Abstract Size tunable nanoparticles (NPs) have played an important role in areas of optoelectronics, drug delivery, magnetism and many others. Moreover, designing size-tunable NPs without exposing them to high temperature and long time thermal treatment is highly desirable to make agglomerate- and defect-free NPs. In this work, we have designed Gd2Hf2O7:Eu3+ (GHOE) NPs using the molten-salt synthesis (MSS) method with the precursors made from varying precipitant concentrations (correspondingly the pH of the precipitating solution). The obtained NPs have a decreasing size as the pH of the precipitating solution rises and a defect fluorite structure with a large fraction of Eu3+ ions localized at GdO8 distorted scalenohedra, a small fraction residing at Hf4+ site, and the presence of oxygen vacacnies in the vicinity of E u H f ' . Maximum photoluminescence and radioluminescence outputs and internal quantum yield have been observed from the GHOE NPs made from 5.0% NH4OH as precipitant due to optimum balance of surface defects and particle clustering. Judd-Ofelt analysis has confirmed that these GHOE NPs have lowest non-radiative transition probability, brightest red emission, largest branching ratio, and highest radiative transition rate. With increasing pH of the precipitating solution, the group symmetry of Eu3+ ions in the GHOE host decreases systematically from D4 → C4v → C3v and then saturates, consistent with the pH dependent asymmetry ratio value. Also, the extent of polarizability enhances and the Eu–O bond becomes more covalent as confirmed by the monotonic increase of Ω2/Ω4 ratio. Our work on these optical materials will assist the scientific community to make size-tunable nanoparticles at low synthesis temperature for efficient luminescent devices.

中文翻译：

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Gd2Hf2O7:Eu3+ 纳米颗粒的 pH 诱导尺寸调节及其对紫外和 X 射线激发发光的影响

摘要 尺寸可调的纳米粒子 (NPs) 在光电子学、药物输送、磁性和许多其他领域发挥了重要作用。此外，设计尺寸可调的 NPs 而不将它们暴露在高温和长时间的热处理中，对于制造无团聚和无缺陷的 NPs 是非常可取的。在这项工作中，我们使用熔盐合成 (MSS) 方法设计了 Gd2Hf2O7:Eu3+ (GHOE) NP，前体由不同的沉淀剂浓度（相应的沉淀溶液的 pH 值）制成。随着沉淀溶液的 pH 值升高，所得 NPs 的尺寸减小，并且存在缺陷萤石结构，其中大部分 Eu3+ 离子位于 GdO8 扭曲的偏三角面体，一小部分位于 Hf4+ 位点，附近存在氧空位E u H f ' 。由于表面缺陷和粒子簇的最佳平衡，从由 5.0% NH4OH 作为沉淀剂制成的 GHOE NPs 中观察到最大的光致发光和辐射发光输出和内部量子产率。Judd-Ofelt 分析证实，这些 GHOE NPs 具有最低的非辐射跃迁概率、最亮的红色发射、最大的分支比和最高的辐射跃迁率。随着沉淀溶液 pH 值的增加，GHOE 主体中 Eu3+ 离子的基团对称性从 D4 → C4v → C3v 系统地降低，然后饱和，与 pH 依赖的不对称比值一致。此外，极化程度增强并且 Eu-O 键变得更加共价，正如 Ω2/Ω4 比值的单调增加所证实的那样。