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Bi3+ films on glass substrates and potential applications in white light emitting diodes
Energy and Buildings 113 (2016) 9–14
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Deposition of NaGd(WO4 )2 :Eu3+ /Bi3+ films on glass substrates and potential applications in white light emitting diodes Zhijian Liu a , Wenlu Xu b , Anni Lin c , Tao He d,∗ , Fan Lin e a
Department of Power Engineering, School of Energy, Power & Mechanical Engineering, North China Electric Power University, Baoding 071003, PR China Department of Electronic & Communication Engineering, School of Electrical and Electronic Engineering, North China Electric Power University, Baoding 071003, PR China c Department of Electrical Engineering, School of Electrical and Electronic Engineering, North China Electric Power University, Baoding 071003, PR China d Oujiang College of Wenzhou University, Wenzhou 325035, PR China e Software School, Xiamen University, Xiamen 361005, PR China b
a r t i c l e
i n f o
Article history: Received 15 November 2015 Received in revised form 21 December 2015 Accepted 21 December 2015 Available online 23 December 2015 Keywords: NaGd(WO4 )2 :Eu3+ /Bi3+ Films Spin-coating Luminescence
a b s t r a c t NaGd(WO4 )2 :Eu3+ /Bi3+ films were deposited on SiO2 glass substrates by spin-coating. The as-prepared films were heat treated at different temperatures. The properties of films were characterized by TG-DSC, XRD, SEM and fluorescence spectrophotometer. The TG-DSC curve indicated that films began to crystallize at about 600 ◦ C. The XRD patterns and SEM images of films annealed at 600 ◦ C, 700 ◦ C and 800 ◦ C showed that a higher temperature benefits to the higher crystallinity and induces the lower thickness. Meanwhile, it was found that the revolution of spin coating influenced the morphology of films obviously. Under the excitation at 396 nm, NaGd(WO4 )2 :Eu3+ /Bi3+ films showed characteristic emission bands originating from 5 D0 → 7 Fj (j = 1, 2, 3, and 4) transitions of Eu3+ ions. It was also found that the excitation wavelength has great effect on the emission intensity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the increasing energy crisis and ecocrisis, more and more researches have focused on low carbon economy. Low carbon economy has been accepted to be one of effective economic development modes for human responding to energy and environmental risks. Several of approaches have been done to accomplish low carbon economy in fields of industry and daily life, such as the appearance of more and more zero energy buildings [1], the use of solar installations in buildings [2], the use of LED in daily [3], the use of energy-saving building materials [4], and the heat recovery in industry [5]. In daily life, white light generation is needed in room lighting, displays, monitors, and other optical devices. And white light emitting diodes (WLEDs) are the most promising white light sources [3]. WLED-based lighting has advantages of lower energy consumption, longer lifetime, higher luminous efficiency and brightness, wider operating temperature range, and better mechanical impact resistance, comparing with traditional incandescent or fluorescent lamps [6,7]. In other words, WLED can be one of attempts to accomplish low carbon economy in daily life.
∗ Corresponding author. E-mail address: [email protected] (T. He). http://dx.doi.org/10.1016/j.enbuild.2015.12.035 0378-7788/© 2015 Elsevier B.V. All rights reserved.
It has been reported that rare earth ions doped garnet luminescence-film grown on single crystal substrates can withstand much higher power densities than powder phosphors without tube degradation [8]. The film-phosphor has uniform thickness, smoother surface and small grain size, which makes it possible to define a smaller pixel spot size to achieve a higher resolution [9]. Moreover, it demonstrates a large separation between the phosphor and the primary LED emitter (referring to as remote phosphor arrangement), which enhances a phosphorescence extraction by reducing optical power absorbed by an LED chip [10]. Generally, the remote phosphor can be obtained by fabricating a phosphor layer on a regular soda lime silicate (SLS) or SiO2 glass substrates [11,12]. The combination of green and red emitting phosphors with a blue chip benefits to the fabrication of WLEDs with high color rendering and high efficiency [13]. In the past decades, Eu3+ ion is used widely as an ideal red comment because of its excellent color purity [14]. When Eu3+ ion presents in a non-centrosymmetric site, phosphors have dominant red emission originating from the 5 D0 → 7 F2 transition. And the red emission intensity can be improved by codoping Bi3+ ions because of an energy transfer between Eu3+ and Bi3+ ions [15–17]. Alkali rare-earth double tungstate compounds have been studied widely for the application of phosphor host [18–23]. NaGd(WO4 )2 , as one of members of double tungstates
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family, belongs to the tetragonal system with space group of C6 4h (I41 /a). As a good host for rare earth ions, NaGd(WO4 )2 shows excellent thermal, hydrolytic, and chemical stability [24]. Herein, we report on the fabrication of NaGd(WO4 )2 :Eu3+ /Bi3+ thick films on SiO2 glass substrates by spin-coating technique. This method has advantages of simpleness and the possibility of large-area coatings with high uniformity on various substrates [11]. The phase, microstructure and luminescent properties of the obtained phosphor-films have been characterized.
