張凌云,聶笑笑,鄭贛鴻,彭張皖,戴振翔

(安徽大學(xué) 物理與材料科學(xué)學(xué)院，安徽省信息材料與器件重點(diǎn)實(shí)驗(yàn)室，安徽 合肥 230039)

Ba2+摻雜Y0.75Bi0.15Sm0.10VO4熒光粉的發(fā)光性質(zhì)

張凌云,聶笑笑,鄭贛鴻*,彭張皖,戴振翔

(安徽大學(xué) 物理與材料科學(xué)學(xué)院，安徽省信息材料與器件重點(diǎn)實(shí)驗(yàn)室，安徽 合肥 230039)

摘要:采用固相法制備Ba(2+)摻雜Y(0.75)Bi(0.15)Sm(0.10)VO4 熒光粉粉末樣品.用X射線衍射和熒光分光光度計(jì)對(duì)樣品的結(jié)構(gòu)和光學(xué)性質(zhì)進(jìn)行了研究.結(jié)果表明：Ba加入并沒有改變樣品的晶體結(jié)構(gòu)，但大大提高了熒光粉的發(fā)光強(qiáng)度.相關(guān)機(jī)理在文中給出了解釋.

關(guān)鍵詞:固相反應(yīng)；Ba(2+)摻雜；熒光性質(zhì)

0Introduction

Recently, the white light-emitting diode (w-LED) has been extensively investigated due to its advantages such as high reliability, high luminous efficiency, long lifetime, low energy consumption, safety and its environment-friendly characteristic. It has great potential applications in the fourth generation lightly sources replacing the incandescent and fluorescent lamps[1-6]. Recent investigations show that YVO4:RE phosphors have a significant promise in the high definition flat display panel, such as plasma display panels (PDP) and field emitting display (FED)[7-8]. In addition, rare earth ions in inorganic host matrix form an important class of phosphors as they possess a few interesting characteristics such as excellent chemical stability, high luminescence efficiency, and flexible emission colors with different activators. So, the selection of the rare earth ion as an activator is a key factor for the preparation of luminescence materials. Among the different rare earth ions, the Sm3+ions as an activator are regarded as one of the most popular and efficient doping ions. Sm3+ions in various hosts show bright emission in orange or red regions because of the transitions from the excited state4G5/2to the ground state6H7/2and also to the higher levels6Hj(j=7/2, 9/2 and 11/2) and found the application in temperature sensors, undersea communications, various fluorescent devices, color display and visible solid-state lasers[9-10].

Wang et al.[11]used a hydrothermal method to prepare YVO4:Ln3+(Ln=Eu, Dy, Sm, Ce) nano-crystals co-doped with Ba2+, and reported a large enhancement in their luminescence. Similarly, Zhang et al. found that the photoluminescence of Sr3Al2O6:Eu3+phosphors can be greatly increased by doping Ba2+ions[12]. Jia et al. used a solvothermal method to synthesize YVO4:Ln3+(Ln=Eu, Dy) and reported that a large enhancement in the luminescence by co-doping Ba2+[13].

In this work, Ba2+-doped Y0.75Bi0.15Sm0.10VO4phosphors was synthesized by a conventional solid-state reaction in air atmosphere. We investigate the effect of Ba addition and different Ba doping concentration on the luminescent properties. Some interesting results have been observed and discussed.

1Experimental

The phosphors Y0.75Bi0.15Sm0.10VO4:xBa (x=0,0.02,0.05,0.08,0.10,0.12,0.15, 0.18) were prepared by conventional solid-sate reaction. The reactants include Y2O3(99.9%)， NH4VO3(99.0%)，Sm2O3(99.9%)，Bi2O3(99%)，and BaCO3(99%). At first, the starting materials with one appropriate ratio were mixed and ground, and subsequently the mixture were heated to 680 ℃ for 10 h. Then these obtained powders were pressed into pellets and intermediately sintered at 850 ℃ in order that the powders can react adequately and then the homogeneous samples were obtained. Finally, in order to obtain the well-crystallized and single phase phosphors, these products were finally sintered at 1 000 ℃ for 12 h.

