NMR as a Probe of the Homogeneity of Paramagnetic Doping/Substitution

晶体结构中的缺陷可以用来改变、调整和控制主体的材料性质。在可以受掺杂影响的特性中，发光效率是其中之一。如果激发态通过一系列掺杂的顺磁原子链以无辐射的方式传输到所谓的杀手位，从而将激发态转换为基态和热量，则猝灭会更有效。显然，猝灭效率取决于主体结构中掺杂的空间分布。因此，在较低的掺杂浓度下，均匀掺杂的样品应该比非均匀掺杂的样品具有更好的发光性能。正如最近的出版物所显示的那样，可以在大约的范围内评估掺杂的均质性。利用核磁共振活性核的相对可见度，从0.5纳米到2纳米。该方法是基于“核磁共振-可见”样品的贡献随掺杂浓度的增加而衰减的，本项目的目的是验证固体核磁共振探测的掺杂均一性对发光性能有正向影响的假设。计划中的实验是为回答以下问题而设计的。核磁共振可见样品体积如何与掺杂剂的电子状态、均匀程度和温度有关？发光性能如何取决于掺杂均匀？先前工作的一个重要见解是，核磁共振探测掺杂均匀的长度与无辐射传输所需的长度相同。该项目的一个要求是合成掺杂系列晶体主体化合物的相纯。故意选择稀土元素作为唯一的掺杂物，以减少化学环境的影响，并容易找到对结果的解释。对于该方法的成功应用，唯一的要求是获得任意原子核的核磁共振信号。因此，该方法可用于多种物质类别的表征，例如硅酸盐(Si-29)、卤化物(F-19、Cl-35/37、Br79/81)、氮化物(N-14/15)、磷酸盐(P-31)、硼酸盐(B-10/11)或铝酸盐(Al-27)。

Defects in crystalline structures may be used to change, adjust and control the materials properties of the host. Among the properties which can be influenced by doping is luminescence efficiency. Quenching is more efficient if the excited state is transported radiationless over a chain of doped paramagnetic atoms to so-called killer sites, which convert the excited state to the ground state and heat. Obviously the efficiency of quenching depends on the spatial distribution of dopands in the host structure. Thus a homogeneously doped sample should have better luminescence properties than a heterogeneously doped sample at a lower doping concentration. As shown in recent publications doping homogeneity can be evaluated on a scale of approx. 0.5 nm to 2 nm using the relative visibility of the NMR-active nuclei. The method is based on the decay of the contribution of the “NMR-visible” sample with rising doping concentration.The target of this project is to test the hypothesis that doping homogeneity as probed by solid-state NMR has a positive influence on luminescence properties. The planned experiments were designed, to answer the following questions.How does the “NMR-visible” sample volume relate to the electronic state of the dopant, the degree of homogeneity and temperature?How do luminescent properties depend on doping homogeneity?An important insight from prior work was the insight, that NMR probes doping homogeneity on the same length scale as the one required for radiationless transport.A requirement for this project is the phase-pure synthesis of doping series of crystalline host compounds. Lanthanides were chosen as the sole dopands deliberately, in order to reduce the influence of the chemical surrounding and to ease finding explanations for the results. For the successful application of the method the only requirement is the availability of an NMR signal of an arbitrary nucleus. Thus the method may be used for the characterization of a number of substance classes, e.g. of silicates (Si-29), halogenides (F-19, Cl-35/37, Br-79/81), nitrides (N-14/15), phosphates (P-31), borates (B-10/11) or aluminates (Al-27).