铒离子掺杂增强钙钛矿型纳米颗粒上转换发光和热灵敏性

稀土上转换发光（UCL）材料因其优异的性能，在彩色显示、温度传感器器件、药物传输、光伏、细胞成像和肿瘤治疗等方面具有巨大的应用价值，从而倍受关注。

一般来说，上转换发光材料的发射效率由激活剂、敏化剂和基质材料决定。其中，基质材料的选择在上转换（UC）发射效率中起到不可忽视的作用。近年来，钙钛矿型氧化物(ABO₃)作为基质材料凭借其优良特性得到了广泛的研究。一方面，钙钛矿型氧化物具有可调性较大的晶体结构，且A位离子(Ca²⁺、Pb²⁺、Ba²⁺和Sr²⁺)和B位离子（Ti⁴⁺）的半径和镧系离子的半径接近。因此，将镧系离子引入钙钛矿氧化物的晶格具有很高的可行性。另一方面，钙钛矿型氧化物具有优异物理性能、化学稳定性和低晶格声子能，是最有前途的多种应用基体材料之一。

为了获得较高的发射效率，通常需要将敏化剂与激活剂共掺杂到UCL基质中。虽然稀土离子掺杂钙钛矿氧化物的上转换发光材料已有报道，但有关稀土离子掺杂纳米尺度PbTiO₃（PTO）的上转换发光以及温度传感特性的报道很少。因此，对稀土离子掺杂的PTO纳米粒子的上转换发光性能研究具有重要意义。

2015年，浙江大学韩高荣教授领导的课题组验证了Er³⁺掺杂PTO纳米纤维的UCL强度受到基质相结构和极化性能影响。当Er³⁺离子掺杂到前钙钛矿（利用高分子辅助水热法制备出新结构的PTO单晶纳米管,其在一定条件下可向普通的钙钛矿PTO转变,从而称这种新结构为“前钙钛矿”）或钙钛矿PTO纳米纤维中时，由于取代位置和化学环境的不同，前钙钛矿结构的纳米纤维没有上转换发光，而钙钛矿结构表现出绿色和红色发光。

从以上研究工作着手，中国计量大学肖珍博士等进行了系列创新研究，相关成果发表于Chinese Optics Letters2021年第2期（Jing Zhu, Shiqing Xu, Lei Lei, Feifei Huang, Zhen Xiao. Empowering perovskite PbTiO₃ nanoparticles with enhanced up-conversion luminescence and thermal sensitivity by introducing Er³⁺ dopant[J]. Chinese Optics Letters, 2021, 19(2): 021601）。

980 nm激光激发下Er-PTO纳米颗粒的浓度依赖和温度依赖荧光发射光谱

该研究工作中采用水热法制备了一系列不同Er3+掺杂浓度的钙钛矿Er-PTO纳米颗粒，详细研究了掺杂浓度对材料形貌、微观结构和UCL性能的影响，发现Er-PTO纳米颗粒拥有绿色和红色发光，辐射中心分别在520，550和660 nm附近。此外，还研究了温度对钙钛矿Er-PTO纳米颗粒上转换发光的影响，根据FIR技术（荧光强度比技术，一种高灵敏度、高信号辨识度和宽范围动态成像的非接触测量方法）发现，当Er³⁺离子浓度为1 mol%时，Er-PTO样品在474 K处的灵敏度为3.1×10^-3 K^-1。同时，Er掺杂铁电材料的上转换发光性能对温度的依赖性，使其有望应用到温度传感器中。

肖珍博士表示，该项工作的研究结果不仅表明Er-PTO材料在室温下具有优异的发光性能，还证实了这些上转换发光材料在传感领域也具有应用潜力。未来的工作将聚焦于钙钛矿氧化物基上转换发光材料的结构和性能的优化上，以充分发掘和实现上转换发光材料的应用潜力。

Empowering Perovskite PbTiO₃ Nanoparticles with Enhanced Up-conversion Luminescence and Thermal Sensitivity by Introducing Er³⁺ Dopant

Up-conversion luminescence (UCL) materials doped with rare earth (RE) ions have deserved soaring attention in the past decades because of their excellent properties and potential applications in color displays, temperature sensor devices, drug delivery, photovoltaic, cell imaging and tumor therapy.

