Giang Lam Thi Kieu, Trejgis Karolina, Marciniak Lukasz, Vu Nguyen, Minh Le Quoc
Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam.
Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam.
Sci Rep. 2020 Sep 4;10(1):14672. doi: 10.1038/s41598-020-71606-6.
This paper presents the use of soft template method to synthesize core and core-shell up-converting nanoparticles usefull for temperature sensing applications. Based on the stock solutions of core β-NaYF:Er,Yb nanoparticles and involving soft template method without any additional process of surface functionalization, it is possible to directly design the core-shell β-NaYF:Er,Yb@NaYF nanoparticles, which can be perfectly dispersed in cyclohexane and surfactants like oleic acid (OA), triethanolamine (TEA) or Cetyltrimethylammonium bromide (CTAB). The morphological, crystalline and elemental characteristics of samples were investigated by Field Emission Scanning Electron Microscopy, X-Ray Diffraction, High Resolution Transmission Electron Microscopy, Selected Area Electron Diffraction patterns and Energy-Dispersive X-Ray Spectroscopy (EDX) measurements. The results showed that the synthesized NaYF:Er,Yb@NaYF core-shell nanoparticles have roughly spherical shape, pure hexagonal β phase with core size of about 35 ± 5 nm and shell thickness of about 40 ± 5 nm. It has been shown that the coating of the β-NaYF:Er,Yb core with NaYF shell layer enables to enhance the green upconversion (UC) emission intensities in respect to red one. Under 976 nm excitation, the synthesized β-NaYF:2%Er,19%Yb@NaYF core-shell nanoparticles revealed three strong emission bands at 520 nm, 545 nm and 650 nm corresponding to H, S and F to I transitions of Er ions with the lifetimes of 215, 193 and 474 µs, respectively. The calculated CIE chromaticity coordinates proved that the emission colour of core-shell nanoparticles was changed from red into yellowish green upon increasing the power density of the 976 nm laser from 0.73 to 9.95 W/cm. The calculated slopes indicated that in the β-NaYF:2%Er,19%Yb@NaYF core-shell nanoparticles, two-photon and three-photon UC processes took place simultaneously. Although the former one is similar as in the case of β-NaYF:Er,Yb bare core nanoparticles, the latter one, three-photon UC process for green emission occurs, due to cross relaxation processes of two Er ions only within nanoparticles with core-shell architecture. Moreover, the energy difference between the H and S levels and associated constant of NaYF@NaYF host lattice were determined and they reached ~ 813 cm and 14.27 (r = 0.998), respectively. In order to investigate the suitability of nanoparticles for optical temperature sensing, the emission spectra were measured in a wide temperature range from 158 to 298 K. An exceptionally high value of relative sensitivity was obtained at 158 K and it amounted to 4.25% K. Further temperature increase resulted in gradual decrease of relative sensitivity, however, it maintained a high value > 1% K in the entire analyzed temperature range.
本文介绍了使用软模板法合成用于温度传感应用的核壳型上转换纳米颗粒。基于β-NaYF:Er,Yb核纳米颗粒的储备溶液，采用软模板法且无需任何额外的表面功能化处理，就能够直接设计出核壳型β-NaYF:Er,Yb@NaYF纳米颗粒，其可完美分散于环己烷以及油酸（OA）、三乙醇胺（TEA）或十六烷基三甲基溴化铵（CTAB）等表面活性剂中。通过场发射扫描电子显微镜、X射线衍射、高分辨率透射电子显微镜、选区电子衍射图谱和能量色散X射线光谱（EDX）测量对样品的形态、晶体和元素特征进行了研究。结果表明，合成的NaYF:Er,Yb@NaYF核壳纳米颗粒大致呈球形，为纯六方β相，核尺寸约为35±5 nm，壳厚度约为40±5 nm。研究表明，用NaYF壳层包覆β-NaYF:Er,Yb核能够增强相对于红色上转换（UC）发射强度的绿色UC发射强度。在976 nm激发下，合成的β-NaYF:2%Er,19%Yb@NaYF核壳纳米颗粒在520 nm、545 nm和650 nm处显示出三个强发射带，分别对应于Er离子的H、S和F到I跃迁，寿命分别为215、193和474 μs。计算得到的CIE色度坐标表明，当976 nm激光的功率密度从0.73 W/cm增加到9.95 W/cm时，核壳纳米颗粒的发射颜色从红色变为黄绿色。计算得到的斜率表明，在β-NaYF:2%Er,19%Yb@NaYF核壳纳米颗粒中，双光子和三光子UC过程同时发生。虽然前者与β-NaYF:Er,Yb裸核纳米颗粒的情况相似，但由于仅在具有核壳结构的纳米颗粒内两个Er离子的交叉弛豫过程，后者即绿色发射的三光子UC过程发生了。此外，还确定了H和S能级之间的能量差以及NaYF@NaYF主体晶格的相关常数，它们分别达到约813 cm和14.27（r = 0.998）。为了研究纳米颗粒用于光学温度传感的适用性，在158至298 K的宽温度范围内测量了发射光谱。在158 K时获得了异常高的相对灵敏度值，达到4.25% K。进一步升高温度导致相对灵敏度逐渐降低，然而，在整个分析温度范围内其仍保持>1% K的高值。
