Investigation of anisotropic spin phenomena in doped single nanocrystals probed by luminescence spectroscopy in a vector magnetic field

本项目的总体目标是探索具有特定晶体和/或形状各向异性的过渡金属掺杂胶体纳米晶体中的集体和个体自旋效应。这需要在方法上进行重大创新，即在矢量磁场中对单纳米晶体进行发光实验。作为一个模型系统，我们将主要集中在纤锌矿或锌闪锌矿结构的掺杂Mn2+的II-VI型工程纳米晶体上。主要的科学问题可分为三个子项。在掺杂过渡金属离子的纳米晶体中，其目的是揭示晶体和/或形状各向异性对激子磁性极化子复合物形成的影响-这种效应已经假设了十多年，但从未得到证实。因此，相对于主要各向异性轴的不同方向的磁场的应用将允许有目的地稳定/不稳定磁极化子的形成。此外，我们打算解决统计磁波动在个别纳米晶体中极化子形成和发射线宽度的作用，区分横向和纵向自旋波动。我们将研究掺杂单一杂质的纳米晶体，以阐述单原子掺杂纳米晶体中s-p和sp-d交换相互作用的联合作用。在这里，我们假设在过渡金属掺杂的情况下，由于明显的sp-d交换相互作用，未掺杂纳米晶体的最低激子态的精细结构最有可能被修改。根据Mn2+的内部4T1 - 6A1跃迁仔细调整激子带隙将允许双重发射，因为这些状态之间的能量转移相当快。计划研究单双发射纳米晶体，其中由于抑制了非均匀展宽，预计发射对温度和磁场的灵敏度会提高。由于单激子-单掺杂相互作用导致激子- Mn2+配合物中不同自旋态之间存在明显的能量分裂，单原子掺杂纳米晶体的双发射研究可能为自旋选择性能量转移提供线索。

The overall goal of this project is to explore collective and individual spin effects in transition metal doped colloidal nanocrystals of defined crystal and/or shape anisotropy. This requires a main innovation in methodology, namely luminescence experiments on single nanocrystals in a vector magnetic field. As a model system, we will predominantly concentrate on Mn2+-doped II-VI shaped-engineered nanocrystals of either wurtzite or zincblende structure. The main scientific questions can be split into three subitems. In nanocrystals doped with a collection of transition metal ions it is intended to unravel the impact of crystal and/or shape anisotropy on the formation of an exciton magnetic polaron complex – an effect which has been hypothesized since more than a decade, but never been proven. Hereby, the application of a magnetic field with varying orientation with respect to the dominant anisotropy axis will allow to purposely stabilize / destabilize magnetic polaron formation. We in addition intend to address the role of statistical magnetic fluctuations on polaron formation and emission linewidth in individual nanocrystals, separating between transversal and longitudinal spin fluctuations. Nanocrystals doped with single impurities will be investigated to elaborate the combined action of s-p and sp-d exchange interactions in single-atom doped nanocrystals. Here, we hypothesize that the fine structure of the lowest exciton state well-established for undoped nanocrystals has most likely to be revised in case of transition metal doping due to pronounced sp-d exchange interactions. Adjusting the exciton bandgap carefully with respect to the internal 4T1 – 6A1 transition of the Mn2+ will allow for dual emission because of the quite rapid energy transfer between these states. It is planned to study single dual emitting nanocrystals, where an enhanced sensitivity of the emission to temperature and magnetic field is expected due to the suppressed inhomogeneous broadening. As the single exciton – single dopant interaction leads to pronounced energy splitting between different Mn2+ spin states within the exciton - Mn2+ complex, the study of dual emission in single-atom doped nanocrystals may shed light on a possible spin-selective energy transfer.