2. Experiments All of raw materials were purchased from Aladdin and used directly without further purification. Gadolinium oxide (Gd2 O3 , 99.99%), europium oxide (Eu2 O3 , 99.99%), bismuth oxide (Bi2 O3 , 99.99%), and sodium tungstate (Na2 WO4 ·2H2 O, 99.99%) were used as raw materials. Deionized water was used as solvent for the synthesis of precursor solution. To obtain A(NO3 )3 (A = Gd, Eu, and Bi) solution (0.5 mol L−1 ), stoichiometric Gd2 O3 , Eu2 O3 , and Bi2 O3 were dissolved into HNO3 solution under heating with agitation. The precursor solution was fabricated using GdCO3 OH:Eu3+ /Bi3+ and Na2 WO4 ·2H2 O as raw materials. For the preparation of GdCO3 OH:Eu3+ /Bi3+ , 2 mL of A(NO3 )3 solution was mixed with 48 mL of deionized water and stirred for about 15 min. Then, 5 g of carbamide (H2 NCONH2 , 99.5%) was added into the above solution. Subsequently, the total volume of the solution was adjusted to be 100 mL by adding appropriate deionized water. The final solution was wrapped in a beaker by polyethylene film and stirred for about 15 min. Finally, the mixed solution was heated at 80 ◦ C for about 4 h under strong stirring. The obtained suspension was separated by centrifugation and collected after washing with deionized water for several times. The concentration of Eu3+ was fixed as 4 mol%. To study the influence of Bi3+ concentration on the luminescence of samples, samples with different concentrations of Bi3+ (0, 4 mol%, 8 mol%, and 12 mol%) were synthesized. In the synthesis of NaGd(WO4 )2 :Eu3+ /Bi3+ precursor solution, the as-prepared GdCO3 OH:Eu3+ /Bi3+ was dispersed into 35 mL of deionized water and treated for about 15 min by ultrasonic. Then, 2 mmol of Na2 WO4 ·2H2 O was added into the above solution. Subsequently, 2 mL of 2-methoxyethanol was added for adjusting viscosity. To keep the precursor solution stable, appropriate amount of acetylacetone (C5 H8 O2 , 99%) was also added into the solution. After an agitation about 30 min, the precursor solution was obtained. The NaGd(WO4 )2 :Eu3+ /Bi3+ films were deposited on SiO2 glass substrates by spin-coating. The substrates were boiled in nitric acid, rinsed with deionized water and air-dried under infrared lamp firstly. Then, the precursor solution was coated onto the substrates at 3000 RPM (revolutions per minute) for about 20 s at room temperature and dried at 300 ◦ C for 10 min in air to eliminate the solvent and organic residue. The same spin coating procedures were repeated 12 times. Finally, the films were annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h in air. To investigate influence of rotation speed on the surface of films, different speeds (1000, 2000, 3000, and 4000 RPM) were used to deposit films. Thermogravimetric and differential scanning calorimetry (TGDSC) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) at a heating rate of 10 ◦ C min−1 . The X-ray diffraction (XRD) patterns of thin films were examined by a Japan RigaKu D/max-g X-ray diffractometer system with graphite monochromatized Cu K␣ irradiation. The surface morphology of thin film was observed by a scanning electron microscope (SEM, JEOL JSM-6301F). The excitation and emission spectra were taken on a Hitachi F4500 fluorescent
Fig. 1. TG-DSC curve of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ precursor.
spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All of measurements were carried out at room temperature. 3. Results and discussion The thermal behavior of the as-prepared NaGd(WO4 )2 : 0.04Eu3+ /0.12Bi3+ precursor is investigated by TG-DSC measurement. The obtained TG-DSC curve is shown in Fig. 1. It can be seen from the TG curve that the weight lessens continually up to 700 ◦ C, corresponding to the two exothermic peaks at 325 ◦ C and 689 ◦ C. Above 700 ◦ C, no abrupt weight loss can be observed. Fig. 2 shows the XRD patterns of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h, respectively. After an annealing at 600 ◦ C, the sample shows the tetragonal phase with ˚ space group of I41 /a and lattice parameter values of a = b = 5.243 A, ˚ and ˛ = ˇ = = 90◦ , which is well according with the c = 11.384 A, JCPDS card No. 25-0829 [25]. The peaks are sharper due to the increases of annealing temperatures, which indicates the increase of crystallinity. No impurity phase can be observed in the XRD patterns. This fact implies clearly that Eu3+ /Bi3+ ions have doped into NaGd(WO4 )2 host entirely and formed solid solution. Eu3+ and Bi3+ ions are expected to substitute Gd3+ sites, as they have same oxidation states and similar ionic radii. Likewise, only single NaGd(WO4 )2 phase is observed for all of films with different doping
Fig. 2. XRD patterns of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h.
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Fig. 3. Cross-section images of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h.
concentrations, further reflecting that Eu3+ and Bi3+ ions are well dispersed as substitutes for Gd3+ in host lattice. To further investigate effects of annealing temperature on microstructure of NaGd(WO4 )2 :Eu3+ /Bi3+ films, the cross-sections of films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C are surveyed. Fig. 3 shows cross-sections of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at different temperatures. Clearly, the films become more and more compact resulting from the increases of annealing temperature. The structure is unconsolidated and the surface is rough when the film is annealed at 600 ◦ C, as shown in Fig. 3A. Increasing the temperature to be 700 ◦ C, the thickness reduces and the surface seems to be smooth, as shown in Fig. 3B. Further increasing the temperature to be 800 ◦ C, NaGd(WO4 )2 :Eu3+ /Bi3+ films with compact structure and smooth surface are obtained, as shown in Fig. 3C. From the XRD patterns, we can conclude that the increases of annealing temperatures benefit to the increase of crystallinity.
The higher crystallinity induces the compact structure and smooth surface of the films. It also can be seen from Fig. 3C that the adhesion of the composite film to the substrate seems to be fairly good. Fig. 4 shows the SEM images of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 800 ◦ C for 3 h, but deposited by different values of RPM. Clearly, one can conclude from Fig. 4 that rotation speed has obvious influence on surface of films. The films deposited at 3000 RPM seem to have a most compact structure. Generally speaking, increase in substrate rotation speed results in decreases of film thickness, Ra (arithmetical mean deviation), RZ (ten point height of irregularities), and S (mean spacing of local peaks) [26]. However, the Ra decreases with the increase of film thickness [27]. Therefore, critical rotation speed benefits to the formation of films with smooth surface. In present study, films become to be smoother and more compact with the increase of rotation speed up to 3000 RPM. As the speed further increases (e.g. 4000 RPM), the centrifugal force
Fig. 4. SEM images of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 800 ◦ C for 3 h under different revolutions of spin coating.
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Fig. 5. Excitation spectra of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films annealed at 800 ◦ C for 3 h.
will cause more particles to be thrown away from the substrate thus leaving only thinner film, which induces the increases of roughness, as shown in Fig. 4D [28]. Fig. 5 shows excitation spectra of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) phosphor films deposited at 3000 RPM after the annealing at 800 ◦ C. The excitation spectra (monitored at 618 nm) show a broad band ranging from 200 nm to 325 nm and centering at 268 nm, which is a charge transfer (CT) band resulting from the O2− → W6+ and O2− → Eu3+ transitions [29]. When there is co-doping Bi3+ , a shoulder appears and the CT band becomes wider, which is induced by 6 S2 → 6s6p transitions of Bi3+ ions [30]. This is a straight evidence for energy transfer from Bi3+ to Eu3+ . The other excitation bands in the range of 350–500 nm correspond to 7 F → 5 L , 7 F → 5 D , and 7 F → 5 D transitions of Eu3+ ions. With 0 6 0 3 0 2 the co-addition of Bi3+ ions, the intensity of CT band decreases but the intensities of Eu3+ excitation bands increase. The Eu3+ excitation intensity strongly depend on Bi3+ concentration, with a maximum for 8 mol% Bi3+ and decreases for higher concentration. Fig. 6 shows emission spectrum of NaGd(WO4 )2 :0.04Eu3+ / 0.12Bi3+ film under an excitation at 396 nm. The spectrum exhibits typical emissions of Eu3+ originating from the 5 D0 → 7 Fj (j = 1, 2, 3, and 4) transitions. No emission from the Bi3+ can be observed, indicating that Bi3+ ion does not act as a luminescence center, but a
Fig. 6. Emission spectrum of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films under the excitation at 396 nm.
