图1 LiSr4-xCax(BN2)3(x=0,1,2,3,4)样品的XRD图像,LSBN和LiCa4(BN2)3的PDF标准卡片作为参考。
Published:05 August 2022,
Received:30 May 2022,
Revised:09 June 2022
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Self-activated phosphors have received a lot of attention from researchers, among which nitridoborate defect phosphors have the potential to become a new generation of phosphors for LEDs due to low toxicity, simple synthesis, and structural diversity, but low thermal stability limits their practical applications. In this paper, a new LiSr2Ca2(BN2)3(LSCBN) phosphor was synthesized by partially replacing Sr in LiSr4(BN2)3(LSBN) using a high-temperature solid-phase one-step method. The phase composition, morphology, and optical properties of the phosphor were characterized by X-ray diffraction, scanning electron microscopy, and fluorescence spectrometry. The results show that the prepared sample LSBN is a cubic crystal system with the space group Im‐3m. It has a wide excitation band in the UV region, the peak of the emission spectrum is located at 561 nm, and the full width at half maximum(FWHM) is about 4 504 cm-1. The luminescence intensity of a LSCBN is twice that of a LSBN. In LSCBN, the partial replacement of Sr by Ca introduces substitutional defects that form new luminescence centers. In the temperature dependence PL spectra, the intensity of LSBN at 150 ℃ is 17% of the initial value, and LSCBN can maintain 57% of the initial intensity at 150 ℃, which exceeds other nitridoborate phosphors that have been reported. This ion-substitution method can effectively regulate the luminescence wavelength and enhance the luminous intensity, improve the thermal stability, and provide a new idea and application prospect for the improvement of the luminescence performance of defect-related nitridoborate phosphors.
近年来,由于白光LED(WLED)具有亮度高、使用寿命长、能耗低和安全环保等优点,已逐渐取代传统光源,成为了新一代固态光源[
氮化物材料因具有优异的热稳定性、高量子效率以及丰富的结构而被应用在LED照明、显示、防伪和生物标记领域。其中,带有[BN]n-、[BN2]3-、[BN3]6-、[B2N4]8-和[B3N6]9-离子的硼氮化物可以通过B和N离子实现不同的阴离子基团。设想当B和N不按既定化学比成键时很容易形成空位缺陷,因此硼氮化物相对于铝氮化物和硅氮化物是非常适合缺陷发光材料的基质[
含有线性[N=B=N]3-的AM4[BN2]3 (A=Li, Na, 1/2Mg;M=Ca,Sr,Ba,Eu)[
通过高温固相反应法合成了一系列LiSr4-xCax⁃(BN2)3(x=0,1,2,3,4)样品。原料为Li3N(Materion, 99.5%)、Sr3N2(Materion,99.9%)、Ca3N2(Materion, 99.9%)、h-BN(Alfa Aesar,99.9%)。首先,在99.99%的氩气氛手套箱中按化学计量比例称量原料,并在手套箱中研磨6 min,使原料充分混合;然后放入BN坩埚;最后,在箱式电阻炉中进行升温反应。升温前,用高纯度氮气对电阻炉洗气两次,在流动氮气的条件下,以5 ℃/min的升温速率加热到1 200 ℃,保温4 h,自然冷却至室温后,取出样品并磨成粉末,等待下一步测试。
采用Panalytical Empyrean型(Philips公司)X射线衍射仪对样品进行晶体结构分析,该仪器采用Cu靶,Kα射线(λ=0.154 06 nm)。在10°~90°范围内采集X射线衍射(XRD)数据,计数时间为0.1 s/step,步长为0.03°。用S-3400N型(Hitachi公司)扫描电子显微镜(SEM)以及配备的能谱仪(EDS)设备分析粉末的形貌及元素。采用FS5稳态瞬态荧光光谱仪(Edinburgh, UK),配合450 W氙光源和双激发单色仪,对样品的激发、发射光谱、衰减曲线及热稳定性进行了测量。采用CASTEP程序的DFT方法对LSCBN和LSBN进行了结构优化,交换关联(XC)函数选择局域密度近似(LDA)。
对制备获得的LiSr4-xCax(BN2)3(x=0, 1, 2, 3, 4)样品进行了X射线衍射(XRD)测试,并与LSBN和LiCa4(BN2)3标准卡片进行了对比。
图1 LiSr4-xCax(BN2)3(x=0,1,2,3,4)样品的XRD图像,LSBN和LiCa4(BN2)3的PDF标准卡片作为参考。
Fig.1 XRD spectra of LiSr4-xCax(BN2)3(x=0, 1, 2, 3, 4) phosphors. As a reference, the standard XRD pattern of LSBN and LiCa4(BN2)3.
利用SEM分析了LSBN和LSCBN样品的形貌和元素组成。
图2 LSBN(a)和LSCBN(b)样品的SEM照片;(c)LSCBN样品的EDS图谱;(d)LSBN与LSCBN荧光粉的晶体结构。
Fig.2 SEM photos of LSBN(a) and LSCBN(b). (c)EDS spectrum of LSCBN. (d)Crystal structure of LSBN and LSCBN phosphors.
