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Abstract
Ag-Eu3+ co-doped fluoroborate glass phosphors doped with various Eu3+-concentrations were prepared by a melt-quenching technique. The luminescent properties of these glass phosphors were characterized by excitation and emission spectra. Broad excitation and emission bands located, respectively, at 300-450 nm and 390-700 nm originating from silver aggregates were observed. Strong red emissions were detected under 404 nm violet light-emitting diode (LED) excitation for those Ag-Eu3+ co-doped samples. It was found that these red emissions of Eu3+ well compensated the deficiency of the red spectral components in glasses containing Ag aggregates. In addition, it was confirmed that stable white light could be achieved from the combination of a specific Ag-Eu3+ co-doped fluoroborate glass phosphor and LEDs with different output wavelengths. By adjusting the luminescence intensity ratio of the glass phosphor to the 404 nm violet LED, tunable emitting color was realized, and the studied glass phosphors showed excellent emitting color stability toward LED drive currents. Our results demonstrated that this kind of easy fabrication, low-cost, and highly stable Ag-Eu3+ co-doped fluoroborate glass phosphors had potential application in white LED.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
White light-emitting diodes (WLEDs), the next generation of solid-state lighting source, have received increasing attention due to their advantages of high brightness, low power consumption and environmental benefits [1–4]. The mainstream of commercial WLEDs is combining GaN-based blue LED chip with proper powder phosphors which can be excited by blue LED. Generally, the WLEDs must be encapsulated by epoxy resins so as to protect the powder phosphors and the LED chip. However, the epoxy resins are easily deteriorated and turn yellow after long term service, thus resulting in a reduction of WLEDs longevity [5]. In addition, the luminescence behavior of the powder phosphors and the LED chips are differently dependent on their serving period, which leads to degradation in the color rendering index and luminous efficiency. To overcome these issues, it is vital to develop novel phosphors to replace the conventional powder phosphors for manufacturing high stability WLEDs.
Rare earth (RE) ions doped glass phosphors with highly thermal stability, good mechanical property, excellent luminescence performance, low cost and ease of mass production have attracted considerable attention in applications for WLEDs [2, 6]. Using glass phosphors instead of the traditional powder phosphors can effectively avoid the influence of the morphology and particle size of the phosphor particles on the performance of the WLED devices, and then obtain homogeneous light emitting. Moreover, the assembling process is extremely simplified due to the free of epoxy resin. Recently, oxide glasses have been extensively employed as host matrixes for RE ions due to their high transparency in ultraviolet (UV) and visible band, high physical and chemical stability, and high solubility of RE ions [3, 5, 7]. However, the absorption transitions of RE ions mostly belongs to f-f transitions in near UV and visible band, many RE ions mono-doped materials cannot meet the needs of practical applications because of the low excitation efficiency, although their emitting energy levels may have high quantum efficiency, suitable emission wavelengths and line widths. In order to obtain near UV and short wavelength visible light-emitting, many useful explorations have been carried out on RE ions doped luminescent materials, such as host sensitization and dopant sensitization [8–12]. In addition, many investigations have also concentrated on the co-doping noble metal ions to improve the luminescence properties of RE ions, for example, Ag/Eu3+ [13, 14], Ag/Sm3+ [15], Cu/Eu3+ [2], Cu/Sm3+ [16, 17] co-doped oxide glass phosphors. Rongfei Wei et al. reported the luminescence enhancement of Eu3+ by Ag species and pointed out that local-field surface plasmon resonance effect of Ag nanoparticles contributed to the enhancement of Eu3+ excited by 464 nm, but very small molecule-like, nonplasmonic Ag particles and isolated Ag+ respectively caused the luminescence enhancement of Eu3+ under 350 nm and 270 nm excitations [14]. Previously, we have also found a fluorescence enhancement of Er3+ in germanate glasses containing Ag particles and the enhancement was ascribed to the surface plasmon oscillations of Ag particles [18]. However, the actual interaction between Ag species and RE ions is unclear. Thus, further efforts should be devoted to the interaction between Ag species and RE ions in various luminescence materials. In addition, much attention should be given to the stability of the luminescent performance of Ag species and RE ions co-doped materials.
