Yan Shirun
Department of Chemistry, Fudan University, Shanghai 200438, China.
Materials (Basel). 2025 Jun 8;18(12):2700. doi: 10.3390/ma18122700.
Thermally enhanced upconversion luminescence (UCL), also known as negative thermal quenching of UCL, denotes a continuous increase in the UCL emission intensity of a particular phosphor with a rising temperature. In recent years, the thermal enhancement of UCL has attracted extensive research attention, with numerous reports detailing this effect in phosphors characterized by varying particle sizes, architectures, and compositions. Several hypotheses have been formulated to explain the underlying mechanisms driving this thermal enhancement. This paper rigorously examines thermally enhanced UCL in fluoride nanoparticles by addressing two key questions: (1) Is the thermal enhancement of UCL an intrinsic feature of these nanoparticles? (2) Can the proposed mechanisms explaining this enhancement be unequivocally supported by the existing literature? Upon analyzing a compilation of experimental observations alongside the concurrent phenomena occurred during spectral measurements, it is postulated that thermally enhanced UCL intensity is likely a consequence of multiple extrinsic factors operating simultaneously at elevated temperatures, rather than being an intrinsic property of nanoparticles. These factors include moisture desorption, laser-induced local heating, and lattice thermal expansion. The size-dependent properties of nanoparticles, such as surface-to-volume ratio, thermal expansion coefficient, and quantum yield, are the fundamental reasons for the size-dependent thermal enhancement factor of UCL. Temperature-dependent emission spectral intensity is not a dependable indicator for assessing the thermal quenching properties of phosphors. This is because it is influenced not only by the phosphor's quantum yield, but also by various extrinsic factors at high temperatures. The nonlinear nature of UCL further magnifies the impact of these extrinsic factors.
热增强型上转换发光（UCL），也被称为UCL的负热猝灭，是指特定磷光体的UCL发射强度随着温度升高而持续增加。近年来，UCL的热增强现象引起了广泛的研究关注，众多报道详细阐述了在具有不同粒径、结构和组成的磷光体中的这种效应。已经提出了几种假设来解释导致这种热增强的潜在机制。本文通过解决两个关键问题，对氟化物纳米颗粒中的热增强型UCL进行了严格研究：（1）UCL的热增强是这些纳米颗粒的固有特征吗？（2）解释这种增强的所提出的机制能否得到现有文献的确切支持？在分析一系列实验观察结果以及光谱测量过程中同时出现的现象后，推测热增强型UCL强度可能是多种外在因素在高温下同时作用的结果，而不是纳米颗粒的固有属性。这些因素包括水分解吸、激光诱导的局部加热和晶格热膨胀。纳米颗粒的尺寸依赖性性质，如表面体积比、热膨胀系数和量子产率，是UCL热增强因子具有尺寸依赖性的根本原因。温度依赖性发射光谱强度不是评估磷光体热猝灭性质的可靠指标。这是因为它不仅受磷光体量子产率的影响，还受高温下各种外在因素的影响。UCL的非线性性质进一步放大了这些外在因素的影响。