Generally, the emission efficiency of the UCL materials are determined by activators, sensitizers and crystalline hosts. Among them, a careful selection of host materials is required to achieve efficient UC emission. Perovskite-type oxides (ABO₃), including of SrTiO₃, are intensively studied as the host materials. On the one hand, the perovskite-type oxides have adjustable crystal structure, and the radius of A-site ions (Ca²⁺, Pb²⁺, Ba²⁺ and Sr²⁺) and B-site ion of Ti⁴⁺ are close to the radius of lanthanide ions. Consequently, the introduction of lanthanide ions into the crystal lattice of perovskite oxides has highly practical viability. On the other hand, the perovskite-type oxides have excellent physical properties, chemical stability and low lattice phonon energies, which are regarded as one of the most promising host materials for various applications.

Meanwhile, the doping concentration has an obvious influence on emission efficiency. To obtain high emission efficiency, a sensitizer is commonly co-doped into UCL hosts along with the activators. While UCL materials doped with rare earth (RE) ions have been reported, their performance has been severely limited by the doping ion concentration, and the UC properties of rare-earth ions doped PbTiO₃ with nanoparticle morphology is rarely reported, as well as the temperature sensing properties. Therefore, developing Er-PTO nanoparticles is of paramount importance.

Recently, the research group led by Han from Zhejiang University verified the UCL intensity of Er³⁺-doped PbTiO₃ nanofibers can be modulated by changing the phase structure and polarization of the host matrix. When the Er³⁺ ions doped into pre-perovskite or perovskite PbTiO₃ nanofibers, due to the differences of substitution sites and chemical environments, the pre-perovskite one had no UC emission, whereas the perovskite one exhibited green and red emissions. Moreover, the UCL emissions could be modulated by adjusting the tetragonality of the perovskite PbTiO₃ nanofibers.

Based on the above work, Dr. Zhen Xiao et al from China Jiliang University have carried out a series of innovative research, and the relevant results are published in the second issue of Chinese Optics Letters, Vol. 19, No. 2, 2021 (Jing Zhu, Shiqing Xu, Lei Lei, Feifei Huang, Zhen Xiao. Empowering perovskite PbTiO₃ nanoparticles with enhanced up-conversion luminescence and thermal sensitivity by introducing Er³⁺ dopant[J]. Chinese Optics Letters, 2021, 19(2): 021601).

Plots of UC emission spectra versus the Er³⁺ doping concentration and temperature of Er-PTO nanoparticles upon 980 nm excitation

In this work, we prepared a serious of perovskite Er-PTO nanoparticles with different Er³⁺ doping concentrations by a facile hydrothermal method. The influences of dopant concentration on the morphology, microstructure and UCL properties were investigated in detail. Green and red emissions centered at 520, 550 and 660 nm are observed. The perovskite Er-PTO nanoparticles fabricated in this work present a great improvement in the UC luminescence. Both green and red emissions of PbTiO₃ samples can be enhanced by doping Er³⁺ ions.

Moreover, the UC luminescence versus temperature also has been investigated. According to the fluorescence intensity ratio technique, which is a noncontact measurement with high sensitivity, high signal discriminability and broad range dynamic imaging, Er-PTO samples with doping 1 mol% concentration of Er³⁺ ions possess a sensitivity of 3.1×10^-3 K^-1 at 474 K. The excellent UCL performance and thermal sensitivity of Er-doped ferroelectric materials indicated the great potential for multifunctional applications. By applying these Er-PTO samples into a temperature sensor link, the transmission rate will be significantly increased, which is a potential application in temperature detection and alarm.

Dr. Zhen Xiao said that her work presented the excellent emission intensity performance at room temperature. Moreover, the application of these UCL materials at high temperature has also been demonstrated. Future work will focus on the further optimizations of the UCL materials and optical set up in order to fully explore and realize the potential of these UCL materials. Furthermore, UCL materials doped with different RE ions at low temperature will be studied and constructed in the future.