Thereby, in this study, Ag-Eu3+ co-doped fluoroborate glass phosphors with different amounts of Ag and Eu3+ were produced by a melt-quenching technique. The luminescent properties of these glass phosphors were systematically studied. The results showed that stable white light emissions were achieved from Ag-Eu3+ co-doped fluoroborate glass phosphors under excitation of various wavelengths. Moreover, tunable white light emissions were also obtained by combining 404 nm violet LED and Ag-Eu3+ co-doped fluoroborate glass phosphors via adjusting the luminescence intensity ratio of the emission from violet LED to the Ag-Eu3+ co-doped fluoroborate glass phosphors.
2. Experimental
2.1 Preparation of Ag-Eu3+ co-doped fluoroborate glass phosphors
Glass phosphors with composition of 70B2O3-10CaF2-10AgNO3- (10-x)Y2O3- xEu2O3 (in mol%, x = 0, 0.5, 1, 3, 5, 7) were prepared by a melt-quenching technique. The samples were named as GA for Ag mono-doped one and GAEi (i = 0.5, 1, 3, 5, 7) for Ag-Eu3+ co-doped ones. All the starting materials used in this study were analytical grade reagents. In a typical preparation process, a certain amount of starting materials were firstly weighed according to the designed stoichiometric ratio, and then well mixed in an agate mortar. Next, the mixtures were transferred into an alumina crucible and melted at 1250 °C for 5 min. Then, the melting liquids were quenched in a pre-heated stainless steel mold to form glasses. Finally, all samples were cut into about 25 mm2 and 2 mm thick, and polished for optical measurements.
2.2 Characterization
The excitation and emission spectra were recorded on Hitachi F-4600 fluorescence spectrophotometer using a 150 W Xenon lamp as the excitation source. The emission spectra were well intensity-corrected. The external quantum efficiency of the glass phosphors were measured by Edinburgh spectrofluorometer FS5 equipped with a SC-30 integrating sphere module. The temperatures of the studied samples were controlled by a self-manufactured temperature control system, DMU TC-450, with a controlling accuracy of about 0.5 °C. A 404 nm violet LED was used in spectral measurements as offered by Dalian University of Technology. The 404 nm violet LED can be worked by the support of current-adjustable direct-current electric source.
3. Result and discussion
Figure 1 shows the excitation spectra of Ag-Eu3+ co-doped fluoroborate glass phosphors with monitored wavelength of 615 nm corresponding to the 5D0→7F2 transition of Eu3+. It can be seen that the excitation spectra of all the Ag-Eu3+ co-doped samples are composed of a broadband located at 200-450 nm and several sharp peaks in the wavelength region of 300-500 nm. The broadband can be divided into two parts: the one centered at 275 nm originates from the well-known O2-Eu3+ charge transfer band [19]; the other one centered at 360 nm may come from glass matrix, which can be confirmed by the excitation spectrum of the sample without Eu3+ (GA). For GA sample, its excitation spectrum only includes one broadband and just centers at 360 nm, as can be seen in Fig. 1. It was reported that the excitation band of the isolated Ag+ located at 270 nm, which originates from 4d10 (1S0) to 4d95s1 (3D1) transition [20]. However, there is no excitation peak around this wavelength for GA sample, which shows the absence of isolated Ag+ or its amount is too little to detect. According to Ref. 15, an excitation band around 351 nm was observed in Ag-Eu co-doped silicate glasses, which was attributed to very small Ag particles, such as Ag2, Ag3 and Ag4. Therefore, the band centered at 360 nm can be ascribed to such Ag aggregates. With an increase in AgNO3 content, much more Ag+ may aggregate together in a high temperature environment and form new luminescence centers. The higher AgNO3 content, the higher aggregated degree of Ag+.
Fig. 1 Excitation spectra of Ag-Eu3+ co-doped fluoroborate glass phosphors measured by monitoring 615 nm emission.
In addition, the existence of the excitation peak of Ag aggregates also indicates that there is an efficient energy transfer from such Ag aggregates to Eu3+. It is worth noting that the wavelength of this excitation band matches well with the output wavelengths of the common violet LED chips, thus it can be concluded that Ag-Eu3+ co-doped fluoroborate glass phosphors may have potential application in generating white light emission by combining them with violet LED.