The crystal structure of the phosphor powders was characterized by X-ray diffraction (XRD) analysis. In the process of XRD analysis, one X-ray diffractometer DX-2000 SSC with CuKαirradiation (λ=0.154 06 nm) is used, with operating voltage being 36 kV and the operating current being 25 mA. The activation and emission spectra were measured on a FL fluorescence spectrophotometer (F-4500). The weight of every sample was equal to 0.8 g. All these operations were carried out at room temperature.

2Results and Discussion

2.1Crystal structures of Y0.75Bi0.15Sm0.10VO4:xBa (0≤x≤0.18) systems

In order to characterize the phase purity and crystallinity of the as-prepared powder samples, the XRD patterns for all samples were examined. The XRD patterns of the as-synthesized Y0.75Bi0.15Sm0.10-VO4:xBa (0≤x≤0.18) phosphors are shown in Fig.1. It can be seen that all samples doped with Ba display the same diffraction peak, which matches well with the standard data of tetragonal phase of YVO4with space group I41/amd141 (JCPDS: 17-0341). No other phase of impurity is detected indicating the samples are well crystallized with a pure single phase. This fact suggests that Ba ions have been successfully built into the host.

Fig.1The XRD patterns of the Y0.75Bi0.15Sm0.10VO4:xBa (0≤x≤0.18) phosphors

2.2The fluorescence properties of the samples Y0.75Bi0.15Sm0.10VO4:xBa2+(0≤x≤0.18)

Fig.2 presents the excitation spectra of Y0.75Bi0.15Sm0.10VO4:xBa (0≤x≤0.18) for monitoring the4G5/2→6H9/2transition (λem=647 nm) of Sm3+, respectively. For Fig.2, in the wavelength region 400—500 nm, several excitation peaks are observed and are located at 411 nm (6H5/2→4F7/2), 423 nm (6H5/2→6P5/2), 447 nm(6H5/2→4G9/2), 468 nm(6H5/2→4I9/2), and 482 nm (6H5/2→4I11/2) which are attributed to f-f transitions of Sm3+charge transfer band of Sm3+-O2-interaction[14-15]. In our Y0.75Bi0.15Sm0.10VO4:xBa2+powder phosphor, it was found that the excitation intensities of f-f transition at 411 nm and 482 nm are higher compared with the other transitions as shown in Fig.2. Therefore, both of the two transitions are chosen for the measurement of emission spectra in our samples as shown as discussed below.

Fig.2The excitation spectra for monitoring the emission at λex=647 nm of the Y0.75Bi0.15Sm0.10VO4:xBa (0≤x≤0.18)

Despite different compositions, all these samples Y0.75Bi0.15Sm0.10VO4:xBa2+show similar excitation spectra. The excitation wavelengths are also found to be independent of Ba addition, indicating that the Ba2+dopingdoes not change the host structure. However, the relative excitation intensity at the highest6H5/2→4F7/2(411 nm) and6H5/2→4I11/2(482 nm) transition of Sm3+is changed obviously with the doped Ba2+content. In order to further detect the change of excitation intensity with Ba2+concentration, we plot the intensity varies with Ba2+concentration as shown in Fig.3. Fig.3 shows the change of excitation intensity of 411 nm and 482 nm with increasing Ba2+concentration. Obviously, the excitation intensity increases with increasing Ba doping and then decreases with increasing Ba concentration further. That is to say, the largest excitation intensity is achieved atx=0.12 for Ba2+doping. In this case, the excitation intensity is enhanced by 6 times due to 12% Ba2+doping.

Fig.3The maximum excitation intensity at 411 nm and 482 nm of Y0.75Bi0.15Sm0.10VO4:xBa
(0≤x≤0.18) versus the Ba concentration