Fig. 7. Dependence of emission intensities on excitation wavelength for NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films.
sensitizer. It is well known that the 5 D0 → 7 F1 transition is caused by magnetic dipole transition of Eu3+ embedded at a site of inversion symmetry, while 5 D0 → 7 F2 transition can be attributed to the electric dipole transition of Eu3+ embedded at a site of noninversion symmetry [31]. The stronger emission band originating from the 5 D0 → 7 F2 transition indicates a strong electric field of low symmetry at the Eu3+ ions. We find that the co-doped concentration of Bi3+ has no influence on the spectrum shape and the positions of emission bands, but has obvious influence on the emission intensity. Fig. 7 gives the relative intensity of emission band peaking at 618 nm for NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) phosphor films under excitation at different wavelengths. It can be seen that the 5 D0 → 7 F2 transition emission intensity increases with increasing Bi3+ concentrations and reaches the optimum level for 8 mol% of Bi3+ . The increases of emission intensity should be induced by the energy transfer between Bi3+ and Eu3+ ions and the higher crystal structural distortion in NaGd(WO4 )2 hosts. There are overlaps between the emission band of Bi3+ and the excitation band of Eu3+ [32,33], which induces the probability of nonradiative energy transfer from Bi3+ ion to Eu3+ ion. As a result, the emission intensity is enhanced highly. The energy transfer between Bi3+ and Eu3+ is very efficient and strongly depends on Bi3+ concentration. Too high Bi3+ concentration may lead to an aggregation of individual sensitizer. These aggregates act as trapping centers and dissipate its absorbed energy as nonradiative transition process and the energy transferring ability decreases while increasing the concentration [34]. Fig. 8 gives decay curves of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films. All of decay curves fit well with a formula of I = A1 exp(−t/ 1 ) + A2 exp(−t/ 2 ) + I0 , where I0 and I is the emission intensity, A1 and A2 are constants, 1 and 2 are short and long lifetimes for the exponential components, respectively. The decay lifetimes () can be calculated as = (A1 1 2 + A2 2 2 )/(A1 1 + A2 2 ). The decay lifetimes for NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) are 5.20, 4.91, 4.73, and 4.06 ms, respectively. As driving current increases, the chip temperature of LED increases accordingly. For high power LED chip, the temperature can reach a value of more than 200 ◦ C. Therefore, it is important to evaluate the thermal quenching of phosphors at the temperature higher than room temperature. Fig. 9 shows the temperaturedependence of emission for NaGd(WO4 )2 :0.04Eu3+ /0.08Bi3+ sample under the excitation at 396 nm. It is obvious that increases of temperature from 25 to 300 ◦ C lead to a decrease of emission intensity. And it shows that the thermal quenching temperature is near
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References
Fig. 8. Decay curves of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films.
Fig. 9. Temperature-dependence of the NaGd(WO4 )2 :0.04Eu3+ /0.08Bi3+ sample.
integral
emission
intensity
for
to 300 ◦ C. The quenching temperature is defined as a temperature at which the emission intensity is reduced to be 50% of the initial intensity. Moreover, three rounds of experiments on cooling and heating cycles indicate that the integral emission intensity can be restored basically to its initial state.
