Photon Science Institute, Manchester University, Oxford Road, Manchester M13 9PL, UK.
J Phys Condens Matter. 2010 May 12;22(18):185403. doi: 10.1088/0953-8984/22/18/185403. Epub 2010 Apr 15.
Yttrium phosphate co-doped with cerium and samarium acts as a charge storage phosphor, but in highly doped material (0.5% co-doping levels), the proximity of defects leads to the uncontrolled non-radiative loss of stored charge through tunnelling. In order to characterize these defects, their mutual interactions and intra-pair charge transfer routes, experiments have been undertaken in which a laser probe is deployed during luminescence excitation using a synchrotron. Two modes of operation are described; in each case, the laser (2.8 eV) probes only Sm(2+) ions, and the detection is set to monitor exclusively Ce(3+) 5d-4f emission. Mode 1: the sample is pumped with monochromatic synchrotron photons in the range 4.5-12 eV, and the resultant charge populations probed with the laser 30 s later; this has the effect of sampling electrons trapped at Sm(2+) that are in quasi-equilibrium. Here, a clear transition between a sub-bandgap Urbach tail region and excitations above the mobility edge is especially apparent, enabling an accurate value of the conduction band energy of YPO(4) to be determined, 9.20 eV. Furthermore, the Sm(2+) and Ce(3+) ground state energies can be positioned within the bandgap (6.8 eV and 3.85 eV above the top of the valence band, respectively). Mode 2: the sample is pumped with monochromatic synchrotron photons in the range 4.5-12 eV and, during this pumping process, the laser probe is activated. This more dynamic process probes direct electron transfer excitation processes between spatially correlated Sm-Ce defect pairs, via their excited states; the laser probe enhances the Ce(3+) emission if direct electron transfer from the Ce(3+) ground state to the excited states of Sm(2+) is being pumped, or quenches the luminescence if the Ce(3+) excited states are pumped. The experiments allow for a precise measure of the difference in energy between the Sm(2+) and Ce(3+) ground states (2.98 eV).
掺铈和钐的磷酸钇作为电荷存储荧光粉，但在高掺杂材料（0.5%的共掺杂水平）中，缺陷的接近导致存储电荷通过隧道无控制地非辐射损失。为了表征这些缺陷、它们的相互作用和内对电荷转移途径，采用了在发光激发过程中使用同步加速器部署激光探针的实验。描述了两种操作模式；在每种情况下，激光（2.8eV）仅探测 Sm(2+)离子，检测设置为仅监测 Ce(3+)5d-4f 发射。模式 1：用范围为 4.5-12eV 的同步加速器单色光子泵浦样品，然后用激光在 30s 后探测所得电荷；这会捕获处于准平衡状态的 Sm(2+)上捕获的电子。在此，亚带隙 Urbach 尾部区域和迁移率边缘以上的激发之间的清晰跃迁特别明显，从而能够准确确定 YPO(4)的导带能量，为 9.20eV。此外，Sm(2+)和 Ce(3+)基态能量可以位于能带内（分别为能带顶部上方 6.8eV 和 3.85eV）。模式 2：用范围为 4.5-12eV 的同步加速器单色光子泵浦样品，并在泵浦过程中激活激光探针。这种更动态的过程通过它们的激发态探测空间相关的 Sm-Ce 缺陷对之间的直接电子转移激发过程；如果从 Ce(3+)基态到 Sm(2+)的激发态泵浦直接电子转移，则激光探针会增强 Ce(3+)发射，或者如果泵浦 Ce(3+)激发态，则猝灭发光。实验允许精确测量 Sm(2+)和 Ce(3+)基态之间的能量差（2.98eV）。
