.516, 0.330). These would fall well within the blue, white and red region respectively as per the 1931 CIE diagram. This phenomenon has shown that the luminescence of the oxygen-deficient glasses is tunable from blue to white to red.
Fig. 7. (a). Photoluminescence spectra of sample F excited by different wavelengths of UV light, which indicates that the luminescence is tunable. (b) CIE chromaticity diagram with coordinates (black circles) shows that the emissions could tunable from blue to white and to red.
4. Conclusion
We have presented a simple but versatile method to control the Ge-related oxygen-deficient defects in calcium aluminium germanate glasses, which produced bright blue, red, and white light luminescence when excited by UV lights. By adjusting the content of Al, we could control the quantities of the defects (Ge2+ and GR) which results in tunable light emission from blue to white and to red.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (Grant No.50672087 and No. 60778039), National Basic Research Program of China (2006CB806000) and National High Technology Program of China (2006AA03Z304). This work is also supported by Program for Changjiang Scholars and Innovative Research Team in University.
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