4. Conclusions In summary, NaGd(WO4 )2 :Eu3+ /Bi3+ films were deposited on SiO2 glass substrates by spin-coating through the GdCO3 OH:Eu3+ /Bi3+ precursor solution. After an annealing at 800 ◦ C, the films have high crystallinity, compact structure and smooth surface. By adjusting revolutions of spin coating, it was found that the value of 3000 RPM benefits to obtain the most compact structure in the deposition. Under the excitation at 396 nm, NaGd(WO4 )2 :Eu3+ /Bi3+ films showed characteristic emission bands originating from the 5 D0 → 7 Fj (j = 1, 2, 3, and 4) transitions of Eu3+ ions. It was also found that excitation wavelength has great effect on intensity of emission. The co-doping Bi3+ ions can improve the emission intensity of films, which can be assigned to the energy transfer from Bi3+ to Eu3+ ions and the higher crystal structural distortion.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Deposition of NaGd(WO4 )2 :Eu3+ /Bi3+ films on glass substrates and potential applications in white light emitting diodes Zhijian Liu a , Wenlu Xu b , Anni Lin c , Tao He d,∗ , Fan Lin e a
Department of Power Engineering, School of Energy, Power & Mechanical Engineering, North China Electric Power University, Baoding 071003, PR China Department of Electronic & Communication Engineering, School of Electrical and Electronic Engineering, North China Electric Power University, Baoding 071003, PR China c Department of Electrical Engineering, School of Electrical and Electronic Engineering, North China Electric Power University, Baoding 071003, PR China d Oujiang College of Wenzhou University, Wenzhou 325035, PR China e Software School, Xiamen University, Xiamen 361005, PR China b
a r t i c l e
i n f o
Article history: Received 15 November 2015 Received in revised form 21 December 2015 Accepted 21 December 2015 Available online 23 December 2015 Keywords: NaGd(WO4 )2 :Eu3+ /Bi3+ Films Spin-coating Luminescence
a b s t r a c t NaGd(WO4 )2 :Eu3+ /Bi3+ films were deposited on SiO2 glass substrates by spin-coating. The as-prepared films were heat treated at different temperatures. The properties of films were characterized by TG-DSC, XRD, SEM and fluorescence spectrophotometer. The TG-DSC curve indicated that films began to crystallize at about 600 ◦ C. The XRD patterns and SEM images of films annealed at 600 ◦ C, 700 ◦ C and 800 ◦ C showed that a higher temperature benefits to the higher crystallinity and induces the lower thickness. Meanwhile, it was found that the revolution of spin coating influenced the morphology of films obviously. Under the excitation at 396 nm, NaGd(WO4 )2 :Eu3+ /Bi3+ films showed characteristic emission bands originating from 5 D0 → 7 Fj (j = 1, 2, 3, and 4) transitions of Eu3+ ions. It was also found that the excitation wavelength has great effect on the emission intensity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the increasing energy crisis and ecocrisis, more and more researches have focused on low carbon economy. Low carbon economy has been accepted to be one of effective economic development modes for human responding to energy and environmental risks. Several of approaches have been done to accomplish low carbon economy in fields of industry and daily life, such as the appearance of more and more zero energy buildings [1], the use of solar installations in buildings [2], the use of LED in daily [3], the use of energy-saving building materials [4], and the heat recovery in industry [5]. In daily life, white light generation is needed in room lighting, displays, monitors, and other optical devices. And white light emitting diodes (WLEDs) are the most promising white light sources [3]. WLED-based lighting has advantages of lower energy consumption, longer lifetime, higher luminous efficiency and brightness, wider operating temperature range, and better mechanical impact resistance, comparing with traditional incandescent or fluorescent lamps [6,7]. In other words, WLED can be one of attempts to accomplish low carbon economy in daily life.
∗ Corresponding author. E-mail address: [email protected] (T. He). http://dx.doi.org/10.1016/j.enbuild.2015.12.035 0378-7788/© 2015 Elsevier B.V. All rights reserved.
It has been reported that rare earth ions doped garnet luminescence-film grown on single crystal substrates can withstand much higher power densities than powder phosphors without tube degradation [8]. The film-phosphor has uniform thickness, smoother surface and small grain size, which makes it possible to define a smaller pixel spot size to achieve a higher resolution [9]. Moreover, it demonstrates a large separation between the phosphor and the primary LED emitter (referring to as remote phosphor arrangement), which enhances a phosphorescence extraction by reducing optical power absorbed by an LED chip [10]. Generally, the remote phosphor can be obtained by fabricating a phosphor layer on a regular soda lime silicate (SLS) or SiO2 glass substrates [11,12]. The combination of green and red emitting phosphors with a blue chip benefits to the fabrication of WLEDs with high color rendering and high efficiency [13]. In the past decades, Eu3+ ion is used widely as an ideal red comment because of its excellent color purity [14]. When Eu3+ ion presents in a non-centrosymmetric site, phosphors have dominant red emission originating from the 5 D0 → 7 F2 transition. And the red emission intensity can be improved by codoping Bi3+ ions because of an energy transfer between Eu3+ and Bi3+ ions [15–17]. Alkali rare-earth double tungstate compounds have been studied widely for the application of phosphor host [18–23]. NaGd(WO4 )2 , as one of members of double tungstates
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family, belongs to the tetragonal system with space group of C6 4h (I41 /a). As a good host for rare earth ions, NaGd(WO4 )2 shows excellent thermal, hydrolytic, and chemical stability [24]. Herein, we report on the fabrication of NaGd(WO4 )2 :Eu3+ /Bi3+ thick films on SiO2 glass substrates by spin-coating technique. This method has advantages of simpleness and the possibility of large-area coatings with high uniformity on various substrates [11]. The phase, microstructure and luminescent properties of the obtained phosphor-films have been characterized.