Those sharp peaks can be ascribed to the f-f transitions of Eu3+ from ground state 7F0 to different excited state levels 5HJ (J = 3, 4, 5, 7) (320 nm), 5D4 (363 nm), 5GJ, 5L7 (J = 2-5) (378 nm), 5L6 (393 nm), 5D3 (412 nm), 5D2 (464 nm) and 5D1 (520 nm). As can be seen in Fig. 1, the one located at 393 nm is dominating in the whole spectra, indicating that this type of glass phosphors can be effectively excited by UV light. It can also be found that with an increase in the amount of Eu2O3, the peak intensity increases first and reaches its maximum value, and then decreases, thus indicating concentration quenching occurs.
Figure 2 shows the emission spectra of Ag-Eu3+ co-doped fluoroborate glass phosphors under 370 and 393 nm excitations. It can be seen that both sets of the emission spectra are similar under the excitation of the two different wavelengths. The emission spectrum of the sample GA without Eu3+ includes only one broadband, which may come from the emission of different states of Ag aggregates [13, 15]. Moreover, it can be found that this broadband almost covers the whole visible light region ranging from 380 nm to 700 nm, which is beneficial for the application in WLEDs. However, it is a pity that the emission spectrum lacks of some long wavelength visible light emission, which may affect the chromatic performance of the solid-state lighting source. Therefore, we choose Eu3+, as red luminescence center, co-doped with Ag into the fluoroborate glass phosphors to compensate the deficiency of the red spectral components in the long wavelength visible region.
Fig. 2 Emission spectra of Ag-Eu3+ co-doped fluoroborate glass phosphors under 370 nm and 393 nm excitations.
From Fig. 2, several sharp emission peaks in the red region can be found for those Ag-Eu3+ co-doped samples other than the broad emission band. These sharp peaks are characteristic emissions from the f-f transitions of Eu3+. The existence of these peaks can effectively compensate the spectral components of Ag mono-doped sample in the long wavelength visible region in a certain extent, which is good for obtaining high quality white light source. As can be seen from Fig. 2, the central emission wavelength of Ag aggregates shifts toward the long wavelength side with an increase in Eu2O3 content, which further indicates the aggregated degree of Ag+ increases. In addition, it can also be seen that with an increase in the amount of Eu2O3, the red emission intensity increases first and reaches its maximum value at 3 mol% Eu2O3. And then the intensity decreases with further increase of Eu2O3 content, indicating concentration quenching happens. It should be noted that there is a hollow (at 464 nm) in the broad emission band, which corresponds to the 7F0→5D2 transition of Eu3+, indicating there is a weak re-absorption of Ag aggregates’ emissions by Eu3+.
In order to further analyze the origin of the broadband emission, the excitation spectra of all the samples were measured with different monitored wavelengths (443, 464 and 500 nm) and are shown in Fig. 3. It can be found that all the excitation band with different monitored wavelengths are broadband and mainly locate at blue-violet light region, indicating these glass phosphors can be effectively excited by a broadband light. As can be seen, the broadband comprises of several absorption peaks, corresponding to different luminescence centers. With an increase in the amount of Eu2O3, all of the excitation band have the same changing trend. The intensity of the broadband located at high energy level decreases continually, but the one located at low energy level increases first and then decreases. And both of the two broad bands shift to long wavelength side gradually, which may be caused by the increasing aggregation of Ag after the introduction of Eu2O3. With an increase in the amount of Eu2O3, the aggregation degree of Ag is also increased.
Fig. 3 Excitation spectra of Ag-Eu3+ co-doped fluoroborate glass phosphors monitored at 443, 464 and 500 nm.
Color coordinates are one of the important factors to evaluate the phosphors’ performance. According to the above intensity-corrected emission spectra, the color coordinates of all the samples under 370, 376 and 393 nm excitations were calculated and they are depicted in Commission International de I’Eclairage (CIE) chromaticity diagram as seen in Fig. 4. As can be seen, the color shifts from white to red with an increase in Eu2O3 content. That is to say, the luminous color of the glass phosphors can be tuned by adjusting the glass component and Eu3+ doping concentration. Notably, the color coordinates of GAE1 (x = 0.3175, y = 0.3053) excited at 370 nm is close to the standard equal energy white light illuminate (x = 0.333, y = 0.333), which is denoted as asterisk in Fig. 4. This result suggests that Ag-Eu3+ co-doped fluoroborate glasses have potential application as white light-emitting phosphors for violet light excited WLEDs. Furthermore, it can be observed that each sample exhibited similar color rendering properties though the excitation wavelengths are different, which is beneficial to the application in solid-state lighting.
Fig. 4 CIE color coordinates of Ag-Eu3+ co-doped fluoroborate glass phosphors under 370, 376 and 393 nm excitations.