Fig.4 reveals that the emission spectra of the Y0.75Bi0.15Sm0.10VO4:xBa2+(0≤x≤0.18) phosphors with different Ba2+concentration on excitation at 411 nm (left )and 482 nm(right), respectively. The emission spectra consist of four sharp emission lines, which are the characteristic of the samarium ions. The four peaks are ascribed to the4G5/2→6H5/2(566 nm),4G5/2→6H7/2(604 nm),4G5/2→6H9/2(647 nm), and4G5/2→6H11/2(713 nm), respectively. Among these, the transition at 647 nm (4G5/2→6H9/2) is having the maximum intensity, which is electric dipole transition (belonging to hypersensitive transitions) that obey the selection rule ΔJ=2. The transition at 604 nm (4G5/2→6H7/2) corresponds to the red emission of Y0.75Bi0.15Sm0.10VO4:xBa2+phosphors. It can be stated that the emitting transition (4G5/2→6H7/2) at 604 nm (ΔJ=±1) is a partly magnetic dipole (MD) and partly electric dipole (ED) nature emission band. The transition at 566 nm (4G5/2→6H5/2) is purely MD natured. The other transition at 647 nm (4G5/2→6H9/2) is purely ED natured, which is sensitive to crystal field. Generally, the intensity ratio of ED and MD transition has been used to measure the symmetry of the local environment of the trivalent 4f ions[16]. The greater the intensity of the ED transition, the more the asymmetry nature. In our present study, the4G5/2→6H9/2(ED) transition of Sm3+ions is more intense than4G5/2→6H5/2(MD) transition, indicating the asymmetric nature of the YVO4host matrix.

Fig.4The emission spectra under excitation at λex=411 nm and 482 excitation of the Y0.75Bi0.15Sm0.10VO4:xBa (0≤x≤0.18)

The emission wavelengths are independent of Ba2+addition, indicating that the Ba2+dopingdoes not change the spectral features of emission spectra. However, the relative emission intensity of Sm3+is changed with the doped Ba2+content, as shown in Fig.5. The intensity of emission peaks greatly increases by doping Ba2+from 0 to 0.12 in Y0.75Bi0.15Sm0.10VO4:xBa2+, and then decreases with 15% Ba2+doping. That is to say, the largest emission intensity is found atx=0.12 for Ba2+doping samples. In addition, due to 12% Ba2+addition, the emission intensity of Y0.75Bi0.15Sm0.10VO4:xBa2+phosphor are strengthened by more than 5 and 6 times under 411 nm and 482 nm excitation, respectively. The alkali-metal ion Ba2+has been proved to be largely enhance the luminescence of YVO4:Eu3+,Ba2+[17].

In addition,the luminescent intensities are significantly enhanced when monovalence ion Ba are incorporated into a host lattice and substitutes for trivalent metallic ion, charge balancing is necessarily required. For appetites, the charge compensation route has been long recognized and no compensating ions are specifically introduced. For Y0.75Bi0.15Sm0.10VO4:Ba system, the incorporation of monovalence Ba can neutralized the charge generated by Ba2+substitution for Sm3+, and thus stabilized the structure and enhanced the luminescence intensity. Maybe, it is considered as following, the degree of enhanced luminescence is related to the electronegative mismatch. The co-doping ion with large electronegativity mismatch possesses of a strong effect on the enhancement of photoluminescent intensity. When we consider the bond structure of Sm-O-Ba, this result might be understood. Due to Ba is a cation with larger radius and larger electronegativity comparing with Sm, it will attract the electrons of O2-more strongly[18], so that the electron cloud density of the O2-ion decreases and the local crystal field surrounding Sm3+ion is also altered. However, when Ba content is more than 0.12, more and more Ba ions are induced into the samples. Subsequently, the distance between two Ba ions would be decreased gradually and be close enough. This will result in one efficient and direct energy transfer process between Ba ions, which reduces the probability of energy transfer from Ba to Sm. And eventually, the emission is limited as shown in Fig 5.

Fig.5The maximum emission intensity at 411 nm and 482 nm of Y0.75Bi0.15Sm0.10VO4:xBa
(0≤x≤0.18) versus the Ba concentration

3Conclusions

TheY0.75Bi0.15Sm0.10VO4:xBa (0≤x≤0.18) phosphors have been synthesized by solid-state reaction method. The corresponding photoluminesce properties are also probed. For these Y0.75Bi0.15-Sm0.10VO4:xBaphosphors, the emission intensity is enhanced firstly with increasing the Ba concentration, and then decrease gradually. The strongest emission intensity is achieved atx=0.12. This is related to the distortion of the crystal field surrounding Sm3+ions, and such a distortion is enhanced by the substituting Ba2+. This distortion leads to the enhancement of the emission intensity. However, the excess distortion caused by the addition of Ba2+over 0.12 may result in the decrease of emission intensity.

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