Stecher Joshua T, Rohlfing Anne B, Therien Michael J
Department of Chemistry, Duke University, French Family Science Center, 124 Science Drive, Durham, NC 27708, USA.
Nanomaterials (Basel). 2014 Jan 8;4(1):69-86. doi: 10.3390/nano4010069.
We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF₃ and BaLaF₅ (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the total cation content; X-ray diffraction (XRD) data indicate a shift in unit cell dimensions consistent with these small codopant ions. Similarly, X-ray photoelectron spectroscopic (XPS) analysis reveals primarily substitution of Na⁺ for La ions (97% of total Na⁺ codopant) in the crystal system, and interstitial Na⁺ (3% of detected Na⁺) and La (3% of detected La) present in (Na)LaF₃ and only direct substitution of Na⁺ for Ba in Ba(Na)LaF₅. In each case, XPS analysis of La 3d lines show a decrease in binding energy (0.08-0.25 eV) indicating a reduction in local crystal field symmetry surrounding rare earth (R.E.) ions, permitting otherwise disallowed R.E. UC transitions to be enhanced. Studies that examine the impact of laser excitation power upon luminescence intensity were conducted over 2.5-100 W/cm² range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm ³F₃ and ³H₄ states was observed and noted as a key initial condition for effective population of the ¹D₂ and ¹I₆ UV emitting states, via Tm-Tm cross-relaxation.
我们报道了一种基于钠共掺杂的LaF₃和BaLaF₅（0.5%Tm；20%Yb）及其相应的核壳衍生物的一锅法溶剂热合成亚10纳米、主要发射紫外（UV）光的上转换纳米晶体（UCNC）。元素分析表明，在这些晶体体系中钠共掺杂剂占总阳离子含量的约20%；X射线衍射（XRD）数据表明晶胞尺寸的变化与这些小的共掺杂离子一致。同样，X射线光电子能谱（XPS）分析表明，在晶体体系中主要是Na⁺取代了La离子（占总Na⁺共掺杂剂的97%），并且间隙Na⁺（占检测到的Na⁺的3%）和La（占检测到的La的3%）存在于(Na)LaF₃中，而在Ba(Na)LaF₅中只有Na⁺直接取代了Ba。在每种情况下，对La 3d谱线的XPS分析显示结合能降低（0.08 - 0.25 eV），这表明围绕稀土（R.E.）离子的局部晶体场对称性降低，使得原本禁戒的R.E.上转换跃迁得以增强。在2.5 - 100 W/cm²范围内进行了研究激光激发功率对发光强度影响的实验，以阐明产生主要紫外发射态的上转换机制。观察到了Tm³F₃和³H₄态的低功率饱和现象，并将其作为通过Tm - Tm交叉弛豫有效填充¹D₂和¹I₆紫外发射态的关键初始条件。