From the excitation spectra, it can be known that these Ag-Eu3+ co-doped fluoroborate glass phosphors can be effectively excited by various wavelengths in a broad range. Therefore, they can be used to realize white light emission by combining them with near UV LED chip with output wavelengths ranging from 270 to 420 nm. In order to further investigate the practicability of this kind of glass phosphors, we combine our samples with a violet LED (emission wavelength is about 404 nm) and make a white light-emitting prototype device, which was fabricated by simply capping the emitting surface of the violet LED using the studied glass slice. In this prototype device, the light emitted from the LED can excite the glass phosphor to generate Stokes emission, and then both these emissions from LED and glass phosphor are combined to form composite white light. Figure 5 shows the emission spectra obtained from the prototype devices with different glass phosphors when the injecting current is 60 mA and the corresponding excitation power density is about 8.20 mW/cm2. It should be noted that all the spectra have been normalized according to the intensity of the 404 nm emission of the violet LED. As can be seen, all the samples can be effectively excited by 404 nm, and the spectra exhibit a broad emission band covering the whole visible light region. With an increase in Eu2O3 content, the emission intensity increases first and then decreases, which just as the former results when excited by Xenon lamp. However, the luminescence intensity of those glass phosphors is weaker than the violet LED owing to the fact that the glass slices are too thin.
Fig. 5 Normalized emission spectra of Ag-Eu3+ co-doped fluoroborate glasses combined with 404 nm violet LED when the injecting current is 60 mA.
To further evaluate the luminescence performance of our fabricated prototype device, the emission spectra for the device with GAE1 glass phosphor (the corresponding external quantum efficiency was measured to be about 44% under 404 nm excitation) were measured when the drive current changes from 5 mA to 100 mA. The normalized spectra according to the intensity of the 404 nm emission of the violet LED are shown in Fig. 6. It can be seen that all the normalized spectra almost coincide, thus indicating that the chromatic property of the device is independent from the injecting current of the violet LED. Thus, it can be concluded that the device has a stable luminescence performance.
Fig. 6 Normalized luminescence spectra of GAE1 sample combined with 404 nm violet LED with different injecting currents.
In order to assess the feasibility of achieving white light emission using those as-prepared Ag-Eu3+ co-doped fluoroborate glass phosphors with violet LED, the emitting colors of the combined devices were investigated by adjusting the luminescence intensity ratio (IG/IL) of the glass phosphors to the violet LED. The corresponding color coordinates were calculated according to the intensity-corrected emission spectra and are shown in Fig. 7. In Fig. 7, the red and yellow arrows, respectively, show the increasing direction of Eu2O3 content and the intensity ratio IG/IL. Here we set the intensity ratio IG/IL to be 0, 0.05, 0.75, 0.10, 0.20, 0.30, 0.50, 1.00, 2.00, 5.00, 10.00, 50.00, 100.00 and 500.00. From Fig. 7, it is easy to find that different emitting colors from the devices can be obtained by adjusting the value of IG/IL. With an increase in IG/IL, the emitting color shifts from blue to white light region when Eu2O3 content is between 1 mol% and 5 mol%. For those samples doped with low Eu2O3 content (lower than 1 mol%), the emitting color can be tuned in blue light region. Once Eu2O3 content is higher than 5 mol%, the emitting color shifts toward blue, pink and yellow region. From Fig. 7, it can also be found that with an increase in the intensity ratio IG/IL, the emitting color of the combined device is much closer to the white light region, particularly for GAEi (i = 1, 3, and 5) samples. This fact tells us that in practical application the luminescence intensity of the glass phosphors can be controlled by adjusting the thickness of the glass phosphors, and then different colors of light can be achieved.
Fig. 7 CIE color coordinates of Ag-Eu3+ co-doped fluoroborate glass phosphors combined with 404 nm violet LED with various luminescence intensity ratio IG/IL .
