School of Materials Science and Engineering, Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University, Nanjing 211189, PR China.
Nanoscale. 2017 Aug 24;9(33):12132-12141. doi: 10.1039/c7nr03682e.
Upconversion nanoparticles (UCNPs) are an excellent choice to construct security features against counterfeiting, owing to their unique NIR-to-VIS upconversion luminescence (UCL) characteristics. However, the application of upconversion materials is limited, due to their single and invariant emission colors. Herein, the temperature-dependent UCL properties of NaGdF:Yb/Ho (or Tm) UCNPs in the solid state have been investigated. An anomalous UCL enhancement at higher temperatures has been demonstrated for these small-sized (<10 nm) UCNPs and the underlying mechanism is discussed herein. Meanwhile, effective UCL with tunable multicolor emissions has been realized by the rational incorporation of Ho and Tm emitters into a single nanostructure. The emission colors of these Ho/Tm co-doped Na(Gd,Yb)F UCNPs can be tuned by changing the laser power or temperature, due to the different spectral sensitivities of the Tm and Ho emitters to the excitation power density and temperature. The power- and temperature-responsive color shifts of these Ho/Tm co-doped UCNPs are favorable for immediate recognition by the naked eye, but are hard to copy, offering the possibility of designing more secure anti-counterfeiting patterns.
上转换纳米粒子（UCNPs）是构建防伪安全特征的绝佳选择，因为它们具有独特的近红外到可见的上转换发光（UCL）特性。然而，由于其单一且不变的发射颜色，上转换材料的应用受到限制。在此，研究了固态下 NaGdF:Yb/Ho（或 Tm）UCNPs 的温度依赖 UCL 性质。这些小尺寸（<10nm）UCNPs 表现出异常的高温 UCL 增强，并且在此讨论了其潜在机制。同时，通过将 Ho 和 Tm 发射器合理地掺入单个纳米结构中，实现了具有可调多色发射的有效 UCL。由于 Tm 和 Ho 发射器对激发光密度和温度的光谱灵敏度不同，这些共掺杂 Ho/Tm 的 Na（Gd，Yb）F UCNPs 的发射颜色可以通过改变激光功率或温度来调节。这些共掺杂 UCNPs 的功率和温度响应的颜色偏移有利于肉眼立即识别，但难以复制，为设计更安全的防伪图案提供了可能性。
