稀土激活碱土硅酸盐LED橘色荧光材料的能带结构、相变与格位选择发光调控机制研究

The alkaline earth silicate phosphor of (Sr,Ba)3SiO5:Eu2+ is a promisingly alternative one in white light-emitting diode (LED) package by replacing the nitride red phosphors, such as CaAlSiN3:Eu2+ and (Sr,Ba)2Si5N8:Eu2+. Compared with the red luminescence of nitride phosphors, however, the silicate phosphor of (Sr,Ba)3SiO5: Eu2+ exhibits poor chromaticity for its orange emission. Moreover, the impurity of the (Sr,Ba)2SiO4:Eu2+ phase invariably coexists with the (Sr,Ba)3SiO5:Eu2+ phosphor. Interestingly, we found that the luminescence of Eu2+ could be controlled from the Sr(4c) to Sr(8f) site in the Sr3SiO5 host by co-doping with Sm2+. In order to reveal the mechanism of phase transformation, we will monitor the dynamic change of phases in variable temperature conditions by using the neutron diffraction. Furthermore, some measures will be explored to suppress the formation of the impuritry phase by constructing different environments of thermodynamic system. Secondly, we will probe the local structure of the phosphor, including the neighboring atoms, the bond lengths and their coordination number, by using the extended x-ray absorption fine structure (EXAFS) spectroscopy. Meanwhile, we will characterize the macroscopic structure by using the neutron diffraction, combining with the Rietveld refinement of the diffraction patterns. Thus, a reasonable structure model about the site occupation of doped ions in crystal lattice could be estabilished. Next, we will study the band structure, the density of states, and the charge density of the phosphor through theoretical calculation. Assisted with the experimental techniques of the synchrotron radiation photoelectron spectroscopy (SRPES) and X-ray absorption near edge structure (XANES), the position of the doped ions in the band strcuture could be identified. Accordingly, the different contribution of doped ions to the band structure and their effect on luminescence could be revealed. Finally, the constitutive relation between the crystal structure and the luminescence property could be determined. These results would provide a solid theoritical basis and be instructive to tune the emission wavelength of the (Sr,Ba)3SiO5:Eu2+ phosphor. Considering the semiconductor lighting and information display is one of the national strategic emerging industries, to some extent, the research carried out about the critical phosphor for LEDs on synchrotron instrumen has a demonstration and significance in enhancing the service of the large-scale science facilities for the demand of national strategic development.

Eu2+激活碱土硅酸盐(Sr,Ba)3SiO5:Eu2+是白光LED氮化物红色荧光粉重要替代品，但其发光颜色偏橙，易伴生(Sr,Ba)2SiO4:Eu2+杂相。有趣的是，利用Sm可调控不同格位Eu发光。为揭示相变机制，项目将在变温条件下利用中子衍射监测(Sr,Ba)3SiO5物相随温度动态变化，尝试构建不同环境热力学体系抑制杂相。其次，利用EXAFS研究包括近邻原子种类、键长和配位数在内的局域环境，结合宏观中子衍射与结构精修，建立正确结构模型。进而，通过理论计算给出能带结构、态密度和电荷密度，结合同步辐射光电子能谱和XANES技术，确定掺杂离子的能带位置，分析不同离子对能带结构的贡献，揭示晶体结构与发光性能的本构关系，从而为调控发射波长提供理论指导和参考依据。半导体照明与信息显示为国家战略新兴产业，采用同步辐射技术研究LED关键原材料有助于提升大科学装置服务国家战略需求，项目具有示范意义。

为满足半导体照明对发光效率高、热稳定性好以及显色强荧光材料的需求，本课题利用大科学装置开展LED荧光粉研究。对Sr3SiO5相变机理研究表明，Sr2SiO4与Sr(OH)2∙2H2O杂相在降温过程中由Sr3SiO5分解产生，相变发生温度在1000℃以上。采取提高降温速率、增加Sr原料配比、部分使用纳米SiO2控制结晶形核等措施有助于抑制杂相，提高发光效率。利用XNAES分析表明，在Sr3SiO5:Eu荧光材料中Eu同时存在+2和+3两种价态，以+3价为主。Sr3SiO5:Eu橘色发光不是真正来自Eu2+，而是源于Eu2+与空穴结合形成的准粒子(Eu3+)*。在Sr3SiO5:Eu晶格点阵中，Eu主要占据Wyck.8f格位，少量占据Wyck.4c格位。对应两种格位Eu，分别形成低能Wyck.4c中心与高能Wyck.8f中心两种陷阱能级。利用Ba与Sm调控Sr3SiO5:Eu发光颜色的机制主要是调节晶体缺陷数量与陷阱能级深度。Ba浓度增加导致浅陷阱能级向高能方向移动、深陷阱能级向低能方向移动。Ba增加导致Eu2+相对含量先增减后减少，所以(Sr,Ba)3SiO5:Eu发光随Ba浓度增加先红移后蓝移。Sm引入深陷阱，Sm浓度增加虽不改变浅陷阱能级深度，但显著减少其数量。所以，随Sm浓度增加(Sr,Sm)3SiO5:Eu位于580nm低能发射逐渐消失而高能475nm发射逐渐增强。能带结构理论计算与吸收光谱证实，(Sr,Ba)3SiO5带隙几乎不随Ba浓度而改变，Ba也主要占据Wyck.8f格位。采用XANES、EXAFS与能带结构理论计算等研究手段证实，Gd掺杂导致Y3Al5O12:Ce3+电子云挤压变形、晶体场增强、电子有效质量减轻、电子铺展能级范围扩大和带隙减小，电子易于发生热离域，从而造成发光颜色红移和热猝灭。利用荧光探针与EXAFS证实，在Li2SrSiO4中存在两种Sr格位，局域结构不同于宏观均匀结构。在CaAlSiN3:Eu2+中，利用EXAFS发现Eu-N键的局域异常膨胀，费米能级移动以及电子在高能激发态相对分布几率的增加导致电子热离域发生是CaAlSiN3:Eu2+发光热猝灭的根本原因。在物质结构研究基础上，本课题利用硅酸盐作为前驱物开展物质结构转化与LED器件封装荧光粉研究。已发表论文9篇、申请发明专利4件，培养博士生2名、硕士生9名，完满完成了各项任务。