Qiang Gang, Golovatenko Aleksandr A, Shornikova Elena V, Yakovlev Dmitri R, Rodina Anna V, Zhukov Evgeny A, Kalitukha Ina V, Sapega Victor F, Kaibyshev Vadim Kh, Prosnikov Mikhail A, Christianen Peter C M, Onushchenko Aleksei A, Bayer Manfred
Experimentelle Physik 2, Technische Universität Dortmund, 44227 Dortmund, Germany.
Nanoscale. 2021 Jan 14;13(2):790-800. doi: 10.1039/d0nr07117j. Epub 2020 Dec 22.
The recombination dynamics and spin polarization of excitons in CdSe nanocrystals synthesized in a glass matrix are investigated using polarized photoluminescence in high magnetic fields up to 30 Tesla. The dynamics are accelerated by increasing temperature and magnetic field, confirming the dark exciton nature of low-temperature photoluminescence (PL). The circularly polarized PL in magnetic fields reveals several unusual appearances: (i) a spectral dependence of the polarization degree, (ii) its low saturation value, and (iii) a stronger intensity of the Zeeman component which is higher in energy. The latter feature is the most surprising being in contradiction with the thermal population of the exciton spin sublevels. The same contradiction was previously observed in the ensemble of wet-chemically synthesized CdSe nanocrystals but was not understood. We present a theory which explains all the observed features and shows that the inverted ordering of the circularly polarized PL maxima from the ensemble of nanocrystals is a result of competition between the zero phonon (ZPL) and one optical phonon-assisted (1PL) emission of the dark excitons. The essential aspects of the theoretical model are different polarization properties of the dark exciton emission via ZPL and 1PL recombination channels and the inhomogeneous broadening of the PL spectrum from the ensemble of nanocrystals exceeding the optical phonon energy.
利用高达30特斯拉的高磁场中的偏振光致发光，研究了玻璃基质中合成的CdSe纳米晶体中激子的复合动力学和自旋极化。通过升高温度和磁场加速动力学过程，证实了低温光致发光（PL）的暗激子性质。磁场中的圆偏振PL呈现出几种不寻常的现象：（i）偏振度的光谱依赖性，（ii）其低饱和值，以及（iii）能量较高的塞曼分量强度更强。后一个特征最为惊人，与激子自旋子能级的热分布相矛盾。之前在湿化学合成的CdSe纳米晶体集合中也观察到了同样的矛盾，但未得到解释。我们提出了一种理论，该理论解释了所有观察到的特征，并表明纳米晶体集合中圆偏振PL最大值的反转顺序是暗激子的零声子（ZPL）和一个光学声子辅助（1PL）发射之间竞争的结果。理论模型的关键方面是通过ZPL和1PL复合通道的暗激子发射的不同偏振特性，以及纳米晶体集合的PL光谱的非均匀展宽超过光学声子能量。