2. Experiments All of raw materials were purchased from Aladdin and used directly without further purification. Gadolinium oxide (Gd2 O3 , 99.99%), europium oxide (Eu2 O3 , 99.99%), bismuth oxide (Bi2 O3 , 99.99%), and sodium tungstate (Na2 WO4 ·2H2 O, 99.99%) were used as raw materials. Deionized water was used as solvent for the synthesis of precursor solution. To obtain A(NO3 )3 (A = Gd, Eu, and Bi) solution (0.5 mol L−1 ), stoichiometric Gd2 O3 , Eu2 O3 , and Bi2 O3 were dissolved into HNO3 solution under heating with agitation. The precursor solution was fabricated using GdCO3 OH:Eu3+ /Bi3+ and Na2 WO4 ·2H2 O as raw materials. For the preparation of GdCO3 OH:Eu3+ /Bi3+ , 2 mL of A(NO3 )3 solution was mixed with 48 mL of deionized water and stirred for about 15 min. Then, 5 g of carbamide (H2 NCONH2 , 99.5%) was added into the above solution. Subsequently, the total volume of the solution was adjusted to be 100 mL by adding appropriate deionized water. The final solution was wrapped in a beaker by polyethylene film and stirred for about 15 min. Finally, the mixed solution was heated at 80 ◦ C for about 4 h under strong stirring. The obtained suspension was separated by centrifugation and collected after washing with deionized water for several times. The concentration of Eu3+ was fixed as 4 mol%. To study the influence of Bi3+ concentration on the luminescence of samples, samples with different concentrations of Bi3+ (0, 4 mol%, 8 mol%, and 12 mol%) were synthesized. In the synthesis of NaGd(WO4 )2 :Eu3+ /Bi3+ precursor solution, the as-prepared GdCO3 OH:Eu3+ /Bi3+ was dispersed into 35 mL of deionized water and treated for about 15 min by ultrasonic. Then, 2 mmol of Na2 WO4 ·2H2 O was added into the above solution. Subsequently, 2 mL of 2-methoxyethanol was added for adjusting viscosity. To keep the precursor solution stable, appropriate amount of acetylacetone (C5 H8 O2 , 99%) was also added into the solution. After an agitation about 30 min, the precursor solution was obtained. The NaGd(WO4 )2 :Eu3+ /Bi3+ films were deposited on SiO2 glass substrates by spin-coating. The substrates were boiled in nitric acid, rinsed with deionized water and air-dried under infrared lamp firstly. Then, the precursor solution was coated onto the substrates at 3000 RPM (revolutions per minute) for about 20 s at room temperature and dried at 300 ◦ C for 10 min in air to eliminate the solvent and organic residue. The same spin coating procedures were repeated 12 times. Finally, the films were annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h in air. To investigate influence of rotation speed on the surface of films, different speeds (1000, 2000, 3000, and 4000 RPM) were used to deposit films. Thermogravimetric and differential scanning calorimetry (TGDSC) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) at a heating rate of 10 ◦ C min−1 . The X-ray diffraction (XRD) patterns of thin films were examined by a Japan RigaKu D/max-g X-ray diffractometer system with graphite monochromatized Cu K␣ irradiation. The surface morphology of thin film was observed by a scanning electron microscope (SEM, JEOL JSM-6301F). The excitation and emission spectra were taken on a Hitachi F4500 fluorescent
Fig. 1. TG-DSC curve of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ precursor.
spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All of measurements were carried out at room temperature. 3. Results and discussion The thermal behavior of the as-prepared NaGd(WO4 )2 : 0.04Eu3+ /0.12Bi3+ precursor is investigated by TG-DSC measurement. The obtained TG-DSC curve is shown in Fig. 1. It can be seen from the TG curve that the weight lessens continually up to 700 ◦ C, corresponding to the two exothermic peaks at 325 ◦ C and 689 ◦ C. Above 700 ◦ C, no abrupt weight loss can be observed. Fig. 2 shows the XRD patterns of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h, respectively. After an annealing at 600 ◦ C, the sample shows the tetragonal phase with ˚ space group of I41 /a and lattice parameter values of a = b = 5.243 A, ˚ and ˛ = ˇ = = 90◦ , which is well according with the c = 11.384 A, JCPDS card No. 25-0829 [25]. The peaks are sharper due to the increases of annealing temperatures, which indicates the increase of crystallinity. No impurity phase can be observed in the XRD patterns. This fact implies clearly that Eu3+ /Bi3+ ions have doped into NaGd(WO4 )2 host entirely and formed solid solution. Eu3+ and Bi3+ ions are expected to substitute Gd3+ sites, as they have same oxidation states and similar ionic radii. Likewise, only single NaGd(WO4 )2 phase is observed for all of films with different doping
Fig. 2. XRD patterns of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h.
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Fig. 3. Cross-section images of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C for 3 h.
concentrations, further reflecting that Eu3+ and Bi3+ ions are well dispersed as substitutes for Gd3+ in host lattice. To further investigate effects of annealing temperature on microstructure of NaGd(WO4 )2 :Eu3+ /Bi3+ films, the cross-sections of films annealed at 600 ◦ C, 700 ◦ C, and 800 ◦ C are surveyed. Fig. 3 shows cross-sections of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at different temperatures. Clearly, the films become more and more compact resulting from the increases of annealing temperature. The structure is unconsolidated and the surface is rough when the film is annealed at 600 ◦ C, as shown in Fig. 3A. Increasing the temperature to be 700 ◦ C, the thickness reduces and the surface seems to be smooth, as shown in Fig. 3B. Further increasing the temperature to be 800 ◦ C, NaGd(WO4 )2 :Eu3+ /Bi3+ films with compact structure and smooth surface are obtained, as shown in Fig. 3C. From the XRD patterns, we can conclude that the increases of annealing temperatures benefit to the increase of crystallinity.
The higher crystallinity induces the compact structure and smooth surface of the films. It also can be seen from Fig. 3C that the adhesion of the composite film to the substrate seems to be fairly good. Fig. 4 shows the SEM images of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 800 ◦ C for 3 h, but deposited by different values of RPM. Clearly, one can conclude from Fig. 4 that rotation speed has obvious influence on surface of films. The films deposited at 3000 RPM seem to have a most compact structure. Generally speaking, increase in substrate rotation speed results in decreases of film thickness, Ra (arithmetical mean deviation), RZ (ten point height of irregularities), and S (mean spacing of local peaks) [26]. However, the Ra decreases with the increase of film thickness [27]. Therefore, critical rotation speed benefits to the formation of films with smooth surface. In present study, films become to be smoother and more compact with the increase of rotation speed up to 3000 RPM. As the speed further increases (e.g. 4000 RPM), the centrifugal force
Fig. 4. SEM images of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films annealed at 800 ◦ C for 3 h under different revolutions of spin coating.
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Fig. 5. Excitation spectra of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films annealed at 800 ◦ C for 3 h.
will cause more particles to be thrown away from the substrate thus leaving only thinner film, which induces the increases of roughness, as shown in Fig. 4D [28]. Fig. 5 shows excitation spectra of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) phosphor films deposited at 3000 RPM after the annealing at 800 ◦ C. The excitation spectra (monitored at 618 nm) show a broad band ranging from 200 nm to 325 nm and centering at 268 nm, which is a charge transfer (CT) band resulting from the O2− → W6+ and O2− → Eu3+ transitions [29]. When there is co-doping Bi3+ , a shoulder appears and the CT band becomes wider, which is induced by 6 S2 → 6s6p transitions of Bi3+ ions [30]. This is a straight evidence for energy transfer from Bi3+ to Eu3+ . The other excitation bands in the range of 350–500 nm correspond to 7 F → 5 L , 7 F → 5 D , and 7 F → 5 D transitions of Eu3+ ions. With 0 6 0 3 0 2 the co-addition of Bi3+ ions, the intensity of CT band decreases but the intensities of Eu3+ excitation bands increase. The Eu3+ excitation intensity strongly depend on Bi3+ concentration, with a maximum for 8 mol% Bi3+ and decreases for higher concentration. Fig. 6 shows emission spectrum of NaGd(WO4 )2 :0.04Eu3+ / 0.12Bi3+ film under an excitation at 396 nm. The spectrum exhibits typical emissions of Eu3+ originating from the 5 D0 → 7 Fj (j = 1, 2, 3, and 4) transitions. No emission from the Bi3+ can be observed, indicating that Bi3+ ion does not act as a luminescence center, but a
Fig. 6. Emission spectrum of NaGd(WO4 )2 :0.04Eu3+ /0.12Bi3+ films under the excitation at 396 nm.
