掺杂Li^(+)的NaGdF_(4)∶Yb^(3+)/Eu^(3+)上转换发光增强及防伪应用

Enhanced Up-Conversion Emission of NaGdF_(4):Yb^(3+)/Eu^(3+)Crystal via Li^(+) Doping for Anti-Counterfeiting Application

稀土发光材料由于其良好的上转换发光性能已经逐渐成为荧光防伪领域的研究热点。首先采用水热法合成了一系列不同Li^(+)掺杂浓度的NaGdF_(4)∶Yb^(3+)/Eu^(3+)晶体,利用X射线衍射(XRD)、扫描电子显微镜(SEM)、上转换发射光谱测试等表征方法,对样品的形貌、尺寸和上转换发光性能进行了分析,并选择发光强度和效果最好的晶体应用于防伪识别。实验结果表明,NaGdF_(4)∶Yb^(3+)/Eu^(3+)/Li^(+)的XRD衍射图谱与标准六方相NaGdF4的衍射峰一致,衍射峰尖锐且没有杂峰,证明合成了高纯度高结晶度的NaGdF4∶Yb^(3+)/Eu^(3+)/Li^(+)晶体。晶体的SEM图可以看出生成的样品为纯六方相,分布均匀,无团聚现象。共掺Yb^(3+)/Eu^(3+)/Li^(+)对晶体的结构、形貌、尺寸几乎没有影响。从上转换发射光谱可以看出,15 mol%Li^(+)掺杂的NaGdF4∶Yb^(3+)/Eu^(3+)晶体绿色发光强度是未掺杂Li^(+)的样品的6倍。考察了980 nm激光器的功率变化对未掺杂Li^(+)以及掺杂15 mol%Li^(+)样品的上转换发光性能的影响,发现随着激发功率从0.8 W增加到2.2 W的过程中,上转换发光强度逐渐增强。拟合后得到样品上转换过程中的所需光子数n值接近于2,为双光子发光过程。对NaGdF4∶Yb/Eu/Li进行了荧光寿命测试,5D_(1)能级的荧光寿命是未掺杂的1.4倍。最后,将形貌均匀、发光强度强的NaGdF_(4)∶0.2Yb/0.02Eu/0.15Li制成防伪油墨,通过丝网印刷技术在纸张、玻璃、塑料等材料上印刷荧光防伪图案。在980 nm激光的泵浦下,防伪图案发出明亮的绿光。在自然环境下,纸张上的防伪图案有很好的隐蔽性。图案中的“safe”字样长度为5.5 mm,字母间距为0.5 mm,在980 nm激光的泵浦下,仍然可以分辨清楚。将印制有防伪图案的塑料暴露在室外自然环境下一个月,图案没有明显改变。表明NaGdF_(4)∶0.2Yb/0.02Eu/0.15Li制成的防伪图案分辨率高,易于识别,受环境影响小,在防伪识别方面具有良好的应用前景。

Rare earth luminescent materials have gradually become a research hotspot in fluorescence anti-counterfeiting because of their high purity of luminous color,long fluorescent life,stable physical-chemical properties,and low toxicity.A series of NaGdF4∶Yb^(3+)/Eu^(3+)microcrystals co-doped with various Li^(+)concentrations were synthesized by the hydrothermal method in this paper.The samples'morphology,size,and up-conversion luminescence properties were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM),up-conversion emission spectroscopy,and fluorescence lifetime tests.The crystal with strong luminous intensity was further applied to anti-counterfeiting identification.It shows that all the diffraction peaks of NaGdF4∶Yb^(3+)/Eu^(3+)/Li^(+)microcrystals are consistent with the standard-NaGdF4 card.No impurity peak was found in the XRD pattern.The hexagonal NaGdF4∶Yb^(3+)/Eu^(3+)/Li^(+)with high purity and crystallinity was synthesized.The SEM image of the crystal shows that the generated sample is a pure hexagonal phase,with uniform distribution,and no reunion.Co-doped Yb^(3+)/Eu^(3+)/Li^(+)has little effect on crystal structure,morphology and size.It can be seen from the up-conversion emission spectrum that the green luminescence intensity of 15 mol%Li^(+)doped NaGdF4∶Yb^(3+)/Eu^(3+)crystal is 6 times higher than that of the undoped Li^(+)sample.Adjust the power range of the laser to 0.8~2.2 W and observe the change in UCL intensity of the samples doped with 0 mol%Li^(+)and 15 mol%Li^(+).It can be observed that with the increase of pump power,the up-conversion intensity gradually increases.The number of photons required to generate the up-conversion luminescence n is close to 2,indicating that the emission process of the sample is a two-photon process.The fluorescence lifetime of the 5D1 level in the sample is about 1.4 times that of the undoped one.Finally,the NaGdF4∶0.2Yb/0.02Eu/0.15Li crystal with uniform morphology and strong luminous intensity was further applied as fluorescent ink.Screen printing technology printed The fluorescent anti-counterfeiting patterns on paper,glass and plastic.The pattern emitted bright green light under the pumping of a 980 nm laser.In the natural environment,the anti-counterfeiting pattern on the paper has good concealment.The word“safe”lenght is 5.5 mm,and the spacing between letters is 0.5 mm.The boundaries between letters are clear and easy to distinguish under 980 nm excitation.The plastic printed with the anti-counterfeiting pattern was exposed to the outdoor natural environment for a month,and the pattern did not change significantly.It shows that the anti-counterfeiting pattern made of NaGdF4∶0.2Yb/0.02Eu/0.15Li has a high resolution,is easy to identify,and is less affected by the environment,and has excellent application prospects in anti-counterfeiting identification.