高浓度敏化剂掺杂的NaYF_4∶Yb^(3+),Er^(3+)纳米材料的合成及发光性能研究

Synthesis and Upconversion Mechanism of NaYF_4∶Yb^(3+),Er^(3+) Nanocrystal Doped with Different Concentration of Sensitizer

为了探究泵浦功率对不同浓度敏化剂离子掺杂的上转换材料发光特性的影响,采用溶剂热法,成功制备了不同浓度敏化剂Yb^(3+)掺杂的NaYF_4∶Yb^(3+), Er^(3+)上转换纳米颗粒。首先对这种纳米晶体的结构和形貌进行了详细的分析,使用X射线粉末衍射仪和透射电子显微镜测试了制备的纳米晶体的结构和形貌。表征结果证明了制备的纳米颗粒均为结晶性良好、形状规则的六方相纳米晶体,随着Yb^(3+)掺杂浓度的提高,纳米晶体的粒径有所增加。在此基础上,通过控制泵浦功率对不同浓度敏化剂Yb^(3+)掺杂的NaYF_4∶Yb^(3+), Er^(3+)上转换纳米颗粒在980 nm激发光下的光致发光特性做了详细的研究。对于不同浓度敏化剂掺杂的样品,随着泵浦功率的提高,上转换发光的强度增强,这可以归因于高泵浦功率促进材料对激发光的吸收。上转换发光的红绿比也得到了提高,值得注意的是,在不同浓度敏化剂Yb^(3+)掺杂的样品中,发光的红绿比改变的程度和可调谐的范围有所不同。为了深入的了解上转换发光机制,对不同浓度样品中可能发生的电子能量传递机制进行了讨论并提出假设,认为上转换发光过程中,不同浓度样品中红绿比变化程度的不同是发光离子组合之间的平均距离和包括多声子弛豫、交叉弛豫和反向能量传递的非弛豫过程的综合作用。在低浓度敏化剂掺杂的样品中,由于掺杂浓度导致Yb^(3+)和Er^(3+)之间的平均距离较大,反向能量传递过程比较微弱。在非弛豫过程中,发生在同一发光中心Er^(3+)上的多声子弛豫和相邻发光中心Er^(3+)之间的交叉弛豫为主要过程。随着泵浦功率的提高,高能级的布居速率增加减弱了非辐射弛豫对发光的影响,材料的红绿比只有微弱的提高,绿光是上转换发光中的主要成分。随着掺杂浓度的提高,敏化剂离子Yb^(3+)和激活剂离子Er^(3+)之间的平均间距减小,反向能量传递过程得到增强,成为非辐射弛豫过程中的主要部分。由于泵浦功率增强而提高的高能级对上转换发光的贡献,通过相邻敏化剂和激活剂离子之间的反向能量传递过程得到迅速的衰减,使红光成为上转换发光中的主要成分。在980 nm的近红外光激发下,在不同浓度Yb^(3+)掺杂的样品中存在不同侧重的非辐射弛豫过程,由于多声子弛豫、交叉弛豫和反向能量传递共同作用,红绿比随着泵浦功率提高而增加。这种发光特性不但使得我们得到红光性能更好的上转换荧光材料,而且可以通过测定材料的红绿比来判定材料的掺杂浓度。经过进一步的设计和修饰,这种纳米材料很有潜力作为一种多功能光动力治疗纳米平台在生物检测领域中得到应用。不同浓度样品中可能发生的电子能量转移过程的提出,有利于对上转换发光机理的了解和稀土发光离子组合的设计和优化。

In order to investigate the effect of pump power on the luminescence properties of upconversion materials doped with different concentrations of sensitizer ions,in this study,NaYF 4∶Yb^3+,Er^3+ upconversion nanoparticles doped with different concentration of sensitizer Yb^3+ were successfully synthesized by solvent-thermal method.The morphology and structure of prepared sample were charactered by XRD and TEM measurements.The results suggested that these samples were all hexagonal nanocrystals with good crystallinity.As the concentration of Yb ^3+ increased, the particle size increased slightly.At the same time,the photoluminescence properties of these prepared nanoparticle excited by 980 nm were studied in detail by collecting the pump power-dependence fluorescence spectrum.For all samples,the intensity of upconversion fluorescence increases with the enchancement of excitation irradiance which can be attributed to the fact that high pump power induced the higher absorption efficiency of nanoparticles.Besides,the red green ratio (RGR) can be tuned by adjusting the excitation irradiance too.And it’s worth noted that the tuning range of RGR depends on the doping ratio of sensitizer Yb^3+ in NaYF 4∶Yb^3+,Er^3+.In order to deeply understand the mechanism of upconversion luminescence,the possible electron energy transfer process was proposed.We assumed that the tuning range of RGR is related to the different average distance between rare earth ions and the comprehensive effect of the process of multi-phonon relaxation,cross-relaxation,and back energy transfer.The sample with low Yb^3+ doping concentration has a negligibly back energy transfer probability due to the fact that average distance between Yb^3+ and Er^3+ is long.The multiphonone relaxation and corss-relaxation are the main processes that convert a part of green emission into red emission.Following the enchancement of excitation irradiation,the benefit of the high excitation irradiance can relief this insufficient,and the red green ratio increases slightly.In heavily doping samples,the back energy transfer process between neighboring Yb^3+ and Er^3+ happened more probably and became the main factor for the nonradiative process.High-lying levels show a decreasing contribution,which leads to a increasing red green ratio followed the enhancement of pump power.The red green ratio increases with the increasing pump power due to different emphases of nonradiative processes in NaYF 4∶Yb^3+,Er^3+ nanoparticles doped with different concentrations of Yb^3+.The luminescence properties of the prepared UCNPs not only allow us obtaining upconversion nanoparticles with better red emission performance,but also determine the doping ratio by measuring the red-green ratio of the material.All results indicated that the material is potentially to be a multifunctional photodynamic therapy nanoplatform used in bio-detection filed through further design and modification.The possible electron energy transfer process is proposed which is helpful in designing and optimizing the doping of rare earth ion-pair,and understand the mechanism of upconversion luminescence.