Fig. 7. Dependence of emission intensities on excitation wavelength for NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films.
sensitizer. It is well known that the 5 D0 → 7 F1 transition is caused by magnetic dipole transition of Eu3+ embedded at a site of inversion symmetry, while 5 D0 → 7 F2 transition can be attributed to the electric dipole transition of Eu3+ embedded at a site of noninversion symmetry [31]. The stronger emission band originating from the 5 D0 → 7 F2 transition indicates a strong electric field of low symmetry at the Eu3+ ions. We find that the co-doped concentration of Bi3+ has no influence on the spectrum shape and the positions of emission bands, but has obvious influence on the emission intensity. Fig. 7 gives the relative intensity of emission band peaking at 618 nm for NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) phosphor films under excitation at different wavelengths. It can be seen that the 5 D0 → 7 F2 transition emission intensity increases with increasing Bi3+ concentrations and reaches the optimum level for 8 mol% of Bi3+ . The increases of emission intensity should be induced by the energy transfer between Bi3+ and Eu3+ ions and the higher crystal structural distortion in NaGd(WO4 )2 hosts. There are overlaps between the emission band of Bi3+ and the excitation band of Eu3+ [32,33], which induces the probability of nonradiative energy transfer from Bi3+ ion to Eu3+ ion. As a result, the emission intensity is enhanced highly. The energy transfer between Bi3+ and Eu3+ is very efficient and strongly depends on Bi3+ concentration. Too high Bi3+ concentration may lead to an aggregation of individual sensitizer. These aggregates act as trapping centers and dissipate its absorbed energy as nonradiative transition process and the energy transferring ability decreases while increasing the concentration [34]. Fig. 8 gives decay curves of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films. All of decay curves fit well with a formula of I = A1 exp(−t/ 1 ) + A2 exp(−t/ 2 ) + I0 , where I0 and I is the emission intensity, A1 and A2 are constants, 1 and 2 are short and long lifetimes for the exponential components, respectively. The decay lifetimes () can be calculated as = (A1 1 2 + A2 2 2 )/(A1 1 + A2 2 ). The decay lifetimes for NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) are 5.20, 4.91, 4.73, and 4.06 ms, respectively. As driving current increases, the chip temperature of LED increases accordingly. For high power LED chip, the temperature can reach a value of more than 200 ◦ C. Therefore, it is important to evaluate the thermal quenching of phosphors at the temperature higher than room temperature. Fig. 9 shows the temperaturedependence of emission for NaGd(WO4 )2 :0.04Eu3+ /0.08Bi3+ sample under the excitation at 396 nm. It is obvious that increases of temperature from 25 to 300 ◦ C lead to a decrease of emission intensity. And it shows that the thermal quenching temperature is near
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References
Fig. 8. Decay curves of NaGd(WO4 )2 :0.04Eu3+ /xBi3+ (x = 0, 0.04, 0.08, and 0.12) films.
Fig. 9. Temperature-dependence of the NaGd(WO4 )2 :0.04Eu3+ /0.08Bi3+ sample.
integral
emission
intensity
for
to 300 ◦ C. The quenching temperature is defined as a temperature at which the emission intensity is reduced to be 50% of the initial intensity. Moreover, three rounds of experiments on cooling and heating cycles indicate that the integral emission intensity can be restored basically to its initial state.
4. Conclusions In summary, NaGd(WO4 )2 :Eu3+ /Bi3+ films were deposited on SiO2 glass substrates by spin-coating through the GdCO3 OH:Eu3+ /Bi3+ precursor solution. After an annealing at 800 ◦ C, the films have high crystallinity, compact structure and smooth surface. By adjusting revolutions of spin coating, it was found that the value of 3000 RPM benefits to obtain the most compact structure in the deposition. Under the excitation at 396 nm, NaGd(WO4 )2 :Eu3+ /Bi3+ films showed characteristic emission bands originating from the 5 D0 → 7 Fj (j = 1, 2, 3, and 4) transitions of Eu3+ ions. It was also found that excitation wavelength has great effect on intensity of emission. The co-doping Bi3+ ions can improve the emission intensity of films, which can be assigned to the energy transfer from Bi3+ to Eu3+ ions and the higher crystal structural distortion.
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