为了进一步分析LSCBN与LSBN的晶格结构,采用CASTEP程序的DFT方法进行了结构优化。采用已经被报道的LSBN晶体结构[
结构优化后LSBN和LSCBN的三维结构如
图3 LiSr4-xCax(BN2)3(x=0,1,2,3,4)的归一化激发光谱(a)、发射光谱(b)、归一化发射光谱(c);(d)LSCBN的发光原理。
Fig.3 (a)-(c)Normalized PLE spectra, PL spectra, Normalized PL spectra of LiSr4-xCax(BN2)3(x=0, 1, 2, 3, 4). (d)Luminescence diagrams LSCBN.
我们尝试着利用简略
进一步,我们发现LSCBN样品具有余辉发光现象。
图4 (a)LSCBN样品的余辉衰减曲线,插图为LiSr4-xCax(BN2)3(x=0,1,2,3,4) 样品的实物图,(ⅰ) 日光照射,(ⅱ) 254 nm紫外光照射,(ⅲ)停止紫外光照射;(b)LSCBN样品的发光原理。
Fig.4 (a)Afterglow decay curve of LSCBN phosphor,the inset presents the physical diagram of LiSr4-xCax(BN2)3(x =0, 1, 2, 3, 4) samples, (ⅰ) under daylight irradiation; (ⅱ) under 254 nm UV irradiation; (ⅲ)after stopping UV irradiation. (b)Luminescence schematic of LSCBN phosphor.
众所周知,阴离子空位能级是阳离子指向阴离子空位的空轨道,它是阳离子-阴离子键断裂的结果。这些能级位于导带(CB)底部以下。同样,阳离子空位能级是围绕阳离子空位的阴离子的孤对轨道,它是阳离子-阴离子键断裂的结果。这些能级位于价带(VB)顶部之上。与这些能级相关的陷阱深度(ET)随着宿主晶格的阳离子-阴离子键的强度增加而变大[
(1) |
(2) |
(3) |
结合上述缺陷形成的理论和余辉发光现象,我们认为,Ca的电负性更强,Ca—N键的强度大于Sr—N键,形成Ca空位的陷阱深度大于Sr空位的陷阱深度,因此就需要更高的能量激发,对应激发光谱蓝移。
通常,LED芯片稳定工作时放热导致发光材料环境温度高达150 ℃,所以荧光粉的热稳定性对于实际应用中的荧光粉很重要。测试了LSBN与LSCBN的发射光谱的温度依赖性曲线。从
图5 (a)~(b)LSBN和LSCBN光致发光光谱的温度依赖性;(c)~(d)LiSr4(BN2)3和LSCBN相对强度的温度依赖性。
Fig.5 (a)-(b)Temperature dependence of the photoluminescence spectra of LSBN and LSCBN. (c)-(d)Temperature dependence of the relative intensities of LSBN and LSCBN.
使用Arrhenius方程拟合了LSBN和LSCBN的热猝灭数据。活化能ΔE可由下式计算[
(4) |
其中I0是室温下的发射强度,IT是不同温度下的发射强度,K是玻尔兹曼常数(8.629×10-5 eV),c是与晶格相关的常数,ΔE是热猝灭的活化能。LSBN和LSCBN样品的
另一方面,随着温度升高,晶格热振动增加。距离导带近的
荧光粉 | 热稳定性/% (150 ℃时) | 参考文献 |
---|---|---|
LiSr2Ca2(BN2)3 | 57 | 本文 |
LiSr4(BN2)3 | 17 | 本文 |
Mg3BN3 | 45 |
[ |
Sr3(BN2)2 | 43 |
[ |
α,β‐Ca3(BN2)2 | <20 |
[ |
CaMg2N2 | <5 |
[ |
LiSr4(BN2)3∶Eu2+ | 20 |
[ |
α‐Sr3(BN2)2∶Eu3+ | <10 |
[ |
α‐Ca3(BN2)2∶Eu3+ | <10 |
[ |
本文通过高温固相反应法合成了一系列LiSr4-xCax(BN2)3(x=0, 1, 2, 3, 4)样品。实验和结构优化结果表明LSCBN具有良好的结晶性和稳定的立方结构。LSCBN样品发光相对强度提高到了LSBN的2倍。LSCBN荧光粉在λex=306 nm处发出峰值波长为561 nm的黄光。与LSBN相比,LSCBN的激发光谱变窄蓝移。同时,LSCBN具有余辉现象,设备可探测的持续发光时间为470 s。在LSBN基质中,本征缺陷引起了自激活发光;而在同样晶体结构的LSCBN中,Ca部分取代Sr时引入了替代式的缺陷,形成了新的发光中心,导致光谱性能的变化。值得一提的是,LSCBN样品在150 ℃时,发射强度约为室温下初始强度的57%,优于已报道的其他硼氮化物发光材料。本文采取阳离子取代的方法,成功改善了荧光粉的性能。这种方法不仅为优化发光材料的性能(尤其是热稳定性能方面)提供了实验依据,也为探索新的、优异的缺陷发光材料提供了实验依据。
本文专家审稿意见及作者回复内容的下载地址:http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20220217.
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