Thermal stability of luminescence is one of the important parameters to assess the performance of phosphors. Usually, the luminescence intensity of the phosphor decreases with an increase in temperature, which has bad effect on the luminescence performance of lighting devices. Therefore, the thermal stability of emissions for the studied GAE3 glass phosphor (the corresponding external quantum efficiency was measured to be about 29.4% under 404 nm excitation) was tested by measuring the emission spectra at various temperatures when the sample excited by 404 nm violet LED with injecting current of 20 mA and the corresponding excitation power density is about 3.45 mW/cm2. In doing so, the GAE3 sample was polished into very thin glass slice and put inside the heater to make the spectral measurements. Both the heater and the violet LED which was adopted as the excitation source were conducted into the sample chamber of the spectrometer. The temperature-dependent spectra are shown in Fig. 8. As can be seen from Fig. 8, with increasing temperature, the luminescence intensity decreases monotonically and the central wavelength of the broadband emission came from Ag aggregates slightly shifts toward longer wavelength.
Fig. 8 Temperature-dependent spectra of GAE3 glass phosphor excited at 404 nm violet LED with injecting current of 20 mA.
To judge the spectral stability, the integrated intensities of those emissions from Ag aggregates and Eu3+ were calculated, respectively, and the temperature-dependent luminescence intensities are shown in Fig. 9. It can be seen that the emissions from Ag aggregate and Eu3+ have an almost identical decay rate in low temperature region (not higher than 100 °C), indicating the emitting color of the glasses almost remain unchanged at temperatures lower than 100 °C. However, when sample temperature is higher than 100 °C, the decay rate of the emissions from Eu3+ is a little faster than that from Ag aggregates, which is probably caused by the different temperature quenching mechanisms for the emissions from Ag aggregates and Eu3+. In general, multi-phonon relaxation, energy transfer and crossover process are three main mechanisms for thermal quenching of luminescence intensity [21]. Considering the large energy separation between the emitting energy level 5D0 and the nearest lower energy level 7F6 and the specific energy level structure of Eu3+, the influence of sample temperature on multi-phonon relaxation and energy transfer rate can be neglected. Crossover process was usually attributed to the temperature dependent luminescence quenching of Eu3+ at high temperatures [21, 22]. But for the Ag-related emissions, both of the multi-phonon relaxation and energy transfer can induce the thermal quenching of luminescence intensity. Figure 10 shows the temperature-dependent luminescence intensities of several different emission wavelengths (470, 500, 592, 617 and 700 nm) of GAE3 glass phosphor which were extracted from the same measured spectra used in Fig. 9. It can be found that the intensity of 617 nm falls down with larger rate versus temperature than those of other wavelengths. The inset shows the same dependences, but each intensity was normalized to its initial luminescence intensity at room temperature. It can be seen that all the emissions from Eu3+ display the same trend owing to the fact that these emissions originate from the same emitting level 5D0. However, those emissions from Ag-related centers have a different decay rate for different emission wavelength, indicating the luminescence of various Ag-related luminescent centers have a different thermal stability.
Fig. 9 Temperature-dependent integrated intensity of emissions from Ag aggregates and Eu3+ of GAE3 glass phosphor excited by 404 nm violet LED with injecting current of 20 mA. The inset shows the normalized result according to the initial luminescence intensity at room temperature.
Fig. 10 Temperature-dependent luminescence intensity of several different emission wavelengths of GAE3 glass phosphor excited by 404 nm violet LED with injecting current of 20 mA. The inset shows the normalized result according to the initial luminescence intensity at room temperature.
4. Conclusion
Ag-Eu3+ co-doped fluoroborate glass phosphors were successfully prepared by melt-quenching technique. Excitation and emission spectra indicated that the broad excitation and emission band were originated from Ag aggregates. Introducing proper amount of Eu3+ into Ag doped glass phosphors can compensate the deficient red spectral components of Ag aggregates in the long wavelength visible light region. By combining those glass phosphors with violet LED, tunable white luminescence emission was achieved via adjusting their luminescence intensity ratio of the glass phosphors to the violet LED. The Ag-Eu3+ co-doped glass phosphors exhibited the emission spectra were independent from the excitation power of the excitation source. Moreover, slightly different temperature dependences of Ag aggregates and Eu3+ emissions were found. The present work suggested Ag-Eu3+ co-doped fluoroborate glass phosphors had significant potential in luminescence materials for different applications such as WLEDs and solid-state display.
Funding
National Natural Science Foundation of China (NSFC) (11104023, 11374044); Natural Science Foundation of Liaoning Province (2015020190); High-level Personnel in Dalian Innovation Support Program (2016RQ037); Fundamental Research Funds for the Central Universities (3132017056, 3132016333).
Acknowledgments
The authors would like to thank the following people for their effort and support: Lili Tong, Han Xiao, and Ming Ye.
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