研究一种具有良好上转换发光性能的稀土掺杂发光材料, 对于防伪技术领域具有非常重要的意义。 为了改善LiYF₄∶Yb³⁺/Ho³⁺微米晶体的上转换发光性能, 采用水热合成法成功制备了一系列Gd³⁺掺杂的LiYF₄∶Yb³⁺/Ho³⁺微米晶体, 并采用X射线衍射(XRD)和扫描电子显微镜(SEM)对样品的相纯度和晶体形貌尺寸进行表征; 在980 nm激光激发下, 通过荧光光谱测试对LiGd_xY_1-xF₄∶Yb³⁺/Ho³⁺微米晶体的上转换发光性能进行分析。 首先, 研究了LiGd_xY_1-xF₄∶Yb³⁺/Ho³⁺微米晶体的晶体结构、 尺寸、 形貌和上转换发光性能的影响。 结果显示, LiGd_xY_1-xF₄∶Yb³⁺/Ho³⁺微米晶体样品的XRD衍射峰与四方相的LiYF₄标准卡(PDF#17-0874)特征峰的位置完全对应且没有其他杂峰, SEM实验结果显示晶体形貌为八面体形状, 表明成功合成了纯四方相的LiGd_xY_1-xF₄∶Yb³⁺/Ho³⁺微米晶体; 荧光光谱测试结果显示, 样品的上转换发光强度随着Gd³⁺掺杂比例的升高呈现出先增强后减弱的趋势, 并且在Gd³⁺掺杂浓度为30 mol%时达到最强。 其次, 进一步研究了Gd³⁺掺杂浓度30 mol%样品的上转换发光性能与激发功率之间的关系, 激发功率为0.5~1.5 W。 LiGd_0.3Y_0.49F₄∶Yb³⁺/Ho³⁺微米晶体的红色和绿色上转换发光强度之比(R/G)随着激发功率的增加只发生大约12%的变化, 样品的上转换发光并没有因为激发功率的增加而发生明显的变色, 仍然可以发出稳定明亮的绿色光。 这一现象表明, Gd³⁺的掺入很好地改善了样品的上转换发光性能, 这种稳定高效的发光性能保证了其良好的防伪性能。 最后, 将Gd³⁺掺杂浓度为30 mol%的LiYF₄∶Yb³⁺/Ho³⁺微米晶体粉末与丝网金属油墨按照一定比例混合制成丝网防伪油墨, 通过丝网印刷技术在玻璃基底上印制了“西安”字样的防伪标识图案, 经过干燥处理后在980 nm激光的激发下, 发出明亮且稳定的绿色可见光, 制成的防伪标识图案具有发光强度高、 易于识别、 不易脱落的特点, 可被广泛应用于防伪领域。

Li ions affect the upconversion efficiency by changing the local crystal field of the luminescent center. Herein, in order to improve the upconversion efficiency of NaYF₄:Yb³⁺/Eu³⁺, a series of NaYF₄:Yb³⁺/Eu³⁺ micro-particles with different Li⁺ doping concentrations were synthesized by the hydrothermal synthesis method, respectively. Firstly, the structure and morphology of NaYF₄:Yb³⁺/Eu³⁺ upconversion micro-particles (UCMPs) with different doping concentrations were analyzed by X-ray diffraction and a scanning electron microscope (SEM). SEM results show that the UCMPs are not only highly crystallized, but also have hexagons with different Li⁺ concentrations of NaYF₄:Yb³⁺/Eu³⁺. X-ray diffraction shows that the crystal field around Eu³⁺ changes with the increase of Li⁺ concentration. Then, the fluorescence spectrum of NaYF₄:Yb³⁺/Eu³⁺ was studied under the irradiation of a 980 nm laser. The results show that the fluorescence intensity of NaYF₄:Yb³⁺/Eu³⁺ with 2% Li⁺ is the strongest, which is twice the intensity of NaYF₄:Yb³⁺/Eu³⁺ without Li⁺. Finally, the fluorescence imaging analysis of NaYF₄:Yb³⁺/Eu³⁺ with 2% Li⁺ concentration was carried out. The UCMPs are used to screen printing to evaluate the imaging effect on different sample surfaces. The results show NaYF₄:Yb³⁺/Eu³⁺ (with 2% Li⁺) has great application prospects in anti-counterfeiting recognition.